Effect of Rayleigh-Taylor Instability on Backward-Facing-Step Stabilized Turbulent Premixed Flames

A computational investigation of centrifugal force effects on backward-facing-step stabilized turbulent premixed propane-air flames is conducted. The centrifugal force acts from the high-density reactants towards the low-density products creating a Rayleigh-Taylor instability. A straight channel with an infinite radius of curvature and a backward-facing step is used as the baseline. A curved channel (referred to as a centrifuge in this work) with a finite radius of curvature and a backward-facing step on the outer radius is used to evaluate the effects of centrifugal force on turbulent flame speeds. Three-dimensional simulations are performed using Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES). The computational results are compared with broadband chemiluminescence and shadowgraph images reported in the literature for similar conditions and geometries. The past experiments and current simulations show that increasing the inlet velocity in the straight channel and centrifuge results in a flame cannot withstand the high stretch rates and the flame is positioned behind the backward-facing step. The measured and computed shadowgraphs for the straight channel and centrifuge demonstrate that by increasing the inlet flow velocity the flame becomes hydrodynamically unstable, characterized by heterogeneous and anisotropic turbulent flow, and a large density variation occurs further downstream. The RANS and LES computations qualitatively capture these trends, but the LES results show better agreement with the experimental broadband chemiluminescence and shadowgraph images. It is challenging to decouple the combined effects of turbulence fluctuations and Rayleigh-Taylor induced centrifugal acceleration on turbulent flames speeds. However, the combined effects of turbulence fluctuations and centrifugal acceleration appear to promote increased turbulent flame speeds and at high enough inlet velocities, allow the flame to exist where it would otherwise blowout.