Lean premixed combustors, such as those used in industrial gas turbines to achieve low emissions, are often susceptible to thermoacoustic combustion instabilities, which manifest themselves as pressure and heat release oscillations in the combustor. These oscillations can result in increased noise and decreased durability due to vibration and flame motion. A physically based nonlinear parametric model has been developed that captures this instability. It describes the coupling of combustor acoustics with the rate of heat release. The model represents this coupling by accounting for the effect of acoustic pressure fluctuations on the varying fuel/air ratio being delivered to the flame, causing a fluctuating heat release due to both fuel air ratio variations and flame front oscillations. If the phasing of the fluctuating heat release and pressure are proper, an instability results that grows into a limit cycle. The nonlinear nature of the model predicts the onset of the instability and additionally captures the resulting limit cycle. Tests of a lean premixed nozzle run at engine scale and engine operating conditions in the UTRC single nozzle rig, conducted under DARPA contract, exhibited instabilities. Parameters from the model were adjusted so that analytical results were consistent with relevant experimental data from this test. The parametric model captures the limit cycle behavior over a range of mean fuel air ratios, showing the instability amplitude (pressure and heat release) to increase and limit cycle frequency to decrease as mean fuel air ratio is reduced.

Nonlinear heat-release/acoustic model for thermoacoustic instability in lean premixed combustors

An eigenvalue based approach for the stabilization of linear time-delay systems of neutral type

http://flyingv.ucsd.edu/papers/PDF/56.pdf

An adaptive algorithm for control of combustion instability

One of the most successful applications of control technology in fluid systems is in the context of dynamic instability in continuous combustion systems. Models of combustion instability have been derived using both physically based and system-identification-based methods. Optimal, time-delay, and adaptive controllers have been designed using these models and demonstrated to lead to an order of magnitude improvement in a range of combustion rigs over empirically designed control methods. We present a survey of the existing theory that includes modeling and control of pressure instability in combustion systems and the practical applications of active control to date in small-, medium-, and large-scale combustion rigs.

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Active control of combustion instability: theory and practice

This paper describes an analytical and experimental investigation to enhance combustion system operability using side branch resonators. First, a simplified model of the combustion system dynamics is developed in which the large amplitude pressure oscillations encountered at the operability limit are viewed as limit cycle oscillations of an initially linear instability. Under this assumption, increasing the damping of the small amplitude combustion system dynamics will increase combustor operability. The model is then modified to include side branch resonators. The parameters describing the side branch resonators and their coupling to the combustion system are identified, and their influence on system stability is examined. The parameters of the side branch resonator are optimized to maximize damping augmentation and frequency robustness. Secondly, the model parameters for the combustor and side branch resonator dynamics are identified from experimental data. The analytical model predicts the observed trends in combustor operability as a function of the resonator parameters and is shown to be a useful guide in developing resonators to improve the operability of combustion systems.

Combustion System Damping Augmentation With Helmholtz Resonators

Presents a case study in system identification for limit cycling systems. The focus of the paper is on (a) the use of a model structure derived from physical considerations and (b) the use of algorithms for the identification of component subsystems of this model structure. The physical process used in this case study is that of a reduced order model for combustion instabilities for lean premixed systems. The identification techniques applied in this paper are the use of linear system identification tools, time delay estimation and qualitative validation of model properties using harmonic balance and describing function methods. The novelty of the paper, apart from its practical application, is that closed loop limit cycle data is used together with a priori process structural knowledge to identify both linear dynamic forward and nonlinear feedback paths.

/pdf/system-identification-for-limit-cycling-systems-a-case-study-4mfc2bxp8w.pdf

A.A. Peracchio

Papers

System identification for limit cycling systems: a case study for combustion instabilities