Computational appraisal of the thermalhydraulic characteristics of supercritical carbon dioxide in heated minichannel for HVAC applications

Abstract

Over last couple of decades, supercritical fluid has found enhanced applications in numerous commercial sectors, encompassing aerospace, chemical, fast reactors, fusion rectors and renewable energy. Supercritical fluid is of major interest to the thermal engineers, primarily owing to its superior heat transport characteristics around the pseudocritical temperature. Supercritical carbon dioxide has particularly been identified as the next-generation coolant in power industry, due to its low critical temperature and reasonable critical pressure. The heating, ventilation and air-conditioning sector has also seen increasing popularity of supercritical CO2 with the phasing out of the conventional halocarbons, as it has zero ozone depletion potential and substantially smaller global warming potential. While decent volume of literature is available regarding the application of supercritical CO2 in macrochannel, thermalhydraulics of supercritical CO2 in minichannel lacks comprehensive investigation, and the present work attempts to fill that specific void through a computational appraisal of a heated horizontal minichannel of 2 mm inner diameter. Systematic simulations have been performed to explore the role of the wall heat flux ranging from 30–50 kW/m2, operating pressure ranging from 8–9 MPa, inlet temperature ranging from 295–315 K and buoyancy parameter on the thermalhydraulic characteristics of supercritical CO2. The Reynolds number in the present simulation ranges from 10000-17000. This article is aimed to explore the phenomena of heat transfer deterioration in horizontal minichannel subjected to uniform wall heat flux. It is observed that increase in wall heat flux leads to lower area-averaged heat transfer coefficient The results show an early heat transfer deterioration on top half surface whereas, normal heat transfer is observed on bottom half surface. Heat flux has significant effect on heat transfer deterioration and higher heat flux leads to reduction in peak value of heat transfer coefficient. At higher inlet temperature heat transfer coefficient decreases and peak of heat transfer coefficient shifted in the upstream direction. Also, peak of heat transfer coefficient vanishes for inlet temperature higher than pseudocritical temperature due to non-significant variation in thermophysical properties at higher temperature. Stratification of temperature, velocity and density is observed along the channel due to local buoyancy produced by non-linear thermophysical property variation of supercritical CO2.