Mechanism of Heat Transfer Enhancement of Air-Water Dispersed Flow in a Pipe

本文研究了垂直加热管内空气-水分散流的强化传热机理。通过多点同时测量，观察到了与以往报道的单组分两相流完全不同的壁温波动的很大幅度和很低的频率，并发现这是由壁面上细流状液膜的曲折运动引起的。用电导探针法测量了加热和不加热条件下的水膜厚度，与层流水膜的分析结果吻合较好。临界水膜厚度被测量为均匀膜的击穿厚度，并且临界厚度与摩擦速度之间的相关性与先前的研究是很好的协议。考虑壁面上均匀或局部周向覆盖液膜的液膜蒸发换热，对传热机理进行了数值模拟分析。在均匀液膜区，传热速率比单相空气流高约1.7倍，本文的分析结果与实测壁温吻合得很好。这种增强主要是由于水膜的蒸发。结果表明，对细流状液膜区的数值分析能很好地预测实验观察到的壁温波动的最大范围。计算结果还表明，由本文分析确定的临界水膜厚度与均匀液膜的液-气界面摩擦速度有很好的相关性。

The mechanism of the heat transfer enhancement of an air-water dispersed flow in a heated vertical pipe has been studied. By simultaneous multi-point measuremetns.a quite large amplitude and a very low frequnce of wall-temperature fluctuations that were completely different from those reported previously for single-component two-phase flows, were observed and were found to be caused by the meandering motion of rivulet-like liquid film on the wall. The water film thickness under heating and no heating was measured by using the conductance probe method, and showed a good agreement with the analytical results for laminar water film. The critical water-film thickness was measured as the breakdown thickness of the uniform film, and the correlation between the critical thickness and the friction velocity was in good agreement with previous studies. A numerical analysis for simulating the heat transfer mechanism was made by taking into account the evaporative heat transfer from the liquid film that covers the wall surface uniformly or partially in circumference as a rivulet-like liquid film. The present analytical results agreed well with the measured wall temperature in the uniform liquid film region, where the rate of heat transfer was approxi-mately seven times higher thanthat for a single-phase air flow. This enhancement was found to be mainly due to the evaporation of teh water film. It was demonstrated that the numerical analysis for the rivulet-like liquid film region predicted well the maximum range of wall-temperature fluctuatins observed experimentally. It was also shown that the critical water-film thickness determined by the present analysis was correlated well with the friction velocity of the liquid-gas interface of the uniform liquid film.