Vibration-induced thermal instabilities in supercritical fluids in the absence of gravity

Abstract

Supercritical fluids (SCFs) are known to exhibit anomalous behaviour in their thermophysical properties such as diverging compressibility and vanishing thermal diffusivity on approaching the critical point. This behaviour leads to a strong thermo-mechanical coupling when SCFs are subjected to simultaneous thermal perturbation and mechanical vibration. The behaviour of the thermal boundary layer (TBL) leads to various interesting dynamics such as thermo-vibrational instabilities, which become particularly ostensive in the absence of gravity. In the present work, two types of instabilities, Rayleigh-vibrational and parametric instabilities, have been numerically investigated under zero-gravity in a 2D configuration using a mathematical model wherein density is calculated directly from continuity equation. Comparison of experimental observations with numerical simulations is also presented. The peculiarity of the model warrants instabilities to be investigated in a more stringent manner (in terms of higher quench percentage and closer proximity to the critical point), unlike the previous studies wherein the equation of state was linearized around the considered state for the calculation of density, resulting in a less precise analysis. In addition to providing physical explanation causing these instabilities, the effect of various parameters on the critical amplitude for the onset of these instabilities is analysed. Furthermore, various attributes such as wavelength of the instabilities, their behaviour under various factors (quench percentage and acceleration) and the effect of cell 2 size on the critical amplitude is also investigated. Finally, a 3D stability plot is shown describing the type of instability (Rayleigh-vibrational or parametric or both) to be expected for the operating condition in terms of amplitude, frequency and quench percentage for a given proximity to the critical point.

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