Abstract
The effects of 10‐MHz ultrasound on heat transfer from a copper block in liquid helium were studied as a function of temperature and acoustic amplitude. The copper block was mounted in the dewar system with a PZT‐4 10‐MHz thickness‐mode resonant transducer positioned perpendicular to its surface. The temperature of the solid was determined using carbon resistance thermometers whose resistance was measured with ac bridge and lock‐in amplifier circuitry. A heater in the copper block was supplied with current to give heat fluxes in the range from 0 to 1.5 mW/cm2. Measurements were made for temperature differences up to 50 mK. In Hell for the temperature range from 1.4 to 1.6 K, the ultrasound had no effect on heat transfer from the sample for amplitudes up to the threshold for collapse cavitation. In contrast, ultrasound produces a significant increase in heat transfer in Heɪ. At a temperature of 2.64 K the effective thermal conductance increased linearly with increasing acoustic velocity amplitude reaching a value which was approximately 2.5 times that of the nonsonated values for a velocity amplitude of 0.8 cm/s. [The work was supported in part by the Air Force Office of Scientific Research.]
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