Conditional analysis of turbulent premixed and stratified flames on local equivalence ratio and progress of reaction

Abstract

Previous studies on the Cambridge/Sandia stratified burner have produced a comprehensive database of line Rayleigh/Raman/CO LIF measurements of scalars, as well as LDA and PIV measurements of velocity, for flames under non-uniform mixture fraction, under moderate turbulent conditions where the ratio of the turbulent velocity fluctuations to the laminar flame speed is of order 10. In prior work, we applied multiple conditioning methods to demonstrate how local stratification increases the levels of CO and H 2 , relative to the corresponding turbulent premixed flame, and enhances surface density function (SDF) and scalar dissipation rate of progress of reaction (SDR), based on extent of temperature rise, at a particular location in the flame where the mixing layer and flame brush cross. In the present study, we examine the global features of selected flames at all locations, by obtaining probability density functions (PDFs) for species concentrations, SDRs, and SDFs, conditioned on local equivalence ratio and location in the flame brush throughout the domain. We find that for most cases, species profiles as a function of temperature are well represented by laminar flame relationships at the local equivalence ratio, with some deviations attributable to either differential diffusion near the flame base and local stratification effects further downstream where the flame brush crosses the mixing layer. In particular, CO 2 is significantly affected by differential diffusion, and CO and H 2 by stratification. However, the stratification effects on the species are relatively minor when conditioned on local equivalence ratio, a simplifying result in the context of modelling. Measurements of the gradient of progress of reaction and scalar dissipation rates, conditioned on local equivalence ratio, show that the thermal zone of the flame is thickened by turbulence: the mean SDF and SDR values are in general lower than those of unstrained laminar flames. The effect is greater under rich conditions, with conditional mean SDR decreasing to less than half of the corresponding laminar value. The extent of flame thickening is the same in the premixed as the stratified case, once the stratified measurements are conditioned on the same equivalence ratio.

Scalar structure of turbulent stratified swirl flames conditioned on local equivalence ratio

“…The influence of higher strain is apparent in both swirling cases (SwB3 and SwB11), as expected from the calculated higher sensitivity of CO 2 to strain, but there is no obvious additional influence of local gradients in equivalence ratio for the stratified case SwB11 compared to SwB3. Thus, as concluded in [1] for the non-swirl cases, Fig. 1 demonstrates that the state space behaviour of CO 2 mass fraction in the stratified swirling flame is also primarily a function of local equivalence ratio.…”

“…In the same way as was done for the non-swirling flames in [1], species mass fractions in the swirl flames were conditioned on the instantaneous local equivalence ratio. The equivalence ratio is reconstructed from the atomic balance of the measured species molar fractions, X i : Fig.…”

“…The equivalence ratio is reconstructed from the atomic balance of the measured species molar fractions, X i : Fig. 7 in [1]. )…”

“…In a recent paper [1], the structure of premixed and stratified turbulent flames was analysed based on the species and temperature database generated from the Cambridge/Sandia burner [2] [3]. A major conclusion of [1] was that the relationship between major species mass fractions and temperature is primarily a function of the local equivalence ratio, with only secondary effects attributed to stratification and differential diffusion. The study also concluded that the mean scalar dissipation rate is lower than the nominal value for a laminar flame, owing to turbulent mixing.…”

Multiple conditioned analysis of the turbulent stratified flame A

“…Sweeney et al [5] conditioned 1D-Raman-Rayleigh data from the Cambridge/Sandia stratified bluff-body burner based on multiple criteria: (a) the instantaneous stratification level (b) the local equivalence ratio (c) the reaction progress variable and (d) a temperature difference over a defined length. Recently, Kamal et al [6] reported on conditioning only on the local equivalence ratio for the same configuration to investigate global features of a flame series.…”