摘 要

人类面临着不容乐观的能源形势以及日益严重的环境污染问题。减少使用汽油、柴油等石油基燃料，大力发展清洁替代燃料是解决能源危机和环境污染问题的有效方案。天然气具备抗爆性好，排放低，价格较低廉等优势，因此是一种理想的清洁替代燃料选择。天然气发动机在运行中会产生NO_x排放等问题，高压缩比天然气发动机的热负荷和机械负荷会加大，可靠性降低。废气再循环（EGR）或在进气中添加稀释气可解决发动机排放与可靠性问题。此外天然气成分中含有稀释气体，其含量随产地变化有所不同，使用天然气燃料时需要考虑稀释气含量。因此本文研究了稀释气体对LNG燃料层流燃烧特性的影响。

利用气相化学动力学软件CHEMKIN探究了N₂、CO₂、H₂O三种稀释气体对LNG燃料层流燃烧特性的影响，计算了不同初始条件下分别添加N₂、CO₂、H₂O三种稀释气体时天然气燃料的层流燃烧速度，并结合火焰结构分析、敏感性分析和基元反应速率分析对三种稀释气体的影响进行了阐释。结果表明：层流燃烧速度随着稀释比例的增大逐渐降低，随着当量比的增大先升高后降低，速度峰值出现在当量比1.0与1.0之间；随着稀释比增大，混合气的可燃当量比范围变窄；绝热火焰温度随稀释比增大而下降，CO₂稀释时下降最明显；加入稀释气后，主要活性基(H、OH、O、CH₃)浓度降低；OH和H是浓度最高的两种活性基，层流燃烧速度与这两种活性基浓度之和的峰值max[H OH]密切相关。

设计了几种虚拟物质，计算了虚拟物质稀释时天然气的层流燃烧速度，将计算结果与实际稀释气的模拟值进行对照，区分出了稀释气体的稀释、吸热与化学效应各自对层流燃烧速度的影响。结果表明：三种稀释气体的吸热效应是占最主要地位的影响效应；N₂稀释气体对天然气的层流燃烧速度没有化学影响，仅有稀释与吸热两种物理影响；CO₂稀释气体的化学影响随着稀释比增大逐渐减小，稀释与吸热效应的贡献率有所增加；H₂O稀释气体的三种效应贡献率在不同稀释比时都保持在比稳定的水平，其中稀释效应比例最小。

利用设计的虚拟物质，分析了CO₂和H₂O的化学效应对天然气层流火焰中活性基浓度、CO的生成等的影响。结果表明：CO₂稀释的化学效应表现为降低OH和H基的浓度，提高CO的浓度；H₂O稀释的化学效应则表现为提高OH基的浓度，降低H基的浓度，降低CO的浓度。而这两种稀释气体对CO的总影响效应均为降低CO的浓度。

最后，本文以当量比为1.0时的完全燃烧产物混合气来代表实际EGR气体，对EGR气体稀释时的层流燃烧速度，活性基浓度和温度与放热率分布等参数进行了计算。研究结果表明EGR气体中N₂占比最大，上述几个主要燃烧参数模拟值均介于N₂和H₂O稀释时的模拟结果之间。

关键词：LNG燃料，稀释气体，层流燃烧特性，CHEMKIN

Abstract

The energy situation and environmental pollution we humans facing are more and more seriously. To solve those two tough problems, one effective approach is reducing the use of gasoline and diesel, promoting the use of alternative clean fuels. Natural gas is an ideal choice of alternative clean fuel for its advantages over gasoline and diesel such as excellent anti-knock quality, low emission as well as low price. When natural gas engine is running, it will produce NOx emissions. High compression ratio will increase natural gas engines’ thermal and mechanical loads and reduce reliability. Using EGR technologies or adding dilution into intake can solve emission and reliability problems. Natural gas often contains diluent gases and its component varies in different producing areas. Considerations should be taken when using natural gas fuel.

Based on CHEMKIN package, this research has explored the effects of three diluent gases including nitrogen, carbon dioxide and water vapor on the laminar burning characteristics of LNG fuel. Calculations are made and laminar burning velocities in different initial condition and different dilution ratio of three gases are acquired. Explanation of the effects of diluent gases are made by means of analyses of flame structure, sensitivity coefficients and reaction rates. The results show that laminar burning velocities decrease with the increase of dilution ratio. When equivalence ratio increases, laminar burning velocities firstly increase and then decrease. Peaks of those velocities appear at equivalence ratio between 1.0 and 1.1. With the increase of dilution ratio, flammability limits of mixtures become narrower. Adiabatic flame temperatures also decrease as dilution ratio increases, and CO₂ dilution shows a sharpest downward trend of this temperature. Mole fraction of major active radicals including OH, H, O, CH₃ decrease due to the presence of three kinds of diluent gas. OH and H are the top two active radicals and the maximum of the sum of their mole fraction max[H OH] is closely related to laminar burning velocities.

In addition, several virtual dilution species are designed in this research and calculations of laminar burning velocities are made when adding these species into fuel/air mixture. By comparing the simulation results of virtual species and true diluent gases, three effects of dilutions in natural gas/air flame including dilution effects, thermal effects and chemical effects have been distinguished. Results show that thermal effects of all three diluent gases play a dominant role in the suppression of laminar burning velocity. Nitrogen have physical effects including thermal and dilution effect on laminar burning velocity, but it have no chemical effect on it. Carbon dioxide’s chemical effect reduces with higher dilution ratio and dilution and thermal effects contribute more. Contributions of three effects of water vapor are relatively stable when dilution ratio varies and dilution effect is the minimal effect.

Using those virtual species designed, this research has analyzed the chemical effects of carbon dioxide and water vapor on active radicals in laminar flame of natural gas and production of carbon monoxide. Results show that chemical effect of carbon dioxide will decrease the mole fractions of OH and H radicals. On the contrary, mole fraction of carbon monoxide will increase. Chemical effect of water vapor can increase the mole fractions of OH radicals and reduce the mole fractions of H radical and carbon monoxide. By the way, the total effects of these two diluent gases will reduce carbon monoxide’s mole fraction.

Finally, this research represent real EGR gas with complete combustion products where equivalence ratio equals to 1.0. Calculations of laminar burning velocities, mole fractions of active radicals, and temperature and heat production rate profiles in the condition of EGR gas dilution have been made. Results show that nitrogen is the main component of EGR gas and calculated values of those properties are between the results of nitrogen dilution and water vapor dilution.

Keywords: LNG fuel, Diluent gases, Laminar burning characteristics, CHEMKIN

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