Studies On Gas Turbulence Modification In Gas-Particle Flows And A Dense Two-Phase Turbulence Model

Presently developed various two-phase turbulence models cannot properly predict gas-phase fluctuation velocities, because the models simulating gas turbulence modification due to particles are still unreasonable. In dense gas-particle flows, there are interactions between particle-particle collision and particle turbulence, however, we are still lack of two-fluid models accounting for both inter-particle collision and anisotropic particle turbulence. In this dissertation, the gas turbulence modification in gas-particle flows and a two-phase turbulence model for dense gas-particle flows are studied using the experimental, theoretical and numerical methods. In experimental studies, the phase-Doppler particle anemometer (PDPA) was used to measure the gas turbulence with and without particles in an axisymmetric sudden expansion duct. The results show that in separated flows 150μm particles enhance gas turbulence in the upstream forward-flow zone and reduce gas turbulence in the reverse flow zone and downstream regions.In theoretical studies, first, a two-time-scale dissipation model of turbulence modification, including a two-time-scale dissipation model for the two-phase velocity correlation and a two-time-scale dissipation model for the dissipation rate of gas turbulent kinetic energy, is proposed. Then, a gas turbulence augmentation model accounting for the finite-size particle wake effect in the gas Reynolds stress equation is proposed. Finally, an anisotropic two-phase turbulence model for dense gas-particle flows, combining the USM model with the particle kinetic theory, is proposed.For numerical simulation, the proposed turbulence modification model is used to simulate gas-particle flows in an axisymmetric sudden expansion with and without swirl and a vertical pipe. The prediction results are in good agreement with the experimental ones. It is shown that the two-time-scale dissipation model gives obviously better predicted gas fluctuation velocities than the original single-time-scale dissipation model, and the gas turbulence modification model accounting for the particle wake effect can predict the gas turbulence enhancement. The anisotropic two-phase turbulence model for dense gas-particle flows is used to predict gas-particle flows in a horizontal pipe and a downer. Predicted particle concentration and velocity are in good agreement with the experimental results and are better than those using the model accounting for only inter-particle collision or only particle turbulence. Predicted particle Reynolds stresses are near to experimental results and more reasonable than those predicted using other models, the redistribution and dissipation of particle turbulence in different directions can be predicted.