Dryout and Post-Dryout Heat Transfer in Horizontal Evaporator Tubes

对沸腾制冷剂HFC-134a和HCFC-22在水平光滑管和水平微翅片管内的流动进行了干燥质量和干燥后区域局部换热系数试验，并建立了干燥质量和换热系数的预测方法1。在一定条件下，干干是在一定质量范围内进行的，因此讨论了两种干干质量；传热开始恶化时的干衣质量和传热结束时的干衣完成质量。澄清了干干开始和完井质量的特点，并将每个干干质量分为光滑管的3个特征区和微翅片管的2个特征区。对两种管进行了无因次相关性分析，结果表明：光滑管的绝对值在±0.03范围内，微翅片管的绝对值在±0.02范围内。在光滑管干后区域，两相流为分散流型，汽液混合物不处于热力学平衡状态。建立了热力学不平衡状态下实际质量的关系式来预测实际蒸汽质量流量。利用实际蒸汽流量，根据假设蒸汽单相流动的格涅林斯基方程计算管壁温度，进而确定换热系数。计算的换热系数与实测值吻合在±10%以内。干燥后微翅片管的换热系数可由Gnielinski方程和由实测压降得到的摩擦系数方程计算为单相换热。

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.