Burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocity was measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean premixed CH₄–air mixtures when turbulent level in unburned mixture is not changed. Moreover, the increase of turbulent burning velocity is faster than that of the corresponding laminar burning velocity at constant equivalence ratio, suggesting that the kinetics effect is not the sole factor that results in the increase in turbulent burning velocity when hydrogen is added. The further analysis of flame surface characteristics and brush thickness indicates that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. The increase in flame brush thickness that results in the increase in total surface area may contribute to the faster increase in turbulent burning velocity, when hydrogen is added. Besides, the stretched local laminar burning velocity may be enhanced with the addition of hydrogen, which may also contribute to the faster increase rate of turbulent burning velocity. Both the variation in flame brush thickness and the enhancement in stretched local laminar burning velocity are due to the decreased fuel Lewis number when hydrogen is added. Therefore, the effects of fuel Lewis number and stretch should be taken into account in correlating burning velocity of turbulent premixed flames.     

Premixed flamelet modelling: Factors influencing the turbulent heat release rate source term and the turbulent burning velocity

A flamelet approach is adopted in a study of the factors affecting the volumetric heat release source term in turbulent combustion. This term is expressed as the product of an instability enhanced burning rate factor, Pbi, and the mean volumetric heat release rate in an unstretched laminar flamelet of the mixture. Included in the expression for Pbi are a pdf of the flame stretch rate and a flame stretch factor. Fractal considerations link the turbulent burning velocity normalised by the effective rms turbulent velocity to Pbi. Evaluation of this last parameter focuses on problems of (i) the pdfs of the flame stretch rate, (ii) the effects of flame stretch rate on the burning rate, (iii) the effects of any flamelet instability on the burning rate, (iv) flamelet extinctions under positive and negative flame stretch rates, and (v) the effects of the unsteadiness of flame stretch rates. The Markstein number influences both the rate of burning and the possibility of flamelet instabilities developing which, through their ensuing wrinkling, increase the burning rate. The flame stretch factor is extended to embrace potential Darrieus-Landau thermo-diffusive flamelet instabilities. A major limitation is the insufficient understanding of the effects of negative stretch rates that might cause flame extinction. The influences of positive and negative Markstein numbers are considered separately. For the former, a computed theoretical relationship for turbulent burning velocity, normalised by the effective rms velocity, is developed which, although close to that measured experimentally, tends to be somewhat lower at the higher values of the Karlovitz stretch factor. This might be attributed to reduced flame extinction and reduced effective Markstein numbers when the increasingly nonsteady conditions reduce the ability of the flame to respond to changes in flame stretch rates. As the pressure increases, Markstein numbers decrease. For negative Markstein numbers the predicted values of Pbi and turbulent burning velocity are significantly increased above the values for positive Markstein numbers. This is confirmed experimentally and these values are close to those predicted theoretically. The increased values are due to the greater stretch rate required for flame extinction, the increased burning rate at positive values of flame stretch rate, and, in some instances, the development of flame instabilities. At lower values of turbulence than those covered by these computations, burning velocities can be enhanced by flame instabilities, as they are with laminar flames, particularly at negative Markstein numbers.     

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 400 K to 700 K. 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.     

Correlation of turbulent burning velocities of ethanol–air, measured in a fan-stirred bomb up to 1.2 MPa

The turbulent burning velocity is defined by the mass rate of burning and this also requires that the associated flame surface area should be defined. Previous measurements of the radial distribution of the mean reaction progress variable in turbulent explosion flames provide a basis for definitions of such surface areas for turbulent burning velocities. These inter-relationships. in general, are different from those for burner flames. Burning velocities are presented for a spherical flame surface, at which the mass of unburned gas inside it is equal to the mass of burned gas outside it. These can readily be transformed to burning velocities based on other surfaces.The measurements of the turbulent burning velocities presented are the mean from five different explosions, all under the same conditions. These cover a wide range of equivalence ratios, pressures and rms turbulent velocities for ethanol–air mixtures. Two techniques are employed, one based on measurements of high speed schlieren images, the other on pressure transducer measurements. There is good agreement between turbulent burning velocities measured by the two techniques. All the measurement are generalised in plots of burning velocity normalised by the effective unburned gas rms velocity as a function of the Karlovitz stretch factor for different strain rate Markstein numbers. For a given value of this stretch factor a decrease in Markstein number increases the normalised burning velocity. Comparisons are made with the findings of other workers.     

Effect of large scale structures on turbulent flame propagation

The effect of laminar burning velocity, Reynolds number, and turbulent fluctuations on the propagation of a turbulent premixed flame stabilized behind a sudden expansion is investigated numerically using a reaction sheet model and line interface calculations of the front motion. Within this framework, flame propagation as an interface between reactants and products is due to the combined action of the advection of products and burning of reactants. Results indicate that turbulent flames have a narrow range of stability when propagating inside channels. At lean conditions, blowoff is caused by the low burning velocity and large turbulent fluctuations. At near-stoichiometric conditions, flashback may occur when the flame blocks the flow and propagates along the wall. Turbulent fluctuations establish an extra mass flux across the flame front that enhances burning at high Reynolds numbers and moderate burning velocities.     

Turbulent burning velocity of hydrogen–air premixed propagating flames at elevated pressures

Lewis number represents the thermo-diffusive effects on laminar flames. That of hydrogen–air mixture varies extensively with the equivalence ratio due to the high molecular diffusivity of hydrogen. In this study, the influences of pressure and thermo-diffusive effects on spherically propagating premixed hydrogen–air turbulent flames were studied using a constant volume fan-stirred combustion vessel. It was noted that the ratio of the turbulent to unstretched laminar burning velocity increased with decreasing equivalence ratio and increasing mixture pressure. Turbulent burning velocity was dominated by three factors: (1) purely hydrodynamic factor, turbulence Reynolds number, (2) relative turbulence intensity to reaction speed, the ratio of turbulence intensity to unstretched laminar burning velocity, and (3) sensitivity of the flame to the stretch due to the thermo-diffusive effects, Lewis and Markstein numbers. A turbulent burning velocity correlation in terms of Reynolds and Lewis numbers is presented.     

Roles of displacement speed on evolution of flame surface density for different turbulent intensities and Lewis numbers in turbulent premixed combustion

Direct numerical simulations (DNS) are conducted in 3D to investigate the evolution of flame surface density (FSD) in turbulent premixed combustion. A parametric study is performed with respect to turbulent intensity and Lewis number to investigate all component terms in the FSD transport equation. A higher turbulent intensity leads to a higher turbulent burning velocity due to increased flame area, while the mean consumption speed remains close to the laminar flame speed. A lower Lewis number leads to a higher turbulent burning velocity, with increases in both total flame area and mean consumption speed. There are two source terms to govern FSD: tangential strain and propagation term, given as a product of displacement speed and curvature. The mean strain rate varies linearly with the turbulent intensity, but shows no noticeable dependence on the Lewis number. The correlation between curvature and displacement speed does not depend on the turbulent intensity, but shows significant influence of the Lewis number. The propagation term decreases with increasing turbulent intensity to become a larger negative sink in the rear of flame brush with flame elements of smaller radii of curvature and higher displacement speeds. A lower Lewis number leads to a larger positive propagation term in the front due to an increased displacement speed to produce more flame area through diffusive thermal instability.     

Generation of PDFS for flame curvature and for flame stretch rate in premixed turbulent combustion

Experimentally derived pdfs of turbulent, premixed, flame curvatures from a variety of sources, for a wide range of conditions are surveyed and a suitable expression sought to generalize these. This proves to be one based on the Damköhler number, Da. This is tantamount to normalizing the curvature by multiplying it by the Taylor scale of turbulence. It enables the distribution of flame curvature when normalized by the laminar flame thickness, to be expressed in terms of the Karlovitz stretch factor, K, and the turbulent Reynolds number, R_l. The value of the pdf at zero curvature is linearly related to Da^1/2.The pdf expressions of Yeung et al. [3] obtained from numerical simulations are used for the strain rate distribution and, on the assumption that these and that for flame curvature are statistically independent, values of flame stretch rate pdfs are generated numerically. It is necessary to define an appropriate surface to define the burning velocity, flame stretch rate, and appropriate Markstein numbers. Two surfaces are considered and employed in the computations, one located at the start of the preheat zone, the other at the start of the reaction zone. The latter seems more rational and gives the better generalisation of the pdfs of flame stretch rate.An assumed linearity of laminar burning velocity with flame stretch rate, extending over both positive and negative stretch rates, enables flame stretch rate pdfs to be generated. It is concluded that negative values of burning velocity are unlikely and that burning velocities should tend to zero rather than attain negative values. This modifies the derivation of flame stretch rate pdfs. These depend on the Markstein number, Karlovitz stretch factor and turbulent Reynolds number. Computations suggest that, for values of K above 0.1 and of R_l above 100, the pdf of stretch rate is similar to that of strain rate. At very low values of K and negative values of Markstein number, pronounced flamelet instability might be anticipated.     

Derivation of burning velocities of premixed hydrogen/air flames at engine-relevant conditions using a single-cylinder compression machine with optical access

With respect to hydrogen internal combustion engines beside turbulence also flame front instabilities of high-pressure combustion provoke an acceleration of the flame. To account for this effect within engine simulations, it is suggested to include the impact of flame front instabilities directly into a “quasi-laminar” burning velocity that is an input for turbulent combustion models. Premixed hydrogen/air flames are investigated in a single-cylinder compression machine using OH-chemiluminescence and in-cylinder pressure analysis. Values of burning velocities are calculated from flame front velocities considering thermal expansion effects. A flame speed correlation is derived which covers temperatures and pressures of the unburned mixture, relevant for internal combustion engines, ranging from 350 K to 700 K and 5 bar to 45 bar. Values of air/fuel equivalence ratio cover lean and rich regimes between 0.4 ≤ λ ≤ 2.8. For an evaluation of stretch and instability effects a comparison to fundamental laminar burning velocities of a one-dimensional flame computed with a detailed chemical kinetic-mechanism is given. At high-pressure conditions flame speed measurements demonstrate that flame front instabilities have an accelerating effect on the value of laminar burning velocities, which cannot be reproduced by computations with a chemical model. A linear stability analysis is applied in order to estimate the magnitude of instabilities. The proposed “quasi-laminar” burning velocity does not account for interaction between turbulence and instability effects. Consequently, at increasing turbulence levels partially counter-balancing of instabilities by turbulence is not followed which may allegorize a possible limitation of the suggested approach.     

汽油机双焰区准维燃烧分析模型

本文针对具有双火焰区燃烧方式的汽油机射流燃烧过程提出了准维分析模型.本模型不仅可以计算出燃烧放热率,燃气平均温度,NO生成规律,还可提供各火焰区的紊流燃烧速度、紊流传播速度和火焰速比等重要参数.计算结果表明,运用本模型可成功地对双焰区汽油机射流燃烧过程进行多方面的综合研究.     

Turbulent burning rates of methane and methane-hydrogen mixtures

Methane and methane-hydrogen (10%, 20% and 50% hydrogen by volume) mixtures have been ignited in a fan stirred bomb in turbulence and filmed using high speed cine schlieren imaging. Measurements were performed at 0.1 MPa (absolute) and 360 K. A turbulent burning velocity was determined for a range of turbulence velocities and equivalence ratios. Experimental laminar burning velocities and Markstein numbers were also derived. For all fuels the turbulent burning velocity increased with turbulence velocity. The addition of hydrogen generally resulted in increased turbulent and laminar burning velocity and decreased Markstein number. Those flames that were less sensitive to stretch (lower Markstein number) burned faster under turbulent conditions, especially as the turbulence levels were increased, compared to stretch-sensitive (high Markstein number) flames.     

Correlations for laminar burning velocities of liquefied petroleum gas–air mixtures

Spherically expanding flames have been employed to determine the laminar flame speeds of liquefied petroleum gas–air mixtures, diluted or not by the combustion exhaust gas, over equivalence ratios from 0.7 to 1.4. The effect of the stretch imposed at the flame front has been explored experimentally, and Markstein lengths are estimated to characterize the flame stretch. After omitting the stretch effect, one has obtained the unstretched laminar burning velocities of liquefied petroleum gas–air flames with or without diluent. Explicit formulas have been obtained to express the laminar burning velocity dependencies on the equivalence ratio and diluent rate.     

Flame chemiluminescence and OH LIF imaging in a hydrogen-fuelled spark-ignition engine

Research into novel internal combustion engines requires consideration of the diversity in future fuels in an attempt to reduce drastically CO₂ emissions from vehicles and promote energy sustainability. Hydrogen has been proposed as a possible fuel for future internal combustion engines. Hydrogen’s wide flammability range allows higher engine efficiency with much leaner operation than conventional fuels, for both reduced toxic emissions and no CO₂ gases. This paper presents results from an optical study of combustion in a spark-ignition research engine running with direct injection and port injection of hydrogen. Crank-angle resolved flame chemiluminescence images were acquired and post-processed for a series of consecutive cycles in order to calculate in-cylinder rates of flame growth. Laser induced fluorescence of OH was also applied on an in-cylinder plane below the spark plug to record detailed features of the flame front for a series of engine cycles. The tests were performed at various air-to-fuel ratios, typically in a range of φ = 0.50–0.83 at 1000 RPM with 0.5 bar intake pressure. The engine was also run with gasoline in direct-injection and port-injection modes to compare with the operation on hydrogen. The observed combustion characteristics were analysed with respect to laminar and turbulent burning velocities, as well as flame stretch. An attempt was also made to review relevant hydrogen work from the limited literature on the subject and make comparisons were appropriate.     

Direct numerical simulation of hydrogen addition in turbulent premixed Bunsen flames using flamelet-generated manifold reduction

Direct numerical simulations of a lean premixed turbulent Bunsen flame with hydrogen addition have been performed. We show the results for a case with equivalence ratio of 0.7 and a molar fractional distribution of 40% H₂ and 60% CH₄. The flamelet-generated manifold technique is used to reduce the chemistry; flamelets with different equivalence ratios and inflow temperature are used to account for stretch effects that are enhanced by preferential diffusion. The three-dimensional simulation clearly shows enhanced burning velocity in regions convex toward the reactants and reduced burning velocity with possible extinction in regions concave toward the reactants. To obtain these effects it was found to be necessary to include two three-dimensional transport equations with essentially different diffusivities. This point is illustrated by comparison of the results with cases in which either a single transport equation was used or two transport equations with minor differences in diffusivities were used. These latter cases incorporated preferential diffusion in the 1D flamelets (and thus in the manifold), but not in the three-dimensional transport. Thus the three-dimensional preferential diffusion effects are shown to enhance curvature and thereby to increase the turbulent burning velocity and reduce the mean flame height. In addition the turbulent burning velocity increases because hydrogen addition leads to a larger laminar flamelet consumption speed. To demonstrate this second effect, results of the cases mentioned above are compared to the results of simulations of the Bunsen flame with 0% hydrogen added to the fuel.     

Experimental study of Markstein number effects on laminar flamelet velocity in turbulent premixed flames

Effects of turbulent flame stretch on mean local laminar burning velocity of flamelets, , were investigated experimentally in an explosion vessel at normal temperature and pressure. In this context, the wrinkling, At/Al, and the burning velocity, ut, of turbulent flames were measured simultaneously. With the flamelet assumption the mean local laminar burning velocity of flamelets, , was calculated for different turbulence intensities. The results were compared to the influence of stretch on spherically expanding laminar flames. For spherically expanding laminar flames the stretched laminar burning velocity, un, varied linearly with the Karlovitz stretch factor, yielding Markstein numbers that depend on the mixture composition. Six different mixtures with positive and negative Markstein numbers were investigated. The measurements of the mean local laminar burning velocity of turbulent flamelets were used to derive an efficiency parameter, I, which reflects the impact of the Markstein number and turbulent flame stretch—expressed by the turbulent Karlovitz stretch factor—on the local laminar burning velocity of flamelets. The results showed that the efficiency is reduced with increasing turbulence intensity and the reduction can be correlated to unsteady effects.     

Early flame development in a spark-ignition engine

The effect of changing the compression ratio from 7 to 3.5 and of different fuels, viz., propane, methane, and isooctane, on early flame development in a spark-ignition engine has been studied using an optical technique. This early phase of combustion is very crucial since cyclic variations in combustion and hence pressure development originate during this phase. The average flame speed increases under the influence of turbulence as the flame grows and appears to reach a fully developed value by the time the flame radius has reached about 11 mm in the engine studied. The evolution of the average flame velocity in this early stage appears to be spherically symmetrical in the engine considered. For the same operating conditions, propane flames are the fastest, followed by those of isooctane and finally of methane, as one would expect from their respective laminar burning velocities. Decreasing the compression ratio reduces the flame velocity sharply, mainly through the increase in residual mass fraction. The estimated initial burning velocity, S₀, differs from the laminar burning velocity, S_L, calculated from previously published correlations. There is considerable cyclic variation in combustion and this decreases as S₀ or S_L increases.     

Evaluation of a combustion model for the simulation of hydrogen spark-ignition engines using a CFD code

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k–? turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations.     

Improving burning speed by using hydrogen enrichment and turbulent jet ignition system in a rotary engine

Low flame speed restrains engine efficiency and increases HC emissions in rotary engines. Hydrogen addition and turbulent jet ignition have a great potential in increasing engine performance as they increase fuel burning speed. In this study, the classical R13b-Renesis Wankel engine and a modified one with a turbulent jet ignition configuration are numerically investigated by using hydrogen as a supplement. Eccentric motion of the rotor was generated by using User Defined Function in ANSYS-Fluent software. Pure methane and methane blended with 3% and 6% hydrogen energy fractions were used as fuels in the calculations. Combustion was modeled by using reduced mechanism of hydrogen-methane combustion having 22 species and 104 reactions. The Wankel engine was simulated at 2000 rpm speed and partial load conditions. At first, classical engine configuration having two spark plugs was simulated with pure methane. Then, hydrogen blended methane simulations were conducted to investigate the benefits of the hydrogen addition. Similar procedure was applied for the turbulent jet ignition application. The results show that both approaches are effective on increasing the burning speed of the fuel. It is revealed that hydrogen addition increases the indicated mean effective pressure (IMEP) by 1.8% and 5.2% for 3% and 6% hydrogen fraction cases respectively in the classical engine. Turbulent jet ignition with pure methane increases IMEP by 4.7% compared to the classical engine. Hydrogen addition only in pre-chamber is effective as much as 6% hydrogen fraction of classical engine. As the burning speed is increased by the application of these methods, CO and HC emissions are reduced and NO emission is increased. It is concluded that benefits of hydrogen addition and turbulent jet ignition applications can be optimized for both reducing harmful emissions and increasing engine performance.     

Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure

Ammonia is one of the most promising alternative fuels. In particular, ammonia combustion for gas turbine combustors for power generation is expected. To shift the fuel for a gas turbine combustor to ammonia step-by-step, the partial replacement of natural gas by ammonia is considered. To reveal the turbulent combustion characteristics, CH₄/NH₃/air turbulent premixed flame at 0.5 MPa was experimentally investigated. The ammonia ratio based on the mole fraction and lower heating value was varied from 0 to 0.2. The results showed that the ratio of the turbulent burning velocity and unstretched laminar burning velocity decreased with an increase in the ammonia ratio. The reason for this variation is that the flame area decreased with an increase in the ammonia ratio as the flame surface density decreased and the fractal inner cutoff increased. The volume fractions in the turbulent flame region were almost the same with ammonia addition, indicating that combustion oscillation can be handled in a manner similar to that for the case of natural gas for CH₄/NH₃/air flames.