Abstract
To explore the effect of stratification on lean premixed combustion in a turbulent flow, an experimental investigation on the TSFA flame of the Darmstadt stratified burner is conducted. Spatially highly resolved major species concentrations and temperature are measured by 1D Raman–Rayleigh. Temperature gradients from line data are corrected to the flame front normal by the means of Crossed Planar Rayleigh Imaging. A conditioning based on multiple criteria relevant for stratified combustion is applied to the large dataset and allows to analyze the impact of stratification on the flame structure. Conditioning criteria are the local equivalence ratio (ϕ=0.75), the local thermal progress variable (c=0.54), the stratification level (instantaneous flame-normal gradient in equivalence ratio), and a temperature difference over the 1D probe volume, ensuring the flame front is included. Conditionally averaged quantities such as temperature profiles in the flame-normal coordinate system, equivalence ratio, H2 mass fractions, and temperature gradients are parameterized on the local equivalence ratio gradient in order to understand the impact of stratification on the flame structure. The temperature profiles in this back-supported configuration are more affected on the reactant side than on the product side by stratification. In contrast to that, equivalence ratios as well as mass fractions of hydrogen are found to be sensitive to stratification on the product side as well as in the reactants. Profiles are altered even in the reaction zone by enhancing the equivalence ratio gradients, which is in contrast to results obtained in the Cambridge/Sandia configuration. This finding indicates the influence of turbulence on stratified combustion in the thin-reaction-zone regime, as characterized for instance by the difference in Karlowitz and Damköhler numbers for the Darmstadt versus the Cambridge/Sandia configuration.