Flow separation from a spherical particle in supercritical water

Flow separation from a spherical particle in supercritical water (SCW) is the basic flow structure in supercritical water fluidized bed (SCWFB). In order to study flow separation from a spherical particle in SCW in detail, a numerical model fully accounting for variations in thermo-physical properties has been developed in the pseudo-critical zone. Flow separation parameters (separation angle, length of wake vortex, width of wake vortex, and drag coefficient) for forced convection, assisting convection, and opposing convection have been obtained at intermediate Reynolds numbers. Results show that variable viscosity has a remarkable effect on flow separation, and the decreasing viscosity results in higher velocity gradient around the sphere particle surface and a larger wake vortex on the rear particle surface. A simple expression of Cd/Cd_c=(μ_w/μ_f)^0.15 is achieved to predicate the drag coefficient of the SCW flow with μ_w/μ_f between 0.7 and 1.0. Free convection inhibits the flow separation of the assisting convection, but enhances the flow separation of the opposing convection. Three flow separation zones (the rear-end separation zone, the transition zone, and the reversed flow zone) are observed for the opposing convection.