Transient Heat Transfer in a Channel of Liquid or Supercritical Helium

Abstract

Two thin-film sensors were used to measure temperatures on opposing (horizontal) surfaces of a rectangular channel, 1.3 cm long, with channel heights of 0.2, 0.94, and 2.54 mm and at pressures of 1, 2, and 3 bar. Power densities exceeding 4 W/cm2 could be obtained in 160 ms pulses without burning out the heater sensor, producing temperature changes as high as 40 K on the active surface and 13 K on the passive surface. This paper reports results for heat fluxes up to 2 W/cm2. Data at 1 bar show the abrupt temperature “takeoff” on the heater surface associated with the transition from nucleate to film boiling; takeoff times exhibit a Q-2 behavior, varying only slightly with gap width, consistent with Schmidt’sl,2 diffusion-vaporization model. At the shorter gap widths temperatures are shifted higher, the height of the takeoff rise changes, the steady state nucleate boiling maximum heat flux decreases, and transition time to steady state behavior is greatly increased. Under 2 and 3 bar conditions the temperature rises more smoothly, and is lower at 3 bar. On the passive surface the effects of different gap widths and pressures are quite pronounced. Response is negligible for a 2.54 mm gap. Temperature rise times in a 0.2 mm gap mostly fall in a 15–30 ms range, relatively independent of heat flux. At early times the 2 bar response climbs slowly, but above a threshold in the 1.0–1.5 W/cm2 range it exhibits an abrupt transition and overtakes the more gradual 3 bar response.