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Combustion dynamics of turbulent swirl flames with hydrogen addition
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 research activity 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.
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