How Fast is the Flame: Displacement vs. Burning Speeds of Fast, Highly Compressible Turbulent Premixed Flames

A comparative analysis of numerically and experimentally determined speeds of fast, highly compressible premixed turbulent flames is presented. Analysis focuses on direct numerical simulations of the Turbulent Shock Tube (TST) facility developed at the University of Central Florida. The TST generates flames, which exhibit highly time-dependent evolution with significant flame acceleration, overpressures, and turbulence generated within the flame brush. Flame speed, a defining characteristic, is generally determined in experiments through measurement of the flame displacement speed, S_D, relative to the upstream flow. It is not currently clear how accurately S_D represents the burning speed, S_T, of a turbulent flame, which is the proper measure of fuel consumption and heat release rate. Here we investigate the differences between numerically calculated S_D and S_T, as well as S_D experimentally determined in the TST. Results show considerable differences between S_D and S_T, which become particularly pronounced as the flame evolution grows increasingly unsteady. This study aims to develop the understanding of various flame-speed measures in fast, highly unsteady turbulent combustion regimes directly relevant to novel combustion systems such as scramjets and detonation-based engines.