Subcooled flow boiling over a thin, low-capacitance surface on a concave wall

Experiments which describe the heat transfer performance from single-phase, forced convection to critical heat flux (CHF) and film boiling were conducted with a thin, heated patch on a concave surface. The heater was a thin-film of 800 angstroms thick and 3 mm×3 mm in area sputtered on a polyimide film to form a boiling surface and temperature sensor. The coolant was circulated through the curved channel and, for reference, a straight channel. Velocities from 2.4 to 6.9 m/s, subcooling from 14.2 to 59 °C, and pressures of either 1.41 or 1.92 atm were tested. Flow visualization in the curved channel characterized the secondary flow and located `upwash' and `downwash' positions of the streamwise oriented vortices. The same test heaters were used in both straight-flow and curved-flow tests. Results show sensitivity to cross-span location. There was little heat spreading within the wall. This was displayed by a co-existence of a film boiling region and a nucleate boiling region on the heater surface. The straight flow and the downwash flow on the curved surface behaved similarly but the upwash flow displayed reduced boiling heat transfer performance. The net effect of curvature was a degradation of nucleate boiling heat transfer, including CHF, when the heated wall was thin, in contrast to the slight increase in CHF with curvature when the heated wall is thick.