Nucleate Pool Boiling Heat Transfer in Microgravity

Abstract

To clarify the effect of gravity reduction on the nucleate boiling heat transfer, pool boiling experiments were conducted along the parabolic trajectory of an aircraft. A transparent flat heating surface with sensors for the measurement of local surface temperature and the liquid film thickness underneath an attached bubble was developed to take account of the increase in the bubble size under microgravity. The structure makes the observation of liquid-vapor behavior at the bubble base and the acquisition of numerical heat transfer data simultaneously possible. Ethanol was used as test liquid at pressure P = 0.02 MPa, heat flux q ≤ 8·104 W/m2 and liquid subcooling ΔTsub ≤ 20K. In the coalesced bubble region, small bubbles generate under a large bubble without producing the thick liquid layer i.e. macrolayer around them. The small bubbles lift the coalesced one and microlayers are formed underneath small bubbles. Accurate evaluation of the surface heat flux distribution shows that there exist both possibilities of heat transfer enhancement and deterioration in microgravity nucleate boiling depending on the behavior of microlayer and extending dry patch underneath the bubble just generated. A heat transfer model was proposed on the basis of the transient heat conduction across the evaporating liquid film. The surface heat flux predicted from the measured microlayer thickness coincides well with the value calculated from the conduction across the heating surface substrate.