These cooling glitches have a velocity-dependent characteristic time, which manifest itself as a broad and energetic peak in the spectrum of heat transfer time series, in the kHz range. Third, the heat transfer time series exhibit highly conductive short-lived events. Surprisingly, the prefactor to this dependence is maximum for an intermediate superfluid fraction or temperature (around 2 K). Second, the velocity dependence of the mean heat transfer is compatible with the square-root dependence observed in classical fluids. First, at the largest superfluid fraction (71%), a new heat transfer regime appears at non-null velocities and it is typically 10 % less conductive than at zero velocity. In contrast, some velocity dependence emerges at larger heat flux, as reported previously, and three nontrivial properties of heat transfer are identified. At low heat fluxes, no velocity dependence is observed, in agreement with expectations. The fluid is He 4 helium with a superfluid fraction varied from 71% down to 0% and an imposed velocity up to 3 m / s, while the characteristic sizes of heaters range from 1.3 μ m up to a few hundreds of microns. Miniature heaters are immersed in flows of quantum fluid and the efficiency of heat transfer is monitored versus velocity, superfluid fraction, and time.
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March 2023
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