FC-72流過一加熱面並以步階方式降低質量流率之流動沸騰熱傳和氣泡特徵研究

Abstract

本研究以實驗方式探討FC-72介電冷卻液在截面寬為50毫米、高8毫米之水平矩形流道中於流動沸騰時進行質量流率驟降，以探討質量流率的改變如何影響暫態強制對流沸騰熱傳及相關氣泡特徵。加熱銅板埋置於測試段之底板中央，其長寬尺寸皆為10毫米。在本次實驗中，固定熱通量下，不同程度的流量下降對暫態沸騰熱傳特性及氣泡特徵之影響將會被詳細的探討。在實驗參數上，介電液FC-72之質量流率改變範圍從300到160kg/m2s，加熱通量為1.0到10 W/cm2。

實驗結果發現，於流動沸騰時，隨著質量流率的下降，加熱的銅板表面壁溫會有下降的現象。當流量改變的程度越大，壁溫下降的幅度越大。值得注意的是，當流量驟降之後，由於測試段入口處之壓力值會隨之下降，此時測試段入口之介電液FC-72溫度保持不變，而使得入口處介電液狀態由飽和態轉變為過熱態。而實驗結果顯示，當流量改變的程度越大，所造成的入口液體過熱度亦越大。除此之外，隨著質量流率的下降，加熱銅板表面氣泡的脫離直徑及成核址密度會隨著時間遞增，而氣泡的脫離頻率則隨著時間遞減。由於氣泡特徵的改變所造成之潛熱熱傳量的增加，最終導致加熱表面壁溫下降的結果。我們亦觀察到當流量改變的程度越大，對氣泡特徵改變的程度也越大。

An experiment is carried out in the present study to investigate transient flow boiling heat transfer and associated bubble characteristics for coolant FC-72 flowing over a small heated copper plate flush mounted on the bottom of a horizontal rectangular channel due to a step decrease in the coolant mass flow rate. In the experiment, the effects of the levels of the step change in the coolant mass flux and imposed heat flux on the time variations of the measured transient flow boiling heat transfer and bubble characteristics are examined in detail. During the tests, the coolant mass flux G ranges from 300 to 160 kg/m2s for the imposed heat flux q varied from 1.0 to 10 W/cm2. The experimental results show that the heated surface temperature decreases with time following a reduction in the coolant mass flux in the transient flow boiling. The effect is more pronounced for a larger reduction in G. Besides, it is noted in the transient flow that the inlet liquid condition changes from a saturated state to a superheated state during the mass flux reduction because of the associated drop in the inlet pressure. Moreover, a larger reduction of the coolant mass flux results in a larger increase in the inlet liquid superheating. Furthermore, as the coolant mass flux reduces with time both the size of the departing bubbles and active nucleation site density increase, but the bubble departure frequency decreases. This results in an increase in latent heat transfer due to the mass flux reduction and hence a drop in the heated surface temperature. We also note that a larger increase in the inlet liquid superheating causes stronger changes in these quantities.