Direct numerical simulations of a high Karlovitz number laboratory premixed jet flame – an analysis of flame stretch and flame thickening

Abstract

This article reports an analysis of the first detailed chemistry direct numerical simulation (DNS) of a high Karlovitz number laboratory premixed flame. The DNS results are first compared with those from laser-based diagnostics with good agreement. The subsequent analysis focuses on a detailed investigation of the flame area, its local thickness and their rates of change in isosurface following reference frames, quantities that are intimately connected. The net flame stretch is demonstrated to be a small residual of large competing terms: the positive tangential strain term and the negative curvature stretch term. The latter is found to be driven by flame speed–curvature correlations and dominated in net by low probability highly curved regions. Flame thickening is demonstrated to be substantial on average, while local regions of flame thinning are also observed. The rate of change of the flame thickness (as measured by the scalar gradient magnitude) is demonstrated, analogously to flame stretch, to be a competition between straining tending to increase gradients and flame speed variations in the normal direction tending to decrease them. The flame stretch and flame thickness analyses are connected by the observation that high positive tangential strain rate regions generally correspond with low curvature regions; these regions tend to be positively stretched in net and are relatively thinner compared with other regions. High curvature magnitude regions (both positive and negative) generally correspond with lower tangential strain; these regions are in net negatively stretched and thickened substantially.

Thin reaction zones in constant-density turbulent flows at low Damköhler numbers: Theory and simulations

“…(35) is consistent with experimental data obtained for reaction-front propagation in aqueous solutions 1 and premixed turbulent flames 32 , as well as with DNS obtained from constant-density single-reaction waves in 2D turbulence 33 , 3D thermonuclear deflagration waves 8 , and 3D highly turbulent lean methane-air and hydrogen-air flames 34 . Furthermore, the present theoretical analysis is also consistent with experimental 31,36,42,68,69 and DNS 34,[70][71][72][73][74][75][76][77][78][79] observations of thin heat-release zones in flames characterized by low Da and high Ka.…”

“…4holding under such conditions. Accordingly, the present study offers an opportunity to reconcile (i) recent experimental 31,36,42,68,69 and DNS 34, [70][71][72][73][74][75][76][77][78][79] data that indicate statistically weak (or the lack of) reaction-zone broadening in flames characterized by high Ka and low Da, and (ii) recent experimental 32 and DNS 8,33,34 data that support Eq. 4at high Ka and low Da.…”

“…4 Karlovitz numbers reported in those papers are significantly lower or higher, respectively, than the values of Da or Ka, respectively, re-calculated using Eqs. Statistically significant broadening of reaction zones was not documented in earlier DNS studies of premixed turbulent flames with single-step [70][71][72] or complex [73][74][75][76][77][78][79] chemistry. Thévenin 73 shows statistical thinning of reaction zones with decreasing Da, but this effect appears to result from Lewis number and preferential diffusion phenomena discussed elsewhere 4,13 .…”

Stretch Rate and Displacement Speed Correlations for Increasingly-Turbulent Premixed Flames

“…In recent years, Direct Numerical Simulation (DNS) has provided evidence of correlations between stretch rate components for moderate-and high-intensity turbulence [13][14][15][16]. In these investigations, the Karlovitz number, a dimensional group defined as…”

Analysis of premixed flame kernel/turbulence interactions under engine conditions based on direct numerical simulation data

“…of (1.6) describes the total area change due to normal-propagation of curved flame regions (kinematic restoration, which could be source or sink for flame area depending on the sign of the curvature), while the third term corresponds to scalar dissipation (Peters 1999). The impact of curvature moments on global flame area dynamics can be shown by adapting the decomposition of s d κ s,Ω proposed by Wang et al (2017a) to the normal-propagation and scalar dissipation terms in (1.6):…”