Development of ultra-compact combustors for low-carbon technology using trapped vortex concepts

到2040年，能源需求将增长四分之一以上（国际能源署数据）。鉴于燃烧在满足这一需求方面的主导地位，必须开发低碳、高效的燃气涡轮机（GT）发动机，以减少排放影响并解决巴黎协定规定的全球变暖问题。近年来，稀薄预混技术由于其潜在的降低排放和高效率而引起了人们的兴趣。然而，稀薄燃烧易于发生不稳定性，这可能导致不必要的振荡、火焰熄灭和回火。使用低碳或零碳燃料（如氢）也受到限制，因为由于高的低热值（LHV），防止回火所需的高速可能会破坏涡流动力学。因此，需要进一步的发展，以实现更好的效率和更低的排放，有效的火焰稳定技术是至关重要的，这一发展。在超紧凑型燃烧室设计中，驻涡（TV）系统被实施在主区域或涡轮间区域中以增加燃烧气体的停留时间，从而导致更好的混合，从而更高的效率和更低的排放。更长的停留时间也意味着更短的燃烧室，因此更轻的发动机和更少的燃料消耗，也有助于多循环装置中的混合过程。TV稳定地锁定在腔内，因此即使在高速下对外部干扰也不太敏感，允许使用具有高LHV的低碳或零碳燃料，如氢。然而，火焰稳定的过程是相当复杂的，因为剪切和边界层（BL）涡动力学，强烈的热传递到壁和火焰传播和自燃过程的同时发生。火焰动力学的有效控制需要对这些过程有深入的了解。本项目旨在加深对用于超紧凑燃烧设计的TV系统中控制火焰稳定性的基本过程的理解，以及它们在贫预混技术背景下在高速（亚音速）条件下提供改进的火焰稳定性和低排放的潜力。特别是，TV物理学将被研究i）在存在代表燃烧室入口处的旋流动力学的径向加速流的情况下;和ii）在存在轴向加速流的情况下，当空腔位于燃烧室出口附近的收敛管道内时。旋转和轴向加速度都可以使涡流动力学不稳定，因此在有效地使用TV系统之前必须了解这种动力学。分析将通过高保真大涡模拟（LES）进行，这是一个具有成本效益的工具相比，昂贵的实验研究。通过这种方式，可以通过参数研究详细探索湍流、当量比和空腔几何形状的影响。此外，不同的替代燃料的性能和它们的含义在火焰稳定性和模型性能可以评估不同的电视设计。一个改进的模型，涉及假定PDF方法的基础上混合火焰/完全搅拌反应器将开发上述物理解释。该项目将通过前所未有的详细的直接数值模拟（DNS）以及项目合作伙伴提供的实验验证数据来获得对该发展的基本理解。该项目的成果将大大有助于现代低碳发动机的开发，并提高对这些设备中基础物理的理解。此外，该项目将导致可用于工业设计周期的CFD代码和模型的开发。因此，该项目对于劳斯莱斯等英国领先行业和其他新兴行业具有及时性和强烈的相关性，并将帮助他们保持在发电行业的领先地位。

Energy demand will be up by more than a quarter by 2040 [International Energy Agency data]. Given the dominance of combustion in meeting this demand, it is imperative to develop low-carbon, efficient gas turbine (GT) engines to reduce emissions impact and tackle the global warming as set by the Paris Agreement. In recent years lean premixed technology has attracted interest due to its potential of reduced emissions and high efficiency. However, lean combustion is prone to instabilities that may lead to unwanted oscillations, flame extinctions and flashbacks. Use of low or zero-carbon fuels like hydrogen is also limited because the high speeds needed to prevent flashbacks due the high low-heating values (LHV) can destabilise the vortex dynamics. Further development is thus required to achieve better efficiency and lower emissions, and effective flame holding techniques are crucial for this development. In ultra-compact combustor design, trapped vortex (TV) systems are implemented either in the primary zone or in the inter-turbine region to increase the resident time of combusting gases, resulting in better mixing, thus higher efficiency and lower emissions. Higher resident times also imply a shorter combustor, thus a lighter engine and less fuel consumption, also helping the process of hybridisation in multi-cycle devices. TV are locked stably within a cavity and thus are less sensitive to external disturbances even at high speeds, allowing use of low or zero-carbon fuels with high LHV like hydrogen. However, the process of flame stabilisation is rather complex because of the shear and boundary layer (BL) vortex dynamics, the strong heat transfer to the wall and the simultaneous occurrence of flame propagation and auto-ignition processes. The effective control of the flame dynamics requires a deep understanding of these processes.This project aims to develop improved understanding of the fundamental processes governing flame stabilisation in TV systems for ultra-compact combustion design, and their potential to deliver improved flame stability and low emissions at high speed (subsonic) conditions in the context of lean premixed technology. In particular, the TV physics will be studied i) in presence of a radially accelerating flow representing the swirled flow dynamics at the entrance of the combustion chamber; and ii) in presence of an axially accelerating flow when the cavity is located within the converging duct near the combustor exit. Both swirled and axial acceleration can destabilise the vortex dynamics, so this dynamics has to be understood before TV systems can be effectively employed. The analyses will be conducted through high-fidelity large eddy simulations (LES), which represents a cost-effective tool as compared to expensive experimental investigations. In this way the effect of turbulence, equivalence ratio and cavity geometry can be explored in details via parametric study. Moreover, the performance of different alternative fuels and their implication in terms of flame holding and model performance can be evaluated for different TV designs. An improved model involving presumed PDF approaches based on mixed flamelets/perfectly stirred reactor will be developed to account for the aforementioned physics. The fundamental understanding for this development will be extracted from unprecedented detailed direct numerical simulation (DNS) and by using validation data from experiments provided by the project partners.The outcomes of this project will significantly help the development of modern, low-carbon engines, and improve the understanding of the fundamental physics within these devices. Moreover, the project will lead to the development of CFD codes and models that can be used in industrial design cycles. Thus, this project is timely and strongly relevant for leading UK industries such as Rolls-Royce and other emerging industry, and will help them to maintain their leading role in the power-generation sector.