Effects of Water Vapor Addition on the Flame Structure and Soot Formation in a Laminar Ethanol/air Coflow Flame
Effects of Water Vapor Addition on the Flame Structure and Soot Formation in a Laminar Ethanol/air Coflow Flame
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DOI：
10.1080/00102202.2019.1667340
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发表时间：
2019-09
影响因子：
1.9
通讯作者：
Liang Qiu;Yuheng Zheng;Yang Hua;Y. Zhuang;Y. Qian;Xiaobei Cheng
中科院分区：
文献类型：
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作者：
Liang Qiu;Yuheng Zheng;Yang Hua;Y. Zhuang;Y. Qian;Xiaobei Cheng
摘要乙醇是一种很有前途的汽油机替代燃料，但乙醇应用于汽油直喷（GDI）发动机时仍然存在颗粒物（PM）排放问题，特别是在高负荷下。喷水是一种有效的降低柴油机微粒排放和爆震的方法。本文采用数值模拟的方法研究了水蒸气对乙醇/空气同向扩散火焰结构和碳烟生成的影响。提出了一种完整的分步解耦方法，通过引入几个虚拟组分，数值分离了水汽的稀释、密度、输运、热效应、辐射效应和化学效应。结果表明，水蒸气的稀释效应和热效应是影响火焰温度分布的主要因素，其次是辐射效应和化学效应。OH自由基的摩尔分数增加是由于H2O + H <=> OH + H2和O + H2O <=> OH + OH反应的促进作用，而CO转化为CO2则是由于CO + OH <=> CO2 + H反应的化学效应。碳烟体积分数也降低，其中稀释和化学效应起着最重要的作用。氢提取碳添加（HACA）的表面生长和OH氧化的碳烟的质量添加和消耗的过程，分别控制。HACA率主要受稀释作用和化学作用的影响。而化学作用对OH氧化速率的影响不大，在化学作用下，OH与碳烟颗粒的碰撞几率几乎不变，OH浓度增加，碳烟体积分数减少.
ABSTRACT Ethanol is a promising alternative fuel to gasoline engines; however, the problem of particulate matter (PM) emissions still exists when ethanol is applied to gasoline direct injection (GDI) engines, especially under high load. Water injection is proved to be an effective way to reduce the PM emissions and engine knocking, simultaneously. In this work, a numerical calculation was carried out in an ethanol/air coflow diffusion flame to investigate the effects of water vapor addition on the flame structure and soot formation. A complete step-by-step decoupling method was proposed to numerically isolate the dilution, density, transport, thermal, radiative and chemical effects of water vapor by introducing several virtual species. Results show that the temperature distribution of the flame is mainly affected by the dilution effect and thermal effect of water vapor, followed by radiative effect and chemical effect. The mole fraction of OH radical increases because of the enhancement of the reaction H2O + H <=> OH + H2 and O + H2O <=> OH + OH, while the conversion of CO to CO2 is promoted through the reaction CO + OH <=> CO2 + H, due to the chemical effect. Soot volume fraction is also reduced, where the dilution and chemical effects play the most important roles. Hydrogen-abstraction-carbon-addition (HACA) surface growth and OH oxidation are the processes controlling the soot mass addition and consumption, respectively. HACA rate is mainly affected by the dilution and chemical effects. However, chemical effect has little influence on the OH oxidation rate, since the collision probability of OH with soot particles remains almost unchanged as a result of the increase of OH concentration and the decrease of soot volume fraction under the chemical effect.