FC-72 在一小水平板上隨時間週期性變化加熱之暫態池沸騰熱傳現象及氣泡特性研究

Abstract

本實驗是探討一個隨時間呈現週期性振盪的矩形波型式的輸入熱，如何影響暫態下飽和態和次冷態介電液體FC-72在一個小水平銅表面上的池沸騰熱傳特性。在實驗中，平均輸入的熱通量和振盪振幅分別為0.5至4.5 W/cm2和0％到100％；而振盪週期，主要是固定在30，60和120秒。工作流體的次冷度主要選擇在5 K和10K的環境下。無論是跟時間相關的沸騰熱傳特性及氣泡特性都會在實驗中將加以研究與探討。而實驗數據中，將表示在各種不同平均的熱通量，振幅和週期的振盪下，隨時間變化下空間平均的銅表面壁溫和熱傳遞係數。 實驗結果中可以發現，不論FC-72為飽和態或次冷態經由時間平均後的暫態沸騰曲線和熱傳係數，都接近穩態的沸騰熱傳。此外，隨時間變化下空間平均的壁溫、熱傳係數、氣泡脫離直徑、脫離頻率和有效成核址密度在不同輸入值的熱通量振盪下，也會隨時間呈現週期性振盪和擁有相同的振盪頻率。再者，由實驗結果中得知如果輸入一個較大的振幅熱通量，或者較長週期和較大的平均熱通量時，由此條件下產生的壁溫，熱傳遞係數和氣泡的特性(如氣泡脫離直徑、脫離頻率的和有效成核址密度)的振幅均會變大。然而，由於熱慣性的影響表現於銅表面的壁溫振盪會造成時間延遲的現象，而這種現象利用無因次的值(延遲的時間和振盪週期的比值)表示，由結果可以知道隨著平均的熱通量越小、週期越短、輸入熱的振盪振幅越大或者較小的液體次冷度均會造成延遲的時間越長。此外，當輸入的平均熱通量約略高於穩態相對應的成核沸騰的熱通量時，間歇性沸騰將會產生。對於目前的暫態加熱條件下會發現:暫態成核沸騰的熱通量會高於為穩態成核沸騰的熱通量。並且，隨著輸入的平均熱通量較大或較長的週期振盪，開始沸騰的時間會較早而停止沸騰的時間會較晚。最後是探索在週期極短和極長的輸入熱下，如何影響暫態壁溫的振盪。由於有限厚度的銅塊中存在熱慣性，在極短的時間下所輸入的熱通量振盪會造成表面壁溫接近於定值類似於穩態的沸騰；在另一方面，在輸入的熱通量振盪為一極長的週期的情況下，在各半週期中空間平均的壁溫會趨於平緩近似穩定的狀態。 隨著熱通量隨時間振盪，氣泡脫離直徑，脫離頻率和有效成核址密度也會隨時間振盪。更具體地說，在前半的週期中，其中輸入的熱通量位於較低的波谷時氣泡脫離的大小、脫離的頻率和有效成核址密度都會隨時間減少；相反地, 在後半週期中，這些氣泡特性的參數都會隨時間增加就如同壁溫增加一樣。此外，當輸入較大平均熱通量、較大的振幅和較長的振盪週期時，氣泡脫離直徑，脫離頻率和有效成核址密度都會振盪地更強烈。

Experiments are carried out here to investigate transient saturated and subcooled pool boiling of liquid FC-72 over a small horizontal copper surface subject to time-periodic heating, intending to explore how a time-periodic imposed heat flux oscillation in the form of a rectangular wave affects the FC-72 pool boiling characteristics. In the experiments, the mean imposed heat flux and amplitude of the imposed heat flux oscillation are respectively varied from 0.5 to 4.5 W/cm2 and from 0% to 100% with the period of the heat flux oscillation mainly fixed at 30, 60 and 120 seconds. The liquid subcooling is chosen to be at 5K and 10K. Both the time dependent boiling heat transfer and associated bubble characteristics will be examined. The measured data are presented in terms of the time variations of the space-average copper surface temperature and heat transfer coefficient for various mean levels, amplitudes and periods of the imposed heat flux oscillation. The experimental results indicate that the time-averaging boiling curves and heat transfer coefficients in the transient FC-72 saturated and subcooled pool boiling are relatively close to that for a constant imposed heat flux. Besides, the temporal variations of the space-average copper surface temperature, heat transfer coefficient, bubble departure diameter and frequency and active nucleation site density also oscillate periodically in time and at the same frequency as the time-varying heat flux imposed to the copper surface. Experiments also show that the resulting oscillation amplitudes of the copper surface temperature, heat transfer coefficient and bubble characteristics, such as dp, f and Nac, get larger for a longer period and a larger amplitude of the heat flux oscillation and for a higher mean imposed heat flux. Moreover, a short time lag in the copper surface temperature oscillation is also noted, which apparently results from the thermal inertia of the copper surface. The dimensionless time lag, which is defined as the actual ratio of the time lag to the period of the imposed heat flux oscillation, is larger at lower mean imposed heat flux, shorter period and higher amplitude of the imposed heat flux oscillation, and smaller liquid subcooling. Furthermore, when the mean imposed heat flux over the heated copper surface is slightly above the heat flux corresponding to that for the onset of stable boiling, intermittent boiling appears. The heat flux for the onset of the intermittent boiling for the present transient heating conditions is significantly higher than that for the stable boiling and the onset of boiling is earlier and the termination of boiling is later at a higher mean imposed heat flux or a longer period of the imposed heat flux oscillation. Finally, the effects of the imposed heat flux oscillation at extremely short and long periods are explored. Due to the existence of the thermal inertia of the heated copper block, the resulting heated surface temperature is close to that in the stable boiling at extremely short period of imposed heat flux oscillation. On the other hand, the space-average heated surface temperature gradually levels off and approaches a nearly stable state at each half cycle of the imposed heat flux oscillation for an extremely long period. As the imposed heat flux oscillates with time, the bubble departure diameter, departure frequency and the active nucleation site density also vary significantly with time. More specifically, the size of the departing bubbles, their departure frequency and active nucleation site density reduce in the first half of the cycle in which the imposed heat flux is at low level and the heated surface temperature decreases with time. While in the second half of the cycle an opposite process is noted since Tw increases with time. Moreover, the dp, f and Nac oscillations are stronger for higher mean level, larger amplitude and longer period of the heat flux oscillation.