Current literature suggests that large spatial gradients of thermophysical properties, which occur in the vicinity of the pseudo-critical thermodynamic state, may result in significant variations in forced-convection heat transfer rates. Specifically, these property gradients induce inertia- and buoyancy-driven flow phenomena that may enhance or deteriorate the turbulence-dominated heat convection process. Understanding of these inertia/buoyancy-driven mechanisms has not been sufficiently established to date. Consequently, the full set of dynamic similarity parameters remains to be identified. Through direct numerical simulations of turbulent boundary layers and channel flows, the present study investigates the characteristics of the flow structures of turbulence in heated flows of supercritical water under buoyant and non-buoyant conditions.
In the absence of buoyancy forces, notable reductions in the density and viscosity in close proximity of the heated wall are observed to promote an increase in the wall shear stress, with resultant loss of coherence of the new near-wall flow structures. This leads to the dominance of larger-scale structures in the wall-normal thermal mixing process that comes at the expense of the smaller-scale thermal mixing, and yields a net reduction in the overall thermal mixing.
Under the influence of wall-normal gravitational acceleration, the wall-normal density gradients are noted to enhance ejection motions due to baroclinic vorticity generation on the lower wall of the channel, thus providing additional wall-normal thermal mixing. Along the upper wall, the same mechanism generates streamwise vorticity of the opposing sense, causing a net reduction in thermal mixing. In the case of downstream-oriented gravitational acceleration, baroclinic vorticity generation driven by spanwise density gradients causes additional wall-normal thermal mixing by promoting larger-scale ejection and sweep motions.
Based on the results of the foregoing direct numerical simulations and complemented by relevant information in the published literature, dynamic similarity criteria for heated flows of supercritical fluids are proposed. Particular consideration is given to the influence of spatial gradients in density in the form of baroclinic forces on such flows. The proposed similarity criteria are successfully validated using data from multiple working fluids, and are used as a basis for a new criterion for the onset of deteriorated heat transfer.