Transient heat transfer in channel of liquid or supercritical helium

Abstract

Thin-film temperature sensors fabricated on alumina chips have been used to study heat transfer across a rectangular channel of liquid or supercritical helium. Step input heating power is applied to a sensor embedded in the bottom channel surface. The temperature response is also monitored at the top of the channel by a purely passive sensor. Temperature traces obtained for varying power inputs, helium pressures (1.0–3.0 bar) and channel heights (0.2 and 0.4mm) display the evolution of steady state conditions. Steady state behaviour suggests that at low powers (in the nucleate boiling regime) only the sensor surface (≈9% of the whole chip face) is heated, and at high powers (in the film boiling regime) the entire chip face is heated; intermediate powers produce large thermal fluctuations, probably corresponding to fluctuations in the extension of the helium film. It is conjectured that the redistribution of heat flow during the onset of film boiling accounts for the comparatively gradual ‘take-off’ observed in the heat pulse temperature responses. Temperature fluctuations are reduced at higher pressures. The time elapsed until the passive sensor temperature begins to rise is comparable to the time required to evaporate enough helium to fill the volume between the sensors. Reducing the channel gap reduces this response time. Take-off times on the active sensor are generally too short for appreciable confinement effects to be observed.