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 rate of a turbulent premixed flame is much higher than that of its laminar counter- part due to the faster consumption of the reactant flow through the enhanced flame surface area. This increased

flame surface area results from the multiscale stretching, folding and continuous surface generation-annihilation processes inherent in turbulence- flame interaction. In the second part of the thesis, we have investigated from where and how do the complex topology and physico-chemical state of a fully developed turbulent premixed

flame generates and evolves in time. To that end, we have developed a back in time version of Flame Particle Tracking (FPT). Flame particles are a class of surface points that move with reactive isoscalar surfaces and represent local states of a premixed flame. To identify the source locations of a turbulent flame surface, we have developed a new tracking method called the Backward Flame Particle Tracking (BFPT). Using BFPT on DNS data, we have found that new flame surface elements in a turbulent flame generate from multiple, positively- curved, leading locations of a flame surface. This observation is in partial consonance with Zeldovich's conjecture of a singular leading point valid for a laminar premixed flame. We have elucidated the generation mechanisms and also developed a relationship between the turbulent flame speed ST of the flame and Sd at the leading points with the help of Finite Strain Theory (FST). We have also investigated the pair-dispersion characteristics of the leading points and found that they obey a modi ed Batchelor's dispersion law, which is used to shed light on why the new surface elements generate from the leading points. In the third and fi nal part, we have investigated models of Sd for turbulent premixed flames. We have again used the concept of tracking flame particles. Using BFPT-FFPT on the DNS data, we have found that we can broadly divide flame particles' lifetime into two phases: non-interacting and interacting. The Phase-I or non-interacting phase forms a signifi cant part of the flame particle lifetime, and during this phase, Sd at

flame particles varies due to stretch and curvature effects. Since the local flame structure is still comparable to a standard premixed flame, the two-parameter Markstein length model gives reasonably accurate results. In the Phase-II or interacting phase, local flame surface elements interact with other portions of the flame. Consequently, the local flame structure is no longer comparable to a standard premixed flame even qualitatively and flame particles observe steep acceleration. We have derived a new interaction model to predict Sd during this phase. It is shown that a combined use of the Markstein length model and the interaction model can reasonably explain the dominant features of the local propagation characteristics for an entire turbulent flame surface, for the fi rst time, to our knowledge.