不同进气氛围耦合废气再循环对双燃料发动机工作过程的影响

基于正庚烷、甲烷、乙烷、丙烷多组分混合物简化动力学机理耦合三维计算流体力学（computational fluid dynamics,CFD）数值模型,模拟研究高替代率时不同进气氛围（H₂、O₂组分）耦合废气再循环（exhaust gas recirculation,EGR）对天然气/柴油双燃料发动机低负荷工作过程的影响机理。研究表明：在不同EGR率下,进气掺氢会使缸内燃烧速率显著加快,OH活性基浓度明显升高,CH₄排放显著降低,但CO排放升高;进气掺氧后,缸压及瞬时放热率峰值、最大压力升高率、最高燃烧温度及OH活性基浓度均升高,碳烟、CO和CH₄后期氧化作用增强使其最终排放降低,但NO_x排放升高。在EGR率小于29%,掺氢比小于2.5%时,在实现较低CO、碳烟排放的同时能显著降低CH₄排放和NO₂/NO_x比例;高EGR率时,进气掺氧能降低CO、碳烟排放,并改善CH₄与NO_x     

CH4/O2同轴射流扩散火焰辐射发光特性

利用分光成像系统,实时收集O2/CH4同轴射流扩散火焰辐射发光信息,重点讨论火焰中3种激发态自由基组分(OH*、CH*和C2*)的发光特征.探讨3种激发态自由基的产生机理;分析OH*、CH*和C2*的辐射强度随氧气/燃料当量比的变化,及其沿扩散火焰轴向分布特征等.结果表明:激发态自由基均由化学激发途径产生,其辐射发光为化学发光;OH*辐射发光强度随氧气/燃料当量比变化显著,可采用[OH*]/[CH*]表征当量比变化;OH*辐射强度可以作为扩散火焰化学反应区及高度的表征.     

带伴流的合成气微混火焰特性研究

选用等体积比的CO和H₂合成气为燃料,研究伴流微混单喷嘴的火焰结构。在Fluent平台上,采用稳态不可压的N-S方程进行求解,探究不同出口流速和当量比对微混单喷嘴火焰结构的影响。结果表明:OH的分布与高温区域基本重合,且化学反应释热率与OH高浓度区域相一致。随当量比增加,空气侧流速降低,火焰前缘逐渐相连,最终火焰附着在喷嘴出口;喷嘴出口流速的增加则强化了射流夹带作用,使其携带环境中的HO₂和O₂,迅速将H氧化为OH,从而收缩反应区域厚度;伴流作用促进CO氧化生成HCO的过程,降低主燃火焰向环境散热。     

氢燃料微混燃烧器燃烧特性研究

针对带中心钝体的四喷嘴微混燃烧器,运用ANSYS FLUENT软件,采用热态小火焰生成流形的方法对燃烧器模型进行数值模拟,并与实验研究相结合,研究了甲烷/氢气混合燃料(体积组分40%CH₄-60%H₂)微混燃烧条件下的燃/空掺混,流场、温度场、火焰形态及污染物排放等基础燃烧特性。研究结果表明：微混燃烧器采用空气和燃料径向进气的结构有利于燃/空掺混,在燃烧器出口的掺混均匀性指数达到0.959;燃烧器钝体结构处存在较明显的小型中心回流区,有助于火焰稳定;当量比在0.4～0.8范围内,火焰根部稳定附着在微混喷嘴的出口,火焰彼此相互独立,实验中燃烧器火焰形态与仿真OH^*场分布基本一致;绝热火焰温度在1 500～2 050 K范围内,模型燃烧室出口NO_x排放浓度均低于16×10^-6,CO排放浓度均低于11×10^-6,表明该微混燃烧器的污染物排放水平较低且燃烧效率极高。     

微型定容燃烧腔内丙烷/空气火焰传播特性

利用高速摄像的方法,在狭缝间距为2,mm的圆盘状微型定容燃烧装置中考察了常温常压、当量比φ为1.0~1.6静止丙烷/空气预混气中心点火后向外传播的火焰传播特性.结果表明:微型定容燃烧腔内形成的火焰面有光滑、褶皱和断裂3种形态;光滑火焰面的火焰传播速度低于常规尺度下的火焰传播速度;火焰传播速度随着当量比的增加先增大后减小;在点火能量影响范围外,火焰传播速度随半径增大而减小;随当量比的增加火焰锋面容易出现褶皱和断裂现象.     

浓-淡火焰相互作用特性的数值研究

为了研究浓淡燃烧中浓火焰与淡火焰之间的作用机制及其对火焰稳定性的影响，建立二维数值计算模型。淡火焰当量比为0.60～0.75时，分析淡火焰两侧的(一侧与浓火焰相邻，一侧与淡火焰相邻)传热传质特性。结果显示：浓火焰为淡火焰提供CH₄、CO、H₂、OH等组分，改变了淡火焰的火焰根部的组分浓度以及化学反应速率。浓火焰的存在使得淡火焰的火焰传播速度加快，火焰根部位置降低，增强了火焰的稳定性。     

高压甲烷射流对层流火焰作用的试验

高压气体燃料射流与引燃的层流火焰间的相互作用决定了天然气直喷发动机的着火稳定性．在定容燃烧弹中,用点火针点燃预混甲烷形成层流火焰,并在不同火焰半径时刻进行高压甲烷射流．采用高速纹影法测试了甲烷不同喷射延时τ对预混层流火焰的影响．结果表明：甲烷喷射延时τ决定了预混层流火焰等效半径R的发展,随着τ增大,预混层流火焰等效半径R增大;射流对层流火焰发展的影响与其作用于层流火焰时火焰等效半径有关,存在一个临界火焰等效半径R₀,当R0时,射流吹熄火焰;R=R₀时,甲烷射流吹熄预混层流火焰后仍可被引燃,火焰传播速度加快;R>R₀时,甲烷射流更容易引燃成湍流燃烧火焰,同时预混火焰未受射流干扰区域仍旧保持层流火焰,此时层流火焰、湍流燃烧火焰并存,火焰传播速度加快．     

焦油组分重整过程中关键反应的机理研究

采用密度泛函理论计算方法,对焦油重整过程中主要发生的C-C键裂解反应、CH₄重整反应和水煤气转化反应的机理和能量变化进行探究。结果表明：在C-C键裂解反应中,C₃H₈首先吸附于催化剂表面形成吸附态C₃H₈~*,进一步裂解生成CH₃~*和CH₂CH₃~*,裂解反应放热,但是反应能垒较大,较难进行;在CH₄重整反应中,CH₄~*发生顺序脱氢反应生成CH₃~*,CH₂~*,CH~*,相比于继续脱氢,CH~*更倾向于与OH~*发生重整反应生成CHO~*,CHO~*脱氢生成CO~*,各步骤产生的H~*结合生成H₂~*,CH₂~*裂解生成CH~*的反应为CH₄重整反应的限速步骤;在水煤气转化反应中,H₂O~*分解后生成的OH~*更倾向于与CO~*结合生...     

一维平面火焰OH基绝对浓度值的测量与分析

针对甲烷/空气预混气体层流平面火焰,利用LIF技术,采用双向光路法对燃烧过程中所产生的OH基进行了测量,得到其一维绝对浓度值及其空间分布,并分析其产生的动力学因素.结果表明,当量比为0.8~1.0范围内,OH基绝对浓度差别不大;当量比在1.1~1.3的范围内,OH基的绝对浓度呈下降趋势,一方面由于OH基消耗反应OH+CH_4→CH_3+H2O,随着CH_4量的增大,OH基的消耗速度增加,另一方面OH基生成反应CH_3+O_2→OH+CH2O,虽然CH_3不断生成,但由于O_2的流量逐渐减小,所以OH基的生成率逐渐下降,二者共同作用导致OH基绝对浓度随着当量比的增大而减小.实验所测OH基绝对浓度值与Versluis等测得的OH基绝对浓度的数量级相差不大.     

旋流预混燃烧室燃烧特性及排放特性的数值模拟研究

为了综合考察燃气轮机燃烧室在高稳定性、低排放以及燃料适应性等方面的新要求,基于旋流预混燃烧技术,通过三维数值模拟方法开展了甲烷/空气、丙烷/空气预混燃烧特性及排放特性研究。结果表明：在一定的预混气进气质量流量条件下,当量比增大易引发回火,燃烧温度更高,同时NO_x排放指数增大,增加预混气质量流量,可在一定程度上提高回/熄火极限;当量比固定,增加预混气进气质量流量可避免潜在的回火现象,且NO_x排放指数线性降低;旋流器的旋流数增大能形成强旋流,稳定火焰,降低NO_x排放指数,但过大的旋流强度会引发回火现象;相比于甲烷/空气预混燃烧,丙烷/空气预混燃烧温度偏高,NO_x排放指数较大,但回熄火边界更宽,对应更广阔的稳定燃烧区间。     

Cross-sectional characteristics of visible emission spectra in partially premixed flames

Visible spectral characteristics of cross-sectional emissions from a partially premixed methane/air flame and a propane/air flame have been investigated. An optical train with a two-axis scanning mirror system was used to record line-of-sight emission spectra from 354nm to 618nm, and inversion technique was applied to obtain cross-sectional emission spectra. By analyzing the reconstructed emission spectra, cross-sectional intensities of CH and C₂ radicals were separated from the background emission. The blue flame edge and yellow flame edge were also obtained by image processing technique for edge detection with color photographs of the flames. These edges were compared with radial distributions of CH, C₂ radicals and background emission. The CH radicals were observed at the blue flame edge. The background emission was generated by soot precursors at upstream of the flame and by soot at downstream of the flame. The C₂ radicals in the propane/air flame were more noticeable than those in the methane/air flame.     

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.     

Effects of hydrogen addition on laminar flame speeds of methane,ethane and propane: Experimental and numerical analysis

The main purpose of this study is to investigate the effects of hydrogen addition on the laminar flame speeds of methane, ethane and propane. In this work, a flat flame method was used to measure the laminar flame speed in a counter-flow configuration combined with particle image velocimetry (PIV) system. The results indicate that with the increase of hydrogen amount, the laminar flame speeds of methane, ethane and propane increase linearly approximately. In addition, as hydrogen is increased, the flame speed of methane has the maximum increasing amplitude among them, which indicates that methane is more sensitive to hydrogen addition in flame speed than the other two fuels.Simulation analysis finds that the reaction R1: H + O₂ ? OH + O can promote the flame speeds of these three kinds of gaseous fuel obviously, and with the increase of hydrogen amount, the promoting effect is more obviously. Therefore, the main reason why hydrogen addition could increase flame speed is that the increase of H radical prompts reaction R1 to proceed in the forward direction. Comparing the flames of methane, ethane and propane mixed with hydrogen, it was found that the promotion of reaction R1 to the methane/hydrogen mixtures flame speed is strongest, and its free radicals concentration in flame increase more obviously. Therefore, hydrogen addition has a greater effect on the flame speed of methane than on that of ethane and propane.     

Laminar burning velocities and transition to unstable flames in H2/O2/N2 and C3H8/O2/N2 mixtures

Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H₂/O₂/N₂ mixtures at values of flame stretch up to 7600 s⁻¹, and existing measurements for C₃H₈/O₂/N₂ mixtures at values of flame stretch up to 900 s⁻¹. Laminar burning velocities varied linearly with increasing Karlovitz numbers—either decreasing or increasing at stable or unstable preferential-diffusion conditions—yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion coditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H₂/O₂/N₂ mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.     

Study on combustion and ignition characteristics of natural gas components in a micro flow reactor with a controlled temperature profile

Combustion and ignition characteristics of natural gas components such as methane, ethane, propane and n-butane were investigated experimentally and computationally using a micro flow reactor with a controlled temperature profile. Special attention was paid to weak flames which were observed in a low flow velocity region. The observed weak flame responses for the above fuels were successfully simulated by one-dimensional computations with a detailed kinetic model for natural gas. Since the position of the weak flame indicates the ignition characteristics as well as the reactivity of each fuel, the experimental and computational results were compared with research octane number (RON) which is a general index for ignition characteristics of ordinary fuels. At 1 atm, ethane showed the highest reactivity among these fuels, although RON of ethane (115) is between those of methane (120) and propane (112). Since the pressure conditions are different between the present experiment and the general RON test, weak flame responses to the pressure were investigated computationally for these fuels. The order of the fuel reactivity by the reactor agreed with that by RON test when the pressure was higher than 4 atm. Reaction path analysis was carried out to clarify the reasons of the highest reactivity of ethane at 1 atm among the employed fuels in this study. The analysis revealed that C₂H₅ + O₂ ⇔ C₂H₄ + HO₂ is a key reaction and promotes ethane oxidation at 1 atm. The effect of the pressure on the fuel oxidation process in the present reactor was also clarified by the analysis. In addition, weak flame responses to various mixing ratios of methane/n-butane blends were investigated experimentally and computationally. The results indicated a significant effect of n-butane addition in the blends on combustion and ignition characteristics of the blended fuels.     

Thermochemical and chemical kinetic data for fluorinated hydrocarbons

A comprehensive, detailed chemical kinetic mechanism was developed and is presented for C₁ and C₂ fluorinated hydrocarbon destruction and flame suppression. Existing fluorinated hydrocarbon thermochemistry and kinetics were compiled from the literature and evaluated. For species where no or incomplete thermochemistry was available, these data were calculated through application of ab initio molecular orbital theory. Group additivity values were determined consistent with experimental and ab initio data. For reactions where no or limited kinetics were available, these data were estimated by analogy to hydrocarbon reactions, by using empirical relationships from other fluorinated hydrocarbon reactions, by ab initio transition state calculations, and by application of RRKM and QRRK methods. The chemistry was modeled considering different transport conditions (plug flow, premixed flame, opposed flow diffusion flame) and using different fuels (methane, ethylene), equivalence ratios, agents (fluoromethanes, fluoroethanes) and agent concentrations. This report provides a compilation and analysis of the thermochemical and chemical kinetic data used in this work.     

Energy-efficient conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 catalyst in the presence of limited oxygen

Oxidative conversion of propane to propylene and ethylene over a V₂O₅/CeO₂/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O₂, has been studied at different temperatures (700–850 °C), C₃H₈/O₂ ratio (4.0), H₂O/C₃H₈ ratio (0.5) and space velocity (3000 cm³ g⁻¹ h⁻¹). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O₂, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O₂. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process.     

Experimental study of propane/air premixed flame dynamics with hydrogen addition in a meso-scale quartz tube

The combustion process of propane/air premixed flame in meso-scale quartz tubes with different hydrogen additions was investigated experimentally to explain the flame-wall interaction mechanism. The ranges of different flame regimes were obtained by changing the flow rates of propane and hydrogen. The effects of hydrogen addition, inlet velocity and equivalence ratio were analyzed. The results show that the hydrogen addition broadens the operation ranges of fast flame regime and slow flame regime significantly. The flame propagation speed is in the same order of the thermal wave speed in solid wall for the slow flames. In fast flame regime, the flame propagation speed has an inverse correlation with the inlet flow velocity irrespective of the equivalence ratio. With the increase of the equivalence ratio, the maximum flame speed in fast flame regime decreases gradually, while the maximum flame speed in slow flame regime increases continually. It indicates that rich fuel condition suppresses the fast flame and promotes the slow flame. In slow flame regime, the output thermal efficiency is dominated by the inlet velocity and equivalence ratio.     

Numerical simulation of one-dimensional laminar flames propagating into reacting premixed gases

In this paper, the propagation of a one-dimensional flame front into a reacting combustible mixture was numerically studied. A simplified mathematical method, splitting the problem into a prereaction part and a flame propagation part, was applied to the completely nonstationary problem. Initially stoichiometric mixtures of H₂/O₂ and C₂H₆/O₂ were investigated for isothermal and adiabatic boundary conditions of the prereactions. In the isothermal case, the laminar burning velocity of the mixture decreased gradually with time. In conclusion, the reasons for this decrease are an increasing amount of combustion products combined with the heat loss necessary to maintain isothermal conditions. Compared with these phenomena, the accelerating effect of radical concentrations in the preflame region is of minor importance. In the adiabatic case, the laminar burning velocity increases steadily, until the ignition delay time of the initial mixture is reached. In this period, the accelerating effect resulting from the temperature increase in the preflame region dominates the decelerating effect of the increasing product concentration. The flame thickness, which was also computed for both boundary conditions, increases here for all examined flames during the time-dependent propagation through the reacting gas mixture. This change in thickness proceeds gradually in the isothermal and spontaneously in the adiabatic case.