Evolution of Flame Curvature in Turbulent Premixed Bunsen Flames at Different Pressure Levels

Abstract

The physical mechanisms underlying the curvature evolution in turbulent premixed Bunsen flames at different thermodynamic pressures are investigated using a three-dimensional Direct Numerical Simulation database. It is found that, due to the occurrence of the Darrieus-Landau instability, the high-pressure flame exhibits higher probability than the low-pressure cases of developing large negative curvature values and saddle concave topologies. The terms in the curvature transport equation due to normal strain rate gradients and curl of vorticity arising from both turbulent flow and flame normal propagation play pivotal roles in the curvature evolution. The mean value of the net contribution of the flame propagation terms dominates over the net contributions arising from the background fluid motion. The net contribution of the source/sink terms tries to reduce the convexity of the flame surface in the positively curved locations. By contrast, the net contribution of the source/sink terms promotes concavity of the flame surface towards the reactants in the negatively curved regions and this effect is particularly strong for the high pressure flame, where the effects of the Darrieus-Landau instability are prominent. This also gives rise to large negative skewness of the probability density functions of curvature in the high-pressure flame with the Darrieus-Landau instability.

Analysis of the Closures of Sub-grid Scale Variance of Reaction Progress Variable for Turbulent Bunsen Burner Flames at Different Pressure Levels

“…It has been shown elsewhere Chakraborty et al 2008) that the model parameter c originates due to the curvature m contribution to the SDR transport and m dependence of S d plays a key role in this contribution. It has been shown and discussed elsewhere (Klein et al 2018b;Alquallaf et al 2019) that the flame curvature statistics are significantly affected by pressure. The likelihood of obtaining DL instability increases with increasing pressure (Klein et al 2018a, b;Alquallaf et al 2019) and this is reflected in the increased skewness of negative curvature values in cases B, C and F (not shown here but refer to Klein et al 2018a, b;Alquallaf et al 2019).…”

“…1 that the flame morphology changes significantly with increasing pressure. The extent of wrinkling in cases C and F is significantly greater than in cases A and E due to DL instability (Klein et al 2018a, b;Alquallaf et al 2019) and interested readers are referred to Klein et al (2018a, b) and Alquallaf et al (2019) for further discussion on this aspect. Figure 1 further shows that the extent of wrinkling of the isosurface decreases with increasing Δ because of the smearing of local information due…”

“…P 0 = 1.0 bar ). Uniform Cartesian meshes of 250 × 250 × 250, 560 × 560 × 560, 795 × 795 × 795 and 1272 × 1272 × 1272 are used for cases A, D, E, case B, case C and case D, respectively, which ensure for all cases the resolution of the flame thickness and the smallest scales of turbulence (Klein et al 2018a, b;Chakraborty et al 2019;Alquallaf et al 2019).…”

“…The flame thickness decreases and turbulent Reynolds number increases (for a given set of values of root-mean-square velocity fluctuation and integral length scale) with increasing pressure and thus it often becomes challenging to analyse high-pressure premixed flames. The advances in high performance computing have enabled detailed analyses of high pressure premixed turbulent combustion and its associated modelling using direct numerical simulations (DNS) data (Savard et al 2017;Klein et al 2018a, b;Chakraborty et al 2019;Alquallaf et al 2019). The present paper focuses on the assessment of the closure of sub-grid scale (SGS) reaction progress variable variance under elevated pressure conditions for turbulent Bunsen burner flames.…”