Premixed Flame Branches Research Articles

Nonlinear Flame Dynamics for Laminar Flame in a Co-flow Mixing Layer with Widely Varying Premixedness: Preferential Diffusion and Stoichiometry Effects

ABSTRACT Intrinsic oscillations arising due to thermo-diffusive mechanism have been studied numerically in the literature for laminar flames having Lewis number greater than unity. In the previous work, the Lewis numbers of fuel and oxidizer are assumed to be equal and the unburnt mixture to be in stoichiometric in proportion. Here, the study is extended to relax these assumptions. In the first part of the present work, the Lewis numbers of fuel and oxidizer are varied individually keeping the overall mixture stoichiometric. The simulations are run for laminar flames stabilized near to a cold burner in a co-flow mixing layer generated by a fuel rich stream and oxidizer rich stream. The premixedness of the unburnt mixture is varied using a premixedness parameter defined in the previous work. This parameter characterizes upstream mixture-fraction gradients such that it approaches unity for zero mixture-fraction gradient and zero when the mixture-fraction gradient is infinite. Thus, upon ignition, different flame structures like, planar premixed flame, triple flames and an edge flame can be observed, as this parameter is varied from zero to unity. Simulations are run to study large time behavior for various values in this range of premixedness. Intrinsic oscillations in the total reaction rate (integrated over the domain) are observed for a combination of values of Lewis numbers of fuel and oxidizer. These are presented as series of contour plots of amplitude of total reaction rate onto a 2D plane with Lewis number of fuel on x-axis and Lewis number of oxidizer on y-axis for various values of premixedness. It is found that the contours of constant amplitude are nearly linear for diffusion flame and partially premixed flames, thus validating the effective Lewis number definition hitherto defined for diffusion flames. Hence, this effective Lewis number can determine the onset of oscillations and also their magnitude. But for weakly curved triple flames, there is significant variation, where the constant amplitude lines are curved. Also, the amplitude of oscillations observed for weakly curved triple flames is much higher than that for other flames. Apart from intrinsic oscillations the shape and structure of triple flame is also studied. It is found that premixed flame branches of a triple flame are highly asymmetric about the trailing edge diffusion flame. The change in the location of triple point is also observed. In the second part of the work, the assumption of the overall stoichiometric ratio being unity is relaxed to understand the effect of the off-stoichiometric mixture on the shape and stability of flame. However, the Lewis numbers of the fuel and oxidizer are maintained to be equal. The maximum intensity of oscillations is observed at an equivalence ratio of 0.3 for partially premixed flames and at around unity for premixed flames.

Ignition dynamics of DME/methane-air reactive mixing layer under reactivity controlled compression ignition conditions: Effects of cool flames

A study of ignition dynamics in a turbulent dimethyl ether (DME)/methane-air mixture under reactivity controlled compression ignition (RCCI) conditions was conducted using direct numerical simulation. Initially, the directly-injected DME and in-cylinder premixed methane-air mixture are partially mixed to form a mixing layer in between. A reduced DME/CH4 oxidization mechanism, consisting of 25 species and 147 reaction steps, is developed and validated. Ignition is found to occur as a two-stage process. Low-temperature autoignition is first initiated in the fuel-rich part of the mixture and then transits to a cool flame, propagating towards the even richer mixture through a balanced reaction-diffusion mechanism. Cool flames not only develop in the mixing layer, but also in the initially stratified DME/methane-air mixture. The formation of high-temperature autoignition kernels is earlier than that in the homogeneous mixture at the same mixture fraction, which is thought to be accelerated by the cool flame. The expanding flames from high-temperature kernels are connected with the neighboring flames before they engulf the stoichiometric mixture iso-lines. The four branches of typical tetrabranchial flames, i.e. cool flame, fuel-rich premixed flame, diffusion flame, fuel-lean premixed flame coexist in the field. The fuel-lean premixed flame branch finally triggers the premixed methane-air flame. The multi-stage and multi-mode nature of the ignition process highlights the intractable challenge to model the RCCI engine combustion.

Structure of tetrabrachial flames in non-premixed autoigniting dimethyl ether/air mixtures

Structures of tetrabrachial flames in two-stage autoigniting dimethyl ether (DME)/air mixture under diesel engine relevant conditions are investigated by direct numerical simulation. Three cases with different initial turbulent integral length scales are studied. Results show that the first stage of autoignition initiates in lean mixtures, and subsequently develops into a diffusion-supported cool flame propagating into rich mixtures; the second-stage autoignition features spatially distributed kernels in fuel-rich mixtures, followed by hybrid autoignition/tetrabrachial flames. The detailed chemical structures of the tetrabrachial flames are analyzed in terms of reactant concentrations and the reaction rate profiles. The cool flame branch is dominated by low temperature reactions, while the other branches are mainly involved in high temperature oxidation of the remaining fuel and intermediate species. The excess DME is consumed in the premixed flame branches and decomposed into more stable fuels including H2, CH4 and CO in the trailing diffusion flame, where H2 and CO are mainly oxidized by intermediate species OH and O. The structures and reaction rates in the tetrabrachial flame exhibit significant asymmetry, which is more distinct in the mixture fraction-temperature phase space. Effects of turbulence on the timing and location of two-stage ignition are then studied. In this study, turbulence tends to advance ignition compared with laminar cases, while the first high-temperature ignition time is similar for the three cases with different initial turbulence integral length scales.

Formation of Soot in Ethylene–Air Partially Premixed Flames Over a Wide Range of Premixedness

This paper reports an investigation of soot formation in ethylene–air partially premixed flames (PPFs) over a wide range of premixedness. An axisymmetric co-flow configuration is chosen to establish PPFs from the fully nonpremixed to fully premixed conditions. Reducing the fuel flow rate as a percentage of the maximum from the core stream and supplying the same to the annular stream leads to stratification of the reactant concentrations. The thermal power, overall equivalence ratio, and the average velocity in both the streams are maintained constant under all conditions. The soot volume fraction is estimated by light attenuation method, and laser-induced incandescence (LII) is performed to map the soot distribution in the flow field. The soot volume fraction is observed to exhibit an “S”-type trend as the conditions are traversed from near the premixed to the nonpremixed regimes. That is, when traversing from the nonpremixed to near-premixed regime, below 60% fuel flow rate in core, the soot volume fraction drops drastically. The onset of sooting in the PPFs is clearly seen to be at the tip of the rich-premixed flame (RPF) branch of their triple flame structure, which advances upstream toward the base of the flame as the premixing is reduced. The S-type variation is clearly the effect of partial premixing, more specifically due to the presence of the lean premixed flame (LPF) branch of the triple flame. LII intensities are insufficient to capture the upstream advance of the soot onset with decreased premixedness. So, a quick and inexpensive technique to isolate soot luminescence through flame imaging is presented in the paper involving quasi-simultaneous imaging with a 650 nm and a BG-3 filter using a normal color camera.

Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.

층류제트 부상화염에서 열손실에 의한 자기진동에 관한 연구

질소 희석된 프로판 부상화염에서 열손실에 의한 자기진동을 기초로 화염안정화선도를 도출하기 위하여 노즐직경 0.3 ㎜, 1.0 ㎜에서 실험적 연구를 수행하였다. 예혼합화염에서 확산화염으로의 전도 열손실에 의한 자기진동 및 매연 복사에 의한 자기진동을 관찰하였다. 0.1 ㎐보다 낮은 주파수 성향을 띄는 열손실에 의한 자기진동은 제안된 메커니즘에 의해 잘 묘사되었고 반면 매연복사에 의한 자기진동은 O(0.1 ㎐)의 주파수 범위를 나타내었으며 제안된 메커니즘은 항온항습실 실험을 통해 입증하였다. 질소 희석된 프로판 부상화염에서 관찰된 열손실에 의한 자기진동의 특성화는 관련된 변수 및 스트라훌 수에 의해 잘 묘사되었다.

A numerical investigation on NO X formation in counterflow n-heptane triple flames

The formation of NO X in counterflow n-heptane/air triple flames was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results indicate that a triple flame produces more NO and NO 2 than the corresponding premixed flames due to not only the appearance of the diffusion flame but also the interaction between different flame branches. The relative contributions of different routes to NO formation in the premixed flame branches change with the variation of the equivalence ratio, but the thermal mechanism always dominates in the diffusion flame branch. The interaction between flame branches is enhanced with the decrease of the distance between them. Both heat and radical exchange between flame branches contribute to the interaction. A new feature that does not exist in methane/air triple flame was observed in n-heptane/air triple flames, i.e. when the rich mixture equivalence ratio is higher, there are two peaks of CH concentration on the rich side of the diffusion flame branch, which leads to that some NO is formed beside the diffusion flame branch by the prompt route.

Numerical investigation of edge flame propagation characteristics in turbulent mixing layers

The statistical behavior of the displacement speed of an edge flame originating from localized ignition of inhomogeneous reactants in decaying isotropic turbulence is studied based on three-dimensional direct numerical simulations (DNS) with simplified chemistry. The ignition is simulated by introducing a source term in the energy transport equation, which deposits a given amount of energy over a specified time duration. The flame structure is shown to exhibit a tribrachial structure where a diffusion flame is stabilized on the stoichiometric mixture fraction isosurface alongside two premixed flame branches on fuel-rich and fuel-lean mixtures. The speed at which the fuel mass fraction YF isosurface moves with respect to an initially coincident material surface at the point where these three flames meet (commonly referred to as triple point) is taken as the edge flame displacement speed. Its statistics are explained in terms of the reaction, normal diffusion, and tangential diffusion components of the density-weighted displacement speed Sd*. It is shown that the statistical behavior of Sd* and its components in response to the scalar dissipation rate and local flame curvature are strongly dependent on the statistical behavior of the fuel mass fraction gradient ∣∇YF∣ in response to local flame curvature and mixture fraction gradient ∣∇ξ∣. It is shown that the extent of premixed and nonpremixed contributions determines the ∣∇YF∣ variation with ∣∇ξ∣, whereas the flow acceleration due to heat release and dilatation field determines the curvature response of ∣∇YF∣. The variation of Sd* in response to scalar dissipation rate and curvature are shown to be non-monotonic in nature and is found to be consistent with previous results based on two-dimensional DNS with detailed chemistry and experimental studies. This nonmonotonic behavior has been shown here to exist even in the absence of detailed chemistry and is explained in terms of the fluid-dynamical aspects of flame-turbulence interaction.

A numerical study of laminar methane/air triple flames in two-dimensional mixing layers

Triple flames formed in methane/air mixing layers with three different mixture fraction gradients were investigated by numerical simulation. The primitive variable method, in which the fully elliptic governing equations were solved with detailed chemistry and complex thermal and transport properties, was used. Radiation heat transfer from CO 2, CO and H 2O was calculated using the discrete-ordinates method coupled to a statistical narrow band correlated-K based wide band model. The results show that with the increase of the mixture fraction gradient, the combustion intensity in the diffusion flame branch of a triple flame is enhanced. In the near-stoichiometric mixture fraction region, the local burning flux of a triple flame is reduced when the mixture fraction gradient is increased. However, when the mixture fraction is significantly different from the stoichiometric value, the local burning flux increases as the mixture fraction gradient is increased. The correlation of the burning speed versus stretch rate established from conventional homogeneous premixed flames cannot completely explain the phenomena observed in triple flames. The interaction between the diffusion and premixed flame branches significantly affects the local burning velocity in regions where the mixture fraction is far from the stoichiometric value in a triple flame. This interaction is caused by both conduction heat transfer and radical exchange. Radiation has negligible effect on local burning properties in a triple flame.

A numerical study on NO x formation in laminar counterflow CH 4/air triple flames

Formation of NO x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO x , compared to the corresponding premixed flames. A triple flame produces more NO and NO 2 than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N 2O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO 2 formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N 2O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.

A numerical study of confined triple flames using a flamelet-generated manifold

Laminar triple flames are investigated numerically using detailed and reduced chemical reaction mechanisms. Triple flames are believed to play an essential role in flame propagation in partially premixed systems. In order to reduce the computational cost of the simulations, the flamelet-generated manifold (FGM) method is used to simplify the chemistry model. The FGM method is based on a library of premixed laminar flamelets. Mixture fraction variations in partially premixed flames are taken into account by using the mixture fraction as an extra degree of freedom. A comparison of the results computed with FGM and with detailed chemistry shows that FGM predicts the structure of triple flames very accurately. The gradient of the mixture fraction in the unburnt mixture is varied, and its influence on the structure and propagation of triple flames is studied. For decreasing gradients, the curvature of the premixed flame branch decreases and the propagation velocity increases. Due to the diffusive nature of the flow and the use of symmetry boundary conditions in the lateral direction, high mixture fraction gradients could not be realized at the position of the flame. As a result, the trailing diffusion flame branch is only weakly present. The heat release in the premixed flame branches is two orders of magnitude higher than in the diffusion flame branch. The premixed and diffusion flame branches are studied using a flamelet analysis. Flame stretch and curvature appear to be significant in the premixed flame branch of a triple flame. These effects cause a decrease in the local mass burning rate of almost 15%. The structure of the diffusion flame branch appears to be similar to the structure of a counterflow diffusion flame with the same scalar dissipation rate.

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge'' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures.

Structure and propagation of triple flames in partially premixed hydrogen–air mixtures

The characteristics of triple flames in a hydrogen–air mixing layer are studied using direct numerical simulation with detailed chemistry. Triple flames are initiated by imposing a temperature ignition source in the center of a scalar mixing layer of nonuniform thickness, thereby forming a pair of freely propagating triple flames. Two different fuel streams are studied: pure hydrogen and hydrogen diluted with nitrogen. During the ignition stage, the initial ignition runaway is followed by a secondary peak as the ignition kernel transitions to a triple flame, consistent with previous observations. For both diluted and undiluted cases, the triple flame structure exhibits more similarity with a diffusion flame than with a premixed flame, such that the triple point, defined as the location of maximum heat release, is always in the proximity of the stoichiometric mixture fraction line. Similar to a previous study of methanol–air triple flames, the enhancement in the stabilization speed is attributed mainly to flow divergence, and its value is proportional to the square root of the density ratio across the flame. In the undiluted case, however, the asymmetric flame structure results in distinct locations where the stabilization speed and the displacement speed are maximum. The effect of unsteady strain rate is also studied by imposing a pair of vortices on the propagating triple flames. The negative strain rate results in the collapse of the premixed flame branches onto the diffusion flame, forming an edge flame structure. Excessive compressive strain and curvature at the triple flame tip leads to a negative displacement speed. A mixture fraction/temperature parameterization is shown to be useful in representing the structure of a triple flame subjected to unsteady strain rate.

Effects of Different Kinds of Fuel and Fuel Equivalence Ratio on Flame Structure of Triple Flame.

In order to clarify the effects of different kinds of fuel and fuel equivalence ratio on flame structure, a numerical simulation of triple flame developed in a co-flowing methane-air or hydrogen-air mixture and air stream was made taking into account the elementary chemical reaction mechanism. The following conclusions were reached: (1)The relation between the apparent burning velocity of the triple flame and the fuel equivalence ratio shows a similar tendency to that of the one-dimensional premixed flame of the corresponding fuel. However, the fuel equivalence ratio at which the apparent burning velocity is the largest is a little larger than that of the one-dimensional premixed flame. The apparent burning velocities are two and three times higher than that of the one-dimensional premixed flame for the methane air or hydrogen-air mixture. (2)The flame thrusts out forward in the downstream of the boundary between mixture and air stream, and a part of the flow is bent and forks out in this protruding flame so that a triple flame is originated; this triple flame is composed of fuel rich and lean premixed flame branches and a diffusion flame branch. The change in shape of the convex part, caused by the effect of the one-dimensional premixed flame, is further promoted by the effect of hydrodynamic instability originated in the expansion brought about by heat release. A considerably strong diffusion flame branch exists almost in the center of the two premixed flame branches for the methane air mixture, while a considerably weak diffusion flame branch approaches the fuel lean premixed flame branch for the hydrogen air mixture. (3)Near the fuel equivalence ratio at which the burning velocity of the one-dimensional premixed flame is the largest, the effect of the one-dimensional premixed flame becomes large and the fuel rich premixed flame advances and becomes vertical to the flow direction. As a result, the effect of hydrodynamic instability is weakened. Thus, both of these effects demonstrate that the fuel equivalence ratio at which the apparent burning velocity is the largest is a little larger than that of the one dimensional premixed flame.

Structure and Propagation of Methanol–Air Triple Flames

The structure and propagation for a methanol (CH 3OH)–air triple flame are studied using direct numerical simulations (DNS). The methanol (CH 3OH)–air triple flame is found to burn with an asymmetric shape due to the different chemical and transport processes characterizing the mixture. The excess fuel, CH 3OH, on the rich premixed flame branch is replaced by more stable fuels CO and H 2 which burn at the diffusion flame. On the lean premixed flame side, a higher concentration of O 2 leaks through to the diffusion flame. The general structure of the triple point features the contribution of both differential diffusion of radicals and heat. A mixture fraction–temperature phase plane description of the triple flame structure is proposed to highlight some interesting features in partially premixed combustion. The effects of differential diffusion at the triple point add to the contribution of hydrodynamic effects in the propagation of the triple flame. Differential diffusion effects are measured using two methods: a direct computation using diffusion velocities and an indirect computation based on the difference between the normalized mixture fractions of C and H. The mixture fraction approach does not clearly identify the effects of differential diffusion, in particular at the curved triple point, because of ambiguities in the contribution of carbon and hydrogen atoms’ carrying species.