LOCAL VOID FRACTION AND WALL TEMPERATURE PREDICTIONS IN FORCED CONVECTION SUBCOOLED BOILING FLOW USING CFD CODE

Abstract

Forced convection subcooled\nboiling flow is an efficient heat transfer process due to the latent heat of\nevaporation as well as the agitated convection induced by the departure of bubbles.\nIn order to predict and hence prevent the possible occurrence of critical heat\nflux (CFH) at the heated wall, efforts are made to improve the predictability\nof wall nucleation boiling models. In this study, simulations are performed using\n3-D CFD code ANSYS CFX to assess the predictive capabilities of 42 combinations\nof nucleation boiling models versus four experimental conditions. Simulation\nresults show that no specific model combination can consistently give the best\nperformance. Also, two major discrepancies are found in the local void fraction\nprofile prediction as well as the wall temperature profile prediction:\n\n1. \nMost of the local\nvoid fraction profile prediction results show smaller void fractions than the\nexperimental data near the heated wall as well as in the bulk region.\n\n2. \nAll of the wall temperature profile prediction\nresults show monotonically increasing trend, which is different from the\ndecreasing-increasing pattern of the experimental data.\n\nThe first discrepancy is improved\nby using the Chen correlation in place of the bubble departure frequency model.\nThe second discrepancy is improved by using modified\nsingle-phase heat transfer coefficient, which accounts for the pumping effect\nand the vapor blanket effect. The influence of condensation is also discussed by using variable bulk bubble diameter in place of the Laplace length. This simulation work is the first one in the\nliterature that captures the decreasing-increasing pattern of the wall\ntemperature profile. Although the local void fraction profile prediction and the\nwall temperature profile prediction are improved, the predicted departure\nfrequency is much higher than the experimental data. In order to understand the\npossible reason for this discrepancy, more data for the important parameters, e.g., departure frequency, departure diameter, active nucleation site density, etc., along the whole flow channel\nare needed as the future work.