Investigation of an Inhomogeneous Mixing Model for Conditional Moment Closure Applied to Autoignition

Autoignition;CMC;Mechanical Engineering

Milford, Adrian

University of Waterloo

关键词: Autoignition;    CMC;    Mechanical Engineering;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/5102/1/Milford_Adrian.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

Autoignition of high pressure methane jets at engine relveant conditions within a shock tubeis investigated using Conditional Moment Closure (CMC). The impact of two commonlyused approximations applied in previous work is examined, the assumption of homogeneousturbulence in the closure of the micro-mixing term and the assumption of negligible radialvariation of terms within the CMC equations. In the present work two formulations ofan inhomogeneous mixing model are implemented, both utilizing the β -PDF, but differingin the respective conditional velocity closure that is applied. The common linear modelfor conditional velocity is considered, in addition to the gradient diffusion model. Thevalidity of cross-stream averaging the CMC equations is examined by comparing resultsfrom two-dimensional (axial and radial) solution of the CMC equations with cross-streamaveraged results.The CMC equations are presented and all terms requiring closure are discussed. So-lution of the CMC equations is decoupled from the flow field solution using the frozenmixing assumption. Detailed chemical kinetics are implemented. The CMC equations arediscretized using finite differences and solved using a fractional step method. To maintainconsistency between the mixing model and the mixture fraction variance equation, thescalar dissipation rate from both implementations of the inhomogeneous model are scaled.The autoignition results for five air temperatures are compared with results obtained usinghomogeneous mixing models and experimental data.The gradient diffusion conditional velocity model is shown to produce diverging be-haviour in low probability regions. The corresponding profiles of conditional scalar dis-sipation rate are negatively impacted with the use of the gradient model, as unphysicalbehaviour at lean mixtures within the core of the fuel jet is observed. The predictions ofignition delay and location from the Inhomogeneous-Linear model are very close to thehomogeneous mixing model results. The Inhomogeneous-Gradient model yields longer ig-nition delays and ignition locations further downstream. This is influenced by the higherscalar dissipation rates at lean mixtures resulting from the divergent behaviour of thegradient conditional velocity model. The ignition delays obtained by solving the CMCequations in two dimensions are in excellent agreement with the cross-stream averagedvalues, but the ignition locations are predicted closer to the injector.