Effective Lewis Number Research Articles

219303 開放空間におけるガス爆発の爆風威力に及ぼす火炎伝ぱ挙動の影響(OS13 安全に寄与する熱工学1,オーガナイズド・セッション)

To understand the consequence of accidental gas explosions in an open space, the effects of flame propagation behavior on the blast wave were examined experimentally. The spherically propagating flames in the mixtures of methane and hydrogen with air were focused. The results show that the flames at equivalence ratios of 0.7, 1.0, and 1.3 were intensively wrinkled and accelerated with the increases of hydrogen addition in methane/air mixtures. The overpressure of the blast wave tends to increase acceleratingly by the flame wrinkling. The flame winkling is considered to be generated by diffusive-thermal instability according to the analysis of the Effective Lewis number. The overpressure can be predicted by the acoustic theory which indicates that the intensity of blast wave is affected, in particular, by burning velocity, volumetric expansion ratio and the flame acceleration. Especially, the intensity of the blast wave is strongly affected by the acceleration of the burning velocity.

Development of Flame Propagation Model Considering Lewis Number Effect for Rich Fuel Mixture for Gasoline SI Engines

The purpose of this research is to include the phenomenon that occurs because of “Wrinkle (Stretch)” which naturally generates to the flame surface under a rich condition that promotes the burning speed. This happens in a gasoline fuel combustion that is heavier than air. In the present study, the unstable flame was formed due to the imbalance of the mass diffusion and the temperature diffusion of the fuel was modeled. The laminar burning speed model was developed by considering the stretch. The model has shown the capability to reproduce the heat generation (heat release rate) at high accuracy in comparison with experimental data. The simulation was carried out for both homogeneous and the stratification combustion and validated with experimental data. Considering the flame stretch made 3-D combustion simulation tool high accuracy from lean to rich fuel mixture condition.

Extinction of Cylindrical Diffusion Flame

An experimental study was performed on the behavior and extinction characteristics of the cylindrical diffusion flame affected by both factors of stretch and curvature. The cylindrical diffusion flame treated here has the convex curvature toward the air stream. The fuels used were propane and methane, and they were diluted with two kinds of inert gas, nitrogen and helium. The obtained results are described as follows. (1) The burner used in this study can form the cylindrical flame with good circularity. The minimum flame diameter is approximately 2.5mm. (2) The flame radius increases (decreases) with the increase in the fuel (air) flow velocity. (3) Flame luminosity has a maximum value when the air flow velocity is varied. On the other hand, the luminosity decreases monotonously with the increase in the fuel flow velocity. (4) Extinction stretch rate of counterflow propane 20%/nitrogen 80% vs. air flame is lower than that of counterflow methane 50%/nitrogen 50% vs. air flame. However, this extinction stretch rate relation is reversed in the case of cylindrical diffusion flame owing to the Lewis number effect caused by the flame curvature. (5) When the Lewis number of fuel flow is considerably larger than unity, the cylindrical diffusion flame can be formed even at the dilution rate with which the counterflow diffusion flame cannot be formed.

Lewis number effect on the propagation of premixed flames in narrow adiabatic channels: Symmetric and non-symmetric flames and their linear stability analysis

The propagation of premixed flames with different Lewis numbers in a planar channel subject to a Poiseuille flow is considered within the diffusive-thermal model for steady and time-dependent cases. It was found that, depending on the Lewis number and the flow rate, symmetric and non-symmetric flames are possible. The existence of multiple steady solutions in cases of the low Lewis number is demonstrated. The time-dependent simulations carried out for high Lewis number flames also showed the symmetric and non-symmetric oscillatory solutions. Linear stability analysis of two-dimensional steady-states was performed using a practical method developed in the paper and applied to calculate the main eigenvalue. It was shown that for symmetric flames with a low Lewis number the increase in the flow rate leads to a loss of stability with subsequent formation of non-symmetric solutions. For flames with a high Lewis number the Poiseuille flow produces a stabilization effect. The results of the stability analysis were successfully compared with the results of direct numerical simulations.

Nonunitary Lewis Number Effects on the Combustion of a Linear Array of Gaseous Fuel Pockets

The numerical solution for the combustion of an infinite linear array of gaseous fuel pockets in a stagnant oxidizing environment under microgravity conditions is discussed. The gas pocket combustion is described using the generalized Shvab-Zel'dovich formulation with nonunitary Lewis number. The combustion process is considered isobaric and the flow is induced by density gradients due to the heat and mass transfer processes (Stefan flow). The model is based on mass, momentum, excess enthalpy, and mixture fraction conservation equations and considers the Burke-Schumann reaction mechanism and ideal gas behavior. The thermophysical properties, except the density, are assumed constant. The finite-volume method is employed in the numerical solution, using a generalized system of coordinates. A nonstaggered grid is used and the SIMPLEC algorithm is employed to solve the modified pressure-velocity coupling. Nonunitary Lewis number and interaction effects on flame behavior and on the fuel consumption are analyzed. Results show that the nonunitary Lewis number can modify the interaction effects on gas pocket linear array combustion. During the combustion process, the flame can evolve from individual flames around each gas fuel pocket to a merged flame, surrounding the merged fuel region. However, under certain conditions, the merged flame and the merged fuel region can be broken, returning to individual flames around each gas fuel pocket.

Effects of Lewis Number on Scalar Variance Transport in Premixed Flames

The influences of differential diffusion rates of heat and mass on the transport of the variances of Favre fluctuations of reaction progress variable and non-dimensional temperature have been studied using three-dimensional simplified chemistry based Direct Numerical Simulation (DNS) data of statistically planar turbulent premixed flames with global Lewis number ranging from Le = 0.34 to 1.2. The Lewis number effects on the statistical behaviours of the various terms of the transport equations of variances of Favre fluctuations of reaction progress variable and non-dimensional temperature have been analysed in the context of Reynolds Averaged Navier Stokes (RANS) simulations. It has been found that the turbulent fluxes of the progress variable and temperature variances exhibit counter-gradient transport for the flames with Lewis number significantly smaller than unity whereas the extent of this counter-gradient transport is found to decrease with increasing Lewis number. The Lewis number is also shown to have significant influences on the magnitudes of the chemical reaction and scalar dissipation rate contributions to the scalar variance transport. The modelling of the unclosed terms in the scalar variance equations for the non-unity Lewis number flames have been discussed in detail. The performances of the existing models for the unclosed terms are assessed based on a-priori analysis of DNS data. Based on the present analysis, new models for the unclosed terms of the active scalar variance transport equations are proposed, whenever necessary, which are shown to satisfactorily capture the behaviours of unclosed terms for all the flames considered in this study.

Lewis number effects in distributed flames

Recent computational studies have simulated a mode of distributed premixed combustion where turbulent mixing plays a significant role in the transport of mass and heat near the reaction zone. Under these conditions, molecular transport processes play a correspondingly smaller role. A consequence of burning in this regime is that changes in the composition can occur within the flame zone that modify the local burning rate. This effect depends on the Lewis number (ratio of molecular heat to mass diffusivity), and so the transition to distributed burning will be different for fuels with different Lewis numbers. In this paper, we examine the role of Lewis number on flames in the distributed burning regime. We use high-resolution three-dimensional flame simulations with detailed transport models to explore the turbulent combustion of lean premixed hydrogen, methane and propane mixtures. Turbulence–flame interactions are found to be more pronounced in hydrogen than in the other fuels.

Effects of Lewis Number on Scalar Dissipation Transport and Its Modeling in Turbulent Premixed Combustion

The effects of Lewis number (Le) on scalar dissipation rate transport of the reaction progress variable have been studied using 3-dimensional DNS data of freely propagating turbulent premixed flames with global Lewis number ranging from 0.34 to 1.2. It has been found that the effects of dilatation rate become increasingly strong with decreasing Le, and is particularly strong for the Le ≪ 1 flames because of greater rate of heat release. Moreover, increasingly strong flame normal acceleration in flames with decreasing Le gives rise to countergradient transport for the turbulent flux of the scalar dissipation rate, whereas this flux exhibits gradient transport for flames with Le ≥ 1. This turbulent flux is observed to correlate with the behavior of the turbulent scalar flux and this behavior is used to propose an algebraic closure for the turbulent flux of the scalar dissipation rate. The modeling of nonunity Le effects on the unclosed terms arising from the density change across the flame front, the turbulence–scalar interaction, the correlation between scalar gradient and the gradient of reaction rate and the molecular dissipation have been addressed. The extent of reaction progress variable gradient alignment with the most extensive (compressive) principal strain rate is found to increase (decrease) with decreasing Le. This alignment behavior gives rise to a sink contribution arising from the turbulence–scalar interaction in the Le ≪ 1 flames. By contrast, the turbulence–scalar interaction acts predominantly as a source term, although this contribution becomes a sink toward the burned gas side for the Le ≥ 1 flames. It has been demonstrated that existing models for the unclosed terms originating from the density change across the flame front and the turbulence–scalar interaction cannot account for strengthening (weakening) of heat release effects when Le < 1 (Le > 1). Suitable modifications to these models have been proposed to account for nonunity Lewis number effects, and validated using direct numerical simulation data. A simple algebraic expression is suggested that successfully relates the scalar dissipation rate of normalized temperature to the scalar dissipation rate of the reaction progress variable in nonunity Lewis number turbulent flames.

Experimental and computational study of the transition to the flame ball regime at normal gravity

Ultra lean flames propagating in a tube of 12.2mm internal diameter in mixtures of (40mol% H2+60mol% CH4) fuel gas with air have been studied experimentally, using OH Planar Lased Induced Fluorescence (PLIF), and by numerical simulations, for equivalence ratios between 0.30 and 0.50. Hypothetical OH LIF signals were reconstructed from the calculated data and compared with measurements. The simulated OH LIF intensity distributions are found to be similar to the measured ones in the experimental flames for equivalence ratios 0.31⩽φ⩽0.44. Within this range, when the equivalence ratio was decreased down to the lean limit value, the shapes of the simulated and experimental flame fronts changed from relatively large egg-shaped to small, about 3mm diameter, nearly spherical flame balls. Simulations showed the presence of a recirculating flow in near-limit flames. The front of the lean limit flame balls resides entirely within the volume of this recirculating flow. The flame front was nearly uniform in lean limit flame balls and became increasingly non-uniform with the departure from the limit, with an eventually vanishing bottom front. The experimental and simulated flammability limits lie far below the theoretical limit for a planar flame, and simulated flame temperatures exceed adiabatic temperatures, indicating strong preferential diffusion (Lewis number) effects in these flames. These effects are qualitatively analyzed based on the behavior of measured and simulated flame parameters. It is suggested that preferential diffusion in non-uniform flames, well-removed from the lean flammability limit, is primarily controlled by flame stretch, while in near-uniform limit flame balls it is dominated by a curvature effect similar to that in microgravity flame balls.

On the critical flame radius and minimum ignition energy for spherical flame initiation

Spherical flame initiation from an ignition kernel is studied theoretically and numerically using different fuel/oxygen/helium/argon mixtures (fuel: hydrogen, methane, and propane). The emphasis is placed on investigating the critical flame radius controlling spherical flame initiation and its correlation with the minimum ignition energy. It is found that the critical flame radius is different from the flame thickness and the flame ball radius and that their relationship depends strongly on the Lewis number. Three different flame regimes in terms of the Lewis number are observed and a new criterion for the critical flame radius is introduced. For mixtures with Lewis number larger than a critical Lewis number above unity, the critical flame radius is smaller than the flame ball radius but larger than the flame thickness. As a result, the minimum ignition energy can be substantially over-predicted (under-predicted) based on the flame ball radius (the flame thickness). The results also show that the minimum ignition energy for successful spherical flame initiation is proportional to the cube of the critical flame radius. Furthermore, preferential diffusion of heat and mass (i.e. the Lewis number effect) is found to play an important role in both spherical flame initiation and flame kernel evolution after ignition. It is shown that the critical flame radius and the minimum ignition energy increase significantly with the Lewis number. Therefore, for transportation fuels with large Lewis numbers, blending of small molecule fuels or thermal and catalytic cracking will significantly reduce the minimum ignition energy.

Inclusion of Preferential Diffusion in Simulations of Premixed Combustion of Hydrogen/Methane Mixtures with Flamelet Generated Manifolds

In this paper we study the possibility to account for preferential diffusion effects in lean turbulent premixed flames in numerical predictions with reduced chemistry. We studied the situation when hydrogen is added to methane at levels of 20% and 40% by volume in the fuel, at lean combustion (ϕ = 0.7) with air. The base case of pure methane was used as a reference. In this case preferential diffusion effects are negligible. First the sensitivity of the mass burning rate to flame stretch was investigated, in one dimensional computations with detailed chemistry, to set reference values. Then the framework of the Flamelet Generated Manifolds (FGM) was used to construct an adequate chemical method to take preferential diffusion into account, without the need for using detailed chemistry. To that end a generalization of the method was presented in which five controlling variables are required. For this system, proper transport equations and effective Lewis numbers where derived. In practice not all five variables are necessary to include and as a first step we limited the amount in the numerical tests in this study to two controlling variables. The method was then tested in configurations in which there was an interaction of coherent vortices and turbulence with flames. It was demonstrated that a minimum of two controlling variables is needed to account for the changed mass burning rate as function of stretch and curvature. It was shown that one-dimensional FGM as well as one-step Arrhenius kinetics can not describe this relation.

Investigation over the recirculation influence on the combustion of micro organic dust particles

This paper investigates the role of recirculation and non-unity Lewis number on the combustion of organic dust particles. Since recirculation effect is more noticeable in micro-combustors, it is necessary to propose a modeling approach of this phenomenon to better simulate the performance of micro-combustors. In this research, in order to model the combustion of organic dust particles, it is assumed that the dust particles vaporize first to yield a known chemical structure which is oxidized in the gas phase, and the chemical structure of this gaseous fuel is assumed methane. To study the flame structure and solve the governing equations, it is considered that the flame structure consists of three zones titled the preheat-vaporization zone, the narrow reaction zone and finally the post flame zone. The recirculation phenomenon is evaluated by entering the exhausted heat from the post flame zone into the preheat zone. The solution is based on the following approach. First, the governing equations in each zone are nondimensionalized. Then the needed boundary and matching conditions are applied in each zone. After that, these equations and the required boundary and matching conditions are simultaneously solved with the analytical model. Consequently, the remarkable effects of recirculation and non-unity Lewis number on the combustion characteristics of the organic dust particles such as burning velocity and temperature profiles for different particle radii are obtained. The results show reasonable agreement with published experimental data.

Numerical simulation of double-diffusive mixed convective flow in rectangular enclosure with insulated moving lid

The present study is concerned with the mixed convection in a rectangular lid-driven cavity under the combined buoyancy effects of thermal and mass diffusion. Double-diffusive convective flow in a rectangular enclosure with moving upper surface is studied numerically. Both upper and lower surfaces are being insulated and impermeable. Constant different temperatures and concentration are imposed along the vertical walls of the enclosure, steady state laminar regime is considered. The transport equations for continuity, momentum, energy and spices transfer are solved. The numerical results are reported for the effect of Richardson number, Lewis number, and buoyancy ratio on the iso-contours of stream line, temperature, and concentration. In addition, the predicted results for both local and average Nusselt and Sherwood numbers are presented and discussed for various parametric conditions. This study was done for 0.1 ≤ Le ≤ 50 and Prandtl number Pr = 0.7. Through out the study the Grashof number and aspect ratio are kept constant at 10 4 and 2 respectively and −10 ≤ N ≤ 10, while Richardson number has been varied from 0.01 to 10 to simulate forced convection dominated flow, mixed convection and natural convection dominated flow.

Extinction of premixed methane/air flames in microgravity by diluents: Effects of radiation and Lewis number

Laminar burning velocities and flammability limits of premixed methane/air flames in the presence of various diluents were investigated by combined use of experiments and numerical simulations. The experiments used a 1-m free-fall spherical combustion chamber to eliminate the effect of buoyancy, enabling accurate measurements of near-limit burning velocities and flammability limits. Burning velocities were measured for CH 4/air flames with varying concentrations of He, Ar, N 2 and CO 2 at NTP. The limiting concentration of each diluent was measured by systematically varying the composition and ignition energy and finding the limiting condition through successive experiment trials. The corresponding freely-propagating, planar 1-D flames were simulated using PREMIX. The transient spherically-expanding flames were simulated using the 1-D Spherical Flame & Reactor Module of COSILAB considering detailed radiation models. The results show that helium exhibits more complex limit behavior than the other diluents due to the large Lewis number of helium mixtures. The near-limit helium-diluted flames require much higher ignition energy than the other flames. In addition, for the spherically expanding helium-diluted flames studied here ( Le > 1), stretch suppresses flame propagation and may cause flame extinction. For the CO 2-diluted flames, the flame speed predicted by the optically-thick model based on the Discrete Transfer Method (DTW) and a modified wide band model has better agreement with measurements in the near-limit region. A significant amount of heat is absorbed by the dilution gas CO 2, resulting in elevation of temperature of the ambient gases. The optically-thick model, however, still overpredicts flame speed, indicating a more sophisticated radiation property model may be needed. Finally, the chemical effect of CO 2 on flame suppression was quantified by a numerical analysis. The results show that the chemical effect of CO 2 is more important than the other diluents due to its active participation in the reaction CO 2 + H = CO + OH, which competes for H radicals with the chain-branching reactions and thus reduces flame speed.

Coupled heat and mass transfer in the entrance region of a circular tube with fully-developed parabolic flow and external convective heating

Coupled heat and mass transfer in the entrance region of a circular tube with fully-developed parabolic flow and external heating is analyzed. The closed form solutions of temperature and concentration distributions, local Nusselt numbers based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Biot and Lewis number on heat and mass transfer were investigated.

Numerical study on combustion of diluted methanol-air premixed mixtures

The effect of nitrogen dilution on the premixed combustion characteristics and flame structure of laminar premixed methanol-air-nitrogen mixtures are analyzed numerically based on an extended methanol oxidation mechanism. The laminar burning velocities, the mass burning fluxes, the adiabatic flame temperature, the global activation temperature, the Zeldovich number, the effective Lewis number and the laminar flame structure of the methanol-air-nitrogen mixtures are obtained under different nitrogen dilution ratios. Comparison between experiments and numerical simulations show that the extended methanol oxidation mechanism can well reproduce the laminar burning velocities for lean and near stoichiometric methanol-air-nitrogen mixtures. The laminar burning velocities and the mass burning fluxes decrease with the increase of nitrogen dilution ratio and the effect is more obvious for the lean mixture. The effective Lewis number of the mixture increases with the increase of nitrogen dilution ratio, and the diffusive-thermal instability of the flame front is decreased by the nitrogen addition. Nitrogen addition can suppress the hydrodynamic instability of methanol-air-nitrogen flames. The decrease of the mole fraction of OH and H is mainly responsible for the suppressed effect of nitrogen diluent on the chemical reaction in the methanol-air-nitrogen laminar premixed flames, and the NOx and formaldehyde emissions are decreased by the nitrogen addition. methanol, dilution, premixed combustion, numerical analysis With increasing concern about fuel shortage and air pollution control, researches on improving engine fuel economy and reducing exhaust emissions have become the major topics in combustion and engine studies. Because of limited reserves of crude oil, the development of alternative-fuel engines has attracted more attention in the engine community. Alternative fuels usually are clean fuels compared with traditional diesel fuel and gasoline fuel in the engine combustion process. The introduction of these alternative fuels is beneficial to the solution of the fuel shortage and reduction of engine exhaust emissions. Nowadays, clean alternative fuels mainly include hydrogen, compressed natural gas (CNG), liquid petroleum gas (LPG), dimethyl ether (DME), and alcohols in internal combustion engines application

Effects of diluents on cellular instabilities in outwardly propagating spherical syngas–air premixed flames

Experiments were conducted in a constant pressure combustion chamber using schlieren system to investigate the effects of carbon dioxide–nitrogen–helium diluents on cellular instabilities of syngas–air premixed flames at room temperature and elevated pressures. The cellular instabilities for the diluted syngas–air flames were interpreted and evaluated in the viewpoint of the hydrodynamic and diffusional-thermal instabilities. Laminar burning velocities and Markstein lengths were calculated by analyzing high-speed schlieren images at various diluent concentrations and equivalence ratios. The measured unstretched laminar burning velocities were compared with the predicted results computed using the PREMIX code with the kinetic mechanism developed by Sun et al. Also, experimentally measured Peclet numbers were compared with the predicted results for fuel–lean flames. Experimental results showed substantial reduction of the laminar burning velocities and of the Markstein lengths with the diluent additions in the fuel blends. Effective Lewis numbers of helium-diluted syngas–air flames increased but those of carbon dioxide- and nitrogen-diluted syngas–air flames decreased in increase of diluents in the reactant mixtures. With helium diluent, the propensity for cells formation was significantly diminished, whereas the cellular instabilities for carbon dioxide- and nitrogen-diluted syngas–air flames were not suppressed.

DISTRIBUTED FLAMES IN TYPE Ia SUPERNOVAE

In the distributed burning regime, turbulence disrupts the internal structure of the flame, and so the idea of laminar burning propagated by conduction is no longer valid. The nature of the burning depends on the turbulent Damkohler number (Da), which steadily declines from much greater than one to less that one as the density decreases to a few 10^6 g/cc. Scaling arguments predict that the turbulent flame speed s, normalized by the turbulent intensity u, follows s/u=Da^1/2 for Da<1. The flame in this regime is a single turbulently-broadened structure that moves at a steady speed, and has a width larger than the integral scale of the turbulence. The scaling is predicted to break down at Da=1, and the flame burns as a turbulently-broadened effective unity Lewis number flame. We refer to this kind of flame as a lambda-flame. The burning becomes a collection of lambda-flames spread over a region approximately the size of the integral scale. While the total burning rate continues to have a well-defined average, s_{T} ~ u, the burning is unsteady. We present a theoretical framework, supported by both 1D and 3D numerical simulations, for the burning in these two regimes. Our results indicate that the average value of s can actually be roughly twice u for Da>1, and that localized excursions to as much as five times u can occur. The lambda-flame speed and width can be predicted based on the turbulence in the star and the turbulent nuclear burning time scale of the fuel. We propose a practical method for measuring these based on the scaling relations and small-scale computationally-inexpensive simulations. This suggests that a simple turbulent flame model can be easily constructed suitable for large-scale distributed supernovae flames.

Studies on the Outwardly and Inwardly Propagating Spherical Flames with Radiative Loss

Outwardly and inwardly propagating spherical flames (OPF and IPF) with radiative loss are studied analytically and numerically. Emphasis is placed on investigating the effects of radiation on flame propagating speed, Markstein length, and flame extinction, as well as on examining whether the reactant can be completely consumed via an IPF. A general correlation between flame propagating speed and flame radius for OPF and IPF of large flame radii is derived and utilized to study the effects of radiative loss and Lewis number on flame propagation and extinction. A correlation for Markstein length at different Lewis numbers and radiative loss is also presented. It is shown that the Markstein length is strongly affected by radiative loss as well as Lewis number, and that only for mixtures not close to their flammability limits and without CO2 dilution is the effect of radiation on the Markstein length measured from expanding spherical flames negligible. Furthermore, the theoretical results are validated by numerical simulations. It is found that when radiative loss is considered, there exists unconsumed reactant after the extinction of IPF for mixture with Lewis number less than unity.

J0801-3-5 DME・水素・メタン混合燃料火炎の構造と局所燃焼速度([J0801-3]エンジン,動力システムにおける燃焼技術(3))

In this study, combustion mechanisms and effects of flame stretch on the local burning velocity of two-component fuel mixtures of DME, hydrogen and methane were investigated by chemical kinetics computations. Lean and rich mixtures having nearly the same laminar burning velocity were prepared by adding nitrogen to mixtures where the fuel mixing rate δ is varied from 0 to 1 at equivalence ratios of 0.4-2.0. As a result, it is elucidated that Markstein numbers of lean two-component fuel mixtures changed monotonically with increasing 8 depending on the effective Lewis number, however, those of rich flames showed a complicated trend with change in the equivalence ratio and δ. These characteristics of response to flame stretch were discussed in light of the chemical reaction mechanisms.