Ohta Haruhiko
Department of Aeronautics and Astronautics, Kyushu University, Higashi-ku, Fukuoka, Japan.
Ann N Y Acad Sci. 2002 Oct;974:463-80. doi: 10.1111/j.1749-6632.2002.tb05925.x.
The objective of this paper is to clarify the mechanisms of heat transfer and dryout phenomena in flow boiling under microgravity conditions. Liquid-vapor behavior in annular flow, encountered in the moderate quality region, has extreme significance for practical application in space. To clarify the gravity effect on the heat transfer observed for an upward flow in a tube, the research described here started from the measurement of pressure drop for binary gas-liquid mixture under various gravity conditions. The shear stress acting on the surface of the annular liquid film was correlated by an empirical method. Gravity effects on the heat transfer due to two-phase forced convection were investigated by the analysis of velocity and temperature profiles in the film. The results reproduce well the trends of heat transfer coefficients varying with the gravity level, quality, and mass velocity. Dryout phenomena in the moderate quality region were observed in detail by the introduction of a transparent heated tube. At heat fluxes just lower and higher than CHF value, a transition of the heat transfer coefficient was calculated from oscillating wall temperature, where a series of opposing heat transfer trends--the enhancement due to the quenching of dried areas or evaporation from thin liquid films and the deterioration due to the extension of dry patches--were observed between the passage of disturbance waves. The CHF condition that resulted from the insufficient decrease of wall temperature in the period of enhanced heat transfer was overcome by a temperature increase in the deterioration period. No clear effect of gravity on the mechanisms of dryout was observed within the range of experiments.
本文的目的是阐明微重力条件下流动沸腾中的传热和干涸现象的机理。在中等干度区域出现的环状流中的液-汽行为对于空间实际应用具有极其重要的意义。为了阐明重力对管内向上流动传热的影响，本文所述研究从测量各种重力条件下二元气-液混合物的压降开始。作用在环状液膜表面的剪应力通过经验方法进行关联。通过分析液膜中的速度和温度分布，研究了重力对两相强制对流传热的影响。结果很好地再现了传热系数随重力水平、干度和质量流速变化的趋势。通过引入透明加热管，详细观察了中等干度区域的干涸现象。在热流率略低于和高于临界热流密度值时，根据振荡壁温计算传热系数的转变，在扰动波通过期间观察到一系列相反的传热趋势——由于干涸区域的淬火或薄液膜蒸发导致的强化以及由于干斑扩展导致的恶化。在传热增强期间壁温下降不足导致的临界热流密度条件，通过恶化期间的温度升高得以克服。在实验范围内未观察到重力对干涸机理有明显影响。
