Effect of Hot Diluted Fuel Flow on Reactive Structures in MILD Combustion

In a survey of potential keys for the mitigation of environmental problems, MILD Combustion represents a flexible and clean process that is the result of a trade-off in the optimization of fuel conversion with respect to efficiency while not requiring drastic changes in the configuration of traditional plants [1]. MILD combustion has been rigorously defined such as a process for which "the inlet temperature of the reactant mixture is higher than mixture self ignition temperature whereas the maximum allowable temperature increase with respect to inlet temperature during combustion is lower than mixture self ignition temperature" [2]. Experimental [3] and numerical [4] works carried out on a steady unidimensional diffusive layer, show an extension of reactive regions by increasing temperature and dilution up to the condition where the reaction zone overcomes the stagnation point. This justifies the classification of the relative reactive structure as Homogeneous Charge Diffusion Ignition [4]. The analysis of the steady diffusive layer requires a higher level of accuracy with respect to the premixed case because different types of conditions has to be considered in order to fit the Mild combustion feed parameters. Experimental works carried out in specific conditions [3, 5-7] evidenced the occurrence of unusual behavior, for instance related to presence of ignition kernels in correspondence of mixture fraction region where they are not expected to be. Hot- Oxidant-Diluted-Fuel fed conditions, was analyzed in the past both for low and high molecular weight paraffins [8, 9]. Results obtained for Hot-Oxidant-Diluted-Oxidant configuration confirm the extension of the mixture fraction region covered by the reaction and the disappearing of the pyrolysis region [9]. The present work aims to conclude the categorization and characterization of the reactive structures in diffusive configuration, dealing with a fed condition corresponding to Hot-Fuel- Diluted-Fuel counterflowing jet. Processes controlled by Hot-Fuel-Diluted-Fuel oxidation can locally occurs in several practical systems. The analysis consists in detailed simulations of the thermochemical patterns in a dense grid of input parameters synthetized in regimes diagrams and it differs from the other ones only in the inlet conditions. Therefore the results are tightly comparable with each others and the discussion on their implication extend the conceptual framework previously outlined.