Numerical investigation of flow and scalar fields of piloted, partially-premixed dimethyl ether/air jet flames using stochastic multiple mapping conditioning approach

Abstract

Turbulent piloted partially-premixed jet flames of dimethyl ether/air have been investigated using the Reynolds-Averaged Navier-Stokes (RANS) based stochastic multiple mapping conditioning (MMC) approach. The MMC model combines the advantages of mapping closure concept, conditional moment closure, and probability density function (PDF) methods. In the present work, the evolution of the mapping function in the reference space is described using a single reference variable based on the mixture fraction. To achieve localness in the composition space, the turbulent mixing is conditioned on the reference space. In addition to localness, MMC preserves the various characteristics of a good mixing model, such as linearity, Gaussianity, independence, and boundedness. A skeletal chemical mechanism for dimethyl ether is used, which consists of 28 species and 24 reversible reactions. Modified Curl's mixing model is adapted to model the micro-mixing term, and standard two-equation k−ε turbulence model is used to model the Reynolds stress term with a modified set of constants. The computed conditional and unconditional statistics demonstrate an excellent agreement with the available experimental data even for flame displaying a large degree of local extinction and re-ignition. For these flames, radical species distribution conditioned on mixture fraction confirms the physical separation between the OH and CH2O species, which was earlier reported using simultaneous laser-induced fluorescence measurements of these radicals. Predicted unconditional scalar dissipation rate for DME-D and DME-F flames reaffirm that near the inlet an overlap between OH and CH2O radicals are observed where the scalar dissipation rate is fairly high. Subsequently, a distinct separation between these radicals becomes evident at downstream locations where the scalar dissipation rate decreases. For the flames investigated here, a strong correlation is noticed between the peak heat release rate and the reaction rate indicator, ROH based on the product of concentrations of OH and CH2O radicals.