FC-72在水平矩形流道中流過一微小加熱面暫態流動沸騰熱傳及氣泡特性研究

Abstract

本實驗是要探討雙相流冷媒循環系統針對介電冷卻液FC-72在截面為寬20毫米、高5毫米之水平矩形流道(水力直徑為8毫米)中進行，隨時間週期振盪的流量如何影響暫態強制對流沸騰熱傳以及相關的氣泡特徵，加熱銅塊埋至於測試段之底板中央其尺寸為直徑10毫米。值得注意的是冷卻液流量隨時間振盪(振盪的形狀接近三角波)，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400 kg/m2s而且流量的振盪振幅為0%、5%、10%，流量的振盪周期為10秒到30秒。除此之外，FC-72於測試段入口處次冷度從0到10 oC、銅塊的加熱通量從0.1到10 W/cm2 而系統壓力為常壓。 由實驗結果發現介電冷卻液的振盪振幅和週期對時間平均的FC-72暫態的飽和態和次冷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。無論如何，在熱通量高於起始成核沸騰的熱通量FC-72的暫態飽和態和次冷態流動沸騰對熱傳係數、氣泡脫離直徑、氣泡脫離頻率、成核址密度都有顯著的振盪影響。除此之外，在高熱通量下和振盪振幅比較大的情況下對熱傳係數、氣泡脫離直徑、氣泡脫離頻率、成核址密度都會造成更強烈的振盪情況發生，但振盪週期的影響並不明顯。因此，當流量隨時間增加的情況下，氣泡脫離的尺寸和成核址密度都會下降，但氣泡脫離頻率會增加。當流量隨時間遞減的情況下，會產生相反的趨勢。並且也發現入口處次冷度的增加會使得沸騰的熱傳係數振盪更劇烈。

This study intends to explore how a time periodic coolant flow rate affects the transient forced convective boiling heat transfer and associated bubble characteristics of FC-72 over a small heated circular copper plate flush mounted on the bottom of a horizontal rectangular channel with a cross section of 20 mm in width and 5 mm in height. The diameter of the copper plate is 10 mm. More specifically, the effects of the coolant flow rate oscillation in the form of nearly triangular wave on the transient flow boiling will be examined in detail. In the experiment the time-average coolant mass flux is varied from 300 to 400 kg/m2s and the amplitude of the coolant mass flux oscillation is fixed at 0%, 5% and 10% of . The period of the coolant mass flux oscillation varies from 10s to 30s. Besides, the liquid subcooling at the inlet of the channel ranges from 0°C to 10°C and the heat flux imposed on the heated plate varies from 0.1 to 10 W/cm2. The experimental results show that the time-average data of the 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 coolant mass flux oscillation. In fact, they resemble these for the stable flow boiling. However, in the transient saturated and subccoled flow boiling of FC-72 significant temporal oscillations in the boiling heat transfer coefficient, bubble departure diameter and frequency, and active nucleation site density appear for the imposed heat flux slightly higher than that for ONB. They oscillate at the same frequency as the mass flux oscillation. Besides, at a higher imposed heat flux and for a larger amplitude of the mass flux oscillation stronger oscillations in , dp, f and Nac are noted. But they are only slightly affected by the period of the mass flux oscillation. Furthermore, in the time duration in which the mass flux rises with time both the size of the departing bubbles and active nucleation site density decrease, but the bubble departure frequency increases. The opposite processes occur for a sink of mass flux with time. We also note that an increases in the inlet liquid subcooling causes stronger oscillations in the boiling heat transfer coefficient.