Prediction of Heat Transfer Coefficient for Flow Boiling in Horizontal Tubes

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At low mass velocity or low quality in horizontal evaporator-tubes, the heat transfer coefficient is not uniform but varies extermely along the tube perimeter, due to the phase separation with the liquid flowing in the lower part of the tube and the vapor in the upper part. Experimental data of the local heat transfer coefficient along the perimenter of horizontal tubes for boiling R22 were obtained under the conditions of different tube diameters, pressures, mass velocities and heat fluxes in the whole quality region. The separation angle, that is, the angle measured from the bottom point of the horizontal tube to the point of separation between the wet part and the dry part of the inside perimeter, was estimated from the heat transfer data. This angle increased with an increase in mass velocity, increased or decreased with increasing quality depending on the mass velocity, and increased with a decrease in tube diameter or pressure. The characteristics of the circumferentially averaged heat transfer coefficient were explained mainly by the behavior of the separation angle. The correlation proposed previously by the authors for the heat transfer in the annular two-phase flow regime was found to be applicable to the prediction of the averaged heat transfer coefficient over the wet perimeter of the horizontal evaporator-tube, while a well-known heat transfer correlation for singlephase fluid flow was to be applicable to the averaged heat transfer coefficient over the dry perimeter. The circumferentially averaged heat transfer rate in horizontal evaporator-tubes could be predicted by using these correlations of heat transfer coefficients and the measured value of the separation angle.