Homogeneous mixtures of CH4/air under moderate or intense low-oxygen dilution（MILD） combustion conditions were numerically studied to clarify the fundamental effects of exhaust gas recirculation（EGR）,espe-cially C...Homogeneous mixtures of CH4/air under moderate or intense low-oxygen dilution（MILD） combustion conditions were numerically studied to clarify the fundamental effects of exhaust gas recirculation（EGR）,espe-cially CO2 in EGR gases,on ignition characteristics.Specifically,effects of CO2 addition on autoignition delay time were emphasized at temperature between 1200 K and 1600 K for a wide range of the lean-to-rich equivalence ratio（0.2~2）.The results showed that the ignition delay time increased with equivalence ratio or CO2 dilution ratio.Fur-thermore,ignition delay time was seen to be exponentially related with the reciprocal of initial temperature.Special concern was given to the chemical effects of CO2 on the ignition delay time.The enhancement of ignition delay time with CO2 addition can be mainly ascribed to the decrease of H,O and OH radicals.The predictions of tem-perature profiles and mole fractions of CO and CO2 were strongly related to the chemical effects of CO2.A single ignition time correlation was obtained in form of Arrhenius-type for the entire range of conditions as a function of temperature,CH4 mole fraction and O2 mole fraction.This correlation could successfully capture the complex be-haviors of ignition of CH4/air/CO2 mixture.The results can be applied to MILD combustion as ＂reference time＂,for example,to predict ignition delay time in turbulent reacting flow.展开更多
中度与极度低氧稀释(moderate or intense low-oxygen dilution,MILD)氧燃烧技术能同时实现低碳和超低NO_(x)排放,是一种创新性的燃烧技术。通过实验方法研究了不同稀释剂(N_(2)、CO_(2)和H_(2)O)、氧气浓度、当量比和氧化剂预热温度下...中度与极度低氧稀释(moderate or intense low-oxygen dilution,MILD)氧燃烧技术能同时实现低碳和超低NO_(x)排放,是一种创新性的燃烧技术。通过实验方法研究了不同稀释剂(N_(2)、CO_(2)和H_(2)O)、氧气浓度、当量比和氧化剂预热温度下甲烷非预混MILD氧燃烧和排放特性。实验发现,N_(2)、CO_(2)和H_(2)O稀释的所有工况中均没有观察到火焰。但N_(2)稀释时,炉内温度和NO排放都比较高,且当氧浓度、当量比或氧化剂温度增加时,NO排放急剧升高(>100×10^(-6)),因此无法实现较好的MILD氧燃烧。与之相比,CO_(2)或H_(2)O稀释下,炉内温度较低,NO排放也非常低(<10×10^(-6));并且NO排放对氧浓度、当量比和氧化剂温度的变化不敏感,能在更宽的范围内建立起低排放的MILD氧燃烧。此外,CO_(2)稀释时会产生较高的CO排放(>20×10^(-6)),但H_(2)O稀释下几乎没有CO排放,且NO排放最低(≈1× 10^(-6))。因此,H_(2)O稀释最有利于实现超低排放的MILD氧燃烧。但实际炉膛应用时需要注意防止H_(2)O稀释造成的炉壁汲水和因此导致的热效率降低、甚至熄火。展开更多
通过数值模拟方法研究了不同氧化剂(O_(2)/N_(2)、O_(2)/CO_(2)和O_(2)/H_(2)O)和燃烧器出口氧浓度(21%~30%)对15kW实验炉内甲烷非预混中度与极度低氧稀释(moderate or intense lowoxygen dilution,MILD)富氧燃烧的流场、燃烧场及湍流...通过数值模拟方法研究了不同氧化剂(O_(2)/N_(2)、O_(2)/CO_(2)和O_(2)/H_(2)O)和燃烧器出口氧浓度(21%~30%)对15kW实验炉内甲烷非预混中度与极度低氧稀释(moderate or intense lowoxygen dilution,MILD)富氧燃烧的流场、燃烧场及湍流–化学相互作用的影响。研究结果表明,不同稀释剂下炉内流动和烟气卷吸情况几乎相同,但在炉内反应方面存在较大差异。各稀释剂下炉内燃烧温度和CO、OH浓度的高低顺序为:N_(2)>CO_(2)>H_(2)O。而且,N_(2)稀释时炉内存在集中的高温区(>1800K),且温度和组分浓度随氧浓度增大而快速升高。而CO_(2)或H_(2)O稀释时炉内温度、组分分布均匀,且对氧浓度变化不敏感。另外,相比CO_(2)或H_(2)O稀释,N_(2)稀释下反应区内的层流火焰速度和Damköhler数(Da)更大,且随氧浓度的升高而急剧增加,30%氧浓度下已经进入传统薄反应区燃烧模式。而CO_(2)或H_(2)O的稀释可以显著降低层流火焰速度,增长化学反应时间,减小Da数,在高氧浓度下依旧保持在分布式反应区,即MILD燃烧区。因此,相比N_(2)稀释,CO_(2)或H_(2)O稀释下更有利于建立MILD富氧燃烧。展开更多
This study investigated the formation and emission characteristics of nitric oxide(NO) from flameless MILD(moderate or intensive low-oxygen dilution) combustion(MILDC) versus traditional visible-flame combustion(TC) i...This study investigated the formation and emission characteristics of nitric oxide(NO) from flameless MILD(moderate or intensive low-oxygen dilution) combustion(MILDC) versus traditional visible-flame combustion(TC) in a 30-k W furnace. Both combustion processes were experimentally operated successively in the same furnace, burning natural gas at a fixed rate of 19 k W and the equivalence ratio of 0.86. Numerical simulations of TC and MILDC were carried out to explain their distinction in the measured furnace temperature and exhaust NO emissions. Present measurements of the NO emission(XNO) versus a varying furnace wall temperature(Tw) have revealed, at the first time, that the relationship of XNO ~ Tw was exponential in both TC and MILDC. By analyzing the simulated results, the average temperature over the reaction zone was identified to be the common characteristic temperature for scaling NO emissions of both cases. Moreover, relative to TC, MILDC had a fairly uniform temperature distribution and low peak temperature, thus reducing the NO emission by over 90%. The thermal-NO formation was found to contribute more than 70%-80% to the total XNO from TC while the N2O-intermediate route dominated the NO emission from MILDC.展开更多
采用标量联合PDF(probability density function)方法结合修正的k-ε湍流模型、EMST小尺度混合模型以及GRI3.0化学反应机理对甲烷-氢气混合燃料(体积比为1∶1)高温伴流射流J HC(jet in hot coflow)火焰进行数值模拟.比较分析了高温伴流...采用标量联合PDF(probability density function)方法结合修正的k-ε湍流模型、EMST小尺度混合模型以及GRI3.0化学反应机理对甲烷-氢气混合燃料(体积比为1∶1)高温伴流射流J HC(jet in hot coflow)火焰进行数值模拟.比较分析了高温伴流中氧气质量分数分别为3%,6%和9%时的3种不同的MILD(moderate andintense lowoxygen dilution)燃烧火焰,3种火焰的计算结果与实验值符合得较好.展开更多
This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow （JHC） from a burner system similar to that of Dally et al. The coflow O2 mass fraction （ Yo2 ...This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow （JHC） from a burner system similar to that of Dally et al. The coflow O2 mass fraction （ Yo2 ） is varied from 3% to 80% and the temperature （Tcof） from 1200 K to 1700 K. The Eddy Dissipation Concept （EDC） model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations. To validate the modeling, several JHC flames are predicted under the experimental conditions of Dally et al. [Proc. Combust. Inst., 29 （1）, 1147-1154 （2002）] and the results obtained match well with the measurements. Results demonstrate that, when Yo2 decreased, the diffusion combustion is likely to transform from traditional combustion to MILD （Moderate or Intense Low-oxygen Dilution） combustion mode. When Tcof is higher, the temperature distribution over the whole domain trends to be more uniform. Reducing yo2 or Tcof leads to less production of intermediate species OH and CO. It is worth noting that if Yo2 is high enough （ Yo2 〉80%）, increasing Yo2 does not cause obvious temperature increase.展开更多
基金Supported by the National Natural Science Foundation of China (50206014)the Shuguang Scholar Program of Shanghai Education Development Foundation (05SG23)
文摘Homogeneous mixtures of CH4/air under moderate or intense low-oxygen dilution（MILD） combustion conditions were numerically studied to clarify the fundamental effects of exhaust gas recirculation（EGR）,espe-cially CO2 in EGR gases,on ignition characteristics.Specifically,effects of CO2 addition on autoignition delay time were emphasized at temperature between 1200 K and 1600 K for a wide range of the lean-to-rich equivalence ratio（0.2~2）.The results showed that the ignition delay time increased with equivalence ratio or CO2 dilution ratio.Fur-thermore,ignition delay time was seen to be exponentially related with the reciprocal of initial temperature.Special concern was given to the chemical effects of CO2 on the ignition delay time.The enhancement of ignition delay time with CO2 addition can be mainly ascribed to the decrease of H,O and OH radicals.The predictions of tem-perature profiles and mole fractions of CO and CO2 were strongly related to the chemical effects of CO2.A single ignition time correlation was obtained in form of Arrhenius-type for the entire range of conditions as a function of temperature,CH4 mole fraction and O2 mole fraction.This correlation could successfully capture the complex be-haviors of ignition of CH4/air/CO2 mixture.The results can be applied to MILD combustion as ＂reference time＂,for example,to predict ignition delay time in turbulent reacting flow.
文摘通过数值模拟方法研究了不同氧化剂(O_(2)/N_(2)、O_(2)/CO_(2)和O_(2)/H_(2)O)和燃烧器出口氧浓度(21%~30%)对15kW实验炉内甲烷非预混中度与极度低氧稀释(moderate or intense lowoxygen dilution,MILD)富氧燃烧的流场、燃烧场及湍流–化学相互作用的影响。研究结果表明,不同稀释剂下炉内流动和烟气卷吸情况几乎相同,但在炉内反应方面存在较大差异。各稀释剂下炉内燃烧温度和CO、OH浓度的高低顺序为:N_(2)>CO_(2)>H_(2)O。而且,N_(2)稀释时炉内存在集中的高温区(>1800K),且温度和组分浓度随氧浓度增大而快速升高。而CO_(2)或H_(2)O稀释时炉内温度、组分分布均匀,且对氧浓度变化不敏感。另外,相比CO_(2)或H_(2)O稀释,N_(2)稀释下反应区内的层流火焰速度和Damköhler数(Da)更大,且随氧浓度的升高而急剧增加,30%氧浓度下已经进入传统薄反应区燃烧模式。而CO_(2)或H_(2)O的稀释可以显著降低层流火焰速度,增长化学反应时间,减小Da数,在高氧浓度下依旧保持在分布式反应区,即MILD燃烧区。因此,相比N_(2)稀释,CO_(2)或H_(2)O稀释下更有利于建立MILD富氧燃烧。
基金support of National Natural Science Foundation of China(No.51776003)is gratefully acknowledged。
文摘This study investigated the formation and emission characteristics of nitric oxide(NO) from flameless MILD(moderate or intensive low-oxygen dilution) combustion(MILDC) versus traditional visible-flame combustion(TC) in a 30-k W furnace. Both combustion processes were experimentally operated successively in the same furnace, burning natural gas at a fixed rate of 19 k W and the equivalence ratio of 0.86. Numerical simulations of TC and MILDC were carried out to explain their distinction in the measured furnace temperature and exhaust NO emissions. Present measurements of the NO emission(XNO) versus a varying furnace wall temperature(Tw) have revealed, at the first time, that the relationship of XNO ~ Tw was exponential in both TC and MILDC. By analyzing the simulated results, the average temperature over the reaction zone was identified to be the common characteristic temperature for scaling NO emissions of both cases. Moreover, relative to TC, MILDC had a fairly uniform temperature distribution and low peak temperature, thus reducing the NO emission by over 90%. The thermal-NO formation was found to contribute more than 70%-80% to the total XNO from TC while the N2O-intermediate route dominated the NO emission from MILDC.
文摘采用标量联合PDF(probability density function)方法结合修正的k-ε湍流模型、EMST小尺度混合模型以及GRI3.0化学反应机理对甲烷-氢气混合燃料(体积比为1∶1)高温伴流射流J HC(jet in hot coflow)火焰进行数值模拟.比较分析了高温伴流中氧气质量分数分别为3%,6%和9%时的3种不同的MILD(moderate andintense lowoxygen dilution)燃烧火焰,3种火焰的计算结果与实验值符合得较好.
基金Supported by the National Natural Science Foundation of China （51276002）, and the Specific Research Fund for the Doctoral Program of Higher Education of China （20110001130014）.
文摘This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow （JHC） from a burner system similar to that of Dally et al. The coflow O2 mass fraction （ Yo2 ） is varied from 3% to 80% and the temperature （Tcof） from 1200 K to 1700 K. The Eddy Dissipation Concept （EDC） model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations. To validate the modeling, several JHC flames are predicted under the experimental conditions of Dally et al. [Proc. Combust. Inst., 29 （1）, 1147-1154 （2002）] and the results obtained match well with the measurements. Results demonstrate that, when Yo2 decreased, the diffusion combustion is likely to transform from traditional combustion to MILD （Moderate or Intense Low-oxygen Dilution） combustion mode. When Tcof is higher, the temperature distribution over the whole domain trends to be more uniform. Reducing yo2 or Tcof leads to less production of intermediate species OH and CO. It is worth noting that if Yo2 is high enough （ Yo2 〉80%）, increasing Yo2 does not cause obvious temperature increase.
