| 1. |
- Kazachkov, Ivan, et al.
(författare)
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Steam flow through the volumetrically heated particle bed
- 2002
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Ingår i: International journal of thermal sciences. - 1290-0729 .- 1778-4166. ; 41:11, s. 1077-1087
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Tidskriftsartikel (refereegranskat)abstract
- The paper is devoted to the problem of ex-vessel debris bed coolability, in particular, coolability and quenching of a particulate corium debris bed by water. A number of experiments were performed in the POMECO (POrous MEdia COolability) facility to measure the dry out heat flux in a self-heated bed. A mathematical model for the description of flow of a compressible fluid (steam) through the volumetrically heated porous bed with particular consideration of the non-thermal local equilibrium is formulated and solved numerically using the split step method. It is shown that initial thermodynamic perturbations which, if they grow, will lead to a temperature escalation at a specific location. Furthermore, the data from the RIT (Royal Institute of Technology) POMECO (POrous MEdia COolability) experiments are used for the validation of the model.
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| 2. |
- Khodabandeh, R., et al.
(författare)
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Influence of system pressure on the boiling heat transfer coefficient in a closed two-phase thermosyphon loop
- 2002
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Ingår i: International journal of thermal sciences. - 1290-0729 .- 1778-4166. ; 41:7, s. 619-624
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Tidskriftsartikel (refereegranskat)abstract
- In recent years heat generation from electronic components has been rapidly increasing. Natural and forced convection air-cooled heat sinks are not well suited for cooling the components generating the highest heat fluxes. Other methods are therefore sought. Recently the use of thermosyphons has attracted attention as a simple and effective cooling system for high heat flux components. In a thermosyphon system the heat is absorbed during boiling and transferred as heat of vaporization from the hot part to the cold part of the system, with relatively small temperature differences. The setup used in this study consists of a thermosyphon loop, including evaporator, condenser, downcomer and riser. The loop has three evaporators, connected in parallel, made from small blocks of copper (10 x 20 x 15) mm in which five vertical channels with diameters 1.5 mm and length 15 min were drilled. Experimental results in terms of heat transfer coefficients at different system pressures and heat inputs are presented and compared to predictions of correlations from the literature. In all tests Isobutane is used as working fluid.
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| 3. |
- Yang, Z. L., et al.
(författare)
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Numerical investigation of bubble coalescence characteristics under nucleate boiling condition by a lattice-Boltzmann model
- 2000
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Ingår i: International journal of thermal sciences. - 1290-0729 .- 1778-4166. ; 39:1, s. 1-17
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Tidskriftsartikel (refereegranskat)abstract
- A numerical study was performed to investigate the characteristics of bubble growth, detachment and coalescence on vertical, horizontal, and inclined downward-facing surfaces. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension (sigma) and contact angle (beta), were implemented through the fluid-fluid (G(sigma)) and fluid-solid (G(t)) interaction potentials. The model predicted a linear relationship between the macroscopic properties (sigma, beta) and microscopic parameters (G(sigma), G(t)). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of bubble coalescence are investigated by simulating the growth and detachment behavior of multiple bubbles generated on horizontal, vertical, and inclined downward-facing surfaces. For the case of horizontal surface, three distinct. regimes of bubble coalescence were represented in the lattice-Boltzmann simulation: lateral coalescence of bubbles situated on the surface; vertical coalescence of bubbles detached in a sequence from a site; and lateral coalescence of bubbles, detached from the surface. Multiple coalescence was predicted on the vertical surface as the bubble detached from a lower elevation merges with the bubble forming on a higher site. The bubble behavior on the inclined downward-facing surface was represented quite similar to that in the nucleate boiling regime on a downward facing surface.
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| 4. |
- Astaneh, Majid, 1990, et al.
(författare)
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Finite-size effects on heat and mass transfer in porous electrodes
- 2022
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- In thin electrode applications, as the ratio of the obstacle size with respect to the system size increases, issues such as finite-size effects become more influential in the transport of heat and mass within a porous structure. This study presents a numerical approach to evaluate the finite-size effects on the heat and mass transfer in porous electrodes. In particular, numerical simulations based on the lattice Boltzmann method (LBM) are employed to analyze the pore-scale transport phenomena. Analyzing the results at both the electrode level and the pore level shows that the mass transfer performance is more influenced by the finite-size effects compared to the transfer of heat. The numerical simulations show that as the parameter m being the ratio of the electrode thickness to the particle diameter is halved, the effective diffusivity increases by 20% while the effective conductivity remains unchanged. We propose a novel analytical tortuosity–porosity (τ−ϕ) correlation as τ=[1−(1−ϕ)m+1]/ϕ where the finite-size effects are taken into account via the parameter m. Besides, particles of small size provide more uniform distributions of temperature and concentration within the porous structure with standard deviations of approximately half of the values obtained from the case made up of large particles. Our findings at the electrode level are compared with the commonly used macroscopic porosity-dependent correlations found in the literature. At the end, by performing a systematic assessment, we provide guidelines for efficient design of porous electrodes.
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| 5. |
- Baigmohammadi, Mohammadreza, et al.
(författare)
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Experimental study on the effect of external thermal pattern on the dynamics of methane-oxygen and methane-oxygen-carbon dioxide premixed flames in non-adiabatic meso-scale reactors
- 2019
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Ingår i: International journal of thermal sciences. - : Elsevier. - 1290-0729 .- 1778-4166. ; 137, s. 242-252
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Tidskriftsartikel (refereegranskat)abstract
- In the current study, the effect of external thermal pattern on the dynamics and characteristics of methane oxygen and methane-oxygen-carbon dioxide premixed flames in non-adiabatic meso-scale cylindrical reactors is investigated experimentally. In this regard, two different external thermal patterns were imposed on the outer surface of the reactors. The results showed that imposing method/direction and also temperature level of the external thermal pattern have impressive effect on flame dynamics and chemiluminescence in the non-adiabatic meso-scale reactors. Also, it was shown that increasing the temperature level of the external thermal pattern could significantly extend the flame stability and its presence range in the meso-scale reactors, especially for the vitiated mixtures (methane-oxygen-carbon dioxide). Moreover, the results demonstrated that decreasing the inner diameter of a meso-scale reactor, which was subjected to an external thermal pattern, could increase the flame controllability and its presence range in the non-adiabatic meso-scale reactors.
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| 6. |
- Boulet, Pascual, et al.
(författare)
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Radiation emission from a heating coil or a halogen lamp on a semitransparent sample
- 2014
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 77:March, s. 223-232
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Tidskriftsartikel (refereegranskat)abstract
- The radiation emission of the heating coil of a Cone Calorimeter and the one of the halogen lamp of a Fire Propagation Apparatus have been studied experimentally for varying power settings. These are two standard apparatuses used for fire calorimetry. The objective is to characterize and compare the radiative flux spectrum received by a fuel sample during pyrolysis experiments. The deviation from the standard assumption of black or gray emission is discussed. It is observed that the emission of the heating coil can be approximated well to an ideal blackbody, especially in the infrared range. On the contrary, the halogen lamp emission is more complex, non gray, with an important contribution in the visible and in the near infrared ranges. The flux received by a sample exposed to these emitters is predicted using ray tracing simulations. This shows that the irradiation flux and spectrum from the cone can be accurately calculated if the coil temperature is known. The non Lambertian irradiation flux from the lamp is modeled with a combination of diffuse and collimated intensities, representing the direct emission from the lamp itself and the reflection by the mirror at the rear side. For both emitters, the irradiation is confirmed to be approximately uniform over the surface of a sample 5 cm large (maximum deviation of ±2% on the incident flux). The uniformity decreases for larger samples, but the ratio of the flux at the center to average flux is still 1.04 for standard 10 cm × 10 cm samples under the cone. For illustration purposes, the influence of the spectral characteristics of the emitter is studied in the case of a sample of PMMA, a non gray translucent medium. Using recently published measurements of PMMA absorptivity, the absorbed flux by a 3 cm thick sample is predicted. In the case of an incident flux of 20 kW/m2, the calculated average absorptivity of the sample is 0.91 under the cone, while it is 0.32 under the FPA lamp. These calculations involve absorption data of a virgin sample at room temperature and consequently the numerical results only hold for the initial instants of irradiation. However, the very large differences in radiative behavior show that important discrepancies in the pyrolysis behavior are expected between the two emitters. This might have consequences for fire testing and inter comparisons of flammability results worth further investigation.
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| 7. |
- Buschmann, M. H., et al.
(författare)
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Correct interpretation of nanofluid convective heat transfer
- 2018
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 129, s. 504-531
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Forskningsöversikt (refereegranskat)abstract
- Engineers and scientist have a long tradition in trying to improve the thermophysical properties of convective heat carriers such as water and transformer oil. Technological developments of the last decades allow the dispersion of particle of sizes ranging between 10 and 100 nm in these liquids. In a large number of recent studies the resulting nanofluids have been reported to display anomalously high increase of convective heat transfer. The present study compiles experiments from five independent research teams investigating convective heat transfer in nanofluid flow in pipes, pipe with inserted twisted tape, annular counter flow heat exchanger, and coil and plate heat exchangers. The results of all these experiments unequivocally confirm that Newtonian nanofluid flow can be consistently characterized by employing Nusselt number correlations obtained for single-phase heat transfer liquids such as water when the correct thermophysical properties of the nanofluid are utilized. It is also shown that the heat transfer enhancement provided by nanofluids equals the increase in the thermal conductivity of the nanofluid as compared to the base fluid independent of the nanoparticle concentration or material. These results demonstrate that no anomalous phenomena are involved in thermal conduction and forced convection based heat transfer of nanofluids. The experiments are theoretically supported by a fundamental similarity analysis of nanoparticle motion in nanofluid flow.
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| 8. |
- Cao, Zhen, et al.
(författare)
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Electrophoretic deposition surfaces to enhance HFE-7200 pool boiling heat transfer and critical heat flux
- 2019
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 146
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Tidskriftsartikel (refereegranskat)abstract
- Modulated nanoparticle-coating surfaces were fabricated by an improved electrophoretic deposition technique in this study. Pool boiling experiments were studied for HFE-7200 on the modulated nanoparticle-coating surfaces, with a smooth surface and uniform coating surfaces as comparison. It was found that the present modulated coating surfaces can enhance the heat transfer coefficient and the critical heat flux by 60% and 20%–40%, respectively, in comparison to the smooth surface, while the uniform coating surface can improve heat transfer coefficients by maximum 100%, but cannot enhance critical heat fluxes. Heat transfer on the modulated nanoparticle-coating surfaces was theoretically analyzed by a mechanistic model which considered free convection, transient conduction and microlayer evaporation. The heat transfer can be predicted by the model, especially at low-to-moderate heat fluxes. Additionally, referring to the bubble visualization at critical heat fluxes, possible mechanisms to trigger critical heat fluxes were discussed. Afterwards, a critical heat flux model originating from the Zuber hydrodynamic instability model, was employed to predict the experimental results, showing a good prediction ability.
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| 9. |
- Du, Wei, et al.
(författare)
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Numerical investigation of flow field and heat transfer characteristics in a latticework duct with jet cooling structures
- 2020
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 158
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Tidskriftsartikel (refereegranskat)abstract
- Interaction effects of the latticework duct and jet cooling structures on the flow structure and heat transfer were numerically investigated. The latticework duct included eleven sub-channels both on the top wall and bottom wall. The crossing angle was set as 90°. The jet cooling structures had a curved target surface and seven jet nozzles. The jet cooling structures and latticework duct were connected by three different jet nozzle locations and two different jet nozzle shapes, respectively. In addition, six different mass flow ratios in the jet cooling structures were considered. Results showed that the flow field and heat transfer characteristics were sensitive to the jet nozzle location, shapes and mass flow ratios. When the mass flow ratio was increased in the jet cooling structure, the heat transfer was augmented in the jet cooling structure while it was decreased in the latticework duct. Using the jet nozzle near the turning region, the upward spiral flow and helical flow in the sub-channel of the latticework became weak, corresponding to a low Nusselt number in the latticework duct. However, the heat transfer in the jet cooling structure was enhanced. Furthermore, the circular jet nozzle provided high friction factor and Nusselt number both for the jet cooling structures and latticework duct.
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| 10. |
- Eriksson, Morgan, et al.
(författare)
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Radiative Heat Transfer in Circulating Fluidized Beds
- 2005
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Ingår i: International journal of thermal sciences. - : Elsevier BV. - 1290-0729 .- 1778-4166. ; 44, s. 399-409
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Tidskriftsartikel (refereegranskat)abstract
- A new model for predicting radiation heat transfer in circulating fluidized bed combustors is proposed. The model assumes two phase structure, the flow at the wall dominated by streamers of clusters traveling mostly downward, interspersed with periods where there is upwards flow of a dilute suspension. Also, the model assumes the intensity distribution to be semi-isotropic in the forward and backward direction. The predictions are in good agreement with the experimental results from the literature. Finally, a parametric study is performed to show the effect of different bed parameters on radiative heat transfer. The results indicate that suspension temperature, wall temperature, wall emissivity, particle emissivity, and suspension density have significant influence on the radiation heat transfer coefficient.
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