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个能源结构中，补充可再生但不稳定的电力来源，如风力涡轮机。显然，使它们尽可能干净、安静和高效是很重要的。不幸的是，燃气涡轮机产生超低NOx排放（NOx引起空气污染和呼吸问题）的条件也使燃烧室极容易发生热声不稳定性。相关的高振幅压力和火焰振荡导致破坏性振动，并使操作不切实际。热声不稳定性是由燃烧室中的声波和不稳定放热相互影响，导致正反馈而引起的。迫切需要能够将其作为燃气涡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缘的湍流，再到非常长的声波。在这个尺度范围内模拟可压缩反应流是非常昂贵的。低阶网络模型提供了一种计算快速的替代方案。他们把燃烧室的几何形状近似为一个简单的连接模块的网络，并利用了声波行为是线性的这一事实（即使对于大的振荡，非线性也只来自火焰）。此外，在低频下，声波的行为最多是二维的。这意味着声波的简单分析模型可以与火焰的计算流动模拟或实验的更复杂模型相结合。最近的工作，其中研究人员发挥了关键作用，已经表明（i）火焰非线性可以典型地通过弱非线性建模方法来捕获，并且（ii）这些弱非线性火焰模型可以通过“不可压缩”大涡模拟（LES），捕捉大的湍流特征（尽管“不可压缩”，但密度可随流动温度变化）。与可压缩模拟相比，使用不可压缩模拟在时间上大约节省一个数量级。对于单湍流火焰燃烧室，最近取得了重大进展，耦合低阶热声网络模型与不可压缩LES的火焰，并准确地预测不仅热声不稳定性的频率，而且其极限环振幅。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。本文将首先探索低阶热声网络建模和不可压火焰大涡模拟的新方法。这些都是需要的，以促进快速和准确的预测多燃烧器环形燃烧室，更有代表性的真实的气体涡轮机设置。我们将使用来自我们的合作者挪威NTNU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，以及当它们足够接近以相互作用时的更复杂的情况。我们将研究起作用的流动物理学-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，我们将开发一种基于英国的可配置光学燃烧装置，使用先进的激光成像测量强制火焰响应。这将有多个燃烧器的选择，并首次，多相燃料，可互换的燃烧室壁材料和可变的出口条件。它将为实际燃烧室的不稳定性提供深入的见解，并为建模者提供高保真的火焰和流动数据。我们的目标是预测热声不稳定性的存在下，这些现象，移动预测能力增加工业相关的燃烧室。

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.