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Non-stationary heat transfer in supercritical fluids at relatively small temporal and spatial scales was studied experimentally. The aim of the study was to clarify the peculiarities of conductive heat transfer mode at significant heat loads. An unexpected stepwise decrease in the instant heat transfer coefficient has been revealed in the course of crossing the vicinity of the critical temperature along the supercritical isobar. This means that the peaks of isobaric heat capacity and excess thermal conductivity, which are known from stationary measurements, do not affect the experimental results. It is assumed that the action of considerable gradient in temperature and the presence of heat-transfer surface in pulse heated system can serve as factors that suppress large-scale fluctuations, leading to a “smoothing” the critical enhancement of the thermophysical properties. As an important consequence, this study gives new insight into selection of the operating pressure of supercritical heat transfer agent.

Results of experimental study of non-stationary heat transfer in supercritical fluids, which were obtained using the method of controlled pulse heating of low-inertia wire probe, are discussed. The aim of this study was to clarify the peculiarities of heat conduction mode at significant heat loads. A threshold decrease in the “instant” heat transfer coefficient, the more pronounced the closer the pressure value to critical pressure, has been found, as well as the absence of impact of the isobaric heat capacity peak known from stationary measurements on the experimental results. These results give new insights into selection of the operating pressure of supercritical heat transfer agent. Small time and spatial scale in the experiments (units of millisecond and units of micrometer) in combination with high-power heat release (up to 20 MW/m2) makes it possible to associate the results with the behavior of boundary layer region of heat transfer agent.

In this paper, we present new data on non-stationary heating of a platinum wire probe, immersed in a binary partially-miscible liquid. A pressure value and a mass fraction of the a polymer in the mixture were the experimental parameters. The characteristic heating time was from 5 to 15 ms. The object of the research was the water/polypropylene glycol-425 (PPG-425) mixture having lower critical solution temperature (LCST). The position of the diffusion spinodal was estimated based on the obtained data on the liquid-liquid binodal in the framework of the Flory-Huggins approximation. An experimental technique to estimate the position of the spinodal of two-component mixtures with LCST on the scale of the mixture component ratio was developed. It was shown that the method of isobaric pulse heating can be used for this purpose. This representation is based on the threshold change in the heat transfer pattern when crossing the phase coexistence curve and the diffusion spinodal.