Abstract
Convective heat transfer of CO2 flows near the critical condition in a 300 μm hydraulic diameter microchannel was visualized and measured. Flow patterns of the gas, liquid, and supercritical phases are presented and discussed. An experimental rig and a microfluidic device were designed and constructed to enable precise control and measurements of temperature, pressure, and mass flux. Heaters and resistive temperature detectors (RTDs) were formed on the microchannel wall to provide heating power and to measure local surface temperature. Flow patterns for the near-critical condition of CO2 were visualized, and local heat transfer coefficient (HTC) of 4.5, 6.1, and 150 kW/m2K were measured, for gas supercritical and liquid cases, respectively. It was observed that there is a distinct difference in flow and heat transfer patterns of different phases near the critical point, which leads to the deviation of the HTC. Also, CO2 presents high HTC coefficients for supercritical and boiling conditions that make it a viable option as thermal fluid for different micro scale applications.