Turbulent Burning Velocity Research Articles

The turbulent burning velocity of methanol–air mixtures

Methanol is a sustainable and versatile alternative fuel for spark-ignition engines and other combustion applications. To characterize the combustion behavior of this fuel, a good understanding of the factors affecting its turbulent burning velocity is required. This paper presents experimental values of the turbulent burning velocity of methanol-air mixtures obtained in a fan-stirred bomb, for u′=2–6m/s, ϕ=0.8–1.4, T=358K and pressures up to 0.5MPa. In combination with laminar burning velocity values previously obtained on the same rig, these measurements are used to provide better insight into the various factors affecting ut of methanol, and to assess to what degree existing turbulent combustion models can reproduce experimental trends. It appeared that most models correctly accounted for the effects of turbulent rms velocity u′. With respect to the effects of ϕ and pressure, however, models accounting for flame stretch and instabilities, through the inclusion of model terms depending on thermodiffusive mixture properties and pressure, had a slight edge on simpler formulations.

Lean premixed opposed jet flames in fractal grid generated multiscale turbulence

The opposed jet configuration presents an attractive canonical geometry for the evaluation of burning properties of turbulent flames with past studies typically limited to low Reynolds numbers. Fractal grid generated turbulence was used to remove the low turbulence level limitations associated with conventional perforated plate generators with the turbulent Reynolds number range moved from 50–120 to 130–318. Optimal grid configurations were determined with particular emphasis on reducing the impact of the flow upstream of the turbulence generators in order to facilitate simpler boundary conditions for computational studies. The resulting flow structures were analysed using proper orthogonal decomposition and conditional proper orthogonal decomposition. Velocity and reaction progress variable statistics, including conditional velocities and scalar fluxes, are reported for fuel lean methane, ethylene and propane flames approaching extinction. The instrumentation comprised particle image velocimetry with the flows to both nozzles seeded with 1μm silicon oil droplets or 3μm Al2O3 particles. Probability density functions were determined for the instantaneous location of the stagnation point to eliminate the possibility of low frequency bulk motion distorting velocity statistics. Probability density functions of flame curvature were determined using a multi-step flame front detection algorithm with estimates of the turbulent burning velocity provided along with a discussion of alternative determination methods. The data sets show that fractal grids generate multi-scale broadband turbulence and present an opportunity for a systematic evaluation of calculation methods for premixed turbulent flames that undergo a transition from non-gradient to gradient turbulent transport while approaching extinction.

I122 H_2/CO火炎の乱流燃焼速度と火炎面形状

Turbulent burning velocity of spherically propagating premixed flame continues increasing during flame propagation. The flame front area is considered to be the most dominant factor for turbulent burning velocity. In this study, fractal analysis was conducted for stoichiometric spherically propagating premixed turbulent H_2/CO/O_2/CO_2 flames. Variations of fractal characteristics, such as fractal dimension, inner-cutoff and outer-cutoff scales with flame radius were investigated. Then, the correlation between turbulent burning velocity and flame front area estimated by fractal analysis was investigated.

乱流予混合火炎におけるFlow Divergence

In order to elucidate the flow divergence in the rich propane-air turbulent premixed flames, the flow fields of lean and rich propane-air and methane-air turbulent premixed flames of the identical conditions of laminar burning velocity and characteristic of turbulence have been closely examined by using a three-color six-beam LDV system. A bi-modal distribution, composed of both the low velocity mode and the high velocity mode, is obtained from gas velocity fluctuation within the turbulent flame brush. The low velocity mode is composed of velocity fluctuations of the unburnt mixture and the high velocity mode is composed of velocity fluctuations of the burnt gas. The reaction progress variable can be calculated from the bi-modal distribution of the radial component of the gas velocity. Distributions of the reaction progress variable clearly show that the rich propane flame is skinner and shorter than the lean propane flame, indicating that the turbulent burning velocity is larger in the rich propane flame. Comparing the burnt gas mode of the radial component of the gas velocity of the lean and rich propane-air and methane-air flames, the flow divergence is found to be remarkable in the rich propane flame. The cellular instability due to the preferential diffusion causes the turbulent burning velocity and/or the heat release to increase in the rich propane flame.

Analysis of flamelet leading point dynamics in an inhomogeneous flow

Several studies have utilized “leading points” concepts to explain the augmentation of burning rates in turbulent flames by flow fluctuations. These ideas have been particularly utilized to explain the strong sensitivity of turbulent burning rates to fuel composition. Leading point concepts suggest that the burning velocity is controlled by the velocity of the points on the flame that propagate farthest out into the reactants – thus, they de-emphasize the classical idea that burning velocity enhancement is due to increases in flame surface area. Rather, within this interpretation, flame area creation is the effect, not the cause, of augmented turbulent burning velocities. However, the theory behind the implementation of leading point concepts in turbulent combustion modeling needs further development and the definition of “leading point” has not been fully clarified. For a certain class of steady shear flows, it is straightforward to demonstrate the leading point concept in an intuitive manner, but the problem becomes more complex when the leading points themselves evolve in time. In this paper, we use the G-equation to describe the flame dynamics and, utilizing results for Hamilton–Jacobi equations from the Aubry–Mather theory, demonstrate both the utility and limitations of leading points interpretations for front propagation, at least for deterministic problems. Specifically, we show how the large-time behavior of the solutions is controlled by discrete points on the flame under certain conditions and is, therefore, independent of the rest of the flow field details – a key hypothesis of leading points theories. However, it is possible to find other conditions where the large time behavior of the flame is not controlled by discrete points on the flame, but rather by the velocity field over its entire surface. Moreover, we also show that even in cases where the burning rate is controlled by discrete points, these points are not necessarily the most forward lying points in the flame front. Finally, we consider the case where the laminar flame speed is a function of flame front curvature and derive exact results for the sensitivity of the front speed to the Markstein length, ℓ, for ℓ>0. These solutions explicitly illustrate how the reduction of front displacement speed for increasing ℓ can be interpreted in terms of leading points dynamics in some cases.

Circle grid fractal plate as a turbulent generator for premixed flame: an overview

This review paper focuses to ascertain a new approach in turbulence generation on the structure of premixed flames and external combustion using a fractal grid pattern. This review paper discusses the relationship between fractal pattern and turbulence flow. Many researchers have explored the fractal pattern as a new concept of turbulence generators, but researchers rarely study fractal turbulence generators on the structure premixed flame. The turbulent flow field characteristics have been studied tand investigated in a premixed combustion application. In terms of turbulence intensity, most researchers used fractal grid that can be tailored so that they can design the characteristic needed in premixed flame. This approach makes it extremely difficult to determine the exact turbulent burning velocity on the velocity fluctuation of the flow. The decision to carry out additional research on the effect circle grid fractal plate as a turbulent generator for premixed flame should depends on the blockage ratio and fractal pattern of the grid.

Measurement on instantaneous flame front structure of turbulent premixed CH4/H2/air flames

Instantaneous flame front structure of turbulent premixed CH4/H2/air flames (hydrogen fraction of 0%, 5%, 10% and 20% by mole fraction) was investigated quantitatively using a nozzle-type Bunsen burner. Hot wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Turbulent burning velocity, ST, flame surface density, Σ, and mean flame volume, Vf, were calculated by processing the OH-PLIF images. Results show that the flame front structures of the turbulent premixed flames are the wrinkled flame front and it becomes much finer with the increase of turbulence intensity as well as hydrogen fraction. The value of ST/SL significantly increases with the increase of u′/SL and it slightly increases with the increase of hydrogen fraction. Flame surface density profile are symmetric and gives its maximum value at about 〈c〉=0.5. Hydrogen addition slightly enhances the Σ and the tendency is more obvious under higher turbulence intensity. The decrease of Σ with the increase of turbulence intensity is mainly due to the effect of flame volume. The mean flame volume of flame region obviously increases with the increase of turbulence intensity within the experimental range due to the increase in depth of the large scale flame wrinkles and flame height. Hydrogen addition is not a predominant factor within the hydrogen fraction range in this study.

The role of low temperature fuel chemistry on turbulent flame propagation

A new high temperature, high Reynolds number, Reactor Assisted Turbulent Slot (RATS) burner has been developed to investigate turbulent flame regimes and burning rates for large hydrocarbon transportation fuels, which exhibit strong low temperature chemistry behavior. The turbulent flow characteristics are quantified using hot wire anemometry. The turbulent flame structures and burning velocities of n-heptane/air mixtures are measured by using planar laser induced fluorescence of OH and CH2O with reactant temperatures spanning from 400K to 700K. It is found for the first time that for n-heptane/air mixtures there are four unique turbulent flame regimes, a conventional chemically-frozen-flow regime, a low-temperature-ignition regime, a transitional regime between the low- to high-temperature-ignition regimes, and a high-temperature-ignition regime, depending on the initial reactant temperature and heated flow residence time prior to the flame. The turbulent burning velocities have been measured for the first two regimes, chemically-frozen-flow and low-temperature-ignition regimes, in order to quantitatively address the role of low temperature ignition on the turbulent burning velocity. In the latter case, large amount of CH2O formation has been observed in the pre-flame zone, signaling a significant change in the reactant composition and chemistry. At a given reactant temperature and turbulent intensity, the normalized turbulent burning velocities can be varied depending on the extent of low temperature fuel oxidation by varying the heated flow residence time and reactant temperature. The present results suggest that contrary to the previous studies, the turbulent flame regimes and burning velocities for fuels with low temperature chemistry may not be uniquely defined at elevated temperatures.

A new approach to model turbulent lifted CH4/air flame issuing in a vitiated coflow using conditional moment closure coupled with an extinction model

In this article, conditional moment closure model (CMC) with detailed chemistry is used to model lifted turbulent methane flame in a high temperature and vitiated coflow and to predict flame lift-off height. The flow and mixing field are predicted by a 2D in-house code employing a k–ε turbulence model (RANS) with modified constant Cε2. The first-order CMC model on its own could not capture the behavior of the lifted flame. Large eddy simulations (LES) coupled with second-order CMC model would be a promising alternative but the objective here was to improve low-cost simulations based on RANS and first-order CMC to address realistic problems. Hence, an extinction model has been incorporated in the first-order CMC to improve its predictions and is referred in this paper as CMCE. In the CMCE model, flame is assumed to be extinguished when the ratio of flow time scale to the chemical time scale falls below a critical value. Predicted lift-off height by the CMCE model agrees very well with the experimental results. There is a significant improvement in temperature and species distributions in both axial and radial directions with the implementation of the CMCE model. Further, the model is extended to predict the flame lift-off height for various coflow temperatures and jet velocities by using scaling ratios. With these modifications, the lift-off heights predicted by the CMCE model match well with the experimental results for a wide range of jet velocities and coflow temperatures. Results from both CMC and CMCE models are compared against the experimental data to show the importance of the extinction model. Flame stabilization process indicates that flame stabilizes on the contour of mean stoichiometric mixture fraction where axial mean velocity equals the turbulent burning velocity.

Relationships for maximum flame surface density and brush thickness through conditional analysis in turbulent premixed combustion

A new conditional averaging method is introduced in terms of the reaction progress variable, c, and its derivatives of successively higher orders, $\Sigma _f^{\prime} $Σf′ (or −∂c/∂n) and ∂2c/∂n2, in high Damköhler number (Da) turbulent premixed combustion. c is continuous and differentiable in space so that conditional averages may be defined for flamelets of finite thicknesses as well as for infinitesimally thin flamelets. A coupled set of transport equations are derived with corresponding conditional coefficient terms for mean reaction progress variable, $\bar c$c¯, and flame surface density (FSD), Σf. Statistics for all component terms are consistent with previous results and show the same qualitative trends irrespective of compressibility by the new averaging practice. It is shown that the maximum FSD, Σmax, occurs at the location where the relationship, $\nabla ^2 \bar c + \langle {\nabla \cdot {\bf n}} \rangle _f \Sigma _f = 0$∇2c¯+⟨∇·n⟩fΣf=0, holds in a flame brush. An analytical expression is derived for Σmax under the assumption of constant tangential strain rate and linear variation of mean curvature and mean displacement speed, ⟨Sd⟩f, in the $\bar c$c¯ space. Σmax becomes equal to one fourth ⟨|∇·n|⟩f or half the mean absolute curvature for fully turbulent flames with the magnitude of ⟨n⟩f close to zero. Once the turbulent burning velocity, ST, is determined by asymptotic behavior at the leading edge, the brush thickness, ℓF, may be obtained from local and integral relationships for the product, ℓFΣmax, in a steadily propagating one-dimensional flame. The newly proposed relationships reproduce proper trends of variation for ST, Σmax, and ℓF against measurement data in literature. They are validated to show good agreement with direct numerical simulation (DNS) results of the Lewis number equal to unity for freely propagating incompressible and stagnating compressible turbulent premixed flames.

Correlation of turbulent burning velocity for syngas/air mixtures at high pressure up to 1.0 MPa

Instantaneous flame front structures of the turbulent premixed flames of syngas/air and CH4/air mixtures were investigated using OH-PLIF technique at high pressure up to 1.0MPa, through which the turbulent burning velocities were derived and correlated with the turbulence intensity. Results show that both syngas/air and CH4/air mixtures, ST/SL increases remarkably with the increase of u′/SL particularly in the weak turbulence region. For the syngas/air mixture, the intensity of flame front wrinkle is promoted with the increase of hydrogen fraction in the syngas due to the increased preferential diffusive-thermal instability. Compared to CH4/air mixture, the syngas flames possess much wrinkled flame front with much smaller fine cusps structure, and with increasing u′/SL, the rate of the increase of ST/SL for the syngas/air mixtures is more significant than that of CH4/air mixtures. This demonstrates that the increase of flame front area due to turbulence wrinkling is promoted by flame intrinsic instability for syngas/air mixtures. The values of ST/SL for all mixtures increase with the increase of pressure because of the decrease of flame thickness which promotes the hydrodynamic instability. A general correlation of turbulent burning velocity for the syngas/air and CH4/air mixtures was obtained in the form of ST/SL∝a[(P/P0)(u′/SL)]n.

On the Application of the Levenberg–Marquardt Method in Conjunction with an Explicit Runge–Kutta and an Implicit Rosenbrock Method to Assess Burning Velocities from Confined Deflagrations

The potential of the Levenberg–Marquardt method combined with an explicit Runge–Kutta method for non-stiff systems, and, an implicit Rosenbrock method for stiff systems to investigate burning velocities using explosion bombs was explored. The implementation of this combination of methods was verified on three benchmark test problems, and, by the application of two integral balance models to laminar hydrogen-air and methane-air explosions. The methodology described here was subsequently applied to quantify the coefficients of a turbulent burning velocity correlation for a methane-air explosion in the decaying flow field of the standard 20-litre explosion sphere. The outcome of this research indicates that the usefulness of the 20-litre sphere can be extended beyond the measurement of practical explosion parameters. When combined with the methodology in this paper, turbulent burning velocity correlations can be assessed in different parts of the Borghi-diagram.

Prediction of confined, vented methane-hydrogen explosions using a computational fluid dynamic approach

Hydrogen is seen as an important energy carrier for the future, with a great benefit being carbon-free emissions at its point of use. A hydrogen transport system between manufacturing sites and end users is required, and one solution proposed is its addition to existing natural gas pipeline networks. A major concern with this approach is that the explosion hazard may be increased, relative to natural gas, should an accidental release occur. This paper describes a mathematical model of confined, vented explosions of mixtures of methane and hydrogen of value in performing consequence and risk assessments. The model is based on solutions of averaged forms of the Navier–Stokes equations, with the equation set closed using k-ɛ and second-moment turbulence models, and the turbulent burning velocity determined from correlations of data on CH4–H2 mixtures reported in the literature. Predictions derived for explosions in a 70 m3 vessel, with and without internal pipe congestion, show reasonable agreement with available data, and demonstrate that hydrogen addition can have a significant effect on overpressure generation. Conclusions drawn from the calculations go some way to identifying safe operating limits for hydrogen addition.

난류 예혼합 화염 선단부의 통계적 특성에 관한 수치적 연구

Leading edge statistics are obtained by direct numerical simulation(DNS) of freely propagating incompressible and stagnating compressible turbulent premixed flames. Conditional averages of velocities in terms of reaction progress variable, c, and local flame surface density, <TEX>${\sum}^{\prime}_f$</TEX>, are defined and compared through the flame brush. It holds asymptotically that <TEX>$</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_f=</TEX><TEX><</TEX><TEX>S_d</TEX><TEX>></TEX><TEX>_f$</TEX> and <TEX>$</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_u-</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_b=D_t/L_w$</TEX> with the characteristic length scale of <TEX>$\bar{c}$</TEX> variation, <TEX>$L_w$</TEX>. It also holds that <TEX>$</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_b=</TEX><TEX><</TEX><TEX>u</TEX><TEX>></TEX><TEX>_f$</TEX> for a freely propagating flame under no mean strain rate. The turbulent burning velocity, <TEX>$S_T$</TEX>, is determined by the conditional statistics at the leading edge under large activation energy.

Flame wrinkle destruction processes in harmonically forced, turbulent premixed flames

AbstractThis paper describes analyses of the nonlinear dynamics of harmonically forced, turbulent premixed flames. A key objective of this work is to analyse the ensemble-averaged dynamics of the flame front position, $\langle \xi \rangle $, excited by harmonic forcing of amplitude $\varepsilon $, in the presence of stochastic flow fluctuations of amplitude $\mu $. Low-amplitude and/or near-field effects are quantified by a third-order perturbation analysis, while the more general case is analysed computationally by solving the three-dimensional level-set equation, extracting the instantaneous flame position, and ensemble averaging the results. We show that different mechanisms contribute to smoothing of flame wrinkles, manifested as progressive decay in the magnitude of $\langle \xi \rangle $. Near the flame holder, random phase jitter, associated with stochastic velocity fluctuations tangential to the flame, is dominant. Farther downstream, propagation of the ensemble-averaged front normal to itself at the time-averaged turbulent burning velocity, $ \overline{{S}_{T, eff} } $, leads to destruction of wrinkles, analogous to the laminar case, an effect that scales with $\mu $. A second, new result is the demonstration that the ensemble-averaged turbulent burning velocity, ${S}_{T, eff} (s, t)$, is modulated by the harmonic forcing, i.e. ${S}_{T, eff} (s, t)= \overline{{S}_{T, eff} (s)} + { S}_{T, eff}^{\prime } (s, t)$, where ${ S}_{T, eff}^{\prime } $ has an inverse dependence upon the instantaneous, ensemble averaged-flame curvature, an effect that scales with $\varepsilon $ and $\mu $. We show that this curvature dependence follows from basic application of Huygens propagation to flames with stochastic wrinkling superimposed upon base curvature. This effect also leads to smoothing of flame wrinkles and is analogous to stretch processes in positive-Markstein-length, laminar flames.

A study of numerical hazard prediction method of gas explosion

A 3-dimensional computational fluid dynamics (CFD) simulation of a premixed hydrogen/air explosion in a large-scale domain is performed. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent burning velocity that characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent burning velocity is evaluated by analyzing the experimental data of hydrogen/air premixed explosion. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement. The effect of mesh size on simulated flame propagation velocity is also tested, showing that the numerical result agrees reasonably well with experiment when the mesh size is less than about 20 cm.

Characterisation of flame development with ethanol, butanol, iso-octane, gasoline and methane in a direct-injection spark-ignition engine

Research into novel internal combustion engines requires consideration of the diversity in future fuels that may contain significant quantities of bio-components in an attempt to reduce CO2 emissions from vehicles and contribute to energy sustainability. However, most biofuels have different chemical and physical properties to those of typical hydrocarbons; these can lead to different mechanisms of mixture preparation and combustion. The current paper presents results from an optical study of combustion in a direct-injection spark-ignition research engine with gasoline, iso-octane, ethanol and butanol fuels injected from a centrally located multi-hole injector. Methane was also employed by injecting it into the inlet plenum of the engine to provide a benchmark case for well-mixed ‘homogeneous’ charge preparation. Crank-angle resolved flame chemiluminescence images were acquired and post-processed for a series of consecutive cycles for each fuel, in order to calculate in-cylinder rates of flame growth and motion. In-cylinder pressure traces were used for heat release analysis and for comparison with the image-processing results. All tests were performed at 1500 RPM with 0.5bar intake plenum pressure. Stoichiometric (ϕ=1.0) and lean (ϕ=0.83) conditions were considered. The combustion characteristics were analysed with respect to laminar and turbulent burning velocities obtained from combustion bombs in the literature and from traditional combustion diagrams in order to bring all data into the context of current theories and allow insights by making comparisons were appropriate.

Effects of Local Flame Displacement Velocity on Turbulent Burning Velocity for Stoichiometric H&lt;sub&gt;2&lt;/sub&gt;-O&lt;sub&gt;2&lt;/sub&gt;-Ar Mixtures

This study was made to establish a prediction method of turbulent burning velocity for stoichiometric hydrogen-oxygen-argon mixtures, with paying special attention to the importance of local burning velocity to turbulent burning velocity. In this study, stoichiometric mixtures with different dilutions, whose laminar burning velocity SL0 was varied from 25 to 90 cm/s, were used under almost the same weak turbulence condition, so that the ratio of turbulence intensity u' to SL0 becomes 0.4 to 2.0. Nitrogen was also used as a conventional dilution gas for comparison with argon. A two-dimensional sequential laser tomography technique was applied to obtain the relationship between the flame shape and the flame displacement in a constant-volume vessel. The local flame displacement velocity SF of turbulent flames was quantitatively measured as the key parameter of local burning velocity. It was found that the trends of the mean values of SF, SF,m, against u'/SL0 corresponded well to its turbulent burning velocities, but SF,m did not increase infinitely with increasing u'/SL0. The turbulent burning velocity also attempted to be predicted by some empirical equations. As a result, the predicted turbulent burning velocities, where the obtained SF,m was employed instead of SL0 as the local burning velocity, were in good agreement with experimental ones.

804 平均反応進行変数に基づく球状伝播予混合乱流火炎の乱流燃焼速度(エンジンシステム1)

804 平均反応進行変数に基づく球状伝播予混合乱流火炎の乱流燃焼速度(エンジンシステム1)

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