斜面通道應用於大面積微結構加熱表面的過冷流沸騰熱傳

Abstract

本研究主要在探討斜面流道以及表面的微結構設計對於尺寸較大的加熱表面之流沸騰熱傳性能。使用斜面上蓋流道、墊片、紅銅加熱塊及下蓋組成微流道裝置，其散熱區域面積為31 mm × 25 mm，並以去離子水作為工作流體，將流體之入口溫度控制在21±1.5°C來進行流沸騰實驗。實驗中採用光滑表面、漸擴型圓形鰭片陣列與平板形鰭片三種加熱表面設計，兩種不同傾斜角度的上蓋斜面流道(2.29°和12.72°)，以及兩種入出口壓力差(55 kPa和85 kPa) 作為變數來探討最佳熱傳性能表現的設計。 實驗結果顯示，表面微結構的設計使散熱面積提高，並增加沸騰時的氣泡成核點，可以降低加熱表面的溫度並且提高熱傳系數，其中以平板形鰭片的加熱表面溫度最低，此外具有微結構的加熱表面其臨界熱通量比較高，圓形鰭片陣列因為有漸擴式設計，可以幫助氣泡排除，在入口壓力為70 kPa的實驗中，加熱表面溫度在107.3°C時可達最大臨界熱通量0.87 W mm-2。而增加上蓋流道的傾斜角度除了可以增加質量流率也可以提升氣泡排除效果，並提高臨界熱通量值，但傾斜角度較大的上蓋流道會產生較大的流道間隙，工作流體從間隙離開微流道，並未全部接觸到加熱表面，對於能夠產生較多氣泡的微結構加熱表面，傾斜角度較大的上蓋流道不易帶走表面剛生成之氣泡，使加熱表面的溫度可能會高於上蓋流道傾斜角度較小的實驗。另外，提高入口端與出口端壓力差可以增加質量流率並提升散熱效果，壓力差較大的熱傳係數會高於壓差較小的實驗。在入口壓力為100 kPa時，使用平板形鰭片加熱表面以及上蓋流道傾斜角度較大的組合，其沸騰熱傳性能最佳，此時的最大熱通量為0.98 W mm-2，對應的加熱表面溫度為101.9°C。

This study investigates the use of the tapered manifold on flow boiling over a large-area microstructured surface. The microchannel device is composed of the tapered manifold, copper heated block with microstructured surface, sealed by a teflon washer. The heat dissipation area is 31 mm × 25 mm. Deionized water is used as the working fluid and held at 21±1.5°C at the inlet. Three microstructure designs are studied: plain surface, shrinking circular fins, and plate fins. The inclined angle of the tapered manifold is 2.29° and 12.72°, and the inlet pressure is kept at 70 kPa and 100 kPa, while the outlet pressure is maintained at 15 kPa to investigate their influence. At the inlet pressure of 100 kPa, using the plate fins and the tapered manifold with a large inclined angle achieves the best heat transfer performance of flow boiling in this study. Because the fins provide extended surfaces of heat transfer and bubble nucleation sites, microstructured surfaces is found to reduce the wall temperature and obtain higher critical heat flux (CHF). Among these designs, the plate-fin surface results in the lowest wall temperature, and the shrinking circular-fin surface leads to the highest CHF. Additionally, increasing the inclined angle of the tapered manifold also helps to improve CHF due to its larger mass flow rate and easier bubble removal. However, a large inclined angle also increases the gap between the manifold and the heated surface. This prevents the fluid from flowing between fins, resulting in the elevation of wall temperature. Increasing the pressure difference between the inlet and outlet increases the mass flow rate and reduces the wall temperature. A maximum heat flux of 0.98 W mm-2 with a wall temperature of 101.9°C is achieved by using the plate-fin surface and the tapered manifold with a large inclined angle under a pressure difference of 85 kPa.