Equivalence Ratio Gradient Effects on Locally Lean, Stoichiometric and Rich Propane/Air and N-Heptane/Air Turbulent Bluff Body Flames

Abstract

The effect of inlet mixture stratification was investigated in propane/air and prevaporised n-heptane/air flames stabilized in the near wake region of a bluff-body burner. The employed axisymmetric burner can sustain flame anchoring at global equivalence ratio values in the range of 0.09 ÷ 0.1 independently of fuel type and permits the variation of fuel concentration along the radial direction. Three distinct stratification gradients were studied for the two fuels considered; One burning from rich to lean, one burning from stoichiometric to lean and one burning from stronger lean to weaker lean mixtures. Particle Image Velocimetry, Mie scattering and OH ∗ and CH* Chemiluminescence were used to investigate flame stabilization characteristics of the two fuels and three stratification gradients, while Fourier – Transform Infrared Spectroscopy was performed to assess the equivalence ratio disposition under non-reacting conditions in the near wake region. 2D hydrodynamic strain rates, Damköhler (Da) and Karlovitz (Ka) numbers and flame brush thickness distributions were estimated and analyzed to elucidate the effects of turbulence, mixture composition and fuel type on the investigated flames. Also, the characteristic size of the reacting fluid pockets was assessed using a two-point sample autocorrelation methodology on the OH* chemiluminescence images. Results suggest that supplying the vicinity of the anchoring region with lean peak equivalence ratio mixtures with Lewis numbers greater than unity reduces the flame’s resistance to strain, while supplying it with rich peak equivalence ratio mixtures of Lewis number ≈1, independently of fuel type, favors resistance to strain, suggesting a connection with preferential diffusion effects.