Effects of equivalence ratio variations on turbulent flame speed in lean methane/air mixtures under lean-burn natural gas engine operating conditions

Abstract

Direct numerical simulations (DNS) of turbulent premixed methane/air flames are carried out to investigate the effects of equivalence ratio on the turbulent flame speed in lean mixtures. Turbulent flames are simulated as statistically stationary following a Lagrangian framework using an inflowoutflow configuration. The inflow velocity is dynamically adjusted at run-time to stabilize the flame brush location within the computational domain. Linear forcing is applied inside the unburned mixtures to maintain the turbulent intensities at desired levels. For the same turbulence properties, several equivalence ratios near the lean limit are selected and it is shown that the normalized turbulent flame speed is a function of the equivalence ratio. Velocity and length scales of the imposed turbulence are then selected in such a way that the Karlovitz and Damköhler numbers remain constant for flames of different equivalence ratios. Simulations are run for more than 80 eddy turnover times and the turbulent flame speed is derived by averaging the inflow velocity. The results show that equivalence ratio does not have an explicit effect on the normalized turbulent flame speed above the lean limit. Examining the flame surface statistics, it is shown that the flame surface normal is preferentially parallel to the most compressive strain rate direction for all equivalence ratios. KeywordsDNS of turbulent premixed flames, premixed flame speed, lean-burn, natural gas engines

Response of heat release to equivalence ratio variations in high Karlovitz premixed H2/air flames at 20 atm

“…Periodic boundary conditions are applied in lateral directions, and non-reflecting Navier-Stokes Characteristics Boundary Conditions (NSCBC) [45] are applied at the inlet/outlet to maintain the high pressure. The domain width is set to be 10 times of the integral length scale, which is higher than that of reported simulations with similar configurations [4,42,43,46]. The parameters of DNS database are listed in Table 1.…”

“…In lean premixed combustion, the peak combustion temperature is reduced, leading to lower NOx emission. However, turbulent lean premixed flames are susceptible to equivalence ratio oscillation, which is one of the most significant mechanisms contributing to combustion instabilities, and local extinction can occur due to variations of equivalence ratio in combustion chambers [2][3][4]. In modern gas turbines, typical equivalence ratios at base load are in the range of 0.45-0.6 [2].…”

“…Wang et al [4,26,27] extended DNS to turbulent combustion at 20 bar with equivalence ratios in the range of 0.39-0.5. Some cases were performed with high turbulence intensity which is relevant to lean-burn natural gas engines.…”

A conservative, thermodynamically consistent numerical approach for low Mach number combustion. Part I: Single-level integration

“…Low Mach number numerical simulation methodology provides a valuable tool for efficiently modelling reacting flow with detailed kinetics and transport; see [1][2][3][4][5][6][7][8] for a variety of recent studies. Low Mach number models are derived from fully compressible equations using low Mach number asymptotics [9,10] and do not include acoustic wave propagation, allowing for much larger time steps based on an advective CFL condition.…”

A hybrid adaptive low-Mach number/compressible method: Euler equations

“…In these engines the region where the burning takes place can be modeled using a low-Mach-number approach, since the short-wavelength acoustic waves generated by the heat release do not carry sufficient information or energy to be of interest. Low-Mach-number modeling of turbulent combustion has been demonstrated to be an efficient way to generate accurate solutions [4,5,6,7,8,9]. However, in large burners, under certain conditions the longwavelength acoustic waves that emanate from the burning region can reflect from the walls of the burner and impinge on the burning region, generating thermoacoustic instabilities which can be violent enough to disrupt the flame, as well as lead to mechanical failures or excessive acoustic noise [10,11,12,13,14,15].…”