Flame Front Area Research Articles

Study on Flame Propagation Characteristics of Natural Gas−Hydrogen−Air Mixtures

Flame propagation characteristics of natural gas−hydrogen−air mixtures were studied in a constant volume combustion bomb at room temperature and low initial pressures. The laminar burning velocities, the mass burning fluxes, and the Markstein number were obtained at various hydrogen fractions and equivalence ratios. The results show that the unstretched laminar burning velocity and the mass burning flux increase with the increase of hydrogen fraction in a natural gas−hydrogen blend. Flame stability tends to increase with the increase of equivalence ratio. For a specific equivalence ratio, the Markstein number decreases with the increase of hydrogen fraction, leading to an increase of thermodiffusively generated flame instability. Flaw and protruding or even honeycomb-like structure occur at the flame front area under lean mixture combustion while rich mixture combustion maintains a smooth flame front surface. Lean mixture increases flame instability while rich mixture increases flame stability. At a large hydrogen fraction (larger than 80%), ρu/ρb is increased and flame thickness is decreased with the increase of hydrogen fraction, leading to the increase of flame instability, and easily forms a cellular surface at flame front.

Theoretical investigation of flame propagation process in an SI engine running on gasoline–ethanol blends

Turbulent flame propagation process in a spark-ignition (SI) engine is theoretically investigated. Fueling with gasoline, ethanol and different gasoline–ethanol blends is considered. A quasi-dimensional SI engine cycle model previously developed by the author is used to predict the thermodynamic state of the cylinder charge during the cycle. The flame is assumed to be spherical in shape and centered at the spark plug. Computations are carried out for an automobile SI engine having a disc-shaped combustion chamber, for which the compression ratio and the nominal speed are 9.2 and 5800 rpm, respectively. Geometrical features (flame radius, flame front area and enflamed volume) of the flame, combustion characteristics (mass fraction burned and burn duration), and cylinder pressure and temperature are predicted as a function of the crank angle. Three different positions of the crank angle are studied: −10°, TC and +10°. It was concluded that ethanol addition to gasoline up to 25 vol% accelerated the flame propagation process.

CELLULAR STRUCTURE OF EXPLOSION FLAMES: MODELING AND LARGE-EDDY SIMULATION

Simulations of deflagration propagation through initially quiescent stoichiometric hydrogen/air mixture inside a large-scale 2.3-m-diameter spherical vessel are performed. A large-eddy simulation (LES) model of premixed combustion is suggested, which is based on a subgrid-scale turbulence model of renormalization group theory, a gradient method for combustion modeling and the dependence of burning velocity on transient pressure and temperature. An unstructured grid with one level of refinement/de-refinement around the flame front area is used. Wrinkled by hydrodynamic instability, the flame front structure has been resolved for the first time in LES of large-scale explosions. The growth of cell sizes with flame radius is reproduced numerically in agreement with theoretical and experimental results. The minimum resolved size of the flame cells is of the order of the simulated flame front thickness, which is about three control volume edges. The fractal dimension of the wrinkled flame front surface grows and reaches a value of 2.15, close to observed in experiments.

Calculation and Consideration of the Lewis Number for Explosion Studies

When a laminar flame front becomes cellular, the flame front area increases and the rate at which the unburnt gases are consumed is increased. If this is not taken into account when modelling vented or non-vented explosions, the pressures that vessels containing explosions are subject to can be greatly under-predicted. For most explosion models, hydrodynamic instabilities are considered as the only driving force for the formation of cellular flames. This paper shows that the transition to cellular flame propagation can occur earlier due to thermo-diffusive instabilities. To predict when thermo-diffusive instabilities should be considered as the main driving force for cellular flame transition, the Lewis number of the reactant mixture should be compared to a critical value. Calculations of the Lewis number for a range of different fuel/air mixtures that are in common use are presented. The fuels studied are methane, ethane, propane and n-butane. For each of these fuel/air mixtures the critical Lewis number was evaluated using an expression from asymptotic analyses. It is shown that the critical Lewis number based on this type of analysis is under-predicted. This is because the flame thickness is not included. It is proposed that a modified form of the critical Lewis number be used which is greater by an order of 1/Zeldovich number.

Linear stability analysis of laminar premixed spray flames

We present an analysis of instability to transverse perturbations of laminar premixed plane spray flames. To this end, a thermal‐diffusional model of a flame front propagating through an overall fuel‐rich or fuel‐lean droplet‐vapour‐air mixture is presented. The fuel droplets are permitted to vaporize at a finite rate so that their interaction with, and possible traversal of, the flame front is accounted for. After establishing steady‐state solutions by means of high activation energy asymptotics, a detailed linear stability analysis is carried out in order to determine neutral stability boundaries. For the fuel‐rich case, it is demonstrated that under certain circumstances a spray flame may be cellular even though its equivalent non‐spray flame is completely stable. In addition, even when the non‐spray flame is itself cellular, the equivalent spray flame will have a finer cellular structure. To our knowledge, these results are the first theoretical qualitative verification of sparse but compelling experimental evidence from the literature. The main effect of the spray on the stability of these flames is due to heat loss from the absorption of heat by the droplets for vaporization. The influence of the initial liquid fuel loading, the latent heat of vaporization and the vaporization coefficient on the critical wavenumber associated with cellularity provides strong evidence of the major role of the heat‐loss mechanism in these sprayrelated phenomena. For fuel‐lean spray flames, it is found that the heat‐loss mechanism manifests itself prominently via the pulsating stability boundary which penetrates into the region of realistic Lewis numbers, thus verifying recent experimental observations of pulsating cellular spray flames. Finally, the cellularity of the spray flames, with their attendant increase in flame front area, suggests a plausible rationale for those circumstances in which burning velocity enhancement, induced by the use of a spray of droplets, was observed experimentally.

Fractal modelling of turbulent combustion

In a previous paper we proposed a new model for turbulent flows, called the fractal model (FM), which is applicable both to RANS and LES formulations. Here, the model is extended to the reactive case with the goal of simulating turbulent flames, both premixed and non-premixed.FM is a subgrid model that describes the physics of the small scales of turbulence building on the phenomenological concept of vortex cascade and on fractal theory. The physics of the small scales is summarized by a turbulent ‘viscosity’ μt, to be added to the molecular one. μt is zero where the flow is laminar and, in particular, goes to zero at solid walls.The fundamental assumption in treating combustion in this work is that chemical reactions take place only at the dissipative scales of turbulence, i.e. near the so-called ‘fine structures’ (the eddy dissipation concept). FM predicts the growth of dissipative scales due to heat release; therefore, it enables a local DNS in the hot regions of the flow where the dissipative scale may grow up to the cell dimension. FM can also estimate the volume fraction γ* occupied by the ‘fine structures’; this quantity is critical for modelling the reaction rate, and therefore the source terms in the energy and species equations. FM can also estimate the local surface of the reactive ‘fine structures’, that is, the local flame front area. It also takes into account, although in approximate manner, the formation of islands of unburnt mixture. In this paper, the model (in the isotropic formulation (IFM)) is used in conjunction with a time-dependent LES (but with the limitations of an isotropic model) approach and is validated through a three-dimensional axisymmetric diffusion flame studied experimentally by Correa and Gulati and numerically by many researchers. The time-dependent results obtained are in good agreement with the experiments. Moreover, the IFM solution offers a possible explanation for the stabilization process of this flame, which undergoes local stretching of the order of 46 000 s−1.

The effect of the stroke-to-bore ratio on combustion, heat transfer and efficiency of a homogeneous charge spark ignition engine of given displacement

This study investigates how the selection of the stroke-to-bore (S/B) ratio affects combustion, heat transfer and overall efficiency in a homogeneous charge spark ignition (SI) engine of a given displacement. Initially, flame front area maps and wall areas in contact with burned gases are examined from a purely geometric point of view, for S/B ratios of 0.7, 1.0 and 1.3. Subsequently, a quasi-dimensional turbulent flame entrainment model is used to quantify the extent to which turbulence versus geometric factors are responsible for the observed combustion, heat transfer and cycle efficiency behaviour, as the S/B ratio varies. Calculations are performed for a range of engine speeds and loads, as well as for operation with 15 per cent exhaust gas recirculation (EGR). Results show that the S/B ratio has a significant effect on both turbulence levels and the geometric interaction of the flame front with the combustion chamber walls. In general, a longer stroke leads to higher thermal efficiency through faster burning and lower overall chamber heat loss. These effects are non-linear, being more dramatic when the S/B ratio is increased from below unity than from above unity. The potential of the long-stroke engine for brake fuel economy improvement can be exploited to the fullest at low speeds, while friction losses gradually diminish it at higher speeds.

Instability of a flame front propagating through a fuel-rich droplet–vapour–air cloud

A simple model of a flame front propagating through a fuel-rich droplet–vapour–air mixture is presented in which the fuel droplets are assumed to evaporate in a sharp front ahead of the reaction front. By performing a linear stability analysis neutral stability boundaries are determined. It is shown that the presence of the spray of droplets in the fresh mixture can have a profound effect by causing cellularization of the flame front. Specifically, we demonstrate that under certain circumstances a spray flame can be cellular when its equivalent non-spray flame is completely stable. Furthermore, it is shown that even when the non-spray flame is itself cellular the equivalent spray flame will have a finer cellular structure. These theoretical predictions verify qualitatively for the first time independent experimental observations from the literature. It is thus shown that the primary effect of the spray on the stability of these flames is due to heat loss from the absorption of heat by the droplets for vaporization. The influence of the initial liquid fuel loading and the latent heat of vaporization on the critical wavenumber associated with cellularity provide further evidence of the responsibility of the heat loss mechanism for these spray-related phenomena. Finally, the cellularity of the spray flames with their attendant increase in flame front area suggest a plausible rationale for the experimentally observed burning velocity enhancement induced by the use of a spray of fuel droplets.

Turbulent Premixed Flames in Closed Combustion Chambers.

A thick burning zone including unburned and burned gas is observed in turbulent premixed flames. In this study, the burning rate and the ion current in the burning zone were investigated in propane-air turbulent flames in a spark-ignition engine cylinder and a constant-volume combustion chamber. The main results are as follows: (1) in propagating turbulent flame, the rate at which mass is burned is not equal to the rate of mass entrainment to the flame front; (2) the ratio of the combustion rate per unit area of turbulent flame front to that of laminar flame, Z=(MbT/AT)/(MbL/AL), is a reasonable expression of the effect of turbulence on the combustion of propagating flame; (3) the ion-current signal has multiple peaks in turbulent flame. The average number of ion current peaks increases with the enlargement of burning zone and the value of Z.

Turbulent Premixed Flames in Closed Combustion Chambers.

A thick Burning zone including unburned and burned gas is observed in the turbulent premixed flames. In this study, the burning rate and the ion-current in the burning zone were investigated in propane-air turbulent flames in a spark-ignition engine cylinder and a constant volume combustion chamber. The main results are as follows : (1) In propagating turbulent flame, the rate of mass burned is not equal to the rate of mass entrainment to the flame front. (2) The ratio of the combustion rate per unit area of turbulent flame front to that of laminar flame, Z=(MbT/Ar)/(MbL/AL), is a reasonable expression of the effect of turbulence on the combustion of propagating flame. (3) The ion-current signal has multiple peaks in the turbulent flame. The average number of ion-current peaks increases with the enlargement of burning zone and the value of Z.

Dynamics of gas combustion in closed vessels with various laws of flame-surface variation

Model problems regarding the combustion of homogeneous fuel mixtures in closed vessels usually consider the case of a divergent spherical flame. The resultant law of variation of the flame front area is of limited applicability, insofar as the area increases in proportion to the square of the radius. As the law of surface variation is of particular importance to the determination of the basic features of combustion kinetics, it may be chosen in the form of laws realized in the propagation of diverging or converging flames with spherical, cylindrical and plane symmetry. Such flames, more closely related to real processes, aid in studying the influence of flame curvature on combustion rate, and are used in analyses of combustion processes in internal combustion engines, rotating gases and tubes. Various fuel-oxidizer mixtures are examined under the different laws and the results are found to be in good agreement with experimental observations. The spatial velocity of a flame as a function of pressure is the same in all cases, usually decreasing in the course of the process. Under certain conditions there is the possibility of acceleration of the flame as a result of change in the normal velocity of the flame.

The nature of cycle-by-cycle variation in the S.I. Engine from high speed photographs

The paper offers an explanation as to the nature and cause of the cyclic combustion variation in the spark ignition engine. This is done from the analysis of photographs of flame initiation, formation, and propagation in a transparent piston engine and in a steady flow transparent tube apparatus in which the turbulence level was controlled. It was found that the main cause of cyclic variation is the small-scale turbulence and the mixing that is caused by this turbulence. The ignition and the combustion process is divided into three main intervals or phases, in order to explain the influence of turbulence, mixing, and chemical kinetics on the overall cyclic variation. The small- and the medium-scale turbulence influence the mixing of the reacting mixture in the small unstable flame kernel with the reactants that surrounds it, causing variation in the reaction rate and hence variations in the delay period. The larger scale turbulence influences the variation of the flame front area in the flame propagation phase.

Geometry of Spherical Flame Propagation in a Disc-Shaped Combustion Chamber

In performing synthesis or analysis of the combustion process in spark-ignition engines by digital computer, it is necessary to know the enflamed volume, the flame front area, and the area of the chamber surface in contact with the enflamed material, as functions of flame travel distance. This Note examines the case of spherical flame propagation in a disc-shaped chamber, with origin at any point on one of the flat surfaces. An ‘exact’ method of computation is outlined, and a simple approximate procedure, giving high accurary for this particular case, is described.

Flame zone studies IV—microstructure and material transport in a laminar propaneair flame front

Experimental data for a stoichiometric, built1 4 atm, propaneair flame are presented. The data include gas velocity normal to the flame front and stream tube area parallel to the flame front measured by the particle-track technique, and a derived temperature profile. Also included is a set of sampled composition profiles for all stable species present obtained with a quartz microprobe and mass spectrometric analysis. The total flux due to both convection and diffusion has been calculated for each species. The data are examined in terms of conservation of mass.