On the Structure and Propagation of Premixed Flames in Turbulence

Abstract

Turbulence interacts with flames over a multitude of lengthscales and timescales. These interactions are nonlinear and are ubiquitous, for example, in propulsion engines operating in space, air, land, and sea. Such interactions also constitute processes involved in astrophysical explosions. The technological development of the engineering devices and understanding of fascinating natural processes thus behoove deeper, fundamental understanding of these complex interactions. Such interactions between turbulence and premixed flames are expressed through a change in the flame structure and propagation rate, characterized by flame thickness, flame surface topology, and the flame speed, respectively. In this thesis, we have identi fied and investigated these crucial interactions and their effects on the flame structure and propagation characteristics for multiple realizations of turbulent premixed flames using Direct Numerical Simulations (DNS). In the first part, we have studied flame thickness and conditional scalar dissipation rate (CSDR) for a temporally evolving turbulent slot-jet flame. Most practical flames encountered either in the swirling flow of a gas turbine combustor or in the wake of a blu body separated flow of an afterburner are stabilized and stretched by the straining action of a turbulent shear layer. Contrary to the general belief, the DNS cases investigated show that the mean flame thickness in turbulence is lesser than that of the corresponding laminar premixed flame. We have explored the alignment of eigenvectors of strain-rate tensor Sij with local normal n to the surface and found that it others an incomplete explanation to the observed phenomena. As such, the flame thickness reduces (or CSDR increases) due to an increase in normal strain-rate n rSd due to flame displacement speed Sd. The strain-rate n rSd dominates over normal strain-rate due to uid ow nn : ru because of the positive stretch-rate response of the local flame structure with sub-unity Lewis number Le. The average propagation r...