掺混H_(2)对CH_(4)-空气对冲扩散火焰温度和NO_(x)生成影响的数值研究北大核心CSCD

文章基于CHEMKIN软件对CH_(4)-空气对冲扩散火焰燃烧过程中掺混H_(2)对火焰温度以及NO_(x)生产量的影响进行了数值研究,分析了不同H_(2)摩尔分数和火焰拉伸率下火焰温度的变化特性以及NO_(x)的生成特性。研究结果表明:受到燃料气体传质能力和燃烧产热能力的综合影响,随着H_(2)摩尔分数的增加,混合燃料主燃烧区的峰值火焰温度点更靠近空气区;随着火焰拉伸率的增大,主燃烧区的范围变窄,反应物在燃烧区的滞留时间缩短,NO的生成受到抑制;NO_(2)和NO的摩尔分数表现出正相关的关系;随着混合燃料中H_(2)摩尔分数的增大,NO和NO_(2)的峰值摩尔分数显著增大。     

CO2稀释对甲烷-高温空气扩散燃烧及NO生成特性影响的化学动力学分析

基于GRI-Mech 3.0详细化学反应机理,利用OPPDIF Code研究了CO2稀释比、预热温度及拉伸率对甲烷-高温空气层流对冲扩散火焰温度、热释放率、组分摩尔分数及NO生成特性的影响.研究结果表明,CO2稀释助燃空气能有效降低火焰中H、O及OH等基团摩尔分数,抑制燃烧过程链传播及链引发反应,从而减缓CH4氧化速率.随着助燃空气中CO2稀释比的增加,火焰最高温度逐渐降低,主氧化区及第二氧化区放热峰值变小,燃烧反应高温区变窄,NO生成指数E显著降低.当稀释比大于20%时,热力型NO随助燃空气温度升高规律并不明显.随着CO2稀释比的增加,快速型NO对NO生成量影响逐渐增强,成为高CO2稀释比下甲烷-高温空气扩散燃烧NO生成的主要路径.     

CO_2稀释对不同燃料无焰燃烧及NO生成特性的影响

为探讨CO_2稀释对不同燃料无焰燃烧机理的影响,通过实验和数值模拟研究了CO_2稀释率对CH_4、C_3H_8和H_2扩散燃烧的火焰温度、NO排放摩尔分数及无焰燃烧的影响.结果表明:随着CO_2稀释率的增大,峰值温度和NO排放摩尔分数逐渐下降,峰值温度距燃烧器喷嘴的距离逐渐增大,炉内温度分布更加均匀,更有利于达到无焰燃烧状态;相同稀释率下,CO_2稀释对降低炉内峰值温度及出口NO排放摩尔分数的效果由好到坏依次为:H_2燃烧、CH_4燃烧、C_3H_8燃烧;当CO_2稀释率足够大时,炉内燃烧处于无焰燃烧状态.     

组分H2/CO的比例对合成气非预混射流火焰燃烧特性的影响

采用一种14组分37步简化机理模型、RNG k-ε湍流模型以及稳态层流小火焰(SLF)燃烧模型，研究了N₂稀释条件下组分H₂/CO的比例对合成气燃烧特性的影响。数值模拟结果表明：当组分H₂/CO的体积比从3:7变化至7:3时，合成气燃烧过程中生成的OH自由基浓度上升，燃烧位置向入口靠近；火焰燃烧峰值温度随H₂/CO体积比的增大而下降，火焰峰值温度所在位置向燃料入口靠近；火焰传播速度随H₂/CO体积比的增大而加快，燃烧反应在更短的距离和时间内完成。     

ODPP/OESC旋流火焰的试验与模拟研究

利用三维旋流燃烧系统,对稀氧部分预混/富氧补燃(ODPP/OESC)火焰结构和污染物生成特性进行了试验研究,降低稀氧体积分数、提高富氧体积分数,动力火焰呈现轴向拉伸趋势,而扩散火焰长度则逐渐缩短;同时,动力燃烧区和扩散燃烧区温度逐渐降低,NO_x排放量显著下降,CO排放量则有所提高。相同工况下数值模拟结果显示,ODPP/OESC改变了动力燃烧区的NO_x生成机理,是NO_x排放量降低的根本原因。ODPP/OESC基于燃料/氧化剂空间体积分数分布的物理过程控制,有效均衡了动力燃烧区与扩散燃烧区的反应速率,可实现CO与NO_x排放的平衡控制。     

进气组分耦合喷油定时对柴油机工作过程的影响机理

通过一台共轨柴油机,基于正庚烷/甲苯/正己烯混合物简化动力学机理耦合三维CFD数值模型,模拟不同进气组分(O_2、H_2和CO_2)耦合喷油时刻对发动机工作过程的影响机理.研究表明:不同进气组分下,随喷油时刻提前,缸内活性自由基(OH、O)质量分数及其分布区域增大,NO生成量增多.但随喷油时刻过度提前,燃烧始点反而推迟,燃烧放热速率、缸内燃烧压力与温度峰值降低,NO也相应减少;相比其他进气组分,进气掺O_2时缸内O自由基质量分数增大,碳烟(Soot)降低且喷油时刻对其影响较小;进气掺H_2时,缸内燃烧压力和温度峰值最高,OH自由基质量分数及分布区域最大,Soot随喷油定时提前大幅降低;进气掺CO_2时,缸内燃烧压力与温度峰值最低,OH和O活性自由基最少,当喷油时刻提前超过24°CA BTDC时,燃油逐渐喷射到压缩余隙容积形成局部过浓区,C_2H_2和多环芳香烃芘(A_4)生成量增多,Soot排放随喷油定时进一步提前明显升高.     

稀释剂对合成气燃烧污染物排放的影响

在自建的气体燃烧实验台上进行了合成气的扩散燃烧实验,研究了H_2/CO体积比对火焰形貌和污染物排放的影响,分析了在3种稀释剂(CO_2、N_2、Ar)作用下合成气火焰中NO和CO排放指数的变化规律及原因。结果表明:当H_2/CO体积比为10∶0(纯H_2)和9∶1时,火焰呈淡黄色,当H_2/CO体积比为8∶2时,开始出现蓝色火焰;随着合成气中CO体积分数的增大,火焰由淡蓝色逐渐变为亮蓝色,火焰清晰度逐步提高;随着H_2/CO体积比的增大,NO的排放指数提高,而CO的排放指数逐渐降低;3种稀释剂均可降低NO的排放指数,其中CO_2的效果最好,N_2和Ar的效果相差不大;稀释剂会提高CO的排放指数,且随着稀释比的增大,CO排放量急剧增大;随着H_2/CO体积比的增大,稀释剂对NO和CO排放指数的影响有所减弱。     

N2和CO2稀释对氢气-空气湍流扩散燃烧及NO生成特性的影响

采用18组分47步H2-N2-CO2反应机理模型、可实现k-ε模型及涡流耗散概念(EDC)模型研究了N2和CO2稀释作用对氢气-空气同轴射流湍流扩散燃烧过程的影响.结果 表明:2种稀释剂均能有效降低氢气燃烧温度,降低NO质量分数,且NO峰值质量分数随着火焰峰值温度的升高而上升;与稀释剂N2相比,CO2对降低氢气燃烧温度和NO质量分数的效果较好;2种稀释剂对火焰峰值温度及NO峰值质量分数的影响是非线性的,随着稀释率的增大,稀释剂降低火焰峰值温度的效果明显增强,而抑制NO生成的效果逐渐减弱;当稀释剂为N2、稀释率为0.5或稀释剂为CO2、稀释率为0.3时,能使火焰峰值温度处于中等水平情况下NO峰值质量分数依然较低,有利于实现氢气的高效低污染燃烧.     

CO_2稀释对合成气扩散火焰中氮氧化物生成排放特性的影响

为揭示合成气燃烧过程中氮氧化物的生成机理和抑制措施,利用详细化学反应机理动力学模型研究了CO2稀释对合成气对冲扩散火焰中氮氧化物生成的影响,结果表明:随着合成气成分的变化及稀释剂CO2的添加,扩散火焰结构及不同NO生成机理对总NOx排放的贡献发生显著变化;低火焰拉伸率下主要表现为热力型NO,但在高火焰拉伸率下,因CH4存在,使总NO生成高于不含CH4的合成气;随CO2稀释剂的添加,NOx的排放指数EI<,NOx>呈单调下降趋势,并且稀释空气的效果优于稀释燃料的效果.     

反应速率对层流扩散火焰特性的影响

为探究燃烧过程中火焰结构和烟黑特性的变化规律,对层流乙烯/空气扩散火焰进行了数值模拟,分析了不同成核过程和表面生长过程中,反应速率常数的指前因子及活化能对层流乙烯/空气扩散火焰温度和烟黑体积分数的影响。结果表明：成核反应速率常数中,指前因子增大,火焰温度降低,烟黑体积分数增大,当指前因子提高50%时,在轴向高度3 cm位置对应的火焰温度峰值减小0.70%,烟黑体积分数的峰值增大37.98%;活化能增加,火焰温度增大,烟黑体积分数减小,当活化能提高50%时,在轴向高度3 cm位置对应的火焰温度峰值增大3.41%,烟黑体积分数的峰值减小78.92%;表面生长反应速率常数中,指前因子增大,火焰温度逐渐减小,烟黑体积分数逐渐增大,当指前因子提高50%时,在轴向高度3 cm位置对应的火焰温度峰值减小2.03%,烟黑体积分数的峰值增大1.65倍;活化能增加,使火焰温度升高,烟黑体积分数减小,当活化能提高50%,在轴向高度3 cm位置对应的火焰温度峰值增大9.61%,当活化能提高12.5%,烟黑体积分数的峰值减小46.68%。     

掺氢比对低排放燃烧室性能影响研究

为了探究传统天然气燃气轮机对氢气燃料的适应性,基于现役某型工业低排放燃气轮机结构和性能,用数值模拟方法分析了燃料中氢气比例对低排放燃烧室性能的影响,确定了燃烧室燃用甲烷和氢气燃料的换用性能。研究表明：在1.0额定工况,掺氢比小于等于30%时,燃烧室不发生回火,喷嘴内部和火焰筒肩部回流区的温度以及燃烧室的总压损失随掺氢比的升高而升高,NOx排放体积分数小幅升高,CO排放体积分数减少;当掺氢比大于30%时,燃烧室发生回火,喷嘴和火焰筒肩部回流区温度、总压损失、NOx排放体积分数大幅升高,CO排放基本为零。在其他工况下,负荷变化对燃烧室边界条件影响较为复杂,对喷嘴回火边界影响无单调性变化规律。     

Combustion and emission characteristics of a spray guided direct-injection spark-ignition engine fueled with natural gas-hydrogen blends

Combustion and emission characteristics of a spray guided direct-injection spark-ignition engine fueled with natural gas-hydrogen blends were investigated. Results show that the brake thermal efficiency increases with the increase of hydrogen fraction and it shows an increasing and then decreasing trend with advancing fuel-injection timing. For later injection timings, the beginning of heat release is advanced with increasing hydrogen fraction, while the beginning of heat release is advanced and then retarded with the increase of hydrogen fraction at earlier injection timings. The flame development duration, rapid combustion duration and total combustion duration decrease with increasing hydrogen fraction. Maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise and maximum heat release rate show an increasing and then decreasing trend with the increase of hydrogen fraction. Brake NOx emission is increased and then decreased, while brake HC, CO and CO₂ emissions decrease with the increase of hydrogen fraction.     

预燃室式天然气掺氢发动机燃烧及排放模拟

为探索掺氢对预燃室式大功率中速天然气发动机燃烧和排放的影响,采用计算流体动力学耦合化学动力学方法,在一台6ACD320型天然气发动机上,对氢气体积分数为0～ 30％的天然气-氢气混合燃料的燃烧过程进行了数值模拟.结果表明:在天然气中掺氢促使缸内产生了更多的0、OH等活性自由基,从而加速了缸内火焰传播,发动机的指示燃气消...     

零碳燃料对燃气锅炉燃烧过程调控作用

利用小型化模拟炉膛开展了零碳燃料氢气对燃气锅炉燃烧过程调控作用实验研究,研究了掺氢比对炉膛内部预混火焰宏观形态、炉膛温度均匀性、炉膛污染物排放规律的影响,并总结了CO及NO_x的排放规律。实验结果表明：随着预混当量比增加,纯甲烷火焰长度逐渐缩短;对于20%掺氢火焰,随着预混程度的提高,火焰长度降低明显;不同火焰条件下,炉膛温度只由燃烧功率控制;改变燃烧条件时,处于壁面附近位置的温度变化较为平稳,而靠近火焰处温度变化较大;天然气中掺入氢气,燃烧时可以有效降低未燃CO排放;在相同预混程度下,全局当量比减小导致未燃空气增加,热量被稀释,火焰温度降低,热力型NO_x的生成降低;随着掺氢比的增加,燃烧时火焰温度升高,导致热力型NO_x排放增加。     

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

Effects of hydrogen addition on structure and NO formation of highly CO-Rich syngas counterflow nonpremixed flames under MILD combustion regime

The present study has numerically investigated the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, combustion processes and NO formation characteristics of the highly CO-rich syngas counterflow nonpremixed flames. To realistically predict the flame properties of the highly CO-rich syngas, the chemistry is represented by the modified GRI 3.0 mechanism. Computations are performed to precisely analyze the flame structure, NO formation rate, and EINO of each NO sub-mechanism. Numerical results reveal that the hydrogen enrichment and oxygen augmentation substantially influence the NO emission characteristics and the dominant NO production route in the CO-rich syngas nonpremixed flames under MILD and high temperature combustion regimes. It is found that the most dominant NO production routes are the NNH path for the lowest oxygen level (3%) and the thermal mechanism for the highest O₂ condition (21%). For the intermediate oxygen level (9%), the most dominant NO production routes are the NNH route for the hydrogen fraction up to 5%, the CO₂ path for the hydrogen fraction range from 5% to 10% and the thermal mechanism for the hydrogen fraction higher than 10%, respectively. To evaluate the contribution of the specific reaction on EINO the sensitivity coefficients are precisely analyzed for NO formation processes with the dominance of NNH/CO₂/Thermal mechanism under the highly CO-rich syngas flames.     

Experimental study of explosion dynamics of syngas flames in the narrow channel

This article introduced the experimental study of the propagation of a syngas premixed flame in a narrow channel. The structural evolution, flame front position and velocity characteristics of lean and rich premixed flames were investigated at different hydrogen volume fractions as the flame was ignited at the open end of the pipe and propagated to the closed end. The comparative study of the syngas fuel characteristics, flame oscillation frequency and overpressure oscillation frequency prove that the syngas explosion flame oscillation in the narrow passage has a strong coupling relationship with overpressure and fuel heat release rate. The results was shown that the flame structure was strongly influenced by the hydrogen volume fraction of the syngas and the fuel concentration. The distorted tulip flame only appears in lean mixture. At 30% of hydrogen volume fraction, the flame exhibits intense and unstable propagation, manifested as the reciprocating and alternating movement of the flame front. As the volume fraction of hydrogen increased, the velocity of flame propagation and the frequency of oscillation increased. When the hydrogen volume fraction γ ≥ 0.4 at the equivalence ratio of Φ = 0.8, the pressure oscillation amplitude gradually increases and reaching the peak after 200–320 ms. Significantly, when γ = 0.3, the pressure peak increases abnormally. This work can provide support for the safe use of syngas in industry by experimental study of various explosion parameters in the narrow channel.     

不同空气稀释剂对合成气扩散火焰NO_x生成特性的影响

针对合成气燃烧中NOx的生成机理,以结构简单的对冲火焰作为研究对象,利用化学反应动力学模型研究了不同稀释剂对火焰特性、自由基浓度及NOx生成的影响.结果表明:3种稀释剂降低NO排放效果的顺序为:CO2＞H2O＞N2,少量的CO2或H2O稀释空气时能有效地降低NOx排放;稀释剂量的增加对合成气中是否存在CH4时的影响趋势基本一致;合成气中CH4的存在降低了火焰温度和热力型NO生成,促进了快速型NO的生成;火焰拉伸率的提高使火焰温度和NO的生成降低.说明采用CO2和H2O稀释空气能有效抑制NOx的生成.     

Effect of different volume fractions of ammonia on the combustion and emission characteristics of the hydrogen-fueled engine

Hydrogen internal combustion engines (ICE) will play an important role in reducing carbon emissions, but low power density and abnormal combustion problems are the main obstacles restricting the promotion of hydrogen ICE. Ammonia is a low-reactivity renewable fuel. The purpose of this study is to study the effect of different ammonia-added volume fractions on hydrogen ICE. In this experimental study, the combustion and emission characteristics of an engine fueled by a hydrogen/ammonia mixture were evaluated at part-load operating conditions. The experiment was carried out on a modified engine, the engine speed was 1300 rpm, the absolute pressure of the manifold was 61 kPa, and the volume fraction of ammonia added was 5.2%, 7.96%, and 10.68%, respectively. The test results show that the addition of ammonia changes the combustion characteristics of hydrogen. As the volume fraction of ammonia added increases, the flame development period and flame propagation period are both prolonged, and the peak heat release rate decreases. The addition of ammonia increases the power of the engine and reduces the indicated thermal efficiency. At the ignition timing of the maximum braking torque, as the volume fraction of ammonia added increases, the indicated mean effective pressure and indicated thermal efficiency increase. Adding ammonia volume fraction has little effect on Nitrogen oxides (NOx) emissions, and NOx emissions gradually increase with the delay of ignition timing.