Mechanical Engineering
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Item Numerical Simulation of Low-Pressure Explosive Combustion in Compartment Fires(2008-11-19) Hu, Zhixin (Victor); Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A filtered progress variable approach is adopted for large eddy simulations (LES) of turbulent deflagrations. The deflagration model is coupled with a non-premixed combustion model, either an equilibrium-chemistry, mixture-fraction based model, or an eddy dissipation model. The coupling interface uses a LES-resolved flame index formulation and provides partially-premixed combustion (PPC) modeling capability. The PPC sub-model is implemented into the Fire Dynamic Simulator (FDS) developed by the National Institute of Standards and Technology, which is then applied to the study of explosive combustion in confined fuel vapor clouds. Current limitations of the PPC model are identified first in two separate series of simulations: 1) a series of simulation corresponding to laminar flame propagation across homogeneous mixtures in open or closed tunnel-like configurations; and 2) a grid refinement study corresponding to laminar flame propagation across a vertically-stratified layer. An experimental database previously developed by FM Global Research, featuring controlled ignition followed by explosive combustion in an enclosure filled with vertically-stratified mixtures of propane in air, is used as a test configuration for model validation. Sealed and vented configurations are both considered, with and without obstacles in the chamber. These pressurized combustion cases present a particular challenge to the bulk pressure algorithm in FDS, which has robustness, accuracy and stability issues, in particular in vented configurations. Two modified bulk pressure models are proposed and evaluated by comparison between measured and simulated pressure data in the Factory Mutual Global (FMG) test configuration. The first model is based on a modified bulk pressure algorithm and uses a simplified expression for pressure valid in a vented compartment under quasi-steady conditions. The second model is based on solving an ordinary differential equation for bulk pressure (including a relaxation term proposed to stabilize possible Helmholtz oscillations) and modified vent flow velocity boundary conditions that are made bulk-pressure-sensitive. Comparisons with experiments are encouraging and demonstrate the potential of the new modeling capability for simulations of low pressure explosions in stratified fuel vapor clouds.Item NUMERICAL MODELING OF MULTIPHASE EXPLOSIONS(2008-11-17) McGrath, Thomas; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work describes the development and application of a compressible multiphase flow model for the numerical simulation of multiphase explosions containing a dispersed particle phase. The model treats all phases as fully compressible, allows full non-equilibrium among phases, and properly models the mathematical characteristics of a dispersed particle phase in both the dense and dilute limits. Using the characteristic equations, a multiphase Riemann solver is developed as the basis for a Godunov-based numerical method. The Riemann solver is approximate, non-iterative, and applicable to all phases. A heuristic equation of state modeling the functional dependence of the dispersed phase pressure on volume fraction is proposed and applied. Using the techniques developed, two multiphase explosion simulations are performed and compared with experiment. Excellent agreement between the numerical and experimental results is found, providing confidence in the solution techniques developed. The sensitivity of the model to correlations for drag, heat transfer, and dispersed phase pressure are also investigated. Results from this analysis indicate that the functional dependence of dispersed phase pressure on volume fraction must be properly represented to obtain accurate simulation results in scenarios where particle-particle interactions are important. Further analyses investigate the effects of physical parameters including particle loading, size, and material on multiphase explosion dynamics. The results of this study indicate the significant effect these parameters have on the overall explosion dynamics, which is important to applications involving both inert and reactive particles.