Combustion instabilities are physical phenomena occurring in a reacting flow (e.g., a flame) in which some perturbations, even very small ones, grow and then become large enough to alter the features of the flow in some particular way.
In many practical cases, the appearance of combustion instabilities is undesirable. For instance, thermoacoustic instabilities are a major hazard to gas turbines and rocket engines. Moreover, flame blowoff of an aero-gas-turbine engine in mid-flight is clearly dangerous (see flameout).
Because of these hazards, the engineering design process of engines involves the determination of a stability map (see figure). This process identifies a combustion-instability region and attempts to either eliminate this region or moved the operating region away from it. This is a very costly iterative process. For example, the numerous tests required to develop rocket engines are largely in part due to the need to eliminate or reduce the impact of thermoacoustic combustion instabilities.
Classification of combustion instabilities
In applications directed towards engines, combustion instability has been classified into three categories, not entirely distinct. This classification was first introduced by Marcel Barrère and Forman A. Williams in 1969. The three categories are
Chamber instabilities - instabilities arising due to the occurrence of combustion inside a chamber (acoustic instabilities, shock instabilities, fluid-dynamic instabilities associated with the chamber, etc.,) Intrinsic instabilities - instabilities arising irrespective of whether combustion occurs inside a chamber or not (chemical-kinetic instabilities, diffusive-thermal instabilities, hydrodynamic instabilities such as Darrieus–Landau instability, Rayleigh–Taylor instability etc.,) System instabilities - instabilities arising due to the interaction between combustion processes in the chamber and anywhere else in the system (feed-system interactions, exhaust-system interactions, etc.,)
Thermoacoustic combustion instabilities
In this type of instabilities the perturbations that grow and alter the features of the flow are of an acoustics nature. Their associated pressure oscillations can have well defined frequencies with amplitudes high enough to pose a serious hazard to combustion systems. For example, in rocket engines, such as the Rocketdyne F-1 rocket engine in the Saturn V program, instabilities can lead to massive damage of the combustion chamber and surrounding components (see rocket engines). Furthermore, instabilities are known to destroy gas-turbine-engine components during testing. They represent a hazard to any type of combustion system.
Thermoacoustic combustion instabilities can be explained by distinguishing the following physical processes:
the feedback between heat-release fluctuations (or flame fluctuations) with the combustor or combustion chamber acoustics the coupling of these two processes in space-time the strength of this coupling in comparison with acoustic losses the physical mechanisms behind the heat-release fluctuations
The simplest example of a thermoacoustic combustion instability is perhaps that happening in a horizontal Rijke tube (see also thermoacoustics): Consider the flow through a horizontal tube open at both ends, in which a flat flame sits at a distance of one-quarter the tube length from the leftmost end. In a similar way to an organ pipe, acoustic waves travel up and down the tube producing a particular pattern of standing waves. Such a pattern also forms in actual combustors, but takes a more complex form. The acoustic waves perturb the flame. In turn, the flame affects the acoustics. This feedback between the acoustic waves in the combustor and the heat-release fluctuations from the flame is a hallmark of thermoacoustic combustion instabilities. It is typically represented with a block diagram (see figure). Under some conditions, the perturbations will grow and then saturate, producing a particular noise. In fact, it is said that the flame of a Rijke tube sings.
The conditions under which perturbations will grow are given by Rayleigh's (John William Strutt, 3rd Baron Rayleigh) criterion: Thermoacoustic combustion instabilities will occur if the volume integral of the correlation of pressure and heat-release fluctuations over the whole tube is larger than zero (see also thermoacoustics). In other words, instabilities will happen if heat-release fluctuations are coupled with acoustical pressure fluctuations in space-time (see figure). However, this condition is not sufficient for the instability to occur.