The Effects of Buoyancy on Laminar Heat Transfer Rates to Supercritical CO₂ in Vertical Upward Flows

Buoyancy effects in vertical, upward laminar flows can result in an augmentation in heat transfer rates to supercritical CO2 (sCO2) near its pseudocritical temperature (TPC). This is in contrast to corresponding flows in the turbulent regime, or laminar sCO2 flows (with minimum buoyancy effects), where a deterioration in heat transfer near TPC, followed by a recovery phase, have been observed. To exploit these sCO2 heat transfer enhancement characteristics and improve heat exchange efficiencies, the location of the TPC pinch point and the variables controlling these buoyancy effects need to be identified. To fill this void, numerical simulations of sCO2 (at inlet: 8.2 MPa, 265 K) in vertical circular tubes of diameters (D) 0.2–2 mm, heated with constant wall heat fluxes (Q) of 1–4 kW/m2) and inlet Reynolds numbers (Re) of 100, 400, were carried out. The tube lengths were varied to maintain an exit temperature of 320 K (TPC~309 K). The results indicated that buoyancy-augmented laminar heat transfer rates may be expected when Gr/Re2.7 > 10−4 (Gr = Grashof number). A modified Nusselt number correlation in terms of (Gr/Re) is proposed and is observed to fit the observed variations within a mean absolute percentage error < 15%, in most regions