DNS of inhomogeneous reactants premixed combustion 
 
Kian Min Lim 
 
The search for clean and efficient combustors is motivated by the increasingly 
stringent emissions regulations. New gas turbine engines are designed to operate 
under lean conditions with inhomogeneous reactants to ensure cleanliness and 
stability of the combustion. This ushers in a new mode of combustion, called the 
inhomogeneous reactants premixed combustion. 
 
The present study investigates the effects of inhomogeneous reactants on premixed 
combustion, specifically on the interactions of an initially planar flame with a field of 
inhomogeneous reactants. Unsteady and unstrained laminar methaneair flames are 
studied in one- and two-dimensional simulations to investigate the effects of normally 
and tangentially (to the flame surface) stratified reactants. A three-dimensional DNS 
of turbulent inhomogeneous reactants premixed combustion is performed to extend 
the investigation into turbulent flames. The methaneair combustion is represented by 
a complex chemical reaction mechanism with 18 species and 68 steps. 
 
The flame surface density (FSD) and displacement speed Sd have been used as the 
framework to analyse the inhomogeneous reactants premixed flame. The flames are 
characterised by an isosurface of reaction progress variable. The unsteady flames 
are compared to the steady laminar unstrained reference case. An equivalence ratio 
dip is observed in all simulations and it can serve as a marker for the premixed 
flame. The dip is attributed to the preferential diffusion of carbonand hydrogen- 
containing species. 
 
Hysteresis of Sd is observed in the unsteady and unstrained laminar flames that 
propagate into normally stratified reactants. Stoichiometric flames propagating into 
lean mixture have a larger Sd than lean flames propagating into stoichiometric 
mixtures. The cross-dissipation term contribution to Sd is small (≈ 10%) but its 
contribution to the hysteresis of Sd is not (≈ 50%). Differential propagation of the 
flame surface is observed in the laminar flame that propagates into tangentially 
stratified reactants. Stretch on the flame surface is induced by the differential 
propagation, which in turn increases the flame surface area. 
 
In the DNS, inhomogeneous reactants were observed to have interacted with the 
turbulent premixed flame in the preheat zone. Results from DNS show similar FSD 
and Sd statistics as previous thin reaction zones regime DNS of turbulent premixed 
flames. Effects of inhomogeneous reactants as observed in the laminar flame 
simulations are not pervalent in the turbulent flames.