Differential Diffusion Effects Research Articles

Assessing turbulence–flame interaction of thermo-diffusive lean premixed H2/air flames towards distributed burning regime

Simultaneous laser-induced fluorescence of OH radicals and particle image velocimetry, and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements are used to investigate the effects of Karlovitz number (by varying the equivalence ratio) and residence time (by varying the axial measurement location) on the internal flame structures of lean premixed hydrogen/air turbulent jet flames. The turbulent flow fields, instantaneous macroscopic flame structures, and thermochemical states of a set of lean premixed hydrogen/air turbulent flames with varying initial equivalence ratio of 0.3, 0.4, and 0.45 at a constant bulk velocity of 100m/s are discussed. With an increasing equivalence ratio, the Karlovitz number derived from the turbulent flow fields decreases rapidly from 7690 to 260 and 100, determined at a downstream location of x/D=7. At the highest Karlovitz number (i.e., the lowest equivalence ratio), a distributed burning is observed in the jet flame as turbulent transport dominates over molecular mixing, and the effects of differential diffusion and flame curvature are suppressed. With decreasing Karlovitz number, intense burning regions characterized by elevated local equivalence ratio, high water mole fraction, and super-adiabatic flame temperatures are observed in association with positive flame curvature. The same combustion diagnostics are applied to another lean premixed hydrogen/air turbulent flame with an initial equivalence ratio of 0.4 and a bulk velocity of 200m/s at selected downstream locations of x/D=3.5, 7, 10.5, and 14 to assess the effects of developing turbulence and residence time. Corresponding Karlovitz numbers are 680, 730, 775, and 690, as the local turbulent intensity changes along downstream locations. While flame structures reveal characteristics towards distributed burning at lower x/D, a locally intense burning region appears at higher x/D together with positive curvature. This is mainly because the turbulence develops with increasing x/D and the flame surface is more disturbed and curved by turbulent eddies with increasing turbulence length scales. The highly diffusive hydrogen is locally concentrated by positively curved flame surfaces, resulting in fuel-rich burning regions at a higher temperature. This indicates that, as velocity fluctuations increase in axial direction, turbulence can also promote thermo-diffusive instabilities to a certain extent, increasing the mixture inhomogeneity in temperature space by interacting with the differential diffusion effect of hydrogen at atmospheric pressure.Novelty and significanceThe novelty of the current work is the focus on turbulence–chemistry interactions of premixed hydrogen/air jet flames at high turbulence and ultra lean conditions. Comprehensive experimental results including the turbulent flow fields (particle image velocimetry), instantaneous flame structures (laser-induced fluorescence of OH radicals) and internal thermochemical states (quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering imaging) of hydrogen/air jet flames are analyzed and discussed. The data sets with well-characterized boundary conditions and high measurement accuracy, are crucial for validation and development of numerical simulation models.

Isolating effects of large and small scale turbulence on thermodiffusively unstable premixed hydrogen flames

Lean turbulent premixed hydrogen/air flames have substantially increased flame speeds, commonly attributed to differential diffusion effects. In this work, the effect of turbulence on lean hydrogen combustion is studied through Direct Numerical Simulation using detailed chemistry and detailed transport. Simulations are conducted at six Karlovitz numbers and four integral length scales. A general expression for the burning efficiency is proposed which depends on the conditional mean chemical source term and gradient of a progress variable, and the amount of superadiabatic burning. At a fixed Karlovitz number, the normalized turbulent flame speed and area both increase almost linearly with the integral length scale ratio. The effect on the mean source term profile is minimal, indicating that the increase in flame speed can solely be attributed to the increase in flame area. At a fixed integral length scale, both the flame speed and area first increase with Karlovitz number before decreasing. The qualitative observations and trends do not change when Soret effects are neglected. Specifically, neglecting Soret diffusion is shown to reduce the flame speed, area, and burning efficiency. At higher Karlovitz numbers, the diffusivity is enhanced due to penetration of turbulence into the reaction zone, significantly dampening differential diffusion effects.Novelty and SignificanceLean premixed hydrogen flames are subject to thermodiffusive instabilities, which can lead to system level instabilities such as flashback. A comprehensive study of the thermodiffusively unstable turbulent flames with detailed chemistry and detailed transport (including Soret diffusion) was conducted across a wide range of turbulent intensities and integral length scales. Varying these two parameters independently was necessary to isolate the effects of large- and small-scale turbulence. Using these results, we propose a general expression for the burning efficiency to explain the relationship between the turbulence intensity, flame speed, and flame area. This work is an important step in developing predictive models which can aid in the design of practical combustion devices.

Evaluation of hydrogen/ammonia substitute fuel mixtures for methane: Effect of differential diffusion

One strategy for utilizing ammonia as an energy vector is to replace conventional hydrocarbons with hydrogen-enriched ammonia in existing or retrofitted combustors. However, the strong differential diffusion effect of hydrogen can significantly alter the combustion properties and cause challenges in combustor operability. Therefore, this numerical study performs a systematic investigation on the effect of differential diffusion on fundamental combustion properties of ammonia/hydrogen fuel blends. The investigated combustion properties span from the initiation of combustion, i.e., ignition, to stretched flame propagation and ultimately to flame stabilization mechanisms. The objective is to evaluate the feasibility of hydrogen/ammonia/air mixtures as substitutes for methane/air particularly considering the implications of differential diffusion effects. To this end, four fuel blends with similar unstretched burning properties are selected: fuel lean ammonia/hydrogen (AH-L), fuel rich ammonia/hydrogen (AH-R), fuel lean methane (M-L) and fuel lean methane/hydrogen (MH-L). First, the forced ignition and stretched flame propagation are evaluated. It is found that the AH-L (AH-R) mixture has a large negative (positive) Markstein length, and thereby among the selected fuel blends, AH-L (AH-R) has the lowest (highest) minimum ignition energy. Then flame stabilization mechanisms are investigated. For a stable flame, AH-L (AH-R) has an intensified (weakened) flame base and a weak (strong) flame tip, which shows a higher flashback (blow-off) propensity. Based on the critical gradient theory, a novel stability regime diagram is proposed. With the regime diagram, the flame stabilization limits (central flashback, boundary layer flashback, stable, and blow-off) and flow conditions can be determined for burners of different sizes.

Steady laminar stagnation flow NH[formula omitted]-H[formula omitted]-air flame at a plane wall: Flame extinction limit and its influence on the thermo-mechanical stress and corrosive behavior of wall materials

The steady laminar stagnation flow flame of NH3-H2-air gas mixture stabilized at a plane wall is numerically investigated. Its interaction with the wall with the consideration of heat loss is the focus of this work. The numerical study of the combustion system is performed by using the full chemical mechanism and detailed transport model including the differential diffusion and Soret effect. The simulation of the solid mechanics is based on the theory of isotropic linear thermo-elasticity. With the numerical simulation, it will be discussed how the wall material would change the flame stability in terms of extinction limit, and how the combustion system such as mixture composition, flame strain rate, and pressure would vary the thermo-mechanical stresses in the solid wall and the corrosive behavior at the surface of the wall.

Numerical Investigation of Partially Premixed Flames Under Transcritical Conditions

Understanding the characteristics of partially premixed flames (PPFs) under transcritical conditions is of critical importance for the development of fuel-rich staged rocket engines. While substantial progress has been achieved for transcritical non-premixed flames (NPFs), comparatively little effort has been made to investigate transcritical PPFs. To this end, a series of transcritical counterflow gaseous hydrogen/liquid oxygen (GH2/LOX) PPFs is simulated to investigate the thermodynamic structure of PPF and to examine the effects of molecular diffusion modeling, strain rate, and the equivalence ratio at the fuel-rich side on PPFs in physical space and mixture fraction space, as well as reduced temperature/reduced pressure space. The comparisons between the NPF and PPF demonstrate that the PPF exhibits a bimodal structure in physical space: a premixed reaction zone at the fuel inlet side and a non-premixed reaction zone at the oxidizer side. In mixture fraction space, the C-shaped structure of PPF is observed owing to the differential diffusion of species. It is found that the choice of molecular diffusion model has a significant impact on PPF structure. The presence of a loop at subcritical pressures in a reduced temperature/reduced pressure space is caused by the differential diffusion of species and the formation of [Formula: see text] in the non-premixed reaction zone. Furthermore, the results indicate that the premixed reaction zones in the PPFs are very sensitive to the change in strain rate and/or equivalence ratio of the premixed mixture at the fuel inlet side. For a given equivalence ratio, increasing strain rate can suppress the differential diffusion effect and the C-shaped structure, while it has a negligible impact on the non-premixed reaction and hence on loop formation.

Effect of differential diffusion on head-on quenching of premixed NH[formula omitted]/H[formula omitted]/air flames within turbulent boundary layers

Direct numerical simulations (DNS) have been done for premixed laminar and turbulent NH3/H2/air flames propagating towards a cold wall. Two turbulent channel flows (with well-characterized turbulent boundary layers) at Reτ = 280 and 313 have been simulated for the turbulent flame cases. Near-wall low temperature chemistry has been considered. Effect of differential diffusion on the flame head-on quenching characteristics (quenching distance and maximum wall heat flux) has been checked in detail. Finally, two underlying mechanisms determining differential diffusion effects have been identified: (a) preferential diffusion of H radical increases the reaction rates of the near-wall low temperature (radical recombination) reactions, resulting in larger quenching distance and wall heat flux; (b) differential diffusion of fuels results in smaller flame heat release, thus, larger quenching distance and smaller wall heat flux. The effect of H radical preferential diffusion is suppressed in rich H2/air turbulent flames and strengthened in lean H2/air turbulent flames, due to the differential diffusion focusing/defocusing effects. The effect of differential diffusion of fuels is suppressed in turbulent flames, due to the enhanced mixing process, especially at low Damköhler numbers typical for NH3/H2/air flames interacting with highly turbulent flows.

Internal flame structures of thermo-diffusive lean premixed H2/air flames with increasing turbulence

Turbulent flow fields, instantaneous flame structures, and internal thermo-chemical states of lean premixed hydrogen/air jet flames at an initial equivalence ratio of 0.4 are experimentally investigated by simultaneous laser-induced fluorescence of hydroxyl radicals (OH-LIF) and particle image velocimetry (PIV), and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements over a range of Karlovitz numbers (Ka) from 50 to 730. At low Ka, intense burning characterized by elevated local equivalence ratio, high water mole fraction (XH2O), and super-adiabatic flame temperature is mainly observed in post-flame regions surrounded by positively curved flame surfaces, where the fast diffusive hydrogen is locally focused. The flame features stronger differential diffusion and curvature effects than that in the planar laminar flame, which indicates that both molecular and turbulent mixing play significant roles, and thermo-diffusive instabilities have synergistic interactions with turbulence at low turbulence level. With increasing Ka, the burning intensity in corresponding regions is weakened, even though the flame surface is more disturbed by the turbulence. At the highest Ka, no intense burning region is observed in the jet flame as the turbulent transport dominates over the molecular diffusion. In the temperature domain, the conditional means of hydrogen mole fraction (XH2) and local equivalence ratio feature effects of diffusive instabilities with broad distributions at low-Ka conditions. Elevated XH2O and local equivalence ratios with super-adiabatic flame temperatures are observed, which is attributed to differential diffusion in hydrogen-containing mixtures and the fuel focusing effect near positively curved flame surfaces. At high Ka, the XH2 shows a more linear decreasing trend and the local equivalence ratio profile becomes flatter over temperature with a narrower distribution, indicating the dominance of turbulent mixing.

A generalized mixture fraction formulation for nonpremixed ammonia–hydrogen flames

For nonpremixed combustion, the mixture fraction variables are widely used as a conserved scalar to map out the flame structure in a unified manner. For ammonia–hydrogen flames, however, a proper definition of the mixture fraction variable is difficult due to the presence of fuel-bound nitrogen in ammonia. This work proposes a revised mathematical definition of the mixture fraction as a modified form of the Bilger’s formula in order to describe the characteristics of nonpremixed ammonia–hydrogen blend flames at general fuel mixture ratios. The newly defined mixture fraction properly accounts for differential diffusion effects in the presence of hydrogen, and thus serves as a convenient invariant coordinate for the flame manifolds. Counterflow nonpremixed flames with different ammonia–hydrogen ratios were computed and the validity of the new formulation is demonstrated. The analysis of the flame structure revealed the presence of double reaction zones, one dominated by the hydrogen oxidation and the other by ammonia oxidation. The role of differential diffusion is crucial in determining the existence of double reactive layers and its impact is evaluated by varying the strain rate. Detailed reaction analysis revealed that the hydrogen flame is related to HO2 reactions, while the ammonia flame to N, NH2 and NH3 reaction groups. As the strain rate is increased, OH reactions are responsible for shifting dominant heat release rate from the hydrogen flame to the ammonia flames. The scalar dissipation rate profile shows two distinct peaks, suggesting that a different functional description is needed in the flamelet modeling framework.

Turbulent burning velocity of hydrogen/n-heptane/air propagating spherical flames: Effects of hydrogen content

This study investigated the flame propagation and turbulent burning velocity of hydrogen/n-heptane/air mixtures subjected to differential diffusion and flame stretch induced by turbulence at different hydrogen molar fractions (XH2 = 0–100%), equivalence ratios (ϕ = 0.5–1.0) and turbulence intensities (u’ = 0–3.62 m/s). The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber generating near-isotropic turbulence. Results show that as hydrogen content increases, the turbulent flame propagation speed tends to be affected by stronger differential diffusion and weakened turbulence, characterized by decreasing Lewis number (Le) and Karlovitz number (Ka) within a similar range of turbulence intensity. Among them, the effect of differential diffusion on the normalized turbulent propagation speed (d<r>/dtσSL) appears to be well correlated with ReT,flameLe-2, where ReT,flame represents the turbulent flame Reynolds number. However, inconsistencies in the fitting parameters for this scale are observed at different XH2, highlighting the impact of substantial variations in flame stretch induced by turbulence. Therefore, the normalized turbulent flame propagation velocity under varying hydrogen contents is proposed to be accurately predicted by d<r>/dtσSL-1 = 0.07Ka0.32(ReT,flameLe−2)0.5. The synergistic effects of Le and Ka can also account for the peak phenomenon in the normalized turbulent burning velocity (ST/SL) observed at 85% hydrogen content. Furthermore, the quantitative characterization of turbulence and differential diffusion under extensive experimental data (Ka = 0.03–90) is studied. As evidenced by the Damköhler theoretical hypothesis, the two-stage trend between ST/SL and (u’/SL)Le-1 is identified with a transition near unity Ka. The modified correlations for turbulent burning velocity in terms of (u’/SL)Le-1 and Ka0.2 are obtained with consideration of the differential diffusion and flame stretch rate induced by turbulence, which show good predictive performance for data at different turbulence regime diagrams regardless of fuel type and equipment size.

Are differential diffusion effects of importance when burning hydrogen under elevated pressures and temperatures?

A ratio of turbulent burning velocity to laminar flame speed is well known to be abnormally high in lean hydrogen-air mixtures, with this phenomenon being commonly attributed to differential diffusion effects. Magnitude of such effects is known to be increased by pressure, but a few recent studies have indicated that the effects are mitigated when reactants are preheated. It is not yet known, however, which of these two counteracting trends is of more importance under elevated pressures and temperatures associated with combustion in engines. Accordingly, it is not yet clear whether or not models of differential diffusion effects are required for research and development of future ultra-clean and highly efficient engines that burn hydrogen (the emphasized effects are typically ignored in engineering computations). To clarify the issue, numerical simulations of lean complex-chemistry hydrogen-air strained laminar flames are performed by varying strain rate, pressure 1≤P≤50 bar, temperature 300≤Tu≤900 K, and equivalence ratio (Φ=0.4, 0.55, and 0.7). This simple problem is selected because maximal consumption speeds reached in critically strained (close to extinction) lean hydrogen-air laminar flames are considered to characterize the influence of differential diffusion on turbulent burning rates within the framework of Zel'dovich's leading point concept, which was well supported in recent studies. Computed results show that, even at Tu=900 K, the aforementioned consumption speeds are significantly larger than the unperturbed laminar flame speeds if the mixture is sufficiently lean (the equivalence ratio Φ=0.55 or lower) and pressure is sufficiently high (P≥30 bar). Therefore, differential diffusion effects are expected to be of importance when burning so lean hydrogen-air mixtures in engines and should be properly modeled.

A pressure-coupled Representative Interactive Linear Eddy Model (RILEM) for engine simulations

The Representative Linear Eddy Model (RILEM) was introduced by Lackmann et al. (2018) as an alternative modeling approach to simulate turbulent non-premixed combustion in engines. The model utilizes a RANS approach for turbulence and the Linear Eddy Model (LEM) with a presumed probability density function (PDF) approach for combustion closure. A distinct feature of RILEM is its potential to handle arbitrary combustion regimes and the consideration of complex physical phenomena such as differential diffusion effects. The original version of RILEM implemented a volume-based coupling between LEM and the flow solver. This work presents a new variant of RILEM, i.e., Multiple Representative Interactive Linear Eddy Model (MRILEM) based on a pressure-based coupling, to overcome some deficiencies of the original RILEM, namely statistical fidelity. Due to the introduced pressure coupling, the effects of heat losses (wall heat fluxes, latent heat of evaporation) on combustion are intrinsically included via the pressure trace. Furthermore, we introduce a new step function PDF for the progress variable defined by its mean value only. Issues with an incomplete solution space for mixture fraction and progress variable due to the stochastic nature of LEM are remedied with a PDF scaling technique, aided by a novel parameterization of the progress-variable PDF The new variant of RILEM is evaluated using part- and full-load cases of a heavy-duty metal engine. The impact of utilizing multiple LEM lines on the completeness of the solution space and its influence on the distribution of scalar values in the CFD domain was demonstrated. Results for pressure trace, flame structure, and CO emissions are analyzed and compared with simulations using the Multi-Zone Well-Mixed Model (MZWM) model and experiments. While pressure traces agree well among the different models and experiments, noteworthy differences are observed between the models regarding CO emissions and temperature. Effects of turbulence chemistry interaction were noticed when comparing MRILEM to the results of the MZWM simulation, namely flame brush and species mass fraction distribution.

Differential diffusion effects in the near field of non-premixed NH3/H2/N2-air jet flames at elevated pressure

Differential diffusion (diff-diff) effects are analyzed in both non-premixed laminar and turbulent NH3-H2-N2 jet flames at 5 bar using 1D quantitative Raman scattering measurements. The two target flames feature fuel compositions simulating a 14% and a 28% NH3 cracking ratio. A diff-diff parameter is introduced for ammonia combustion, analogous to the definition used for hydrocarbon flames. Measurements in laminar flames, compared to Chemkin simulation featuring multi-component transport show that the instrument is capable of measuring the mean diff-diff parameter, but that single-shot measurements are compromised by the large sensitivity of the parameter to errors in N2 and NH3 mass fractions. The analysis of the diff-diff parameter in turbulent flames shows that both flames have strong differential diffusion in the near field up to an axial distance of 10 diameters (Z/D = 10), then the turbulent mixing becomes dominant further downstream. The flame with a 28% NH3 cracking ratio exhibits a higher diff-diff parameter especially in the low-temperature region (<1000 K) due to the higher H2 content. The competition between differential diffusion and turbulent mixing in the near field is also reflected in the NH3, H2, and NH3/H2 profiles, falling in between prediction for Le = 1 and multi-component transport. Interestingly the NH3/H2 profiles in the far field converge to a profile matching that of Le = 1, but with an increased NH3/H2 ratio, showing the persistence of the differential diffusion effects in the region dominated by turbulent mixing.

Combustion studies of MMA/[formula omitted] for a hybrid rocket motor

Poly(methyl methacrylate) (PMMA) is the synthetic polymer of methyl methacrylate (MMA), which is used in a multitude of everyday applications, can also be used as a solid fuel in hybrid rockets. When used as a fuel in a combustion chamber, PMMA undergoes thermal decomposition and phase-change (solid-to-gas), where gaseous MMA (C5H8O2) is the primary product. The gaseous MMA then undergoes combustion with an oxidizer. Experimental studies of this combustion chamber have been performed in literature, and this study helps produce high-fidelity simulations, which can match experimental observations while giving access to more data in the combustion chamber. Simulations of turbulent diffusion flames are performed with gaseous MMA introduced through the combustion chamber walls. Two different diffusion models are used alongside a finite-rate chemistry model to observe the effects of differential diffusion. The rate of inflow of MMA is controlled by the temperature field in the combustion chamber, and the results from the 3D simulation are comparable to 1D counterflow diffusion flame simulations. Simulations are also performed with a tabulated chemistry model developed to improve the computational efficiency and with two different internal diameters to analyze the geometric effects. If one wishes to capture the flow field profiles and fuel regression rate, chemistry effects need to be included. If one wishes to capture the internal chemistry profiles of the combustion products, differential diffusion effects are important. The fuel regression rate and the radial chemistry profiles from the simulations are compared with the experimental results and show good agreement.

Effects of turbulence on variations in early development of hydrogen and iso-octane flame kernels under engine conditions

The understanding and prediction of the early development of flame kernels are of high practical importance for the robust relight of aviation gas turbines and the control of cycle-to-cycle variations (CCV) of spark-ignition engines. CCV are known to correlate strongly with early flame kernel development and complicate the optimization of such engines in terms of safety, thermal efficiency, and engine emissions. The flame kernel initiated by a spark is initially small, in the very early combustion phase typically smaller than the size of the turbulent integral length scales. Therefore, the development of the flame kernel is dominated by local, intermittent flow fluctuations and can vary under the same nominal conditions. In this study, the effects of turbulence on the early development of premixed iso-octane and hydrogen turbulent flame kernels under realistic engine conditions are investigated through direct numerical simulations. Multiple realizations were simulated under the same nominal conditions for both fuels. Significant variations in flame kernel interactions with turbulence can be identified among different realizations. The fuel consumption rate varies by a factor of two, which is remarkable considering that only statistical differences in the local flow field are present between different realizations. Effects of different flow features of the initial flow fields on the flame kernel development were analyzed. It was found that the flow motion on the scale of the ignition radius, specifically the fluid deformation, which is characterized by the invariants of the strain rate tensor, determines the global shape of the kernel, while the variations of the kernel growth rate are mostly driven by the variations of the smallest turbulent scales. In particular, turbulence influences the flame surface area growth mainly through the tangential strain rate at the flame surface, which is shown to result from the small-scale turbulent motion. Due to differential diffusion effects, hydrogen and iso-octane exhibit significantly different flame responses to curvature, which is comprehensively studied for both fuels. The findings in this study will guide the development of combustion models that are capable to capture variations of the early flame kernels based on the local turbulence dissipation rate.

Self-similar propagation and flame acceleration of hydrogen-rich syngas turbulent expanding flames

In order to investigate the flame self-similar propagation and flame acceleration characteristics of hydrogen-rich syngas turbulent expanding flames, the turbulent combustion experimental studies of H2/CO/air mixtures with XH2 = 55%-95%, φ = 0.6–1.0, P = 1 bar-3 bar, u’=0.809–2.533 were conducted in a fan-stirred turbulent combustion chamber with an inner diameter of 380 mm. Results show that the normalized flame propagation speed ST/SL and turbulent flame Reynolds number ReT,flame could be represented by power-law correlations. With the increase of hydrogen fraction from 55% to 95%, the power exponent decreases from 0.413 to 0.301. The increase of hydrogen fraction makes the flame acceleration weaker because that the enhancing of laminar flame speed is more obvious than the differential-diffusion effect by decreasing Le. With the decrease of equivalence ratio from 1.0 to 0.6, the power exponent increases from 0.304 to 0.411 in this study. The relative high power exponent under low equivalence ratio is due to the coupling between differential-diffusion and the flame stretch on the local wrinkled flame structures. In addition, the influence of turbulence and flame intrinsic instabilities on flame acceleration propagation process of hydrogen-enriched syngas were studied. The acceleration exponent was extracted to quantitatively investigate the flame self-acceleration propagation in the transition stage, and the relationships between acceleration exponent and turbulence intensity (u’), Kovasznay number (Kz), turbulence integral length scale (LT), flame thickness (δ), thermal expansion ratio (ρu/ρb), effective Lewis number (Leeff) were analyzed

Numerical analysis and flamelet modeling of NO[formula omitted] formation in a thermodiffusively unstable hydrogen flame

In a thermodiffusively unstable premixed hydrogen flame, cellular flame structures exist due to the strong differential diffusion of hydrogen featuring wide ranges of curvature and strain rate. NOx formation pathways are directly affected by the complex cellular flame structures as different species have different sensitivities to differential diffusion effects and curvature. In this work, the NOx formation characteristics of a thermodiffusively unstable laminar premixed hydrogen flame are investigated. At first, the NOx formation characteristics in the tangential and normal directions of the flame front are analyzed, focusing on the effects of curvature. Then, reaction path analyses are conducted to identify the relevant pathways and elementary reactions that pre-dominantly contribute to the formation of NOx pollutants. To investigate the effects of curvature on NOx formation, the reaction fluxes and the elementary reaction rates are calculated by conditioning on the curvature value. Finally, the performance of a flamelet model in predicting the NOx species in the thermodiffusively unstable premixed hydrogen flame is evaluated through an a priori analysis. The results show that curvature directly affects NOx formation, particularly the dominant NNH and N2O reaction pathways while the contribution of the thermal-NO pathway is found to be overall negligible. The NOx species are mainly formed in positively-curved flame segments where the flamelet model gives good predictions, while discrepancies with respect to the model exist in the negatively-curved flame regions. The reason for this is clarified through a chemical timescale analysis and a conditional reaction path analysis.

A species-weighted flamelet/progress variable model with differential diffusion effects for oxy-fuel jet flames

A flamelet/progress variable (FPV) model accounting for the differential diffusion (DD) effects is proposed and applied to large eddy simulation (LES) of turbulent oxy-fuel flames (Sevault et al. 2012). Based on tabulations with the detailed molecular diffusion and the equal diffusion (ED) assumption, a species-weighted flamelet (SWF) model is developed to represent the DD effects. The influences of combustion progress, mixture fraction, and turbulence on variable Lewis numbers are incorporated into the model. The model assessments are first conducted on a laminar coflow flame, showing that the effect of DD on temperature and species is accurately captured, in that the importance of DD is seen in the laminar flame. Subsequently the model is implemented in a fully coupled LES of oxy-fuel jet flames. The LES results show that DD plays an important role in the reaction zone and regions near the fuel nozzle, and its effect decreases farther downstream, consistent with the experimental observations. The LES with the SWF model yields good predictions on the mean temperature and major species at the fuel-rich side while slight deviation (less than 6%) at the fuel-lean side. In comparison, LES with the original unity Lewis numbers flamelet model (Pierce et al. 2004) predicts a full extinction because of neglecting the DD characteristics, while the variable Lewis number flamelet model provides the maximum deviation of 26% because of ignoring the ED characteristics produced by turbulent disturbance. The trend and location of localized extinction of oxy-fuel flame are also well predicted using the SWF model.

A thick reaction zone model for premixed flames in two-dimensional channels

Direct interactions between the flow field and the chemical reaction in premixed flames occur when the reaction zone thickness is comparable to, or greater than flow length scales. To study such interactions, a laminar model is considered that has direct bearings to steadily propagating deflagrations in a Hele-Shaw channel with a background plane Poiseuille flow. The study employs asymptotic analyses, pertaining to large activation energy and lubrication theories and considers a distinguished limit where the channel width is comparable to the reaction zone thickness, with account being taken of thermal-expansion and heat-loss effects. The reaction zone structure and burning rates depend on three parameters, namely, the Peclet number, , the Lewis number, and the ratio of channel half-width to reaction zone thickness, . In particular, when the parameter is small wherein the reaction zone is thick, transport processes are found to be controlled by Taylor's dispersion mechanism and an explicit formula for the effective burning speed is obtained. The formula indicates that for , which interestingly coincides with a recent experimental prediction of the turbulent flame speed in a highly turbulent jet flame. The results suggest that the role played by differential diffusion effects is significant both in the laminar and turbulent cases. The reason for the peculiar dependence can be attributed, at least in our laminar model, to Taylor dispersion. Presumably, this dependence may be attributed to a similar but more general mechanism in the turbulent distributed reaction zone regime, rather than to diffusive-thermal curvature effects. The latter effects play however an important role in determining the effective propagation speed for thinner reaction zones, in particular, when is large in our model. It is found that the magnitude of heat losses at extinction, which directly affects the mixture flammability limits, is multiplied by a factor in comparison with those corresponding to the no-flow case in narrow channels.

The Effects of Differential Diffusion on Turbulent Non-Premixed Flames LO2/CH4 under Transcritical Conditions Using Large-Eddy Simulation

In this paper, a large-eddy simulation (LES) of turbulent non-premixed LO2/CH4 combustion under transcritical conditions is performed based on the Mascotte test rig from the Office National d’Etudes et de Recherches Ae´rospatiales (ONERA), and the aim is to understand the effects of differential diffusion on the flame behaviors. In the LES, oxygen was injected into the environment above the critical pressure while the temperature was below the critical temperature. The flamelet/progress variable (FPV) approach was used as the combustion model. Two LES cases with different species diffusion coefficient schemes—i.e., non-unity and unity Lewis numbers—for generating the flamelet tables were carried out to explore the effects of differential diffusion on the flame and flow structures. The results of the LES case with non-unity Lewis numbers were in good agreement with the experimental data. It was shown that differential diffusion had evident impacts on the flame structure and flow dynamics. In particular, when unity Lewis numbers were used to evaluate the species diffusion coefficient, the flame length was underestimated and the flame expansion was more significant. Compared to laminar counterflow flames, turbulence in jet flames allows chemical reactions to take place in a wider range of mixture fractions. The density distributions of the two LES cases in the mixture fraction space were very similar, indicating that differential diffusion had no significant effects on the phase transition under transcritical conditions.

Effects of density ratio and differential diffusion on flame accelerative propagation of H2/O2/N2 mixtures

The effects of density ratio and differential diffusion on premixed flame propagation of H2/O2/N2 mixtures are investigated by constant volume combustion chamber. The density ratio and differential diffusion are controlled independently by adjusting the O2/N2 ratio and equivalence ratio. Results show that the density ratio has no effect on turbulent burning velocity while the differential diffusion has a promotion effect on turbulent burning velocity. The onsets of laminar flame acceleration are promoted by both density ratio and differential ratio. The turbulent flames perform a continuous acceleration propagation and the dependence between flame propagation speed and flame radius can be characterized as (dR/dt)/(σ·SL) ∼ R0.33∼0.37, which is lower than the 1/2 power law. The acceleration parameters of laminar flames and turbulent flames (u’/SL = 1) are around 0.17 and 0.36 respectively, and both of them are not affected by density ratio and differential diffusion. The empirical formula m = 0.19·(u’/SL)0.4+0.17 is concluded to quantitatively describe the accelerative characteristics of laminar and turbulent flames. The current study indicates that the acceleration of laminar flames is mainly induced by flame intrinsic instability, and the latter can affect the acceleration onset but not affect the fractal excess. The acceleration of turbulent flames is dominated by turbulent stretch, while the effects of density ratio and differential diffusion can be ignored.