Heat transfer of air‐water dispersed flow in a vertical pipe

Abstract

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. 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 larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998