Experimental Investigation of Orientation Effects on Pseudocritical Transition Dynamics in Heated Microchannel With Side-View Schlieren Imaging

Abstract The heat transfer characteristics of carbon dioxide flow in microchannels in close vicinity of the critical point have drawn the interest of researchers in recent years as a promising approach to achieve high heat flux (500 W cm−2) thermal management. Enhancements to the supercritical convective heat transfer coefficients are observed to accompany the pseudocritical transition due to the significant thermophysical property variations around the critical point. In addition, CO2 is a dielectric fluid with a low global warming potential (GWP), further increasing the appeal for thermal management applications. Supercritical fluid convection heat transfer in microchannel configurations relevant to thermal management requires further study. Specifically, there is a limited understanding of how density gradients induced by the pseudocritical transition influence the heat transfer in microscale flows and what role orientation with respect to gravity plays. The present study developed a microchannel (Dh = 500 μm) test article to allow heat transfer measurements and side-view schlieren imaging of carbon dioxide. High-speed visualization of density gradients from schlieren imaging and heat transfer characteristics for non-uniform boundary heating to a single channel wall will be discussed as a function of orientation. Schlieren images reveal the development of a region influenced by pseudocritical transition near the heated wall that grows with increased heat flux or decreased mass flux. The observed density fluctuations that comprise this region are strong functions of experimental conditions and may aid in generating a predictive capability for supercritical heat transfer. The insights derived from this study are ultimately aimed at developing a mechanistic understanding of near-critical flows in microchannels and are crucial to assessing the potential for applications of electronic thermal management.