Diffusion Ignition Processes in MILD Combustion: A Mini-Review

Giancarlo Sorrentino,Raffaele Ragucci,Pino Sabia,Antonio Cavaliere,Mara De Joannon

doi:10.3389/fmech.2020.00010

Giancarlo Sorrentino, Raffaele Ragucci + Show 3 more

Open Access

https://doi.org/10.3389/fmech.2020.00010

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Abstract

MILD Combustion processes belong to new combustion technologies developed to achieve efficient and clean fuel conversion. The basic concept behind its implementation is the use of high levels of hot exhausted gas recirculation within the combustion chamber. They simultaneously dilute fresh reactants, to control system temperatures and pollutants emission, while promoting fuel complete oxidation. The combination of low maximum system working temperatures and high diluted mixtures with intense pre-heating delineate an oxidation process with unique chemical and physical features, such as uniformity of scalars at macroscale related to distributed reacting regions at microscale, extremely different from conventional flames. In turn, this requires the definition and characterization of new elementary processes, not ascribable to traditional deflagrative or diffusive flame structures, that in literature have been identified as “Diffusion Ignition”. The present Mini-Review reports on several literature characterization of such reactive structures under steady and unsteady conditions combining evidences from numerical, experimental and/or theoretical studies. Both premixed and non-premixed configurations were analyzed in terms of system temperature, heat release and species distributions as key parameters to describe the intrinsic nature of such new elementary processes. Analyses were realized changing the main system external parameters (mixture pre-heating temperature, dilution level in several feedings configurations) moving from traditional to MILD conditions. Results highlighted the “distributed ignition” nature of Ignidiffusive structures, with implication on the thickness of the oxidation structures in the mixture fraction space, the presence/absence of a pyrolysis region and the correlation of the maximum heat release with the mixture stoichiometric.

Highlights

Elementary structures in combustion such as laminar diffusion layers have been characterized in the literature (Tsuji, 1982; Chao et al, 1991; Darabiha, 1992; Chen et al, 2012)
Diffusion ignition can be described in 1-D spatial conditions, but usually they are very difficult to mimic in simple experiments due to such low dimensionality. 2-D experiments where fuel jets are injected in co-flowing or cross-flowing oxidizer streams are easier to realize for diluted and preheated conditions
Jet in hot flows configuration was used to reproduce diffusion ignition processes (Adelaide or Delft jet in hot coflow) in both the laminar (LJHC) and turbulent (JHC) cases when inlet reactant temperatures are higher than ignition ones

Summary

Introduction

Elementary structures in combustion such as laminar diffusion layers have been characterized in the literature (Tsuji, 1982; Chao et al, 1991; Darabiha, 1992; Chen et al, 2012). The characterization of diffusion-controlled combustion processes developing in a steady 1-D layer was reported in the literature with several numerical studies by means of the opposed jets configuration (de Joannon et al, 2007; Cheong et al, 2017; Mameri et al, 2018).

Results

Conclusion