Existence of Supercritical "Liquid-Like" State in Subcritical Region, Optimal Heat Transfer Enhancement, and Argon as a Nonreacting, Noncorroding Sc Heat Transfer Fluid

Abstract

Supercritical (SC) fluid, which was discovered exactly two hundred years ago, was initially thought to have only one state with no distinction between the liquid and vapor. It is now considered to have two distinct regions --"liquidlike" and "gas-like," with thermophysical properties exhibiting anomalous behavior near the critical point (CP). This work highlights that the anomalous behavior, as observed above CP in the Widom region, extends to pressures and temperatures much below the critical pressure P c and/or critical temperature T c . Indeed, the anomalous region starts within the subcritical liquid state and extends well into the SC region. Motivated by many applications, extensive research has been devoted to thermal transport by SC water and CO 2 . However, both of these fluids are reactive and corrosive, and require high pressures (P c = 217.8 and 72.8 atm, respectively); SC water also needs high temperature (T c = 373.95°C for water). This paper presents, for the first time, argon as an alternative SC fluid for thermal transport, whose P c is lower (47.994 atm) and it can work at low temperatures (T c = -122.46°C). Argon is easily-available, nonreactive, noncorrosive, and environmentally benign. A thermal transport analysis of fully developed flow of SC Ar through an isothermally heated duct and its comparison with SC CO 2 and water demonstrates that for similar rates of heat transfer, the increase in cost of using argon is small, in terms of flow work required, whereas the benefits are substantial with regard to complexity (due to P c being much lower), operational maintenance, and life of the thermal systems. It is also revealed that 2 to 3 orders-of-magnitude enhancement in gaseous state heat transfer is possible by moving from subcritical to high-supercritical pressures; the effect being more pronounced at lower SC temperatures, even at cryogenic temperatures in contrast to SC CO 2 (T c = 30.98°C) and water.