Investigation of Mixing Models and Finite Volume Conditional Moment Closure Applied to Autoignition of Hydrogen Jets
Buckrell, Andrew James Michael
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In the present work, the processes of steady combustion and autoignition of hydrogen are investigated using the Conditional Moment Closure (CMC) model with a Reynolds Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) code. A study of the effects on the flowfield of changing turbulence model constants, specifically the turbulent Schmidt number, Sct, and C epsilon 1 of the k − epsilon model, are investigated. The effects of two different mixing models are explored: the AMC model, which is commonly used in CMC implementations, and a model based on the assumption of inhomogeneous turbulence. The background equations required for implementation of the CMC model are presented, and all relevant closures are discussed. The numerical implementation of the CMC model, in addition to other techniques aimed at reducing computational expense of the CMC calculations, are provided. The CMC equation is discretised using finite volume (FV) method. The CFD and CMC calculations are fully coupled, allowing for simulations of steady flames or flame development after the occurrence of autoignition. Through testing of a steady jet flame, it is observed that the flowfield calculations follow typical k − epsilon model trends, with an overprediction of spreading and an underprediction of penetration. The CMC calculations are observed to perform well, providing good agreement with experimental measurements. Autoignition simulations are conducted for 3 different cases of turbulence constants and 7 different coflow temperatures to determine the final effect on the steady flowfield. In comparison to the standard constants, reduction of Sct results in a reduction of the centreline mixing intensity within the flowfield and a corresponding reduction of ignition length, while reducing C 1 results in an increase of centreline mixing intensity and an increase in the ignition length. All scenarios tested result in an underprediction of ignition length in comparison to experimental results; however, good agreement with the experimental trends is achieved. At low coflow temperatures, the effects of mixing intensity within the flowfield are seen to have the largest influence on ignition length, while at high coflow temperatures, the chemical source term in the CMC equation increases in magnitude, resulting in very little difference between predictions for different sets of turbulence constants. The inhomogeneous mixing model is compared using the standard turbulence constants. A reduction of ignition lengths in comparison to the AMC model is observed. In steady state simulation of the autoigniting flow, the inhomogeneous model is observed to predict both lifted flames and fully anchored flames, depending on coflow temperature.