Mixing Time Scale Models for Multiple Mapping Conditioning with Two Reference Variables

C Straub,O T Stein,A Kronenburg,S Galindo-Lopez,M J Cleary

doi:10.1007/s10494-020-00188-0

C Straub, O T Stein + Show 3 more

Open Access

https://doi.org/10.1007/s10494-020-00188-0

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Abstract

A novel multiple mapping conditioning (MMC) approach has been developed for the modelling of turbulent premixed flames including mixture inhomogeneities due to mixture stratification or mixing with the cold surroundings. MMC requires conditioning of a mixing operator on characteristic quantities (reference variables) to ensure localness of mixing in composition space. Previous MMC used the LES-filtered reaction progress variable as reference field. Here, the reference variable space is extended by adding the LES-filtered mixture fraction effectively leading to a double conditioning of the mixing operator. The model is used to predict a turbulent stratified flame and is validated by comparison with experimental data. The introduction of the second reference variable also requires modification of the mixing time scale. Two different mixing time scale models are compared in this work. A novel anisotropic model for stratified combustion leads to somewhat higher levels of fluctuations for the passive scalar when compared with the original model but differences remain small within the flame front. The results show that both models predict flame position and flame structure with good accuracy.

Highlights

Many premixed flames are not burning in a truly premixed combustion mode due to mixing with the surrounding air and/or stratification of the fuel mixture
A generalized multiple mapping conditioning (MMC) mixing model has been applied to configuration TSF-A of the turbulent stratified flame series
The introduction of the filtered mixture fraction as an additional reference variable leads to the distinction of fluid elements with different equivalence ratios that originate from the stratification of the flame

Summary

Introduction

Many premixed flames are not burning in a truly premixed combustion mode due to mixing with the surrounding air and/or stratification of the fuel mixture. The latter is a common design feature as it enhances ignition probability and flame stability in overall lean premixed environments that are favoured due to their relatively benign emission characteristics of pollutants such as NOx and soot. Thereby, the FDF methods are not specific to any combustion regime and do not discriminate between non-premixed and premixed combustion They should be able to predict flamelet-like structures if turbulence levels are low to moderate but they should be able to predict any deviations thereof if turbulence is strong and the preheat or reaction zones are locally thickened. Despite the in principle regime independence of FDF models the governing equation contain an unclosed term for the subfilter conditional scalar dissipation which needs to be modelled via a mixing model and some of the common models are not suitable for all regimes due to their violation of the principle that mixing should be local in composition space (Subramaniam and Pope 1998)

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