Geometry and orientation effects in non-uniformly heated microchannel heat exchangers using supercritical carbon dioxide

There is growing interest to use supercritical carbon dioxide (sCO₂) as a working fluid in thermal management applications. This study investigates the thermal-hydraulic performance of microchannel heat sinks as a function of flow channel geometry and orientation at operating conditions representative of electronics cooling applications. Three different experimental test sections, subject to non-uniform heat flux boundary conditions, were investigated. Two of the test sections contained parallel arrays of rectangular microchannels with a hydraulic diameter of 750 μm and aspect ratios of 1 and 2, respectively. The third test section had a staggered array of diamond shaped micro-pins with a hydraulic diameter, based on the minimum flow area, of 525.2 μm. Data were collected for varying inlet temperature (16⩽T_in⩽50 °C), mass flux (315⩽G⩽1000 kg m⁻² s⁻¹), and heat flux (20⩽q^″⩽50 W cm⁻²) at a fixed reduced pressure (P_R) of 1.1. A data analysis method using 2-D and 3-D heat transfer models of the test sections was used to calculate the average heat transfer coefficients for each experimental condition. Additionally, a pressure drop model was developed to resolve the total measured pressure drop into its components. The results of this study indicate that the turbulent convective heat transfer was independent of orientation (top versus bottom heating) for square microchannel (aspect ratio = 1) for the conditions investigated. Increasing the channel aspect ratio from 1 to 2 led to an enhancement in thermal transport. Finally, the heat transfer performance of the staggered pin array flow geometry was superior to the rectangular channels, but this enhancement in heat transfer was commensurate with the increase in pressure drop. Based on these results, this paper concludes with general design recommendations for those considering the early adoption of supercritical carbon dioxide for thermal management applications.