不同初始压力下掺氢天然气的燃烧特性

在定容燃烧弹内研究了不同初始压力下天然气-氢气-空气混合气的火焰传播规律,得到了不同掺氢比例和初始压力下,不同燃空当量比时混合气的层流燃烧速率,并分析了火焰的稳定性及其影响因素.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,而马克斯坦长度值则随着掺氢比例的增加而减小,即火焰的稳定性下降.不同初始压力下,随着燃空当量比的增加,马克斯坦长度值在不同掺氢比例下均增加,显示火焰的稳定性增加.无拉伸层流燃烧速率随着初始压力的增加略有减小,且在化学当量比附近,变化的初始压力和掺氢比对无拉伸层流燃烧速率的影响最为明显.     

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。     

初始压力对天然气-氢气-空气混合气火焰传播特性的影响

使用定容燃烧弹研究了不同初始压力下天然气-氢气-空气混合气的火焰传播规律,得到了初始压力、掺氢比和燃空当量比对无拉伸层流燃烧速率、质量燃烧流量的影响,结合高速纹影图片分析了影响火焰稳定性的因素(马克斯坦长度、火焰面两侧密度比和火焰厚度).结果表明,掺氢天然气无拉伸层流燃烧速率以及火焰的不稳定性受掺氢比、初始压力和燃空当量比的综合影响.结合高速纹影图片,得出火焰的稳定性会随初始压力的增加而减小;在相同的燃空当量比和掺氢比下,初始压力对密度比的影响不大,但是对火焰厚度的影响比较明显.     

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。     

二甲醚／空气预混合气燃烧特性

利用定容燃烧弹研究了不同当量比和初始压力下的二甲醚/空气预混合气的燃烧特性,并基于准维双区模型计算了二甲醚/空气预混合气燃烧特征参数.研究结果表明:在各初始压力条件下,化学计量比附近的混合气压力升高率和混合气质量燃烧速率最大.对于稀混合气和当量比混合气,火焰传播速率随初始压力的降低而增加;而对于浓混合气,火焰传播速率随初始压力的降低而降低.最高燃烧压力出现在化学计量比附近而与初始压力无关.对于给定的当量比,最高燃烧压力随初始压力的增加而明显增加.在化学计量比附近,燃烧持续期和火焰发展期最短且基本上不随初始压力变化.     

二甲醚-空气混合气层流燃烧速度的测定

在定容燃烧弹中利用高速纹影摄像法系统地研究了不同燃空当量比和初始压力下二甲醚-空气混合气的层流燃烧特性.利用球形扩散火焰理论分析纹影照片,获得了不同初始压力和当量比下的二甲醚-空气混合气层流燃烧速率.结果表明：随着初始压力的增大,层流燃烧速率显著减小,层流燃烧速率的峰值向浓混合气侧偏移.拉伸层流燃烧速率随拉伸率的增加而增加,拉伸层流质量燃烧速率随拉伸率的增加而减小.根据球形扩散火焰模型得到混合气的马克斯坦长度值表明：在各初始压力下,随着当量比的增加,二甲醚-空气混合气的马克斯坦长度值逐渐减小,火焰前锋面的不稳定性增加.     

EGR中二氧化碳对氢气层流燃烧特性的影响

为了更深入地理解废气中二氧化碳对掺氢燃料燃烧特性的影响,在定容燃烧弹中利用高速摄像系统研究了不同燃空当量比φ(0.6～1.4)和稀释比(0%～40%)下CO2稀释氢气-空气混合气的层流燃烧特性.结果表明:氢气-空气混合气的火焰传播速率随着燃空当量比的增大而增大;马克斯坦长度随着当量比的增大而增大,即火焰的稳定性增强;随稀释比的增大,无拉伸火焰传播速率S1明显减小;同时得到层流火焰燃烧速率,并分析了稀释比对火焰稳定性的影响.通过对试验结果数据拟合,获得了计算氢气-CO2-空气混合气的无拉伸层流燃烧速率的拟合多项式.     

利用定容燃烧弹研究天然气掺氢混合燃料直喷燃烧循环变动

利用定容燃烧弹开展了天然气掺混0%～40%氢气混合燃料直喷燃烧循环变动研究,高压气体燃料(8.0 MPa)喷入定容燃烧弹模拟直喷发动机燃烧条件.在整体当量比为0.6和0.8下,试验采集了火焰发展图片和燃烧过程容弹内压力,从火焰发展图片和燃烧特征参数两个方面分析了掺氢和混合气分层分布对天然气直喷燃烧循环变动的影响.结果表明:燃烧循环变动起始于火焰发展初期阶段.随着掺氢比增加,火焰形态更规则且更集中于点火电极.同时,由于直喷燃烧方式混合气分层分布,能够实现低循环变动的稳定稀燃.循环变动随着掺氢比的增加而减小,这种趋势在稀燃工况((b=0.6)下更加明显.在直喷燃烧方式下,由于混合气分层分布减弱了火焰发展初期阶段对后续燃烧过程的影响,因此燃烧特征参数间呈现相互独立的关系.     

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。     

二乙醚-空气预混合气层流燃烧速率测定

在定容燃烧弹内利用高速纹影摄像法系统地研究了不同初始压力、不同初始温度和不同燃空当量比下二乙醚-空气预混合气的层流燃烧特性。利用球形发展火焰分析得到了不同初始压力、不同初始温度和不同燃空当量比下二乙醚-空气预混合气的无拉伸层流火焰燃烧速率、马克斯坦长度等层流燃烧参数。研究结果表明:无拉伸层流火焰燃烧速率随初始温度的增加而增加,随初始压力的增加而降低;马克斯坦长度随着初始温度的增加而减小,随初始压力的增加而减小,随当量比的增加而减小,表明火焰前锋面不稳定性随初始温度和初始压力的增加而增加,随混合气浓度的增加而增加。基于试验数据获得了二乙醚-空气预混合气无拉伸层流燃烧速率的关系式。     

The effect of equivalence ratio,temperature and pressure on the combustion characteristics of hydrogen-air pre-mixture with turbulent jet induced by pre-chamber sparkplug

The pre-chamber sparkplug mode can increase the combustion velocity because it can induce the turbulent jet into the cylinder. Higher combustion velocity can increase the brake thermal efficiency and decrease the knock tendency for hydrogen engines. To explore the effect of pre-chamber sparkplug mode on the combustion characteristics of the hydrogen-air mixture, different equivalence ratios, initial pressures and temperatures were selected to study in a constant volume combustion chamber working with pre-chamber sparkplug mode and normal sparkplug mode. The results showed that the pre-chamber sparkplug mode can accelerate the combustion velocity, increase maximum combustion pressure and decrease the combustion duration at all initial conditions. The maximum combustion pressure of pre-chamber sparkplug mode occurred at the equivalence ratio of 1.0 while it occurred at the equivalence ratio of 1.2 with normal sparkplug mode, which means pre-chamber sparkplug mode can increase the higher brake thermal efficiency and power. The combustion intensity of pre-chamber sparkplug mode was bigger than 1 and the biggest value occurred at the equivalence ratio of 0.6. Moreover, the combustion intensity of pre-chamber sparkplug mode was higher with lean equivalence ratios than that of rich equivalence ratios. Increasing the initial pressure can increase maximum combustion pressure and combustion velocity obviously for pre-chamber sparkplug mode, which was different from the normal sparkplug mode. The initial temperatures had little impact on the combustion intensity. These results showed the pre-chamber sparkplug mode was more suitable to be used in the boosting hydrogen engines to improve the performance.     

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.     

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.     

Effect of partially premixed and hydrogen addition on natural gas direct-injection lean combustion

Effect of partially premixed mixture and hydrogen addition on natural gas direct-injection lean combustion was studied experimentally using a constant volume vessel. Flame propagating photos and pressure derived combustion parameters were analysed at different premixed ratios (from 0% to 80%) and hydrogen fractions (from 0% to 40%) at overall equivalence ratio of 0.6, 0.8 and 1.0, respectively. The results show that the flame kernel is concentrated to the spark position with the increase of premixed ratio and/or hydrogen fraction. Flame propagating speed is decreased with the increase of premixed ratio while it increases as hydrogen is added to natural gas. Hydrogen addition has little effect on the partially direct-injection natural gas combustion at the stoichiometric fuel-air mixture condition and all premixed ratios. However, hydrogen addition significantly enhances the combustion rate of natural gas direct-injection combustion at lean mixture condition. Both the initial and main combustion durations are increased with the increase of premixed ratio, while they show the decreasing trend as hydrogen is added to natural gas at the lean mixture condition. Partially premixed direct-injection combustion combining with hydrogen addition can achieve the stable spark ignition and fast combustion at the lean mixture condition.     

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.     

Combustion characteristics of methanol–air and methanol–air–diluent premixed mixtures at elevated temperatures and pressures

Combustion characteristics of the methanol–air premixed mixtures were studied in a constant volume bomb at different equivalence ratios, initial pressures and temperatures, and dilution ratios. The results show that the combustion pressure, the mass burning rate and the burned gas temperature get the maximum value at the equivalence ratio of 1.1 while the flame development duration and the combustion duration get the minimum value at the equivalence ratio of 1.1. The flame development duration, the combustion duration and the peak combustion pressure decrease with the increase of the initial temperature, while the maximum burned gas temperature increases with the increase of the initial temperature. The peak combustion pressure and temperature increase with the increase of the initial pressure. The flame development duration and combustion duration increase with the increase of the dilution ratio, while the peak combustion pressure and temperature decrease with the increase of the dilution ratio.     

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.     

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.     

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.     

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.