Combustor thermoacoustics for multi-burner low emissions gas turbines (CHAMBER)

燃气轮机发电并为我们的飞机提供动力。它们将长期存在于整体能源结构中，作为风力涡轮机等可再生但不稳定的能源的补充。显然，让它们尽可能干净、安静和高效是很重要的。不幸的是，燃气轮机产生超低氮氧化物排放（氮氧化物引起空气污染和呼吸问题）的条件也使燃烧室极易产生热声不稳定。伴随的高振幅压力和火焰振荡导致破坏性振动，使操作不切实际。热声不稳定性是由声波和燃烧室内的非定常放热相互影响，导致正反馈引起的。迫切需要能够将其作为燃气轮机设计过程的一部分进行预测，以便能够“设计出来”。这在目前是不可能的。计算模拟热声不稳定性需要解决许多长度尺度-从微小的化学反应尺度，到火焰前起皱，再到非常长的声波。在这个范围内模拟可压缩的反应流是非常昂贵的。低阶网络模型提供了一种计算速度快的替代方案。他们将燃烧器的几何形状近似为一个简单连接模块的网络，并利用声波行为是线性的这一事实（即使是大的振荡，非线性也只来自火焰）。此外，在低频时，声波最多表现为二维。这意味着声波的简单分析模型可以与火焰的更复杂的计算流模拟或实验模型相结合。研究人员在最近的工作中发挥了关键作用，表明(i)火焰非线性通常可以通过弱非线性建模方法捕获，（ii）这些弱非线性火焰模型可以通过“不可压缩”大涡模拟（LES）获得，该模拟可以捕获大湍流特征（尽管“不可压缩”，但密度可以随流动温度变化）。与可压缩模拟相比，不可压缩模拟的使用在时间上节省了大约一个数量级。对于单湍流火焰燃烧室，最近取得了重大进展，将低阶热声网络模型与火焰的不可压缩LES耦合，不仅可以准确预测热声不稳定的频率，还可以准确预测其极限环幅值。现在是时候尝试快速和准确的预测越来越现实的燃烧室设置。提议的工作有两个方面。它将首先寻求开发低阶热声网络建模和火焰不可压缩LES的新方法。这些都是促进多燃烧器环形燃烧器的快速和准确的预测所必需的，更能代表真实的燃气轮机装置。我们将使用来自我们的合作伙伴挪威理工大学的世界级实验燃烧设施的数据进行验证。我们将考虑独立燃烧的火焰，以及更复杂的情况，当它们靠近到足以相互作用时。我们将研究流动物理的作用-周向声波，火焰非线性和火焰-火焰相互作用的综合效应。同时，我们将开发一个基于英国的可配置光学燃烧装置，使用先进的激光成像来测量强迫火焰响应。这将有多个燃烧器的选择，第一次，多相燃料，可互换的燃烧室壁材料和可变的出口条件。它将提供深入了解不稳定的实际燃烧器，高保真火焰和流动数据的建模。我们的目标是在这些现象的存在下预测热声不稳定性，将预测能力转向增加工业相关的燃烧器。

Gas turbines generate electricity and power our aeroplanes. They will be a long term presence in the overall energy mix, complementing renewable but inconsistent sources of power, such as wind turbines. It is clearly important to make them as clean, quiet and efficient as possible. Unfortunately, the very conditions under which gas turbines produce ultra low NOx emissions (NOx cause air pollution and breathing problems) also make the combustor extremely prone to thermoacoustic instability. The associated high amplitude pressure and flame oscillations lead to damaging vibration, and make operation impractical. Thermoacoustic instability is caused by acoustic waves and unsteady heat release in the combustor mutually affecting one another, leading to positive feedback. There is an urgent need to be able to predict it as part of the gas turbine design process, so that it can be "designed out". This is currently not possible.Computationally simulating thermoacoustic instability requires many length scales to be resolved - from the tiny chemical reaction scales, to flame-front wrinkling, to the very long acoustic waves. Simulating compressible reacting flow over this range of scales is prohibitively expensive. Low order network models provide a computationally fast alternative. They approximate the combustor geometry as a network of simple connected modules, and exploit the fact that the acoustic wave behaviour is linear (even for large oscillations, nonlinearity comes only from the flame). Furthermore, at low frequencies the acoustic waves behave at most two-dimensionally. This means that simple analytical models for the acoustic waves can be coupled with more complex models, from computational flow simulations or experiments, for the flame.Recent work, in which the investigators have played a key role, has shown that (i) the flame nonlinearity can typically be captured via a weakly nonlinear modeling approach and (ii) that these weakly nonlinear flame models can be obtained by "incompressible" large eddy simulations (LES), which capture large turbulent flow features (although "incompressible", the density can change with flow temperature). The use of incompressible simulations saves roughly an order of magnitude in time compared to compressible simulations. For single turbulent flame combustors, major progress was recently achieved by coupling a low order thermoacoustic network model with incompressible LES of the flame, and accurately predicting not only the frequency of thermoacoustic instability, but also its limit cycle amplitude. It is now timely to attempt fast and accurate predictions for increasingly realistic combustor setups.The proposed work is two-fold. It will firstly seek to develop new methods for low order thermoacoustic network modeling and for incompressible LES of flames. These are needed to facilitate fast and accurate predictions for multi-burner annular combustors, more representative of real gas turbine set-ups. We will use data from a world-class experimental combustion facility at our collaborators, NTNU in Norway, for validation. We will consider flames which burn independently, and more complicated cases when they are close enough to interact with one another. We will investigate the flow physics at play - the combined effect of circumferential acoustic waves, flame nonlinearity and flame-flame interactions.At the same time, we will develop a UK-based configurable optical combustion rig to measure forced flame response using advanced laser imaging. This will have the option for multiple burners, and for the first time, multi-phase fuel, interchangeable combustor wall materials and variable exit conditions. It will provide insights into instability in practical combustors, and high-fidelity flame and flow data for modellers. We aim to predict thermoacoustic instability in the presence of these phenomena, moving predictive capability towards increasing industrially relevant combustors.