Thermal-diffusive instabilities, soot and carbon nanotube formation in unstrained diffusion flames

燃烧系统是一个复杂的机械系统，其中大量的化学反应同时发生，并与流体力学过程密切耦合。因此，在简单的结构中收集关于每种现象的基本知识，不受扰动影响，往往是有用的。在这个项目中，我们使用了一个独特的实验燃烧器来研究无应变扩散火焰中的燃烧相关现象。在这种配置中，反应物被单独提供给反应室，并且火焰具有一维内部结构，其允许与理论模型进行直接定量比较。虽然这种配置似乎远离实际应用，但它被广泛用于理论模型的发展，例如热扩散不稳定性。它也与小火焰概念有关，小火焰概念广泛用于燃气轮机中遇到的湍流部分预混燃烧的数值模拟。 这里介绍的研究计划针对两个主要课题，其中使用这种独特的工具可以提供新的洞察复杂的燃烧相关的现象特别感兴趣的加拿大能源转换部门。在期望减少来自化石或生物来源的烃燃烧的排放的背景下，若干产品是密集研究的主题。例如，二氧化碳（CO2）是主要的温室气体，烟尘等细颗粒物是严重的健康危害，氮氧化物（NOx）是城市污染和酸雨的原因。然而，减少这些排放中的一种的尝试通常是以增加另一种排放为代价的，或者可能导致不可接受的性能下降。 该提案中包括的第一个研究课题是调查接近消光极限的热扩散不稳定性（TDI）。目前的趋势是用贫混合气操作燃烧室，这是一种广泛用于降低燃气涡轮机应用中排放的技术，使其更容易受到TDI发展的影响，有必要进行研究以确定贫混合气中的安全操作条件。我们的工作旨在为扩散火焰稳定性理论模型的定量验证提供实验结果。第二个研究主题是扩散火焰中碳烟颗粒的形成和聚集。这些现象可以有利地研究使用我们的实验配置，其中可以形成稳定的烟灰层，并且可以精确地控制颗粒的停留时间。 此外，在上述燃烧器中进行观察后，又启动了两个研究项目。该提案的第三个研究轴侧重于火焰环境中碳纳米管形成的研究。更具体地说，我们的反应室中流动的无应变和一维性质为各种碳纳米结构的生长提供了独特的条件，包括没有催化剂的纳米管。该提案的第四个也是最后一个研究方向是需要开发技术，以浓缩和分类气溶胶，例如根据颗粒大小进行分类。我们正在实施一种基于使用声力的方法，对多分散亚微米气溶胶进行高级浓缩和分选。虽然最初的动机是我们希望从火焰同时产生的烟灰中分离碳纳米管，但这种基于声学的技术与能量转换领域的广泛工业应用有关。

Combustion systems are complex machines in which a large number of chemical reaction take place simultaneously and closely coupled with fluid mechanics processes. It is therefore often useful to gather fundamental knowledge on each phenomenon in simple configuration uncoupled from perturbing influences. In this project we are using a unique experimental burner to investigate combustion-related phenomenon in unstrained diffusion flames. In this configuration, the reactants are provided separately to the reaction chamber and the flame has a one-dimensional inner structure which allows for direct quantitative comparison with theoretical models. Although this configuration can seem far from practical applications it is used extensively for the development of theoretical models, for instance on thermal-diffusive instabilities. It is also relevant to the flamelet concept, used extensively for the numerical modelling of the turbulent partially-premixed combustion encountered in gas turbines. The research program presented here targets two main topics where the use of this unique tool can provide novel insight into complex combustion-related phenomena of particular interest for the canadian energy-conversion sector. In the context of the desire to reduce emissions from the combustion of hydrocarbons from fossil or biological origin, several products are the subject of intense research. For instance, carbon dioxide (CO2) is the dominant greenhouse gas, fine particulate such as soot are a serious health hazard and nitrogen oxides (NOx) are responsible for urban pollution and acid rain. However, attempts to reduce one of these emissions is often made at the expense of an increase in another or can result in an unacceptable decrease in performance. The first research topic included in this proposal is the investigation of thermal-diffusive instabilities (TDIs) that form close to the extinction limit. The current trend to operate combustors with lean mixtures, a technique used extensively to to lower emissions in gas turbine applications, makes them more susceptible to the development of TDIs and research is necessary to to identify safe operating conditions in lean mixtures. Our work aims to provide experimental results for the quantitative validation of theoretical models for diffusion flame stability. The second research topic focuses on the formation and aggregation of soot particles in diffusion flame. These phenomena can be advantageously investigated using our experimental configuration where a stable soot layer can be formed and the residence time of the particles can be controlled precisely. Additionally, two more research projects have been initiated following observation made in the burner mentioned above. The third research axis of this proposal focuses on the investigation of carbon nanotube formation in flame environments. More specifically, the unstrained and one dimensional nature of the flow in our reaction chamber offers unique conditions for the growth of a wide variety of carbon nano-structures, including nanotubes without catalysts. The fourth and final research axis of this proposal addresses the need to develop techniques to concentrate and sort aerosols, for instance as a function of the particle size. We are implementing an approach based of the use of acoustic forces to perform advanced concentration and sorting tasks on polydisperse sub-micron aerosols. Although initially motivated by our desire to separate the carbon nanotubes from the soot simultaneously produced by the flame, such acoustic-based techniques are relevant to a wide range of industrial applications in the energy conversion sector.