FC-72流過一可變功率加熱面之暫態流動沸騰熱傳及氣泡特性研究

Abstract

本實驗主要探討雙相流冷煤循環系統對介電冷卻液FC-72在截面為寬20毫米、高五毫米之水平矩形流道(水力直徑為8毫米)中進行，隨時間作週期性震盪之加熱功率如何影響暫態強制對流沸騰熱傳以及相關氣泡特徵，加熱銅塊埋置於測試段之底板中央其尺寸為直徑10毫米。值得注意的是加熱功率隨時間振盪(振盪的形狀接近矩型波)，是如何影響暫態流動沸騰將會被詳細的探討。在實驗參數範圍上，介電液FC-72平均質量通率從300到400kg/m2s，加熱通量振幅為0%、10%、30%、50%，振盪週期為10秒到30秒。此外，FC-72於測試段入口次冷度從0到10oC、銅塊加熱通量從0.1到10 W/cm2而系統壓力維持在常壓。 由實驗發現當熱通量在振盪過程中，在相對應的瞬時壁溫、熱傳遞係數、氣泡脫離直徑、脫離頻率和成核址密度也會有振盪的現象發生，但是當熱通量振盪的振幅和週期經過時間平均化之後的飽和態和次冷態流動的暫態流動沸騰熱傳特徵沒有明顯的影響，類似穩態的流動沸騰。除此之外，在高熱通量情況下和振盪振幅、加熱週期比較大的情況下對其熱傳遞係數、氣泡脫離直徑、脫離頻率和成核址密度都會造成更強烈的振盪情況發生以及在壁溫所需要的反應時間也相對的縮短。當加熱通量隨時間減少的情況下，氣泡脫離直徑、脫離頻率和成核址密度都會跟著遞減。當加熱通量隨時間增加的情況下，會產生相反的趨勢。此外，在暫態飽和態流動沸騰以及次冷態流動沸騰過程中當平均熱通量非常接近穩態沸騰啟始熱通量時，我們觀察到在暫態流動沸騰中有間歇性沸騰的現象。並且也發現入口次冷度的增加會使得沸騰的熱傳係數振盪更劇烈。

An experiment is carried out in this study to investigate the transient flow boiling heat transfer performance and associated bubble characteristics of FC-72 flow over a small circular plate subject to a time varying heat flux with the plate flush mounted on the bottom of a horizontal rectangular channel. The imposed heat flux oscillates with time in the form of rectangular waves. The effects of the FC-72 inlet liquid subcooling, mass flux, and imposed heat flux oscillation on the FC-72 transient flow boiling characteristics are explored. In the experiment the inlet liquid subcooling △Tsub ranges from 0 to 10 ℃, the coolant mass flux G varies from 300 to 400 kg/m2s, mean imposed heat flux ranges from 0 to 10 W/cm2 and the amplitude of the heat flux oscillation is fixed at 10%, 30% and 50% of with the period of the heat flux oscillation varied from 10sec. to 30sec., covering the saturated and subcooled flow boiling. The transient flow boiling heat transfer characteristics are illustrated by the measured time variations of the heated plate temperature and boiling heat transfer coefficient. The experimental results show that the time-average FC-72 transient saturated and subcooled flow boiling heat transfer characteristics are not affected to a significant degree by the amplitude and period of the imposed heat oscillation. In fact, they resemble that for the stable flow boiling. However, the imposed heat flux oscillation causes significant temporal oscillations in the boiling heat transfer coefficient, bubble departure diameter and frequency, and active nucleation site density. These physical quantities oscillate at the same frequency as the heat flux oscillation and at a higher , a larger △q/ , and a longer tp they exhibit stronger oscillations except for tp=30 sec. Besides, a slight time lag in Tw oscillation is seen. Moreover, the size of departing bubbles, active nucleation size density and bubble departure frequency decrease as the heat flux is reduced to the low level of -△q. The opposite processes take place for the heat flux raised to the high level of +△q. Furthermore, at the mean imposed heat flux close to that for the ONB in the stable boiling we observe intermittent boiling in the transient saturated flow boiling and subcooled flow boiling. We also find that an increase in the inlet liquid subcooling results in stronger oscillations of these quantities.