Wy Y. Lin

Presumed joint probability density function model for turbulent combustion

Abstract A presumed joint probability density function (pdf) model of turbulent combustion is proposed in this paper. The turbulent fluctuations of reactant concentrations and temperature are described using a presumed joint pdf of three-dimensional Gaussian distribution based on first and second-order moments of reactant concentration and temperature. Mean reaction rates in both premixed and diffusion combustion are obtained by mean of integration under the presumed joint pdf. This model is applied to predict turbulent premixed combustion of sudden-expansion flow and turbulent jet diffusion methane/air flame. For turbulent premixed combustion, the predicted results of temperature distribution and maximum temperature using the proposed model agree better with the experiment than that using the conventional eddy-breakup (EBU)–Arrhenius model. For the turbulent jet diffusion methane/air flame, the predicted results of velocity, temperature and species concentrations using the proposed model, the Arrhenius, EBU–Arrhenius, and laminar flamelet models are compared with experiment data. Results obtained with the presumed pdf model and that obtained by the laminar flamelet model both agree well with experiments, while results using the other models have a significant difference. The presumed joint pdf model is used to predict the NO formation process, which also agrees well with the experiment data. A unified turbulent combustion model, in which both effects of turbulent diffusion and chemical dynamics are considered, is established for both premixed and diffusion combustion, especially for the process of NO formation.

Structures of scalar transport in 2D transitional jet diffusion flames by LES

Abstract In this paper, large eddy simulation of a two-dimensional spatially developing transitional free methane diffusion jet at moderate Reynolds number is performed. The solver of the governing equations is built based on a projection method and time integration is carried out using a second-order Adams–Bashforth scheme. A dynamic eddy viscosity model is utilized for the turbulent subgrid scale terms and a similar dynamic method is applied for modeling the filtered reaction rate. The direct solver for pressure correction Poisson equation is based on the Buneman variant of cyclic odd–even reduction algorithm. A reduced four-step chemical kinetic mechanism is applied for the simulation of methane combustion. Ignition process is well described by the simulation. Detailed description of transient vortical structures in the entire flow field is given along with transient vortex–flame interactions. The development of a diffusion jet flame is found to involve two distinct phases of “turbulence dominated” and “reaction dominated” respectively. The “turbulence dominated” phase exists only for a very short time at the initial stage of the flame.

Large eddy simulation of coherent structures in rectangular methane non-premixed flame

Large eddy simulation of a three-dimensional spatially developing transitional free methane non-premixed flame is performed. The solver of the governing equations is based upon a projection method. The Smagorinsky model is utilized for the turbulent subgrid scale terms. A global reaction mechanism is applied for the simulation of methane/air combustion. Simulation results clearly illustrate the coherent structure of the rectangular non-premixed flame, consisting of three distinct zones in the near field. Periodic characteristics of the coherent structures in the rectangular non-premixed flame are discussed. The predicted structure of the flame is in good agreement with the experimental results. Distributions of species concentrations across the flame surfaces are illustrated and typical flame structures in the far field are analyzed. Local mass fraction analysis and flow visualization indicate that the black spots of the flames are due to strong entrainment of oxygen into the central jet by streamwise vortices, and breaking up of the flame is caused by an enormous amount of entrainment of streamwise vortices as well as stretching of spanwise vortices at the bottom of the flame.

Large Eddy Simulation of Heavy Gas Dispersion around an Obstacle

Trials of heavy gas dispersion at Thorney Island were simulated by large eddy simulation (LES). By comparing with experimental data, reasonable model constants of C s and ScT in sub-grid scale models are determined for simulating the hazardous gas dispersion process. Computational results are in good agreement with experimental data indicating that LES provide a reliable means of estimating gas dispersion in real terrains. Dispersion processes of hazardous gases with different density are studied, the results showed that the density have remarkable effects on the hazardous gas dispersion process.