Conditional moment closures for turbulent reacting flows
Mathematical modelling of the turbulent combustion process is becoming increasingly applied in calculations to assist in the design and analysis of practical combustion devices for efficiency-improvement and emission reduction. The current requirement to accurately predict pollutant emissions in many applications has increased the need for linking turbulent flow calculations and finite- rate chemistry effects in a rigorous way. Several methodologies are available for modelling such interactions, including the transported probability density function (PDF) approach and the conditional moment closure (CMC) method. Although in the early stages of its development, the CMC method has been shown to be a promising technique for predicting a wide range of practical problems. These include both premixed and non-premixed combustion, relatively slow chemistry effects, and ignition and extinction phenomena. This study concerns the CMC approach, and addresses the application of a number of models to a wide range of flows displaying varied compositions and geometries, including hydrogen and methane, and rim-stable and lifted jets. The impact of the choice of chemistry mechanism is considered for all the flows, and a higherorder CMC chemistry closure is investigated for the hydrogen flames. Analysis is made as to the ability of a parabolic CMC model to predict such flows, and the performance of the sub-model interactions is also reported on. The method of coupling the turbulent mixing field and the chemical kine tics is also investigated, and the effects of Reynolds stress and k - E turbulence closures upon subsequent CMC calculations are compared in all the flows considered. Overall, the results shown and conclusions drawn are very promising with respect to the possible future development of CMC. Requirements essential for this step forward of CMC methodologies for use in modelling practical geometries are specified, and an outline for the continuation of these studies is presented.