Analysis of near wall flame and wall heat flux modeling in turbulent premixed combustion

TL;DR

This work investigates premixed near-wall flame behavior and wall heat flux in turbulent, wall-bounded flows using DNS for two configurations: wall-normal flushing and inclined sweeping flames. It introduces a skin-friction tensor framework to link flame curvature $\kappa$ with wall vorticity $\omega_1$ and analyzes how normal and tangential flame-strain rates $S_n$ and $S_t$ relate to $\kappa$, while revealing strong alignment of the progress-variable gradient with the most compressive direction near the wall and a species alignment index $G_{\mathrm{align}}$ that captures cross-species misalignment in thickened near-wall flames. A practical wall heat flux model is developed by reducing the energy and species balance to a local flame-zone description and is validated against laminar references and DNS data for both configurations, showing robustness to finite thickness, detailed chemistry, wall-parallel heat transfer, and flame orientation. The model’s axisymmetric, orientation-insensitive formulation suggests it can be integrated into LES to predict wall heat losses in realistic turbulent combustion devices. Overall, the paper provides new physical insights into FWI, introduces scalable diagnostic tools, and delivers a predictive, DNS-informed wall heat flux predictor for engineering applications.

Abstract

Reactive flows in confined spaces involve complex flame-wall interaction (FWI). This work aims to gain more insights into the physics of the premixed near-wall flame and the wall heat flux as an important engineering relevant quantity. Two different flame configurations have been studied, including the normal flushing flame and inclined sweeping flame. By introducing the skin friction vector defined second-order tensor, direct numerical simulation (DNS) results of these two configurations show consistently that larger flame curvatures are associated with small vorticity magnitude under the influence of the vortex pair structure. Correlation of both the flame normal and tangential strain rates with the flame curvature has also been quantified. Alignment of the progress variable gradient with the most compressive eigenvector on the wall is similar to the boundary free behavior. To characterize the flame ordered structure, especially in the near-wall region, a species alignment index is proposed. The big difference in this index for flames in different regions suggests distinct flame structures. Building upon these fundamental insights, a predictive model for wall heat flux is proposed. For the purpose of applicability, realistic turbulent combustion situations need to be taken into account, for instance, flames with finite thickness, complex chemical kinetics, non-negligible near-wall reactions, and variable flame orientation relative to the wall. The model is first tested in an one-dimensional laminar flame and then validated against DNS datasets, justifying the model performance with satisfying agreement.

Figures (18)

• Figure 1: Spatial distribution of: (a) instantaneous hydrogen-based progress variable field; (b) instantaneous temperature field together with the flame isosurface defined by $c=0.8$.
• Figure 2: Spatial distribution of an instantaneous DNS temperature field superimposed with a hydrogen-based progress variable $c=0.8$ isosurface.
• Figure 3: Distributions of the flame temperature, progress variable (represented by the hydrogen mass fraction), and the heat release rate distributions with respect to the non-dimensional wall distance $\delta/\delta_z$ under different non-dimensional inlet strain rates $\varepsilon$. Results from the mixture-averaged transport model and the unity Lewis number model are shown by the solid and dashed lines, respectively.
• Figure 4: Comparison of the non-dimensional diffusion terms in the temperature equation at $\varepsilon=8.92$. The solid and dashed lines indicate the results from the mixture-averaged transport model and the unity Lewis number model, respectively.
• Figure 5: Joint PDFs between $\mathrm{H}_2$ contour line defined curvature $\kappa_{\mathrm{H}_2}$ and $\omega_1$ for the normal flushing flame: (a) $0.65< c\leq0.8$; (b) $0.8 <c\leq0.95$, and for the inclined sweeping flame: (c) $0.65 <c\leq0.8$; (d) $0.8< c\leq0.95$.