A Quantitative Survey of Combustion Intermediates towards Understanding of Plasma Assisted Combustion Mechanisms

这项研究的目标是了解非热等离子体辅助燃烧(PAC)的基本机制，并促进不同人群对等离子体、燃烧和清洁能源领域的兴趣。在等离子体引发的自由基、离子和激发的中性物种中，自由基可能在提高燃烧效率、减少污染物排放、缩短点火延迟时间、减少燃料混合物等方面发挥最重要的作用。然而，涉及自由基和其他活性物种的反应的速率常数和动力学机理在很大程度上仍然鲜为人知，这在很大程度上是因为难以获得点火前、点火中和点火后反应物/产物的绝对数密度的现场实验数据。这项建议对燃烧中间体的绝对数密度进行了调查，从而更好地理解了自由基在PAC中的作用。该项目开发的技术将可移植到研究其他类型的PAC，该项目产生的科学数据库将使更大的PAC社区受益。这项工作将有助于解释等离子体如何提高燃烧性能。研究工作包括：1)开发了环境空气中低速连续微波等离子体放电和超音速射流室内高速脉冲直流辉光放电辅助的两种新型燃烧系统；2)采用高灵敏度激光腔振荡谱(CRDS)技术以高空间分辨率近实时、原位测量多种燃烧中间体的绝对数密度；3)利用生成的粒子密度数据库建立关键基础反应的速率常数和动力学路径，这将为PAC新的动力学机理的发展奠定基础；和4)通过一套功能组合的诊断技术表征等离子体源和燃烧特性来验证动力学机制。新颖性有三个方面：1)CRDS能够近实时、就地测量绝对数密度；2)CRDS与宽可调(UV-MID-IR)、窄线宽激光光源相结合，使我们能够测量几乎所有重要的自由基和其他反应物种；以及3)简单燃料混合物和两种不同的等离子体的组合将有助于初始动力学建模和两个主要燃烧属性的研究：火焰行为和点火延迟时间。这些研究活动将有助于推动目前对PAC机理的理解，从完全依赖于已经建立的用于在没有低温等离子体的情况下燃烧的高温反应机理，到使用新的反应机理，这些新的反应机理建立在由低温非热等离子体增强燃烧的时间和空间上的实验数据库上。这项研究的社会效益在于燃油经济性、污染控制和各种应用中等离子体/燃烧操作的技术限制。例如，美国48%的电力来自燃烧过程，燃烧效率、燃料重整和燃烧污染是关键问题。在航空航天应用中，燃烧过程的技术极限受到高速和高海拔快速点火的苛刻需求的挑战。两名应用工程物理博士生和一名本科生将在这个项目中接受培训。从当地一所高中挑选出来的有数学和科学天赋的高中生也将参与这个项目。将建立一个小规模的等离子和燃烧研究研究站，以吸引本科生进行暑期研究；在当地社区的一系列年度外联活动期间，它将向高中生和K-12科学教师开放参观。拟议的研究将加强东南部清洁能源和激光诊断与控制方面的研究基础设施。

1066486WangThe goal of this study is to understand the fundamental mechanisms of nonthermal plasma-assisted combustion (PAC) and to promote the interests of a diverse population in the fields of plasma, combustion, and clean energy. Among plasma-initiated radicals, ions, and excited neutral species, the radicals may play the most important role in the enhancement of PAC for increased combustion efficiency, reduced pollutant emissions, short ignition delay time, leaner fuel mixtures, etc. However, rate constants and kinetic mechanisms of the reactions involving radicals as well as other reactive species remain little known, to a large extent, due to the challenge of obtaining time-resolved, in situ experimental data on the absolute number densities of reactants/products of the reactions pre-, during-, and post-ignition. This proposal conducts a survey of absolute number densities of the combustion intermediates, leading to a better understanding of the role of radicals in PACs. The technologies developed in this project will be transportable to study other types of PACs and the scientific database generated in this project will benefit the greater PACs community. This work will help to explain how plasma can enhance combustion performance. The research effort includes: 1) Develop two novel combustion systems assisted by a continuous microwave plasma discharge with low-speed flows in ambient air and by a pulsed DC glow discharge with high-speed flows in a supersonic jet chamber; 2) Employ high sensitivity laser cavity ringdown spectroscopy (CRDS) technique to measure absolute number densities of multiple combustion intermediates in near real-time, in situ with high spatial resolution; 3) Use the generated particle densities database to establish the rate constants and kinetic pathways of key fundamental reactions, which will lead to the development of new kinetic mechanisms for PACs; and 4) Validate the kinetic mechanisms through characterization of the plasma sources and combustion properties by using a suite of functionally-combined diagnostic techniques.The novelty is three-fold: 1) CRDS is capable of measuring absolute number densities in near real-time, in situ; 2) CRDS in combination with a widely tunable (UV - mid-IR), narrow-linewidth laser source enables us to measure almost all important radicals and other reactive species; and 3) Combination of the simple fuel mixtures with two distinct plasmas will facilitate the initial kinetic modeling and the investigation of two major combustion attributes: flame behavior and ignition delay time. The research activities will help advance the current understanding of PAC mechanisms from entirely depending on high-temperature reaction mechanisms that have been established for combustion without nonthermal plasma to using the new reaction mechanisms that are built on an experimental database obtained temporally and spatially in combustions enhanced by a low-temperature nonthermal plasma. The social benefits of this study lie in the areas of fuel economy, pollution control, and technological limits of plasma/combustion operation in various applications. For instance, 48% of electricity in the US comes from combustion processes, where combustion efficiency, fuel reforming, and combustion pollution are critical issues. In aerospace applications, technological limits of combustion processes are challenged by the demanding needs of fast ignition at high speed and high altitudes. Two Ph.D. students and one undergraduate student in Applied Engineering Physics will be trained in this project. Selected math and science talented high school students from a local high school will also be involved in this project. A small-scale Research Station for Plasma and Combustion studies will be established to attract undergraduate students for summer research; and it will be open for tours to high school students and K-12 science teachers during a series of annual outreach events in the local community. The proposed research will enhance research infrastructure in clean energy and laser diagnostics and control in the southeast.