Effects of buoyancy and thermophysical property variations on the flow of supercritical carbon dioxide

Abstract The flow and heat transfer behaviours of fluids at supercritical pressure have been studied using direct numerical simulations (DNS), in which one or more thermal properties are artificially frozen to discern the various physical mechanisms from each other so as to better understand the complex phenomena. Different from previous similar studies on this topic, this study focuses on the axial flow development resulted from the large variations of thermophysical properties. The contribution of the flow inertia has been quantified by analysing the momentum balance for each case studied, which has been found to be significant throughout the entire length of the pipe in cases when buoyancy is considered. The effect of the inertia on momentum in turn impacts on turbulence production, generally delaying flow laminarisation. Such an influence of flow development is non-trivial and cannot be omitted in flow analysis and heat transfer calculations. This suggests that the results of simplified analyses based on a spatially developed flow cannot be directly applied to such flows despite they can be very useful in developing fundamental understanding of the physics. Similarly, this also explains that in some cases, buoyancy parameters based on local flow quantities cannot describe heat transfer deterioration accurately. The effect of variable viscosity alone can cause turbulence reduction by flattening the velocity profile, but it will not turn the velocity profile to an M-shape, which can only be achieved by buoyancy.