Surface density function evolution and the influence of strain rates during turbulent boundary layer flashback of hydrogen-rich premixed combustion

Abstract

The statistical behavior of the magnitude of the reaction progress variable gradient [alternatively known as the surface density function (SDF)] and the strain rates, which govern the evolution of the SDF, have been analyzed for boundary layer flashback of a premixed hydrogen-air flame with an equivalence ratio of 1.5 in a fully developed turbulent channel flow. The non-reacting part of the channel flow is representative of the friction velocity based Reynolds number Reτ = 120. A skeletal chemical mechanism with nine chemical species and twenty reactions is employed to represent hydrogen-air combustion. Three definitions of the reaction progress variable (RPV) based on the mass fractions of H 2 , O 2 , and H 2 O have been considered to analyze the SDF statistics. It is found that the mean variations of the SDF and the displacement speed S d depend on the choice of the RPV and the distance away from the wall. The preferential alignment of the RPV gradient with the most extensive principal strain rate strengthens with an increase in distance from the cold wall, which leads to changes in the behaviors of normal and tangential strain rates from the vicinity of the wall toward the middle of the channel. The differences in displacement speed statistics for different choices of the RPV and the wall distance affect the behaviors of the normal strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of the effective normal strain rate experienced by the progress variable isosurfaces for different wall distances and choices of RPVs.

Assessment of Bray Moss Libby formulation for premixed flame-wall interaction within turbulent boundary layers: Influence of flow configuration

“…It has recently been shown in a DNS study of statistically planar methane-air flames with detailed chemistry and transport [60] that the global Lewis number remains close to unity and the leading-order response of the flame speed to turbulence is primarily driven by the global Lewis number even under very intense turbulence conditions which justifies the use of unity Lewis number in this work. The heat release parameter τ is taken to be 2.3 for the flames considered here, which corresponds to preheating of reactants to a temperature of T R = 730 K. This value of T R and the resulting τ is consistent with the earlier detailed chemistry DNS of FWI in the case of a premixed V-flame in a turbulent channel flow [13], turbulent boundary layer flashback [15,[17][18][19] and single-step chemistry premixed V-flame DNS with FWI [1,2] and without FWI [61]. In order to perform wall bounded flow simulations the isothermal no-slip wall boundary condition is implemented in SENGA+ according to the Navier Stokes Characteristic Boundary Conditions (NSCBC) technique [62].…”

“…At the same time the vortical structures transport unburned fluid away from the wall and carry it into the burned gases, consequently creating pockets of fresh gases in the burnt gas regions. The work on FWI within turbulent channel flows with isothermal inert walls has been extended by Gruber et al [13] and Ahmed et al [14] in the case of a premixed V-flame, and by Gruber et al [15,16], Kitano et al [17] and Ahmed et al [18,19] in the case of turbulent boundary layer flashback. More recently, FWI has also been investigated under adiabatic wall boundary conditions by Ahmed et al.…”

Energy Integral Equation for Premixed Flame-Wall Interaction in Turbulent Boundary Layers and its Application to Turbulent Burning Velocity and Wall Flux Evaluations

“…Only a brief discussion is provided here. The simulation has been carried out using a uniform Cartesian gridbased compressible DNS code SENGA+ [6][7][8][9][10][11][12][14][15][16]24,25,28] where all the spatial derivatives are evaluated using a high-order finitedifference scheme (10th order central difference scheme for the internal grid points but the order of accuracy gradually reduces to 2nd order at the non-periodic boundaries) and time advancement has been carried out using a low storage 3rd order Runge-Kutta scheme. A non-reacting turbulent plane channel flow driven by a constant streamwise pressure gradient (i.e.…”

“…[9,11,15,16] for further information in this regard. Moreover, several previous analyses [5][6][7][8][9][10][11][12][14][15][16][20][21][22]24,25,27,29,33] provided important insights into premixed FWI using simple chemistry and the same approach has been adopted here. Note that the derivation of Eqs.…”

“…To date, most analyses on premixed turbulent combustion have been conducted for flames without any wall effects, and recent advancements in experimental diagnostics [2][3][4] and computational simulations [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] have enabled insightful analyses of FWI for turbulent premixed com-bustion. These analyses provided valuable insights into the flame structure and flow dynamics [2,8,12,20] , wall heat flux [2][3][4][5]7,[20][21][22][23][24] , reactive scalar gradient [3,6,7,11,[14][15][16][24][25][26] , kinetic energy [9,13,27] , turbulent scalar flux [10,27] and displacement speed [13,…”