The dynamic transition from combustion noise to combustion instability was investigated experimentally in two laboratory-scale turbulent combustors (namely, swirl-stabilized and bluff-body-stabilized backward-facing-step combustors) by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near-random fashion amidst regions of aperiodic low-amplitude fluctuations. These excursions to periodic oscillations last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. A continuous measure to quantify this bifurcation in dynamics can be obtained by defining an order parameter as the probability of the signal amplitude exceeding a predefined threshold. A hysteresis zone was observed in the bluff-body-stabilized configuration that was absent in the swirl-stabilized configuration. The recurrence properties of the dynamics of intermittent burst oscillations were quantified using recurrence plots and the distribution of the aperiodic phases was examined. From the statistics of these aperiodic phases, robust early-warning signals of an impending combustion instability may be obtained.

Intermittency route to thermoacoustic instability in turbulent combustors

Nine decades of rocket engine and gas turbine development have shown that thermoacoustic oscillations are difficult to predict but can usually be eliminated with relatively small ad hoc design changes These changes can, however, be ruinously expensive to devise This review explains why linear and nonlinear thermoacoustic behavior is so sensitive to parameters such as operating point, fuel composition, and injector geometry It shows how nonperiodic behavior arises in experiments and simulations and discusses how fluctuations in thermoacoustic systems with turbulent reacting flow, which are usually filtered or averaged out as noise, can reveal useful information Finally, it proposes tools to exploit this sensitivity in the future: adjoint-based sensitivity analysis to optimize passive control designs and complex systems theory to warn of impending thermoacoustic oscillations and to identify the most sensitive elements of a thermoacoustic system

Sensitivity and Nonlinearity of Thermoacoustic Oscillations

The transition in dynamics from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, combustion instability in confined, combustion environments was studied experimentally in a laboratory-scale combustor with two different flameholding devices in a turbulent flow field. We show that the low-amplitude, irregular pressure fluctuations acquired during stable regimes, termed ‘combustion noise’, display scale invariance and have a multifractal signature that disappears at the onset of combustion instability. Traditional analysis often treats combustion noise and combustion instability as acoustic problems wherein the irregular fluctuations observed in experiments are often considered as a stochastic background to the dynamics. We demonstrate that the irregular fluctuations contain useful information of prognostic value by defining representative measures such as Hurst exponents that can act as early warning signals to impending instability in fielded combustors.

Multifractality in combustion noise: predicting an impending combustion instability

We investigate the scale invariance of combustion noise generated from turbulent reacting flows in a confined environment using complex networks. The time series data of unsteady pressure, which is the indicative of spatiotemporal changes happening in the combustor, is converted into complex networks using the visibility algorithm. We show that the complex networks obtained from the low-amplitude, aperiodic pressure fluctuations during combustion noise have scale-free structure. The power-law distributions of connections in the scale-free network are related to the scale invariance of combustion noise. We also show that the scale-free feature of combustion noise disappears and order emerges in the complex network topology during the transition from combustion noise to combustion instability. The use of complex networks enables us to formalize the identification of the pattern (i.e. scale-free to order) during the transition from combustion noise to thermoacoustic instability as a structural change in topology of the network.

Combustion noise is scale-free: transition from scale-free to order at the onset of thermoacoustic instability

The invention is directed to a method and system for supporting MIMO technologies which can require the transport of multiple spatial streams on a traditional Distributed Antenna System (DAS). According to the invention, at one end of the DAS, each spatial stream is shifted in frequency to a pre-assigned band (such as a band at a frequency lower than the native frequency) that does not overlap the band assigned to other spatial streams (or the band of any other services being carried by the DAS). Each of the spatial streams can be combined and transmitted as a combined signal over a common coaxial cable. At the other “end” of the DAS, the different streams are shifted back to their original (overlapping) frequencies but retain their individual “identities” by being radiated through physically separate antenna elements.

Distributed antenna system for MIMO technologies

Combustion noise has been traditionally thought of as stochastic fluctuations present in the background of the dynamics in combustors amongst the flow, heat release and the chamber acoustics. Through a series of determinism tests, we show that these aperiodic fluctuations are in fact chaotic of moderately high dimensions (d0 ≈ 8-10). These chaotic fluctuations then transition to high amplitude combustion instability when the operating conditions are varied towards leaner equivalence ratios. Precursors to such a transition from chaos to dynamics dominated by periodic oscillations are of interest to designers and operators of combustors in estimating the boundaries of operability. We introduce a test for chaos, known as 0-1 test for chaos in the literature, as a measure of the proximity of the combustor to an impending instability. The measure is robust and shows a smooth transition for variation in flow conditions towards instability enabling thresholds to be set for operational boundaries.

Loss of Chaos in Combustion Noise as a Precursor of Impending Combustion Instability

A system for early detection of onset of oscillatory instabilities in practical devices is described. The system consists of a measuring device, an instability detection unit and a control unit. The measuring device is configured to generate signals corresponding to the dynamics happening inside the practical device. The instability detection unit is configured to diagnose the stability of the practical device from the signals that are generated by the measuring device. Further, the control unit is configured to control various operating parameters in the practical device based on the information obtained from the instability detection unit.

System and method for predetermining the onset of impending oscillatory instabilities in practical devices

System und verfahren zur vorherbestimmung bevorstehender oszillierender instabilitäten bei praktischen vorrichtungen

La presente invention concerne un systeme pour la detection precoce de l'apparition d'instabilites oscillatoires dans des dispositifs pratiques. Le systeme selon l'invention est constitue d'un dispositif de mesure, d'une unite de detection d'instabilite et d'une unite de commande. Le dispositif de mesure est configure pour generer des signaux correspondant a la dynamique qui se produit a l'interieur du dispositif pratique. L'unite de detection d'instabilite est configuree pour diagnostiquer la stabilite du dispositif pratique a partir des signaux qui sont generes par le dispositif de mesure. En outre, l'unite de commande est configuree pour commander divers parametres de fonctionnement dans le dispositif pratique sur la base des informations obtenues a partir de l'unite de detection d'instabilite.

Sulochana Karuppusamy

Papers

Loss of Chaos in Combustion Noise as a Precursor of Impending Combustion Instability

System and method for predetermining the onset of impending oscillatory instabilities in practical devices

System und verfahren zur vorherbestimmung bevorstehender oszillierender instabilitäten bei praktischen vorrichtungen

Système et procédé de prédétermination d'apparition d'instabilités oscillatoires imminentes dans des dispositifs pratiques