Quantification of combustion regime transitions in premixed turbulent DME flames

The current study quantifies the probability of encountering up to five fluid states (reactants, combustion products, mixing fluid, fluids with low and high reactivity) in premixed turbulent DME flames as a function of the Damkohler number. The flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re similar or equal to 18,400 and Re-t > 370). The chemical timescale was varied via the mixture stoichiometry resulting in a wide range of Damkohler numbers (0.08 <= Da <= 5.6). The mean turbulent strain (>= 3200 s(-1)) exceeded the extinction strain rate of the corresponding laminar flames for all mixtures. Simultaneous Mie scattering, OH-PLIF and PIV were used to identify the fluid states and supporting computations show that the thermochemical state (e.g. OH and CH concentrations) at the twin flame extinction point correlates well with flames in the back-to-burnt geometry at the corresponding rate of heat release. For mixtures where the bulk strain (similar or equal to 750 s(-1)) was similar to (or less than) the extinction strain rate, fluids with low and high reactivity could accordingly be segregated by a threshold based on the OH concentration at the extinction point. A sensitivity analysis of the distribution between the fluid states was performed. The flow conditions were further analysed in terms of Damkohler and Karlovitz numbers. The study provides (i) the evolution of multi-fluid probability statistics as a function of the Damkohler number, including (ii) the flow direction across fluid interfaces and OH gradients, (iii) mean flow field statistics, (iv) conditional velocity statistics and (v) a tentative combustion regime classification. (C) 2017 Published by Elsevier Inc. on behalf of The Combustion Institute.