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Simulations of autoignition and laminar premixed flames in methane/air mixtures diluted with hot products


Type

Article

Change log

Authors

Sidey, J 
Mastorakos, Epaminondas  ORCID logo  https://orcid.org/0000-0001-8245-5188
Gordon, RL 

Abstract

This paper considers constant-pressure autoignition and freely propagating premixed flames of cold methane-air mixtures mixed with equilibrium hot products at high enough dilution levels that the combustion enters the MILD combustion regime. The analysis is meant to provide further insight on MILD regime boundaries and to identify the effect of hot products speciation. As the mass fraction of hot products in the reactants mixture increases, autoignition occurs earlier. Species profiles show that the products-reactants mixture approximately equilibrates to a new state over a quick transient well before the main autoignition event, but as dilution becomes very high, this equilibration transient becomes more prominent and eventually merges with the primary ignition event. The dilution level at which these two reactive zones merge corresponds well with that marking the transition into the MILD regime, as defined according to conventional criteria. Similarly, premixed flame simulations at high dilutions show evidence of significant reactions involving intermediate species prior to the flame front. Since the premixed flame governing equations system demands that the species and temperature gradients be zero at the “cold” boundary, flame speed cannot be calculated above a certain dilution level. Up to this point, which again agrees reasonably well with the transition into the MILD regime according to convention, the laminar burning velocity was found to increase with hot product dilution while flame thickness remained largely unchanged. Some comments on the MILD combustion regime boundary definition for gas turbine applications are included.

Description

Keywords

Autoignition, High dilution, Laminar premixed flames, MILD combustion

Journal Title

Combustion Science and Technology

Conference Name

Journal ISSN

0010-2202
1563-521X

Volume Title

186

Publisher

Informa UK Limited
Sponsorship
This work has been partly funded by Rolls-Royce plc.