Two-Phase Heat Transfer Enhancementn Porous Microchannels
Date Issued
2009
Date
2009
Author(s)
Liu, Kuang-Tsu
Abstract
The microchannel evaporators possess the advantages of high heat transfer coefficient, good temperature uniformity, and small coolant flow rates requirement, were considered as potential cooling technology. In recent years, the heat dissipation rate of products like electronic devices or light emitter diodes keeps raising. The heat transfer enhanced microchannels are even valuable for applications.n present study, The flow boiling experiments were conducted with a plane microchannel evaporator with 62 225μm x 660μm channels carved into a 1 square inch copper substrates and porous microchannel evaporators sintered with copper dendritic powder, using R-134a as coolants, under conditions of coolants flow rates 133~200ml/min and the saturated pressure of 800kpa. The comparison of heat transfer characteristics, pressure drop, pressure instability, and heat transfer enhanced effects were made between plane and porous microchannel evaporators. The effects of the particle size dp and coating thickness δ over the heat transfer performance were also investigated.he results showed that when the quality was smaller than 0.4, the heat transfer coefficient mainly increased with increasing heat flux and did not vary with mass flow rate or quality. This region was dominated by the nucleation boiling. On the other hand, when the quality was larger than 0.4, the heat transfer coefficient decreased with increasing quality. This was an opposite trend to that of the conventional size flow boiling. The experiment results were substituted into the Cooper’s pool boiling correlations, the mean average error was 34.3% for full range of quality and 8.2% before quality reached 0.4. The critical heat flux enhanced with increasing flow rates and the pressure drop were raised with increasing flow rates and heat flux. The pressure rop oscillation suggested the presence of instability inside the plane microchannels, and the maximum amplitude of oscillation were found to be near the onset of nucleation.he porous microchannel evaporators were sintered under the fixed temperature and time, dp and δ ranged from 18~70μm and 150~375μm, respectively. The experiment results showed that the heat transfer coefficient reached the peak value at low quality and decreased with increasing quality but did not varied with the mass flow rate. This is apparently different from plane microchannels. The investigation of the effect of dp and δ indicated the ratio of the thickness to the particle size δ/ dp had a significant effect over the heat transfer performance. This ratio must be properly chosen in order to reach better performance. In the range of parameter studied in present study, when δ/dp is between 2~12，the heat transfer coefficient enhanced with increasing δ/dp. The highest pressure drop of porous microchannels reached 16kPa, about 45% larger than plane microchannels. The average pressure drop oscillation amplitude near the onset of nucleation was 47% smaller than that of plane microchannels, presented a much stable boiling behavior when the nucleation began. The present study showed that the porous microchannel evaporators could effectively enhance the heat transfer performance of plane microchannels, the best performance were achieved by the porous microchannel with dp = 32μm and δ = 375μm, δ/ dp = 12. The average heat transfer coefficient was 597.5kW/m2K, about 5 times larger than the plane microchannel. The critical heat flux reached 143W/cm2, gained about 10%. To conclude the results of the study, the porous microchannel evaporator is highly potential for the industrial applications.
Subjects
microchannel
boiling heat transfer enhancement
porous structure
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-98-R96522318-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):2596b0ce79a530ae27b6db1f20015f5a