ORIENTATION EFFECTS ON CRITICAL HEAT-FLUX FROM DISCRETE, IN-LINE HEAT-SOURCES IN A FLOW CHANNEL

The effects of flow orientation on critical heat flux (CHF) were inves tigated on a series of nine in-line simulated microelectronic chips in Fluorinert FC-72. The chips were subjected to coolant in upflow, down flow, or horizontal flow with the chips on the top or bottom walls of the channel with respect to gravity. Changes in angle of orientation a ffected CHF for velocities below 200 cm/s, with some chips reaching CH F at heat fluxes below the pool boiling and flooding-induced CHF value s. Increased subcooling was found to dampen this adverse effect of ori entation slightly. Critical heatflux was overwhelmingly caused by loca lized dryout of the chip surface. However, during the low velocity dow nflow tests, low CHF values were measured because of liquid blockage b y vapor counterflow and vapor stagnation in the channel. At the horizo ntal orientation with downward-facing chips, vapor/liquid stratificati on also yielded low CHF values. Previously derived correlations for wa ter and long, continuous heaters had limited success in predicting CHF for the present discontinuous heater configuration. Because orientati on has a profound effect on the hydrodynamics of two-phase flow and, c onsequently, on CHF for small inlet velocities, downflow angles should be avoided, or when other constraints force the usage of downflow ang les, the inlet liquid velocity should be sufficiently large.