Unburned Side Research Articles

Stretch Effects in Large Eddy Simulation of Turbulent Premixed Bunsen Flames

ABSTRACT The stretch effects in large eddy simulation (LES) of a turbulent lean propane-air premixed Bunsen flame in the wrinkled flamelet regime are investigated. A simple approach to model the subgrid-scale flame stretch is proposed. It is based on the analysis which shows that, when the surface-filtered strain rate in a subgrid-scale stretch model is approximated using the volume-filtered velocity field solved for in LES, heat release and associated gas expansion in the filtered flame brush tend to artificially alter the wrinkling of resolved flame fronts. To minimize such artifacts, it is suggested that the volume-filtered strain rate on the unburned side of the filtered flame brush be used to approximate the surface-filtered strain rate and projected through the filtered flame brush. The results show the importance of mitigating the artificial heat release effects when considering the strain effects in LES. The relative importance of the curvature stretch is also investigated in terms of the mean and local effects. For a Bunsen flame with a positive Markstein number, the mean flame curvature effect tends to increase the total burning rate by enhancing the laminar flame speed near the flame tip, while the local flame curvature effect tends to decrease the total burning rate by suppressing the wrinkling of the resolved flames. It is found that the competition between the two makes the overall curvature effects not influence the total burning rate much for the flame investigated here, as compared with the strain effects.

Local effects in vortex-flame interactions: Implications for turbulent premixed flame scaling and observables

A computational investigation was carried out to assess the effects of highly energetic vortices on the local structure of laminar premixed flames. The study involved a lean atmospheric methane-air flame interacting with vortex sizes ranging from five times to half of the laminar flame thickness. The characteristic velocities of the vortices were chosen to correspond to extreme turbulence intensities. Vortices smaller than the flame thickness were found to have a minimal effect on the flame structure due to the high rate of dissipation. Vortices with sizes greater than or equal to the flame thickness were found to induce flame siphoning and/or product-side local flame annihilation and as a result transport considerable amounts of formaldehyde and thermal energy to the unburned side thereby modifying the state of the reactants. The results of the present study for high activation energy reacting flows raise questions regarding the general validity of: (1) conjectured mechanisms of the so-called flame broadening under intense turbulence conditions; and (2) qualitative arguments regarding sub-flame-thickness vortex effects on the preheat zone in the context of the Borghi–Peters regime diagram, as originally postulated by Damköhler.

Comparison of the Reactive Scalar Gradient Evolution between Homogeneous MILD Combustion and Premixed Turbulent Flames

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technique that can simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the differences in statistical behaviours of the surface density function (SDF = magnitude of the reaction progress variable gradient) between conventional premixed flames and exhaust gas recirculation (EGR) type homogeneous-mixture combustion under MILD conditions using direct numerical simulations (DNS) data. The mean values of the SDF in the MILD combustion cases were found to be significantly smaller than those in the corresponding premixed flame cases. Moreover, the mean behaviour of the SDF in response to the variations of turbulence intensity were compared between MILD and premixed flame cases, and the differences are explained in terms of the strain rates induced by fluid motion and the ones arising from flame displacement speed. It was found that the effects of dilatation rate were much weaker in the MILD combustion cases than in the premixed flame cases, and the reactive scalar gradient in MILD combustion cases preferentially aligns with the most compressive principal strain-rate eigendirection. By contrast, the reactive scalar gradient preferentially aligned with the most extensive principal strain-rate eigendirection within the flame in the premixed flame cases considered here, but the extent of this alignment weakened with increasing turbulence intensity. This gave rise to a predominantly positive mean value of normal strain rate in the premixed flames, whereas the mean normal strain rate remained negative, and its magnitude increased with increasing turbulence intensity in the MILD combustion cases. The mean value of the reaction component of displacement speed assumed non-negligible values in the MILD combustion cases for a broader range of reaction progress variable, compared with the conventional premixed flames. Moreover, the mean displacement speed increased from the unburned gas side to the burned gas side in the conventional premixed flames, whereas the mean displacement speed in MILD combustion cases decreased from the unburned gas side to the middle of the flame before increasing mildly towards the burned gas side. These differences in the mean displacement speed gave rise to significant differences in the mean behaviour of the normal strain rate induced by the flame propagation and effective strain rate, which explains the differences in the SDF evolution and its response to the variation of turbulence intensity between the conventional premixed flames and MILD combustion cases. The tangential fluid-dynamic strain rate assumed positive mean values, but it was overcome by negative mean values of curvature stretch rate to yield negative mean values of stretch rate for both the premixed flames and MILD combustion cases. This behaviour is explained in terms of the curvature dependence of displacement speed. These findings suggest that the curvature dependence of displacement speed and the scalar gradient alignment with local principal strain rate eigendirections need to be addressed for modelling EGR-type homogeneous-mixture MILD combustion.

Effects of body forces on vorticity and enstrophy evolutions in turbulent premixed flames

The effects of body forces (alternatively, the Froude number) on both vorticity and enstrophy evolutions within the flame brush have been analyzed using direct numerical simulation data of freely propagating statistically planar turbulent premixed flames subjected to different turbulence intensities. The turbulence parameters are taken to represent the thin reaction zone regime of premixed turbulent combustion. The enstrophy has been found to decay significantly from the unburned to the burned gas side of the flame brush for high turbulence intensities, and this trend becomes particularly prominent with the strengthening of the body force promoting unstable stratification. However, local instances of enstrophy generation have been observed, and in some cases, the decay of enstrophy is arrested across the flame brush for small turbulence intensities. This trend strengthens with the increasing magnitude of the body force promoting stable stratification. The enstrophy generation due to the baroclinic torque is primarily responsible for this local enstrophy generation for small turbulence intensities especially under the body force promoting stable stratification. This baroclinic torque contribution is also found to be responsible for anisotropic behavior of vorticity components within the flame brush. The vortex stretching and viscous dissipation terms have been found to be the leading order source and sink terms, respectively, in the enstrophy transport for high turbulence intensities especially in the case of body force promoting unstable stratification. However, baroclinic torque and the sink term due to dilatation rate continue to play significant roles even for high turbulence intensity cases considered here, but their relative importance increases with a decreasing turbulence intensity especially under the body force that promotes stable stratification. The surface-weighted entrainment velocity has been found to be mostly unaffected by the body force in this analysis, and a minor influence can be discerned in the case of small turbulence intensities where an unstable stratification tends to promote high negative values of entrainment velocity only toward the unburned gas side of the flame brush.

Effects of Microbial Biomineralization Surface Erosion Control Treatments on Vegetation and Revegetation along Highways

Microbial induced calcite precipitation (MICP) has been widely studied in laboratories to test changes to soil strength and density. Rarely studied is the biogeotechnology’s influence on real-world conditions. Consideration for the natural environment coexisting with treated soil is important, particularly vegetative responses to biochemical and physical changes from treatments. In this factorial designed study, vegetative response from one-time biochemical surficial treatments is observed in four soil growth mediums: two variants burned soil, unburned side slope construction soil, and Ottawa sand. Treatment objectives are to create a light crust that provides short-term erosion control, protects concurrently applied seeds and provides a beneficial plant environment (BPE). The crust creates a BPE through increased soil water retention and shear soil strength allowing better root and plant stability. An overly dense crust prevents root penetration and is avoided because established root systems are crucial in long-term erosion control. This study successfully created such a crust in all soil types treated. Also studied were influences of solution components on germination rates. Component influence proved highly dependent on soil type as calcium chloride inclusion was highly detrimental to seedling success in clean sand, somewhat detrimental to burned soil with ash layer, insignificant in unburned soil, and beneficial to burned soil without ash layer. These results give an indication of the complex biochemical soil reactions occurring from MICP treatment. This study gives evidence that a one-time application of a seeded biochemical solution has real-world potential as a balanced short-term and long-term erosion control technology for burned and construction soils.

Analysis of flame curvature evolution in a turbulent premixed bluff body burner

The physical mechanisms responsible for flame curvature evolution of a methane-air premixed flame attached to a bluff-body burner have been investigated using a high-fidelity flame-resolved three-dimensional simulation database. The contributions to the mean curvature generation due to the fluid flow motion and due to a combination of flow and flame propagation induced strain rates have been analyzed in detail and dominant contributions in different zones (reactants, flame, and products) of the flame have been identified. The effect of fluid flow on the mean curvature evolution is important on the unburned gas side, whereas the flame propagation dominates the mean curvature evolution in the reaction region and toward the hot products. The statistical contributions of the mean curvature transport equation have been analyzed in terms of the iso-scalar surface geometry, characterized by the mean and Gauss curvatures. This information has subsequently been used to provide physical insights into the dominant mechanisms of curvature evolution for different flame topologies.

The effects of hydrogen addition, inlet temperature and wall thermal conductivity on the flame-wall thermal coupling of premixed propane/air mixtures in meso-scale tubes

The effects of hydrogen addition, inlet temperature, wall thermal conductivity and wall thickness on the flame-wall coupling of the propane/air flames in a meso-scale tube are numerically investigated using a two dimensional model along with the detailed chemical mechanism. Higher wall thermal conductivity can result in preheating the fresh mixture uniformly in strongly flame-wall coupled system, which is vital to enhance the burning rate of fuel mixture. With the increase of wall thermal conductivity or hydrogen addition, the leading edge of the flame shifts from the wall to the axis, meanwhile the flame is more convex towards the unburned side near the leading edge. As the hydrogen addition and inlet temperature increase, the flame propagation speed increases significantly, while the maximum temperature and maximum total enthalpy decrease due to the reduced heat recirculation power. The flame propagation speed has a negative correlation with heat loss. The chemical reactions in preheat zone are enhanced at low wall thermal conductivity due to the higher inner wall temperature. Thinner combustor wall leads to higher flame speed and higher heat loss simultaneously. Results have implications on the choice of solid wall material and heat recirculation design in a stable meso-scale combustor for different fuels.

Local Strain Rate and Curvature Dependences of Scalar Dissipation Rate Transport in Turbulent Premixed Flames: A Direct Numerical Simulation Analysis

The statistical behaviours of the instantaneous scalar dissipation rateNcof reaction progress variablecin turbulent premixed flames have been analysed based on three-dimensional direct numerical simulation data of freely propagating statistically planar flame and V-flame configurations with different turbulent Reynolds numberRet. The statistical behaviours ofNcand different terms of its transport equation for planar and V-flames are found to be qualitatively similar. The mean contribution of the density-variation termT1is positive, whereas the molecular dissipation term(-D2)acts as a leading order sink. The mean contribution of the strain rate termT2is predominantly negative for the cases considered here. The mean reaction rate contributionT3is positive (negative) towards the unburned (burned) gas side of the flame, whereas the mean contribution of the diffusivity gradient term(D)assumes negative (positive) values towards the unburned (burned) gas side. The local statistical behaviours ofNc,T1,T2,T3,(-D2), andf(D)have been analysed in terms of their marginal probability density functions (pdfs) and their joint pdfs with local tangential strain rateaTand curvaturekm. Detailed physical explanations have been provided for the observed behaviour.

Statistical Analysis and a-priori Modelling of Flame Surface Density Transport in Turbulent Stratified Flames: A Direct Numerical Simulation Study

Statistically planar turbulent premixed and stratified flames for different initial intensities of decaying turbulence have been simulated for global equivalence ratios = 0.7 and = 1.0 using three-dimensional simplified chemistry based Direct Numerical Simulations (DNS). The simulation parameters are chosen such that the thin reaction zones regime combustion is realised in all cases and a random bi-modal distribution of equivalence ratio ϕ is introduced in the unburned gas ahead of the flame to account for the mixture inhomogeneity for stratified flames. The modelling of the unclosed terms (i.e. the turbulent transport term T1, the tangential strain rate term T2, the propagation term T3, and the curvature term T4) of the generalised FSD transport equation has been addressed in the context of RANS simulations. It has been found that the turbulent transport term T1 remains small in comparison to the leading order contributions of the tangential strain rate and curvature terms (i.e. T2 and T4, respectively) in the globally stoichiometric cases, but T1 begins to play a more important role in the globally fuel-lean cases. The strain rate term T2 remains positive throughout the flame brush and acts as a leading order source term for all the flames considered in this analysis. It is has been found that the magnitude of T2 decreases with decreasing root-mean-square velocity fluctuations u′ ( ) for a given value of (u′). The contribution of the propagation term T3 remains generally positive towards the unburned gas side of the flame brush but assumes generally negative values towards the burned gas side of the flame brush. Moreover, whilst the order of magnitude of the propagation term T3 is comparable in all cases, T3 remains small in comparison to the leading order contributors (i.e. T2 and T4) in the globally stoichiometric cases however it plays a more important role in the globally fuel-lean cases. The curvature term T4 acts as a leading order sink term in all cases except towards the unburned gas side of the flame brush in low u′ globally stoichiometric (i.e. = 1.0) flames. Furthermore, it has been demonstrated that the magnitude of T4 decreases with decreasing u′ ( ) for a given value of (u′). Appropriate model expressions have been identified for T1, T2, T3 and T4 based on an a-priori analysis of the DNS data.

Radical fingering in turbulent premixed flame classified into thin reaction zones

Direct Numerical Simulations (DNSs) of hydrogen-air turbulent premixed planar flames in homogeneous isotropic turbulence are conducted to investigate the local flame structure in thin reaction zones. The detail kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen-air reaction. DNSs are conducted at Reλ=37.4 and 60.8, where Reλ denotes Reynolds number based on Taylor micro-scale (λ). In the thin reaction zones, the heat release rate is highly fluctuating in the low temperature region and the maximum heat release rates reach to 1.3–1.4 times that of the laminar flame. The high heat release rate regions are surrounded by the low heat release regions and are shown like pocket and reed-shape structures. In the unburned side, the radical fingering of HO2 is also generated, which suggests that the strong coherent-fine scale eddies in the unburned side transport reactive species in the preheat zone into the unburned side for the case of thin reaction zones. It is implied that the radical fingering to the unburned mixture is one of the important flame structures in the reaction zones or in broken reaction zones. To clarify effects of turbulence on the local flame structure, the statistical characteristics of flame elements are also investigated.

J054012 Thin reaction zones における乱流予混合火炎の局所火炎構造

Direct Numerical Simulation (DNS) of hydrogen-air turbulent premixed planar flames in homogeneous isotropic turbulence has been conducted to investigate local flame structures in thin reaction zones. The detail kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen-air reaction. DNS is conducted at Re_λ= 37.4 and 60.8, where Re_λ denotes Reynolds number based on Taylor micro-scale (λ). In the thin reaction zones, heat release rate is highly fluctuating in low temperature region. The high heat release rate regions are surrounded by the low heat release regions and are shown like pocket and reed-shape structures. In the unburned side, the radical fingering of HO_2 is also generated, which suggests that the strong coherent-fine scale eddies in the unburnedside transport reactive species in the preheat zone into the unburned side for the case of thin reaction zones.

A Simple Model for Evaluating Conditioned Velocities in Premixed Turbulent Flames

A simple model is proposed to evaluate (a) the divergence of velocity vector conditioned on unburned mixture, and (b) the vector component normal to the mean flame brush in the flamelet regime of premixed turbulent combustion. The model involves a single constant and does not invoke an extra balance equation. To perform the first test of the model, six flames stabilized in impinging jets and experimentally investigated by 4 research groups were numerically simulated. In the computations, (a) approximations of the measured axial profiles of the mean combustion progress variable were invoked, (b) the well-known (Bray et al., 1998, and 2000) statistically steady and 1-dimensional Favre-averaged continuity and Euler equations were numerically integrated in order to approximate the measured axial profiles of the mean axial velocity, and, then, (c) the approximations were utilized in order to evaluate conditioned velocities and turbulent scalar flux using the proposed model supplemented with the BML approach and balance equation for the Favre-averaged combustion progress variable. The obtained agreement between the measured and computed axial profiles of the conditioned axial velocities or axial turbulent scalar flux was encouraging, thus, indicating that the proposed simple model is promising. Since the correlation between fluctuations of velocity and unity normal vectors, conditioned to flamelet surface, plays a key role in the model, the encouraging test results call for studying this correlation in future DNS. Moreover, further research into the difference in velocity conditioned on unburned mixture and velocity conditioned on the unburned side of flamelets is necessary for improving the model at the leading edge of a turbulent flame brush.

Statistical Analysis of Cross Scalar Dissipation Rate Transport in Turbulent Partially Premixed Flames: A Direct Numerical Simulation Study

Statistically planar turbulent partially premixed flames for different initial intensities of decaying turbulence have been simulated for global equivalence ratios = 0.7 and = 1.0 using three-dimensional simplified chemistry based Direct Numerical Simulations (DNS). The simulation parameters are chosen such that the combustion situation belongs to the thin reaction zones regime and a random bi-modal distribution of equivalence ratio Φ is introduced in the unburned gas ahead of the flame to account for mixture inhomogeneity. The DNS data has been used to analyse the statistical behaviour of the transport of the cross-scalar dissipation rate based on the fuel mass fraction YF and the mixture fraction ξ fluctuations \(\,\tilde{\varepsilon}_{Y\xi}={\overline{\rho D\nabla Y_{F}^{\prime\prime}.\nabla \xi^{\prime\prime}} } \big/ {\bar {\rho }}\) (where \(\bar{q}\), \(\tilde{q}={\overline{\rho q} } \big/ {\bar {\rho }}\) and \(q^{\prime\prime} =q-\tilde {q}\) are Reynolds average, Favre mean and Favre fluctuation of a general quantity q) in the context of Reynolds Averaged Navier–Stokes simulations where ρ is the gas density and D is the gas diffusivity. The statistical behaviours of the unclosed terms in the \(\tilde{\varepsilon }_{Y\xi } \) transport equation originating from turbulent transport T1, density variation T2, scalar–turbulence interaction T3, chemical reaction rate T4 and the molecular dissipation rate D2 have been analysed in detail. It has been observed that the contributions of T2, T3, T4 and D2 play important roles in the \(\tilde{\varepsilon }_{Y\xi } \) transport for the globally stoichiometric cases, but in the globally fuel-lean cases the contributions of T2 and T4 become relatively weaker in comparison to the contributions of T3 and D2. The term T1 remains small in comparison to the leading order contributions of T3 and D2 for all cases, but the contribution of T1 plays a more important role in the low Damkohler combustion cases. The term T2 behaves as a sink term towards the unburned gas side but becomes a source term towards the burned gas side. The scalar–turbulence interaction term T3 has been found to be generally positive throughout the flame brush, but in globally stoichiometric cases the contribution of T3 becomes negative in regions of intense heat release. The combined contribution of (T4 – D2) remains mostly as a sink in all cases studied here. Models are proposed for the unclosed terms of the \(\tilde{\varepsilon }_{Y\xi } \) transport equation in the context of Reynolds Averaged Navier–Stokes simulations, which are shown to satisfactorily predict the corresponding quantities extracted from the DNS data for all cases.

Modelling of the tangential strain rate term of the Flame Surface Density transport equation in the context of Reynolds Averaged Navier–Stokes simulation

The modelling of the tangential strain rate term in the Flame Surface Density (FSD) transport equation in the context of Reynolds Averaged Navier–Stokes (RANS) simulations of turbulent premixed combustion has been addressed by a priori analysis of a Direct Numerical Simulation (DNS) database of statistically planar freely propagating flames with wide variations of Damköhler number Da, heat release parameter τ and global Lewis number Le. It has been found that the dilatation rate contribution to the FSD transport strengthens with increasing value of τ and decreasing value of Le. The behaviour of the normal strain rate term shows significant differences in response to Da and Le. It has been found that the normal strain rate contribution to the FSD transport remains a sink term for the flames with high and small values of Da and Le, respectively, where the effects of strain rate induced by heat release achem overcome the effects of turbulent straining aturb. By contrast, the effects of aturb overcomes the effects of achem for low Da flames with Le⩾1, which leads to a positive value of the normal strain rate term towards the unburned gas side, but this term becomes negative towards the burned gas side due to strong achem overcoming aturb in the regions of intense heat release. The strengthening of the dilatation rate and achem at small and large values of Le and Da, respectively, is explicitly taken into account to propose new models for the strain rate contributions to the FSD transport. The new model is shown to satisfactorily capture the effects of Damköhler number Da, heat release parameter τ, and global Lewis number Le, on the tangential strain rate term of the FSD transport equation for all the cases considered in this study.

Effects of soret diffusion on the laminar flame speed of n-butane-air mixtures

The effect of Soret diffusion on flame speed of the freely-propagating planar premixed flames was computationally studied for n-butane/air mixtures using a detailed reaction mechanism and transport. The results show that Soret effect almost has no influence on the n-butane/air flame speed, and the sensitivity analysis of flame speed shows that only very light species H radical is involved in the ranked key reactions. That is to say, the Soret diffusion of H radical has no influence on the flame speed. However, the Soret effect of H plays an important role on hydrogen/air flame speed. We found that the Soret diffusion flux of the kth species just exists in the overlap region which consists of the rear part of “driving force” gradT/T profile and the front part of mole fraction profile of the kth species in the freely-propagating planar premixed flame. Compared with hydrogen/air flame, the investigation shows that for the n-butane/air flame, due to the inherent feature of total heat release rate, the high temperature largely reduces the “driving force” gradT/T especially on the rear part of its profile and compared with the peak of temperature gradient the location of “driving force” peak is closer to the unburned side, on the other hand the broader consumption layer of H weakens the front part of mole fraction of H and makes the peak of H mole fraction far away from the unburned side. Thus, the overlap region is narrow and low values of “driving force” and mole fraction of H embed in it.

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.

2123 希薄予混合燃焼器における燃料濃度分布と火炎温度分布計測に関する研究(J18-1 ガスタービンにおける熱流動問題(1),ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

In heavy-duty gas turbine engines, the turbine inlet temperature increases year by year to improve thermal efficiency. But, at the same time, this leads to the increase of NOx emissions. One of the most promising approaches for gaseous fuel(natural gas) to reduce NOx emissions is the adoption of lean premixed combustion. With lean premixed combustion, locally high temperature regions which are attributed to mixture inhomogeneity are suppressed, and so it has large effect in reducing NOx. However, in the case that it is applied to combustor, since fuel and air are separately supplied because of preventing flashback, it is needed to burn the mixture after sufficient mixing. In this study, we examined the air-fuel mixing characteristics in the unburned region where is adjacent to flame zone in Lean Premix Combustor with Swirling Flow. The mixing characteristics were evaluated by measuring the instantaneous concentration of mixed gaseous fuel which contains acetone as a tracer, using planar laser-induced fluorescence(P-LIF). Simultaneously, we measured the instantaneous flame temperature by Rayleigh scattering method, and investigated correlation between mixing characteristics in the unburned side and temperature distribution in the burned side.

An Improvement of Lean Combustion Characteristics with Hydrogen Addition in a Rotating Tubular Flame (Burned Gas Temperature and Burned Gas Composition)

A rotating tubular flame has received considerable attention in combustion research because it experiences flame curvature and flame stretch simultaneously. It is also interesting to note that the rotating tubular flame is very stable, and hence, addition of small amount of hydrogen enables lean combustion of heavy hydrocarbon fuels with this flame. In our previous report, it has been found that hydrogen addition of 3% by volume enables lean combustion up to the effective equivalence ratio of 0.1 for methane, and up to the ratio of 0.2 for propane flame. In this experiment, probing of the burned gas temperature and the burned gas composition were made. The results show that in the cases when hydrogen is added by 1, 2, and 3% by volume to methane/air mixture, and by 3% by volume to propane/air mixture, the burned gas temperature exceeds the temperature obtained by chemical equilibrium calculation (CEC) in some low fuel concentration ranges. In these ranges, the oxygen concentration in the burned gas becomes lower than that of CEC, while the water concentration exceeds that of CEC. These facts substantiate the validity of the previously postulated mechanism for extension of lean combustion that due to the effects of flame curvature and flame stretch, hydrogen preferentially diffuses across stream tube from the upstream, unburned gas side into the reaction zone, resulting in an increase of gas temperature and also of water concentration ; then lean combustion is achieved.

Response of flame speed to positively and negatively curved premixed flames

We address flame stretch effects on two-dimensional premixed ‘regular’ (RF) and ‘inverted’ (UF) flames burning methane–air and propane–air mixtures through an experimental investigation. The regular flames have a negative curvature and are concave to the unburned mixture, while the inverted flames are positively curved and convex. The response to stretch (that addresses the influence of curvature) differs along the planar and curved regions of a premixed flamefront. A curved flame behaves as a lens that focuses (negative curvature) or defocuses heat from the burned to the unburned side depending upon its curvature, but defocuses (negative curvature) or focuses the concentration of the deficient reactant from the unburned side into the premixed reaction zone. The focusing of heat into the unburned side of the negatively curved RF raises the local temperature in the unburned region and increases the upstream velocity. The positive curvature of the UF induces a positive stretch rate, which decreases the flame propagation speed by lowering the local reaction and heat generation rates. However, the combined effects of stretch and curvature are more complex. The planar and curved regions have different responses to stretch. The planar regions behave in accord with the Markstein S u(κ) linear relation and the unstretched flame speeds S u 0 thus inferred are in accord with the literature. The flame speed, however, changes dramatically along the curved regions due to curvature effects. Although curvature effects are included in the definition of stretch, they are not fully accounted for by the S u(κ) Markstein linear relation. Even after considering the effect of curvature in the definition of κ, strong negative curvature raises the value of the flame speed above the prediction of the S u(κ) expression, while strong positive curvature reduces its magnitude below that predicted by the relation. This implies that the curvature has an influence on S u through κ and through the sign of curvature and Lewis number.

Interaction of adjacent flame surfaces on the formation of wrinkling laminar premixed flame

To gain further understanding of the flamelet regime of turbulent combustion, the interaction betweenadjacent flame surfaces of a wrinkling laminar premixed flame has been investigated experimentally. The wrinkling flame was formed with three adjacent laminar V-shaped flames stabilized on a multiple-slitburner of a propane/air mixture. In addition, a single V-shaped flame was also formed on the burner. We compared flame shapes and flow velocity distributions between those two types of flames. The results show that the unburned flow field was expanded outwardly by the convex flame surface toward unburned side. In the case of the wrinkling flame, however, the outward deviation of the flow field caused by the existence of the inner convex was prevented by the existence of the surrounding flames. Consequently, the inner convex flame surface of the wrinkling flame is more oblique, and the curvature is sharper than that of a single convex flame (single V-shaped flame) due to the interaction of adjacent flame surfaces. Moreover, the formation of the inner convex flame of the wrinkling flame is more unstable than that of a single convex flame. This is due to the curvature of the inner convex of the wrinkling flame being sharper than that of the single convex flame so that the burning velocity is decreased as a result of the curvature effect.