Simulation of a bluff-body-stabilied diffusion flame using second-moment closure and monte carlo methods

Abstract

A bluff-body-stabilized methane diffusion flame is studied using second-moment closure and Monte Carbo methods. A finite-volume method (FVM) is used to solve the Favre-averaged transport equations for mass and momentum using an assumed-shape probability density function (PDF) to model turbulent scalar fluctuations. A hybrid Monte Carlo method is used to solve for the Favre joint velocity-scalar PDF. Both methods are two-dimensional fully elliptic. The implications of second-mement closures and stochastic Langevin models for the predictions of Reynolds stresses, mean flow, and scalar fields are discussed. In the FVM, turbulence is modeled by the k-e model, Rotta's return-to-isotropy model (Rotta), and isotropization of production model (Basic Model). In the Monte Carlo method, the simplified Langevin model (SLM) and teneralized Langevin model (GLM) are used. Micromixing is modeled by the interaction by exchange with the mean (IEM) model and chemistry is modeled by a constrained equilibriumrium model. A bluff-body-stabilized flame was chosen as a test because the flow pattern exhibits complex phenomena such as recirculation regions and stagnation points. For the hybrid Monte Carlo method, consistency between the FVM and Monte Carlo submodels is checked for an inert and a reacting case. The combination SLM/k-e yields an inconsistent treatment of Reynolds stresses. Consistent results are obtained with the hybrid models SLM/Rotta and GLM/Basic Model. For the reacting flow, scalar transport differs substantially between the FVM and Lagrangian formulation because, in the Monte Carlo submodel, the flow and density fields are not coupled directly. Because the FVM submodel is sensitive to density variations, the hybrid method converges slowly. FVM predictions are compared to experimental data. The Basic Model predictions are superior to standard k-e results. The overall flame predictions are poor because of finite-rate kinetic effects and partial premixing that are not captured, by our conserved-scalar chemistry model.