Numerical study on mixing of cold-water jet with supercritical bulk flow in a pipe using an improved algorithm

Numerical instability and inaccuracy usually occur when using existing pressure-based algorithms for low Mach number compressible flow to study the supercritical buoyant flow with large temperature gradient due to the drastic thermophysical property changes with temperature. In this study, an improved algorithm for low Mach number variable density flow has been implemented using the C++ open source CFD library OpenFOAM. The thermophysical property of water is directly calculated using IAPWS-97 formula and a better estimate of the physical property of water before and during the pressure-velocity decoupling process enhances the algorithm for low Mach number variable-density flow to allow for large density ratio in the flow field. The improved algorithm shows better numerical stability for a coarse initial field and significantly improved resolutions of flow perturbations. Numerical study on cold water mixing with supercritical water (SCW) indicate that cold water jet "pulse" into the bulk SCW with large temperature fluctuation. Three main factors affecting the mixing characteristics including Reynolds numbers of cold-water jet and bulk SCW flow, direction of the gravity and temperature difference between the cold-water jet and bulk SCW have also been investigated in this study.