EFFECTS OF BUOYANCY ON THE CRITICAL HEAT-FLUX IN FORCED-CONVECTION

The critical heat flux (CHF) for R-113 was measured in forced convection over a flat surface at various orientations for relatively low flow velocities corresponding to Reynolds numbers ranging between 3000-6500 in the test section. As expected, the CHF was found to depend upon the orientation of the buoyancy. Although the buoyancy force acting on the vapor generally dominates over the flow inertia in this flow range, the inertia would continue to be substantial if the gravity was to be reduced significantly. In the experiments of this study, the CHF was determined for heating surface orientations ranging from 0 to 360 deg, for flow velocities between 4-35 cm/s, and for subcoolings between 2.8-22.2-degrees-C. The results presented here demonstrate the strong influence of buoyancy at low flow velocities, which diminishes as the flow velocity and subcooling are increased. In addition, a simple model in which the effects of varying the buoyancy orientation in pool boiling is incorporated is proposed to correlate the CHF at low flow velocities, which finally leads to an analogy between the CHF under adverse gravity and that under microgravity conditions.