Combustion dynamics of turbulent swirl flames with hydrogen addition

燃烧不稳定性是阻碍低排放航空和工业燃气轮机燃烧室发展的最严重问题之一。为了实现高效、低排放的性能，必须有稀油和最好是预混合的操作条件。然而，这些稀薄燃烧室的缺点是特别容易受到热声不稳定性的影响。这些不稳定性的特征是燃烧室中由于热声和流体动力过程之间的复杂相互作用而产生的强烈压力振荡。当压力或速度振荡与非定常放热有利耦合时，可能会产生较大幅度的自持振荡。这些高幅度的振荡会对燃烧室的性能产生不利影响，并可能导致系统的灾难性故障。为了减少排放和提高燃油经济性，燃气轮机发动机制造商越来越多地采用稀油预混概念。尽管燃料贫化条件通过降低火焰温度来减少NOx排放，但贫化火焰特别容易受到燃烧振荡和喷出的影响。氢气富集法是提高燃烧室在极端贫化条件下稳定运行的一种很有前途的方法。富氢还改善了火焰的可燃性和对应变和曲率的响应。这些好处表明，氢气浓缩在低排放燃气轮机燃烧技术的发展中具有很好的作用。然而，在燃烧不稳定的背景下，富氢火焰的响应还没有完全被理解。因此，这项研究的主要动机是理解和支持燃烧振荡背景下氢添加对热释放的调节机制。有几个众所周知的机制可以促进稀薄火焰中放热的波动，即混合比的变化，火焰对压力/速度振荡的敏感性，以及涡旋的形成和脱落。这些机制中的任何一种都可以通过正反馈导致燃烧振荡的幅度增加，直到达到自我维持的极限环幅度。然而，驱动不稳定性线性增长的机制和那些导致热释放振荡饱和到极限环条件的机制往往有明显的区别。为了有效地预测和控制燃烧不稳定性，必须更好地了解从线性增长到非线性饱和的转变以及控制这一转变的机理，特别是在添加氢气的工业型非/部分预混火焰中。这一建议的目的是：a)研究和比较钝体和涡流稳定湍流火焰中放热振荡的机制，b)研究火焰锚定和时空混合物变化的影响，这与实际燃烧室的极限环振荡有关，以及c)利用先进的激光诊断技术同时测量流动和放热，评估和了解添加氢在改善燃烧室动态稳定性方面的作用。该项目的预期结果是支持与实际燃烧室相关的湍流火焰中燃烧振荡的机制。特别是，所提出的实验将突出火焰稳定、当量比变化和氢气添加对非线性火焰响应的作用，这对于提高对实际燃烧系统中极限环振荡的基本理解和预测具有重要意义。这项研究将导致用于声学分析的非线性火焰模型的发展，并有助于开发新的控制策略来消除工业燃烧器中的燃烧振荡。

Combustion instabilities represent one of the most serious problems hindering the development of low-emission aero- and industrial- gas turbine combustors. In order to achieve efficient, low-emissions performance fuel-lean and preferably premixed operating conditions are necessary. However, these lean combustors have the drawback of being particularly susceptible to thermo-acoustic instability. These instabilities are characterised by strong pressure oscillations in the combustion chamber due to a complex interaction between thermo-acoustic and fluid-dynamic processes. When the pressure or velocity oscillations couple favourably with the unsteady heat release, large-amplitude self-sustained oscillation may result. These high amplitude oscillations can have a detrimental effect on combustor performance and may cause catastrophic failure of the system. Lean premix concept is increasingly adopted by gas turbine engine manufacturers to reduce emissions and increase fuel economy. Although fuel lean conditions reduce NOx emissions by decreasing the flame temperature, lean flames are particularly susceptible to combustion oscillations and blow-off. Hydrogen enrichment is one of the promising methods that can be used to improve the stable operation of the combustor under extremely lean conditions. Hydrogen enrichment also improves the ignitability and the response of the flame to strain and curvature. These benefits suggest a promising role for hydrogen enrichment in the development of low-emission gas turbine combustion technology. However, the response of the hydrogen enriched flames in the context of combustion instability is not fully understood. Thus, the primary motivation of this study is to understand and underpin the mechanisms of heat release modulation with hydrogen addition in the context of combustion oscillations. There are several well known mechanisms that can promote fluctuations in the heat release in lean flames; namely, variations in mixture ratio, sensitivity of the flames to pressure/velocity oscillations, and the formation and shedding of vortices. Any of these mechanisms can cause combustion oscillations to grow in amplitude through positive feedback until a self-sustaining limit-cycle amplitude is reached. However, there is often a clear distinction between the mechanisms driving linear growth of instability and those which cause the heat release oscillations to saturate to limit-cycle conditions. In order to predict and control combustion instabilities effectively the transition from linear growth to non-linear saturation and the mechanisms governing this transition has to be better understood, especially in industrial type non-/partially premixed flames with hydrogen addition. This proposal aims: a) to study and compare mechanisms of heat release oscillations in bluff-body and swirl stabilised turbulent flames, b) to investigate the effect of flame anchoring and that of spatial and temporal mixture variation, which are relevant to limit-cycle oscillation in practical combustors, and c) to assess and understand the role of hydrogen addition in improving the dynamic stability of the combustor, using simultaneous measurements of flow and heat release via advanced laser diagnostic techniques. The expected outcome of this project is to underpin the mechanisms of combustion oscillations in turbulent flames relevant to practical combustors. In particular, the proposed experiments will highlight the role of flame stabilisation, equivalence ratio variation and hydrogen addition on the non-linear flame response, which is of significant importance for improving the fundamental understanding and prediction of the limit-cycle oscillations in practical combustion systems. This research will lead to development of non-linear flame models for acoustic analysis and also aid the development of new control strategies for elimination of combustion oscillations in industrial combustors.