Prediction of Heat Transfer Coefficient for Flow Boiling in Horizontal Tubes

在低质量速度或低质量的水平蒸发器管内，由于流体在管内下部流动和蒸汽在管内上部流动的相分离，传热系数不是均匀的，而是沿管内周长变化很大。在整个质量区不同管径、压力、质量速度和热流密度的条件下，获得了R22沸腾水平管周边局部换热系数的实验数据。分离角，即从水平管底部到内周湿部和干部分离点所测得的角度，由传热数据估算。该角度随质量速度的增加而增大，随质量速度的增加而增大或减小，随管径或压力的减小而增大。周向平均换热系数的特性主要用分离角的变化来解释。作者先前提出的环形两相流的传热关系式可用于预测水平蒸发器管湿周的平均传热系数，而众所周知的单相流体流动的传热关系式可用于预测干周的平均传热系数。利用传热系数与分离角测量值的相关关系，可以预测水平蒸发器管内的周向平均换热率。

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.