Numerical study of turbulent non-premixed cool flames at high and supercritical pressures: Real gas effects and dual peak structure

To study the formation of cool flames at high/supercritical pressures, adaptive mesh refinement (AMR)-based simulations of dimethyl ether (DME)-air pesudo-turbulent reacting mixing layer were performed with detailed chemistry at 80 and 150 atm. These pressures span both the subcritical (80 atm) and supercritical (150 atm) regimes of the fuel mixture stream (composed of 70% DME and 30% N2 by mass). The simulations are performed using both ideal gas and Soave–Redlich–Kwong (SRK) equations of state (EoS) to study real gas effects on auto-igniting mixing layers at high/supercritical pressures. Various Quantities of Interest (QoIs), such as temperature, heat release rate (HRR), and low-temperature chemistry (LTC)-specific intermediate radicals of DME (e.g., CH3OCH2O2 and OCH2OCHO), indicate that the characteristic two-stage ignition of DME is retained at these pressures. At 150 atm, “supercritical cool flame” is formed, and subsequently transits into a “spotty” hot flame. The increase in pressure results in a reduction of the first stage ignition delay time. A dual peak structure is observed in the means of the QoIs conditioned on mixture fraction. HRR and mass fractions of intermediate species (such as CH3OCH2O2 and CH2O) attain minima at the stoichiometric point. However, temperature and mass fractions of H2O and CO2 attain maxima there. The conditional means of net production rates of selected species (CH3OCH2O2, CH2O, HO2, OH, H2O, CO2) are then used to explain the location of each peak in the dual peak structure, concluding it to be a result of multiple reaction pathways, namely, LTC (600–800 K), intermediate-temperature chemistry (ITC) (900–1300 K), and high-temperature chemistry (HTC) (>1500 K), which are active at different temperature regimes in the mixture fraction space. Comparisons between the SRK and ideal gas EoS are made using both the spatial distribution and conditional means of the QoIs. We observe significant differences in the mixing-layer profiles and hence the formation of stoichiometric pockets, which also manifests as deviations in the order of 5–10% in predicting thermodynamic quantities and species mass fractions in the mixture fraction space. It is also found that these deviations increase with pressure.