LOCAL AND MEAN HEAT-TRANSFER COEFFICIENTS IN BUBBLY AND SLUG FLOWS UNDER MICROGRAVITY CONDITIONS

Experimental local heat transfer data were collected on-board NASA's K C-135 reduced gravity aircraft for two-phase, air-water flow in vertic al, upward, co-current flow through a 9.53 mm circular tube. It was fo und that in the bubbly and slug flow regimes (surface tension dominate d regimes) reduced gravity has a tendency to lower the heat transfer c oefficient by as much as 50% at the lowest gas qualities. As the gas-q uality increases (transition to annular flow), the difference between the 1 - G and mu - G heat transfer coefficients is much less: signific ant. Due to minimal slip between the two-phases at mu - G conditions a nd a thermal entry length heat transfer coefficient profile similar to that for single-phase flows, it is proposed to predict the two-phase heat transfer coefficients with analytical single-phase thermal entry length solutions. This method was found to predict coefficients within +/-26% for bubbly and slug flow regimes for 3000 < Re-TP < 10,000 usi ng superficial liquid Reynolds numbers. For Re-TP > 10,000, empirical single-phase turbulent correlations provide a reasonable match to the experimental data. Copyright (C) 1996 Elsevier Science Ltd.