Gravity effects on subcooled flow boiling heat transfer

•Subcooled flow boiling heat transfer measurements were made in a 6 mm ID tube at various gravity levels, flow rates, heat fluxes, and subcoolings using temperature sensitive paints to obtain local heat transfer.•The average heat transfer coefficient typically increased with heat flux, mass flux, and absolute gravity level.•In microgravity, the lack of mixing at low heat fluxes due to the absence of natural convection and bubble slip velocity resulted in a decrease in heat transfer coefficient compared to downward and upward conditions.•Mechanisms by which heat is transferred under various conditions are discussed. Subcooled flow boiling measurements using HFE-7000 were obtained in a vertical 6 mm ID sapphire tube during upward and downward flow at various gravity levels including hypergravity and microgravity. Temperature sensitive paint (TSP) applied to the inside of the tube was used to measure time and space resolved temperature and heat transfer distributions at the wall–fluid interface, and this data along with flow visualization were used to characterize the heat transfer for different flow patterns. Time-averaged heat transfer coefficients were compared at nine gravity levels, four mass fluxes, six heat fluxes, and two subcoolings. The average heat transfer coefficient typically increased with heat flux, mass flux, and absolute gravity level. In microgravity, the lack of mixing at low heat fluxes due to the absence of natural convection and bubble slip velocity resulted in a decrease in heat transfer coefficient compared to downward and upward conditions. The heat transfer was strongly dependent on the flow regime, causing certain data points at high mass flux or low gravity to deviate from the typical trends due to deactivation of nucleation sites. The heat transfer coefficient became less dependent on gravity as the mass flux and heat flux increased. Flow regimes were very sensitive to the competition between buoyancy and inertial forces, which in turn affected the heat transfer. Mechanisms by which heat is transferred under various conditions are discussed.