Supercritical fluids when subjected to simultaneous quench and vibration have been known to cause various intriguing flow phenomena and instabilities depending on the relative direction of temperature gradient and vibration. Here we describe a surprising and interesting phenomenon wherein temperature in the fluid falls below the imposed boundary value when the walls are quenched and the direction of vibration is normal to the temperature gradient. We define these regions in the fluid as sink zones, because they act like sink for heat within the fluid domain. The formation of these zones is first explained using a one-dimensional (1D) analysis with acceleration in constant direction. Subsequently, the effect of various boundary conditions and the relative direction of the temperature gradient to acceleration are analyzed, highlighting the necessary conditions for the formation of sink zones. It is found that the effect of high compressibility and the action of self-weight (due to high acceleration) causes the temperature to change in the bulk besides the usual action of piston effect. This subsequently affects the overall temperature profile thereby leading to the formation of sink zones. Though the examined 1D cases differ from the current two-dimensional (2D) cases, owing to the direction of acceleration being normal as compared to parallel in case of former, the explanations pertaining to 1D cases are judiciously utilized to elucidate the formation of sink zones in 2D supercritical fluids subjected to thermal quench and vibrational acceleration. The appearance of sink zones is found to be dependent on several factors such as proximity to the critical point and acceleration. A surface three-dimensional plot illustrating the effect of these parameters on onset time of sink zones is presented to further substantiate these arguments.

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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The phenomenon of piston effect is well known in supercritical fluids wherein the thermal homogenization of the bulk occurs on a very short time scale due to pressure change caused by expansion or contraction of the fluid in the thermal boundary layer. In this article, we highlight an interesting phenomenon wherein by the application of external forces (vibration) normal to the temperature gradient, see-saw motion of the thermal boundary layer is observed in weightlessness conditions. This is attributed to the thermomechanical coupling caused by the temperature change due to external forces. We term this change in the temperature field due to external forces as forced piston effect (FPE). A detailed investigation of this intriguing behavior shows that the see-saw motion is attributed to the variation of the relative thickness of the thermal boundary layer, defined on the basis of relative local bulk temperature, along the direction of vibration. This change in the temperature field, which is observed to be caused by FPE in vibration, is shown to depend on the compressibility (and thus proximity to the critical point), the imposed acceleration and the cell size. It is also found that see-saw motion persists in the presence of gravity and thus is described ubiquitous in nature for all conditions. A plot illustrating the maximum change in the temperature as a function of these parameters is further proposed.

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The performance of liquid gallium and water in the double layer microchannel has been analysed using three-dimensional conjugate heat transfer analysis. The effect of flow rate on the counter and parallel arrangement of each fluid is studied for three different lengths. Furthermore, cooling capability of liquid gallium and water is compared at the same length with flow rate and pumping power as governing parameters. The performance of fluid was judged on the basis of maximum temperature attained and minimal temperature variations at the heated region. Interesting results have been found showing the effect of specific heat on the type of arrangement for liquid gallium with similar observation for water for low Reynolds number and relatively longer length. Among liquid gallium and water, above certain pumping power use of liquid gallium is found to be favourable for a shorter length of the double layer microchannel. Furthermore, the range of flow rate and pumping power showing superior performance with water was found to increase with the length.

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