The challenges of lean premixed combustion

Clean combustion, with low levels of pollutants, can be achieved by mixing the fuel and air well before they enter a combustion chamber and burning lean with a high ratio of air to fuel. Unfortunately, these are just the conditions that cause self-excited oscillations which can become so intense that they break the combustor. Many gas-fuelled power stations have suffered damage in this way and been unable to achieve their designed power and low emission levels. Self-excited oscillations are also being experienced in the development of the next generation of aeroengines as they strive to achieve lower emissions. These instabilities occur due to the interaction between sound and flames: the unsteady combustion generates acoustic waves and these in turn alter the inlet flow rates of fuel and air and lead to further unsteady combustion. An overview of the physics of the interaction between sound and flames is presented and will be illustrated by visualisations of flame dynamics through experiment and computer modelling. The elements needed for effective prediction and modelling are discussed, together with the potential of ‘anti-sound’ and passive acoustic absorbers to eliminate the instability. INTRODUCTION Gas turbine combustors which burn lean and premixed lead to very low levels of nitrous oxides. Manufacturers of industrial gas turbines, which have to meet stringent emission requirements, have therefore vigorously pursued lean premixed (LP) technology. Unfortunately LP combustion has proved to be notoriously susceptible to instability leading to large-amplitude pressure oscillations. These may cause structural damage, either because the resulting flow perturbations enhance the heat transfer or through vibration. More than one power station has been unable to achieve its designed power and low emissions because of the onset of instability, leading to long commissioning times as a series of ad hoc fixes are tried. Lean premixed combustion is also of interest to aeroengine manufacturers since the next generation of aero gas turbines is also required to achieve very low emissions. Self-excited oscillations are being experienced in their development. Historically, self-excited combustion instabilities have been experienced in combustors with a high specific heat input like afterburners and rockets, and occur as the fuel-air ratio is increased above a critical level. Lean premixed combustion is different, with intense oscillations at low fuel-air ratios. The physics of this mechanism are now clear: pressure oscillations cause changes in the flow rates of inlet air or fuel, thereby changing the fuel-air ratio. At lean premixed conditions, the rate of combustion is strongly influenced by fuel-air ratio, indeed if the fuel-air ratio drops significantly local extinctions can occur. The resulting change in the rate of combustion is an acoustic source, generating pressure waves. Instability is then possible because while the unsteady combustion generates sound, the resulting pressure waves cause yet more unsteady combustion. If the phase relationship is suitable, linear perturbations grow in amplitude into a finite amplitude self-excited oscillation. In this paper, we present an overview of the physics of the interaction between sound and flames. Methods to predict the susceptibility to self-excited oscillation and the frequency of these oscillations are presented. The complementary roles of experiments, wave analysis, Computational Fluid Dynamics and low-order models for unsteady combustion are discussed. These approaches have been validated by experiments on simple lean premixed flames and can be applied to predict oscillations in gas turbine combustors. Many LP combustion systems are unstable and good design will include passive or active control features. Passive control usually takes the form of either a redesign of the premix passages and combustor to reduce the sensitivity of the combustion to changes in the inlet flow, or the introduction of additional acoustic absorbers. Active control, in which the fuel is added unsteadily in response to a measured fluctuating signal, is an effective way of stabilising combustion oscillations. It is over ten years since the first full-scale demonstration of the feasibility of such feedback control. However, practical implementation requires controllers that have guaranteed performance across a range of combustor operating conditions. The development of such controllers needs the integration of ideas from acoustics, fluid mechanics, combustion, control and system identification. Current work on adaptive controllers will be summarised, with particular emphasis on ensuring that the controller will operate effectively as the engine conditions change. INTERACTION OF HEAT AND SOUND Rayleigh (1896) gave a clear physically based description of the interaction between sound waves and unsteady heat input. In essence, just as in any thermodynamic cycle, the addition of heat at high pressure leads to a net energy input, which is available to do work. If the energy gained from the unsteady combustion exceeds the rate of work done, the acoustic waves grow in amplitude. Mathematically, for small amplitude disturbances in a simple combustor with a fluctuating rate of heat addition q within a volume V with bounding surface S, acoustic waves grow in amplitude if