Numerical Analysis of Mixed Convection Phenomena in Heat Transfer to Supercritical Pressure Carbon Dioxide Inside a Horizontal Miniature Tube

To clarify and evaluate the influence of buoyancy force on the heat transfer to SCO2 flowing in a horizontal miniature tube, in this study, mixed convection phenomena with strong variations in thermophysical properties were numerically studied using an in-house code. The numerical model was validated by comparing the predicted wall temperature and available experimental data obtained for SCO2 at different heat fluxes, and the results showed good agreement. The effects of the heat-to-mass flux ratio on the heat transfer characteristics and heat transfer mechanism were investigated under operating conditions having mass flux, heat flux, pressure, and inlet temperature of 600-1200 kg center dot m(-2)center dot s(-1), 25-300 kW center dot m(-2), 8 MPa and 303 K, respectively. A transition in the heat transfer regime from enhanced to normal was found as the heat-to-mass flux ratio increased to 83.33 J center dot kg(-1). A pseudo-trans-critical process occurred in the tube at a heat-to-mass flux ratio above 125 J center dot kg(-1), and the heat transfer to SCO2 could be classified into three stages based on the reduced bulk fluid temperature. Particularly, the wall temperature was inversely proportional to the mass flux within the reduced bulk fluid temperature ranging of 0.992-1.0. Analysis of the heat transfer mechanism showed that the location of the large heat capacity in the boundary layer and the buoyancy force were responsible for the variation in the heat transfer characteristics. The applicability of the existing buoyancy criterion was examined, and Bu-P = 6 was found to be reasonable for predicting the onset of the influence of the buoyancy force on heat transfer.