Direct numerical simulation of thermo-diffusively unstable premixed hydrogen-air flames in a fully-developed turbulent channel flow at $Re_τ=530$

TL;DR

This paper addresses how thermo-diffusive instabilities influence premixed hydrogen–air flames interacting with realistic near-wall turbulence in a canonical wall-bounded flow. It employs direct numerical simulations of anchored V-shaped flames in fully developed turbulent channel flow at $Re_\tau=530$ for $\varphi=0.25$ and $0.35$, using detailed kinetics and diffusion (including the Soret effect) to resolve TD-driven variability and its coupling with turbulence. Key findings show a strong TD–turbulence synergy, with the stretch factor $I_0$ increasing as flames encounter stronger near-wall turbulence and local Karlovitz number $Ka$, particularly in the buffer layer, leading to elevated flame speeds and richer local reactivity; TD effects also shape flame topology and microstructure, with positive curvature regions exhibiting higher mixture enrichment and heat release, while quenching distances are reduced relative to 1D laminar HOQ references. The results yield high-fidelity data for TD-aware modeling of hydrogen flames in shear-dominated turbulence and have practical implications for combustor design and safety in near-wall environments.

Abstract

Direct Numerical Simulations (DNS) of premixed hydrogen-air flames anchored in a fully-developed turbulent channel flow (TCF) are performed at a friction Reynolds number of $\mathrm{Re}_τ=530$ and thermochemical conditions susceptible to the emergence of intrinsic thermo-diffusive (TD) phenomena acting on the turbulent flame. Two premixed flames are studied: a slower flame ($\varphi=0.25$), predominantly propagating within the core flow, and a faster one ($\varphi=0.35$), reaching closer to the channel walls and intermittently quenching on it. The present DNS database provides new insights into the characteristics of premixed flames susceptible to TD phenomena and propagating in realistic near-wall shear turbulence. The influence of varying turbulence intensity, and of wall-distance dependent time and length scales, on the flame propagation characteristics is evaluated through a detailed analysis of the local stretch factor $I_0$, quantifying reactivity enhancements caused by TD phenomena. At $\varphi=0.25$, the flame response to the fluid motions is mainly forced by the weaker turbulence present in the core flow. This results in an augmented $I_0$ compared to the laminar reference value, suggesting reactivity enhancement by the strongly non-linear interaction of TD phenomena with (relatively) weak turbulent motions present within the core flow. At $\varphi=0.35$, as the flame propagates from the core flow towards the channel walls, the flame response is forced by turbulence of increasing intensity, resulting in a corresponding augmentation of the Karlovitz number. Crucially, as the flame propagates into the near-wall region, the peak value of $I_0$ is co-located with the peak Reynolds stresses ($y^+ \sim 10$). This observation suggests a strong (local) synergistic interaction between TD phenomena and wall turbulence, ultimately resulting in significantly enhanced flame speed.

Figures (20)

• Figure 1: $(a)$ Turbulent jet flame, taken from Nicolai2025; $(b)$ V-shaped flame anchored in a TCF configuration, with hatched lines indicate the channel walls. The flames are colored by the hydrogen fuel mass fraction $Y_{\ce{H2}}$, and the unburnt and burnt regions are denoted by 'u' and 'b', respectively.
• Figure 2: Schematic representation of the simulated V-shaped flame anchored in a TCF configuration. The orange surface denotes the flame front, and the flame anchor is depicted as a cylinder.
• Figure 3: Comparison of the non-reactive, fully-developed turbulent channel flow precursor DNS at $Re_\tau = 530$ with DNS data from moser_direct_1999 at $Re_\tau = 590$. $(a)$ Non-dimensional mean streamwise velocity $U^+$. $(b)$ Non-dimensional streamwise rms velocity fluctuation $u^+_\mathrm{rms}$.
• Figure 4: Instantaneous and averaged flame structures for both simulated flames $\varphi=0.25$$(a,c,e,g)$ and $\varphi=0.35$$(b,d,f,h)$. Panels $(a,b)$ show the normalized fields of temperature $T_\mathrm{norm}$; $(c,d)$ the Bilger's mixture fraction $Z_\mathrm{Bilger,\,norm}$; $(e,f)$ the OH mass fraction $Y_{\ce{OH},\,\mathrm{norm}}$; and $(g,h)$ the heat-release rate $HRR_\mathrm{norm}$ in physical space. The colored lines in the temperature field $(a,b)$ denote different iso-values of the normalized progress variable $C_\mathrm{norm}$, where $C_\mathrm{norm}=0.7$ is used to define the flame front. In panels $(c,d)$ the vertical lines mark the boundaries of the four spatial bins $A$-$D$, positioned at different longitudinal locations (in the streamwise direction), that will be used in the statistical analysis. White boxes highlight the region which is magnified and shown in figure \ref{['fig: avg_instant_flame_zoom']}.
• Figure 5: Zoomed view of instantaneous flame structures corresponding to the white boxes in figure \ref{['fig: avg_instant_flame']} for the $\varphi=0.25$ flame $(a$-$d)$ and the $\varphi=0.35$ flame $(e$-$h)$.