Numerical investigation on turbulent mixed convective heat transfer of CO2 in a horizontal miniature tube at supercritical pressure

Abstract

Mixed convective heat transfer behaviors of turbulent supercritical CO2 (SCO2) flowing in a horizontal miniature tube (di = 2 mm) have been investigated using an in-house code. The computational model adopted was verified against experimentally determined data with a number of operating conditions. The numerical simulations were carried out for pressure of 8 MPa, inlet Reynolds number of 42,521, and wall heat fluxes ranging from 50 to 320 kW m−2. A significant influence of natural convection in horizontal forced turbulent SCO2 flow was discerned, despite the small diameter and high turbulence intensity. The mechanism of mixed convection heat transfer was studied by analyzing the cross-sectional distributions of thermal properties, eddy diffusivity of heat, and secondary flow vectors. The effect of wall heat flux on the non-uniformity in heat transfer was further investigated, covering q/G ratios ranging from 41.67 to 266.67 J kg−1. The regime of heat transfer was identified by comparing the numerical results with the reference Nusselt number calculated using the Dittus-Boelter correlation. A transition from enhanced to deteriorated heat transfer regime, as well as subsequently heat transfer recovery was found with increasing bulk fluid temperature. In addition, the applicability of three existing buoyancy parameters (Buc, BuJ, and BuP) was evaluated. The Petukhov buoyancy criteria agreed best with the simulation results. A new threshold of 6.0 for BuP was determined by performing a comparative study with cases without gravity, above which natural convection would have a considerable influence on forced turbulent heat transfer.