Local and mean heat transfer coefficients in bubbly and slug flows under microgravity conditions

Abstract

Experimental local heat transfer data were collected on-board NASA's KC-135 reduced gravity aircraft for two-phase, air-water flow in vertical, upward, co-current flow through a 9.53 mm circular tube. It was found that in the bubbly and slug flow regimes (surface tension dominated regimes) reduced gravity has a tendency to lower the heat transfer coefficient by as much as 50% at the lowest gas qualities. As the gas-quality increases (transition to annular flow), the difference between the 1 − G and μ − G heat transfer coefficients is much less significant. Due to minimal slip between the two-phases at μ − G conditions and 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 using superficial liquid Reynolds numbers. For Re TP > 10,000, empirical single-phase turbulent correlations provide a reasonable match to the experimental data.