多孔性結構微流道之二相熱傳增強研究

Abstract

微流道蒸發器具有高熱傳係數、高均溫性與低工質需求量等優點，被視為極具潛力的散熱技術。近年來，電子元件、雷射發光二極體等產品的發熱量日益增高，熱傳增強之微流道更具其應用價值。 本研究於1平方英吋銅表面加工具有62條寬深為225×660μm流道的平板微流道蒸發器，並以樹枝狀銅粉燒結具相同尺寸的多孔性結構微流道蒸發器，以R-134a為工質，在流量範圍133~200ml/min、飽和壓力800kpa下進行熱測試，比較平板與多孔性結構微流道蒸發器的熱傳特性、壓降、壓力不穩定性，與熱傳增強效果，並探討銅粉粒徑與結構底厚對熱傳性能的影響。 實驗結果顯示平板微流道蒸發器的熱傳係數在乾度小於0.4前，主要隨熱通量上升而增加，不隨質量流率、乾度變化，應屬於核沸騰機制；乾度大於0.4後熱傳係數隨乾度上升而下降，此趨勢與傳統管流相反。實驗結果代入Cooper池沸騰經驗式之總平均誤差約為34.3%，在乾度小於0.4前為8.2%。臨界熱通量隨流量上升而增加。而壓降則隨流量與熱通量上升而增加。壓降震盪顯示平板微流道具有不穩定性，其震盪幅度在接近起始沸騰時最大。 多孔性結構微流道蒸發器的其銅粉粒徑變化範圍為18~70μm，底厚範圍150~375μm，以固定燒結溫度與持溫時間製成。實驗顯示其熱傳係數在低熱通量時就達到峰值，隨熱通量與乾度上升而下降，不受質量流率影響，與平板流道趨勢明顯不同。探討銅粉粒徑與底厚影響的實驗結果發現，厚度對粒徑的比值對熱傳性能有很大影響，該比值需要適當的配合以達到最佳的熱傳性能，在研究參數範圍內，當厚度粒徑比介於2~12，熱傳係數隨厚度粒徑比上升而增加。多孔性結構微流道蒸發器的最高壓降達16kpa，約較平板微流道增加45%。而鄰近沸騰起始時壓降平均振幅較平板微流道下降47%，顯示其在沸騰起始時較為穩定。本研究顯示多孔性結構微流道蒸發器能有效提升平板微流道熱傳能力，具最佳熱傳性能的 多孔性結構微流道蒸發器銅粉粒徑為32μm，底厚375μm，厚度粒徑比為12，其熱傳係數較平板微流道平均為597.5kW/m2K，較平板微流道增加5倍。臨界熱通量達143W/ cm2，約提升10%。總結本研究成果，多孔性結構微流道蒸發器極具工業應用潛力。

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. In 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. The 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 drop 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. The 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.