MILD combustion in diffusion-controlled regimes of Hot Diluted Fuel

Abstract

Reactive structures have been characterized in a steady laminar, unidimensional mixing layer on a dense grid of parameters in moderate or intense low-oxygen dilution (MILD) combustion conditions with hot and diluted fuel. The structures have been studied in terms of temperature and heat release profiles in a mixture fraction space for various ranges of stretch rates and for two reference pressures (1 and 10bar) using a standard code and standard kinetic scheme.In the analysis of reactive structure three synthetic characteristics have been pointed out in previous works as discriminative for the occurrence of different combustion regimes. Such characteristics are the thickness of the oxidation structures, the presence/absence of a pyrolysis region and the correlation/no correlation of the regions of maximum heat release with those at which the mixture is stoichiometric. Following the same criteria, maps of regimes have been built up on a inlet fuel temperature – temperature increase plane for fixed stretch rates and different pressures.It has been pointed out that in diffusion controlled regimes of Hot Diluted Fuel, analysed in this paper, MILD combustion conditions are characterized by flame thickening and pyrolysis depression, which are also typical of flameless combustion. In addition, the region of maximum heat release has a generally high level of correlation with the stoichiometric regions. Thus, MILD combustion regime differs from MILD combustion regime found in other diffusion controlled regimes where the region of maximum heat release is generally not correlated with the stoichiometric regions. In the case presented in this paper, only a second solution, corresponding to a very low conversion occurring in a very wide stretch-rate range, shows no correlation between heat release and stoichiometric mixture fraction. This behavior has been attributed to the oxidative pyrolysis of methane.At atmospheric pressure, the flameless characteristics extend to a low level of preheating, provided that the fuel is diluted, with a consequently low level of temperature increase, thus confirming that MILD and flameless regimes are not coincident in all possible feeding conditions.In general, the results obtained in these Hot-Fuel-Diluted-Fuel conditions are consistent with and extend those reported in the literature for Hot-Oxidant-Diluted-Fuel, Hot-Oxidant and Diluted-Fuel conditions, supporting the assertion that these inlet parameters are a suitable choice for the definition of MILD combustion.