微重力下光滑表面上FC-72的池沸腾实验研究

Experimental Study of Pool Boiling of FC-72 Over Smooth Surface Under Microgravity

本文利用小尺度光滑芯片(10 mm×10 mm×0.5 mm),通过控制加热电流方法,在北京落塔进行了持续3.6 s有效微重力时间的过冷池沸腾实验研究。在低热流和中等热流区,微重力条件下可以观察到稳态或准稳态池沸腾现象,传热特性基本维持不变;在中等热流区域,气泡的横向合并会引起合并后的大气泡表面剧烈震荡,进而引发合并大气泡从加热表面脱落。在高热流密度区,微重力条件下加热表面很快形成半球形状并几乎完全覆盖其表面的聚合大气泡;随后,由于高过冷液体冷凝作用,聚合大气泡收缩为半椭球形状,界面也变得更为光滑,且完全覆盖了加热表面,壁温曲线明显升高,可能提前发生临界热流或转变为膜态沸腾,导致微重力条件下沸腾换热显著恶化。

Experiments of subcooled pool boiling of FC-72 on a small scale smooth silicon chip with the dimensions of 10 mm×10mm×0.5 mm were studied in short-term microgravity utilizing Drop-Tower Beijing with duration of about 3.6 s.The smooth silicon chip was heated by means of controlling heating current for the desired heat flux.The results indicate that steady or quasi-steady nucleate pool boiling was observed in the experiments in low and intermediate heat flux regions.The performance of boiling heat transfer can be maintained unchanged.It is found that the oscillation due to coalescence of adjacent bubbles is the primary reason of bubbles departure in microgravity condition in intermediate heat flux regime.At high heat fluxes,a large coalesced bubble forms quickly and covers the heater surface completely in microgravity,followed by shrinking to an oblate in shape and smooth in contour due to the highly subcooled condensation.The mean heater surface temperature increases significantly,which indicates the deterioration of boiling heat transfer.It is possible for the occurrence of local dry-out or transition to film boiling at the bottom of the large coalesced bubble.