The effects of wall heat fluxes and tube diameters on laminar heat transfer rates to supercritical CO2

Abstract

Nusselt number (Nu) correlations associated with constant property fluids have been deemed to be invalid when quantifying heat transfer rates to supercritical CO2 (sCO2) in the vicinity of its pseudocritical temperature (TPC). For laminar sCO2 flow, under constant heat flux conditions, previous studies have demonstrated a sudden deterioration in heat transfer near TPC, followed by a recovery phase where Nu approaches a value greater than 4.36 (associated with fully developed flow conditions for constant property fluids). Through numerical simulations of sCO2 heat transfer in horizontal circular tubes of diameters (D) from 0.2 mm to 2 mm, subject to constant wall heat fluxes (Q) in the range 0.25 – 25 kW/m2 and for inlet Reynolds (Re) numbers of 40, 100 and 400, this study demonstrates for the first time that the extent of the heat transfer deterioration and its subsequent recovery is critically dependent on the magnitude of the imposed heat flux. However, the Nu variations were independent of inlet Re across the different simulation scenarios as long as the product QD was held constant. Therefore, a modified Nu correlation in terms of thermophysical properties evaluated at both the bulk (TBULK) and wall temperatures (TWALL) is proposed to represent the observed variations.