Dryout and Post-Dryout Heat Transfer in Horizontal Evaporator Tubes

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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.