Effects of initial pressure and hydrogen concentration on laminar combustion characteristics of diluted natural gas–hydrogen–air mixture

In this study, the experiment study about the laminar burning velocity and the flame stability of CO₂ diluted natural gas–hydrogen–air mixture was conducted in a constant volume combustion vessel by using the high-speed schlieren photography system. The unstretched laminar burning velocity and the Markstein length at different hydrogen fractions, dilution ratios and equivalence ratios and with different initial pressures were obtained. The flame stability was studied by analyzing the Markstein length, the flame thickness, the density ratio and the flame propagation schlieren photos. The results showed that the unstretched laminar burning velocity would be reduced with the increase of the initial pressure and dilution ratio and would be increased with the increase of the hydrogen fraction of the mixture. Meanwhile, the Markstein length would be increased with the increase of the equivalence ratio and the dilution ratio. Slight flaws occurred at the early stage. At a specific equivalence ratio, a higher initial pressure and hydrogen fraction would cause incomplete combustion.     

Measurements of laminar burning velocities and flame stability analysis for dissociated methanol–air–diluent mixtures at elevated temperatures and pressures

The laminar burning velocities and Markstein lengths for the dissociated methanol–air–diluent mixtures were measured at different equivalence ratios, initial temperatures and pressures, diluents (N₂ and CO₂) and dilution ratios by using the spherically outward expanding flame. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of dissociated methanol–air mixture increases with an increase in initial temperature and decreases with an increase in initial pressure. The peak laminar burning velocity occurs at equivalence ratio of 1.8. The Markstein length decreases with an increase in initial temperature and initial pressure. Cellular flame structures are presented at early flame propagation stage with the decrease of equivalence ratio or dilution ratio. The transition positions can be observed in the curve of flame propagation speed to stretch rate, indicating the occurrence of cellular structure at flame fronts. Mixture diluents (N₂ and CO₂) will decrease the laminar burning velocities of mixtures and increase the sensitivity of flame front to flame stretch rate. Markstein length increases with an increase in dilution ratio except for very lean mixture (equivalence ratio less than 0.8). CO₂ dilution has a greater impact on laminar flame speed and flame front stability compared to N₂. It is also demonstrated that the normalized unstretched laminar burning velocity is only related to dilution ratio and is not influenced by equivalence ratio.     

氮气稀释气对天然气燃烧特性的影响

为获得氮气稀释气对天然气燃烧特性的影响规律,在定容燃烧反应器中对不同当量比与初始压力下天然气的火焰传播特性、燃烧稳定性及燃烧特性进行了试验测试,并分析了氮气稀释度对天然气火焰传播特性、燃烧稳定性及燃烧特性的影响规律。研究结果表明:随着初始压力与氮气稀释度的升高,火焰前锋面将出现细小裂纹,火核逐渐向定容燃烧反应器上部漂移,火焰稳定性变差;随着初始压力的提高,马克斯坦长度明显变短,火焰稳定性变差,无拉伸火焰传播速度与层流燃烧速度明显降低,但最大燃烧压力显著升高。随着当量比的提高,层流燃烧速度与最大燃烧压力出现先增加后降低的趋势,两者的最大值出现在当量比为1.0时。马克斯坦长度随氮气稀释度的增加逐渐变短,表明火焰逐渐趋于不稳定;同时,无拉伸火焰传播速度、层流燃烧速度与最大燃烧压力随氮气稀释度的增加显著降低。     

Laminar burning velocity and Markstein length of nitrogen diluted natural gas/hydrogen/air mixtures at normal,reduced and elevated pressures

Flame propagation of premixed nitrogen diluted natural gas/hydrogen/air mixtures was studied in a constant volume combustion bomb under various initial pressures. Laminar burning velocities and Markstein lengths were obtained for the diluted stoichiometric fuel/air mixtures with different hydrogen fractions and diluent ratios under various initial pressures. The results showed that both unstretched flame speed and unstretched burning velocity are reduced with the increase in initial pressure (except when the hydrogen fraction is 80%) as well as diluent ratio. The velocity reduction rate due to diluent addition is determined mainly by hydrogen fraction and diluent ratio, and the effect of initial pressure is negligible. Flame stability was studied by analyzing Markstein length. It was found that the increase of initial pressure and hydrogen fraction decreases flame stability and the flame tends to be more stable with the addition of diluent gas. Generally speaking, Markstein length of a fuel with low hydrogen fraction is more sensitive to the change of initial pressure than that of a one with high hydrogen fraction.     

Effect of initial pressure on laminar combustion characteristics of hydrogen enriched natural gas

Flame propagation of premixed natural gas–hydrogen–air mixtures was studied in a constant volume combustion bomb. Laminar burning velocities and mass burning fluxes were obtained under various hydrogen fractions and equivalence ratios with various initial pressures, while flame stability and their influencing factors (Markstein length, density ratio and flame thickness) were obtained by analyzing the flame images at various hydrogen fractions, initial pressures and equivalence ratios. The results show that hydrogen fraction, initial pressure as well as equivalence ratio have combined influence on both unstretched laminar burning velocity and flame instability. Meanwhile, according to flame propagation pictures taken by the high speed camera, flame stability decreases with the increase of initial pressures; for given equivalence ratio and hydrogen fraction, flame thickness is more sensitive to the variation of the initial pressure than to that of the density ratio.     

Effects of hydrogen concentration on premixed laminar flames of hydrogen–methane–air

The unstretched laminar burning velocities and Markstein numbers of spherically propagating hydrogen–methane–air flames were studied at a mixture pressure of 0.10 MPa and a mixture temperature of 350 K. The fraction of hydrogen in the binary fuel was varied from 0 to 1.0 at equivalence ratios of 0.8, 1.0 and 1.2. The unstretched laminar burning velocity increased non-linearly with hydrogen fraction for all the equivalence ratios. The Markstein number varied non-monotonically at equivalence ratios of 0.8 and 1.0 and increased monotonically at equivalence ratio of 1.2 with increasing hydrogen fraction. Analytical evaluation of the Markstein number suggested that the trends could be due to the effective Lewis number, which varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0 and increased monotonically at 1.2. The propensity of flame instability varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0.     

Effects of hydrogen addition on the premixed laminar-flames of ethanol–air gaseous mixtures: An experimental study

The effects of hydrogen addition on the laminar premixed-flame characteristics of ethanol–air gaseous mixtures were investigated experimentally by using outwardly propagating spherical flames. The experiments were conducted in a constant-volume combustion vessel with a central ignition at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 100%, and an equivalence ratio from 0.6 to 1.6, and the flame images were obtained by a high-speed schlieren camera system. The results show that the unstretched flame propagation speeds and burning velocities increase exponentially with the increase in hydrogen fraction for a constant equivalence ratio. When the hydrogen fraction is equal to or less than 60%, the burned gas Markstein length reduces with the increase of equivalence ratio, indicating a positive correlation between the flame instability and hydrogen fraction, while the opposite effect is observed when the hydrogen fraction is greater than 60%. At an equivalence ratio below 1.4, the Markstein length decreases with increased hydrogen fraction, indicating that the flame instability is exacerbated with hydrogen addition, while the reverse holds in the case of equivalence ratio above 1.4. Finally, an empirical formula is developed to estimate the laminar burning velocity of ethanol–hydrogen–air flames on the basis of present experimental data.     

Effects of hydrogen addition and nitrogen dilution on the laminar flame characteristics of premixed methane–air flames

The effect of hydrogen addition and nitrogen dilution on laminar flame characteristics was investigated. The spherical expanding flame technique, in a constant volume bomb, was employed to extract laminar flame characteristics. The mole fraction of hydrogen in the methane–hydrogen mixture was varied from 0 to 1 and the mole fraction of nitrogen in the total mixture (methane–hydrogen–air–diluent) from 0 to 0.35. Measurements were performed at an initial pressure of 0.1 MPa and an initial temperature of 300 K. The mixtures investigated were under stoichiometric conditions. Based on experimental measurements, a new correlation for calculating the laminar burning velocity of methane–hydrogen–air–nitrogen mixtures is proposed. The laminar burning velocity was found to increase linearly with hydrogen mass fraction for all dilution ratios while the burned gas Markstein length decreases with the increase in hydrogen amount in the mixture except for high hydrogen mole fractions (>0.6). Nitrogen dilution has a nonlinear reducing effect on the laminar burning velocity and an increasing effect on the burned gas Markstein length. The experimental results and the proposed correlation obtained are in good agreement with literature values.     

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.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.     

天然气-氢气-空气混合气的层流燃烧速度测定

在定容燃烧弹内研究了常温常压下天然气-氢气-空气混合气的火焰传播规律,得到了不同掺氢比例（氢气在天然气中的体积掺混比例为0％～100％）和燃空当量比（0．6～1．4）下混合气的层流燃烧速率和马克斯坦长度,通过对马克斯坦长度的测量,分析了拉伸对火焰传播的影响。结果表明,随着天然气中掺氢比例的增加,混合气的燃烧速率呈指数规律增加,马克斯坦长度值减小,火焰的稳定性下降。各掺氢比例下,随当量比的增加,马克斯坦长度值增加,火焰的稳定性增强。通过对试验结果的数据拟合,得到了计算天然气-氢气-空气混合气层流燃烧速率的关系式。     

稀释气对掺氢天然气层流预混燃烧燃烧速率的影响

在定容燃烧弹内研究了初始压力为0.1 MPa、初始温度为285 K时掺氢天然气-空气-稀释气体混合气层流预混燃烧的火焰传播规律,获得了分别使用氮气和二氧化碳作为稀释气时,在不同掺氢比、稀释度和当量比下的无拉伸层流燃烧速率,并分析了氮气和二氧化碳稀释气对燃烧速率的影响.结果表明:无拉伸层流燃烧速率随着掺氢比的增加而增加,随着稀释度的增加而减小,二氧化碳对燃烧速率的作用强于氮气.     

Measurements of laminar burning velocities and onset of cellular instabilities of methane–hydrogen–air flames at elevated pressures and temperatures

An experimental study on laminar burning velocities and onset of cellular instabilities of the premixed methane–hydrogen–air flames was conducted in a constant volume combustion vessel at elevated pressures and temperatures. The unstretched laminar burning velocity and Markstein length were obtained over a wide range of hydrogen fractions. Besides, the effects of hydrogen addition, initial pressure and initial temperature on flame instabilities were analyzed. The results show that the unstretched flame propagation speed and the unstretched laminar burning velocity are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. Early onset of cellular instability is presented and the critical radius and Markstein length are decreased with the increase of initial pressure, indicating the increase of hydrodynamic instability with the increase of initial pressure. Flame instability is insensitive to initial temperature compared to initial pressure. With the increase of hydrogen fraction, significant decrease in critical radius and Markstein length is presented, indicating the increase in both diffusional-thermal and hydrodynamic instabilities as hydrogen fraction is increased.     

Study of laminar combustion characteristics of gasoline surrogate fuel-hydrogen-air premixed flames

This paper investigated the effects of hydrogen addition to gasoline surrogates fuel-air mixture on the premixed spherical flame laminar combustion characteristics. The experiments were carried out by high speed Schlieren photography on a constant-volume combustion vessel. Combining with nonlinear fitting technique, the variation of flame propagation speed, laminar burning velocity, Markstein length, flame thickness, thermal expansion coefficient and mass burning flux were studied at various equivalence ratios (0.8–1.4) and hydrogen mixing ratios (0%–50%). The results suggested that the nonlinear fitting method had a better agreement with the experimental data in this paper and the flame propagation was strongly effected by stretch at low equivalence ratios. The stretched propagation speed increased with the increase of hydrogen fraction at the same equivalence ratio. For a given hydrogen fraction, Markstein length decreased with the increase of equivalence ratio; flame propagation speed and laminar burning velocity first increased and then decreased with the increase of equivalence ratio while the peaks of the burning velocity shifted toward the richer side with the increase of hydrogen fraction.     

Laminar burning velocities,Markstein lengths,and flame thickness of liquefied petroleum gas with hydrogen enrichment

In this paper, experimental data of laminar burning velocity, Markstein length, and flame thickness of LPG flames with various percentages of hydrogen (H₂) enrichments have been presented. The experiments were conducted under the conditions of 0.1 MPa, 300 K in a constant volume chamber. The tested equivalence ratios of air/fuel mixture range from 0.6 to 1.5, and the examined LPG contains 10%–90% of hydrogen in volume. Experimental results show that hydrogen addition significantly increase the laminar burning velocity of LPG, and the accelerating effectiveness is substantial when the percentage of hydrogen is larger than 60%. Effect of hydrogen addition on diffusion thermal instability, as indicated by Markstein length, was analyzed at various equivalence ratios. Hydrogen addition decreases the flame thickness. Equivalence ratio has more dominating effect on flame thickness than hydrogen does. For the fuel with 10% LPG and 90% hydrogen, the flame thickness values are close for all equivalence ratios.     

天然气-氢气-空气混合气火焰传播特性研究

在定容燃烧弹内研究了初始条件为常温常压的灭然气-氢气-空气混合气火焰传播规律,得到了不同掺氢比例和燃空当量比下混合气的层流燃烧速率、质量燃烧流量和马克斯坦长度,结合火焰传播照片,分析了火焰的稳定性并预测了大尺寸火焰稳定性的演变趋势。研究结果表明,随着天然气中掺氢比例的增加,混合气的燃烧速率增加,且增长速率逐渐加快,马克斯坦长度值减小,火焰的稳定性下降。各种掺氢比例下,随当量比的增加,马克斯坦长度值增加,火焰的稳定性增加。掺氢比例高于80％时,随着火焰的传播,其不稳定性将明显增加。     

高温高压下掺氢天然气的燃烧特性

在定容燃烧弹内研究了高温(450,K)、高压(0.75,MPa)条件下天然气-氢气-空气混合气的火焰传播过程,获得了不同掺氢比和不同当量比下掺氢天然气的无拉伸层流燃烧速率,并分析了火焰的稳定性。结果表明,高温高压下随着掺氢比的增加,掺氢天然气的燃烧速率增加,且增长速率逐渐加快;马克斯坦长度则随着掺氢比的增加而减小,即火焰的稳定性下降。随着当量比的增加,无拉伸层流燃烧速率呈现先增大后减小的趋势,且最大无拉伸层流燃烧速率所对应当量比的位置随着掺氢比的提高而向浓混合气移动;马克斯坦长度随当量比的增加而增大,即火焰稳定性随当量比的增加而提高。     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Experimental and kinetic study of laminar flame characteristics of H2/O2/diluent flame under elevated pressure

Laminar burning velocity, Markstein length, and critical flame radius of an H₂/O₂ flame with different diluents, He, Ar, N₂ and CO₂, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of R_cr. It can be found that Le_eff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N_2, and CO₂ diluted mixture. Interestingly, the CO₂ diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Le_eff of CO₂ diluted mixture increased rapidly with the equivalence ratio.     

异丁醇/辛酸甲酯混合燃料预混层流火焰速度的研究

针对生物柴油与醇类混合燃料燃烧机理研究的需求,采用高速纹影光学诊断方法和定容燃烧弹系统试验研究了异丁醇/辛酸甲酯混合燃料的预混层流燃烧特性。测量了不同当量比和初始压力条件下的不同配比混合燃料—空气预混合气的层流燃烧火焰速度,火焰拉伸率以及马克斯坦长度。分析了燃烧初始条件及异丁醇掺混比例对混合燃料的无拉伸层流燃烧速度及火焰不稳定性的影响规律。结果表明:异丁醇/辛酸甲酯混合燃料的拉伸层流火焰传播速度和层流火焰燃烧速度随着当量比的增加先增加后减少,随着初始压力的增加而减小;马克斯坦长度随着当量比和初始压力的增加而减小;异丁醇掺混比例的增加加快了层流火焰燃烧速度,但使得火焰的不稳定性倾向增加。