Shape Of Flame Front Research Articles

Application of modified eddy dissipation concept with large eddy simulation for numerical investigation of internal combustion engines

Computational methodology of large eddy simulation (LES) is tested in a real internal combustion engine geometry. Premixed turbulent combustion and the behavior of flame propagation are studied and compared with different LES and combustion process models. KIVA-3V, an open source Fortran code, is modified to implement large-eddy simulation technique based on one-equation sub-grid scale (SGS) turbulent kinetic energy and Smagorinsky models. For combustion process, improved turbulent-mixing combustion models based on eddy-dissipation model (EDM) and modified eddy-dissipation concept (EDC) are implemented and studied in case of using LES. The models with modified combustion processes of EDM and EDC are validated using experimental data of premixed combustion of laboratory burner with application of converging optimum and desired combustion model constants. Also experimental in-cylinder pressure data are used to validate the numerical CFD results of engine geometry. The physics of flow structure, flame propagation, heat energy released and fuel mass consumption during combustion are studied and compared by LES models. It is observed that large-scale of unsteady turbulent flow motions are developed before spark ignition that makes different behavior of flame front shape in combustion process with different turbulent models. Also numerical simulations indicate that different LES turbulence models may not capture the same flame propagation structure, whereas LES dynamic SGS viscosity models in comparison with standard constant models, almost capture the same flame front shape and propagation structure in the combustion chamber.

Vaporization Damköhler Number and Enrichment Effects in Spray Diffusion Flames in an Oscillating Flow Field

The sensitivity to the vaporization Damköhler number of the behavior of a coflow laminar spray diffusion flame in an oscillating flow field is investigated. Droplet grouping induced by the host gas flow oscillations is accounted for, and its effect is described through a specially constructed model for the vaporization Damköhler number that responds to the proximity of the droplets as they cluster. A formal analytical solution is developed, and the dynamics of the spray flame front shapes and thermal fields are deduced. Computed results demonstrate how strongly the vaporization Damköhler number impacts on the type of primary homogeneous flame formed and on the possible existence of multiple flame sheets as the flow field oscillates. In addition, isolated regions of high fuel vapor concentrations are produced by fuel droplet enrichment. The presence of resulting parallel fluctuating thermal fields indicates a potential impact on undesirable pollutants production.

Variations of Turbulent Burning Velocity and Flame Front Shape of Spherically Propagating Premixed Turbulent Flame with Effective Turbulence Intensity

Variations of Turbulent Burning Velocity and Flame Front Shape of Spherically Propagating Premixed Turbulent Flame with Effective Turbulence Intensity

1010 乱れが球状伝播予混合火炎の火炎面形状に及ぼす影響(燃焼(火炎))

1010 乱れが球状伝播予混合火炎の火炎面形状に及ぼす影響(燃焼(火炎))

Spray flame dynamics with oscillating flow and droplet grouping

The co-flow laminar spray diffusion flame in an oscillating flow field is investigated. Mild slip is permitted between the droplets and their host surroundings and droplet grouping resulting from the host flow oscillations is accounted for. The spray is modelled using the sectional approach and a perturbation analysis using a small sectional Stokes number is utilised for solving the liquid phase governing equations. The effect of droplet grouping is described through a specially constructed model for the vaporisation Damkohler number. The large chemical Damkohler number assumption is adopted and a formal analytical solution is developed for Schwab-Zeldovitch parameters through which the dynamics of the spray flame front shapes and thermal fields are deduced. Computed results based on the solutions demonstrate how the phenomenon of droplet grouping can lead to the existence of multiple flame sheets as a result of the dynamic change in the type of the main homogeneous flame from under- to over-ventilated as the flow field oscillates. Concomitant fluctuating thermal fields are also shown to be present indicating a potential impact on undesirable pollutants production.

The influence of droplet grouping on a Burke-Schumann spray diffusion flame in an oscillating flow field

A new mathematical analysis of a laminar Burke-Schumann type of spray diffusion flame in an oscillating flow field is presented within a framework in which mild slip is permitted between the droplets and their host surroundings. A perturbation analysis using a small Stokes number is used for solving the liquid phase governing equations. The effect of droplet grouping in the oscillatory flow field is accounted for by constructing an appropriate model for the vaporization Damkohler number. A formal analytical solution is developed for the Schwab-Zeldovitch parameter through which instantaneous flame front shapes are found. Computed results based on the solution expose the strong impact that the phenomenon of droplet grouping can have on flame characteristics such as flame height, shape and type (over- or under-ventilated). Despite the models simplicity its predictions offer an opening insight into the mechanisms prevalent in more complex spray-combustion situations in which droplet grouping may occur.

Heat release and flame structure measurements of self-excited acoustically-driven premixed methane flames

An open–open organ pipe burner (Rijke tube) with a bluff-body ring was used to create a self-excited, acoustically-driven, premixed methane–air conical flame, with equivalence ratios ranging from 0.85 to 1.05. The feed tube velocities corresponded to Re = 1780–4450. Coupled oscillations in pressure, velocity, and heat release from the flame are naturally encouraged at resonant frequencies in the Rijke tube combustor. This coupling creates sustainable self-exited oscillations in flame front area and shape. The period of the oscillations occur at the resonant frequency of the combustion chamber when the flame is placed ∼¼ of the distance from the bottom of the tube. In this investigation, the shape of these acoustically-driven flames is measured by employing both OH planar laser-induced fluorescence (PLIF) and chemiluminescence imaging and the images are correlated to simultaneously measured pressure in the combustor. Past research on acoustically perturbed flames has focused on qualitative flame area and heat release relationships under imposed velocity perturbations at imposed frequencies. This study reports quantitative empirical fits with respect to pressure or phase angle in a self-generated pressure oscillation. The OH-PLIF images were single temporal shots and the chemiluminescence images were phase averaged on chip, such that 15 exposures were used to create one image. Thus, both measurements were time resolved during the flame oscillation. Phase-resolved area and heat release variations throughout the pressure oscillation were computed. A relation between flame area and the phase angle before the pressure maximum was derived for all flames in order to quantitatively show that the Rayleigh criterion was satisfied in the combustor. Qualitative trends in oscillating flame area were found with respect to feed tube flow rates. A logarithmic relation was found between the RMS pressure and both the normalized average area and heat release rate for all flames.

Experimental study of flame microstructure and propagation behavior of mine-gas explosion

The high speed cameral and schlieren images methods were used to record the photograph of flame propagation process. Meanwhile, the ionization current probes were set up to detect the reaction intensity of the reaction zone. The characteristics of methane/air flame propagation and microstructure were analyzed in detail by the experimental results coupled with chemical reaction thermodynamics. The high speed schlieren image showed the transition from laminar flame to turbulence combustion. The ion current curves disclosed the reaction intensity and combustion characteristic of flame front. In the test, the particular tulip flame was formed clearly, which was induced to some extent by turbulent combustion. Based on the schlieren images and iron current result, it can be drawn that the small scale turbulence combustion also appears in laminar flame, which thickens the flame front, but makes little influence on the flame front shape. During the laminar-turbulent transition, the explosion pressure plays an important role on the flame structure change.

The effect of combustion chamber geometry layout on combustion and emission

In this paper some results concerning the combined effect of the tumble flow and combustion chamber geometry layout variations on flame front shape and its propagation through homogenous mixture of isooctane and air are presented. Spatial distributions of NO in different combustion chamber geometries are presented as well. The basic combustion chamber geometry layout considered consists of the flat head with two vertical valves and a cylindrical bowl subjected to variations of depth and squish area. All results presented were obtained by dint of multidimensional modeling of reactive flows in arbitrary geometry with moving objects and boundaries with modified KIVA3 and KIVA3V source codes. Two additional computer codes were applied to generate boundary conditions for KIVA3V calculations with moving valves. The AVL TYCON code was used for the calculation of valve lift profiles, and AVL BOOST code was used for the calculation of relevant data set in the valve regions. Different combustion chamber geometry layouts generate different levels of squish, and the combustion effects in essence depend on the interaction of that flow with tumble. It was found that for particular combustion chamber shapes with different diameter/depth aspect ratios entirely different flame front shapes and propagation velocities were encountered primarily due to variations of fluid flow patterns in the vicinity of top dead center.

Flame front detection and characterization using conditioned particle image velocimetry (CPIV)

We investigate the ability of the conditioned particle image velocimetry technique (CPIV) to derive the actual flame front position in turbulent premixed flames. In CPIV, the flame front shape is deduced from the step in the particle number density in PIV images caused by the steep temperature increase in the reaction zone of premixed flames. In a validation experiment the true flame front position is deduced for comparison from simultaneous heat release measurements using planar LIF measurements of OH and CH(2)O. It is found that CPIV yields nearly the same spatial position as the heat release measurements or the steepest slope in the OH distribution. Furthermore, statistical quantities, derived from the extracted flame front shape, like the spatially resolved turbulent flux, the flame surface density and the flame front curvature are compared, showing negligible differences between the applied methods.

Experimental investigation into the effect of reformer gas addition on flame speed and flame front propagation in premixed, homogeneous charge gasoline engines

The effect of reformer gas addition to gasoline in internal combustion engines is assessed based on in-cylinder measurement techniques. These include ion sensors, an optical spark plug and heat release analysis from the cylinder pressure. A detailed analysis of these measurements is presented, giving insight into the combustion process and into the energy release. The flame front shape and propagation in the combustion chamber are reconstructed and the flame speed is estimated. The laminar flame speed has been observed to increase linearly with the energy fraction of reformer gas in the fuel blend. From pure gasoline to pure reformer gas the laminar flame speed increases by a factor of 4.4. The relative increase in the turbulent flame speed is lower. These results confirm what can be observed from the heat release analysis, that reformer gas addition mainly shortens the first phase of the combustion process. Different reformer gas compositions were tested, varying the ratio of hydrogen to inert species. Finally, flame propagation and flame speed at EGR-burn limit and at lean-burn limit are investigated.

917 衝撃波の干渉を受けた火炎の挙動に関する研究(2)(OS9-4 高速流れと衝撃波現象の解明と制御,OS9高速流れと衝撃波現象の解明と制御)

When a flame front is interacted with a shock wave, a shape of flame front is changed largely and a complicated pressure wave is formed accompanied with a generation of high-pressure and high-temperature regions. Moreover, a turbulent combustion arises and the flame is known to transit the detonation wave, which propagate supersonic velocity. Therefore, a mechanism of the interaction between a shock wave and a flame is significant from a safety-engineering point of view. In this study, an interaction behavior of a flame interacted with a shock wave was investigated by using schlieren method with the aid of high-speed video camera and image converter camera. Furthermore, numerical simulation is conducted in order to investigate the temperature and pressure distributions around the flame. Experiments are conducted by using a shock tube of square cross section of 100 mm. The flame produced by an ignition of methane-air mixture of 30 kPa is interacted with a planar shock wave of propagation Mach number 1.54. The flame front is largely deformed and unburned gas is observed to penetrate into the flame and Richtmyer-Meshkov instabilities are caused. In numerical, similar behavior with experimental are simulated.

The nature of early flame development in a lean-burn stratified-charge spark-ignition engine

The operating range of lean-burn SI engines is limited by the level of cycle-by-cycle variability in the early flame development, which typically corresponds to the 0–5% mass fraction burned. An experimental investigation was undertaken to study this flame variability in an optical, stratified-charge, SI engine close to the lean limit of stable operation (A/F=22). Double-exposed flame images acquired through either a pentroof window (“tumble plane” of view) or the piston crown (“swirl plane” of view) were processed to calculate the intra-cycle flame growth and convection rates under 1500 RPM low-load conditions. Projected flame-boundary analysis was also performed to investigate the effect of flame shape/wrinkling on the subsequent timing of 5% mass fraction burned on a cycle-by-cycle basis. The images showed that the flame always preserved its shape while growing in size (even when it had been initiated with a highly convoluted shape); image processing demonstrated the manner with which the flame-growth speed varied as the flame propagated and approached the pentroof and piston-crown walls for slow, “typical” or fast burning cycles. It was found that it was beneficial to have a high convection velocity in the swirl plane of flow during the first 10° CA after ignition timing (corresponding to less than 0.1% mass fraction burned), but after this stage it was beneficial to have a moderate convection velocity for the flame. However, on the tumble plane of flow, a high convection velocity was preferable up to 30° CA after ignition timing (corresponding, typically, to 1% mass fraction burned). Slow development of a flame was associated with higher stretch rates for the same flame radius than fast-developing flames during the period of growth from 3 to 6 mm in radius (about 0.1–1% mass fraction burned). Extended analysis of the projected flame front's shape and its wrinkling showed that the fastest lean-condition flames had contour characteristics similar to those of the flames recorded for stoichiometric conditions. This suggested that the fastest lean flames on a cycle-by-cycle basis might have been richer than the average in the vicinity of the spark plug at ignition.

Isolated and sinergic effects of organized flows on combustion in s.i. engines

This paper presents some results concerning the interaction between squish and tumble and its effect on flame front shape and its displacement through the combustion chamber geometry of the s.i. engine consisting of o flat head with two vertical valves and the cylindrical bowl. These results obtained by the KIVA 3V code were compared with the results of the flame front shape and its displacement obtained for the case with no valves i.e. without tumble. In order to alleviate the application of the KIVA 3Vcode and to enhance its flexibility, two additional computer codes were applied as well, i.e. the AVL TYCON code intended for cam design calculations and calculations of the dynamic behavior of timing drives and gear transmission units and the AVL BOOST code intended for engine cycle calculations, including 1.5D fluid flow calculations through pipelines. The first code was used for the calculation of the valve lift curve while the other was used for the calculation of the relevant data set in valve regions. It was found that for particular combustion chamber shapes considered the entirely different flame front shapes and propagation velocities were encountered for these two cases ensuing primarily from the entirely different fluid flow patterns in the vicinity of TDC.

519 燃料噴霧の突入による火炎面形状の変化の観察(実験)(GS-6 燃焼(2))

519 燃料噴霧の突入による火炎面形状の変化の観察(実験)(GS-6 燃焼(2))

The Structure of Premixed Flames in a Spatially Evolving Turbulent Flow

Large eddy simulations of spatially propagating premixed flames in a three-dimensional dump combustor have been conducted at relatively high Reynolds numbers. A thin flame model is used to simulate the propagating flame front and the propagation speed is estimated using the subgrid turbulence intensity obtained from a one-equation subgrid turbulence model. Analysis of the simulations show that various statistics, such as flame front shape factor, stretch effects, vorticity/strain rate, and strain rate/surface normal alignments, agree reasonably well with data from constant density, direct numerical simulations of passive scalars and premixed flame propagation in simpler, temporally evolving low Reynolds number flows. This suggests that the general characteristics of propagating scalars in turbulent flows are relatively independent of Reynolds number and further that these features can be captured using relatively coarse grid LES at high Reynolds numbers. However, detailed analysis showed that in spatially developing flows, there is a significant dependence on the spatial location for some statistical properties. As the flame propagates downstream the probability density of the shape factor showed a decreasing probability for cylindrical shaped flames and the strain rate/flame normal alignment showed a transition from the alignment seen in shear driven flow to alignment seen in isotropic turbulence. Strain rate (in the plane of the flame) was maximum near the flame holder (the rearward facing step) and decreases in the downstream direction with the pdf becoming more symmetric similar to isotropic flows. The implications of the observed spatial dependence for investigating flame stretch effects are discussed.

Flame propagation along 90° bend in an open duct

Experiments of flame propagation in a small, open square duct including a 90° bend were performed for a methane-air mixture using high-speed cine camera and schlieren techniques. In a slow flame propagation from open end to closed end, a blunt flame front shape is kept along the bend. However, in the case of closed-to open-end rapid propagation, from the bend the sharpened flame tip proceeds near the inner wall with succeeding flame front elongated towards the axial direction, and in serious cases transition to turbulent flame occurs. The latter propagation mode was examined in detail. The flame tip speed takes a large value in the bend. In the latter straight section, after a short deceleration period, the flame propagation speed (in axial direction) continuously increases for a while. The relation between the flame behavior and flow field ahead of the flame was investigated further by observing time line movement in the unburned mixture. From the similarity between the flame front shape and time line shape variations, it was revealed that the flame behavior around the bend is determined primarily by the nature of unburned gas flow driven toward the open end. The observed time line movements were compared further with those obtained by numerical simulation based on two-dimensional incompressible inviscid or viscous cold flow models. It was shown that around the bend, the mixture flow pattern and also features of the flame behavior are determined primarily by potential flow characteristics. It was suggested that the temporary flame speed increase at the bend is mainly due to large axial flow velocity of unburned mixture near the inner wall, while the increase of flame propagation speed at the downstream straight section is primarily due to the increase of the flame area.

214 高圧乱流予混合火炎のレーザートモグラフィーによる可視化とフラクタル解析

Visualization of flame front shape of high pressure turbulent premixed flame by using laser tomograpy was made for the purpose of exploring the influence of the turbulence intensity and ambient pressure on the turbulent flame structure. The high pressure combustion chamber was used, where the continuous combustion was possible. In order to quantify the complexity of the flame front shape, the fractal analysis was made. As a result, the fractal dimension increased with increasing the turbulence intensity and ambient pressure, and it was saturated to about 2.15.

Translated from Nensho no Kagaku to Gijutsu, 1, 59 )1992( Detailed Structure of Flame Front Disturbance

Abstract The detailed three-dimensional structure of the flame front disturbance induced by a weak pressure wave has been studied experimentally using a small combustion chamber. The flame front disturbance was recorded by a newly developed high-speed Schiieren system, which gives a view from the direction normal to the flame front. The results were compared with those induced by preferential diffusion mechanisms. The image of a flame front disturbance induced by acceleration caused by weak pressure waves was found to be circular when it was observed from the direction normal to the flame front. This image was much different from that induced by the preferential diffusion mechanisms, which was observed as a net composed of thick lines. These images of the flame front disturbances were compared with those observed in our previous studies, in which the disturbances were observed from the direction normal to the flame propagating direction. Consequently, the flame front disturbances induced by acceleration were inferred to be horn-shaped, while those induced by the preferential diffusion mechanisms were inferred to be ridge-shaped. It is postulated that this difference of the flame front shapes should be attributed to the difference of the mechanisms which, sustain or enhance the disturbances.

CARS Measurements and Numerical Simulation Results for Methane/Air Counterflow Flames

AbstractIn extension of previous work a comparison has been made between experiments and numerical simulation results across the flame front of an atmospheric pressure methane/air counterflow diffusion stabilized in the forward stagnation region of a cylindrical porous burner plate. Temperature profiles have been measured by CARS spectroscopy on molecular nitrogen. The experimental data are compared with results of numerical calculations using a computer code based on a one‐dimensional boundary layer approximation. This work covers pure diffusion flames at different strain rates as well as partially premixed flames at two stoichiometries and strain rates. The results indicate good agreement between measured and calculated temperature profiles with regard to peak temperature location of the flame front, temperature gradients and overall shape of the non‐premixed and partially premixed flames, respectively.