Heat Transfer of Air-Water Dispersed Flow in a Vertical Pipe.

Abstract

The heat transfer and its enhancement of an air-water dispersed flow in a heated vertical pipe have been studied. The axial and circumferential wall temperature distributions were measured for various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular point in the direction of the stream with a notable variation in the circumferential profile. This sharp rise was thought to be caused by the breakdown of the water film rather than by its dryout. A separate experiment in which no heating was used was carried on to estimate the droplet deposition velocity and the water-film flow rate. Numerical analysis taking into account the heat and mass transfer from the water film was performed to estimate the mean wall temperature, and good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times higher than that for a single phase air flow. This enhancement was shown to be mainly due to the evaporation of the water film. The mechanism of the heat transfer enhancement is discussed in detail using the numerical analysis results.