阻塞比对氢-空预混火焰加速影响的可视化研究

在顶置点火定容燃烧弹内布置网孔板,通过火焰传播路径上的孔板诱导实现火焰加速。利用纹影法和压力采集系统,研究了阻塞比对氢-空预混气孔板诱导火焰加速的影响规律。试验结果表明:阻塞比的增加可增大孔板对火焰的扰动,使火焰传播速度大幅增加;任意初始工况下均存在一个最佳阻塞比使燃烧持续期达到最短,在试验范围内,较低初始压力、较小当量比和较高初始温度的最佳阻塞比为0.90,其余工况的最佳阻塞比均在0.84;孔板诱导燃烧加速的效果非常显著,在试验工况范围内,各阻塞比孔板诱导下的燃烧持续期均缩短45%以上。     

初始压力对氢-空气预混诱导湍流燃烧的影响

在顶置点火定容燃烧弹内加入一个网孔板,采用纹影法和压力采集系统,研究不同初始压力下氢气-空气预混合气穿过网孔时的火焰传播特性以及压力特性.结果表明:火焰通过网孔板时产生射流,网孔间火焰相互干涉,火焰前锋面积增加,诱导成湍流燃烧;相对于未添加网孔板而言,最大燃烧压力值几乎不变,达到最大压力的时刻提前,燃烧持续期缩短;火焰穿过网孔板后,传播速度提高,压力升高率增加;随着初始压力的增加,添加网孔板后的燃烧持续期变化率先增大后减小,在初始压力为0.10,MPa时达到最大值.     

湍流条件下高当量比合成气着火实验研究

为了研究高当量比合成气着火极限条件下初始湍流环境对火焰初始发展阶段的影响规律,在湍流定容燃烧实验装置中开展了常温、常压条件下50%CO/50%H2合成气相关湍流燃烧实验,并研究了火焰等效半径和火焰传播速度变化的规律及影响因素.研究结果表明:在本文实验条件下层流环境中,火焰传播速度呈现先减小后增加的趋势,并且随着当量比的增加而逐渐减小,合成气着火极限当量比为5.8;在高当量比混合气条件下,初始湍流强度的增加可以拓宽可燃混合气的着火极限;在高当量比着火极限条件下,火焰等效半径随着湍流强度增大而增加,火焰传播速度随着湍流强度的增加而增大,同一湍流强度环境中,火焰传播速度总体呈现出先减小后增大的趋势.     

用光学纹影摄影术观察分析定容燃烧室内氢燃料的燃烧过程

论述了采用纹影摄影术和高速摄影法观察分析氢气和空气预混合燃料在定容燃烧室内的火花点火燃烧过程,定性地分析了预混合氢气燃料的火焰形态和变化过程,以及燃烧室内的初始压力和空燃比对火焰传播速度及其燃烧压力的影响,通过采用纹影摄影术方法,初步揭示了预混合氢气燃料在定容燃烧室内燃烧时火焰初期紊流产生的机理,以及由开始的层流状火焰发展到湍流状火焰的过程,研究结果表明,预混合氢气燃料燃烧的火焰传播速度及燃烧压力明显地受燃烧室内的初始压力和空燃比的影响。     

电增压及EGR共同作用下对降低汽油机燃油消耗率的试验研究

基于光学定容燃烧弹试验平台,通过高速纹影摄像系统在相同甲烷燃料初始温度、压力及混合气浓度下,定量分析了不同结构预燃室湍流射流点火（turbulent jet ignition, TJI）的燃烧特性,包括火焰传播速度、火焰面积、火焰形态及燃烧压力等参数。研究结果表明,预燃室孔径越小,相同时间内火焰传播得越远,火焰传播速度和火焰面积增长速度越快,燃烧压力峰值越高。随着预燃室孔径减小,着火机理会由射流中带有火焰的火焰点火转变为火焰过孔时熄灭的喷射点火。喷射点火着火时刻延迟,初始火焰速度减慢,但燃烧压力峰值受影响不大。多级加速预燃室压力升高率与压力峰值与单孔预燃室相比变化不大。虽然火焰出口时速度较慢,但是火焰出口时刻提前且速度衰减较弱,因此多级加速预燃室火焰速度在短时间内超过单孔预燃室,并且压力和火焰面积也更早达到最大值。     

不同预燃室结构对甲烷湍流射流点火燃烧的影响

基于光学定容燃烧弹试验平台,通过高速纹影摄像系统在相同甲烷燃料初始温度、压力及混合气浓度下,定量分析了不同结构预燃室湍流射流点火(turbulent jet ignition,TJI)的燃烧特性,包括火焰传播速度、火焰面积、火焰形态及燃烧压力等参数。研究结果表明,预燃室孔径越小,相同时间内火焰传播得越远,火焰传播速度和火焰面积增长速度越快,燃烧压力峰值越高。随着预燃室孔径减小,着火机理会由射流中带有火焰的火焰点火转变为火焰过孔时熄灭的喷射点火。喷射点火着火时刻延迟,初始火焰速度减慢,但燃烧压力峰值受影响不大。多级加速预燃室压力升高率与压力峰值与单孔预燃室相比变化不大。虽然火焰出口时速度较慢,但是火焰出口时刻提前且速度衰减较弱,因此多级加速预燃室火焰速度在短时间内超过单孔预燃室,并且压力和火焰面积也更早达到最大值。     

1-戊烯层流火焰传播速度的测量

1-戊烯是国产93号汽油的重要烯烃组成成分,通过定容弹燃烧实验系统测量了1-戊烯在初始压力0.1,MPa和0.3,MPa,初始温度350,K和450,K、当量比从0.5~1.6的层流火焰传播速度,结果表明初始压力的升高对1-戊烯的层流火焰传播速度有抑制作用;而初始温度的升高则对其有促进作用;随着当量比的增大,层流火焰传播速度先增大,在当量比为1.1处取得最大值,而后随之下降.对比1-戊烯反应机理计算结果发现,机理计算结果无法较好地与实验结果吻合,并且机理过于庞大,无法应用于实际CFD计算.     

不同初始压力下氢气燃烧的胞状不稳定性及自加速性

采用高速纹影摄像系统和定容燃烧弹对不同初始压力下(0.1~0.5,MPa)氢气燃烧的不稳定性和自加速性进行了实验研究,分析了火焰胞状不稳定性的发展过程和变化规律,分别对比了火焰轮廓及火焰传播速度的自加速表现.研究结果表明,在火焰没有达到一开始就完全胞状化之前,随初始压力的增大,氢气燃烧的不稳定性增强;胞状不稳定的火焰会出现自加速,而稳定火焰不会出现自加速;火焰的加速特性在均布的胞状结构形成后便会出现,其始点与胞状不稳定的火焰临界半径一致,始点过后,火焰的传播速度(或燃烧速度)随着燃烧半径的增加(或燃烧时间的增加)而不断地自加速.     

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。     

氢氧加湿燃烧的数值模拟

介绍了利用CHEMKIN 4.1软件中封闭的全混同性反应器模型模拟加湿情况下氢氧燃烧的方法,同时基于详细的氢氧燃烧化学反应动力学机理分析了初始反应温度、反应压力及水蒸气添加量等对氢氧燃烧的影响。模拟结果表明,初始反应温度、反应压力及水蒸气添加量等因素均会对氢氧燃烧的点火延迟时间和燃烧最高温度产生一定程度的影响,其中水蒸气添加量的变化对氢氧燃烧的最高温度及火焰传播速度的影响较为显著。这对后续在实验室中进行氢氧加湿燃烧研究可提供有益的指导。     

Experimental study on induced accelerated combustion of premixed hydrogen-air in a confined environment

The study on induced accelerated combustion of premixed hydrogen-air in a confined environment is of great significance for the efficient utilization of hydrogen energy in internal combustion engines. The accelerated flame induced by the orifice plate is more stable and easy to control, which is beneficial to achieve controlled and rapid turbulent combustion. In this work, the accelerated combustion process induced by the orifice plate, and the influence of the orifice structure and initial conditions on the flame propagation and combustion characteristics were investigated by constant volume combustion bomb and schlieren method. The results show that the combustion process induced by the orifice plate consists of three stages: the initial stage of propagation, the accelerated stage of the orifice plate, and the end combustion stage. The reduction in aperture induces greater turbulence intensity and increases the perturbation of the orifice plate to the flame, resulting in a substantial increase in flame propagation speed through the orifice plate. As the initial pressure and the equivalence ratio increase, the velocity of turbulent flame induced by the orifice plate and the change rate of the velocity before and after the orifice plate increase. As the initial temperature increases, the turbulent flame propagation velocity does not change much, and the velocity change rate before and after the orifice plate decreases. The effect of the initial conditions on flame acceleration induced by the orifice plate is essentially the influence of flame propagation speed and instability. The greater the flame propagation speed and the stronger the flame instability, the stronger the induced turbulence and the greater the influence of the turbulent flow disturbance, and the greater the velocity of the turbulent flame induced by the orifice plate. There exists an optimum aperture for the shortest combustion duration at any initial conditions, but the optimal diameter is not sensitive to changes in initial conditions. The effect of orifice-induced combustion acceleration is remarkable, and the combustion durations induced by each orifice plate are shortened by more than 50%.     

Effect of turbulence on flame propagation in cornstarch dust-air mixtures

Following the quantitative determination of dust cloud parameters, this study investigated the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160×160 mm square cross section, and gave particular attention to the effect of turbulence on flame characteristics. The turbulence induced by dust dispersion process was measured using a particle image velocimetry (PIV) system. Upward propagating dust flames were visualized with direct light and shadow photography. The results show that a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition is about 10 cm/s. The measured propagation speed of laminar flames appears to be in the range of 0.45–0.56 m/s, consistent with the measurements reported in the literature. For the present experimental conditions, the flame speed is little sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of dust combustion process.      

Impacts of plate slits on flame acceleration of premixed methane/air in a closed tube

The paper aims at revealing the interaction of various numbers of premixed methane/air jet flames in a closed duct. In the experiment, a high-speed video camera and pressure transducers are used to study the flame structure and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with Power-Law combustion model is employed to investigate the interaction between the moving flame and vortices induced by the thin plate. The results demonstrate that the flame propagation for all plate configurations can be divided into four typical stages, i.e. hemispherical flame, finger-shaped flame, jet flame and bidirectional propagation flame. For three plate configurations, the jet flames merge together under the effect of the vortices, and the more slits with the same blockage ratio (BR) do not mean the stronger deflagration. It is observed that the peaks of flame tip speed and pressure growth rate decrease with the increase of the number of slits. The sub-grid scale combustion model, Power-Law model, coupled with sub-grid scale viscosity model, dynamic Smagorinsky-Lilly eddy viscosity model can well reproduce the flame propagation. By analyzing the numeric flow structure, the flame propagation mechanism of premixed methane/air flame propagation in a tube with various slits can be explained in the view of pure hydrodynamics.     

Study on the dynamic process of in-duct hydrogen-air explosion flame propagation under different blocking rates

To effectively analyses the flame propagation of premixed hydrogen-air explosion, this paper carries out a numerical study on the dynamics of flame propagation during hydrogen explosions in a closed duct under different blocking rates. The study shows that flame structure is roughly the same when the flame passes through an obstacle under different blocking rates. The difference in blocking rates only shows a slight difference in the degree of flame deformation. When the flame passes through the obstacle, Rayleigh -Taylor (R-T) instability accompanies the entire flame propagation process and corresponds to each stage flame acceleration. Kelvin-Helmholtz(K-H) instability has a more prominent influence on the tip flame propagation. When the explosion flame propagates, instabilities lead to difference in density gradient and pressure gradient in the duct. Interaction between density gradient and pressure gradient leads to formation of baroclinic torque, which is the main cause of the vorticity. During the flame propagation, the vorticity at the front of the flame is roughly zero, whereas the vorticity formed at the obstacle or in the burned gases is more apparent. The larger the blocking rate, the more prominent the turbulence intensity during the flame propagation.     

Effect of the turbulence intensity on knocking tendency in a SI engine with high compression ratio using biogas and blends with natural gas,propane and hydrogen

This research presents the test results carried out in a diesel engine converted to spark ignition (SI) using gaseous fuels, applying a geometry change of the pistons combustion chamber (GCPCC) to increase the turbulence intensity during the combustion process; with similar compression ratio (CR) of the original diesel engine; the increase in turbulence intensity was planned to rise turbulent flame speed of biogas, to compensate its low laminar flame speed. The research present the test to evaluate the effect of increase turbulence intensity on knocking tendency; using fuel blends of biogas with natural gas, propane and hydrogen; for each fuel blend the maximum output power was measured just into the knocking threshold before and after GCPCC; spark timing (ST) was adjusted for optimum generating efficiency at the knocking threshold. Turbulence intensity with GCPCC was estimated using Fluent 13, with 3D Combustion Fluid Dynamics (CFD) numerical simulations; 12 combustion chamber geometries were simulated in motoring conditions; the selected geometry had the greatest simulated turbulent kinetic energy (TKE) and Reynolds number (Re) during combustion. The increased turbulence intensity was measured indirectly through the periods of combustion duration to mass fraction burn 0–5%, 0–50% and 0–90%; for almost all the fuel blends the increased turbulence intensity of the engine, increased the knocking tendency requiring to reduce the maximum output power to keep engine operation just into the knocking threshold. Biogas was the only fuel without power derating by the conditions of higher pressure and higher turbulence during combustion by GCPCC and improve its generating efficiency. Peak pressure, heat release rate, mean effective pressure and exhaust temperature were lower after GCPCC. Tests results indicated that knocking tendency was increased because of the higher turbulent flame speed; fuel blends with high laminar flame speed and low methane number (MN) had higher knocking tendency and lower output power.     

Effect of orifice plates spaces on flame propagation in a square cross-section channel

This paper investigates the effect of orifice plate separation distance on flame behavior as well as pressure-time histories. Experiments were conducted in a 1 m long, 7 cm by 7 cm square cross-section channel, employing schlieren photography with high-speed camera for visualization to qualitatively identify the propagation mechanisms, and piezoelectric pressure transducers to measure pressure evolution. With two orifice plates in the path of the flame, the flame presents complicated propagating characteristics, i.e., compared to a planar flame, and a corresponding more intricate velocity time-history. It is found that acoustic waves generated in-between orifice plates, after reflection off the second plate, interact with the rear flame front to produce an approximately planar flame. This phenomenon vanishes with decrease in orifice plate separation distance, whereas, with increasing blocking effect the effect is enhanced. In addition, the pressure difference across the second orifice plate correlates with the jet flame length.     

用紊流卷吸燃烧模型计算研究点火时刻紊流参数对燃烧循环的影响

利用准维紊流卷吸燃烧模型计算研究了点火时刻气缸内紊流参数的变化对汽油机燃烧循环变动的影响。结果表明，点火时刻紊流强度u′及紊流长度积分标尺L的变动对汽油机燃烧循环变动均有很大影响，增大u′或L均有利于加快燃烧速度及火焰传播速度，缩短火焰发展期；u′或L的变动加大，燃烧循环变动也随之加大，其中，u′对燃烧循环变动的影响尤其大。