Quasi-Direct Numerical Simulation of Turbulence–Flame Interactions in a High-Karlovitz-Number Ammonia Premixed Jet Flame

Ammonia has been identified as a promising fuel to diminish greenhouse gas emission. However, ammonia combustion presents certain challenges including low reactivity and high NO emission. In the present study, three-dimensional direct numerical simulations (DNS) of ammonia/air premixed slot jet flames with varying Karlovitz numbers (Ka) and cracking ratios were performed. Three cases were considered, including two pure ammonia/air flames with different turbulence intensities and one partially cracked ammonia/air flame with high turbulence intensity. The effects of turbulence intensity and partial ammonia cracking on turbulence–flame interactions and NO emission characteristics of the flames were investigated. It was shown that the turbulent flame speed is higher for the flames with high turbulence intensity. In general, the flame displacement speed is negatively correlated with curvature in negative curvature regions, while the correlation is weak in the positive curvature regions for highly turbulent flames. Most flame area is consumed in negatively curved regions and produced in positively curved regions. It was found that the NO mass fraction is higher in the flame with partial ammonia cracking compared to the pure ammonia/air flames. The NO pathway analysis shows that the NH → NO pathway is enhanced, while the NO consumption pathway is suppressed in the partially cracked ammonia/air flame. The NO mass fraction is higher in regions of negative curvature than positive curvature. Interestingly, the NO mass fraction is found to be negatively correlated with the local equivalence ratio, which is consistent in both the DNS and the corresponding laminar premixed flames.

LES/PDF based modeling of soot–turbulence interactions in turbulent flames

: The interaction between individual vortices and a premixed laminar flame was investigated in order to characterize the underlying dynamics of flame-turbulence interactions and thereby gain an improved understanding of premixed turbulent flames. In addition, previous two-dimensional flame structure measurements made in turbulent premixed flames were re-analyzed in order to obtain flame curvature and orientation statistics.... Premixed Turbulent Flames, Flame-Vortex Interactions, Turbulence-Flame Interactions, Turbulent Flame Structure.

Turbulence–flame interaction and fractal characteristics of H2–air premixed flame under pressure rising condition

: Countercurrent flow control was applied as a novel flame stabilization technique on a premixed jet flame. In the experimental setup a converging nozzle burner was placed concentrically in a suction collar to generate suction driven countercurrent flow. Flame blow off velocity was measured at different suction velocity and equivalence ratio values. Tests show that by applying countercurrent flow around the periphery of a jet flame it can be stabilized up to jet exit velocity of 40 m/s. Particle Image Velocimetry (PIV) measurements and shadowgraph visualizations reveal that, application of suction changes the shear layer structure. An optical temperature measurement technique, refereed to as Laser Speckle Displacement (LSD) was developed for instantaneous temperature field measurements. It is shown that the technique is easy to implement and can be used in conjunction with PIV for simultaneous velocity and temperature measurements in reacting flows.

Flashback is a key operability issue for low emission premixed combustion systems operated on high hydrogen content fuels. Previous work investigated fuel composition impacts on flashback propensity and found that burner tip temperature was important in correlating flashback data in premixed jet flames. An enclosure around the jet flame was found to enhance the flame–burner rim interaction. The present study further addresses these issues using a jet burner with various geometric configurations and interchangeable materials. Systematic studies addressing the quantitative influence of various parameters such as tip temperature, burner material, enclosure size, and burner diameter on flashback propensity were carried out. A comprehensive overview of the flashback limits for all conditions tested in the current study as well as those published previously is given. The collective results indicate that the burner materials, tip temperature, and flame confinement play significant roles for flashback propensity and thus help explain previous scatter in flashback data. Furthermore, the present work indicates that the upstream flame propagation during flashback is affected by the burner material. The material with lower thermal conductivity yields larger flashback propensity but slower flame regression inside the tube. These observations can be potentially exploited to minimize the negative impacts of flashback in practical applications.

A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame

This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation model to accurately predict gas temperatures and thus soot formation rates. When including such a radiation model, the LES model predicts mean soot concentrations within 30% in the ethylene jet flame.

Visualization of the heat release zone of highly turbulent premixed jet flames

Experimental study of vorticity-strain interactions in turbulent premixed counterflow flames

This study presents a numerical investigation of conditional source-term estimation (CSE) with direct integration of chemical kinetics, applied to one turbulent DME jet flame. This new CSE framework eliminates the need for pre-tabulated chemistry, therefore greater flexibility and accuracy are added when more complex fuels are considered. Two chemical mechanisms are considered: a detailed mechanism with 42 species and a tailored 21-species reduced mechanism. Both simulations are evaluated against a comprehensive experimental dataset including temperature and species concentration fields. Results show that simulations using both mechanisms yield nearly identical predictions for major scalars, with only minor differences observed in the conditional and Favre-averaged profiles. Discrepancies in peak temperature and species concentrations correlate with local deviations in predicted mixing statistics. While the detailed mechanism increases computational cost by nearly tenfold, the reduced mechanism retains accuracy at a fraction of the expense. These findings confirm that direct chemistry integration CSE, when combined with an optimized skeletal mechanism, offers an accurate and computationally efficient approach for modeling DME combustion in turbulent flows. Novelty and significance statement This study includes two novel components. One is focused on the assessment of a recent conditional source-term estimation (CSE) formulation with direct chemistry integration, in principle, capable of dealing with any chemical kinetics, without pre-tabulated chemistry. For the first time, this method is applied to the simulation of a turbulent flame burning DME with two different chemical mechanisms including over 20 species. A suitable stiff solver is added. A rigorous analysis is performed using experimental data. The second novelty is the derivation of a new reduced mechanism for DME, consisting of only 21 species, thoroughly validated over a range of combustion conditions for laminar flame speeds, species concentrations and ignition delays, and included in the CSE turbulent flame simulations, with excellent performance. This study, including direct chemistry integration CSE and optimized skeletal DME kinetics, provides significant contributions towards the advancement of accurate and efficient combustion simulation tools for industry-relevant conditions.

Flame turbulence interaction is one of the leading order terms in the scalar dissipation $\left (\widetilde {\varepsilon }_{c}\right )$ transport equation [35] and is thus an important phenomenon in premixed turbulent combustion. Swaminathan and Grout [36] and Chakraborty and Swaminathan [15, 16] have shown that the effect of strain rate on the transport of $\widetilde {\varepsilon }_{c}$ is dominated by the interaction between the fluctuating scalar gradients and the fluctuating strain rate, denoted here by $\overline {\rho }\widetilde {\Delta }_{c}= \overline {\rho {\alpha }\nabla c^{\prime \prime }S_{ij}^{\prime \prime }\nabla c^{\prime \prime }}$ ; this represents the flame turbulence interaction. In order to obtain an accurate representation of this phenomenon, a new evolution equation for $\widetilde {\Delta }_{c}$ has been proposed. This equation gives a detailed insight into flame turbulence interaction and provides an alternative approach to model the important physics represented by $\widetilde {\Delta }_{c}$ . The $\widetilde {\Delta }_{c}$ evolution equation is derived in detail and an order of magnitude analysis is carried out to determine the leading order terms in the $\widetilde {\Delta }_{c}$ evolution equation. The leading order terms are then studied using a Direct Numerical Simulation (DNS) of premixed turbulent flames in the corrugated flamelet regime. It is found that the behaviour of $\widetilde {\Delta }_{c}$ is determined by the competition between the source terms (pressure gradient and the reaction rate), diffusion/dissipation processes, turbulent strain rate and the dilatation rate. Closures for the leading order terms in $\widetilde {\Delta }_{c}$ evolution equation have been proposed and compared with the DNS data.

A Direct Numerical Simulation (DNS) of a turbulent non-premixed flame interacting with a Gaussian acoustic wave is carried out in this work. This numerical simulation takes into account detailed transport phenomena including the Soret effect as well as complete chemical kinetics on a very fine mesh. Turbulent non-premixed flame calculations are carried out both with and without an acoustic wave and results are recorded at the same time. By a simple difference it is then possible to obtain the influence of the acoustic wave/turbulent flame interaction. Using an extension of the non-linear Rayleigh criterion to a system with many species and elementary reactions, the obtained results can be further analysed. The initially planar acoustic wave develops strong perturbations along its transverse direction because of the interaction process, even at very early times. The amplitude of the pressure perturbation presents locally high positive as well as negative values, demonstrating the importance of focussing/defocussing effects and local amplification (resp. damping) phenomena. In the same way, the heat release rate is locally modified (either increased or decreased) during the interaction process. Finally, the presented Rayleigh criterion is used to identify regions where local amplification (respectively damping) takes place. Both amplification and damping zones coexist directly close to each other inside the reaction zone. The observed, resulting global effect is thus based on an average over highly varying local conditions within the flame front, leading to a smoothing effect. The complexity of the coupling procedure leading to this global wave amplification or damping is demonstrated by the present analysis.

Morphology and structure of spherically propagating premixed turbulent hydrogen - air flames

ABSTRACTA numerical study of premixed flame-turbulence interaction is performed to investigate the effects of turbulence on the structural features of the flame and the subgrid-scale (SGS) effects on vorticity dynamics, energy transfer mechanism, and turbulent transport across the flame. We consider a freely propagating methane-air turbulent premixed flame interacting with a decaying isotropic turbulence under three different initial conditions corresponding to the corrugated flamelet (CF), the thin reaction zone (TRZ), and the broken/distributed reaction zone (B/DRZ) regimes. We employ the well-established linear eddy mixing (LEM) model in large-eddy simulation (LEMLES), a new subgrid closure for reaction-diffusion occurring in the small-scales based on LEM (RRLES), and a quasi-laminar chemistry based closure in large-eddy simulation (QLLES) to simulate flame-turbulence interactions and to compare predictions with direct numerical simulation (DNS). We assess the accuracy and robustness of the closures by comparing statistical features to highlight their abilities and limitations. The newly proposed RRLES subgrid closure uses a dual-resolution grid for solving the species transport equations. Such an approach is shown to improve the existing LEMLES subgrid model especially at low Reynolds numbers. All SGS closures reveals good agreement, although there are some differences due to the closure used for convective transport of the scalar field and the reaction rate. Further analysis of the DNS dataset shows that there is a significant contribution by dilatation and baroclinic torque terms across the flame. In particular, at higher Karlovitz number, there is an abrupt change in the sign of the dilatation term, which is related to the competing effects of thermal expansion due to heat release and enhanced molecular mixing by the turbulence across the flame brush region. The enhanced mixing leads to localized pockets of cold reactants surrounded by hot products, which is only partly captured by the employed closures. The analysis of SGS kinetic energy and scalar dissipation rates indicates the presence of back-scatter of turbulent kinetic energy, and we also observe counter-gradient transport across the flame. The results suggest that further improvement of the traditional closures is needed to accurately capture the dynamics of flame-turbulence interaction.