Experimental Study on Gas Cooling Heat Transfer for Supercritical CO₂ in Microchannels

The conventional refrigerants have considerable ozone depleting effect (CFC/HCFC) and global warming impact (HFC). Carbon Dioxide (CO₂) is being investigated as an alternative refrigerant for vapor compression systems. In addition to its environmental benefits, Carbon Dioxide offers certain attractive thermal characteristics such as small surface tension, small liquid viscosity and large refrigerant capacity. Furthermore, when used with micro channels CO₂ heat exchangers provide additional advantage of high compaction, low weight/low volume, while yielding excellent thermal performance. The objective of the present work was to study the heat transfer and pressure drop characteristics of supercritical CO₂ gas cooling process in microchannels. A 10 ports microchannels tube with ID of 0.79mm was tested for the pressure range of 8 to 10MPa and mass flux range of 300 to 1200 kg/m² s. As expected, mass flux has a significant influence both on the supercritical heat transfer and pressure drop coefficients. Pseudo-critical temperature (temperature at which the specific heat has maximum value for the given pressure) is found to play an important role in the CO₂ heat exchanging process as well. Conventional forced convection heat transfer correlations fail to accurately predict the heat transfer coefficients of supercritical CO₂ with deviations as much as 50% from experimental data, especially near pseudo-critical temperature. As the gas cooling pressure increases, the pressure drop decreases, which is due to the lower viscosity & higher density. Employing average specific heat along the entire tube length, a semi-empirical correlation was developed to predict the supercritical gas cooling process of CO₂ in microchannels, within an error of 20%.