多孔性結構微流道蒸發器流動沸騰熱傳增強研究

Abstract

微流道蒸發器具有高熱傳係數、高均溫性、單位散熱面積大與低工質需求量等優點，被視為極具潛力的散熱技術，多孔結構具有大量成核址與連通孔洞，預期能提升微流道蒸發器熱傳性能。 本研究於1平方英吋無氧銅製作62條深寬為225×660μm流道之多孔微流道蒸發器和平板微流道，以水為工質，在質量通率範圍103~207kg/m^2 s、飽和壓力1.4bar進行實驗。探討銅粉粒徑、結構底厚對熱傳性能的影響，與平板微流道比較熱傳特性、壓降、壓力不穩定性與熱傳增強效果。最後比較工質為水及R-134a兩種不同工質於微流道中對熱傳性能、壓降、壓降不穩定性的影響。 實驗結果與考慮表面張力之熱傳經驗式比較平均絕對誤差為16.5%。壓降隨熱通量上升而增加，改變流量下壓降變化不大。實驗壓降與考慮表面張力之分離流模型經驗式比較平均絕對誤差為21.3%。壓降震盪顯示平板微流道具有流道之間不穩定性，其振盪幅度在接近起始沸騰最大。 探討燒結底厚(225~375μm)、銅粉粒徑(1~100μm)與其比值(3~20)等製程參數實驗結果發現，底厚225μm和粒徑為53μm，微流道熱傳增強幅度最大，厚度對粒徑比值為3~4時在微流道有較佳的熱傳性能，熱傳係數平均較平板微流道提升300%。 多孔性結構微流道蒸發器的熱傳結果與平板微流道蒸發器不同，熱傳係數隨質量流率改變。多孔性結構微流道蒸發器壓降隨熱通量上升而增加，整體較平整表面高，但增加最大不超過50%。多孔表面微流道能有效抑制流道不穩定性，最大壓降震盪振幅降低66%，顯示其在相變化操作較為穩定。 於多孔性結構微流道中，工質為水，多孔結構微流道在厚度粒徑比為3~4，有最佳的熱傳性能表現，工質為R-134a則有所不同，厚度粒徑比為8~12，熱傳有較好的表現，水與R-134a兩種不同工質表面張力的差異可能為厚度粒徑比最佳選擇的不同。比較工質為水及R-134a兩種不同工質於微流道中，於平板微流道中，壓降實驗結果顯示以水為工質微流道壓降震盪比R-134a還大。壓降實驗結果顯示最大壓降振幅皆有大幅的下降。 總結本研究成果，多孔性結構微流道蒸發器在熱傳有明顯提升，可提高二相操作之穩定性，在有限的壓降增幅下極具工業應用潛力。

The microchannel evaporator,which possesses the advantage of high heat transfer coefficient,good temperature uniformity,and small requirement for coolant flow rates,is considered as a potential cooling technology.The porous structure with a large number of nucleation site density as well as the reentrant grooves is to enhance the heat transfer performance in the microchannels evaporator. In present study,the flow boiling experiments were conducted with a plane and porous microchannels evaporator on one square inch copper substrates. Using water as working fluid,the mass flux from103~207 kg/m^2 s and the saturated pressure of 140kpa. Both microchannels have 62 channels(225μm in width；and 660μm in depth).The effects of powder size,thickness of structure upon heat transfer performance are investigated.The comparsions of heat transfer characteristics,pressure drop, pressure instability,and heat transfer enhanced effects between the plane and the porous microchannels evaporator are made.Finally,the comparisons of heat transfer performance,pressure drop,pressure instability between two different working fluid water and R-134a in microchannels. The experiment results were substituted into the heat transfer correlations in which the surface tension force was taken into consideration.The mean average error was16.5%. Pressure drop raised by increasing heat fluxes,but did not vary with increasing mass flux.The experiment results were substituted into the separation model incorporating surface tension force. The mean average error was 21.3%. The pressure drop oscillation suggested that the presence of instability inside plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation. The porous microchannel evaporators were sintered under the following parameters: the powder diameter dp ranged from 1~100μm, thickness of porous structure δ ranged from 225~375μm, and δ/dp ranged from 3~20, respectively. The investigation on the effect of particle size dp as well as thickness δ indicated that the ratio of the thickness to the particle size δ/dp had a significance in the heat transfer performance. This ratio must be properly chosen in order to reach a better heat transfer performance. The better ratio of δ/dp was between 3~4 in our work,withδ 225μm and dp 53μm.The average heat transfer coefficient enhanced about 3 times larger than the plane microchannels. For the porous microchannels evaporator,the heat transfer results different from the plane microchannels evaporator,heat transfer coefficient varied with varing mass flux.Pressure drop in porous microchannel evaporator was raised by increasing heat fluxes.The pressure drop was higher than plane microchannels；however,the maximum pressure drop was not over 50%. The maximum amplitude of oscillation was 66% lower than plane microchannels.This result presented that the porous microchannels evaporator provided a stable boiling behavior when nucleation began. For the porous microchannels： Working fluid water,the better ratio ofδ/dp was between 3~4；however, the better ratio ofδ/dp was between 8~12 when R-134a as working fluid.Surface tension force was probably the different choose between the better ratio ofδ/dp .The comparisons between two different working fluid water and R-134a in microchannels： The pressure results showed that water in the plane microchannels,its maximum amplitude of oscillation was larger than R-134a.The maximum amplitude of oscillation was obviously lower than the plane microchannels in two different working fluids. To conclude the present study, the porous microchannel evaporator is highly potential for the industrial applications