On coupled hydrodynamic and diffusional-thermal instabilities in flame propagation.

摘要

An accurate numerical solver was first developed for a 2D compressible reactive flow with consistent nonreflective boundary conditions. Using this solver the coupling between hydrodynamic and diffusional-thermal instabilities for freely propagating premixed flames were studied for the effects of Lewis number (Le), pressure, and turbulence intensity.; Specifically, we first derived a set of highly accurate nonreflective boundary conditions by considering a general equation of state. The derivation fully couples the fluid dynamic equations and the species equations.; Employing these boundary conditions with a direct numerical simulation (DNS) solver, the coupling between hydrodynamic instability and diffusional-thermal instability was investigated. For Le ⩽ 1, the elementary nonlinear evolution processes, such as cell growth, cell splitting, cell merging, cell lateral movement, and local extinction were identified and studied. For Le > 1, effects of diffusional-thermal pulsation in both linear and nonlinear stages were investigated. It is shown that in the linear stage, pulsating instability promotes hydrodynamic instability, whereas in the nonlinear stage we have identified three different flame dynamic regimes with increasing activation energy, namely stable cellular flame propagation, periodic pulsating cellular flames, and irregular pulsating cellular flames.; We then investigated the effects of pressure up to 3 atm on flame front instability for both the linear and nonlinear growth stages for Le ⩽ 1. Results show that elevated pressure extends the unstable range of flame fronts and generate a fine flame cell structure in the linear growth stage, while the critical wavenumber is reduced and small cells appear over large cells with increasing pressure in the nonlinear growth stage.; Finally, effects of turbulence and flame instabilities on the flame front evolutions for Le = 1.0 and 0.7 flames were investigated. It is shown that hydrodynamic instability dominates the growth of the flame cells when the turbulence intensity is weak (u' = 1%-5%), whereas the turbulent motion wrinkles the flame front and dominates the evolution process when the turbulence intensity is large (u' = 50%). Curvature stretch dominates the total stretch rate for flames with both weak and strong turbulence intensities, and therefore plays a significant role in turbulent flame modeling.