Experimental And Theoretical Study On Flow Boiling In Microchannel

The rapid development of MEMS, microelectronics, laser technology, and new material manufacturing bring about new needs of cooling technologies for high heat flux in micro-channel. The thermal exchange based on the flow boiling in micro-channel is an important challenge of these technologies. In recent years, many researches have paid much attention on the micro flow boiling study. However, the published experimental results and the flow boiling mechanism lead to obvious confusions. The main purpose of the present study is to clarify the mechanisms of micro flow boiling. Based on prevailing knowledge of flow boiling patterns, pressure drop and heat transfer performance, the experimental study and theoretical analysis on flow boiling in micro-channel is performed in this work.Firstly, the flow boiling pattern transitions such as bubbles' forming, expanding and merging etc. in the horizontally placed and uniformly evenly heated micro capillary tube and rectangular micro-channel are observed by using a high-speed camera with the maximum speed of 38000 frames per second and a microscope. The experimental results show that the main flow regimes in these micro-tubes are the periodic changes of single-liquid flow, slug flow, and annular flow with liquid film surrounded in the micro-tube with inner diameter of 520 and 315μm. However, bubbly flow is not observed during sub-cooled and saturated flow boiling for the micro tube with inner diameter of 242μm, the flow pattern is only consist of single phase liquid flow and two-phase elongate slug flow. In the process of sub-cooled flow boiling, the alternations of the pressure drop and the outlet temperature, the trends of which are opposite, are almost identical. This sub-cooled flow boiling process is consist of alternation of saturated vapor phase and sub-cooled liquid phase, which is very different from that in macro-channel. For the situation in rectangular micro-channel, the transitions of flow boiling patterns are almost the same as the situation inmicro-tube.Secondly, based on the knowledge of flow boiling patterns in the micro-tube and rectangular micro-channel, the surface temperature and pressure drop are measured by the Infrared Radiometry, thermocouples, and silicon absolute pressure sensors to study the performance of pressure drop and heat transfer. The experimental results evidence that, by using the Infrared Radiometry, the evaluation of sub-cooled flow boiling patterns in an opaque micro-tube can be estimated from the thermographs. During the process of saturated flow boiling, the experimental two-phase frictional pressure drop strongly depends on heat flux and mass flux and the two-phase frictional multipliers Q^fl decreased with Lockhart-Martinelli parameter and changed with mass flux. According to the experimental results and the effects of mass flux and heat flux, a new pressure drop correlation is suggested, which agrees well with our experimental data. The boiling heat transfer coefficient decreases slightly or remains unchanged with increasing of the vapor quality in the micro-tube, but expresses the opposite trend in the rectangular micro-channel due to the different heating method. It is implied that this trend is attributed to the influence of liquid droplet entrained between the adjoining annular flow regions and deposited within this region. Based on the comparison between the experimental results and the predicted correlations, a new boiling heat transfer correlation is obtained.Finally, following the experimental results, the proposed combined theoretical model using the effects of liquid droplet entrainment and deposition within the annular flow region and intermittent saturated single water for heat transfer of saturated flow boiling in a micro-tube is established and could predict the experimental data well.