HFE-7000於微流道熱沉之流動沸騰熱傳增強研究

Abstract

本研究以實驗方法探討於微流道之流動沸騰，以HFE-7000作為工作流體，控制系統出口壓力為150 kPa，並改變不同的實驗參數，探討在水力直徑1050 μm之微流道熱沉與另一水力直徑1114 μm之漸擴微流道內的流動沸騰熱傳及壓降特性；其控制實驗參數分別為質量通率(100, 200 kg/m2s)、加熱通率(37.5, 75 kW/m2)、蒸汽乾度(0.1-0.8)及不同流道擺放方式( 90°垂直向上流動、45°傾斜向上流動、0°水平流動與-90°垂直向下流動)，再配合高速攝影機觀察本實驗系統在不同效應下所對應的結果。實驗結果顯示，本實驗系統其流動沸騰機制有兩種熱傳機制共同存在，在低蒸汽乾度約0.2時，以成核沸騰的機制為主導，改變加熱通率對於熱傳係數有所影響；而隨著乾度增加時，轉換為強制對流沸騰的機制為主導時，改變質量通率在此時熱傳係數有影響。在角度效應上，在質量通率100 kg/m2s時較為明顯，熱傳係數在向上流動的擺放方式略大於水平流動的擺放方式，因浮力增益了蒸汽的速度；而熱傳係數在向下流動的擺放方式時效果最差，蒸汽柱受到浮力的阻力，容易造成流量不均，而發生乾化現象；當質量通率增加後角度效應變小。此外，本研究提出一漸擴微流道以增強兩相流動沸騰之效果，作法為在中段開始壁面往下漸擴至出口，利用增大空間與表面張力來使汽液分流並穩定蒸汽柱在狹小流道內的流動，並使得後段液膜變厚，活化成核址，成核的氣泡在藉由蒸汽柱挾帶而離開通道，進而增益熱傳效率。結果顯示，漸擴微流道熱沉，除了可以增益兩相流動沸騰外，亦可降低壓降；其熱傳係數平均增加約23.9%，壓降平均降低約42.5%。

This study experimentally investigated the flow boiling of dielectric fluid HFE-7000 within a micro-channel heat sink with a hydraulic diameter of 1050 μm and a fixed outlet pressure of 150 kPa. The mass flux ranges from 100 to 200 kg/m2s with vapor quality from 0.1 to 0.9, heat flux from 37.5 to 75 kW/m2 and the effect of inclination is also investigated. The inclination angles include 90∘(vertical upward), 45∘(inclined upward), 0∘(horizontal forward), and -90∘(vertical downward). In addition, the present study proposed a novel micro-channel with diverging cross-section from the middle of the channel. The flow boiling experiments were conducted in rectangular micro-channel in comparison with the proposed new micro-channel under similar operating conditions. The results show that the heat transfer mechanism in micro-channel heat sink is governed by the nucleate boiling and the convective boiling. The effect of inclination shows that the heat transfer coefficient for the upward arrangement is slightly better than that of horizontal arrangement. On the contrary, the downward arrangement always impairs the heat transfer coefficient. With the rise of mass flux, the effect of inclination is also reduced. On the other hand, the micro-channel heat sink with diverging cross-section from the middle of the channel shows a better heat transfer performance than the rectangular micro-channel heat sink with the similar operating condition. Through increasing the height of the channel downstream, the diverging cross section will provide a larger space to occupy the vapor and the liquid film at the downstream is prone to evaporate more; thereby the vapor will tend to flow smoothly and expand at the downstream for heat transfer augmentation. The increasing of flow area in the flow direction not only helps to improve the stability of two-phase flow but also reduce the pressure drop considerably. The results show that the micro-channel heat sink with diverging cross-section from the middle of the channel can enhance the heat transfer coefficient as much as 23.9% while reduce the pressure by 42.5% as compared to the rectangular micro-channel heat sink.