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

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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.