Analysis of Markers for Combustion Mode and Heat Release in MILD Combustion Using DNS Data

N.A.K Doan,N Swaminathan

doi:10.1080/00102202.2019.1610746

Abstract

ABSTRACTVarious commonly used markers for heat release are assessed using direct numerical simulation (DNS) data for Moderate or Intense Low-oxygen Dilution (MILD) combustion to find their suitability for non-premixed MILD combustion. The laser-induced fluorescence (LIF) signals of various markers are synthesized from the DNS data to construct their planar (PLIF) images which are compared to the heat release rate images obtained directly from the DNS data. The local OH values in heat releasing regions are observed to be very small compared to the background level coming from unreacted mixture diluted with exhaust gases. Furthermore, these values are very much smaller compared to those in burnt regions. This observation rises questions on the use of OH-PLIF for MILD combustion. However, the chemiluminescent image obtained using is shown to correlate well with the heat release. Two scalar-based PLIF markers, and , correlate well with the heat release. Flame index (FI) and chemical explosive mode analyses (CEMA) are used to identify premixed and non-premixed regions in MILD combustion. Although there is some agreement between the CEMA and FI results, large discrepancies are still observed. The schlieren images deduced from the DNS data showed that this technique can be used for a quick and qualitative identification of MILD combustion before applying expensive laser diagnostics.

Highlights

Considerable progress has been made since 1980s on turbulence-combustion interaction, turbulent combustion modelling (Pope, 2013) and large eddy simulations of reacting flows in practical engines with complex geometries (Menon, 2018)
A homogeneous temperature field is observed for the Moderate or Intense Low-oxygen Dilution (MILD) case, which is similar to those observed in photographs from MILD combustion experiments
The contribution of rich premixed combustion to the total heat release rate was shown to be smaller than 10% by Doan et al (2018) for the non-premixed MILD combustion cases used for this study

Summary

Introduction

Considerable progress has been made since 1980s on turbulence-combustion interaction, turbulent combustion modelling (Pope, 2013) and large eddy simulations of reacting flows in practical engines with complex geometries (Menon, 2018). The applicability of these diagnostics to combustion under MILD conditions is unclear since the heat releasing regions in MILD combustion appear different from those in conventional combustion Radicals such as OH, CH and HCO used commonly as HRR markers are present in unreacted mixture of MILD combustion because of dilution using exhaust gases containing these species. Our interest is to extend those assessments of HRR markers for MILD combustion with mixture fraction variation using DNS data of Doan et al (2018) This specific interest is because the inception of MILD combustion does not follow the classical routes due to the chemical kinetic role of radicals present in the unreacted mixture as has been shown by Doan & Swaminathan (2019). A summary of the main findings is provided in the final section

DNS of MILD combustion

Results and discussion

Markers for Heat Release

Premixed or Non-premixed mode identification

Schlieren approach

Conclusions