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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

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. Source

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

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. Source

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

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. Source

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

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. Source

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

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. Source

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