COMPARATIVE ANALYSIS OF THERMAL AND HYDRAULIC PERFORMANCE OF A MINI-CHANNEL HEAT SINK WITH SUPERCRITICAL CARBON DIOXIDE AND WATER COOLANTS

This study investigates the heat transfer performance of supercritical carbon dioxide (s-CO2) in comparison to water as a coolant within circular tubes. The study was conducted numerically using the commercially available software FLUENT. The study was conducted for the s-CO2 for the temperature range of 305 K to 316 K at 8 MPa covering the pseudocritical temperature for the CO2. The thermal and hydraulic performance, including heat transfer coefficient and pressure drop, under varying inlet temperature, mass flux, and heat flux conditions, were evaluated and compared with those of water. The study finds that for s-CO2, the heat transfer coefficient reaches its peak value at the pseudocritical temperature, which is around 307.9K at an operating pressure of 8 MPa. The bulk fluid temperature ranging from 306 K to 308 K offers better thermal and hydraulic performance for s-CO2, with a higher heat transfer coefficient and lower pressure drop than water. However, beyond this temperature range, both the heat transfer coefficient and pressure drop of s-CO2 become worse than water. Furthermore, the study observes that the heat transfer coefficient for s-CO2 increases with mass flux, similar to water. However, the slope is steeper than water, and it becomes steepest at an inlet temperature of 308K. An interesting trend for s-CO2 is noted, as the heat transfer coefficient decreases with an increase in heat flux beyond the peak temperature (the temperature at which the maximum heat flux was observed). Conversely, the reverse scenario has been observed prior to the peak temperature. Water, on the other hand, does not show any significant change in this regard. The thermophysical properties of s-CO2, including specific heat capacity, density, thermal conductivity, and viscosity, around its pseudocritical temperature at 308 K (at 8 MPa) are critical for interpreting these results. The study identifies the optimum conditions at which s-CO2 proves to be better than conventional coolants such as water.