A Priori Direct Numerical Simulation Assessment of MILD Combustion Modelling in the Context of Reynolds Averaged Navier–Stokes Simulations

Abstract

A priori Direct Numerical Simulation (DNS) assessment of mean reaction rate closures for reaction progress variable in the context of Reynolds Averaged Navier–Stokes (RANS) simulations has been conducted for MILD combustion of homogeneous (i.e., constant equivalence ratio), methane-air mixtures. The reaction rate predictions according to statistical (e.g., presumed probability density function), phenomenological (e.g., eddy-break up (EBU), eddy dissipation concept (EDC) and the scalar dissipation rate (SDR) based approaches), and flame surface description (e.g., Flame Surface Density) based closures are compared. The performance of the various reaction rate closures has been assessed by comparing the models’ predictions to the corresponding quantities extracted from DNS data. It has been found that the usual presumed probability density function (PDF) approach using the beta-function predicts the PDF of the reaction progress variable in homogenous mixture MILD combustion throughout the flame brush for all cases considered here provided that the scalar variance is accurately predicted. The accurate estimation of scalar variance requires the solution of a modelled transport equation, which depends on the closure of Favre-averaged SDR. A linear relaxation based algebraic closure for the Favre-averaged SDR has been found to capture the behaviour of the Favre-averaged SDR in the current homogenous mixture MILD combustion setup. It has been found that the EBU, SDR and FSD-based mean reaction rate closures do not adequately predict the mean reaction rate of the reaction progress variable for the parameter range considered here. However, a variant of the EDC closure, with model coefficients expressed as functions of micro-scale Damköhler and turbulent Reynolds numbers, has been found to be more successful in predicting the mean reaction rate of reaction progress variable compared to other modelling methodologies for the range of turbulence intensities and dilution levels considered here.

Flame self-interactions in MILD combustion of homogeneous and inhomogeneous mixtures

“…A notable exception to this trend is the inhomogeneous mixture case at low dilution where the peak frequency of FSI events is clearly shifted further towards the burned gas side. A possible explanation for this behaviour is that the combination of mixture inhomogeneity, which introduces increased levels of reactivity 23 , 24 , and higher oxygen concentration leads to this shift in peak FSI frequency to higher c values. However, it remains difficult to categorically establish the exact reason for, or the mechanism behind, this behaviour.…”

“…It can also be seen from Table 3 that mixture inhomogeneity acts to reduce the frequency of FSI events for both dilution levels and turbulence intensities. This can be attributed to an increase in the progress variable reaction rate due to mixture inhomogeneity 23 , 24 , which results in the combustion process being completed in smaller portion of the domain. Thus, the region fills up a smaller portion of the domain which reduces the opportunity for FSI events to occur.…”