具多孔性結構微流道蒸發器之熱傳增強研究

Abstract

微流道蒸發器具有體積小、工質需求量低、高熱傳係數、高臨界熱通量等優點，而國內外在這方面的研究也有很多，但大多集中在沸騰熱傳機制、流動不穩定性、壓降分析等方面，而對於其沸騰熱傳增強的研究則很少。因此，本研究利用多孔性結構能提供較多有效成核址及較大內部交錯面積的優點，將其與微流道蒸發器結合，以期能提升傳統微流道蒸發器之熱傳性能。 本研究以平均粒徑30μm的樹枝狀銅粉做為燒結用粉末，並藉由調整燒結溫度製作出孔隙度41%~66%的多孔性結構微流道蒸發器，其流道的水力直徑為292μm~327μm。實驗以R-134a為工質，操作壓力為800kpa，並藉由改變流量的方式(100 ml/min~166 ml/min)針對不同孔隙度進行測試。 實驗結果指出，傳統微流道蒸發器的熱傳係數會受到熱通量、乾度以及流量的影響，其中熱傳係數會隨著的乾度以及熱通量增加呈現先上升後下降的趨勢，而隨著流量的增加其峰值會越早發生，且整體的熱傳性能也會隨著流量的增加而上升。而多孔性結構微流道蒸發器的熱傳係數則會受到熱通量 、乾度、流量以及孔隙度的影響，其中熱傳係數也是隨著乾度以及熱通量的增加呈現先上升後下降的趨勢，而當孔隙度與流量越高時其峰值越早發生。在熱傳增強方面，孔隙度越高其增強幅度越大，以孔隙度66%之多孔性結構微流道蒸發器增強幅度最大，熱傳性能為傳統微流道蒸發器的1~5倍。在臨界熱通量方面，與傳統微流道蒸發器的趨勢相同，流量越大時其臨界熱通量也越高，臨界熱通量可達93W/cm2~143 W/cm2，提升幅度為5%~23%。 在壓降方面，相較於傳統微流道蒸發器壓降除了隨著流量以及熱通量的增加而上升外，多孔性結構微流道蒸發器的壓降也會隨孔隙度的增加而上升。其壓降提高約30%到260%，最高的壓降為孔隙度66%之多孔性結構微流道蒸發器在流量為166 ml/min時，壓降為30 kpa。

Microchannel evaporator has the advantages of compactness, minimal coolant usage, high heat transfer coefficient, and high critical heat flux. Most of recent researches emphasize heat transfer mechanism, flow instability, and pressure drop analysis but not too much focuses on heat transfer enhancement in microchannel evaporator. For the purpose of enhancing boiling heat transfer in microchannel evaporator, this research takes advantage of porous media with more nucleation sites and larger inner areas to produce microchannels. This study manufacture microchannels with porosity range between 41% and 66% by adjusting the sintered temperature and we use dendritic copper powders in average particle size of 30μm. The hydraulic diameter’s range of these porous microchannels is 292μm to 327μm. The working fluid used is refrigerant R-134a, operating pressure is 800 kpa, and volume flow rate ranges from 100 ml/min to 166 ml/min. The result reveals that heat transfer coefficient of solid microchannel evaporator is primarily affected by heat flux、quality and volume flow rate. As quality and heat flux increase, the heat transfer coefficient will rise up first and then go down. The peak value will happen earlier as long as flow rate higher and the overall boiling heat transfer capacity of solid microchannel evaporator is better in higher volume flow rate. The same trend can be observed in porous microchannel evaporator. Furthermore, with the increase of the porosity, the peak value will be shown up earlier. In the aspects of boiling heat transfer enhancement, the porous microchannel evaporator with the highest porosity of 66% performs best in heat transfer enhancement and the heat transfer enhancement ratios are 1 to 5 times, comparing with solid microchannel evaporator. As far as CHF is concerned, porous microchannel evapoator have the same tendency of solid microchannel evaporator. CHF will go higher as volume flow rate increases. CHF can reach a value of 93W/cm2 to 143W/cm2 and it is enhanced 5% to 23%. With the same of characteristics between solid and porous microchannel evaporator, that pressure drop rises with the increase of volume flow rate and heat flux, the latter’s pressure drop will rise up around 30% to 260% as porosity increases. When the volume flow rate is 166ml/min, the highest pressure drop happens to porous microchannel evaporator with porosity of 66% and it is 30kpa.