| Summary: | 碩士 === 臺灣大學 === 機械工程學研究所 === 98 === Phase-change heat transfer provides advantages such as good temperature uniformity, less coolant flow rates requirement and high cooling efficiency. Porous media surface possesses a lot of nucleate sites and the connected pores which can enhance heat transfer coefficient. In present study, the flow boiling experiments were conducted with porous microchannels evaporators sintered with copper dendritic powder on 1 square inch copper substrates. Working fluid R-134a was used at volume flow rate from 133~417ml/min and the saturated pressure of 800kpa. The purpose of this research is to investigate heat transfer characteristics, pressure drop, pressure instability, heat transfer enhanced in different pore distributions and to compare with plane microchannels evaporators.
The results of the plane microchannels evaporator showed the heat mechanism was dominated by the nucleation boiling before quality 0.4. The experiment data were substituted into the Cooper’s pool boiling correlation, the mean average error was 8.2%. On the other hand, when the quality was over 0.4, the heat mechanism was dominated by the forced convection boiling. Critical heat flux (CHF) increased with increasing volume flow rate and reached to 127.9W/cm2. The pressure drop increases with increasing heat flux and volume flow rate. The experiment results also substituted into the correlation considered surface tension and viscosity, and the average error was 17%. Pressure drop oscillation suggested the presence of instability inside the plane microchannels. The maximum amplitude of oscillation was found to be near the onset of nucleation and independent of volume flow rate.
The experiment results showed that the heat transfer coefficient of porous microchannels reached the peak value at low quality, than decreased with increasing quality and increased with increasing volume flow rate. The trend is almost the same with plane microchannels when the quality is over 0.4. The investigation of different parameters indicated that pore distribution affects the heat transfer characteristic directly. The heat transfer coefficient can be enhanced with dual- pore distribution structure. The best performance were achieved by the porous microchannels and the heat transfer coefficient enhanced about 10 times larger than the plane microchannels. Pressure drop and the maximum pressure instability amplitude of porous structure channel was 20% higher and 47% lower than plane channel near the onset of nucleation respectively. To conclude the results of the study, the porous microchannels evaporator is highly potential for the industrial cooling applications and flow stability.
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