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1.
  • Johansson, Bert, et al. (författare)
  • Application of wedge-shaped hot-film probes in a gas-particle flow
  • 2003
  • Ingår i: Experimental Thermal and Fluid Science. - 0894-1777. ; 27:2, s. 187-191
  • Tidskriftsartikel (refereegranskat)abstract
    • A wedge-shaped hot-film probe is used to perform measurements of turbulence intensity and an estimate of the turbulent kinetic energy as well as the Eulerian spectra, macro- and micro-scales of the gas phase in a horizontal pipe flow carrying pulp fibres or spherical particles. In addition, the paper gives a summary of a method for handling the poor frequency response of the probe and discusses how to sort out "bad samples", occurring when the probe is hit by particles.
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2.
  • Jørgensen, F.E., et al. (författare)
  • A multi-sensor hot-wire anemometer system for investigation of wall-bounded flow structures
  • 2003
  • Ingår i: Experimental Thermal and Fluid Science. - 0894-1777. ; 27:2, s. 207-214
  • Tidskriftsartikel (refereegranskat)abstract
    • This work describes a hot-wire anemometer system and an experimental procedure designed for mapping of the flow structure in disturbed boundary layers. It also briefly presents results from periodically disturbed wing boundary layers. The hot-wire system consists of a number of individual anemometer units, which can be combined to operate multi-sensor probes for simultaneous measurement of velocities in many points. In the present paper, however, only a single probe was used mounted in a dedicated traverse system in order to demonstrate how ensemble averaging can provide knowledge about the growth and decay of organised flow structures, as long as they are periodic in both time and space. The information from this flow structure can then be used in the design of multi-array hot-wire probes for further studies of the non-deterministic stages of the boundary layer transition.
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3.
  • Owhaib, Wahib Suleiman, et al. (författare)
  • Experimental investigation of single-phase convective heat transfer in circular microchannels
  • 2004
  • Ingår i: Experimental Thermal and Fluid Science. - 0894-1777 .- 1879-2286. ; 28:2-3, s. 105-110
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, the heat transfer characteristics of single-phase forced convection of R134a through single circular micro-channels with 1.7, 1.2, and 0.8 mm. as inner diameters were investigated experimentally. The results were compared both to correlations for the heat transfer in macroscale channels and to correlations suggested for microscale geometries. The results show good agreement between the classical correlations and the experimentally measured data in the turbulent region. Contrary, none of the suggested correlations for microchannels, agreed with the test data. In the laminar regime, the heat transfer coefficients were almost identical for all three diameters.
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4.
  • Theofanous, T. G., et al. (författare)
  • The boiling crisis phenomenon - Part I : nucleation and nucleate boiling heat transfer
  • 2002
  • Ingår i: Experimental Thermal and Fluid Science. - 0894-1777 .- 1879-2286. ; 26:6-7, s. 775-792
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper (Part I) and the companion paper (Part II, Exp. Therm. Fluid. Sci. 26 (6-7) (2002) 793- 810) present results of an experimental study on nucleate pool boiling. The experiments were conducted under highly-controlled conditions, using electrically heated, vapor-deposited sub-micron metallic films. A high-speed. high-resolution infrared camera was used to visualize dynamic thermal patterns on the heater's surface over a broad range of heat fluxes, starting from the onset of nucleation and up to boiling crisis. Both fresh heaters and aged heaters were experimented with. The heaters' surface nanomorphology and chemistry were characterized with atomic force microscopy, scanning electron microscopy, and X-ray diffraction spectroscopy. First-of-a-kind experimental data on nucleation and boiling heat transfer at high heat fluxes are presented, and a stark difference between fresh and aged heaters is revealed. Remarkable are the origin, evolution and dynamics of the heater dryout process (leading to burnout), identified quantitatively and captured in action for the first time.
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5.
  • Theofanous, T. G., et al. (författare)
  • The boiling crisis phenomenon - Part II : dryout dynamics and burnout
  • 2002
  • Ingår i: Experimental Thermal and Fluid Science. - 0894-1777 .- 1879-2286. ; 26:6-7, s. 793-810
  • Tidskriftsartikel (refereegranskat)abstract
    • This is Part II of a two-part paper on the boiling crisis phenomenon. Here we report on burnout experiments conducted on fresh and aged heaters in pool boiling. Critical heat fluxes (CHFs) were found to vary from 50% to 140% of the hydrodynamic limit, previously thought to exist at well-wetting conditions. The burnout events were captured in action (for the first time), using highspeed, high-resolution infrared thermometry. Based on these observations and in conjunction with the levels of CHF reached, we are led to conclude that the phenomenon cannot be (macro)hydrodynamically limited, at east at normal pressure and gravity conditions. Based on infrared thermometry, and aided by X-ray radiography data on void fraction, the case for a scale separation phenomenon in high heat flux pool boiling is argued. This indicates that boiling crisis is controlled by the microhydrodynamics and rupture of an extended liquid microlayer, sitting and vaporizing autonomously on the heater surface. Further. the detailed dynamics of this microlayer, as revealed by our experiments. demonstrates that all previous thermally based models of boiling crisis are inappropriate.
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6.
  • Ahangar Zonouzi, S., et al. (författare)
  • Experimental investigation of the flow and heat transfer of magnetic nanofluid in a vertical tube in the presence of magnetic quadrupole field
  • 2018
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 91, s. 155-165
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, the effects of applying magnetic field on hydrodynamics and heat transfer of Fe3O4/water magnetic nanofluid flowing inside a vertical tube have been studied experimentally. The applied magnetic field was resulted from quadrupole magnets located at different axial positions along the tube length. The variations of the local heat transfer coefficient and also the pressure drop of the ferrofluid flow along the length of the tube by applying the magnetic quadrupole field have been investigated for different Reynolds numbers. The obtained experimental results show maximum enhancements of 23.4%, 37.9% and 48.9% in the local heat transfer coefficient for the magnetic nanofluid with 2 vol% Fe3O4 in the presence of the quadrupole magnets located at three different axial installation positions for the Reynolds number of 580 and the relative increase in total pressure drop by applying the magnetic field is about 1% for Re = 580. The increase of the heat transfer coefficient is due to the radial magnetic force toward the heated wall generated by magnetic quadrupole field acting over the ferrofluid flowing inside the tube so that the velocity of the ferrofluid in the vicinity of the heated wall is increased. It is also observed that the enhancement of heat transfer coefficient by applying magnetic quadrupole is decreased with increasing the Reynolds number.
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7.
  • Andersson, Klas, 1977, et al. (författare)
  • Radiation intensity of lignite-fired oxy-fuel flames
  • 2008
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 33:1, s. 67-76
  • Tidskriftsartikel (refereegranskat)abstract
    • The radiative heat transfer in oxy-fuel flames is compared to corresponding conditions in air-fuel flames during combustion of lignite in the Chalmers 100 kW oxy-fuel test facility. In the oxy-fuel cases the flue-gas recycle rate was varied, so that, in principle, the same stoichiometry was kept in all cases, whereas the oxygen fraction in the recycled flue-gas mixture ranged from 25 to 29 vol.%. Radial profiles of gas concentration, temperature and total radiation intensity were measured in the furnace. The temperature, and thereby the total radiation intensity of the oxy-fuel flames, increases with decreasing flue-gas recycle rate. The ratio of gas and total radiation intensities increases under oxy-fuel conditions compared to air-firing. However, when radiation overlap between gas and particles is considered the ratios for air-firing and oxy-fuel conditions become more similar, since the gas-particle overlap is increased in the CO2-rich atmosphere. A large fraction of the radiation in these lignite flames is emitted by particles whose radiation was not significantly influenced by oxy-fuel operation. Therefore, an increment of gas radiation due to higher CO2 concentration is not evident because of the background of particle radiation, and, the total radiation intensities are similar during oxy-fuel and air-fuel operation as long as the temperature distributions are similar.
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8.
  • Anwar, Z., et al. (författare)
  • Dryout characteristics of natural and synthetic refrigerants in single vertical mini-channels
  • 2015
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 68, s. 257-267
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental results on dryout of seven refrigerants (R134a, R1234yf, R152a, R22, R245fa, R290 and R600a) in small, single vertical tubes under upward flow conditions are reported in this study. The experiments were conducted under a wide range of operating conditions in stainless steel tubes (0.64-1.70. mm and 213-245. mm heated length). The effects of operating parameters like mass flux, vapor quality, saturation pressure and channel size are discussed in detail. In general, dryout heat flux increased with increasing mass flux, and with increasing tube diameter. No effect of varying saturation temperature was observed. The experimental findings were compared with well-known macro and micro-scale correlations from the literature and it was found that Wu's correlation (in modified form) quite satisfactorily predicted the whole database. A new correlation for prediction of heat flux at dryout conditions is also proposed.
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9.
  • Anwar, Z., et al. (författare)
  • Flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel
  • 2014
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 53, s. 207-217
  • Tidskriftsartikel (refereegranskat)abstract
    • This article reports on flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel. The experiments were carried out with a resistively heated stainless steel tube (1.60mm in diameter and 245mm heated length) at 27 and 32°C saturation temperature. Five mass fluxes in the range 100-500kg/m2s with heat fluxes from 5 to 245kW/m2 were tested. Under similar operating conditions experiments were repeated with R134a in the same setup to compare thermal performance of R152a. The results showed that the heat transfer was strongly influenced by the applied heat flux with insignificant convective contributions. The dryout heat flux increased with increasing mass flux but no effect of varying operating pressure was noticed. The experimental results for heat transfer and dryout heat flux were compared with well-known macro and micro-scale correlations from the literature.
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10.
  • Anwar, Zahid, et al. (författare)
  • Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf : Experimental results and predictions
  • 2015
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 66, s. 137-149
  • Tidskriftsartikel (refereegranskat)abstract
    • Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf in a vertical stainless steel test section (1.60mm inside diameter and 245mm heated length) under upward flow conditions are reported in this article. The experiments were carried out at 27 and 32°C saturation temperatures with five mass fluxes in the range of 100-500kg/m2s while the applied heat flux was in the range of 5-130kW/m2. The experiments were carried out with gradual increase of the applied heat flux til completion of dryout. Under similar conditions, tests were repeated with R134a in the same test setup to compare thermal performance of these two refrigerants. The results showed that boiling heat transfer was strongly controlled by the applied heat flux and operating pressure with insignificant dependence on mass flux and vapor quality. The frictional pressure drop increased with mass flux and vapor quality and decreased with increasing saturation temperature as expected. Signs of dryout first appeared at vapor qualities of 85%, with the values generally increasing with increasing mass flux. The effect of varying system pressure was insignificant. The experimental results (boiling heat transfer, pressure drop and dryout heat flux) were compared with the predictions from well-known correlations (for macro and micro-scale channels) from the literature.
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11.
  • Baudoin, Antoine, 1984-, et al. (författare)
  • Temperature and velocity measurements in a buoyant flow induced by a heat source array on a vertical plate
  • 2017
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 88, s. 234-245
  • Tidskriftsartikel (refereegranskat)abstract
    • Heat source arrays are common in engineering applications. Natural convection is a reliable and silent cooling strategy. Therefore, an array of flush-mounted heat sources has been studied under conjugate conduction and natural convection condition. This studies was performed for a system with relatively large dimensions, typical for power electronics, and a modified Rayleigh number up to 2 . 10(10) A modular set of heaters was designed to vary the distribution of heat sources on the plate and investigate the influence of the spacing. Velocity and temperature were measured in the convective flow with particle image velocimetry and micro-thermocouple. The velocity field was analyzed with proper orthogonal decomposition. The first instabilities of the convective flows were described. The results gave abetter understanding of the heat transfers in these configurations and are valuable for model validation.
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12.
  • Benini, Ernesto, et al. (författare)
  • Experimental and numerical analyses to enhance the performance of a microturbine diffuser
  • 2006
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 30:5, s. 427-440
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper describes design and off-design behavior of a centrifugal compressor of a 100 kW gas turbine used for small scale power generation and establishes the guidelines to improve diffuser performance. The first part of the paper deals with the experimental and numerical tests on the overall machine: An extensive series of tests at different operating points and rotational speeds is performed using steady probe measurements at impeller exit and diffuser exit; the numerical model features a mixing plane at impeller-diffuser interface and therefore neglects the effect of unsteadiness due to rotor-stator interaction. In the second part of the paper, the true time-dependent rotor-stator interaction is investigated by means of a numerical model where a sliding mesh technique is adopted instead. The unsteady results are then processed and compared with the computed steady flow in the diffuser. Finally, the geometry of the compressor diffuser is optimized using an evolutionary algorithm coupled with a CFD code.
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13.
  • Bitaraf Haghighi, Ehsan, et al. (författare)
  • Cooling performance of nanofluids in a small diameter tube
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 49, s. 114-122
  • Tidskriftsartikel (refereegranskat)abstract
    • This article reports convective single-phase heat transfer performance in laminar flow for some selected nanofluids (NFs) in an open small diameter test section. A 0.50 mm inner diameter, 30 cm long stainless steel test section was used for screening single phase laminar convective heat transfer with water and five different water based NFs. Tested NFs were; Al2O3 (two types), TiO2 (two types) and CeO2 (one type), all 9 wt.% particle concentration. The effective thermal conductivity of the NFs were measured with Transient Plane Source (TPS) method and viscosity were measured with a rotating coaxial cylindrical viscometer. The obtained experimental results for thermal conductivity were in good agreement with the predicted values from Maxwell equation. The local Shah correlation, which is conventionally used for predicting convective heat transfer in laminar flow in Newtonian fluids with constant heat flux boundary condition, was shown to be valid for NFs. Moreover, the Darcy correlation was used to predict the friction factor for the NFs as well as for water. Enhancement in heat transfer for NFs was observed, when compared at equal Reynolds number, as a result of higher velocity or mass flow rate of the NFs at any given Reynolds number due to higher viscosity for NFs. However, when compared at equal pumping power no or only minor enhancement was observed.
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14.
  • Bitaraf Haghighi, Ehsan, et al. (författare)
  • Experimental study on convective heat transfer of nanofluids in turbulent flow : Methods of comparison of their performance
  • 2014
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 57, s. 378-387
  • Tidskriftsartikel (refereegranskat)abstract
    • Turbulent convective heat transfer coefficients of 9 wt% Al2O3/water and TiO2/water nanofluids inside a circular tube were investigated independently at the Royal Institute of Technology, KTH (Sweden) and at University of Birmingham (UK). The experimental data from both laboratories agreed very well and clearly show that Nusselt numbers are well correlated by the equations developed for single phase fluids with the thermophysical properties of nanofluid. The heat transfer coefficients of nanofluids can be compared with those of the base fluids at the same Reynolds number or at the same pumping power. As the same Reynolds number requires higher flow rate of nanofluids therefore such comparison shows up to 15% increase in heat transfer coefficient. However, at equal pumping power, the heat transfer coefficient of Al2O3 nanofluid was practically the same as that of water while that of TiO2 was about 10% lower. Comparing performance at equal Reynolds number is clearly misleading since the heat transfer coefficient can always be increased by increased pumping power, accordingly, the comparison between the fluids should be done at equal pumping power.
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15.
  • Bäckström, Daniel, 1985, et al. (författare)
  • Particle composition and size distribution in coal flames - The influence on radiative heat transfer
  • 2015
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 64, s. 70-80
  • Tidskriftsartikel (refereegranskat)abstract
    • Radiative heat transfer in a 77 kWth swirling lignite flame has been studied. The aim is to characterize different particle types present in a coal flame and to determine their influence on the radiative heat transfer. The study combines extractive particle measurements, radiative intensity measurements and detailed radiation modelling. The size distribution of the extracted particles was measured with a low pressure impactor and some of the size fractions were analysed with SEM–EDX. The measured total radiative intensity is compared with the modelled intensity based on the particle measurements in the same cross-section of the flame. The particle properties were calculated with Mie theory and the gas properties with a statistical narrow-band model. The results show that the contribution of coal/char particles dominates the radiative heat transfer in the investigated cross-section of the flame. The methodology applied in this work shows promising results for characterization of particle radiation in flames of practical size.
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16.
  • Dehaj, M. S., et al. (författare)
  • Experimental analysis of natural gas combustion in a porous burner
  • 2017
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 84, s. 134-143
  • Tidskriftsartikel (refereegranskat)abstract
    • The study aims to evaluate a novel porous medium burner (PMB) with a heat exchanger for household heating systems. In this regard, a PMB together with a heat exchanger is designed and built in the laboratory. The changes of temperature in the porous medium combustion chamber have been recorded, and the pressure losses in the burner, thermal efficiency of the burner, and the concentration of NO polluting gasses have been measured. The obtained results indicate that the maximum temperature is at the beginning of the combustion zone and the end of the preheating zone; but, with the increase of power and the excess air ratio, the flame front moves forward. At a constant power, with the increase of the excess air ratio, the total flow rate increases, followed by the increase of pressure loss. The thermal efficiency results indicate that the highest efficiency is achieved at the excess air ratio of 1/2. This is due to the fact that at this ratio, the combustion products have their highest temperature. The volume of excess air has a significant influence on the amount of NO produced. With the increase of excess air, the volume of NO gasses diminishes.
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17.
  • Discetti, S., et al. (författare)
  • Characterization of very-large-scale motions in high-Re pipe flows
  • 2019
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; , s. 1-8
  • Tidskriftsartikel (refereegranskat)abstract
    • Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number.
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18.
  • Gao, Xiufang, et al. (författare)
  • PIV measurement of the flow field in rectangular ducts with 60 circle parallel, crossed and V-shaped ribs
  • 2004
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 1879-2286 .- 0894-1777. ; 28:6, s. 639-653
  • Tidskriftsartikel (refereegranskat)abstract
    • The flow characteristics in rectangular ducts with aspect ratio of 1:8 were investigated experimentally using particle image velocimetry to reveal the function of the inclined ribs as turbulators or generators of favorable flows. Circular ribs were attached on the wide walls and the rib height-to-hydraulic diameter ratio was kept as 0.06 with an attack angle of 60degrees. The obtained results showed strong effects of the inclined ribs on the flow behavior by the development of secondary flows. The magnitude of the streamwise velocity component was detected to have spanwise variation. The flow characteristics were correlated with the previous corresponding heat transfer results obtained by the same authors. (C) 2003 Elsevier Inc. All rights reserved.
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19.
  • Ghanbarpour, Morteza, et al. (författare)
  • Thermal properties and rheological behavior of water based Al2O3 nanofluid as a heat transfer fluid
  • 2014
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 53, s. 227-235
  • Tidskriftsartikel (refereegranskat)abstract
    • An experimental investigation and theoretical study of thermal conductivity and viscosity of Al2O3/water nanofluids are presented in this article. Various suspensions containing Al2O3 nanoparticles were tested in concentration ranging from 3% to 50% in mass and temperature ranging from 293K to 323K. The results reveal that both the thermal conductivity and viscosity of nanofluids increase with temperature and particle concentration accordingly while the increase in viscosity is much higher than the increase in thermal conductivity. The thermal conductivity and viscosity enhancement are in the range of 1.1-87% and 18.1-300%, respectively. Moreover, the results indicate that the thermal conductivity increases nonlinearly with concentration, but, linearly with the increase in temperature. In addition, the experimental results are compared with some existing correlations from literature and some modifications are suggested. Finally, the average heat transfer coefficient at different basis of comparisons including equal Reynolds number, fluid velocity and pumping power is studied based on the experimental thermal conductivity and viscosity in fully developed laminar and turbulent flow regimes. It is found that equal Reynolds number as a basis of comparison is highly misleading and equal pumping power can be used to study the advantage of using nanofluid instead of the base fluid.
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20.
  • Giri, Asis, et al. (författare)
  • Analysis of bubble dynamics in explosive boiling of droplet with fine fragmentation
  • 2005
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 29:3, s. 295-303
  • Tidskriftsartikel (refereegranskat)abstract
    • If a high temperature liquid comes into contact with cold and volatile liquid, rapid (or explosive) evaporation may occur spontaneously or triggered by the impact of a pressure wave. This event generating a shock wave is called a steam explosion. It involves many multiphase flow and heat transfer phenomena. One of the more important phenomena in a steam explosion is the fine fragmentation of the hot liquid, which determines the explosive heat transfer from the hot liquid to the cold liquid and the vaporisation rate of the cold liquid. When a small hot single drop (similar to1 mm) interacts with the coolant, a vapour bubble is formed around the drop. It was observed experimentally that these vapour bubbles grow and collapse. During this process, the small hot droplet fragments and generates finer particles. To understand the fine fragmentation process during a steam explosion, in this study, this phenomenon was examined by using non-linear stability analysis of vapour bubble dynamics based on a concept developed by Inoue et al. [Chem. Eng. Commun. 118 (1992) 189]. From the analysis, it was observed that higher spherical modes were very much unstable during collapse process, which decided the size of the fragmented particles. Vapour shell between molten metal and coolant was considered unstable if the amplitude of one of the spherical modes was greater than vapour shell thickness. In addition, the mass of fragmented particles during each cycle of vapour bubble dynamics was predicted from the analysis. The calculated results were found to be in reasonable agreement with the previously reported [J. Non-Equil. Thermodyn. 13 (1988) 27] experimental results.
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21.
  • Gourdon, Mathias, 1980, et al. (författare)
  • Qualitative investigation of the flow behaviour during falling film evaporation of a dairy product
  • 2015
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 60, s. 9-19
  • Tidskriftsartikel (refereegranskat)abstract
    • Falling film evaporation is an important technology in the dairy industry for producing powders. In this paper, flow details of liquid falling films have qualitatively been investigated using a pilot evaporator, and in particular using a high-speed camera. Variations with different dry solids contents, flow rates and driving temperature differences were investigated. The flow characteristics were seen to be considerably affected by all three variables. Two of the main observations were the formation of bubbles under evaporative conditions and that the flow, bubble formation and evaporative heat transfer coefficient was observed to be heat flux dependent.
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22.
  • Grozdek, Marino, et al. (författare)
  • Experimental investigation of ice slurry flow pressure drop in horizontal tubes
  • 2009
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 33:2, s. 357-370
  • Tidskriftsartikel (refereegranskat)abstract
    • Pressure drop behaviour of ice slurry based on ethanol-water mixture in circular horizontal tubes has been experimentally investigated. The secondary fluid was prepared by mixing ethyl alcohol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 degrees C). The pressure drop tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 30% depending on test conditions. Results from flow tests reveal much higher pressure drop for higher ice concentrations and higher velocities in comparison to the single phase flow. However for ice concentrations of 15% and higher, certain velocity exists at which ice slurry pressure drop is same or even lower than for single phase flow. It seems that higher ice concentration delay flow pattern transition moment (from laminar to turbulent) toward higher velocities. In addition experimental results for pressure drop were compared to the analytical results, based on Poiseulle and Buckingham-Reiner models for laminar flow, Blasius. Darby and Melson, Dodge and Metzner, Steffe and Tomita for turbulent region and general correlation of Kitanovski which is valid for both flow regimes. For laminar flow and low buoyancy numbers Buckingham-Reiner method gives good agreement with experimental results while for turbulent flow best fit is provided with Dodge-Metzner and Tomita methods. Furthermore, for transport purposes it has been shown that ice mass fraction of 20% offers best ratio of ice slurry transport capability and required pumping power.
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23.
  • Guemes, A., et al. (författare)
  • Flow organization in the wake of a rib in a turbulent boundary layer with pressure gradient
  • 2019
  • Ingår i: Experimental Thermal and Fluid Science. - : ELSEVIER SCIENCE INC. - 0894-1777 .- 1879-2286. ; 108, s. 115-124
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of a streamwise pressure gradient on the wake developed by wall-attached square ribs in a turbulent boundary layer is investigated experimentally. Favourable-, adverse- and zero-pressure-gradient conditions (FPG, APG and ZPG, respectively) are reproduced at matched friction Reynolds number and non-dimensional rib height. Flow-field measurements are carried out by means of Particle Image Velocimetry (PIV). Turbulence statistics are extracted at high resolution using an Ensemble Particle Tracking Velocimetry approach. Modal analysis is performed with Proper Orthogonal Decomposition (POD). We demonstrate that a non-dimensional expression of the pressure gradient and shear stress is needed to quantify the pressure-gradient effects in the wake developing past wall-attached ribs. We suggest the Clauser pressure-gradient parameter beta, commonly used in the literature for the characterization of turbulent boundary layers under the effect of a pressure gradient, as a suitable parameter. The results show that, in presence of an adverse pressure gradient, the recirculation region downstream of the rib is increased in size, thus delaying the reattachment, and that the peak of turbulence intensity and the shed eddies are shifted towards larger wall-normal distances than in the ZPG case. The observed changes with respect to the ZPG configuration appear more intense for larger magnitude of beta, which are more likely to be obtained in APG than in FPG due to the reduced skin friction and increased displacement thickness.
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24.
  • Guo, Qiang, et al. (författare)
  • An experimental study on quenching of metallic spheres in seawater
  • 2023
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 148
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by quenching of melt droplets and debris particles in seawater during a hypothetical severe accident of light water reactors (LWRs), the quenching process of stainless-steel spheres in a seawater pool was investigated in the present study. The polished spheres were pre-heated up to 1000celcius in an induction furnace with inserted atmosphere, and then immersed into the subcooled water pool in a chamber made of transparent quartz. A thermocouple was embedded in the center of the sphere to measure the history of the sphere's temperature, while a high-speed camera was employed to record the quenching process and vapor film dynamics. Quantitative data, e.g. film thickness and oscillation, of the vapor film evolution during the quenching process were obtained through an image processing program developed on the MATLAB platform.The experimental results indicated that the quenching rate was higher in seawater than in deionized water, and the vapor film collapsed at a temperature higher than the Leidenfrost temperature in deionized water. The trend appeared more significant with increasing subcooling of water. The comparison of the quenched spheres suggested the surface of the sphere in seawater achieved higher degree of discoloration and roughening than that in deionized water, probably due to the additives of salt which change water properties. The image processing and analysis revealed that the vapor film had different thickness profile along the upper and lower hemispheres, and the averaged film thickness was smaller in seawater than in de-ionized water during the stage of stable film boiling. The vapor film was thinning and oscillating with time, and its fluctuations appeared different frequencies and amplitudes at the upper and lower locations, which may explain the mechanism of the earlier collapse of vapor film in the quenching process of a high-temperature sphere in seawater.
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25.
  • Hansson, Roberta Concilio, et al. (författare)
  • Simultaneous high speed digital cinematographic and X-ray radiographic imaging of a intense multi-fluid interaction with rapid phase changes
  • 2009
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 33:4, s. 754-763
  • Tidskriftsartikel (refereegranskat)abstract
    • As typical for the study of the vapor explosions, the qualitative and quantitative understanding of the phenomena requires visualization of both material and interface dynamics. A new approach to multifluid multiphase visualization is presented with the focus on the development of a synchronized highspeed visualization by digital cinematography and X-ray radiography. The developed system, named SHARP (simultaneous high-speed acquisition of X-ray radiography and photography), and its image processing methodology, directed to an image synchronization procedure and a separate quantification of vapor and molten material dynamics, is presented in this paper. Furthermore, we exploit an intrinsic property of the X-ray radiation, namely the differences in linear mass attenuation coefficients over the beam path through a multi-component system, to characterize the evolution of molten material distribution. Analysis of the data obtained by the SHARP system and image processing procedure developed granted new insights into the physics of the vapor explosion phenomena, as well as, quantitative information of the associated dynamic micro-interactions.
  •  
26.
  • Hosseinalipour, S. M., et al. (författare)
  • Experimental investigations and correlation development of convective heat transfer in a rotating smooth channel
  • 2018
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 94, s. 316-328
  • Tidskriftsartikel (refereegranskat)abstract
    • An experimental study has been carried out to investigate the effects of Coriolis force and centrifugal buoyancy force on convective heat transfer in a smooth cooling channel under rotating condition in a new rotation facility. The main motivation behind this work is to obtain required data for developing heat transfer correlations based on rotating state. In accordance with the open literature, four parameters, i.e.s Reynolds number, Rotation number, coolant Density Ratio and main flow direction were selected to study the influence on local heat transfer coefficient. Therefore, the investigation considered Reynolds number (Re) ranging from 5000 to 12,000, Rotation number (Ro) in the range of 0–0.15, Inlet Density Ratio (DR) between 0.04 and 0.1 and at the entry developing region of a square smooth channel in a new rotation test bench. Comprehensive experiment have been conducted to collect data on the local Nusselt number at the leading and trailing walls. The results show that the local heat transfer rate is enhanced by rotation on the trailing surface and reduced on the leading surface in outward flow due to symmetrical secondary vortex generated by Coriolis force. The centrifugal buoyancy effect is favorable to the heat transfer enhancement on both walls. Also, by utilizing the extensive experimental data, correlations for the local Nusselt number have been updated for the leading and trailing walls in ranges of the mentioned flow parameters, which can be used as an internal cooling boundary condition for numerical analysis of thermal stresses and cooling efficiency maximization for gas turbine blades.
  •  
27.
  • Huang, Dan, et al. (författare)
  • Effects of hybrid nanofluid mixture in plate heat exchangers
  • 2016
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 1879-2286 .- 0894-1777. ; 72, s. 190-196
  • Tidskriftsartikel (refereegranskat)abstract
    • Heat transfer and pressure drop characteristics of a hybrid nanofluid mixture containing alumina nanoparticles and multi-walled carbon nanotubes (MWCNTs) were experimentally investigated in a chevron corrugated-plate heat exchanger. A MWCNT/water nanofluid with a volume concentration of 0.0111% and an Al2O3/water nanofluid with a volume concentration of 1.89% were mixed at a volume ratio of 1:2.5. A small amount of MWCNTs was added in order to increase the mixture thermal conductivity. Experiments with water used as both hot and cold fluids were carried out first to obtain a heat transfer correlation for fluids flowing in the chevron plate heat exchanger. The results of the nanofluid mixture were compared with those of the Al2O3/water nanofluid and water. Results show that the heat transfer coefficient of the hybrid nanofluid mixture is slightly larger than that of the Al2O3/water nanofluid and water, when comparison is based on the same flow velocity. The hybrid nanofluid mixture also exhibits the highest heat transfer coefficient at a given pumping power. The pressure drop of the hybrid nanofluid mixture is smaller than that of the Al2O3/water nanofluid and only slightly higher than that of water. Therefore, hybrid nanofluid mixtures might be promising in many heat transfer applications.
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28.
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29.
  • Innings, Fredrik, et al. (författare)
  • Analysis of the flow field in a high-pressure homogenizer
  • 2007
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 1879-2286 .- 0894-1777. ; 32:2, s. 345-354
  • Tidskriftsartikel (refereegranskat)abstract
    • Numerous theories have been developed describing the drop break-up in a high-pressure homogenizer (HPH), but they are generally based on quite rudimentary descriptions of the flow fields. As the flow in a real HPH is very extreme, with gaps of 10–100 μm and velocities of hundreds of m/s, it is practically impossible to measure the velocity fields. In this study, a scale model of an HPH made of acrylic plastic has been developed making measurements possible. Great care was taken to keep the relevant dimensionless numbers constant during the scale-up. The flow field at the gap entrance shows a steady acceleration and total turbulence suppression, in the gap the flow field is flat with thin boundary layers, and at the exit a turbulent jet is formed. The jet was found to be very unsteady, and in very similar flow situations could either be attached to the walls or continue straight ahead after the gap.
  •  
30.
  • Konnov, Alexander, et al. (författare)
  • 2D effects in laminar premixed flames stabilized on a flat flame burner
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 1879-2286 .- 0894-1777. ; 47, s. 213-223
  • Tidskriftsartikel (refereegranskat)abstract
    • In the present work non-idealities of flat burner-stabilized flames at atmospheric and low pressures are examined using PIV measurements, which are supported by and analyzed with the help of CFD modeling. Radial and axial velocity profiles measured in the cold flow and in the flames at atmospheric and at lower pressures are presented. Two possible reasons for the drop in the apparent laminar burning velocity at low pressures (below 20 kPa) were considered: the non-uniformity of the velocity profile issuing from the burner mouth and diffusion of the gas mixture from the original gas flow. At very low pressures an outward velocity at the edge of the burner plate is detected. A part of the fuel-air mixture therefore seems to escape the flow at the edge of the burner. Further experiments, however, revealed that diffusion of the fuel from the fresh mixture and of the ambient atmosphere into the flame is less important as compared to the diffusive loss of H atoms from the flame at low pressures. This was substantiated by the numerical analysis and assessment of the earlier observations from the literature. A critical relation between the size of the burner and the lowest pressure for ensuring the stabilization of a flat flame is demonstrated. (C) 2013 Elsevier Inc. All rights reserved.
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31.
  • Lal, Sreeyuth, et al. (författare)
  • Turbulent airflow above a full-scale macroporous material : Boundary layer characterization and conditional statistical analysis
  • 2016
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 74, s. 390-403
  • Tidskriftsartikel (refereegranskat)abstract
    • Convective drying of macroporous materials is governed by the complex interaction between airflow above and into the material, the roughness of the air material interface and the characteristics of the material pore system. In this study, we experimentally investigate this interplay in detail and at fullscale, using porous asphalt (PA) as a model material. The characteristics of the turbulent flow in the immediate vicinity of the material surface are studied with full-scale wind tunnel experiments at three flow speeds over two types of PA with different surface porosities and surface pore sizes, and are compared to similar measurements over a smooth and impermeable reference material. It is shown that, above a certain wall-normal distance, turbulence profiles can be scaled to make them independent of the flow speed. However, at low speed, the scaling breaks down due to a combination of organized turbulent structures of high intensity and a low turbulence background. No generally valid scaling applicable at all tested air speeds is found close to the surface, where drying occurs. Hence, realistic drying experiments must be performed at full scale and for the entire range of velocities of interest.
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32.
  • Li, Haohan, et al. (författare)
  • Investigation of evaporating sprays in a medium speed marine engine
  • 2021
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 121
  • Tidskriftsartikel (refereegranskat)abstract
    • The understanding of diesel sprays is very important to enable a better and cleaner marine engine design, but unfortunately little knowledge is openly available on marine engine fuel sprays. In this paper, evaporating sprays for medium speed marine engines were studied in a constant volume combustion chamber by performing optical measurements through Schlieren and Mie diagnostic techniques. The effects of ambient gas temperature and ambient gas density on vapor and liquid penetration were investigated by changing the target condition in the combustion chamber. A comparative study of two injectors with different nozzle diameters (0.38 mm and 0.44 mm) was also carried out at ambient density of 22.5 kg/m3. Some empirical correlations of spray penetration have been modified to fit the spray measurement data. Due to the transient characteristics of the pump-line-nozzle injection system, a time-dependent injection pressure profile is suggested for calculation of spray penetration. The spray tip penetration at large distance under low density (7.6 and 15.2 kg/m3) conditions is expected to be proportional to t2/3, which is supported by the model considering spray-induced gas turbulence effect. The t1/2 law, where turbulence is not taken into account, is still valid under high density (22.5 kg/m3) conditions with higher engine load. The comparison of two models demonstrates that the effect of gas turbulence is influenced by the ambient gas density and engine load.
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33.
  • Li, Liangxing, et al. (författare)
  • Experimental investigation on the moving characteristics of molten metal droplets impacting coolant
  • 2008
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 32:4, s. 962-972
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper reports the results of an experimental investigation on the moving characteristics of molten metal droplets impacting coolant free surface. A visualization experimental facility of molten fuel coolant interactions (MFCI) is designed and set up in the present study. The lead–bismuth (Pb–Bi) alloys are employed as the metal materials. An automatic control circuit is designed and applied to control the release of the molten droplets. High-speed camera is employed to record the movement of the molten metal droplets falling down and into a coolant pool. Based on the analysis of the experimental data, a so-called “J-region” of the droplet’s velocity–time curves was found and the melt droplet enters the “J-region” when it impacts the free water surface. In the “J-region”, the velocity of the melt droplet decreases rapidly and suddenly at first, and then increases again. The droplet gradually reaches a comparatively steady velocity when it leaves the “J-region”. The present study provides essential information for further study on the fragmentation behavior of high-temperature molten droplets in coolant.
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34.
  • Li, Xingchen, et al. (författare)
  • Breakup dynamics of gas-liquid interface during Taylor bubble formation in a microchannel flow-focusing device
  • 2020
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 113
  • Tidskriftsartikel (refereegranskat)abstract
    • This work aims to investigate the breakup dynamics of the gas-liquid interface during bubble formation in a microchannel flow-focusing device. An interface tracking method is developed to capture the profiles of the gaseous thread evolution. The results show that the pinch-off period can be further divided into a liquid squeezing stage and a free pinch-off stage in both the radial and axial directions. The time domain criterion between these two stages in a low viscous liquid, with Ohnesorge numbers Oh≪1, is proved to be shorter than the capillary time. The effects of surface tension, viscosity and gas inertial force exerting on the interface during the free pinch-off stage are proved similar to those in a quiescent liquid pool. The power law of the minimum diameter at the gaseous thread to the pinch-off remaining time in the present experiments agrees with previous studies in both ranges (1/3 to 1/2) and tendency.
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35.
  • Lim, H.D., et al. (författare)
  • Visual-hull based 3D reconstruction of shocks in under-expanded supersonic bevelled jets.
  • 2018
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 99, s. 458-473
  • Tidskriftsartikel (refereegranskat)abstract
    • Three-dimensional shock structures produced by Mach 1.45 supersonic bevelled jets were digitally reconstructedbased on schlieren photography and a voxel-based visual hull technique. By taking advantage of the strong edgefeatures commonly found in schlieren images of shock waves, the proposed technique demonstrates the possibilityof performing shock wave reconstruction in supersonic jet applications without prior knowledge of theglobal density or velocity field. Semi-synthetic camera parameters were introduced as a method to circumventcamera calibration issues faced in the reconstruction procedure. This is key to achieving accurate and highresolutionreconstructed shock waves for both axisymmetric and asymmetric test cases with an average of 2.5%error when validated against raw schlieren images. When applied to bevelled jets with non-uniform nozzle exitgeometries, an additional assumption was made to address the problem of schlieren line-of-sight blockage by thenon-conventional nozzle, and reconstruction errors were found to be larger near regions of poorer shock wavecontrast. Current results indicate that the technique is robust and fast during image calibration and processing,with accuracy of reconstructed shock waves in both conventional and non-conventional nozzles strongly dependenton shock wave contrast. Compared to existing techniques that can be used to reconstruct 3D shockstructures, the proposed technique has the advantage of being totally non-intrusive as compared to point orparticle-based measurements, requires significantly less computation than tomographic methods, offers highresolution reconstruction even with limited camera resolution and projected schlieren views, and is easy and costeffective to implement.
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36.
  • Lin, C., et al. (författare)
  • Velocity characteristics in boundary layer flow caused by solitary wave traveling over horizontal bottom
  • 2016
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 76, s. 238-252
  • Tidskriftsartikel (refereegranskat)abstract
    • The characteristics of horizontal velocity in the bottom boundary-layer flow induced by a solitary wave propagating over a horizontal bottom are presented experimentally, using high-speed particle image velocimetry (HSPIV). The ratio of wave height to water depth varies from 0.096 to 0.386 and the flow inside the boundary layer is laminar. The results show that the horizontal velocity profiles can be mainly classified into two categories with respect to the passing of the solitary wave-crest at the measuring section: the pre-passing (or acceleration) phases under favorable pressure gradient and post-passing (or deceleration) phases under adverse pressure gradient. For the velocity distributions obtained during the pre-passing phases, a nonlinear regression analysis was used to precisely determine the time-dependent characteristic length and velocity scales underlying these profiles. A similarity profile of the horizontal velocity is established first using the time-dependent free-stream velocity and boundary layer thickness as the characteristic velocity and length scales, respectively. In addition, the displacement thickness, the momentum thickness, and the energy thickness are also considered as alternative length scales. All these four representative thicknesses are closely related to each other, demonstrating that any one amongst them can be regarded as the characteristic length scale. The forms of similarity profiles for the non-dimensional velocity distributions are somewhat analogous to the results of steady boundary layer flow over a thin plate under with pressure gradient, but with different coefficients or powers. While during the post-passing phases, flow reversal which acts like an unsteady wall jet and moves in the opposite direction against the wave propagation occurs after the passage of solitary wave-crest. The thickness of flow reversal layer increases with time. A similarity profile is proposed for the velocity distributions corresponding to occurrence of the extreme value in the maximum negative velocity of flow reversal. Variations of the maximum negative velocity and the thickness of flow reversal with the time right after the start of flow reversal are also discussed in detail. Moreover, the non-dimensional time leads of the horizontal velocities at different heights in the boundary layer over the free-stream velocity are evidenced to be more noticeable toward the bottom, and also in lower ratio of wave height to water depth. A similarity profile for the non-dimensional time lead versus the non-dimensional height above the bottom surface is also presented.
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37.
  • Lundström, Hans (författare)
  • Investigation of heat transfer from thin wires in air and a new method for temperature correction of hot-wire anemometers
  • 2021
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 128
  • Tidskriftsartikel (refereegranskat)abstract
    • Variations in fluid temperature are the most important error source in hot-wire anemometer measurements and must sometimes be compensated for. Although many temperature compensation schemes have been proposed over the years, no consensus seems to be reached regarding correction methods. In this paper new precision measurements on heat transfer from heated wires have confirmed that, provided the fluid properties are evaluated at the mean temperature between the air and sensor temperatures, the Nusselt number is a consistent function of the Reynolds number without any further corrections. Based on this knowledge a new calibration function which accounts for temperature dependencies in fluid properties is proposed. The aim has been to come up with a calibration function that is useful in practical anemometry. The only parameters in the function are air- and sensor temperatures. Velocity calibration is only necessary at one air temperature because temperature dependencies on conductivity and viscosity are incorporated in the calibration function. 
  •  
38.
  • Lundström, Hans, et al. (författare)
  • Radiation influence on indoor air temperature sensors : Experimental evaluation of measurement errors and improvement methods
  • 2020
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 115
  • Tidskriftsartikel (refereegranskat)abstract
    • Radiation influence on commonly used temperature sensors for measuring indoor air temperatures can be significant, especially at the typically low air velocities occurring indoors. Conceptually, a physical sensor may not read the true air temperature, it only reads its own temperature, and, being a solid body, it will exchange energy with the surrounding surfaces (walls, windows etc.) through radiation. In the present study, radiation influence on indoor air temperature measurements was investigated experimentally and errors were quantified in simple terms. Measures to reduce the impact on some common temperature sensors were explored. A special test rig was built to simulate typical airflow and radiation environments indoors. It is suggested that the radiation impact on a temperature sensor is quantified by a radiation sensitivity factor defined as RSF = hrad/hconv, where hrad and hconv are heat transfer coefficients for radiation and convection, respectively. As this definition infers, the radiation sensitivity is dependent on size, geometry and emissivity of the temperature sensor. The radiation sensitivity factor, thus being unique for each type of sensor, was measured for some common types of thermistors and thermocouples. It is demonstrated that radiation errors may be reduced by 60 – 80 % on thermistors by reducing their emissivity through gold sputtering, and on thermocouples by stripping the insulation at the outermost part of their sensor leads.
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39.
  • Lundström, Hans, et al. (författare)
  • Temperature dependence of convective heat transfer from fine wires in air : A comprehensive experimental investigation with application to temperature compensation in hot-wire anemometry
  • 2007
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 32:2, s. 649-657
  • Tidskriftsartikel (refereegranskat)abstract
    • Heat transfer from a fine wire to air has been experimentally investigated. High accuracy measurements, where both the air temperature and wire temperature have been varied systematically and independently have made it possible to map the behavior of the heat transfer process for different velocities, air temperatures, and wire temperatures. Based on these results a compensation method is proposed which makes it possible to temperature compensate hot wires of large aspect ratio and at low Reynolds numbers for anemometry measurements with velocity calibration only at one air temperature.
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40.
  • Mallor, Fermin, et al. (författare)
  • Modal decomposition of flow fields and convective heat transfer maps : An application to wall-proximity square ribs
  • 2019
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; , s. 517-527
  • Tidskriftsartikel (refereegranskat)abstract
    • In this work the modal decomposition of convective heat transfer distributions in turbulent flows is explored. The organization and thermal footprint of the turbulent flow features generated downstream of wall-proximity two-dimensional square ribs immersed in a turbulent boundary layer are investigated experimentally. This study employs modal decomposition to investigate whether this analysis can allow identifying which characteristics of the flow topology are responsible for the Nusselt-number augmentation, aiming to uncover the underlying physics of heat-transfer enhancement. Heat transfer and flow velocity measurements are performed at a Reynolds number (based on the free-stream velocity and rib side-length) equal to 4600. Square ribs are tested for two different gap spacings from the wall (0.25 and 0.5 ribs side-length) and in wall-attached configuration. A low-thermal-inertia heat transfer sensor coupled with high-repetition-rate Infrared (IR) thermography is designed to study the unsteady variation of the convective heat-transfer coefficient downstream of the obstacles. Flow-field measurements are performed with non-time-resolved Particle Image Velocimetry (PIV). A modal analysis with Proper Orthogonal Decomposition (POD) is applied to both convective heat-transfer maps and velocity-fields. The comparison of the Nusselt-number spatial modes of the clean turbulent boundary layer configuration and of the configurations with the ribs shows a variation of the spatial pattern associated with oscillations with strong spanwise coherence, opposed to the thin elongated streaks which dominate the convective heat transfer in the clean turbulent boundary layer. In configurations where the convective heat transfer is enhanced by coherent structures located close to the wall, similar eigenspectra are observed for both flow field and convective heat transfer modes. The results of the modal analysis support a picture of a direct relation between the coherence of near-wall flow features and heat-transfer augmentation, providing a statistical evidence for the fact that near-wall coherent eddies are extremely efficient in enhancing heat transfer.
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41.
  • Maqbool, Muhammad Hamayun, et al. (författare)
  • Experimental investigation of dryout of propane in uniformly heated single vertical mini-channels
  • 2012
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 37, s. 121-129
  • Tidskriftsartikel (refereegranskat)abstract
    • This article presents dryout results of propane in single vertical circular minichannels made of stainless steel with internal diameters of 1.70 mm and 1.224 mm. The heat flux is increased in steps up to occurrence of dryout. The effects of different parameters such as mass flux, saturation temperature, vapour quality and internal diameter on the dryout are investigated. The results show that the dryout heat flux increases with the increase in mass velocity, with the decrease of vapour quality and with the increase of internal diameter. Almost no effect of saturation temperature on dryout heat flux is observed. Generalised CHF correlations developed for macro and micro scale from the literature are also compared with the experimental results.
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42.
  • Maqbool, Muhammad Hamayun, et al. (författare)
  • Investigation of two phase heat transfer and pressure drop of propane in a vertical circular minichannel
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 46, s. 120-130
  • Tidskriftsartikel (refereegranskat)abstract
    • This article reports the flow boiling heat transfer and pressure drop results of propane in a vertical circular stainless steel minichannel having an internal diameter of 1.70 mm and a heated length of 245 mm. Two phase heat transfer and pressure drop experiments have been performed at saturation temperatures of 23, 33 and 43 degrees C. Heat flux is varied from 5 to 280 kW/m(2) and mass flux is varied from 100 to 500 kg/m(2) s. The results show that the two phase frictional pressure drops, as expected, are increased with the increase of mass flux, vapour qualities and with the decrease of saturation temperature. The heat transfer coefficients are showed to increase with the increase of heat flux and saturation temperature while the influence of mass flux and vapour quality is observed as insignificant. After incipience of dryout, the decrease in heat transfer coefficient and also the two phase frictional pressure drop, especially at higher mass fluxes, is observed. The two phase frictional pressure drop correlations of Muller-Steinhagen and Heck and Friedel and two phase flow heat transfer correlations of Cooper and Liu and Winterton well predicted the experimental results.
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43.
  • Mariani, Raffaello, et al. (författare)
  • Shock-Free Compressible Vortex Rings Impinging on a Stationary Surface: Effects of Surface Angle Variation.
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 47, s. 126-142
  • Tidskriftsartikel (refereegranskat)abstract
    • An experimental study has been conducted on the impingement of compressible vortex rings on stationary, inclined, smooth surfaces. Four surface angle inclinations were evaluated: 90° (perpendicular to theflow longitudinal axis), 75°, 60°, and 45°. The flow was assumed axisymmetric for the 90° benchmark case. A single pressure ratio of ~4 was used, corresponding to an experimental incident shock wave Mach number of ~1.31, generating a shock-free compressible vortex ring.Results showed two main flow processes: the interaction of the reflected shock wave with the approaching vortical structure, and the impingement of the vortex ring on the surface. At surface inclinationangles other than 90, these processes are asymmetrical and the shape of the reflected shock ismodified by the approaching vortex ring. The lower outer section of the reflected shock wave is initiallydiffracted by the lower core of the vortex ring, its central section is then captured by the central section ofthe torus, and ultimately the upper outer section is diffracted by the upper core. The vortex ring subsequently impinges on the surface with the lower core impacting first, undergoing an asymmetrical stretching and deformation.Flow field studies showed an initial decrease in velocity magnitude and vorticity field as the flow transitionedfrom a symmetrical to an asymmetrical impingement, possibly due to the uneven formation of the boundary layer. The velocity magnitude and vorticity field of the upper core subsequently increase again, as a result of the upwards deflection of the flow. Oppositely, the lower core is seen to dissipate. Surface pressure measurements confirmed the asymmetry in the flow development. As the surface angle is varied, the pressure at the central measurement location is seen to decrease as the position ofthe stagnation point is altered. An interesting feature within the surface measurements is a strong lateral acceleration due to the formation of vortex lines which roll around the main vortex core.
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44.
  • Martínez-Galván, E., et al. (författare)
  • Effect of the spray cone angle in the spray cooling with R134a
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 50, s. 127-138
  • Tidskriftsartikel (refereegranskat)abstract
    • Spray cooling experiments were performed to study the effect of the spray cone angle on heat transfer and film thickness. The experiments were carried out for three different types of nozzles. Results for heat transfer and film thickness for different flow rates are presented working with the dielectric refrigerant R134a and on a heater with low surface roughness. The behavior of the heat transfer with respect to the spray cone angle shows that as the spray cone angle decreases, there is a delay in the onset of the nucleated boiling regime. As a result, thermal performance worsens as the spray cone angle decreases. On the other hand, in the nucleate boiling regime film thickness increases as the spray cone angle decreases. The qualitative analysis of these variations serves to better understand the heat transfer mechanisms that occur during the spray cooling technique.
  •  
45.
  • Martinez-Galvan, Eduardo, et al. (författare)
  • Influence of surface roughness on a spray cooling system with R134a. Part I : Heat transfer measurements
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 46, s. 183-190
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the heat transfer measurements are presented. The analysis of the results explains the influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the spray cooling boiling curve, on the Nusselt number and on the efficiency. It has been found that the effect of a smooth roughness is to delay the onset of the nucleate boiling, but once this regime has started, boiling is so fast that the CHF (Critical Heat Flux) is reached at lower heater temperatures and lower heat fluxes. In a companion paper the sprayed refrigerant film thickness measurements and its relation with the heat transfer measurements are presented.
  •  
46.
  • Martinez-Galván, Eduardo, et al. (författare)
  • Influence of surface roughness on a spray cooling system with R134a. Part II : Film thickness measurements
  • 2013
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 48, s. 73-80
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the sprayed refrigerant film thickness measurements are presented. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is analyzed and discussed. In a companion paper, results of the heat transfer measurements are presented. It has been found that there is a relation between the variations of the average Nusselt number and of the film thickness along the spray cooling boiling curve. The heat transfer regimes along that curve are related not only to a variation in the average Nusselt number but also to changes in the film thickness. The qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling.
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47.
  • Mei, Y., et al. (författare)
  • A study on steam-water two phase flow distribution in a rectangular channel with different channel orientations
  • 2018
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier Inc.. - 0894-1777 .- 1879-2286. ; 99, s. 219-232
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental study on steam-water two phase vertical and inclined upward flow (15–90°) was performed in a rectangular channel with cross section of 17 mm × 10 mm under atmospheric pressure to investigate the phase distribution and the average void fraction in the cross section which were obtained from the local void fraction measurement by a conductivity probe. The inlet superficial velocities of the steam and water varied from 0.72 to 3.85 m/s and from 0.11 to 0.3 m/s respectively. A high speed camera was used to identify the flow patterns. Experimental results show that the phase distribution curves are significantly affected by channel orientation and the average void fraction first decreases and then increases with the increase of orientation. Based on the drift-flux model, two parameters, namely, the distribution parameter (C0) and the drift velocity (Ugm) have been studied in detail. Both the distribution parameter and the drift velocity are found to be functions of orientation. The distribution parameter decreases with the increase of orientation while the drift velocity first increases and then decreases with the increase of orientation., Based on the experimental data, an improved drift-flux model is proposed especially for the slug and churn flow, which predicts the void fraction in an inclined channel with good accuracy.
  •  
48.
  • Morteza, Ghanbarpourgeravi, 1978-, et al. (författare)
  • Thermal performance of screen mesh heat pipe with Al2O3 nanofluid
  • 2015
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 66, s. 213-220
  • Tidskriftsartikel (refereegranskat)abstract
    • This study presents the effect of Al2O3 nanofluid (NF) on thermal performance of screen mesh heat pipe in cooling applications. Three cylindrical copper heat pipes of 200 mm length and 6.35 mm outer diameter containing two layers of screen mesh were fabricated and tested with distilled water and water based Al2O3 NF with mass concentrations of 5% and 10% as working fluids. To study the effect of NF on the heat pipes thermal performance, the heat input is increased and then decreased consecutively and the heat pipes surface temperatures are measured at steady state conditions. Results show that using 5 wt.% of Al2O3 NF improves the thermal performance of the heat pipe for increasing and decreasing heat fluxes compared with distilled water, while utilizing 10 wt.% of Al2O3 NF deteriorates the heat pipe thermal performance. For heat pipe with 5 wt.% Al2O3 NF the reduction in thermal resistance of the heat pipe is found to be between 6% and 24% for increasing and between 20% and 55% for decreasing heat fluxes, while the thermal resistance increased between 187% and 206% for increasing and between 155% and 175% for decreasing steps in heat pipe with 10 wt.% of Al2O3 NF.
  •  
49.
  • Mura, Ernesto, 1974, et al. (författare)
  • Interfacial shear stress, heat transfer and bubble appearance in falling film evaporation
  • 2016
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 79, s. 57-64
  • Tidskriftsartikel (refereegranskat)abstract
    • In the falling film evaporation of dairy products, the phase change process occurs via two major phenomena: surface convective evaporation and boiling evaporation. Previous studies have shown that under certain conditions, the heat transfer mechanism can be greatly improved when the evaporation is dominated by the presence of bubbles or foam clusters. In the present work, the influence of the surface bubbling phenomenon on the heat transfer coefficient has been studied. The effect of the co-flowing vapor rate inside the evaporative tube has been experimentally related to the presence of the surface bubbles for dairy products characterized by different dry solid contents (DC = 0%, 13%, 30%, 40%, 51%). The results show that at low dry solid contents (DC
  •  
50.
  • Omrane, Alaa, et al. (författare)
  • Temperature measurements of combustible and non-combustible surfaces using laser induced phosphorescence
  • 2004
  • Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 1879-2286 .- 0894-1777. ; 28:7, s. 669-676
  • Tidskriftsartikel (refereegranskat)abstract
    • In recent years, thermographic phosphors used for surface temperature measurements have been developed to suit combustion applications. Pulsed ultraviolet laser radiation at 266 nm and 7 ns duration was used to excite a thermographic phosphor, Mg4FGeO6:Mn, which was adapted to the surface of the investigated material. The laser-induced emission from the phosphor was recorded. A calibration of the phosphorescence lifetime and spectral properties against temperature allowed surface temperature measurements between 298 and 873 K. The techniques were successfully used in a flame spread scenario for two-dimensional measurements of surface temperature on a low-density fiber board. The phosphorescence images were obtained with eight consecutively gated CCD detectors enable pixel by pixel-lifetime evaluation of the phosphorescence by interpolating an exponentialdecay curve to the counts of the corresponding pixel positions of the sequential CCD images. The best precision, better than +/-5 K, was obtained at these temperatures. The technique was also applied to temperature measurements of decomposing materials in a pyrolysis oven. The material investigated was birch particles. Recent development of the technique has allowed other applications, e.g., to such as engine valve and droplets temperature measurements. (C) 2004 Elsevier Inc. All rights reserved.
  •  
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