Impact of adiabatic entrance length on numerical simulations of supercritical CO2 heat transfer in horizontal circular tubes

Abstract

Horizontal supercritical carbon dioxide (sCO2) pipe-flow studies in the heat transfer literature commonly use a fixed-length adiabatic hydraulic development section prior to the heat transfer region. It is expected that the length of the adiabatic entrance region is long enough to generate hydrodynamically fully developed flow, such that the influence of the entrance is eliminated. However, despite the criteria for subcritical flows being well established, the criteria defining the adiabatic entrance length for sCO2 flows are not as clear. In this paper, 6-mm-diameter round tubes with heat-flux-to-mass-flux ratios (q/G) equal to 60 J/kg are considered. The tubes have adiabatic entrance lengths between 0 and 1000 mm (0 and 167D), followed by a 500 mm (83D) heat transfer cooling section. An assessment of the effect of adiabatic entrance lengths on flow development and heat transfer behavior is given. An analysis of several flow features such as centerline pressure, centerline velocity, axial velocity profile, wall shear stress, and local heat transfer coefficient are presented. Based on findings for a 6-mm-diameter tube with cooling heat flux boundary conditions and a q/G ratio equal to 60 J/kg, it has been determined that a critical adiabatic length, which is the minimum entrance length necessary to achieve hydrodynamically fully developed flow, of greater than 60D is required.