A Consistent Flamelet Formulation for Non-Premixed Combustion Considering Differential Diffusion Effects

Large eddy simulations of non-reacting H2/CO2 jets mixing with air are performed and the calculations are compared with the experiments reported by Smith et al. [“Laser Raman scattering measurements of differential molecular diffusion in non-reacting turbulent jets of H2/CO2 mixing with air,” Phys. Fluids 7, 1455–1466 (1995)]. The influence of differential diffusion effects for Reynolds numbers Re = 1000–8000 is analyzed and a differential diffusion parameter, ξ, is defined on the basis of normalized H2 and CO2 concentrations in order to quantify the effects of differential diffusion with increasing Reynolds number. The analysis is made not only in physical space but also with scatter plots and histograms. The simulation results reveal that differential diffusion effects are significant at downstream locations (more than 15 nozzle diameters away from the inlet) only for the lower Reynolds numbers (Re = 1000–2000). However, differential diffusion effects are present for all Reynolds numbers examined close to the inlet (closer than 10 nozzle diameters). This is not only confirmed by the mean results of the differential diffusion parameter, ξ, but also by looking at the histograms of ξ. This is an important indication that differential diffusion can be important in turbulent reacting flows if laminarization of the flow or weakening of turbulent diffusion occurs. Including differential diffusion effects in turbulent reactive flows involving mixtures with vastly different mass diffusivities can, therefore, improve the accuracy of numerical simulations. Results obtained assuming equal species mass diffusivities revealed that differential diffusion effects do not have any significant influence in the velocity field.

Differential diffusion effects in counter-flow premixed hydrogen-enriched methane and propane flames

Flame structure analysis of turbulent premixed/stratified flames with H2 addition considering differential diffusion and stretch effects

Broken reaction zone and differential diffusion effects in high Karlovitz n-C7H16 premixed turbulent flames

Effects of differential diffusion on turbulent burning velocity and flame surface area in NH3/H2/N2-air premixed flames

Rate-ratio asymptotic analysis of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed methane flames with comparison to experiments

A new imaging technique was developed that provides two-dimensional images of the mixture fraction (ξ), scalar dissipation rate (χ), temperature (T), and fuel consumption rate \((\dot{\omega}_{\rm F})\) in a turbulent non-premixed jet flame. The new method is based on “seeding” nitric oxide (NO) into a particular carbon monoxide–air flame in which it remains passive. It is first demonstrated that the mass fraction of NO is a conserved scalar in the present carbon monoxide–air flame configuration, using both laminar flame calibration experiments and computations with full chemistry. Simultaneous planar laser-induced fluorescence (PLIF) and planar Rayleigh scattering temperature imaging allow a quantitative determination of the local NO mass fraction and hence mixture fraction in the turbulent jet flame. The instantaneous mixture fraction fields in conjunction with the local temperature fields are then used to determine quantitative scalar dissipation rate fields. Advantages of the present technique include an improved signal-to-noise ratio over previous Raman scattering techniques, improved accuracy near the stoichiometric contour because simplifying chemistry assumptions are not required, and the ability to measure ξ and χ in flames experiencing localized extinction. However, the method of measuring ξ based on the passive NO is restricted to dry carbon monoxide–air flames due to the well-controlled flame chemistry. Sample imaging results for ξ, χ, T, and \(\dot{\omega}_{\rm F}\) are presented that show high levels of signal-to-noise while resolving the smallest mixing scales of the turbulent flowfield. The application, accuracy, and limitations of the present technique are discussed.

Syngas is an attractive alternative to currently popular hydrocarbon fuels due to its ability to be synthesized from multiple sources and lower carbon content. Direct Numerical Simulation (DNS) studies on premixed and non-premixed syngas flames have recently received attention. In this light, DNS of turbulent premixed syngas has been performed. Influence of turbulence and differential diffusion effects on chemical pathways of fuels like Hydrogen and methane has been studied in the past. Similar study on syngas flame has not been reported in the literature. Two cases with variation in the intensity of turbulence have been reported in this study. Effect of differential diffusion and turbulence on heat release rate and fuel consumption rate has been discussed. The behavior of heat release rate and fuel consumption rate was largely similar between laminar and turbulent flames considered in this study. Influence of species Lewis number was found to be more pronounced than that of turbulence.

Direct numerical simulation of auto-ignition of a hydrogen vortex ring reacting with hot air

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.

Assessment of a methodology to include differential diffusion in numerical simulations of a turbulent flame

A multi-scale asymptotic scaling and regime analysis of flamelet equations including tangential diffusion effects for laminar and turbulent flames

Effects of differential diffusion on hydrogen flame kernel development under engine conditions

Laminar triple flames are investigated numerically using detailed and reduced chemical reaction mechanisms. Triple flames are believed to play an essential role in flame propagation in partially premixed systems. In order to reduce the computational cost of the simulations, the flamelet-generated manifold (FGM) method is used to simplify the chemistry model. The FGM method is based on a library of premixed laminar flamelets. Mixture fraction variations in partially premixed flames are taken into account by using the mixture fraction as an extra degree of freedom. A comparison of the results computed with FGM and with detailed chemistry shows that FGM predicts the structure of triple flames very accurately. The gradient of the mixture fraction in the unburnt mixture is varied, and its influence on the structure and propagation of triple flames is studied. For decreasing gradients, the curvature of the premixed flame branch decreases and the propagation velocity increases. Due to the diffusive nature of the flow and the use of symmetry boundary conditions in the lateral direction, high mixture fraction gradients could not be realized at the position of the flame. As a result, the trailing diffusion flame branch is only weakly present. The heat release in the premixed flame branches is two orders of magnitude higher than in the diffusion flame branch. The premixed and diffusion flame branches are studied using a flamelet analysis. Flame stretch and curvature appear to be significant in the premixed flame branch of a triple flame. These effects cause a decrease in the local mass burning rate of almost 15%. The structure of the diffusion flame branch appears to be similar to the structure of a counterflow diffusion flame with the same scalar dissipation rate.

Auto-ignition processes of hydrogen, diluted with nitrogen, in heated air are numerically investigated by means of an unsteady laminar flamelet approach in mixture fraction space. The focus is on the auto-ignition delay time and the most reactive mixture fraction as obtained with five chemical mechanisms. Two strongly different levels of dilution, corresponding to experiments in the open literature, are considered. This concerns low-temperature chemistry at atmospheric pressure. The temperature of the air stream is much higher than the temperature of the fuel stream in the cases under study. We extensively investigate the effect of the co-flow temperature, the conditional scalar dissipation rate and the resolution in mixture fraction space for one case. With respect to the conditional scalar dissipation rate, we discuss the Amplitude Mapping Closure (AMC) model with imposed maximum scalar dissipation rate at mixture fraction equal to 0.5, as well as a constant conditional scalar dissipation rate value over the entire mixture fraction value range. We also illustrate that an auto-ignition criterion, based on a temperature rise, leads to similar results as an auto-ignition criterion, based on OH mass fraction, provided that the hydrogen is not too strongly diluted.