Investigation of heat transfer to near- and super-critical fluids.

摘要

Heat transfer to near- or super-critical fluids is investigated through experimental and computational studies. Flow visualization by means of shadowgraph and light scattering is conducted to study the transport phenomena of mixed convection in a channel flow. The flow is shown to be time-dependent, three dimensional, and with vortex structures. The system is natural-convection dominated. Critical opalescence over the entire cell is observed at conditions in the near-critical regime. Temperature measurements of the fluid show a thin thermal boundary layer along the heated surface; the bulk flow temperature above this layer is nearly uniform. The experimental observation suggests that the effects of buoyancy cannot be neglected in the near-critical heat transfer in the near-wall region.; Laminar boundary-layer flow calculations with the wall temperature ranging from subcritical to supercritical temperatures yield similarity solutions when the buoyant acceleration is not considered, and the prescribed profiles are specified at the inlet. The results suggest that the near-critical property variations do not alter the flow characteristics if a prescribed profile is used as the initial condition. Calculations with uniform flow at the inlet show oscillations in the wall shear stress and wall heat flux near the inlet and do not yield similarity solutions. The oscillations are not observed when an elliptic equation solver is used. The axial diffusion is important in the flow developing region. Reference temperatures are determined for use with the conventional heat-transfer correlations established for constant-property flows. The results of wall shear stress and wall heat flux show that the constant-property correlations are appropriate for the subcritical regime and the supercritical regime of {dollar}rm Tsb{lcub}w{rcub}/Tsb{lcub}c{rcub}>1.6.{dollar} For wall temperatures over the range of {dollar}rm 1.1