Experimental investigation of supercritical carbon dioxide in horizontal microchannels with non-uniform heat flux boundary conditions

Supercritical carbon dioxide (sCO₂) experiences a drastic change in its thermophysical properties near the thermodynamic critical point. A non-linear thermophysical property variation can influence the heat transfer behavior of sCO₂ which is not predicted well by conventional single phase heat transfer theory. This can become a major hindrance in the effective design of heat exchangers using sCO₂ as a heat transfer fluid and operating in the vicinity of the critical point. Previous investigations of sCO₂ heating have been primarily focused on macroscale, circular and uniformly heated channels at relatively low heat fluxes. It is unclear if models and correlations developed from large circular tube data can be scaled down to the microscale, non-circular channels subject to non-uniform heating. The present study experimentally investigates the turbulent heat transfer performance of sCO₂ in a microchannel heat exchanger operating in a horizontal configuration with a single wall non-uniform heat flux boundary condition. The test section has five parallel channels with a 0.75 mm hydraulic diameter and an aspect ratio of 1. The channels are fabricated using computer numerical control machining and the test section sealed using a diffusion bonding approach. Data analysis techniques which employ 2-D and 3-D heat transfer models of the experimental test section are developed to calculate the average heat transfer coefficients for a given set of experimental conditions. Data are obtained over a wide range of experimental parameters including test section applied heat flux (20⩽q^″⩽40 W cm⁻²), mass flux (500⩽G⩽1000 kg m⁻² s⁻¹), reduced pressure (1.03⩽P_R⩽1.1), and inlet temperatures (16⩽T_in⩽50 °C). The heat transfer data were screened for the presence of buoyancy and flow acceleration effects and then compared against correlations developed for turbulent subcritical and supercritical fluid flows.