Quantification of External Enthalpy Controlled Combustion at Unity Damköhler Number

The use of external enthalpy support (e.g. via heat recirculation) can enable combustion beyond normal flammability limits and lead to significantly reduced emissions and fuel consumption. The present work quantifies the impact of such support on the combustion of lean (\(\varPhi = 0.6\)) turbulent premixed DME/air flames with a Damköhler number around unity. The flames were aerodynamically stabilised against thermally equilibrated hot combustion products (HCP) in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (\(Re \simeq 18{,}400\) and \(Re_t > 370\)). The bulk strain (\(a_b = 750\) s\(^{-1}\)) was of the order of the extinction strain rate (\(a_q = 600\) s\(^{-1}\)) of the corresponding laminar opposed twin flame with the mean turbulent strain (\(a_I = 3200\) s\(^{-1}\)) significantly higher. The HCP temperature (\(1600 \le T_{HCP}\)(K) \( \le 1800\)) was varied from close to the extinction point (\(T_{q} \simeq 1570\) K) of the corresponding laminar twin flame to beyond the unstrained adiabatic flame temperature (\(T_{ad} \simeq 1750\) K). The flames were characterised using simultaneous Mie scattering, OH-PLIF and PIV measurements and subjected to a multi-fluid analysis (i.e. reactants and combustion products, as well as mixing, weakly reacting and strongly reacting fluids). The study quantifies the (i) evolution of fluid state probabilities and (ii) interface statistics, (iii) unconditional and (iv) conditional velocity statistics, (v) conditional strain along fluid interfaces and (vi) scalar fluxes as a function of the external enthalpy support.