Mixed Convection at Supercritical Pressures

FluidsDraskic, M.Westerweel, J.Pecnik, R. display sharp, non-linear variations of thermodynamic properties when they are heated at a supercritical pressure. As such, near-pseudo-critical heat transfer is often characterized by large variations in density, leading to sharp near-wall accelerations or strong stratifications when buoyancy becomes dominant. We study the modulation of heat transfer and turbulence by non-negligible buoyancy in such property-variant flows, for the development of near-pseudo-critical heat exchangers for supercritical energy conversion systems. In particular, a liquid-like, horizontal base flow of carbon dioxide at 88.5 bar and 32.6 ^∘C is considered, which is subjected to a vertical heat flux of up to 12.0 kW/m² at Reynolds numbers of up to Re_Dh≤10.000. Here, optical- and surface temperature measurements are used concurrently to evaluate the flow. Integratced visualizations of the flow field show the onset of strong stratifications with limited heating rates in the near-pseudo-critical region. During unstable stratification, the channel flow is dominated by the upward motion of thermal plumes. When the stratification is stable, any vertical motion and turbulence present in an equivalent neutrally buoyant flow is suppressed. As a result, wall heat is removed more effectively in the unstably stratified configuration than in a forced convective flow, whereas the opposite is true for a stably stratified flow. The difference in the perceived heat transfer between the considered configurations increases as buoyancy becomes more dominant.