| 1. |
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| 2. |
- Pettersson, M, et al.
(författare)
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Modelling of an electric IR heater at transient and steady state conditions Part I: model and validation
- 2000
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 43:7, s. 1209-1222
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Tidskriftsartikel (refereegranskat)abstract
- A model for an electric infrared (IR) heater has been developed. The model includes non-grey radiative heat transfer between the different parts of the IR heater, as well as conduction in reflector material and convective cooling of surfaces. The geometry is simplified into one dimension. Using IR module voltage, as the only input, the model predicts the temperature of heater components and cooling air, as well as the net radiation heat transfer to the surroundings at steady state and transient conditions. The model has been validated against both steady state and transient experimental results from a small electric IR heater. The model predictions are in good agreement with experimental data both regarding steady state results and the transient response over a wide range of voltages. (C) 2000 Elsevier Science Ltd. All rights reserved.
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| 3. |
- Pettersson, M, et al.
(författare)
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Modelling of an electric IR heater at transient and steady state conditions Part II: modelling a paper dryer
- 2000
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 43:7, s. 1223-1232
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Tidskriftsartikel (refereegranskat)abstract
- A model for an electric infrared (IR) paper dryer has been developed. The model includes non-grey radiative heat transfer between the different parts of the IR heater, as well as conduction in reflector material and convective cooling of surfaces. Such heat transfer calculations are combined with energy balances to provide a system of equations that simulates the behaviour of an electric III dryer. Using IR module voltage as the only input, the model predicts the temperature of dryer components and cooling air, as well as the net radiation heat transfer to the paper sheet at steady state and transient conditions. The model has been used to investigate trends in efficiency and component temperature with changing voltage and paper grade. Emphasis has been on back reflector temperature and dryer efficiency. Also, the transients during start-up of an IR paper dryer have been investigated. The study indicates that the transients of the back reflector is important for the time needed to reach steady state heat flux at the paper sheet. (C) 2000 Elsevier Science Ltd. All rights reserved.
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| 4. |
- Yan, Zhenghua, et al.
(författare)
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A two-equation turbulence model and its application to a buoyant diffusion flame
- 1999
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 42:7, s. 1305-1315
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Tidskriftsartikel (refereegranskat)abstract
- A modified k–ε two-equation turbulence model was developed to improve the consideration of the important buoyancy effect on turbulence and turbulent transport, which is a serious deficiency of the standard buoyancy-modified k–ε model. The present model was tested against both plane and axisymmetric thermal plumes and a buoyant diffusion flame. The model was found to be stable, computationally economic, promising and applicable to complex situations. The predicted plume spreading rates and velocity and temperature profiles agreed well with experimental measurements. When compared with the standard buoyancy-modified k–ε turbulence model, this model gives significantly improved numerical results.
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| 5. |
- Yuan, Jinliang, et al.
(författare)
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Simulation of Fully Developed Laminar Heat and Mass Transfer in Fuel Cell Ducts with Different Cross Sections
- 2001
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 44:21, s. 4047-4058
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Tidskriftsartikel (refereegranskat)abstract
- The fully developed laminar flow and heat transfer for ducts with rectangular and trapezoidal cross section in fuel cells have been numerically simulated with one porous wall with a uniform mass injection or suction, while the other three walls being impermeable. The new concept of thermal boundary conditions of combined constant heat flux and constant temperature on the walls was implemented. Based on the constant thermal-physical property assumption, the wall friction factor and Nusselt number for different mass transfer rates, aspect ratios and base angles are obtained. Comparisons of these numerical results with published data are also presented.
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| 6. |
- Yuan, Jinliang, et al.
(författare)
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Three-dimensional computational analysis of gas and heat transport phenomena in ducts relevant for anode-supported solid oxide fuel cells
- 2003
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 46:5, s. 809-821
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Tidskriftsartikel (refereegranskat)abstract
- Various transport phenomena occurring in an anode duct of medium temperature solid oxide fuel cell (SOFC) have been simulated and analyzed by a fully three-dimensional calculation method. The considered composite duct consists of a thick porous layer, the gas flow duct and solid current interconnector. Unique fuel cell boundary and interfacial conditions, such as the combined thermal boundary conditions on solid walls, mass transfer associated with the electrochemical reaction and gas permeation across the interface, were applied in the analysis. Based on three characteristic ratios proposed in this study, gas flow and heat transfer were investigated and presented in terms of friction factors and Nusselt numbers. It was revealed that, among various parameters, the duct configuration and properties of the porous anode layer have significant effects on both gas flow and heat transfer of anode-supported SOFC ducts. The results from this study can be applied in fuel cell overall modeling methods, such as those considering unit/stack level modeling. (C) 2002 Elsevier Science Ltd. All rights reserved.
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| 7. |
- Bjurström, Henrik, et al.
(författare)
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Thermal Conductivity of a Microporous Particulate Medium
- 1984
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Ingår i: International journal of heat and mass transfer. - : Elsevier BV. - 0017-9310. ; 27:11, s. 2025-2036
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Tidskriftsartikel (refereegranskat)abstract
- A systematic study of the thermal conductivity of beds of moist silica gel is presented. The influence of porosity, water content, total gas pressure and temperature is determined through measurements under transient conditions with the transient hot-strip (THS) method and under static conditions in a bench-scale reactor. The predictions of the effective thermal conductivity of the beds from four different simple models (Russell, geometric mean value, unit-cell model and stochastic model) agree reasonably well with the experimental results. The unit-cell model is extended in order to account for the water sorbed in the micropores and describes satisfactorily the dependency of the effective thermal conductivity on the water content.
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| 8. |
- Breitholtz, Claes, 1967, et al.
(författare)
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Heat balance over the fluid-dynamic boundary layer of a circulating fluidized bed furnace
- 1998
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 41:8-9, s. 1013-1024
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Tidskriftsartikel (refereegranskat)abstract
- A heat balance over the fluid-dynamic boundary layer of a circulating fluidized bed furnace hasbeen formullated in order to calculate the heat transfer to the walls. The convective heat transfer is relatedto a horizontal particle flow from the core to the wall region, and an additive radiative heat transferconstituent is calculated for the particulate medium. An empirical description of the vertical distributionof solids and the transversal temperature profile are needed for the evaluation. The calculated heat transfercoefficients were compared with measurements in three commercial boilers and the deviation was less than40%, which is in the same order as the uncertainty of the experimental data.
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| 9. |
- Löfgren, Hans B., et al.
(författare)
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Initial solidification in liquid metal film flow over a moving bondary
- 2001
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310 .- 1879-2189. ; 44:4, s. 837-842
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Tidskriftsartikel (refereegranskat)abstract
- The initial solidification problem of a two-dimensional liquid metal film flow over a heat extracting moving boundary is studied. Analytical solutions in the limit of large Peclet numbers are found. It is shown that the point of initial solidification depends on the Peclet number, the Biot number and the superheat. The initial growth of the solidified phase is found to have a quadratic dependence of the distance from the point of initial solidification. The results are applicable to continuous strip casters.
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| 10. |
- Rahm, Lars, 1948-
(författare)
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On the thermal adjustment of an almost-enclosed fluid region with through-flow
- 1986
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 29:10, s. 1479-1485
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of a heat-up process in a horizontally enclosed fluid region, with an imposed through-flow and subject to weak thermal forcing, is investigated. Based on a boundary layer approach, a simple one-dimensional model for the interior fluid region is derived. The predicted response of the temperature field agrees well with numerical solutions of the corresponding undegenerate one-dimensional problem. The analytical results are in reasonably close agreement with numerical results presented in a companion paper by Hyun and Hyun (Int. J. Heat Mass Transfer 29, 1487–1493 (1986)).
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| 11. |
- Yang, Z. L., et al.
(författare)
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Numerical investigation of bubble growth and detachment by the lattice-Boltzmann method
- 2001
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310 .- 1879-2189. ; 44:1, s. 195-206
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Tidskriftsartikel (refereegranskat)abstract
- A numerical study has been performed to investigate the characteristics of bubble growth on, and detachment from, an orifice. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension (a) and contact angle (beta), were implemented through the fluid-fluid (G(sigma)) and fluid-solid (G(t)) interaction potentials. The model was found to possess a linear relation between the macroscopic properties (sigma, beta) and microscopic parameters (G(sigma), G(t)). The separate effects of the body force (gravity), gas injection rate, surface tension, and wettability were analyzed for both horizontal and vertical surfaces. It is shown that results of the lattice-Boltzmann modeling exhibit correct parametric dependencies of the departure diameter of bubbles generated on the horizontal surface on the above factors as previously established in experiments. For the case of bubble growth and departure on the vertical surface, the different effects of hydrodynamic parameters, except gas generation rate, were predicted.
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| 12. |
- Yang, Z. L., et al.
(författare)
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Numerical simulation of bubbly two-phase flow in a narrow channel
- 2002
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310 .- 1879-2189. ; 45:3, s. 631-639
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Tidskriftsartikel (refereegranskat)abstract
- An advanced numerical simulation method on fluid dynamics - lattice-Boltzmann (LB) method is employed to simulate the movement of Taylor bubbles in a narrow channel, and to investigate the flow regimes of two-phase flow in narrow channels under adiabatic conditions. The calculated average thickness of the fluid film between the Taylor bubble and the channel wall agree well with the classical analytical correlation developed by Bretherton. The numerical simulation of the behavior of the flow regime transition in a narrow channel shows that the body force has significant effect on the movement of bubbles with different sizes. Smaller body force always leads to the later coalescence of the bubbles, and decreases the flow regime transition time. The calculations show that the surface tension of the fluid has little effect on the flow regime transition behavior within the assumed range of the surface tension. The bubbly flow with different bubble sizes will gradually change into the slug flow regime. However, the bubbly flow regime with the same bubble size may be maintained if no perturbations on the bubble movement occur. The slug flow regime will not change if no phase change occurs at the two-phase interface.
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| 13. |
- Abdi, Amir, et al.
(författare)
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Experimental investigation of thermo-physical properties of n-octadecane and n-eicosane
- 2020
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Ingår i: International Journal of Heat and Mass Transfer. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0017-9310 .- 1879-2189. ; 161
-
Tidskriftsartikel (refereegranskat)abstract
- Reliable knowledge of phase change materials (PCM) thermo-physical properties is essential to model and design latent thermal energy storage (LTES) systems. This study aims to conduct a methodological measurement of thermo-physical properties, including latent enthalpy, isobaric specific heat, thermal conductivity and dynamic viscosity, of two n-alkanes, n-octadecane and n-eicosane. The enthalpy and isobaric specific heat of the materials are measured via differential scanning calorimetry (DSC) technique, using a pDSC evo7 from Setaram Instrumentation with a sample mass of 628.4 mg. The influence of the scanning rates, varying from 0.5 K/min to 0.025 K/min, in dynamic continuous mode within temperature range of 10-65 degrees C is investigated. The thermal conductivity and the dynamic viscosity are measured via Hot Disk TPS-2500S instrument and Brookfield rotational viscometer, respectively, up to 70 degrees C. The thermal analysis results via the pDSC show that the isothermal condition can be approached at a very low scanning rate, however at the cost of a higher noise level. A trade-off is observed for n-octadecane, achieving the lowest deviation of 0.7% in latent heat measurement at 0.05 K/min, as compared to the American Petroleum Table values. For n-eicosane, the lowest deviation of 1.2% is seen at the lowest scanning rate of 0.025 K/min. The thermal conductivity measured values show good agreements with a number of documented literature studies in the solid phase, within deviations of 2%. Larger deviations of 5-16% are found for the measurement in the liquid phase. The viscosity values also show a good agreement with the literature values with maximum deviations of 2.9% and 6.3%, with respect to the values of American Petroleum Tables, for n-octadecane and n-eicosane, respectively. The good agreements achieved in measurements establish the reliable thermo-physical properties contributing to the future simulations and designs.
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| 14. |
- Ahmadpour, Ali, et al.
(författare)
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Thermal-hydraulic performance evaluation of gas-liquid multiphase flows in a vertical sinusoidal wavy channel in the presence/absence of phase change
- 2019
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 138, s. 677-689
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent gas-liquid multiphase flows with and without phase change in a vertical wavy channel are addressed. The multiphase flow field is resolved using the volume of fluid method (VOF), and the flow equations are discretized and numerically solved by the well-known finite volume method. As a multiphase system without mass transfer, air/water flow is considered. It is shown that numerical simulation is well capable of predicting the various multiphase flow regimes ranging from slug to bubbly flows inside wavy channels. Moreover, accurate predictions of overall pressure drop are provided by numerical solutions for various air and water flow rates and the phase shift angle between wavy channel walls. Additionally, condensing flows of refrigerant R134a are simulated inside wavy channels. It is found that for almost all the cases considered in the present study, the convective heat transfer coefficient is higher in wavy channels in respect to straight channels. However, a significant pressure drop penalty is observed especially for high mass fluxes across wavy channels. Therefore, the use of the wavy channels for the enhancement of condensing heat transfer is only advisable for low mass fluxes with the phase shift angle of 180°. © 2019 Elsevier Ltd
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| 15. |
- Albernaz, Daniel L., 1984-, et al.
(författare)
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Simulation of a suspended droplet under evaporation with Marangoni effects
- 2016
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier. - 0017-9310 .- 1879-2189. ; 91, s. 853-860
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the Marangoni effects in a hexane droplet under evaporation and close to its critical point. A lattice Boltzmann model is used to perform 3D numerical simulations. In a first case, the droplet is placed in its own vapor and a temperature gradient is imposed. The droplet locomotion through the domain is observed, where the temperature differences across the surface is proportional to the droplet velocity and the Marangoni effect is confirmed. The droplet is then set under a forced convection condition. The results show that the Marangoni stresses play a major role in maintaining the internal circulation when the superheated vapor temperature is increased. Surprisingly, surface tension variations along the interface due to temperature change may affect heat transfer and internal circulation even for low Weber number. Other results and considerations regarding the droplet surface are also discussed.
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| 16. |
- Ali, Rashid, et al.
(författare)
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Dryout Characteristics During Flow Boiling of R134a in Vertical Circular Minichannels
- 2011
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 54:11-12, s. 2434-2445
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.
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| 17. |
- Andersson, Bengt-Åke, et al.
(författare)
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Experimental methods of estimating heat transfer in circulating fluidized bed boilers
- 1992
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 35:12, s. 3353-3362
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Tidskriftsartikel (refereegranskat)abstract
- Four different experimental methods have been used for the estimation of the bed-to-membranewall heat transfer in a 12 MW,, circulating fluidized bed boiler. The methods are compared for a case ofnormal operating conditions and the measured heat transfer coefficients are presented. In the central partof the combustion chamber where most of the cooling surface is located, the cross-sectional averagesuspension density normally varies in the range of 10-20 kg m -’ and the heat transfer coefficient is around130 W m ’ K-’ with a scatter of + 15% due to the different methods. The methods are critically analyzedand the heat transfer data are compared with relevant literature data.
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| 18. |
- Andersson, Martin, et al.
(författare)
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Comparison of Humidified Hydrogen and Partly Pre-Reformed Natural Gas as Fuel for Solid Oxide Fuel Cells applying Computational Fluid Dynamics
- 2014
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 77, s. 1008-1022
-
Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional computational fluid dynamics (CFD) approach based on the finite element method (FEM) is used to investigate a solid oxide fuel cell (SOFC). Governing equations for heat, gas-phase species, electron, ion and momentum transport are implemented and coupled to kinetics describing electrochemical as well as internal reforming reactions. The model cell design is based on a cell from Ningbo Institute of Material Technology and Engineering in China and the electrochemical area-to-volume ratios are based on experimental work performed at Kyushu University in Japan. A parameter study is performed focusing on the inlet fuel composition, where humidified hydrogen, 30 % pre-reformed natural gas (as defined by IEA) and 50 % pre-reformed natural gas (as defined by Kyushu University) are compared. It is found that when 30 % pre-reformed natural gas is supplied as fuel the air mass flow rate is halved, compared to the case with humidified hydrogen, keeping the inlet and outlet temperatures constant. The current density is decreased but the fuel utilization is kept at 80 %. It is found that the cathode support layer has a significant oxygen gas-phase resistance in the direction normal to the cathode/electrolyte interface (at positions under the interconnect ribs), as well as an electron resistance inside the cathode (at positions under the air channel) in the same direction. The methane steam reforming reaction is shown, both according to the experiments and to the models, to proceed along the main flow direction throughout the cell.
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| 19. |
- Andersson, Martin, et al.
(författare)
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SOFC Modeling Considering Electrochemical Reactions at the Active Three Phase Boundaries
- 2012
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 55:4-5, s. 773-788
-
Tidskriftsartikel (refereegranskat)abstract
- Abstract in UndeterminedIt is expected that fuel cells will play a significant role in a future sustainable energy system, due to their high energy efficiency and the possibility to use renewable fuels. A fully coupled CFD model (COMSOL Multiphysics) is developed to describe an intermediate temperature SOFC single cell, including governing equations for heat, mass, momentum and charge transport as well as kinetics considering the internal reforming and the electrochemical reactions. The influences of the ion and electron transport resistance within the electrodes, as well as the impact of the operating temperature and the cooling effect by the surplus of air flow, are investigated. As revealed for the standard case in this study, 90% of the electrochemical reactions occur within 2.4 mu m in the cathode and 6.2 mu m in the anode away from the electrode/electrolyte interface. In spite of the thin electrochemical active zone, the difference to earlier models with the reactions defined at the electrode-electrolyte interfaces is significant. It is also found that 60% of the polarizations occur in the anode, 10% in the electrolyte and 30% in the cathode. It is predicted that the cell current density increases if the ionic transfer tortuosity in the electrodes is decreased, the air flow rate is decreased or the cell operating temperature is increased.
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| 20. |
- Anghel, Ionut Gheorghe, et al.
(författare)
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Post-dryout heat transfer to high-pressure water flowing upward in vertical channels with various flow obstacles
- 2012
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 55:25-26, s. 8020-8031
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Tidskriftsartikel (refereegranskat)abstract
- Post-dryout heat transfer to high pressure water was investigated experimentally in vertical tubes and annuli containing various flow obstacles. The operational conditions during the experiments were as follows: mass flux from 500 to 1750 kg/m(2) s. pressure from 5 to 9 MPa, inlet subcooling from 10 to 40K and heat flux up to 1.5 MW/m(2). Five different test sections were used in experiments: three annular test sections with inner diameter 12.7 mm and outer diameter 24.3 mm, containing cylindrical and grid flow obstacles in the upper part, and two tubular test sections with inner diameter 24.3 mm with and without pin flow obstacles. The heated length in all test sections was 3650 mm. The wall temperature was measured with 88 thermocouples located along the inner rod and the outer tube surfaces. Due to the presence of flow obstacles, only developing post-dryout heat transfer was observed. Selected post-dryout heat transfer correlations were compared to the experimental data. It has been concluded that all tested correlations predict significantly higher wall temperatures than those obtained in the present experiment. A simple correction function to the Saha model has been suggested which significantly improves the agreement between the correlation and the present data.
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| 21. |
- Aryal, Pradip, et al.
(författare)
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Comparative study of the main electromagnetic models applied to melt pool prediction with gas metal arc : Effect on flow, ripples from drop impact, and geometry
- 2022
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier. - 0017-9310 .- 1879-2189. ; 194
-
Tidskriftsartikel (refereegranskat)abstract
- The present work concerns the electromagnetic force models in computational fluid dynamics simulations of melt pools produced with electric arcs. These are commonly applied to gas metal arcs with metal transfer, in welding and additive manufacturing. Metal drop impact on the melt pool is thus included in this study. The electromagnetic force models applied in literature use either numerical solutions of Poisson equations or one of the two analytical models developed by Kou and Sun, or Tsao and Wu. These models rely on assumptions for which the effect on the melt pool predictions remains to be understood. The present work thoroughly investigates those assumptions and their effects. It has been supported by dedicated experimental tests that did provide estimates of unknown model parameters and validation data. The obtained results show that the assumptions that fundamentally distinguish these three models change the electromagnetic force, including the relation between its components. These changes, which can also be spatially non-uniform, are large. As a result, these models lead to significantly different recirculation flow pattern, thermal convection, melt pool morphology, bead dimensions, and free surface response to the metal transfer. We conclude by proposing conditions in which each of these models is suited or questionable.
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| 22. |
- Aryal, Pradip, et al.
(författare)
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Melt pool electromagnetic force model extended to account for free surface deformation : Application to gas metal arc
- 2024
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310 .- 1879-2189. ; 221, s. 124987-124987
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Tidskriftsartikel (refereegranskat)abstract
- Computational fluid dynamics models with free surface tracking intended to simulate the melt pool produced by an electric arc usually model the electromagnetic force ignoring the deformation of the free surface. However, with an arc heat source, the electromagnetic force is known to be among the leading-order forces, especially at high currents. In addition, the free surface can undergo significant deformations, especially in the presence of metal transfer. In the present study, a generalization of the electromagnetic force model that accounts for the deformation of the free surface is therefore proposed. Test cases with a pulsed gas-metal arc that transfers one metal drop per pulse were investigated experimentally at three different travel speeds to provide validation data. The cases were simulated with both the proposed and the earlier model to assess the influence of the new developments. The results showed that, in the regions where both models determine the force, the discrepancy between the models’ results can reach up to an order of magnitude. Especially, the earlier model overestimates the electromagnetic force deep into the melt pool. On the other hand, it neglects it in the liquid metal that is located at an elevation above the original upper surface of the workpiece, while the proposed model showed that in this area the intensity of the electromagnetic force is the largest. These significant discrepancies result in non-negligible differences in the predicted melt pool thermal flow and geometry. Especially, the propose dextended model provides an improved prediction of the fingertip-shaped fusion boundary.
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| 23. |
- Asadi, Masoud, et al.
(författare)
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A review of heat transfer and pressure drop characteristics of single and two-phase microchannels
- 2014
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 79, s. 34-53
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Forskningsöversikt (refereegranskat)abstract
- An impressive amount of investigations has been devoted to enhancing thermal performance of microchannels. The small size of microchannels and their ability to dissipate heat makes them as one of the best choices for the electronic cooling systems. In this paper, a comprehensive review of available studies regarding single and two-phase microchannels is presented and analyzed. 219 articles are reviewed to identify the heat transfer mechanisms and pressure drops in microchannels. This review looks into the different methodologies and correlations used to predict the heat transfer and pressure drop characteristics of microchannels along the channel geometries and flow regimes. The review shows that earlier studies (from 1982 to 2002) were largely conducted using experimental approaches, and discrepancies between analytical and experimental results were large, while more recent studies (from 2003 to 2013) used numerical simulations, correlations for predicting pressure drop and heat transfer coefficients were considerably more accurate. (C) 2014 Elsevier Ltd. All rights reserved.
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| 24. |
- Askfelt, Henrik, et al.
(författare)
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Experimental and numerical analysis of adhesion failure in moist packaging material during excessive heating
- 2017
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 108, s. 2566-2580
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Tidskriftsartikel (refereegranskat)abstract
- In the presented paper experimental and numerical analyses are made of the failure due to blister formation in moist packaging material when exposed to excessive heating. The basic concept of the experimental analysis is to expose a package material to a hot air jet and measure the time until the material experience internal failure. For the numerical analysis, a triphasic macroscale hybrid mixture theory approach is adopted. The in-plane permeability and the static and dynamic sorption properties of the paperboard are believed to have significant influences on the blister formation. For this reason in-plane permeability experiments and sorption experiments are performed to retrieve proper constitutive coefficients for the numerical simulations. Results from numerical simulations are compared with the experimental tests in order to better understand the physics behind the forming of a blister. The results indicate that the blister formation primarily depends on the in-plane permeability and the sorption properties of the paperboard as well as the properties of the adhesion between the paperboard and the Al-foil.
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| 25. |
- Attarzadeh, Reza, et al.
(författare)
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Design analysis of the "Schwartz D" based heat exchanger : A numerical study
- 2021
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 177
-
Tidskriftsartikel (refereegranskat)abstract
- Triply Periodic Minimal Surfaces (TPMS) have promising thermophysical properties, which makes them a suitable candidate in the production of low-temperature waste heat recovery systems. A TPMS thermal performance is connected to the complex flow patterns inside the pores and their interactions with the walls. Unfortunately, the experimental study's design analysis and optimization of TPMS heat exchangers are complicated due to the flow pattern complexity and visual limitations inside the TPMS. In this study, three-dimensional steady-state, conjugate heat transfer (CHT) simulations for laminar incompressible flow were carried out to quantify the performance of a TPMS based heat exchanger. TPMS Lattices based on Schwartz D architecture was modeled to elucidate the design parameters and establishing relationships between gas velocity, heat transfer, and thermal performance of TPMS at different wall thicknesses. In this study, four types of lattices from the same architectures with varying wall thickness were examined for a range of the gas velocity, with one design found to be the optimized lattice providing the highest thermal performance. The results and methodology presented here can facilitate improvements in TPMSheat exchangers' fabrication for recycling the waste heat in low pitch thermal systems.
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| 26. |
- Bahador, Mehdi, et al.
(författare)
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Investigation on the effects of fly ash particles on the thermal radiation in biomass fired boilers
- 2008
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 51:9-10, s. 2411-2417
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Tidskriftsartikel (refereegranskat)abstract
- Ash is produced in combustion of biomass. Sonic part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also call affect the thermal radiation. The effects of fly ash particles oil the thermal radiation are considered ill this investigation. By analyzing sampled data in all electrostatic filter, a realistic particle size distribution is found. Although the optical data oil biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fc coal fly ash can be applied for file biomass fly ash. The Mie theory is used to predict scattering and absorption coefficients and phase function. The mean Planck scattering and absorption coefficients and phase function are predicted by averaging over the particle size distribution and Planck function, respectively. The effects of fly ash particles oil thermal radiation are evaluated by a three-dimensional test case. It is assumed that the medium is a mixture of non-grey gases and different level of particle loading. Predicted results from the test case showed that the fly ash can be influential oil the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important oil the radiative heat Source and radiative heat flux to the wall whereas their absorption effect is important and call increase the radiative heat Source and wall heat fluxes.
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| 27. |
- Bal, Nicolas, et al.
(författare)
-
Experimental study of radiative heat transfer in a translucent fuel sample exposed to different spectral sources
- 2013
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 61:1, s. 742-748
-
Tidskriftsartikel (refereegranskat)abstract
- Radiative heat transfer to a solid is a key mechanism in fire dynamics, and in-depth absorption is especially of importance for translucent fuels. The sample-heater interaction for radiative heat transfer is experimentally investigated in this study with two different heaters (electric resistance and tungsten lamp) using clear PolyMethylMethAcrylate (PMMA) samples from two different formulations (Plexiglass and Lucite). First, the significant effects of the heater type and operating temperature on the radiative heat transfer are revealed with broadband measurements of transmittance on samples of different thicknesses. Then, the attenuation coefficient in Beer-Lambert's law has been calculated from detailed spectral measurements over the full wavelength range encountered in real fires. The measurements present large spectral heterogeneity. These experimental results and calculation of in-depth absorption are used to explain the reason behind the apparent variation of the fuel absorbance with the sample thickness observed in past studies. The measurement of the spectral intensity emitted by the heaters verifies that the common assumption of blackbody behavior is correct for the electric resistance, whereas the tungsten lamp does not even behave as a greybody. This investigation proofs the necessity of a multi-band radiation model to calculate accurately the fire radiative heat transfer which affects directly the in-depth temperature profiles and hence the pyrolysis process for translucent fuel.
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| 28. |
- Balz, Reto, 1985, et al.
(författare)
-
Experimental and numerical investigation of cavitation in marine Diesel injectors
- 2021
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 169
-
Tidskriftsartikel (refereegranskat)abstract
- To further increase the efficiency and decrease emissions of large two-stroke marine Diesel engines, the understanding of the fuel injection, spray breakup and the resulting combustion plays a vital role. Investigations have shown that the strongly asymmetrically and eccentrically arranged nozzle bores of the fuel injectors can lead to undesirable spray deflections that provoke increased component temperatures, emissions and fuel consumption. In order to investigate the origin of these spray deviations, transparent nozzles have been used to qualitatively visualize the in-nozzle flow under realistic geometrical and fuel pressure conditions. Three different, 0.75 mm diameter, single-hole nozzle geometries that represent typical geometrical characteristics have been used in cavitating nozzle flow experiments. The optical measurement technique Shadowgraphy has been applied to visualize the in-nozzle flow over the complete fuel injection process. The experiments have been performed with Diesel fuel at a rail pressure of 50 MPa with ambient back-pressure and temperature. Impingement measurements have been executed to compare the nozzle performance and validate CFD simulations using URANS with cavitation modeling in order to provide qualitative and quantitative support to the experimental results. The volume of fluid (VOF) method has been applied to simulate the multiphase flow with High Resolution Interface Capturing (HRIC). The cavitation model is based on a flash-boiling method with rapid heat transfer between the liquid and vapor phases. A Homogeneous Relaxation Model (HRM) has been utilized to describe the rate at which the instantaneous quality, the mass fraction of vapor in a two-phase mixture, will approach its equilibrium value. The numerical modeling of the cavitation inside the nozzle bore and the evaluated momentum flux have been compared to the experimental findings and show good agreement for the qualitative comparison of the cavitation patterns and differences of less than 6% for the quantitative momentum flux comparison.
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| 29. |
- Bandaru, S V Ravikumar, et al.
(författare)
-
Upward-facing multi-nozzle spray cooling experiments for external cooling of reactor pressure vessels
- 2020
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 163
-
Tidskriftsartikel (refereegranskat)abstract
- Cooling by water spray is a well-known technology that can reach significantly higher Critical Heat Flux (CHF) compared to other cooling methods. For the light water reactor safety, the in-vessel retention (IVR) by external reactor vessel cooling (ERVC) is a comprehensive severe accident management strategy to arrest and confine the corium in the lower head of the reactor pressure vessel. Heat fluxes up to 1.5 MW/m2 have already been assumed attainable in low-power nuclear reactors while cooling required in high-power reactors is expected to reach 2.5 MW/m2. Instead of reactor lower head flooding and relying on cooling due to natural convection, a viable and more efficient alternative is to spray the external surface of the vessel. Given all the advantages of spray cooling reported in the literature, a lab-scale experimental facility was built to validate the efficiency of multi-nozzle spray cooling of a downward-facing heated surface inclined at different angles up to 90o. The facility employed a 2×3 matrix of spray nozzles to cool the FeCrAl alloy foil with an effectively heated surface area of 96 cm2 using water as the coolant. Heat loads and surface inclinations were varied parameters in the test matrix. The results show that no significant variations in spray cooling performance concerning the inclination of the heated surface. A surface heat flux of 2.5 MW/m2 was achieved at every inclination of the downward-facing surface. The results also indicate that more uniform liquid film distribution could be obtained for some inclinations, which in turn leads to maintaining low surface temperature. The obtained surface heat flux margin by spray cooling indicates that it is feasible to adopt IVR-ERVC strategy for a large power reactor.
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| 30. |
- Belyakov, Nikolay, et al.
(författare)
-
Analytical solution of non-stationary heat conduction problem for two sliding layers with time-dependent friction conditions
- 2016
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier. - 0017-9310 .- 1879-2189. ; 98, s. 624-630
-
Tidskriftsartikel (refereegranskat)abstract
- In this article we conduct an overview of various types of thermal contact conditions at the sliding interface. We formulate a problem of non-stationary heat conduction in two sliding layers with generalized thermal contact conditions allowing for dependence of the heat-generation coefficient and contact heat transfer coefficient on time. We then derive an analytical solution of the problem by constructing a special coordinate integral transform. In contrast to the commonly used transforms, e.g. Laplace or Fourier transforms, the one proposed is applicable to a product of two functions dependent on time. The solution is validated by a series of test problems with parameters corresponding to those of real tribosystems. Analysis shows an essential influence of both time-dependent heat-generation coefficient and contact heat transfer coefficient on the partition of the friction heat between the layers. The solution can be used for simulating temperature fields in sliding components with account of this influence.
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| 31. |
- Bergagio, Mattia, et al.
(författare)
-
An iterative finite-element algorithm for solving two-dimensional nonlinear inverse heat conduction problems
- 2018
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier. - 0017-9310 .- 1879-2189. ; 126, s. 281-292
-
Tidskriftsartikel (refereegranskat)abstract
- It is often useful to determine temperature and heat flux in multidimensional solid domains of arbitrary shape with inaccessible boundaries. In this study, an effective algorithm for solving boundary inverse heat conduction problems (IHCPs) is implemented: transient temperatures on inaccessible boundaries are estimated from redundant simulated measurements on accessible boundaries. A nonlinear heat equation is considered, where some of the material properties are dependent on temperature. The IHCP is reformulated as an optimization problem. The resulting functional is iteratively minimized using a conjugate gradient method together with an adjoint (dual) problem approach. The associated partial differential equations are solved using the finite-element package FEniCS. Tikhonov regularization is introduced to mitigate the ill-posedness of the IHCP. The accuracy of the implemented algorithm is assessed by comparing the solutions to the IHCP with the correct temperature values, on the inaccessible boundaries. The robustness of our method is tested by adding Gaussian noise to the initial conditions and redundant boundary data in the inverse problem formulation. A mesh independence study is performed.
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| 32. |
- Bergagio, Mattia, et al.
(författare)
-
Analysis of temperature fluctuations caused by mixing of non-isothermal water streams at elevated pressure
- 2017
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier. - 0017-9310 .- 1879-2189. ; 104, s. 979-992
-
Tidskriftsartikel (refereegranskat)abstract
- Temperatures were measured at the inner surface of an annulus between two coaxial tubes, where three water streams mixed. These temperatures were sampled at either 100 Hz or 1000 Hz. The acquisition time was set to 120 s. Two water streams at 549 K, with a Reynolds number between 3.56 × 105 and 7.11 × 105, descended in the annular gap and mixed with a water stream at 333 K or 423 K, with a Reynolds number ranging from 1.27 × 104 to 3.23 × 104. Water pressure was kept at 7.2 MPa. Inner-surface temperatures were collected at eight azimuthal and five axial positions, for each combination of boundary conditions. To better analyze these temperatures and mixing in the vicinity of the wall, scalars estimating the mixing intensity at each measurement position were computed from detrended temperature time series. Fourier and Hilbert–Huang marginal spectra were calculated for the time series giving rise to the highest values of a mixing estimator of choice. The relationship between temperature and velocity was explored by examining the results of an LES simulation using the same boundary conditions as in one of the experimental cases.
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| 33. |
- Bian, Boshen, et al.
(författare)
-
Direct numerical simulation of internally heated natural convection in a hemispherical geometry
- 2024
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 220
-
Tidskriftsartikel (refereegranskat)abstract
- Internally heated (IH) natural convection can be found in nature, industrial processes, or during a severe accident in a light water reactor. In this accident scenario, the nuclear reactor core and some internal structures can melt down and relocate to the lower head of the reactor pressure vessel (RPV) and interact with the remaining coolant. Subsequent re-heating and re-melting under decay and oxidation heat creates a transition from a debris bed to a molten pool. The molten pool, which can involve more than hundred tons of dangerously superheated oxidic and metallic liquids, imposes thermo-mechanical loads on the vessel wall that can lead to a thermal and/or structural failure of the vessel and subsequent release of radioactive materials to the reactor pit, and can possibly make its way to the environment. This study uses Direct Numerical Simulation (DNS) to investigate homogeneous IH molten pool convection in a hemispherical domain using Nek5000, an open-source spectral element code. With a Rayleigh number of 1.6 × 1011, the highest reached through DNS in this confined hemispherical geometry, and a Prandtl number of 0.5, which corresponds to a prototypic corium, the study provides detailed information on the thermo-fluid behavior. The results show a turbulent flow with three distinct regions, consistent with the general flow observations from the BALI experiments. The study also presents detailed information on turbulence, such as turbulent kinetic energy (TKE), turbulent heat flux (THF), and temperature variance. Additionally, the study provides 3D heat flux distributions along the boundaries. The heat fluxes along the top boundary fluctuate due to the turbulent eddies in the vicinity, while along the curved boundary the heat fluxes increase nonlinearly from the bottom to the top.
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| 34. |
- Bobbili, Prabhakara Rao, et al.
(författare)
-
Thermal analysis of plate condensers in presence of flow maldistribution
- 2006
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 49:25-26, s. 4966-4977
-
Tidskriftsartikel (refereegranskat)abstract
- Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.
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| 35. |
- Bovo, Mirko, 1979, et al.
(författare)
-
Direct comparison of les and experiment of a single-pulse impinging jet
- 2015
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 88, s. 102-110
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental results are commonly used to validate numerical simulations. Often, it is challenging to accurately reproduce the experimental setups in the numerical environment, thereby leaving uncertainty in the validation process. An experiment on impinging jets was designed with boundary conditions well suited for implementation in the numerical environment. In this work, LES was implemented to specifically match the experiment of a single-pulse jet impinging on a surface oriented normally to the jet axis. The experiment was designed primarily to study the thermal effects on the impingement zone with high space-time resolution. The temperature evolution of the impingement target was measured via IR camera. The space-time resolved jet velocity-field was measured with PIV and was used as boundary condition for the simulation. The focus of the LES was to replicate the experiment as faithfully as possible in the virtual environment. LES was run multiple times to allow statistical evaluation of the results, as done in the experimental procedure. High levels of agreement were found between the LES and the experimental results, both from a qualitative and a quantitative point of view. This work could therefore be considered as a successful validation of the LES in the study of this type of flows.
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| 36. |
- Brethouwer, Gert
(författare)
-
Strong dissimilarity between heat and momentum transfer in rotating Couette flows
- 2023
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 205
-
Tidskriftsartikel (refereegranskat)abstract
- Rotation influences flows and transport processes in many engineering applications, however, even in canonical flow cases, the effects of rotation are not fully understood. Here, we present the results of di-rect numerical simulations of heat transfer plane Couette and Taylor-Couette flows subject to rotation about the spanwise and axial axis, respectively. Temperature is a passive scalar since buoyancy is ne-glected. The Reynolds number Re and the rotation rate Rn are systematically varied to thoroughly inves-tigate the influence of rotation on heat and momentum transfer and the Reynolds analogy. We find that with increasing anti-cyclonic rotation, the Nusselt numbers for the momentum transfer Num and heat transfer Nuh both increase at first before declining and approaching unity at rapid rotation rates when the flow becomes fully laminar. The Reynolds analogy factor RA = N uh/N um is near unity for non-rotating Couette flows, but it grows significantly with increasing rotation rate. RA reaches a maximum of approx-imately 2 at low Re up to 6 and 8 near Rn = 1 at higher Re in plane Couette and Taylor-Couette flow, respectively. The simulations thus show that the Reynolds analogy between heat and momentum trans-fer breaks down and that the heat transfer can become much faster than moment transfer when plane Couette and Taylor-Couette flows are subject to anti-cyclonic rotation. This happens at low Re as well as higher Re when the flows are fully turbulent. The turbulent Prandtl becomes much smaller than unity and the mean velocity and temperature profiles differ when the Reynolds analogy breaks down. We also present empirical models for Num and RA , which agree reasonably well to very well with the data within a limited Rn range.
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| 37. |
- Cao, Zhen, et al.
(författare)
-
Heat transfer prediction and critical heat flux mechanism for pool boiling of NOVEC-649 on microporous copper surfaces
- 2019
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 141, s. 818-834
-
Tidskriftsartikel (refereegranskat)abstract
- Pool boiling performance of NOVEC-649 was experimentally studied on microporous surfaces prepared by an electrochemical deposition method. Microporous structures contribute to large surface roughness values and provide large quantities of cavities ranging from several hundreds of nanometers to several microns for bubble nucleation. The results show that a maximum enhancement of 600% in heat transfer coefficient and a maximum enhancement of 55% in critical heat flux are achieved on the deposited surfaces, compared with a smooth copper surface. Experimental heat transfer coefficients were compared with literature correlations, considering the effects of roughness and surface-liquid combination. Then a fitted Rohsenow correlation was discussed and developed to predict the present results. Experimental critical heat fluxes were compared with classical models. It was found that the critical heat flux on the smooth surface could be predicted by the lift-off model and the Kandlikar model, but these models cannot predict the critical heat fluxes on the deposited surfaces well. Following, the Kandlikar model was modified by further considering a wicking force and a roughness-factor-dependent surface tension force. The present modified CHF model was validated by comparing with present experimental data and the literature, with a deviation around ±30%.
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| 38. |
- Cao, Zhen, et al.
(författare)
-
Model-based assessment of boiling heat transfer enhanced by coatings
- 2022
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 196
-
Tidskriftsartikel (refereegranskat)abstract
- The present study aims to investigate boiling heat transfer enhancement on coated surfaces, based on mechanistic models. An electrochemical deposition method was used to fabricate coatings on copper surfaces which enhance critical heat flux and heat transfer coefficient of deionized water by 35.5% and 40.1%, respectively, compared with a smooth surface. Bubble dynamics indicates that regardless of surfaces, scaling laws of Db*∝t*0.5 and Db*∝t*0.2 are followed in the inertia-controlled growth stage and the heat-diffusion-controlled growth stage, respectively, concerning normalized bubble diameter (Db*) and normalized bubble time (t*). The coating decreases the bubble departure diameter to one-third to half of that on the smooth surface and increases the departure frequency to three times that on the smooth surface. In addition, the coated surface provides more active nucleation sites which are 1, 2 order magnitude higher than the smooth surface. With these insights, a mechanistic heat transfer model was established by quantifying natural convection, transient heat conduction, and microlayer evaporation, which matches well with the measured pool boiling curve. In the end, critical heat flux was explored experimentally and theoretically. Inspired by the coalesced bubble behavior at high heat flux and the Kandlikar force model, a new force-balance model was proposed by incorporating a surface-dependent surface tension force and adding a new capillary wicking force. The present model presents a better prediction of critical heat flux, verified by the current measurement and the literature.
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| 39. |
- Cao, Zhen, et al.
(författare)
-
Pool boiling heat transfer of FC-72 on pin-fin silicon surfaces with nanoparticle deposition
- 2018
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 126, s. 1019-1033
-
Tidskriftsartikel (refereegranskat)abstract
- In the present study, two types of micro-pin–fin configurations were fabricated on silicon surfaces by a dry etching method, i.e., staggered pin fins (#1) and aligned pin fins with empty areas (#2). The micro-pin–fin surfaces were then further modified by depositing FeMn oxide nanoparticles (∼35 nm) electrostatically for 8 h and 16 h, respectively, namely #1-8h, #1-16h, #2-8h and #2-16h. Subcooled pool boiling heat transfer was experimentally studied on these surfaces at atmospheric pressure, using FC-72 as the working fluid. The results showed that in comparison to the smooth surface, pool boiling heat transfer was significantly enhanced by the micro-pin-fin surfaces and the maximum superheat was considerably decreased. Additionally, critical heat fluxes were also greatly improved, e.g., the critical heat flux on #1 was almost twice of that on the smooth surface. Generally, the nanoparticle deposition could further enhance pool boiling heat transfer, including the heat transfer coefficient and critical heat flux (CHF). High speed visualizations were taken to explore the mechanisms behind the heat transfer performance. The bubble behavior on the micro-pin–fin surfaces with and without nanoparticles was compared at low, moderate and high heat fluxes, respectively. The wickability of FC-72 on the test surfaces was measured, based on which, a modified CHF model was proposed to predict the experimental CHFs. Accordingly, a possible mechanism of CHF enhancement was described.
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| 40. |
- Cao, Zhen, et al.
(författare)
-
Pool boiling of HFE-7200 on nanoparticle-coating surfaces: Experiments and heat transfer analysis
- 2018
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 133, s. 548-560
-
Tidskriftsartikel (refereegranskat)abstract
- In the present study, an electrophoretic deposition method was employed to modify copper surfaces withCu-Zn (100 nm) nanoparticles. Pool boiling heat transfer of HFE-7200 on the modified surfaces was experimentally studied. The results showed that the heat transfer coefficient on the modified surfaces was significantly enhanced compared with that on a smooth surface, e.g., a maximum 100% enhancement,while the maximum superheat on the modified surfaces was around 20 K lower than that on the smooth surface. However, the critical heat flux (CHF) was not improved considerably, and supplementary tests indicated that the wickability of HFE-7200 was almost the same on the modified surfaces and the smooth surface. The departure diameters of bubbles were recorded by a high speed camera, which were compared with several models in literature. Active nucleation site sizes were evaluated by the Hsu nucleation theory and active nucleation site densities were estimated by appropriate correlations.In addition, a heat transfer model, considering natural convection, re-formation of thermal boundary layer and microlayer evaporation, was formulated to predict the heat transfer on the modified surfaces and the smooth surface. A relatively good prediction was achieved.
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| 41. |
- Chaze, W., et al.
(författare)
-
Heat flux reconstruction by inversion of experimental infrared temperature measurements – Application to the impact of a droplet in the film boiling regime
- 2019
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 128, s. 469-478
-
Tidskriftsartikel (refereegranskat)abstract
- An Inverse Heat Conduction Problem (IHCP) is considered in order to estimate the transient heat flux extracted from a hot solid surface by an impinging droplet. The resolution of the IHCP is made with the so-called quadrupole method, which provides an analytical expression of the temperature and the heat flux at the front surface of the solid wall, where the drop impact takes place. In the experiments, the thermal response of the front surface is recorded using IR thermography. For that, sapphire is chosen as the material of the solid wall, and the front surface is coated with a thin TiAlN ceramic layer (thickness of 300 nm). The latter is highly emissive and opaque in the IR while sapphire is transparent at the same wavelengths. This feature allows the surface impacted by the droplet to be viewed from the bottom by the IR camera. This approach has been implemented to gain some insights into the heat transfer from the solid surface as well as the formation and growth of the vapor film, which appears under the droplet in the regime of film boiling, when the solid temperature is much higher than the boiling temperature of the liquid. Due to the small thickness of the vapor film, heat conduction is predominant in the vapor layer. Hence, the thickness of the vapor film can be deduced from the value of the reconstructed local heat flux, assuming a linear profile of temperature between the liquid interface of the droplet at the saturation temperature and the solid surface measured by IR thermometry.
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| 42. |
- Chen, Qiuyang, et al.
(författare)
-
Effect of bi-layer interconnector design on mass transfer performance in porous anode of solid oxide fuel cells
- 2011
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 54:9-10, s. 1994-2003
-
Tidskriftsartikel (refereegranskat)abstract
- We propose a novel interconnector design, termed bi-layer interconnector, for solid oxide fuel cells (SOFCs). It can disturb the fuel gas and air on the planes normal to the SOFC three-phase-boundary (TPB) layer. In this paper, a two-dimensional half-cell model is developed to study the concentration overpotentials in the fuel side of the SOFC stack with conventional and novel bi-layer interconnectors. The numerical results show that the novel bi-layer interconnector can increase the velocity of the fuel gas in the porous anode. The results of mole fraction distribution illustrate that the novel bi-layer interconnector can effectively disturb the fuel flow. The average H-2 mole fraction in the porous anode of SOFC with bi-layer interconnector is about 4.7% higher than that of conventional SOFC. The average H-2 mole fraction at TPB interface is about 9.2% higher. The concentration overpotential of the novel SOFC design is lower than that of the conventional SOFC design by 5%. It can enhance the mass transfer in porous electrode and improve the performance of SOFC. (C) 2011 Elsevier Ltd. All rights reserved.
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| 43. |
- Chourushi, T., et al.
(författare)
-
Thermal and flow characteristics of nonequilibrium monatomic, diatomic, and polyatomic gases in cylindrical Couette flow based on second-order non-Navierâ-Fourier constitutive model
- 2022
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier Ltd. - 0017-9310 .- 1879-2189. ; 187
-
Tidskriftsartikel (refereegranskat)abstract
- The thermal and flow characteristics of nonequilibrium monatomic, diatomic, and polyatomic gases in cylindrical Couette flow are investigated using first- and second-order Boltzmann-Curtiss-based constitutive models. The mixed modal discontinuous Galerkin scheme is used for solving the conservation laws in conjunction with the Maxwell velocity-slip and Smoluchowski temperature-jump boundary conditions. Also derived are new analytic solutions for compressible cylindrical Couette gas flow including the temperature profile, and they are used to verify the numerical scheme. Further, the second-order non-Navier-Fourier constitutive relations are derived for the cylindrical coordinates. Various abnormal behaviour is found in the second-order constitutive model, such as non-zero normal stress and excess normal stress, non-zero tangential heat flux, and flattened pressure and density profiles. The physical mechanisms behind this abnormal behaviour are found to be similar to the Knudsen layer in planar Couette gas flow, and the curvature of the cylindrical geometry does not affect the fundamental second-order physics. Moreover, two new abnormal mechanisms are found in diatomic and polyatomic gases: (i) the subtle interplay of excess normal stress (and bulk viscosity) with the nonlinear coupled constitutive relation, and (ii) the combined role of the bulk viscosity ratio and the specific heat ratio.
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| 44. |
- Cintolesi, Carlo, et al.
(författare)
-
Numerical simulation of conjugate heat transfer and surface radiative heat transfer using the P 1 thermal radiation model: Parametric study in benchmark cases.
- 2017
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 107, s. 956-971
-
Tidskriftsartikel (refereegranskat)abstract
- A parametric investigation of radiative heat transfer is carried out, including the effects of conjugate heattransfer between fluid and solid media. The thermal radiation is simulated using the P1 -model. Thenumerical model and the thermal coupling strategy, suitable for a transient solver, is described. Suchnumerical coupling requires that the radiative equation is solved several times at each iteration; hence,the computational cost of the radiative model is a crucial issue. The P1 -model is adopted because of itsparticularly fast computation. First, a collection of benchmark cases is presented and used to carefullyvalidate the radiation model against literature results and to analyse the model prediction limits.Despite the simplicity of the model, it satisfactorily reproduces the thermal radiation effects. Some lackof accuracy is identified in particular cases. Second, a number of benchmark cases are described andadopted to investigate fluid–solid thermal interaction in the presence of radiation. Three cases aredesigned, to couple radiation with: pure conduction, conduction and forced convection, conductionand natural convection. In all the cases, the surface radiative heat transfer strongly influences the systemthermodynamics, leading to a significant increase of the fluid–solid interface temperature. The main non-dimensional numbers, related to the mutual influence of the different heat transfer modes, are intro-duced and employed in the analyses. A new conduction-radiation parameter is derived in order to studythe conductive boundary layer in absence of convective heat transfer.
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| 45. |
- Deng, Yucheng, et al.
(författare)
-
An Experimental study on steam explosion of multiple droplets in different chemical solutions
- 2024
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 226
-
Tidskriftsartikel (refereegranskat)abstract
- Motivated by the interest in steam explosion in chemical solutions and seawater, a series of tests were carried out in the MISTEE facility at KTH to investigate steam explosion characteristics as multiple molten droplets of tin were falling through a coolant pool containing deionized water, boric acid solution, neutral solution of boric acid and sodium phosphate, and seawater, separately. The experimental results revealed distinct and complex characteristics of steam explosion of multiple droplets, which were not observed in previous single-droplet steam explosion experiments. The tin melt samples of 5 g and 20 g were employed to formulate different numbers of multiple droplets. In the test with 5 g melt, steam explosion was more energetic at a deeper explosion location − a similar trend found in the single-droplet steam explosion test with 1 g melt. However, the test of 20 g melt did not show a clear trend in a wide range of explosion depth. The peak pressure and impulse increased with increasing mass of melt sample. The steam explosion occurred more closely to the coolant pool surfaces in the seawater and chemical solutions than in deionized water. Steam explosion intensity was significantly reduced in a neutral solution containing 1.2 wt.% boric acid and sodium phosphate. The influence of the chemical solutions on steam explosion was diminishing in the tests with multiple droplets.
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| 46. |
- Deng, Yucheng, et al.
(författare)
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An experimental study on the effect of chemical additives in coolant on steam explosion
- 2024
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier Ltd. - 0017-9310 .- 1879-2189. ; 218
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Tidskriftsartikel (refereegranskat)abstract
- In assessment of severe accident risk in light water reactors (LWRs), steam explosion is a nonnegligible phenomenon following a relocation of core melt (corium) into coolant, and thus various research efforts have been paid to steam explosion. There had been numerous studies showing that the occurrence of steam explosions is influenced by several factors such as melt and coolant temperatures, melt materials, non-condensable gasses, etc. However, most of the existing experiments used deionized (DI) water or tap water as coolant, with little consideration of the effect of chemicals (e.g. boric acid, sodium hydroxide, sodium phosphate) commonly applied in reactor coolant. To examine the effect of the chemical additives in coolant on steam explosion, the present study performs a series of molten Tin droplet-coolant interaction tests using DI water and different chemical solutions, including H3BO3 solutions, NaOH + H3BO3 neutral solutions, and Na3PO4 + H3BO3 neutral solutions. The experimental results show that adding NaOH and Na3PO4 in boric acid solution significantly affects the occurrence probability of spontaneous steam explosion, because of the presence of PO43− and H+ ions. When different solutions have equivalent concentrations of H3BO3, the peak pressure values of the spontaneous steam explosion of Sn droplets are similar among various solutions. Compared with those in DI water, steam explosion in the chemical solutions occurs predominantly within a narrow range of depth from 28 mm to 40 mm and produces a much higher peak pressure. This implies that more energetic steam explosions may occur in the chemical solutions.
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| 47. |
- Dong, Shichang, et al.
(författare)
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Theoretical model for subcooled upward flow boiling heat transfer and critical heat flux for an inclined downward heated surface
- 2023
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 213
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Tidskriftsartikel (refereegranskat)abstract
- The in-vessel retention system and ex-vessel retention system are very important to the safety of nu-clear power plants under severe accidents. While the success of such safety systems relies on well un-derstanding the corresponding physical mechanisms of boiling heat transfer and critical heat flux (CHF). Challenges till remain in accurately predicting the subcooled flow boiling curve especially in the low-pressure and low-flow conditions due to its complex boiling phenomenon. The present study introduces a theoretical model to predict the boiling curve and critical heat flux for subcooled flow boiling in in-clined downward heated rectangular channel. The proposed model well estimates the transition from forced convection, isolated bubble nucleate boiling to fully developed boiling regime by considering the growth and interaction of bubbles. Through probability analysis of bubbles' interaction, the proportion of heat flux in different boiling regimes is determined. In addition, the flow boiling CHF is predicted based on the probability analysis of dry spots. The new model is validated by the subcooled flow boil-ing experiments with vertical single-side heated channel under low-pressure and low-flow conditions. The predicted boiling curves are consistent with experimental results corresponding to different thermal-hydraulic parameters, such as pressure, mass flux, inlet subcooling and wall wettability (hydrophilic and hydrophobic), and the prediction error of CHF is within & PLUSMN;15%. Furthermore, the inclination effect on CHF is validated by the subcooled flow boiling experiments in inclined channel with the inclination angle varying from 0 & DEG; to 90 & DEG;, which shows the good applicability of the developed model.
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| 48. |
- Dovic, D., et al.
(författare)
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Generalized correlations for predicting heat transfer and pressure drop in plate heat exchanger channels of arbitrary geometry
- 2009
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 52:19-20, s. 4553-4563
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Tidskriftsartikel (refereegranskat)abstract
- Characteristics of the flow in chevron plate heat exchangers are investigated through visualization tests of channels with beta = 28 degrees and beta = 61 degrees. Mathematical model is then developed with the aim of deriving correlations for prediction off and Nu for flow in channels of arbitrary geometry (beta and 1511). Thermal and hydraulic characteristics are evaluated using analytical solutions for the entrance and fully developed regions of a sinusoidal duct adapted to the basic single cell. The derived correlations are finally adjusted so as to agree with experimental results from tests on channels with beta = 28 degrees and beta = 65 degrees. f and Nu calculated by the presented correlations are shown to be consistent with experimental data from the literature at Re = 2-10,000, beta = (15-67)degrees and b/l = 0.26-0.4.
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| 49. |
- Du, Kun, et al.
(författare)
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Effect of non-axisymmetric endwall contouring and swirling inlet flow on film cooling performance of turbine endwall
- 2024
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- Non-axisymmetric endwall contouring (NEC) is one of the verified approaches to suppress secondary flows and improve aerodynamic performance. However, the design of NEC brings significant challenges to the design of endwall cooling structures. Herein, a pressure-sensitive paint experimental approach was used to obtain the film cooling effectiveness of the NEC endwall with a purge slot in this study. Three NEC types were adopted: NEC (COS), NEC (SIN), and NEC (−SIN). In addition, lean premixed combustion technology was used to achieve lower levels of NOx emissions. The turbine inlet was characterized by high turbulence and strong swirling. The effects of different swirling angles (±10, ±20, and ±30°) and densities were further explored. Due to the NEC profiling changing the secondary flow near the endwall area, coolant from the purge slot was better attached to the slot exit position, leading to a significant increase in the size of the high-cooling-efficiency region. With the mass flow ratio (MFR) varying from 0.5 to 2%, the film cooling effectiveness of the flat and NEC endwalls had similar variation characteristics. When the MFR = 0.5%, the area-averaged cooling efficiencies of the NEC (COS), NEC (SIN), and NEC (−SIN) endwalls could be improved by 2, 12.5, and 20%, respectively. Positive swirling and smaller negative swirling inflow could improve the film cooling effectiveness inside the channel. The case of SA = +20° had the best improvement, where the film cooling effectiveness of the NEC (COS), NEC (SIN), and NEC (−SIN) endwalls could reach up to 29, 35, 36, and 34%, respectively. The NEC (−SIN) endwall was less sensitive to the effects of the swirling inflow.
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| 50. |
- Du, Kun, et al.
(författare)
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Effects of the cooling configurations layout near the first-stage vane leading edge on the endwall cooling and phantom cooling of the vane suction side surface
- 2018
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 123, s. 1021-1034
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Tidskriftsartikel (refereegranskat)abstract
- Increasing the turbine inlet temperature can enhance the thermal efficiency of a gas turbine. Therefore, modern gas turbines operate at a relatively high level of temperature and endure heavy thermal load. It is important to ensure the modern gas turbine works at a high performance and safe condition. Advanced cooling techniques are implemented in the gas turbine system. In the current study, effects of the cooling configurations layout near the first-stage vane leading edge on the endwall cooling and phantom cooling of the vane suction side surface were numerically investigated. Three-dimensional (3D) Reynolds-averaged Navier-Stokes (RANS) equations combined with the shear stress transport (SST) k-ω turbulence model were solved to perform the simulations on basis of validation by comparing the experimental data and computational results. The results indicate that the layout of the cooling configurations has a significant influence on the endwall cooling, but a limited effect on the phantom cooling of the suction side surface and the aerodynamic performance. For each type, the endwall cooling and phantom cooling of the suction side surface are enhanced with the increase of the blowing ratio (M) of the leading edge coolant injection. Meanwhile, the thermodynamic loss is gradually enhanced. Overall, the Type B which has a partly blocked upstream slot achieves the best performance in terms of the coolant mass flowrate, endwall cooling, phantom cooling performance of the suction side surface and the aerodynamic performance at M=1.0.
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