To model high-pressure reactive flows, the most promising methodology is large eddy simulation (LES) in which one solves the large scales of the problem and models the small scales. There is currently no guidance as to the relative importance of small scales originating from different phenomena.
The concept of irreversible entropy production was used here, which is the dissipation, to determine the relative importance of the small scales originating from different phenomena in high-pressure reactive flows. These small scales were obtained from Direct Numerical Simulations (DNS) in which all dissipation-relevant scales in the continuum regime are resolved.
Three DNS realizations were created to study fundamental features of high-pressure combustion with particular focus on the requirements for Subgrid-Scale (SGS) modeling in LES. The configuration was that of a temporal mixing layer. The DNS realizations were produced to (1) mimic exhaust gas recirculation (EGR) during auto-ignition and combustion, and study its features compared to a case without EGR, and (2) to evaluate the role of the initial pressure. The conservation equations included a real-gas equation of state and computed transport properties for multispecies mixtures at high-pressure. For each realization, a transitional state was achieved that was taken as the initial condition for reaction. As the domain-average-p reached a peak, simulations were stopped and the analysis was performed at the pressure-peak state. Examination of that state reveals that a primary diffusion flame is established surrounded by regions of premixed burning. The phenomenon of uphill diffusion was observed for both H2O and CO2, the result of which is turbulence enhancement. Without EGR, uphill diffusion is overwhelmingly reduced, but not eliminated, and the flow exhibits less turbulent aspects than with EGR; therefore, at high-pressure, EGR not only modifies the chemistry, but it also modifies the dynamics of the flow.
Inspection of the dissipation and its four contributions, gvisc (viscous dissipation), gmass (dissipation due to mass diffusion), gtemp (dissipation due to thermal conduction), and greac (dissipation due to reaction), was made from the viewpoints of homogeneous-plane averages and rms, and of spatial distributions in a spanwise plane. Although the magnitude of greac dominates that of gvisc, gmass, and gtemp because their spatial distribution is different and the largest values of each are encountered at locations representing different phenomena in the flow, the emphasis should be in LES on the accurate modeling of all SGS-producing terms.