High Karlovitz Number Research Articles

Differential diffusion effects, distributed burning, and local extinctions in high Karlovitz premixed flames

Direct numerical simulations of premixed n-heptane/air flames at different Karlovitz numbers are performed using detailed chemistry. Differential diffusion effects are systematically isolated by performing simulations with both non-unity and unity Lewis numbers. Different unburnt temperatures and turbulence intensities are used and their effects on the flame structure and chemical source terms are investigated. As the unburnt gases are preheated, the viscosity ratio across the flame is reduced and the Karlovitz number at the reaction zone is increased. The increase in turbulence intensity suppresses differential diffusion effects on the flame structure (i.e. species dependence on temperature). However, differential diffusion effects on the chemical source terms are still noticeable even at the highest Karlovitz number simulated. Simulations with differential diffusion effects exhibit lower mean fuel consumption and heat release rates than their unity Lewis number counterparts. However, the difference is reduced as the reaction zone Karlovitz number is increased. Transition to distributed burning is characterized by a broadening of the reaction zone resulting from enhanced turbulent mixing. Local extinctions in the burning rate are observed only in non-unity Lewis number simulations and their probability decreases at high Karlovitz numbers. These results highlight the importance of using the reaction zone Karlovitz number to investigate the effect of turbulence on the chemical source terms and to compare flames at different unburnt temperatures.

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

A series of direct numerical simulations of a high Karlovitz number, n-C7H16, turbulent premixed flames performed previously in Savard et al. (2014) are further analyzed in this paper. Two flames are considered: one with unity Lewis numbers to isolate the effect of turbulence on the flame, and one with non-unity Lewis numbers to study the influence of turbulence on differential diffusion. In this paper, the focus is put on the reaction zone and how it is affected by turbulence. First, the reaction zone is shown to be thin for both flames, yet large chemical source term fluctuations are observed. In particular, for the non-unity Lewis number flame, while being thin, the reaction zone is also broken. Second, differential diffusion is shown to have limited effect on the distributions of strain rate and curvature at the reaction zone. Due to the high level of turbulence, the flame behaves more like a material (i.e. passive) surface than a propagating surface. Third, the fuel consumption rate is found to be correlated (yet weakly) with strain rate in the unity Lewis number flame, whereas a stronger correlation with curvature is found in the non-unity Lewis number flame. All these results explain the apparent turbulent flame speeds. It is found that the contribution of the fluctuations in the fuel consumption rate averages away in the unity Lewis number flame. On the other hand, for the non-unity Lewis number flame, the non-linear correlation between fuel consumption rate and curvature has a strong impact on the turbulent flame speed.

Direct numerical simulation of lean premixed CH4/air and H2/air flames at high Karlovitz numbers

Three-dimensional direct numerical simulation with detailed chemical kinetics of lean premixed CH4/air and H2/air flames at high Karlovitz numbers (Ka ∼ 1800) is carried out. It is found that the high intensity turbulence along with differential diffusion result in a much more rapid transport of H radicals from the reaction zone to the low temperature unburned mixtures (∼500 K) than that in laminar flamelets. The enhanced concentration of H radicals in the low temperature zone drastically increases the reaction rates of exothermic chain terminating reactions (e.g., H + O2+M = HO2 + M in lean H2/air flames), which results in a significantly enhanced heat release rate at low temperatures. This effect is observed in both CH4/air and H2/air flames and locally, the heat release rate in the low temperature zone can exceed the peak heat release rate of a laminar flamelet. The effects of chemical kinetics and transport properties on the H2/air flame are investigated, from which it is concluded that the enhanced heat release rate in the low temperature zone is a convection–diffusion-reaction phenomenon, and to obtain it, detailed chemistry is essential and detailed transport is important.

Flame structure analysis for categorization of lean premixed CH4/air and H2/air flames at high Karlovitz numbers: Direct numerical simulation studies

This paper presents direct numerical simulation studies of lean CH4/air and H2/air flames at high Karlovitz numbers utilizing detailed chemical kinetic mechanisms. Identical Karlovitz numbers are applied to the two flames; however, as the interaction of high turbulence intensity small scale structures with the reaction zones is considered, a significant difference in the effect of the turbulence structures on the inner reaction layer is observed. The heat release rate layer as well as layers of different species reaction rates is found to be more distributed for the H2/air flame compared with those in the CH4/air flame, owing to the difference in the Kolmogorov length scale and the differential diffusion effects. A modified, species-specific Karlovitz number is proposed, where the chemical time scale in the definition is a characteristic species-specific chemical time scale, which is obtained from stationary 1D simulations. The difference between the two flames for different intermediate species corresponds well to the difference in the obtained species-specific Karlovitz numbers. A validation study of the species-specific Karlovitz number is also conducted, where a turbulent effective chemical time scale is quantified for different intermediate species from instantaneous data sets. It is shown that in the preheat zone and reaction zone of the flame, the ratio between the laminar and turbulent chemical time scales of intermediate species corresponds well to the species-specific Karlovitz numbers for the two different flames.

Linear Eddy Mixing Model Studies of High Karlovitz Number Turbulent Premixed Flames

Turbulent premixed flames exhibit different structural and propagation characteristics with increasing upstream turbulence intensity starting from thin wrinkled flames in the Corrugated Flamelet regimes to a flame with a thicker preheat zone in the Thin Reaction Zone Regime (TRZ) and finally, becoming more disorganized or broken in the Distributed or Broken Reaction Zone (D/BRZ) regimes under intense turbulence. A single comprehensive predictive model that can span all regimes does not currently exist, and in this study we explore the ability of the stand-alone one-dimensional linear-eddy mixing (LEM) model to simulate the flames in all these regimes. Past applications of this 1DLEM model have demonstrated reasonable predictions in the flamelet and TRZ regimes and here, new experiments in the TRZ regime are specifically addressed to evaluate the predictive capability of this model. Additional simulations in the D/BRZ regimes (where no data is currently available) are performed to determine if the model can be extended to the high turbulence regime. Comparison with the data in the TRZ regime shows satisfactory agreement. Analysis suggests varying levels of preheat zone broadening in all the TRZ and D/BRZ cases. While the average heat release distribution for the TRZ cases is nearly identical to the laminar unstrained baseline, changes to the species and heat release distribution are observed only at a high Karlovitz Number K a > 103. In the D/BRZ regime it is shown that the transition is related to enhanced turbulent diffusion that dominates molecular diffusion effects causing deviations from the laminar baseline.

LES of a premixed jet flame DNS using a strained flamelet model

Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence’s perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed flamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slot-jet direct numerical simulation (DNS). In the premixed regime diagram this slot-jet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model under-predicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes.

Simultaneous visualization of OH, CH, CH2O and toluene PLIF in a methane jet flame with varying degrees of turbulence

This paper presents simultaneous, single shot planar laser-induced fluorescence (PLIF) imaging of four species: OH, CH, CH2O (formaldehyde) and toluene (C6H5CH3) in methane/air jet flames. The jet flames were stabilized by a flat pilot flame generated using a McKenna type burner. Both flames were operated with a stoichiometric premixed methane/air mixture at room temperature and atmospheric pressure. Several flames, with varying jet flow speeds, were investigated, spanning from laminar (10m/s jet exit velocity) up to highly turbulent flame conditions, with high Karlovitz numbers, (150m/s jet exit velocity). Measuring the four species presented above provides detailed information on jet flame structures including the transition from fuel (indicated by the fuel tracer toluene) via the preheat zone (indicated by CH2O) and the inner layer of the flame front (indicated by CH) to the oxidation layer and the postflame zone (both indicated by OH). Furthermore, the simultaneously recorded PLIF images enable the study of correlations between these key species. Especially, overlapping regions between the species in the flames is of interest. The result indicates that turbulence in the present jet flames affects primarily the mixing in the preheat zone and the wrinkling of the reaction layers. It does however not significantly affect the inner flame front structures, represented by the CH radicals, as the thickness of the CH layer remains fairly constant under the investigated flame conditions.

Large Eddy Simulations of a piloted lean premix jet flame using finite-rate chemistry

A Large Eddy Simulation (LES) model capable of accurately representing finite-rate chemistry effects in turbulent premixed combustion is presented. The LES computations use finite-rate chemistry and implicit LES combustion modelling to simulate an experimentally well-documented lean-premixed jet flame stabilized by a stoichiometric pilot. The validity of the implicit LES assumption is discussed and criteria are expressed in terms of subgrid scale Damköhler and Karlovitz numbers. Simulation results are compared to experimental data for velocity, temperature and species mass fractions of CH4, CO and OH. The simulation results highlight the validity and capability of the present approach for the flame and in general the combustion regime examined. A sensitivity analysis to the choice of the finite-rate chemistry mechanism is reported, this analysis indicates that the one and two-step global reaction mechanisms evaluated fail to capture the reaction layer with sufficient accuracy, while a 20-species skeletal mechanism reproduces the experimental observations accurately including the key finite-rate chemistry indicators CO and OH. The LES results are shown to be grid insensitive and that the grid resolution within the bounds examined is far less important compared to the sensitivity of the finite-rate chemistry representation. The results are analyzed in terms of the flame dynamics and it is shown that intense small scale mixing (high Karlovitz number) between the pilot and the jet is an important mechanism for the stabilization of the flame.

High resolution imaging of flameless and distributed turbulent combustion

Planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements were used for the study of turbulence/combustion interactions in distributed reaction regimes including flameless or MILD combustion. A novel laboratory scale burner (Distributed and Flameless Combustion Burner – DFCB) was used to reach uniquely high Karlovitz numbers, presently reported up to 14,400. It consists of a highly turbulent piloted high speed jet burner with a vitiated coflow. Six cases are reported whereas two of them (leaner cases) led to an invisible reacting zone, though still emitting light in the UV and near infrared range. Simultaneous OH/CH 2O PLIF image with 50 μm spatial resolution were achieved to capture the variation of intermediate species in the reaction layer. When complemented with temperature images obtained by Rayleigh scattering measurement, it provided insights of the reaction front structures as well as measures of the flame brush thicknesses. In particular, variations in the jet velocity highlighted the influence of turbulent mixing (hence turbulence/chemistry interaction) on the flame structures as depicted by the formation of relatively large pools of CH 2O. Further, variations in the jet stoichiometry impacted on the reaction zone visibility but only marginally on the intensity and moderately on the overall shape of the OH and CH 2O signals.

05/02232 Direct numerical simulation of hydrogen-enriched lean premixed methane-air flames

Large eddy simulation (LES) is employed to investigate the effects of hydrogen within the fuel mixture for lean CH4–H2 turbulent premixed flames in the distributed reaction zone (high Karlovitz number). The subfilter combustion term representing the interaction between turbulence and chemistry is modelled using the partially stirred reactor (PaSR) model, along with complex chemistry using a skeletal mechanism based on the GRI-MECH3.0, to handle mixtures up to 60% of hydrogen in volume. The influence of hydrogen enrichment at high turbulence levels is studied by means of the local flame structure, and the assessment of species formation inside the flame. Results show that with higher hydrogen content, the flame displays a higher degree of wrinkling and is shorter because of a higher turbulent flame speed. The local structure of the flame is mainly affected by the turbulence but differential diffusion effects still play a role in the flame front aspect, especially when hydrogen addition becomes important. Concerning the formation of intermediate species and pollutants, by increasing the amount of hydrogen in the fuel mixture at fixed equivalence ratio, more emissions are reported. This is expected as the mixture reactivity (heat release) increases when substituting methane by hydrogen in the unburned mixture.

The transient response of strained laminar-premixed flames

Modeling and simulation of turbulent combustion in premixed gases, for relatively large-scale and low-intensity turbulence, have traditionally been based on the assumption that the flame response to strain is instantaneous. In this paper, we revisit the validity of this assumption by examining the time-dependent response of a premixed laminar flame when subjected to a sudden change in strain and a periodic strain. We find that at unity Lewis number and for a stepwise increase in strain, the settling time of the flame varies between the chemical time, the flame time and the flow time as the Karlovitz number changes from low to intermediate to high Karlovitz numbers and over the entire range of flame temperatures. For given Lewis and Karlovitz numbers, the settling time decreases as the flame temperature increases. Thus, in a flamelet or thin flame modeling, and over the entire range of Lewis number, the response of a premixed flame can be considered instantaneous only for high flame temperatures. The same is found to be true for intermediate flame temperatures when the Lewis number is unity. Otherwise, for low and intermediate flame temperatures, and nonunity Lewis numbers, corrections reflecting the lag between the flow and the flame should be considered. The response of the flame to oscillating strains whose maximum value is below unity Karlovitz number is also investigated for two values of the flame temperatures. It is found that the average burning velocity is close to the burning velocity at the average strain. For low frequency oscillations, the phase shift between the strain and the burning velocity is close to 0 for L e < 1 and near 90° for L e ≥ 1. For high frequency oscillating strains, and over the entire range of Lewis number and flame temperature, the phase shift is of order of 140°.