Umair Ahmed

The influence of the fuel Lewis number LeF on Reynolds stresses and their transport during flame–wall interaction (FWI) is studied using a direct numerical simulation database of oblique wall-quenching of V-shaped premixed flames in a turbulent channel flow. Simulations are conducted for LeF=0.6,1.0, and 1.4. Results indicate that LeF significantly affects the magnitude and anisotropy of the Reynolds stresses. In these cases, the location of the FWI is affected by LeF, with the flame at LeF=0.6 interacting closest to the flame holder, while the flame with LeF=1.4 interacts farthest from the flame holder. The wall-normal distributions of the Reynolds stress components deviate significantly from those recorded in the non-reacting turbulent channel flows, with deviations strongly influenced by LeF. The production of the streamwise Reynolds stress component by the mean velocity gradient exceeds sink contributions from other terms, especially downstream of the FWI region. The pressure–strain term plays a dominant role in Reynolds stress transport in the spanwise direction, facilitating Reynolds stress redistribution. In particular, local equilibrium between the generation and dissipation of Reynolds stresses is not achieved, particularly as LeF decreases, leading to anisotropy of Reynolds stresses. These findings highlight the impact of LeF on near-wall flow dynamics and the production, redistribution, and dissipation of Reynolds stresses. The study provides valuable information for improving turbulence models for Reynolds-averaged Navier–Stokes simulations of FWIs under varying LeF conditions, advancing the understanding of turbulent boundary layer flows with premixed flames.

The statistical behaviors of the two-point spatial correlations of the turbulent velocity fluctuations based on Reynolds decomposition, Favre decomposition, and density-compensated Favre decomposition have been analyzed at different wall-normal distances for different stages of head-on quenching of premixed flames due to wall heat loss within turbulent boundary layers. Using a Direct Numerical Simulation database for friction Reynolds numbers Reτ of 110 and 180, it has been demonstrated that Favre fluctuation-based correlations differ quantitatively from Reynolds and density-compensated Favre fluctuation correlations, particularly in the early stages of quenching and when the flame is away from the wall. Despite these differences, the overall qualitative trends remain unaffected by the choice of definition of spatial correlations. As quenching progresses, the quantitative differences reduce due to weakening thermal expansion effects and increasing presence of burned gas in the boundary layer. This effect is strongest for wall-normal velocity fluctuations, as flame-normal acceleration due to thermal expansion primarily influences this direction in this configuration. However, momentum redistribution due to this flame normal acceleration also impacts streamwise and spanwise velocity correlations. These effects manifest in the integral length scales derived from two-point spatial correlations, with their variations evolving as quenching progresses. The longitudinal integral length scale in the streamwise direction exhibits the highest peak value among all considered length scales. While qualitative behaviors of correlations and integral length scales remain largely unaffected by the variation in Reτ considered here, a higher Reτ results in a broader wavenumber range before viscous roll-off occurs in the one-dimensional velocity spectrum.

Direct numerical simulation of flame-wall interaction (FWI) for two flame configurations under different laminar burning velocity to nonreacting wall friction velocity ratio, S L / u τ NR , have been performed. The first configuration considered is a V-flame in a turbulent channel flow with isothermal inert walls and represents oblique wall interaction. The second configuration is representative of head-on interaction (HOI) of a planar flame interacting with a wall in a fully developed turbulent boundary layer. In the case of the V-flame with S L / u τ NR = 0.4 , a smaller flame angle is obtained when compared with that of the V-flame with S L / u τ NR = 0.7 . This is a consequence of the differences in the turbulent burning velocities between the two V-flame cases, where the case with S L / u τ NR = 0.4 has a lower turbulent burning velocity when compared with that of the case with S L / u τ NR = 0.7 . By contrast, the ratio of turbulent to laminar burning velocity is higher for the S L / u τ NR = 0.4 HOI case when compared to the corresponding HOI case with S L / u τ NR = 0.7 . The flame orientations with respect to the streamwise component of velocity and wall-normal direction in addition to the S L / u τ NR values have been found to have a significant impact on the variations of wall heat flux, wall shear stress, and wall friction velocity in premixed FWI within turbulent boundary layers. The behaviors of nondimensional streamwise velocity, u + , and nondimensional temperature using wall units, T + , have also been investigated for different S L / u τ NR ratios and it is found that the standard log-law profiles in the case of u + and T + are not valid for the two configurations and the two S L / u τ NR ratios considered. Published by the American Physical Society 2024

Direct numerical simulation (DNS) data of flame–wall interaction (FWI) has been utilized to analyze the multiscale nature of turbulent Reynolds stresses and dissipation rate tensor anisotropies within turbulent reacting flow boundary layers across a broad range of scales. The DNS data of head-on quenching of premixed flames propagating through turbulent boundary layers, representative of friction Reynolds numbers Reτ of 110 and 180, has been explicitly filtered using both two- and three-dimensional Gaussian filter kernels for the purpose of multiscale analysis. The low-pass filter results demonstrate the transition from a 2-component limit to a 1-component limit near the wall with increasing filter width, accompanied by a decrease in isotropy, suggesting a significant alteration in dominant flow patterns and a diminishing tendency toward isotropy. The high-pass filter results indicate a progressive increase in anisotropy with the progress of FWI at the channel center, emphasizing the anisotropy of the large scales with the progress of FWI. Furthermore, behaviors of the second and third invariants of the Reynolds stress tensor remain qualitatively similar to that of the dissipation rate tensor at all stages of FWI, suggesting a link between viscous dissipation and Reynolds stress distributions; notably, there is a stronger isotropic tendency in the dissipation rate tensor when the flame is away from the wall, intensifying with an increase in Reynolds numbers. However, as FWI progresses, the shift in the trend toward the 1-component limit indicates an increase in anisotropy within the turbulent reacting flow for the region near the center of the channel.

A recently proposed scalar forcing scheme that maintains the mixture fraction mean, root-mean-square and probability density function in the unburned gas can lead to a statistically quasi-stationary state in direct numerical simulations of turbulent stratified combustion when combined with velocity forcing. Scalar forcing alongside turbulence forcing leads to greater values of turbulent burning velocity and flame surface area in comparison to unforced simulations for globally fuel-lean mixtures. The sustained unburned gas mixture inhomogeneity changes the percentage shares of back- and front-supported flame elements in comparison to unforced simulations, and this effect is particularly apparent for high turbulence intensities. Scalar forcing does not significantly affect the heat release rates due to different modes of combustion and the micro-mixing rate within the flame characterised by scalar dissipation rate of the reaction progress variable. Thus, scalar forcing has a significant potential for enabling detailed parametric studies as well as providing well-converged time-averaged statistics for stratified-mixture combustion using Direct Numerical Simulations in canonical configurations.