【作者】 赵阳；

【导师】 王鹏翔；

【作者基本信息】 哈尔滨工业大学 ， 动力工程（专业学位）， 2022， 硕士

【摘要】 随着清洁燃烧技术的不断发展,第二代富氧燃烧技术表现出了无氮氧化物、硫氧化物排放优势的同时还具有较高的二氧化碳捕集率的优势从而得到众多研究机构及学者青睐。但是高含量CO₂或者H₂O作为稀释气体的引入也为燃烧带来了燃尽率降低、燃烧稳定性较差等挑战。因此,对如何改善利用CO₂或H₂O作为稀释气体条件下的燃烧特性研究具有重要意义。低温等离子体因其含有较多的自由基及激发态物质具有较强的化学活性,可以有效促进燃烧反应的进行,同时其作用效果在较广的参数范围内都较为明显,具有很强的适用性,在航空发动机、超燃冲压发动机及部分汽车引擎中都得到了广泛的研究并取得了较好的作用效果,因此在利用CO₂或H₂O作为稀释气体的近零排放系统燃烧中引入该技术是解决该系统中存在的燃烧稳定性及点火特性等问题的重要手段。在低温等离子体中,因为纳秒脉冲DBD等离子体具有能量效率高、放电稳定、产生的活性物质含量高等优点尤为适合用来进行强化点火及辅助燃烧方面的研究,因此本文利用纳秒脉冲DBD等离子体探究对利用CO₂或H₂O作为稀释气体条件下甲烷的点火特性及等离子体对燃烧的作用机制。在等离子体放电过程中约化场强值及气体成分等参数都会对放电的稳定性及放电能量去向产生巨大影响,进而影响到后续的燃烧反应,因此本文首先采用BOLSIG+免费程序对不同约化场强及气体成分下平均电子能量、汤生系数、能量损失系数比等重要放电参数进行了计算,计算结果表明相较于H₂O,利用CO₂作为稀释气可以使更多的电子能量用于激发燃料和氧气分子,从而更有效的促进燃烧反应进程,但汤生电离系数会有所下降汤生吸附吸收有所上升,导致放电稳定性较低。对于利用H₂O作为稀释气条件下约化场强约为150-200Td左右时沉积到燃料与氧气相关方向能量比例最高,利用CO₂作为稀释气条件下约化场强约为200-300Td左右时沉积到燃料与氧气相关方向能量比例最高。在对放电参数进行初步研究后,本文利用开源等离子体反应动力学程序ZDPlaskin与开源燃烧反应动力学程序CHEMKINⅡ联合求解,获得了不同初始温度、稀释气比例、放电约化场强下各工况点火延迟时间及点火增强倍数,主要自由基及激发态物质随时间演变规律和反应体系气体温度的变化规律,并利用敏感性分析和路径通量分析方法对影响点火特性的基元反应及激发态物质作用路径进行了研究。研究结果表明,当约化场强较低时（50Td）增强效果并不明显,但随着约化场强的升高,对点火的强化效果显著提高;等离子体对初始温度较低的条件下点火强化的效果要明显高于初始温度较高的条件下。基元反应H+O₂=OH+O具有最高的负敏感性,且随着放电约化场强的增加呈快速增加趋势,提高该反应的速率可有效降低点火延迟时间。等离子体中含有的主要参与反应的激发态物质中振动激发态甲烷可以有效参与化学反应生成CH₃,电子激发态的O₂及O可以有效参与化学反应中,通过O₂（A1）+CH₃=CH₂O+OH等反应生成大量OH等自由基,有利于燃烧反应的进行,但振动激发态O₂却基本全通过非弹性碰撞逐级衰减回基态再参与反应,等离子体放电中沉积的能量将以热能的形式散失,并不能很好的促进燃烧基元反应的进行,因此应当调整等离子体放电参数,使更多能量沉积至氧相关的电子激发态以起到更好的点火效果。更多还原

【Abstract】 With the continuous development of clean combustion technology,the second-generation oxy-fuel combustion technology has the advantages of no nitrogen oxides and sulfur oxides emissions,and also has a high carbon dioxide capture rate,and has been favored by many research institutions and scholars.However,the introduction of high content of CO₂ or H₂O as diluent also brings challenges such as reduced burnout rate and poor combustion stability.Therefore,it is of great significance to study how to improve the combustion characteristics under the condition of using CO₂ or H₂O as diluent gas.Low-temperature plasma has strong chemical activity because it contains more free radicals and excited species,which can effectively promote the combustion reaction.It has been widely studied in aero engines,scramjet engines and some automobile engines and achieved good effects.Therefore,the introduction of this technology in the combustion of near-zero emission systems using CO₂ or H₂O as diluent gas is an important method to solve the problems of combustion stability and ignition characteristics in the system.In low-temperature plasma,NSP-DBD plasma is especially suitable for research on enhanced ignition and assisted combustion because of its high energy efficiency,stable discharge,and high content of active species.Therefore,this paper uses NSP-DBD plasma.The ignition characteristics of methane under the condition of using CO₂ or H₂O as diluent gas and the mechanism of plasma on combustion were investigated in detail.In the process of plasma discharge,parameters such as the reduced electric field and gas composition will have a huge impact on the stability of the discharge and the deposit direction of the discharge energy,which affects the subsequent combustion reactions.The important discharge parameters such as the electron average energy,Townsend coefficient,and energy loss coefficient ratio under the different reduced electric field and gas composition were calculated.The calculation results show that compared with H₂O,the use of CO₂ as a diluent can make more electron energy used to excite fuel and oxygen molecules,thereby promoting the combustion reaction process more effectively,but the Townsend ionization coefficient will decrease and the Townsend attachment will increase,resulting in lower discharge stability.For the condition of using H₂O as the diluent gas,when the reduced electric field is about 150-200Td,the deposition of energy in the direction related to the fuel and oxygen is the highest,for the condition of using CO₂ as the diluent gas,when the reduced electric field is about 150-200Td,the deposition of energy in the direction related to the fuel and oxygen is the highest.After preliminary research on the discharge parameters,this paper uses the open source plasma reaction kinetics program ZDPlaskin and the open source combustion reaction kinetics program CHEMKINⅡto jointly solve the problems,and obtains rhe ignition delay time and ignition enhancement factor,the evolution of the main radicals and excited state species with time,and the change of the gas temperature of the reaction system under different initial temperatures,dilution gas ratios and reduced electric field.The sensitivity analysis and path flux analysis methods are used to analyze the elementary reactions and excited species that affect the ignition characteristics.The results show that the enhancement effect is not obvious when the reduced electric field is low（50Td）,but with the increase of the reduced field strength,the enhancement effect on ignition is significantly improved;The ignition enhancement effect of plasma under the condition of lower initial temperature is obviously higher than that under the condition of higher initial temperature.The elementary reaction H+O₂=OH+O has the highest negative sensitivity,and with the increase of the reduced electric field,increasing the rate of this reaction can effectively reduce the ignition delay time.In the excited species are mainly involved in the reaction contained in the plasma,the vibrationally excited methane can effectively participate in the chemical reaction to generate CH₃,and the electronically excited O₂ and O can effectively participate in the chemical reaction,through O₂（A1）+CH₃=CH₂O+OH The reaction generates a large number of free radicals such as OH,which is conducive to the progress of the combustion reaction,but the vibrationally excited O₂ basically quench to the ground state step by step through inelastic collisions and then participates in the reaction,and the energy deposited in the plasma discharge will be dissipated in the form of thermal energy,which can not play a good role in promoting the combustion elementary reaction,so the plasma discharge parameters should be adjusted so that more energy is deposited into the oxygen-related electronically excited species to achieve a better ignition effect.更多还原