Dryout and Post-Dryout Heat Transfer in Horizontal Evaporator Tubes

对沸腾制冷剂HFC-134 a和HCFC-22在水平光管和水平微肋管内的干涸特性和干涸后区域局部换热系数进行了实验研究，并建立了干涸特性和局部换热系数的预测方法.在给定条件下，干燥在一定的质量范围内进行，因此，讨论了两种干燥质量;热传递开始恶化时的干燥开始质量和热传递恶化结束时的干燥完成质量。阐明了干化开始和完成质量的特征，并且将每种干化质量分为光管的3个特征区域和微翅片管的2个区域。建立了相应管的两种质量的无偏差关联式，光滑管的绝对值与测量值在±0.03以内一致，微肋管的绝对值与测量值在±0.02以内一致.在光管的干燥后区域，两相流流型为分散流型，汽液混合物不处于热力学平衡。建立了热力学非平衡状态下实际干度的关联式，预测了实际蒸汽质量流量。利用实际蒸汽流量，从仅假设蒸汽单相流动的Gnielinski方程计算管壁温度，然后确定传热系数。传热系数的计算值与实测值在±10%以内吻合。干燥后的微肋管的传热系数可以计算为单相传热从Gnielinski方程与摩擦系数的方程从测量的压降。

Experiments were made on dryout quality and local heat transfer coefficient in the post-dryout region for the flow of boiling refrigerants HFC-134a and HCFC-22 inside horizontal smooth tubes and a horizontal micro-fin tube, and prediction methods were developed for the dryout quality and the heat transfer coefficient.1. Dryout proceeded over a certain quality range at given conditions and, therefore, two dryout qualities were discussed ; the dryout inception quality at which the heat transfer began to deteriorate and the dryout completion quality at which the heat transfer deterioration ended. The characteristics of the dryout inception and completion qualities were clarified, and each of the dryout qualities was classified into three characteristic regimes for the smooth tube and two regimes for the micro-fin tube. Dimensionless correlations of both the qualities were developed for the respective tubes, and they agreed with the measurements within ±0.03 in the absolute values for the smooth tube and within ±0.02 for the micro-fin tube.2. In the post-dryout region of the smooth tube, the two-phase flow pattern is a dispersed one, where the vapor-liquid mixture is not in a thermodynamic equilibrium. A correlation of the actual quality in the thermodynamic nonequilibrium was developed to predict the actual vapor mass flow rate. By the use of the actual vapor flow rate, the tube wall temperature is calculated from the Gnielinski equation assuming the single-phase flow of the vapor only, and then the heat transfer coefficient is determined. The calculated heat transfer coefficient agreed within ±10% with the measured one. The post-dryout heat transfer coefficient in the micro-fin tube can be calculated as a single-phase heat transfer from the Gnielinski equation with an equation of friction factor obtained from the measured pressure drop.