Identification, Modeling and Control of Nonlinear Thermoacoustic Instability

燃烧系统(通常用于陆基燃气轮机、航空发动机和加热炉)中的自发压力和放热率振荡是由于系统的声学模式与燃烧源的非稳态放热率之间的耦合而引起的。这些自激振荡导致高噪声水平，增加NOx气体的排放，并缩短燃烧系统的使用寿命。在极端情况下，这种振荡的出现可能会导致系统完全故障。因此，对实际燃烧系统中热声不稳定性的研究具有巨大的推动力。最近对燃烧系统的研究表明，自激热声振动的固有动力学可以经历分叉，导致包括混沌动力学在内的高度复杂的非线性行为。以前，自激热声振动被限制在极限环行为。这一根本性的发现需要重新定义应用于热声系统的常规分析和控制策略。该项目的重点是研究热声振动的非线性动力学，并制定考虑自激振动非线性的主动控制策略。该项目确定了三个主要目标：a)开发识别热声振动的非线性特征的技术；b)开发和实施专门设计用于控制分叉和复杂(周期、准周期和混沌)非线性状态的基于模型的非线性控制策略；以及c)根据非线性振荡研究描述函数方法，并制定扩展以处理自激热声振荡的复杂行为以及极限环行为。这项研究不仅是对一种最有趣的自然发生的非线性耦合的基础研究，而且对解决动力和推进行业中最关键和最长期存在的问题之一也具有重要的现实意义。

Spontaneous pressure and heat release rate oscillations can get excited in combustion systems (employed commonly in land-based gas turbines, aero-engines and furnaces) due to coupling between the acoustic modes of the system and unsteady heat release rate from the source of combustion. These self-excited oscillations result in high noise levels, increased emission of NOx gases and reduced life span of the combustion system. In extreme cases, the emergence of such oscillations can lead to complete system failure. Hence, there is a huge impetus for investigations on thermoacoustic instability in practical combustion systems. Recent research on combustion systems has revealed that inherent dynamics of self-excited thermoacoustic oscillations, previously believed to be restricted to limit cycle behavior, can undergo bifurcations leading to highly complex nonlinear behavior, including chaotic dynamics. This fundamental discovery creates the need to redefine conventional analyses and control strategies applied in thermoacoustic systems. The focus of the proposed project is on investigating nonlinear dynamics of thermoacoustic oscillations and formulating active control strategies that take nonlinearities of self-excited oscillations into account. Three main objectives have been defined for the project: a) development of techniques for the identification of nonlinear characteristics of thermoacoustic oscillations, b) development and implementation of nonlinear, model-based control strategies, designed specifically to control bifurcations and complex (periodic, quasi-periodic and chaotic) nonlinear states, and, c) investigating the describing function approach in the light of nonlinear oscillations and formulating an extension to tackle complex behavior of self-excited thermoacoustic oscillations, in addition to limit cycle behavior. The proposed research is not only a fundamental study of one of the most interesting naturally occurring nonlinear coupling, but is also of a high and immediate significance to address one of the most crucial and long-standing issues in the power and propulsion industry.