NUMERICAL SIMULATION OF SUPERCRITICAL PRESSURE FLUIDS WITH VARIABLE TURBULENT PRANDTL NUMBER AND MODIFIED DAMPING FUNCTION (1/2)

摘要

In most numerical simulations of a fluid flow with constant properties or negligible variations, the value of Pr, has been considered to be unity or close to it. However, numerical attempts with a constant Pr, have never been successful in estimating the wall temperature in highly-buoyant supercritical fluids through vertical tubes. Several experimental data and numerical studies have indicated that Pr, can be very smaller or larger than unity in a region of severe property variation. Recent research, both numerical and experimental, indicate that Pr, is very likely a function of fluid-thermal variables, when the gradients of the physical properties of the fluid are significant. In this regard, a new concept of a variable turbulent Pr, according to the property variation was developed. Another point to be considered is that the turbulent boundary layer (TBL) deforms so severely that the damping function included in the turbulence modeling is no longer a fixed function of the turbulence properties. When a velocity overshoot (or peak) appears in or near the TBL, a new TBL may develop between the point of the velocity peak and the wall. Accordingly, the damping function in the new TBL will be different from that for the TBL without a velocity peak. A new formulation for the damping function, actually the thickness of the viscous sublayer, A~+, was introduced to accommodate the TBL deformation. The new formulations for Pr, and A~+ were tested in the numerical simulation, and the results agreed well with the experimental data.