【作者】 李星；

【导师】 贾力；

【作者基本信息】 北京交通大学 ， 载运工具运用工程， 2014， 博士

【摘要】 采用当前燃烧领域较完善的研究手段,对甲烷(CH4)在高温氧化剂中着火、拉伸熄灭及燃烧特性进行了研究。采用确定温度边界微型反应器,对CH4/air和CH4/O2/CO2(Xo2/Xco2=0.62)混合物的着火和燃烧特性进行研究。实验结果表明来流速度较高和较低条件下混合物在微型反应器内存在两种稳态的火焰响应,常态火焰(normal flame)和弱火焰(weak flame),来流速度在中间区域时对应一种动态火焰,即快速着火熄灭火焰(FREI, flame with repetitive extinction and ignition)。研究表明随着来流速度的降低常态火焰向上游区域迁移,弱火焰的位置受来流速度影响不大。CH4/air和CH4/O2/CO2(XO2/Xco2=0.62)混合物弱火焰对应着火温度非常接近。采用一维稳态计算程序和详细化学反应机理,对确定温度边界微型反应器内火焰燃烧和着火特性进行了计算,得到两种稳态火焰,常态火焰和弱火焰的数值解。数值计算结果和实验结果吻合较好。分析了CH4在常态火焰和弱火焰中的反应过程,并对C02和N2对常态火焰和弱火焰内CH4反应过程的影响进行了详细的讨论。利用对冲火焰结构研究了氧化剂温度对非预混对冲火焰拉伸熄灭极限的影响。研究了燃料混合物为常温(300K)的CH4/N2与air(O2/N2)对冲火焰和CH4/CO2与O2/CO2对冲火焰,其中CH4/CO2与O2/CO2对冲火焰氧化剂中氧摩尔分数采用0.40和0.35。得到不同氧化剂温度条件下(300K,700K,1000K)不同火焰的熄灭特性。分别采用光学薄模型(OTM, optical thin model)和绝热模型(ADI, adiabatic)讨论了辐射散热对拉伸熄灭极限的影响,研究结果表明燃料浓度较高时辐射散热对两种对冲火焰拉伸熄灭极限的影响不大。氧化剂温度为常温时(300K),氧化剂中氧摩尔分数为0.35的CH4/CO2与02/C02对冲火焰的熄灭曲线靠近CH4/N2与air (O2/N2)对冲火焰的熄灭曲线。而当氧化剂温度较高时(1000K), CH4/N2与air (O2/N2)对冲火焰的熄灭曲线靠近氧化剂中氧摩尔分数为0.40的CH4/CO2与02/C02对冲火焰的熄灭曲线。对影响拉伸熄灭的参数进行了理论研究,分析表明拉伸熄灭极限主要由焓流密度和动力学特性两个影响因素决定。通过对数值计算结果中化学链起始反应和主要吸放热反应的分析,得到了影响熄灭极限的关键基元为OH,结合理论研究定量的比较了OH生成速率对拉伸熄灭极限的影响。提出估算拉伸熄灭极限的方法,对拉伸熄灭曲线随氧化剂温度的变化进行了合理的解释。对高温助燃空气旋流和平行射流两种流动模式下CH4射流火焰燃烧特性进行了比较。研究表明两种高温空气流动模式下射流火焰燃烧特性截然不同,采用高温空气旋流可以稳定射流火焰,并有效的降低NO生成。在此基础上研究了燃烧室外采用空气对流换热和水对流换热条件下甲烷在旋流高温空气中燃烧特性。研究表明当燃烧室外采用空气对流换热条件时,随着助燃空气温度增加,燃烧室气体温度升高,壁面平均热流和出口NO浓度增大；随着过量空气系数的增加,燃烧的最高温度和NO生成量增加,气体平均温度及热流密度先增加后减小,过量空气系数为1.2时温度均匀性好,传热量最大；随助燃空气中氧浓度减小,燃烧室温度均匀性增强,壁面平均热流增大,NO生成量显著减小。更多还原

【Abstract】 Ignition, stretch extinction and combustion characteristics of methane with high temperature oxidizers were studied by using well developed research methods in this work.Combustion and ignition characteristics of a stoichiometric CH4/air and CH4/O2/CO2(Xo2/Xco2=0.62) mixtures were tested in a micro flow reactor with a controlled temperature profile. Two kinds of steady flame responses, namely normal flame and weak flame, were observed experimentally at conditions of high and low inlet velocities. Dynamic flame which termed as FREI (flame with repetitive extinction and ignition) was observed in intermediate inlet velocity regimes. It was observed that the flame position of normal flame is shifted to the upstream with the increase of inlet flow velocity, and the flame location of weak flame is not sensitive to the inlet flow velocity. The ignition temperatures of CH4/air and CH4/O2/CO2(XO2/XCO2=0.62) weak flames are very close. One dimensional steady-state computation code with detail reaction mechanism was used to extend the experimental understanding. Steady flame responses, normal flame and weak flame, were well captured by this computation code. And the reaction path of CH4in both normal flame and weak flame were analyzed, and the effect of CO2and N2on the oxidation process of CH4were studied.The effect of oxidizer temperature on the stretch extinction limits of non-premixed counterflow flames was investigated. Extinction stretch rates of CH4/N2(at300K) versus air (O2/N2) flames and CH4/CO2(at300K) versus O2/CO2flames with oxygen mole fractions of0.35and0.40at the oxidizer temperatures of300K,700K and1000K were obtained. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air-flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show the influence of radiative heat loss on stretch extinction limits is not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of0.35is close to that of the air flames at the oxidizer temperature of300K. However, the extinction curve of air flames with high temperature oxidizer (1000K) is comparable with that of oxygen-enriched flames with oxygen mole fraction of0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that the stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. The OH radicals which play important roles in chain propagating and main endothermic reactions is used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames is well explained by the present scaling analysis.Combustion characteristics of methane-jet-flames with swirling and parallel high temperature airs were compared. It was found that the combustion characteristics of methane-jet-flames are totally different in swirling and parallel high temperature airs. Swirling high temperature air results low NO formation and uniform temperature distribution. Furthermore, the combustion characteristics of methane-jet-flame in swirling high temperature air under air convection heat transfer boundary and water convection boundary conditions were studied. In the air convection heat transfer boundary case, the effects of important parameters such as air temperature, excessive air ratio and oxygen concentration on combustion characteristics were systematic discussed. The maximum flame temperature, average wall heat flux and NO formation increase with the air temperature. The maximum gas temperature and NO formation increase with the excessive air ratio, the average gas temperature and heat flux increased firstly and then decreased, the maximum heat flux can be obtained at excessive air ratio of1.2. With the decrease of oxygen concentration of high temperature air, NO generation is significantly decreased, and the uniformity of gas and average wall heat flux are increased.更多还原