高压下氢气-乙醇球形膨胀火焰的层流燃烧速度和火焰不稳定性研究

在初始温度为400 K、不同的初始压力（0.1 MPa、0.4 MPa）、氢气比例（70%、80%）和当量比（0.7～1.4）条件下进行氢气-乙醇预混燃烧实验,使用高速纹影技术记录火焰传播图像。对氢气-乙醇球形膨胀火焰中的层流燃烧速度（LBV）进行实验研究,发现LBV随着氢气比例的增加而增加,压力升高却有着负影响。对火焰发展不同阶段的火焰形貌进行了研究。当火焰表面的大裂纹分裂出现小裂纹并且导致新细胞再生时,火焰变得不稳定。通过热膨胀比、火焰厚度和刘易斯数等参数考察了流体动力学效应和热扩散效应对火焰固有不稳定性的影响。结果表明,流体动力不稳定性随着压力的增加而增加,热扩散不稳定性对压力变化的敏感性较低。此外,增加氢气比例或初始压力会导致火焰更早遭受不稳定。     

Experimental study on self-acceleration characteristics of unstable flame of low calorific value gas blended with hydrogen

The self-acceleration characteristics of cellular flame of low calorific value (LCV) gas in a constant volume combustion bomb were studied, the propagation process of spherical flame with different hydrogen (H₂) addition and initial pressure was analyzed, and the flame radius versus time was also discussed. The experimental results show that the self-acceleration of the cellular flame of LCV gas blended with hydrogen appears at high pressure, high hydrogen ratio and lean burn. The acceleration index increases with the increase of hydrogen addition and the reduction of equivalence ratio, and increases with the increase of initial pressure, but the acceleration index does not infinitely increase with the increase of initial pressure. With the increase of the hydrogen addition and the reduction of the equivalence ratio, the critical radius of the cellular flame decreases, which is shown that increase of the hydrogen addition and the lean burn will make appearance of cellular flame in advance. When the ratio of hydrogen is less than 60%, the critical Peclet number decreases with increase of hydrogen addition, when the ratio of hydrogen continues to increase to 80%, it increases slightly. The research in this paper provides an experimental basis for the in-depth study of engine combustion of LCV gas blended with hydrogen.     

Study on laminar combustion characteristic of low calorific value gas blended with hydrogen in a constant volume combustion bomb

In this paper, the laminar combustion velocity of low calorific value gases blended with hydrogen was experimentally studied in the constant volume combustion bomb. An experimental system of constant volume combustion bomb was set up, and the variation trend of laminar flame velocity of low calorific value blended hydrogen was analyzed under different initial conditions. The experimental results show that increasing the initial pressure will reduce the laminar combustion velocity of the flame, while increasing the initial temperature will increase the laminar combustion velocity of the flame. With the increase of hydrogen ratio, the laminar combustion velocity of flame and instability of flame increases. The influence of equivalence ratio on laminar combustion velocity of flame is quite complex, generally, the burning speed of the rich mixture is greater than that of the lean mixture. This research can provide experimental basis for the design and development of low calorific value gas blended with hydrogen engine.     

Parameter extraction from spherically expanding flames propagated in hydrogen/air mixtures

The characteristics of hydrogen/air flame were studied by using the spherical expanding flame propagated in a constant volume chamber. The influence of ignition induced blast wave and the flame instability on flame propagation was investigated. The nonlinear evaluation method for laminar flame parameter evaluation was established. By using the nonlinear evaluation method and the experimental results of flame propagation, the laminar flame speed and Markstein length were extracted and the difference between the nonlinearly evaluated laminar flame speed and the linearly evaluated one was analyzed. The influence of initial pressure and equivalence ratio on laminar flame speed and flame thickness was investigated. The laminar flame speed varies with equivalence ratio and initial pressure. There exists an equivalence ratio at which the laminar flame speed gets its maximum value. And there also exists an initial pressure at which the laminar flame speed gets its maximum value. The critical radius, Markstein length and flame instability of hydrogen/air flame with different equivalence ratio at different initial pressure had been studied. In hydrogen/air flame the flame stability decreases with the increase of initial pressure, while it increases with the increase of equivalence ratio. The global stability of flame is determined by the combination of the stabilizing effect of stretch effect, thermodiffusive instability mechanism and hydrodynamic instability mechanism.     

Investigations on the cellular instabilities of expanding hydrogen/methanol spherical flame

A comprehensive measurement and investigation of the cellularization of methanol/hydrogen flame is important for the thorough understanding of the transition of turbulent flame. In this work, a constant volume combustion bomb with schlieren photography technology is used to study the flame evolution of methanol/hydrogen fuel. By investigating the flames smooth laminar flame to a certain degree of cellular flame, the effect of hydrogen addition on the cellular instability of the hydrogen/methanol spherical flame is revealed. The experiments were conducted with different hydrogen mixing ratios (0%–80%) at different equivalence ratios (0.8–1.5) under a series of initial temperature (375 K–450 K) and pressure (1.0 bar–3.0 bar). The results showed that the process of flame cellular instability advanced in general as the hydrogen mixing ratio increased. The promoting effect of hydrogen addition was more significant in lean flames. The cellularization in lean flames was dominated by the instability of thermal diffusion, while that in the rich flames was dominated by the hydrodynamic instability. The initial pressure impacts the flame cellar instability mainly through the hydrodynamic instability.     

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.     

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。     

Experimental study on premixed combustion of spherically propagating methanol-air-nitrogen flames

The outward propagation and development of surface instability of the spark-ignited spherical premixed flames for methanol-air-nitrogen mixtures were experimentally studied by using a constant volume combustion chamber and a high-speed schlieren photography system. The laminar burning velocities, the mass burning fluxes, and the Markstein lengths were obtained at different equivalence ratios, dilution ratios, initial temperatures, and pressures. The laminar burning velocities and the mass burning fluxes give a similar curve versus the equivalence ratios. They increase with the increase of initial temperature and decrease with the increase of dilution ratio. The laminar burning velocity decreases with elevating the initial pressure, while the mass burning flux increases with the increase of the initial pressure. Markstein length decreases slightly with the increase of initial temperature for the rich mixtures. High initial pressure corresponds to low Markstein length. Markstein length increases with the increase of dilution ratio, which is more obvious when the mixture becomes leaner. Equivalence ratio has a slight impact on the development of the diffusive-thermal cellular structure at elevated initial pressures. The initial pressure has a significant influence on the occurrence of the flame front cellular structure. At the elevated pressures, the cracks on the flame surface branch and develop into the cell structure. These cells are bounded by cracks emitting a bright light, which may indicate soot formation. For very lean mixture combustion, the buoyancy effect and cooling effect from the spark electrodes have a significant impact on the flame propagation. The hydrodynamic instability, inhibited with the increase of initial temperature around the stoichiometric equivalence ratio, is enhanced with the increase of initial pressure and suppressed by mixture dilution.     

Experimental study on premixed combustion of spherically propagating methanol-air-nitrogen flames

The outward propagation and development of surface instability of the spark-ignited spherical premixed flames for methanol-air-nitrogen mixtures were experimentally studied by using a constant volume combustion chamber and a high-speed schlieren photography system. The laminar burning velocities, the mass burning fluxes, and the Markstein lengths were obtained at different equivalence ratios, dilution ratios, initial temperatures, and pressures. The laminar burning velocities and the mass burning fluxes give a similar curve versus the equivalence ratios. They increase with the increase of initial temperature and decrease with the increase of dilution ratio. The laminar burning velocity decreases with elevating the initial pressure, while the mass burning flux increases with the increase of the initial pressure. Markstein length decreases slightly with the increase of initial temperature for the rich mixtures. High initial pressure corresponds to low Markstein length. Markstein length increases with the increase of dilution ratio, which is more obvious when the mixture becomes leaner. Equivalence ratio has a slight impact on the development of the diffusive-thermal cellular structure at elevated initial pressures. The initial pressure has a significant influence on the occurrence of the flame front cellular structure. At the elevated pressures, the cracks on the flame surface branch and develop into the cell structure. These cells are bounded by cracks emitting a bright light, which may indicate soot formation. For very lean mixture combustion, the buoyancy effect and cooling effect from the spark electrodes have a significant impact on the flame propagation. The hydrodynamic instability, inhibited with the increase of initial temperature around the stoichiometric equivalence ratio, is enhanced with the increase of initial pressure and suppressed by mixture dilution.     

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.     

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。     

Effect of initial pressure,temperature and equivalence ratios on laminar combustion characteristics of hydrogen enriched natural gas

In this study, the flame propagation characteristics of premixed natural gas–hydrogen–air mixtures were studied in constant volume combustion bomb by using the high-speed schlieren photography system. The flame radius, laminar flame propagation speed and the flame stretch rate were obtained under different initial pressure, temperature, equivalence ratios and hydrogen fractions. Meanwhile, the flame stability and their influencing factors were obtained by analyzing the Markstein length and the flame propagation schlieren photos under various combustion conditions. The results show that the stretched laminar propagation speed increases with the increase of the initial temperature and hydrogen fraction of the mixture, and will decreases with the increase of the initial pressure. Meanwhile, according to the Markstein length and the flame propagation pictures, the flame stability decreases with the increase of the temperature and hydrogen fraction, and the slight flaws occurred at the early stage; at larger flame radius, the flame stability is more sensitive to the variation of the initial temperature and hydrogen fraction than to that of initial pressure and equivalence ratio.     

乙醇-氢气-空气混合物层流燃烧特性研究

氢气是一种高效的添加剂,可以改善生物质燃料的层流燃烧特性。为研究氢气对乙醇-空气预混层流火焰燃烧特性的影响,利用定容燃烧弹结合高速纹影摄像技术,系统研究了初始温度为400 K,初始压力为0.1 MPa和0.4 MPa,氢气含量为0%、10%、30%、50%、70%和90%,当量比为0.7 ~ 1.4时的氢气-乙醇-空气混合燃料的层流燃烧速度（LBV）、火焰厚度和马克斯坦长度等参数,并采用辐射校正公式使LBV更加精准。通过数值仿真构建预混火焰模型,与实验结果进行对比。结果表明,氢气比例的增加可以提高混合燃料的层流燃烧速度。当氢气比例小于50%时,LBV随氢气比例的增加线性增长。而当氢气比例大于50%,LBV随氢气的增加呈指数增长。初始压力的上升虽然降低了LBV,但提高了LBV的增长率。此外,随着氢气比例和初始压力的增加,火焰厚度减小,马克斯坦长度降低,火焰的不稳定性增强。     

Inerting effect of carbon dioxide on confined hydrogen explosion

Taking maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse as index, this paper is aimed at evaluating the inerting effects of carbon dioxide on confined hydrogen explosion by varying initial pressure, carbon dioxide addition and equivalence ratio. The results indicated that under enhancing hydrodynamic instability, the stronger flame destabilization occurs with the increase of initial pressure. At Φ = 0.8 and Φ = 1.0, the destabilization effect of thermodiffusive instability continues to increase with the increase of carbon dioxide addition. At all equivalence ratios, the destabilization effect of hydrodynamic instability decreases monotonously with the increase of carbon dioxide addition. All of maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse reach the peak value at Φ = 1.5, and decrease significantly with increasing carbon dioxide addition. The inerting effect of carbon dioxide could be attributed to the reduction of thermal diffusivity, flame temperature and active radicals. The chemical effect of carbon dioxide reaches the peak value at Φ = 1.0. With the increase of carbon dioxide addition, the chemical effect continues to decrease at Φ = 0.8 and Φ = 1.0, and increase monotonously at Φ = 2.5.     

Experimental investigation on the onset of cellular instabilities and acceleration of expanding spherical flames

We experimentally investigated the cellular instabilities of expanding spherical propagation of hydrogen–air, methane–air, and propane–air flames. Using image-thresholding technique, the formations and developments of a cell on a flame surface were investigated. The size of the observed cell due to the hydrodynamic instability was larger than those generated by the diffusional–thermal instability. The critical flame radius and critical Peclet number for the onset of instability were evaluated. These critical values for hydrogen–air and methane–air flames increased with increasing concentration. The values decreased with increasing initial pressure because the flame thickness decreased with increasing initial pressure. The ratio of the increase in the burning velocity increased with increasing initial pressure, although that of the hydrogen–air flames only increased with decreasing concentration. The results demonstrated that acceleration of the flame speed is affected by the intensity of the diffusional–thermal and hydrodynamic instabilities.     

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.     

沼气掺氢发动机燃烧稳定性试验

在一台单缸火花点火发动机上开展了燃用不同组分配比的沼气模拟气体的掺氢混合气的燃烧稳定性试验研究.研究结果表明:在15%～35%的掺氢比范嗣内,随着混合气中掺氢比的增加,发动机循环变动变小,燃烧稳定性提高.掺氢导致平均指示压力的循环变动系数减小,燃烧放热率加快,火焰发展期缩短.其中35%掺氢比的混合气比15%掺氢比的混合...     

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。     

Effect of hydrogen enrichment and electric field on lean CH4/air flame propagation at elevated pressure

Electric assisted combustion for hydrogen enriched hydrocarbons may even extend the lean burn limit and provide the further improvement on combustion stability. This study investigates the effect of hydrogen enrichment and DC electric field on lean CH₄/air flame propagation. Electric field inside the chamber was generated by mesh and needle electrodes. Effect of hydrogen enrichment on the ion mole fraction in the flame was discussed based on reaction mechanism included neutral and ion reactions. The flame propagation images, flame displacement speed were used to evaluate the combined influences of hydrogen enrichment and electric field on propagating flame. Results showed that the hydrogen addition would increase positive ions mole fraction and the peak value is mainly determined by H₃O⁺. This would be due to that CH increases with hydrogen fraction, which is the main species in the initial reaction for the ion reactions. Electric field effect about flame propagation was suppressed with hydrogen addition due to the competition between the increment in ion mole fraction and the decrement in flame time. Electric assisted combustion is more evident at leaner conditions and elevated pressure. The ratio of ionic wind velocity to flow velocity may be the determined factor to predict the electric field effect about propagating flame. The tendency based on this ratio is in accordance with the experimental results for various hydrogen fraction and equivalence ratio at elevated pressure.     

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.