Physical Insights of Non-Premixed MILD Combustion using DNS
Moderate or Intense Low-oxygen Dilution (MILD) combustion is a combustion technology that can simultaneously improve the energy efficiency and reduce the pollutant emissions of combustion devices. It is characterised by highly preheated reactants and a small temperature rise during combustion due to the large dilution of the reactant mixture with products of combustion. These conditions are generally achieved using exhaust gas recirculation. However, the physical understanding of MILD combustion remains limited which prevents its more widely spread use.
In this thesis, Direct Numerical Simulation (DNS) is used to study turbulence, premixed flames and MILD combustion to obtain these additional physical insights. In a first stage, the scale-locality of the energy cascade is analysed by applying a multiscale analysis methodology, called the bandpass filter method, on DNS of homogeneous isotropic turbulence. Evidence supporting this scale-locality were obtained and the results were found to be similar for Reynolds numbers ranging from 37 to 1131. Using the same method in turbulent premixed flames, the scale-locality of the energy cascade was still observed despite the presence of intense reactions. In addition, it was found that eddies of scales larger than the laminar flame thickness were imparting the most strain on the flame.
In a second part, a methodology was developed to conduct the DNS of MILD combustion with mixture fraction variations. This methodology included the effect of mixing of exhaust gases with fuel and oxidiser in unburnt, burnt and reacting states. In addition, a specific chemical mechanism that includes the chemistry of
The reaction zones in MILD combustion were also analysed and extremely convoluted reaction zones were observed with frequent interactions among them. These interactions yielded the appearance of volumetrically distributed reactions. Furthermore, the adequacy of some species to identify these reaction zones was assessed and
Because of the presence of radicals and high reactant temperatures, auto-igniting regions and propagating reaction zones are both observed locally. The balance between these phenomena was investigated and it was found that this was strongly influenced by the typical lengthscale of the mixture fraction field, with a smaller lengthscale favouring sequential autoignition. Finally, using the bandpass filtering method, the effect of heat release rate in MILD combustion on the energy cascade was studied and this showed that the energy cascade was not unduly affected.