进气加湿对船用柴油机燃烧和排放的影响

运用CFD数值模拟软件AVL Fire建立了船用柴油机燃烧过程模型,研究不同相对湿度的进气成分对船用柴油机燃烧和排放特性的影响,并着重研究对NO_x和碳烟排放的影响规律和作用机理.结果表明:随着进气加湿率的增加,缸内压力和燃烧温度均有降低;当加湿率达到100%,时,峰值压力相比进气为干空气时下降0.67,MPa,最高燃烧温度下降220,K;滞燃期和燃烧持续期延长,预混燃烧比例增多;燃烧重心CA50向后偏移,燃烧定容度和热效率下降.NO_x排放不断减少,燃烧火焰温度的降低和水蒸气对富氧区的稀释是NO_x排放下降的主要原因;碳烟排放随着加湿率的增加不断恶化,进气加湿虽然可以促进油气混合、抑制碳烟生成,OH基团加速碳烟前驱物的氧化,但加湿后氧气质量分数的减少和燃烧温度的降低导致碳烟后期氧化能力严重减弱.     

混合气初始状态对微型燃烧室内HCCI燃烧的影响

基于微型燃烧室内自由活塞单次压缩实现均质充量压燃(homogeneous charge compress ignition,HCCI)燃烧的可视化试验,结合甲烷的详细化学反应动力学机理及动网格技术,建立了三维动网格模型,将自由活塞运动与燃烧过程相耦合,对不同初始状态下微型燃烧室内HCCI燃烧特性进行了数值模拟,得到了不同初始温度、初始压力、当量比及混合气泄漏下的燃烧特性及动力特性的变化规律。研究结果表明:初始温度、初始压力及当量比对微型燃烧室内HCCI燃烧影响较大,随着初始温度的升高,微型燃烧室内HCCI压缩着火范围扩大,但随着初始压力的增大,压缩比降低,压缩着火范围减小,当量比的变化显著影响微型燃烧室内HCCI压缩燃烧的最高温度和最高压力,混合气泄漏主要影响膨胀过程,对动力性能影响非常显著。     

氢混天然气燃气轮机加湿低氮燃烧性能研究

为了解贫预混燃烧室天然气掺氢加湿燃烧时的性能变化和容许加湿范围,解决氢混燃气轮机NO_x排放超标问题,以某燃气轮机燃烧室为研究对象,数值研究了掺氢比和加湿比对燃烧性能及污染物排放特性的影响。结果表明：燃料无加湿条件下,燃烧室出口CO和CO₂排放值随着掺氢比的增加而减小,较高燃烧温度将导致热力型NO_x排放值增加,掺氢比达到0.2以上时,NO_x排放已超出环保限值;燃料加湿条件下,随着加湿程度增加,燃气出口平均流速及水蒸气组分含量均增加,燃烧筒内全局温度、CO₂和NO_x排放值均降低,CO排放值先降低后增加;掺氢天然气加湿可实现低氮燃烧,考虑到低掺氢工况燃气轮机功率输出效能和高掺氢工况燃烧性能恶化问题,水蒸气加湿量不宜过多,当掺氢比为0.3时,推荐燃料加湿比为0.463。     

加湿热空气对流冷凝换热冷凝液量的实验研究

以冷凝式燃气锅炉的冷凝受热面为原型,采用加湿热空气模拟燃气锅炉的尾部烟气,通过单列光管间壁式换热器进行冷凝换热,在较宽广的加湿热空气温度(100～200℃)和水蒸气体积分数(4%～16%)范围内研究了冷凝液量的生成规律.实验结果表明:水蒸气分压力、冷却水流量和加湿热空气流量是影响水蒸气冷凝液量的主要因素,水蒸气的凝结率为40%～75%.由实验数据进行多元线性回归分析,提出了一个新的计算冷凝液量的经验关系式.     

稀释气体对天然气掺甲醇裂解气定容预混燃烧的影响

用H_2、CO体积比为2∶1的混合气来模拟甲醇裂解气(dissociated methanol,DM),在温度343K、压力0.3MPa的条件下基于燃烧压力、最高燃烧压力、爆炸指数及临界半径等参数研究了CH_4(60%)-DM(40%)预混气在添加稀释气体(N_2、CO_2)后的燃烧的变化情况。在不添加稀释气体的条件下,进行了同样初始温度和压力下的天然气(100%/60%/20%)-甲醇裂解气(0%/40%/80%)-空气预混燃烧的对比试验。结果表明:甲醇裂解气能加快压力上升,促进燃烧,缩短燃烧持续期,加速火焰胞状结构的出现,但在化学计量比及附近会降低最高燃烧压力;稀释气体则有降低最高燃烧压力和爆炸指数等抑制燃烧的作用,其中CO_2对燃烧的抑制作用强于N_2。     

生物质热解气化特性分析

通过对玉米秆、小麦秆、棉花秆、稻草、松木屑等生物质进行热重分析和差热分析,分析了生物质气化过程的裂解机理。研究中采用松木屑作为气化原料,得出了气化温度、压力、水蒸气加入量等反应条件对气化产物的产率、组成成分及焦油产率的影响规律,研究结果表明,温度对生物质气化的影响最大,当温度为800℃时气体的产率最高。     

基于铁矿石载氧体加压煤化学链燃烧的试验研究

在反应温度为970℃、压力范围为0.1～0.6 MPa的条件下,以铁矿石为载氧体,采用固定床反应器,对煤化学链燃烧进行了试验研究,考察了加压对燃料反应器内水蒸气气氛下煤化学链燃烧的反应特性.结果表明:加压能加快煤水蒸气气化速率,加强水气转换反应,并对煤气组分产生影响,使CO浓度降低,CO2和H2浓度升高;加压后还原反应烟气中不再含有H2,CO和CH4的浓度也变得很低,说明加压可提高还原反应中煤气的转化率;随着压力的升高,碳转化率先升高后又降低,存在着一个中间压力值,使碳转化率最高.     

热力参数对微尺度发动机燃烧特性影响机理分析

为了研究初始温度和压力对微尺度活塞式内燃机微燃烧特性的影响机理,采用层流有限速率模型和甲醇燃烧化学反应机理及动态网格技术,对其闭口热力系的瞬态预混层流燃烧过程进行动态多维仿真。结合化学反应机理温度敏感性分析和产物反应速率分析技术,从宏观和微观两个角度分析仿真结果,结果表明仿真结果与试验结果吻合较好。随着气体初始温度提高,自由活性基团浓度增加,以具有最大正温度敏感系数的反应为代表的基元反应速率增大,燃烧过程加快,缸内压力和温度增加。随着气体初始压力提高,由于H基竞争基元反应速率随压力增加的增加规律不同,使得部分曲轴转角下燃烧速率反而下降,缸内温度有所下降,但压力显著增加。从整体过程来看,燃烧速率随着初始压力的增大而增大。随着初始温度的增加,以具有最大生成排放物反应速率的反应为代表的基元反应速率增大,排放增加。     

加湿燃烧研究现状

首先从两个方面详细介绍了燃料加湿燃烧的研究成果,一方面是加湿对燃气轮机燃烧室内燃烧特性的影响,另一方面是加湿对燃料着火特性、可燃极限的影响。分析了目前加湿燃烧研究所取得的结论,空气加湿燃烧可以降低火焰燃烧温度、改变燃烧过程中的化学反应以及传热传质过程,影响火焰的流场和结构,使火焰趋于不稳定。同时燃烧区温度的降低有利于降低NOx排放,对HAT、IGCC等热力循环中降低NOx排放有重要意义,最后指出了目前加湿燃烧存在的不足,并结合本课题组的研究结论为后续研究方向提出了建议。     

纳米铝粉在水蒸气中的燃烧特性

针对粒径对纳米铝粉燃烧性能的影响,研究了不同粒径的纳米铝粉在水蒸气气氛中的燃烧特性,同时与其在空气气氛中的燃烧特性进行比较.结果表明,纳米铝粉粒径对其在水蒸气气氛中的着火温度影响较大,但对其燃烧的最高温度影响较小.纳米铝粉在水蒸气气氛中的着火温度比在空气气氛中的着火温度低,且最高温度也低.从燃烧现象可以发现,纳米铝粉在空气气氛中比在水蒸气气氛中燃烧剧烈,火焰亮度更强.此外,还采用扫描电镜与能谱分析联用技术对纳米铝粉在不同气氛中的燃烧产物进行了分析.     

天然气/氢气燃烧特性研究

在定容燃烧弹中研究了不同氢气掺混比例、燃空当量比和初始压力下的大然气／氢气混合气的燃烧特性，建立了适合用于容弹计算的准维双区模型。研究结果表明：在各种当量比和初始压力下，随着掺氢比例的增加，混合气的质量燃烧速率明显增加，燃烧持续期和火焰发展期娃著缩短。随着掺氢比例的增加，短的燃烧持续期所对应的当量比范围变宽，稀混合气和浓混合气条件下天然气掺氢对火焰发展期缩短的效果更明显。化学计量比附近（1．0—1．1）掺氢燃烧对燃烧最大压力值影响不大，浓混合气（燃空当量比大于1．1）和稀混合气燃烧时，随着掺氢比例的增加，最大燃烧压力值增加。     

Counter-current hydrogen–oxygen vortex combustion chamber. Thermal physics of processing

The expediency is substantiated for the use of a vortex counter-current circulation flow of water steam and the stoichiometric hydrogen-oxygen mixture as a source of heat generation. That flow is essential for the purpose of making efficient models of high-temperature combustion chambers for the cogeneration cycles.An experimental investigation was carried out of hydrogen-oxygen mixture combustion in a counter-current combustion chamber-superheater. Operating range values of heat power were obtained. A maximum temperature of the superheated steam at the combustion chamber outlet was 1350 K. There were determined the optimal modes of hydrogen-oxygen mixture outflow ensuring its steady combustion in the counter-current flow core of the water steam.     

Effect of excess hydrogen on hydrogen fueled internal combustion engine under full load

Detailed hydrogen-air chemical reaction mechanisms were coupled with three dimension grids of an experimental hydrogen fueled internal combustion engine (HICE) to establish a combustion model based on CONVERGE software. The influence of excess hydrogen coefficient on the combustion and emission characteristics of HICE under full load was studied based on the CFD model. Simulation results showed that excess hydrogen leaded to higher concentration of OH species in flame front, and quicker hydrogen-oxygen reaction and flame propagation speed, which in turn leaded to higher pressure and temperature in cylinder. The rise of pressure and temperature in turn contributed to the increase of indicate power but un-burned hydrogen leaded to decrease of efficiency. NOx, especially NO emissions decreased significantly with excess hydrogen under full load not only because increased of H concentration, and decreased of O and OH concentration, which leaded to reverse reaction of NO formation through thermal NO routes. Low excess hydrogen coefficient can achieve a good trade-off between power and emissions under full load.     

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Effect of ignition timing and hydrogen fraction on combustion and emission characteristics of natural gas direct-injection engine

An experimental study on the combustion and emission characteristics of a direct-injection spark-ignited engine fueled with natural gas/hydrogen blends under various ignition timings was conducted. The results show that ignition timing has a significant influence on engine performance, combustion and emissions. The interval between the end of fuel injection and ignition timing is a very important parameter for direct-injection natural gas engines. The turbulent flow in the combustion chamber generated by the fuel jet remains high and relative strong mixture stratification is introduced when decreasing the angle interval between the end of fuel injection and ignition timing giving fast burning rates and high thermal efficiencies. The maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise and maximum heat release rate increase with the advancing of ignition timing. However, these parameters do not vary much with hydrogen addition under specific ignition timing indicating that a small hydrogen fraction addition of less than 20% in the present experiment has little influence on combustion parameters under specific ignition timing. The exhaust HC emission decreases while the exhaust CO₂ concentration increases with the advancing of ignition timing. In the lean combustion condition, the exhaust CO does not vary much with ignition timing. At the same ignition timing, the exhaust HC decreases with hydrogen addition while the exhaust CO and CO₂ do not vary much with hydrogen addition. The exhaust NO_x increases with the advancing of ignition timing and the behavior tends to be more obvious at large ignition advance angle. The brake mean effective pressure and the effective thermal efficiency of natural gas/hydrogen mixture combustion increase compared with those of natural gas combustion when the hydrogen fraction is over 10%.     

Effects of hydrogen and carbon dioxide on the laminar burning velocities of methane–air mixtures

The effects of different mole fractions of hydrogen and carbon dioxide on the combustion characteristics of a premixed methane–air mixture are experimentally and numerically investigated. The laminar burning velocity of hydrogen-methane-carbon dioxide-air mixture was measured using the spherically expanding flame method at the initial temperature and pressure of 283 K and 0.1 MPa, respectively. Additionally, numerical analysis is conducted under steady 1D laminar flow conditions to investigate the adiabatic flame temperature, dominant elementary reactions, and NO formation. The measured velocities correspond with those estimated numerically. The results show that increasing the carbon dioxide mole fraction decreases the laminar burning velocity, attributed to the carbon dioxide dilution, which decreases the thermal diffusivity and flame temperature. Conversely, the velocity increases with the thermal diffusivity as the hydrogen mole fraction increases. Moreover, the hydrogen addition leads to chain-branching reactions that produce active H, O, and OH radicals via the oxidation of hydrocarbons, which is the rate-determining reaction. Furthermore, an increase in the mole fractions of hydrogen and carbon dioxide decreases the NO production amount.     

燃烧边界条件对乙醇均质压燃燃烧的影响

在一台由CA6110柴油机改造而成的单缸发动机上进行了燃烧边界条件对乙醇燃料均质压燃（HCCI）燃烧过程影响的试验研究。结果表明，在转速和进气温度一定时，随着过量空气系数的增加，着火始点推迟，燃烧持续期变长，缸内的最大燃烧压力降低，放热率降低，φ50（50％乙醇燃烧放热量所在的曲轴转角）位置推迟，燃烧效率降低；在发动机转速、进气温度和过量空气系数一定时，随着EGR率的升高，着火始点推迟，燃烧持续期延长，φ50位置推迟，放热速率降低，压力升高率变小，缸内最大燃烧压力减小，燃烧效率降低。在转速和供油量一定时，随着进气温度的升高，着火始点提前，燃烧持续期变短，压力升高率变大，缸内的最大燃烧压力变大。得到了发动机转速、过量空气系数和对应于最大指示热效率点的进气温度间的MAP图。