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Fu B.A.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | Chen M.Q.,Beijing Jiaotong University | Huang Y.W.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Fuel | Year: 2015

Heat transfer characteristics of the lignite thin-layer during the hot air forced convective drying were investigated experimentally as a function of hot air temperatures (100, 110, 120, 130, 140, 150, and 160 °C) and speeds (0.6, 1.4, and 2.0 m s-1). The average temperature and surface temperature of the thin-layer increased rapidly in the first falling rate period, whereas those rose slightly in the second falling rate period. The stabilized temperature of the thin layer at hot air temperature range of 100-160 °C, increased by about 4.2% to 14.8% when the wind speed rose from 0.6 m s-1 up to 2.0 m s-1. The average surface heat transfer coefficients in the first falling rate period were about 2-3 times of those in the second falling rate period. With an increase of the hot air temperature from 100 to 160 °C, the average surface heat transfer coefficients increased by about 71.9% to 80.9% in the first falling rate period and about 56.8% to 146.0% in the second falling rate period. The dimensionless surface heat transfer correlation of the lignite thin-layer was obtained for the first falling rate period. © 2015 Elsevier Ltd.


Yin L.,Beijing Jiaotong University | Jia L.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Experimental Thermal and Fluid Science | Year: 2016

Bubble confined growth during boiling in micro-scale space is experimentally investigated. Degassed deionized water (DI water) is used to study the confined characteristics of growing bubble in microchannel. Mechanisms of bubble confinement in microscale space are discussed. Effects of heat flux and microchannel size on bubble confined growth are examined. The cross-sections of the tested rectangular microchannel are 0.5. mm. ×. 1 mm and 1 mm. ×. 1 mm respectively. The bubble growth process is observed and recorded by high speed CCD camera with 250 frames per second. The confined characteristics of bubble in microchannel are represented by the fluctuated variation of bubble root contact angle and other two bubble shape parameters, namely the maximal local void fraction of bubble and the bubble aspect ratio. The increased bubble growth force caused by miniaturization of space size is the chief mechanism for bubble confined characteristics in microchannel. Moreover, it is found that whether a growing bubble in microchannel will show confined characteristics is irrelevant to whether or not it will finally occupy the whole microchannel cross-section. The bubble confined characteristics are jointly controlled by the bubble growth rate and the size of microscale space. In some cases the bubble confined growth features are not displayed throughout the growth period even the bubble completely occupies the whole microchannel cross-section. © 2016 Elsevier Inc.


Yin L.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | Jia L.,Beijing Jiaotong University
International Journal of Heat and Mass Transfer | Year: 2016

Bubble behaviors are closely related to the heat transfer performance during flow boiling in microchannel, however, the effect of channel cross-section decreasing on the bubble growth is still not fully understood at present. In this work, an experimental investigation is conducted to investigate the bubble growth characteristics during flow boiling in a single microchannel with 0.5 mm × 1 mm rectangular cross-section, and the heat transfer performance of flow boiling and its influencing factors are studied. Experiments are conducted with subcooled deionized water and the bubble behaviors are visualized by a high speed CCD camera installed upon the test section. Depending on the heat flux, different growth features are observed in the bubble growth process. Two kinds of bubble growth model are identified: the power law model in initial growth period and the linear law model in later period. The confinement effect of the microchannel is deemed as the mechanism causing the alteration of bubble growth models during its growth process. The deformation features of confined bubble are discussed to illustrate the intensification of evaporation on the liquid-vapor (LV) interface at bubble root, which increases the growth rate of bubble in its confined growth period as well as the heat transfer capability of bubble. Therefore, the maximum local heat transfer coefficient along the channel is found in the region where confined bubble and/or short elongated bubble flow pattern are dominant. Moreover, the heat flux is found to have great influence on the overall heat transfer performance of flow boiling in microchannel, but the effect of mass flux is much less. © 2016 Elsevier Ltd. All rights reserved.


Guan C.,Beijing Jiaotong University | Jia L.,Beijing Jiaotong University | Yin L.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Applied Thermal Engineering | Year: 2016

Theoretical and experimental study is conducted to investigate the condensation heat transfer characteristics of mixed gas with fine coal particles in this paper. The visualization experiments of fine coal particles striking the film and mixed gas condensation heat transfer experiments in a vertical channel are carried out. Mixed gases contain different proportions of N2, steam and fine coal particles. The theoretical model of particles flicking condensate film is established. Results show that fine coal particles can penetrate the condensate film and flow inside the film when the particle diameter and the incidence velocity meet the conditions of penetrating judgment. The Nu number of heat transfer and the mass of the condensation decrease with the increase of the diameter and the concentration of the particles in the mixed gases. © 2016 Elsevier Ltd.


Yang Y.,Beijing Jiaotong University | Jia L.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Yingyong Jichu yu Gongcheng Kexue Xuebao/Journal of Basic Science and Engineering | Year: 2016

The liquid film flowing characteristics of new refrigerant R410A flowing upward condensation in a vertical rectangular channel were observed by high speed camera. The saturation temperature of refrigerant R410A vapor is 28℃. The hydraulic diameter of the rectangular channel is 14.34mm and the length is 160mm. The test was conducted at mass fluxes 1.8~23kg·m-2·s-1. It is shown that tadpole flow and laminar wave flow occurred with the increase of mass flux. The thickness of the liquid film and the wave frequency were found to be increased and enhanced with increasing mass flux. The average condensation heat transfer coefficient increases at first, and then decreases with mass flux. Also, the variation of the average condensation heat transfer coefficients of the different segments divided by the temperature measuring points with the different segments of the channel were obtained. © 2016, The Editorial Board of Journal of Basic Science and Engineering. All right reserved.


Wang Z.,Beijing Jiaotong University | Wang Z.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | Zhang Z.,Beijing Jiaotong University | Zhang Z.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | And 4 more authors.
Applied Thermal Engineering | Year: 2015

The temperature of battery modules in electric vehicles (EVs) must be controlled adequately to keep within a specified range for optimum performance. The paraffin/aluminum foam composite phase change material (PCM) was investigated experimentally. The experimental results indicate that paraffin/aluminum foam composite PCM has an ideal cooling effect in limiting the temperature rise of the Li-ion battery during the discharge process. The heat storage properties of pure paraffin and the composite PCM are also experimentally studied. The inner temperature distribution of the foam and paraffin is monitored during the melting process. The addition of aluminum foam can largely improve the effective thermal conductivity of the PCM, although its existence suppresses the local natural convection. The experimental results indicate that the use of aluminum foam can speed up the melting process and improve the temperature uniformity of the PCM. When the heat fluxes are 7000 W/m2 and 12,000 W/m2, the heat storage time of the composite PCM is 73.6% and 74.4% of that of pure paraffin, respectively. © 2015 Published by Elsevier Ltd.


Li X.,Beijing Jiaotong University | Li X.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | Jia L.,Beijing Jiaotong University | Jia L.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Journal of Thermal Science | Year: 2015

The present study reports an experimental evaluation of heat transfer characteristic of R134a flow boiling in micro-channel heat sink. The heat sink is composed of 30 parallel rectangular micro-channels with cross-sectional dimensions of 500μm width and depth, as well as total length 30mm. Experiments were conducted with heat flux up to 80.212 W/cm2, mass velocity ranging from 373.33 to 1244.44 kg/m2s, vapor quality ranging from 0.06 to 0.9. The wall temperature of heat sink heated could be controlled at around 50°C. Heat transfer coefficient could be up to 180 kW/m2K. Two dominating flow patterns were observed by analyzing boiling curves. The heat transfer characteristics of nucleate boiling and convective boiling were presented in the study. Revised correlations of R134a flow boiling in micro-channel heat sink were carried out with the consideration of nucleate boiling and convective boiling mechanisms. © 2015, Science Press, Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg.


Yang L.,Beijing Jiaotong University | Yang L.,Beijing Key Laboratory of flow and heat transfer of phase changing in micro and small scale | Zhao N.,Beijing Jiaotong University | Zhao N.,Beijing Key Laboratory of flow and heat transfer of phase changing in micro and small scale | And 2 more authors.
Journal of Thermal Science | Year: 2015

In this study, a visualization-based experiment was performed to measure the motion of the nanowire under a magnetic field. A simulation method based on a multiple reference frame model (MRF model) was used to calculate fluid torque. Here, it was validated with the experimental data and theoretical results. Fluid torque of steady rotated nanowire was simulated and compared using experiment and theoretical models. The unsteady rotated condition was studied using transient simulation to compare with theory and the results showed that the acceleration of nanowire did not affect the flow field, indicating that the theoretical models based on the steady condition were still valid. The influence of solid walls on nanowire rotation was also studied here. The results showed that if the nanowire was placed close to the wall, the viscous force of wall would increase the velocity gradient around the nanowire, causing higher torque predictions. The fluid torque decreased quickly when the vertical distance between nanowire and wall exceeded 5 times the diameter of the wire. © 2015, Science Press, Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg.


Yang Y.,Beijing Jiaotong University | Yang Y.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | Jia L.,Beijing Jiaotong University | Jia L.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Journal of Thermal Science | Year: 2015

This paper presents an experimental investigation on condensation of R410A upward flow in vertical tubes with the same inner diameter of 8.02mm and different lengths of 300 mm, 400 mm, 500 mm and 600mm. Condensation experiments were performed at mass fluxes of 103–490 kg m−2s−1. The saturation temperatures of experimental condition were 31°C, 38°C and 48°C, alternatively. The average vapor quality in the test section is between 0.91 and 0.98. The effects of tube length, mass flux and condensation temperature on condensation were discussed. Four correlations used for the upward flow condensation were compared with the experimental data obtained from various experimental conditions. A modified correlation was proposed within a ±15% deviation range. © 2015, Science Press, Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg.


Huang Y.W.,Beijing Jiaotong University | Huang Y.W.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale | Chen M.Q.,Beijing Jiaotong University | Chen M.Q.,Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale
Measurement: Journal of the International Measurement Confederation | Year: 2015

The back-propagation (BP) and generalized regression neural network models (GRNN) were investigated to predict the thin layer drying behavior in municipal sewage sludge during hot air forced convection. The accuracy of the BP model to predict the moisture content of the sewage sludge thin layer during hot air forced convective drying was far higher than that of the GRNN model. The GRNN models could automatically determine the best smoothing parameters, which were 0.6 and 0.3 for predicting the moisture content and average temperature, respectively. The model type for predicting the average temperature of the sewage sludge thin layer was selected for different sample groups by comparing their MSE values or R2 values. The GRNN model was suitable for predicting the average temperature corresponding to the sample groups at hot air velocity of 0.6 m/s, and drying temperatures of 100 °C, 160 °C; hot air velocity of 1.4 m/s, and drying temperatures of 130 °C, 140 °C; hot air velocity of 2.0 m/s, and drying temperatures of 150 °C, 160 °C. The average temperature for the other sample groups was best predicted by the BP model. © 2015 Elsevier Ltd. All rights reserved..

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