EXISTENCE OF SUPERCRITICAL LIQUID-LIKE STATE IN SUBCRITICAL REGION, OPTIMAL HEAT TRANSFER ENHANCEMENT, AND ARGON AS A NONREACTING, NONCORRODING SC HEAT TRANSFER FLUID

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 - liquid-like 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 CO2. 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 degrees 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 degrees 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 CO2 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 CO2 (T-c = 30.98 degrees C) and water.

Automated summary

The study reveals that the anomalous behavior of supercritical fluid extends to subcritical liquid state and shows that argon can be used as an alternative supercritical fluid for thermal transport with lower pressure and temperature requirements, as well as nonreactive and noncorrosive properties. The research also indicates that there is a significant enhancement in heat transfer at high-supercritical pressures, especially at lower temperatures.