Turbulent Non-premixed Jet Flames Research Articles

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Lifting of Transversely Forced Turbulent Nonpremixed Coaxial Jet Flames

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior.

Large eddy simulation of turbulent non-premixed oxy-fuel jet flames with different Reynolds numbers

Due to differences in the physical and chemical properties of CO2 and N2, along with a reduction in the momentum ratio between oxidant and fuel streams, the local extinction of oxy-fuel flames is significantly more pronounced compared to conventional air–fuel flames, which poses challenges in the design and operation of oxy-fuel burners. This study further validates the species-weighted flamelet/progress variable (FPV) model proposed in previous work [Jiang et al., 2023], particularly in oxy-fuel flames characterized by highly local extinction. Large eddy simulations were conducted on the Sandia oxy-fuel jet diffusion flame at various Reynolds numbers. The predictions are systematically compared with experimental data, and the influence of Reynolds number on local extinction is thoroughly analyzed. The results demonstrate that the numerical simulation effectively predicts mean temperature, species mass fractions, differential diffusion parameters (ZHC), and local extinction in oxy-fuel jet flames across a wide range of Reynolds numbers. The errors in predicted mean temperature and mass fractions exhibit a slight increase with rising Reynolds numbers, yet remain below 15 %. As the Reynolds number increases from 12,000 to 18,000, the predicted peak ZHC decreases by 30 %, and the beneficial effect of preferential diffusion of H2 weakens, while the adverse effect of CO2 on combustion becomes stronger.

Modeling Edge Flame Propagation for Flame Particle Tracking in the Forced Ignition Process of Turbulent Nonpremixed Jet Flame

ABSTRACT To ensure safe and reliable operation, ignition performance in propulsion and power devices needs to be thoroughly assessed. Reduced order models (ROMs) for ignition have gained increasing attention, given that first-principles models are restricted by immense computational costs that preclude rapid assessment for the design purpose. Currently, flame self-propagation is generally neglected in the ROM descriptions of ignition process and the impact on the flame front dynamics as well as the ignition probability map is not clear for low swirl or jet flames which are of interest for industrial applications. In this study, an empirical model for flame self-propagation is proposed to augment the capability of Lagrangian flame particle tracking method, a ROM model to predict ignition probability based on nonreacting flow field. The model adopts the classic scaling law of turbulent flame speed together with fuel gradient correction and flame self-propagation to mimic the edge flame dynamics during the ignition process. The model performances in terms of flame front dynamics and ignition probability map are quantified in a turbulent nonpremixed methane-air jet flame. Results reveal that only with flame self-propagation, the flame particle tracking can well capture the transient evolution of the leading flame front as observed in experiments. It is found that the predicted ignition probability map agrees well with experimental data with high ignition probability around the vicinity of the stoichiometric region at the downstream axial distance of 6 to 40 times the jet diameter. This study illustrates the importance to account for flame self-propagation to correctly predict ignition probability in jet dominant nonpremixed flames.

LES investigation of soot formation in a turbulent non-premixed jet flame with sectional method and FGM chemistry

This study proposes a detailed soot modeling framework for large-eddy simulation (LES) to accurately predict soot formation and particle size distributions (PSD) in turbulent reacting flows. The framework incorporates Flamelet Generated Manifold (FGM) chemistry and a soot model based on the discrete sectional method (DSM) to predict both qualitative and quantitative sooting behavior while keeping the computational cost affordable. Two elementary modeling strategies are considered in the LES formalism for describing soot formation rates. These strategies rely on an a-priori tabulation of soot formation rates and their run-time computation. The LES formalism is applied to the simulations of a well-characterized, non-premixed, turbulent jet flame. A comparative analysis of strategies employed for filtered soot source term treatment is conducted to investigate their impact on the prediction of soot quantities and the evolution of soot PSDs. The LES results for the gas phase and soot phase are compared against the available experimental data. A good prediction of soot evolution is achieved with the two methodologies. The tabulation of soot formation rates leads to a significant reduction in computational cost compared to the model based on their explicit runtime computation. The LES results reveal that the modeling of filtered soot source terms has a significant impact on the quantitative prediction of soot formation. The possible reasons for the observed differences in the soot prediction are discussed. The run-time computation-based model provides a more consistent treatment of the non-linear interactions between the gas and soot phases in soot source terms compared to the tabulated soot chemistry approach. On the other hand, the tabulated soot chemistry model is an interesting and efficient modeling approach for predicting soot formation in turbulent conditions. Overall, both approaches have their strengths and limitations, and the choice of approach may depend on the specific needs of the application.

The Effects of Cracking Ratio on Ammonia/Air Non-Premixed Flames under High-Pressure Conditions Using Large Eddy Simulations

Ammonia is a promising carbon-free fuel. However, one of the main challenges for ammonia combustion is the high level of NO emissions. In this study, simulations were conducted for ammonia/air laminar counterflow flames and turbulent non-premixed jet flames in the KAUST high-pressure combustion duct (HPCD) at a pressure of 5 bar, with two ammonia cracking ratios of 14% and 28%. The influence of ammonia cracking ratio on the flame structure and NO formation mechanism were examined. The laminar counterflow flame results showed that HNO is one of the most critical species related to NO formation and NO is mainly generated through the path of NH2→NH→HNO→NO. For the turbulent flames, the flamelet/progress variable (FPV) approach was employed in the context of large eddy simulations (LES) for high-fidelity simulations. The simulation results were compared with the measured data with promising agreements, which proves the accuracy of the FPV method for the present flames. It was shown that with increasing cracking ratio, not only the flame reactivity is enhanced, but also the generation of NO is increased. The correlation between NO and HNO is weaker when compared to that between NO and radicals such as O, H and OH in the entire flame. Through the distribution of NO source terms, it was found that the NO source term has a higher absolute value in the upstream region and the absolute value rapidly decreases with increasing streamwise distance. The total NO source term is positive in the fuel-lean zone and shows negative values in the fuel-rich zone.

Turbulence effects on the formation and growth of nano-particles in three-dimensional premixed and non-premixed flames

This study examines the impact of turbulence on particle-forming flames via three-dimensional direct numerical simulations. The simulations employ the finite rate chemistry approach to simulate both premixed and non-premixed methane-air turbulent planar jet flames. These flames are doped with titanium tetraisopropoxide (TTIP) to form titanium dioxide TiO2 nanoparticles. The sectional model is employed to solve the population balance equation governing particle dynamics. Through these simulations, a number of the Batchelor scales pertaining to the smaller nanoparticle structures are effectively captured. The analysis conducted on these simulations is to identify and quantify the respective influences of diffusion, coagulation, and inception on variations in particle concentration across different regions within the flame. Several regions of the computational domain with different turbulence intensities are analyzed. Results show that the physical mechanisms that contribute to particle growth are not negligible on the particle concentrations. The impact of normal aN and tangential aT strain rates, which govern the thickness of particle-loaded zones affected by turbulence effects, is also evaluated. Conditional mean values of the strain rates upon the particle number concentration fields and the PDFs of aN and aT confirm that on the iso-surfaces of the particle fields the compressive and stretching effects are predominant. The aforementioned information is used to gain a deeper understanding of the influence of turbulence on premixed and non-premixed particle-forming flames, which will help us to develop transferable models for the simulation of nanoparticle synthesis.

Flamelet/transported PDF simulations of ethylene/air jet turbulent non-premixed flame using a three-equation PAH-based soot production model

This article reports flamelet/transported PDF (TPDF) simulations of the well-documented ethylene/air turbulent non-premixed jet flame investigated experimentally at Sandia. The transported PDF equation is solved with the Stochastic Eulerian Field method. The soot production is modelled by a validated three equation PAH-based soot model that predicts the mean soot aggregate properties at low computational time and includes a detailed description of the soot production processes. Gas and soot radiation is modelled using the rank-correlated full-spectrum k model. The turbulence/chemistry/soot production/radiation interactions are taken into account by means of the PDF method. Simulations are run by considering or not soot differential mixing. Based on recent conclusions drawn from Direct Numerical Simulations (Zhou et al., Proc. Combsut. Inst. 38 (2021) 2731–2739), soot differential mixing is modelled by neglecting soot mixing owing to sufficiently large mixing timescales. When soot differential mixing is considered, model predictions reproduce reasonably well the exhaustive set of experimental data, including flame structure, soot statistics and radiative outputs without adjusting parameters. In particular, the predictions demonstrate for the first time the capability of RANS/TPDF models to capture the soot intermittency. On the other hand, neglecting the soot differential mixing produces notable reductions in mean and fluctuating soot volume fraction and soot intermittency. Scatter plot analysis shows that the effects of soot differential mixing are more pronounced in regions of the mixture fraction space where soot surface growth and soot oxidation dominate the soot production, affecting these processes in a non-negligible manner. In an opposite way, soot nucleation and PAH condensation are much less significantly affected. Model results show also that disregarding soot differential mixing reduces the mean soot emission as well the soot emission turbulence/radiation interaction.

An experimental study and analysis of lift-off length in inclined nonpremixed turbulent jet flames

The lifted flame behavior of inclined turbulent jets, considering the relative angle between the fuel jet momentum and flame buoyancy was investigated experimentally by varying the inclination angle of nonpremixed fuel jets. Variations of lift-off length from the flame base to the nozzle exit was quantified experimentally with nozzles of various diameters (2, 3, and 5 mm) and inclination angles (range of −90° to 90°). The data was analyzed based on the experimental finding of upstream preheating effect depending on inclination angles. Major findings are as follows: (1) The lift-off length (h) increases linearly with the increase in initial fuel jet velocity (ue) at a fixed inclination angle. The proportionality slope κ of the linear relationship between h versus ue decreases appreciably with jet inclination angle for the negatively inclined flames; while for the positively inclined flames, the lift-off length decreases relatively weakly. (2) Physical analysis on the flow characteristics of inclined jets was conducted, and the preheating effect was proposed based on the combustion behaviors, especially for the negatively inclined jet flames. The preheating temperatures of unburned fuel/air mixtures at the flame base and nozzle exit were experimentally quantified, revealing that the negatively inclination angle can have a significant influence on the preheating temperatures. (3) Based on the proposed preheating mechanism, a physical model accounting for the effect of jet inclination angle was developed to quantify the lift-off length of inclined jet flames. The proposed model successfully represented lift-off lengths at all the experimental conditions with various inclination angles and nozzle diameters. The present findings provide new data set and a reasonable physical model for lifted flame behavior of inclined turbulent jet flames, revealing the effect of the relative angle between fuel jet momentum and flame buoyancy.

Large Eddy Simulation of turbulent nonpremixed sooting flames: Presumed subfilter PDF model for finite-rate oxidation of soot

Modeling soot evolution in turbulent reacting flows using Large Eddy Simulation (LES) is challenging due to the complex subfilter soot-turbulence-chemistry interactions. Soot particles form at fuel-rich mixtures and are subsequently oxidized as they are transported toward fuel-lean mixtures. In previous work, this phenomenology was explicitly encoded into a presumed subfilter PDF model for soot by confining soot strictly to mixtures where growth rates exceed oxidation rates. However, this model implicitly assumed that oxidation is infinitely fast. In this work, a new presumed subfilter PDF model for soot is proposed to account for finite-rate soot oxidation. The distribution of soot with respect to the flame structure (mixture fraction) is modeled by comparing the local relative motion of diffusionless soot particles with respect to mixture fraction iso-contours with the local oxidation rate. When the oxidation rate is suppressed or the transport rate is very fast, soot is allowed to penetrate further into fuel-lean mixtures. This model can allow for soot leakage across the flame, a critical phenomenon in smoking flames. The new model is validated a priori against Direct Numerical Simulation (DNS) databases of turbulent nonpremixed jet flames and then a posteriori against experimental measurements in a laboratory-scale turbulent jet flame. A priori results show remarkably good agreement with filtered DNS data and are shown to correctly allow for soot leakage at low Damköhler number. Finally, LES results of the turbulent sooting jet flame show an improvement of soot prediction using the new soot subfilter model compared to previous works. In this flame, as in experiments, evidence of soot leakage is found near the beginning of the sooting region, and this soot leakage cannot be predicted with previous subfilter models that presume infinitely fast oxidation.

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.

A comparison between turbulent non-premixed jet flames of CH4 and the 50%NH3+50%H2 blend

A challenge of application of ammonia in combustion devices is due to its low chemical reactivity. Addition of hydrogen into ammonia can increase drastically the premixed laminar flame speed of the blends. In practical combustion devices, however, turbulent non-premixed jet flames are usually employed, while researches on ammonia jet flames are scarce. In this study, firstly, the premixed laminar flame speeds of CH4 and the 50%NH3+50%H2 blend in volumetric fraction are investigated by the Schlieren imaging. It is confirmed that the laminar flame speeds of the 50%NH3+50%H2 blend are remarkably faster than those of CH4 for equivalence ratios above 1.1, while they are similar for lower equivalence ratios. Then, the Schlieren imaging and planar laser induced fluorescence imaging of hydroxyl of the burning turbulent gas jets are conducted. It is found that the flame propagation speed and OH intensity in the CH4 jets are remarkably higher than those in the 50%NH3+50%H2 jet. To unveil the mechanism, the equivalence ratio and fuel mole fractions along the jet axis are investigated by the laser induced breakdown spectroscopy. The equivalence ratios of the 50%NH3+50%H2 blend are significantly lower than those of CH4. Compared to that of CH4, the volumetric flow rate of 50%NH3+50%H2 blend is higher for the choked flow of the under-expanded jets. Therefore, the lower equivalence ratios are attributed to the higher stoichiometric fuel-air ratio of the 50%NH3+50%H2 blend. Stratification of NH3 and H2 due to their distinct molecule transfer properties is experimentally observed, which probably lowers the flame propagation speed in the jet. These findings are expected to be a valuable reference for development of simulation models and design of practical combustion devices with zero-carbon fuels.

Quantitative laser-induced fluorescence of NO in ammonia-hydrogen-nitrogen turbulent jet flames at elevated pressure

Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A2Σ+−X2Π (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH3–H2–N2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH3–H2–N2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH3–H2–N2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH3–H2–N2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames.

Relative influence of soot oxidation kinetics and subfilter soot-turbulence interactions on soot evolution in turbulent nonpremixed flames

Oxidation of soot is a critical process in determining the evolution of soot in turbulent reacting flows. On the one hand, the kinetic rates of soot oxidation remain one of the most significant uncertainties in soot models. On the other hand, turbulent transport plays a significant role in the distribution of soot with respect to the flame structure so the location of soot in regions where oxidation can actually occur. The objective of this work is to assess the relative importance of small-scale turbulent transport and oxidation kinetics on the prediction of soot evolution in turbulent nonpremixed flames. A series of Large Eddy Simulations has been performed in a turbulent nonpremixed piloted jet flame of ethylene and air using a series of soot subfilter PDF models (with different assumptions regarding the small-scale turbulent transport of soot) and a series of kinetic rates for soot oxidation (with differences in kinetic rates for both OH and O2 oxidation). Similar to previous work, the results show that accounting for the disappearance of soot in lean mixtures due to soot oxidation is extremely important to avoid excessive “spurious oxidation” of soot otherwise artificially present in lean mixtures. In addition, the sensitivity to soot oxidation kinetics is found to be significantly less important than this effect. However, once this effect is taken into account in the soot subfilter PDF, the sensitivity to soot oxidation kinetics becomes rather significant compared to further refinements to the soot subfilter PDF models. More specifically, the assumption in the soot subfilter PDF models whether soot oxidation is infinitely fast or finite rate compared to small-scale turbulent transport is a less significant model sensitivity compared to the sensitivity to the kinetic rates of soot oxidation. Furthermore, oxidation by OH and O2 are revealed to play fundamentally different roles in soot evolution.

Thermal Runaway Propagation in Li-ion Battery Packs Due to Combustion of Vent Gases

Accurate understanding of propagation of thermal runaway is of much importance for developing safe battery pack designs. Combustion of vent gases emerging from a trigger cell undergoing thermal runaway has not been studied in sufficient detail, even though the additional heat generated during combustion likely plays an important role in thermal runaway propagation. This work presents comprehensive numerical modeling and simulations of thermal runaway propagation in a pack of cylindrical cells. The model accounts for multiple coupled non-linear phenomena, including vent gas flow and combustion, radiation and thermal runaway. Non-premixed combustion of venting gas is modeled using k-ε turbulence model and finite rate chemical kinetics. Simulation results are shown to be in good agreement with experimental data for a benchmark turbulent non-premixed jet flame. Simulations show that hot combustion products are rapidly transported in gaps between cells, potentially leading to self-sustained thermal runaway propagation to adjacent cells. Results demonstrate the critical importance of combustion in determining the nature of propagation of thermal runaway. The vent hole location is identified as an important parameter that influences whether and the extent to which thermal runaway propagation occurs. This work contributes towards the practical understanding of thermal runaway safety of Li-ion battery packs.

Conditional analysis of temperature and strain rate effects on dissipation structure in turbulent non-premixed jet flames

This work presents results from simultaneous high-resolution temperature and velocity measurements in a series of turbulent non-premixed jet flames. The filtered Rayleigh scattering (FRS)-based temperature measurements demonstrate sufficient signal-to-noise (SNR) and spatial resolution to estimate the smallest scalar length scales and accurately determine dissipation rate fields. A comprehensive set of conditional statistics are used to characterize the small-scale structure, including the dependence of dissipation layer widths on Reynolds number, temperature, and dissipation magnitude. In general, the dissipation layer thickness decrease with increasing Reynolds number and increase with increasing temperature. However, dissipation layer widths show two distinct behaviors with respect to dissipation magnitude. For small dissipation values, increases in magnitude results in broadening of the dissipation layer, while for larger magnitude values of dissipation, the layer widths are thinned, highlighting the complexity of small-scale turbulent mixing. Additionally, measured ratios of the dissipation layer width to the Batchelor length scale are consistent across all Reynolds numbers and agree with previous studies in non-reacting flows. The unique aspect about the current set of measurements is the ability to examine the interaction of dissipation structure with turbulent flow parameters for the first time in turbulent non-premixed flames. Particularly, the strain rate/dissipation relationship is examined and compared to previous studies in non-reacting flows. It is found that the dissipation layers tend to align normal to the principal compressive strain axis and this tendency increases with increasing Reynolds number. For the lowest Reynolds number case, no dependence of the dissipation layer width nor dissipation rate magnitude on strain rate is found. However, for higher Reynolds numbers, a strong dependence of the dissipation layer width and dissipation rate magnitude on the principal compressive strain rate is observed. These results indicate the direct role of the compressive strain rate field on small-scale mixing structure in reacting flows.

Ensemble Combustion of Liquid Oxygen / Gaseous Hydrogen Coaxial Flames with Transverse Acoustics

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames, under the influence of transverse acoustic forcing. The results are systematically compared to the results from a predecessor study of the same ensemble of flames under the same conditions, but without acoustic forcing. Both pressure nodes (PN) and pressure antinodes (PAN) were investigated as a function of the outer-to-inner momentum flux ratio. Shadowgraphs and OH* emission imaging showed that PNs distort the flames more than PANs, similar to previous observations in single flames. A wave amplification mechanism (WAM) identified previously to be present both in single flames and in the unforced ensemble of flames was found to also be present under acoustic forcing. In the predecessor study without acoustics, the WAM tended to self-organize within the ensemble into an out-of-phase varicose nesting that minimized large-scale collisions. In the present study, PN forcing tended to organize the WAM structures into a sinuous nesting, which also minimized collisions. PAN forcing, on the other hand, promoted destructive in-phase varicose nesting that promoted large-scale collisions, causing subsequent breakdown into smaller scales. Dynamic Mode Decomposition revealed natural frequencies and modes in addition to the acoustic quantities. Discrete combustion waves related to the WAM propagated at the same normalized average velocities regardless of whether acoustics were present in the ensemble or not. However, the trajectories are grouped into different curves than for single-element flames.

Ensemble Combustion of Liquid Oxygen/ Gaseous Hydrogen Coaxial Flames

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames. The experimental conditions were carefully controlled to duplicate those of a previous single element experiment as closely as possible, so that the single element flame could be closely compared with the present three element flame. All ensemble and inter-element effects were due to the turbulent combustion only; acoustic effects were intentionally suppressed. Abundant interactions were found both in the time averaged sense as well as in the dynamics of the larger scale motions. The multielement flames were found to be axially longer, and dominant frequencies were found to emerge for the multielement flames that were not present for the single element flames. A dynamic wave amplification mechanism previously identified for single element flames was found to remain present for multielement flames. However, the mechanism did not operate independently for each element in the ensemble, but instead entered into phase relationships with the neighboring elements. The phase relationship possibly encouraged the dynamics between jet flames that allowed the dominant frequencies to emerge. Several indications of such a phase relationship were found, but not all indications were observed to occur all of the time. It is possible that the coordination of the wave amplification mechanism between the flames was weak or intermittent. Other simultaneous processes probably also contributed to the overall flame behavior.

The partially stirred reactor model for combustion closure in large eddy simulations: Physical principles, sub-models for the cell reacting fraction, and open challenges

For their ability to account for finite-rate chemistry, reactor-based models are well suited Turbulence–Chemistry Interactions (TCI) Sub-Grid Scale (SGS) closures for Large Eddy Simulations (LES). The SGS closure in the Partially Stirred Reactor (PaSR) model relies on the determination of the reacting fraction of each computational cell, whose definition is based on estimates of the characteristic mixing and chemical time scales. Direct Numerical Simulations (DNS) of turbulent combustion can supply key information on TCI for the development, validation, and comparison of combustion models. In particular, a priori testing allows the direct validation of model assumptions. In the present work, an a priori assessment of the PaSR model is conducted. Its ability to reconstruct thermo-chemical quantities of interest is investigated along with model assumptions. Sub-grid quantities are extracted from the DNS to investigate the role of the cell reacting fraction. Various definitions are then proposed to estimate the characteristic chemical timescale in the PaSR model. Modeled chemical source terms and heat release rates are compared against the filtered quantities from DNS data of a two-dimensional, spatially developing, turbulent nonpremixed jet flame with detailed kinetics. The results demonstrate the importance of accounting for the fine structures quantities in the context of reactor-based models. A new formulation of the chemical timescale is proposed and provides improved overall predictions. Several issues are raised in the discussion, representing realistic prospects for further developments of the PaSR model as a SGS combustion closure for LES.

Implementation of the Scalar Dissipation Rate in the REDIM Concept and its Validation for the Piloted Non-Premixed Turbulent Jet Flames

In order to address the impact of the concentration gradients on the chemistry – turbulence interaction in turbulent flames, the REDIM reduced chemistry is constructed incorporating the scalar dissipation rate, which is a key quantity describing the turbulent mixing process. This is achieved by providing a variable gradient estimate in the REDIM evolution equation. In such case, the REDIM reduced chemistry is tabulated as a function of the reduced coordinates and the scalar dissipation rate as an additional progress variable. The constructed REDIM is based on a detailed transport model including the differential diffusion, and is validated for a piloted non-premixed turbulent jet flames (Sandia Flame D and E). The results show that the newly generated REDIM can reproduce the thermo-kinetic quantities very well, and the differential molecular diffusion effect can also be well captured.