Numerical simulations of heat transfer phenomena with turbulent supercritical carbon dioxide flow in heated horizontal minichannels with different shapes

This study aims to clarify and evaluate the influence of buoyancy force on heat transfer to supercritical carbon dioxide flowing in horizontal minichannels. Numerical simulations are carried out on the turbulent mixed convective heat transfer of supercritical carbon dioxide in horizontal semicircular, circular, and rectangular minichannels (dh = 2 mm) for the low-pressure side of a closed Brayton system heat exchanger (p = 8 MPa, Tin = 303 K, G = 1200 kg‧m−2‧s−1, Rein = 42,521–42,860, q = 50–30 kW‧m−2). The heat transfer mechanism in different channels is analyzed and the effect of heat flux is investigated. The heat transfer in the bottom wall is stronger than that in the top wall due to the buoyancy force. The heat transfer at the corners of semicircular and rectangular channels is greatly reduced due to blockage compared to the circular channel. A transition in the heat transfer regime from enhanced to normal is found as the heat-to-mass flux ratio increases to 83.33 J·kg−1. Starting from q/G = 125 J‧kg−1, there is a significant heat transfer deterioration, and the difference between the top and bottom walls gradually becomes apparent under the influence of buoyancy. In addition, the applicability of three existing buoyancy parameters (Buc, BuJ, and BuP) is evaluated. The BuP buoyancy criterion agrees best with the simulation results. A comparative study with the case without gravity identifies a threshold value of 6.0 for BuP in the circular channel and a threshold value of 1.0 for BuP in the semicircular and rectangular channels. Above this threshold, natural convection will have a considerable effect on forced turbulent heat transfer.