CFD Modelling of the acoustic response of sprays

在可预见的未来，在能源生产和运输部门，特别是在航空业，燃烧化石燃料将是必不可少的。减轻这一点对环境的影响是一个关键的优先事项。为了满足对NOx排放的重要且日益严格的监管限制，“稀燃”燃烧室被视为至关重要的一步。稀燃燃烧器在燃烧区使用更多的空气，以降低火焰温度和污染物的形成。这样做的缺点是降低了火焰的“热声”稳定性。热释放的波动导致压力波，压力波影响燃料和空气进入燃烧室的流动，进而影响热释放。这种双向耦合可能会导致反馈回路，导致巨大的、破坏性的压力振荡。为了成功开发稀薄燃烧技术，必须控制这种热声不稳定性。在这方面，数值模拟工具起着至关重要的作用。如果可以在计算模拟中预测设计的稳定性或其他方面，而不需要测试物理原型，则可以节省大量资源。与单纯的实验相比，精确的数值模拟还可以更深入地了解所涉及的物理过程。大多数燃气轮机和所有航空发动机都使用液体燃料。燃料被分解成喷雾，与旋转的气流混合并蒸发。阻碍这类燃烧器中热声行为精确模拟的主要障碍之一是如何模拟声压波对喷雾形成和传输的影响。目前，必须使用假设和经验关系来定义时变的空气和燃料入口条件，这些假设和经验关系的适用性值得怀疑。特别是，它们没有考虑到气流对压力波的空气动力学响应中的延迟或“相移”，这对于捕捉燃烧系统的真正非稳定行为是至关重要的。这一建议旨在通过开发适当验证的计算方法来克服这一障碍，以模拟声波影响下复杂旋流中的喷雾。验证该方法需要获得适当的实验数据。这样的数据最近在拉夫堡大学(LU)产生。一种典型的稀燃燃料雾化器，燃料在两个同环状的旋转气流之间喷射，已经用声强迫进行了测试。可以获得流动中几个位置上随时间变化的燃料液滴大小、数量和速度的数据。这些数据将用于以下方面：1.将开发出在一定范围的声激励频率范围内获得准确的喷雾和进气条件的方法。这些结果将与喷嘴附近的实验数据进行比较，并与目前简化的计算喷雾入口条件的方法进行比较。我们将系统地研究所有方法中与假设相关的误差。通过使用路易斯安那大学开发的可压缩CFD方法，将改进喷油器独立气流通道中的气流对压力波响应的预测。2.将使用提案第一部分中制定的入口条件，模拟存在声强迫时的喷雾输送过程。这些模拟，根据进一步下游的实验数据进行验证，将是对进气条件和所采用的模拟方法的彻底测试。它们还将提供对所涉及的流动物理的更深层次的理解。这将包括湍流混合和声学强迫对不同大小的液滴的影响，以及喷嘴中不同气流的相对重要性。这一认识，加上改进的边界条件，将对未来低排放燃烧室的设计具有重要价值。

For the foreseeable future burning fossil fuels will be essential in the energy generation and transport sectors, particularly in aviation. Mitigating the environmental impacts of this is a key priority. In order to meet important, and increasingly stringent, regulatory limits on NOx emissions "lean burn" combustors are seen as a vital step. Lean burn combustors use more air in the combustion zone to reduce the flame temperature and the formation of pollutants. The downside to this is that the "thermoacoustic" stability of the flame is reduced. Fluctuations in heat release lead to pressure waves which affect the flow of fuel and air into the combustor which in turn can affect the heat release. This two-way coupling can lead to a feedback loop causing large, and damaging, pressure oscillations. For lean burn technology to be successfully developed this thermoacoustic instability must be controlled. Numerical simulation tools have a vital part to play in this. If the stability or otherwise of a design can be predicted in a computational simulation, without the need for testing a physical prototype then a great deal of resources can be saved. Accurate numerical simulation can also give a deeper understanding of the physical processes involved than experiments alone. Most gas turbines and all aero engines use liquid fuel. The fuel is broken up into a spray which mixes with and evaporates into a swirling air flow. One of the major hurdles standing in the way of accurate simulation of thermoacoustic behaviour in burners of this type is how to model the effect of acoustic pressure waves on the formation and transport of the spray. Currently assumptions and empirical relations have to be used to define the time varying air and fuel inlet conditions and these are of questionable applicability. In particular they take no account of the delay or "phase shift" in the aerodynamic response of the airflow to a pressure wave, which is vital for capturing the true unsteady behaviour of a combustion system. This proposal aims to overcome this hurdle by developing properly validated computational methods for simulating sprays in complex swirling flows under the influence of acoustic waves.Validation of this method requires access to appropriate experimental data. Such data has recently been produced at Loughborough University (LU). A typical lean burn fuel atomizer, in which fuel is injected between two co-annular swirling air streams, has been tested with acoustic forcing. Data is available for the time varying fuel droplet size, number and velocity at several locations in the flow. This data will be used in the following ways:1. Methods of deriving accurate spray and air inlet conditions for a range of acoustic excitation frequencies will be developed. These will be tested by comparison with the experimental data from close to the injector and also compared to the current simplified method of deriving the spray inlet conditions. The errors associated with the assumptions in all methods will be studied systematically. The prediction of air flow response to a pressure wave in the separate airflow passages of the fuel injector will be improved by the use of a compressible CFD method developed at LU. 2. Simulations of the spray transport process in the presence of acoustic forcing will be carried out using the inlet conditions developed in the first part of the proposal. These simulations, validated against experimental data taken further downstream, will be a thorough test of the inlet conditions and of the modelling methods employed. They will also provide a deeper understanding of the flow physics involved. This will include the effects of turbulent mixing and acoustic forcing on droplets of different sizes and the relative importance of the different airflow streams in the injector. This understanding, coupled with improved boundary conditions will be of great value in designing low emission combustors of the future.