Onset Of Thermoacoustic Instability Research Articles

Thermoacoustic instabilities with varying geometries of the main-stage exit in a centrally staged burner

The present study investigates the effect of the exit geometry of the main stage on thermoacoustic instabilities in a model combustor with a dual-swirl layout. In practical combustors, the main stage has a much larger airflow than the pilot stage, therefore dominating the aerodynamics of the global flame. In this paper, six different geometries of the main-stage exit are designed to cover the common styles in engineering applications. Their effects on thermoacoustic instability and flame shape are examined experimentally. Sudden transitions from the attached flame to the blurred flame are found in most of the exit geometries, excluding the outer-rough case. This transition of flame shape triggers the onset of strong thermoacoustic oscillation, leading to a jump in the pressure fluctuation amplitude. Frequency drop and the acoustic mode shift are also found at the transition point. Flame dynamics are further analyzed using the dynamic mode decomposition method. It is concluded that the outer-rough case has the best thermoacoustic stability within the tested range. The mechanisms of the observed phenomena are analyzed by measuring the reacting flow fields. It is found that the increased roughness of the outer wall can prevent the flame shape transition and the onset of thermoacoustic instability, possibly due to the weakened vortex structures and strain rate in the outer shear layer.

Coupled interaction between acoustics and unsteady flame dynamics during the transition to thermoacoustic instability in a multi-element rocket combustor

Rocket engine combustors are prone to transverse instabilities that are characterized by large amplitude high frequency oscillations in the acoustic pressure and the heat release rate. In this work, we study the coupled interaction between the acoustic pressure and the CH* intensity (representative of heat release rate) oscillations in a 2D multi-element self-excited model rocket combustor during the transition from a stable state to thermoacoustic instability through intermittency. We show the emergence of synchronization between these oscillations from desynchronization through intermittent phase synchronization during the onset of thermoacoustic instability. We find substantial evidence that the intensities of the jet flames close to the end wall is higher than that observed near the center of the combustor as a result of its strong coupling to the local acoustic field. Using concepts from recurrence theory, we distinguish the type of synchronization between the acoustic pressure and the CH* intensity oscillations at the end wall and the center of the combustor during thermoacoustic instability. Analyzing the local CH* intensity oscillations, we observe that the longitudinally propagating jet flames experience substantial transverse displacement with flame merging effects during thermoacoustic instability. Furthermore, we differentiate the interaction of the jet flames with the shock wave during intermittency and thermoacoustic instability at both the end wall and the center of the combustor. We also discerned the change from stochastic to deterministic nature in the local CH* intensity oscillations during the transition to thermoacoustic instability. Our results demonstrate that only the first few transverse modes contribute to the generation of acoustic power from the reacting flow, in spite of the pressure oscillations featuring several harmonics. The insights gained from such coupled analysis during the transition from stable state to thermoacoustic instability can be leveraged to aid modeling efforts of thermoacoustic instability in rocket engines with multiple injectors.

Non-adiabatic modulation of premixed-flame thermoacoustic frequencies in slender tubes

This paper presents an experimental study of the influence of heat losses on the onset of thermoacoustic instabilities in methane–air premixed flames propagating in a horizontal tube of diameter,$D = 10$mm. Flames are ignited at the open end of the tube and propagate towards the closed end undergoing strong oscillations of different features owing to the interaction with acoustic waves. The frequency of oscillation and its axial location are controlled through the tube length$L$and the intensity of heat losses. These parameters are respectively modified in the experiments by a moveable piston and a circulating thermal bath of water prescribing temperature conditions. Main experimental observations show that classical one-dimensional predictions of the oscillation frequency do not accurately describe the phenomena under non-adiabatic real scenarios. In addition to the experimental measurements, a quasi-one-dimensional analysis of the burnt gases is provided, which introduces the effect of heat losses at the wall of the tube on the interplay between the acoustic field and the reaction sheet. As a result, this analysis provides an improved description of the interaction and accurately predicts the excited flame-oscillation harmonics through the eigenvalues of the non-adiabatic acoustics model. Unlike the original one-dimensional analysis, the comparison between the flame oscillation frequency provided by the non-adiabatic extended theory and the frequencies measured in our experiments is in excellent agreement in the whole range of temperatures considered. This confirms the importance of heat losses in the modulation of the instabilities and the transition between different flame oscillation regimes.

Recurrence network analysis exploring the routes to thermoacoustic instability in a Rijke tube with inverse diffusion flame.

Inverse diffusion flame (IDF) is a reliable low NOx technology that is suitable for various industrial applications including gas turbines. However, a confined IDF may exhibit thermoacoustic instability, a kind of dynamic instability, which is characterized by catastrophically large amplitude pressure oscillations. Transition to such instability for an inverse diffusion flame is less explored compared to other types of flame. In the present study, thermoacoustic instability in a Rijke tube with IDF is achieved by varying air flow rate and input power independently, and the onset of thermoacoustic instability is examined using the framework of recurrence network (RN). During the transition to thermoacoustic instability, we find new routes and two new intermediate states, here referred to as "amplitude varying aperiodic oscillations" and "low amplitude limit cycle-like oscillations." Furthermore, we show that recurrence network analysis can be used to identify the dynamical states during the transition to thermoacoustic instability. We observe an absence of a single characteristic scale, resulting in a non-regular network even during thermoacoustic instability. Furthermore, the degree distributions of RN during combustion noise do not obey a single power law. Thus, scale-free nature is not exhibited during combustion noise. In short, recurrence network analysis shows significant differences in the topological information during combustion noise and thermoacoustic instability for IDF with those for premixed flames, reported earlier.

Predicting Onset of Combustion Instability in Gas Turbines Using Fiber Optic Sensors

A data driven method based on cross-entropy derived from symbolic time series analysis of the chemiluminescence and pressure variations has recently been found to be robust and computationally efficient among existing methods for predicting instability. In this article, we report the development and testing of compact and field-deployable optical fiber-based modules for sensing chemiluminescence and pressure variations in the combustor. The time-series data obtained from the above sensor modules are used to deduce the D-Markov cross-entropy for different protocols corresponding to typical combustor operation conditions. Such cross-entropy data is compared with similar data obtained from standard high speed camera and PZT-based pressure sensors. It is found that the fiber optic sensor modules compare favorably with the conventional sensors thereby providing an exciting new pathway for field-deployable solutions to monitor the onset of thermo-acoustic instability in combustors.

On the emergence of large clusters of acoustic power sources at the onset of thermoacoustic instability in a turbulent combustor

In turbulent combustors, the transition from stable combustion (i.e. combustion noise) to thermoacoustic instability occurs via intermittency. During stable combustion, the acoustic power production happens in a spatially incoherent manner. In contrast, during thermoacoustic instability, the acoustic power production happens in a spatially coherent manner. In the present study, we investigate the spatiotemporal dynamics of acoustic power sources during the intermittency route to thermoacoustic instability using complex network theory. To that end, we perform simultaneous acoustic pressure measurement, high-speed chemiluminescence imaging and particle image velocimetry in a backward-facing step combustor with a bluff body stabilized flame at different equivalence ratios. We examine the spatiotemporal dynamics of acoustic power sources by constructing time-varying spatial networks during the different dynamical states of combustor operation. We show that as the turbulent combustor transits from combustion noise to thermoacoustic instability via intermittency, small fragments of acoustic power sources, observed during combustion noise, nucleate, coalesce and grow in size to form large clusters at the onset of thermoacoustic instability. This nucleation, coalescence and growth of small clusters of acoustic power sources occurs during the growth of pressure oscillations during intermittency. In contrast, during the decay of pressure oscillations during intermittency, these large clusters of acoustic power sources disintegrate into small ones. We use network measures such as the link density, the number of components and the size of the largest component to quantify the spatiotemporal dynamics of acoustic power sources as the turbulent combustor transits from combustion noise to thermoacoustic instability via intermittency.

Chaos, synchronization, and desynchronization in a liquid-fueled diffusion-flame combustor with an intrinsic hydrodynamic mode.

We experimentally investigate the nonlinear dynamics of a thermoacoustically self-excited aero-engine combustion system featuring a turbulent swirling liquid-fueled diffusion flame in a variable-length combustor. We focus on the steady-state dynamics via simultaneous measurements of the acoustic pressure in the combustor and the heat release rate (HRR) from the flame. When the combustor length is increased following the onset of thermoacoustic instability, we find that the pressure signal transitions from a period-1 limit cycle to chaos, whereas the HRR signal remains chaotic owing to the presence of an intrinsic hydrodynamic mode in the flame. When the hydrodynamic mode is filtered out of the data, we find that the pressure and HRR signals are in generalized synchronization. However, when the hydrodynamic mode is retained in the data, we find that the pressure and HRR signals are either weakly phase synchronized or desynchronized. This study has two main contributions: (i) it shows that a liquid-fueled diffusion-flame combustor can exhibit dynamics as complex as those of its gaseous-fueled premixed-flame counterparts and (ii) it highlights the need to be exceptionally careful when selecting a diagnostic signal from which to calculate nonlinear measures of self-excited thermoacoustic oscillations, because our experiments show that the pressure and HRR signals can be desynchronized by the presence of a hydrodynamic mode in the flame at a frequency different from that of the dominant thermoacoustic mode.

Early Warning Signs of Imminent Thermoacoustic Oscillations Through Critical Slowing Down

Critical slowing down (CSD) is a phenomenon that is common to many complicated dynamical systems as they approach critical transitions/bifurcations. We demonstrate that pressure signals measured during the onset of thermoacoustic instabilities in a gas turbine engine test exhibit evidence of CSD well before the oscillation amplitude increases. CSD was detected through both the variance and the lag-1 auto-regressive coefficient in a rolling window of the pressure signal. Increasing trends in both metrics were quantified using Kendall's τ, and the robustness and statistical significance of the observed increases were confirmed. Changes in the CSD metrics could be detected several seconds prior to changes in the oscillation amplitude. Hence, real-time calculation of these metrics holds promise as early warning signals of impending thermoacoustic instabilities.

Temporal and Spatiotemporal Analyses of Synchronization Transition in a Swirl-Stabilized Combustor

The coupled behavior of the acoustic pressure and the heat release rate fluctuations is studied in a swirl-stabilized combustor during the transition of system dynamics from combustion noise to thermoacoustic instability via intermittency. The framework of synchronization is used to analyze the temporal as well as the spatiotemporal behavior of these oscillations. The results of the synchronization transition observed in this combustor are further compared and contrasted with those recently reported by Pawar et al. (“Thermoacoustic Instability as Mutual Synchronization Between the Acoustic Field of the Confinement and Turbulent Reactive Flow,” Journal of Fluid Mechanics, Vol. 827, Sept. 2017, pp. 664–693) and Mondal et al. (“Onset of Thermoacoustic Instability in Turbulent Combustors: An Emergence of Synchronized Periodicity Through Formation of Chimera-like States,” Journal of Fluid Mechanics, Vol. 811, Jan. 2017, pp. 659–681) in a bluff-body stabilized combustor. A rigorous analysis of the synchronization transition of these oscillations in the frequency domain is also presented. The temporal analysis reveals that the synchronization states and the synchronization transition to thermoacoustic instability of the coupled acoustic pressure and heat release rate fluctuations are similar for both swirl-stabilized and bluff-body stabilized combustors. On the other hand, the spatiotemporal analysis of both the combustors during the state of strongly correlated periodic oscillations shows that, despite the acoustic power production being higher during this state, an increased amount of spatial incoherence is visible in the instantaneous and phase-conditioned phasor field of the reaction zone of the swirl-stabilized combustor as compared to that of the bluff-body stabilized combustor. In short, the present study unifies the synchronization transition of coupled oscillations in these turbulent combustors, wherein the mechanisms behind the onset of thermoacoustic instabilities are apparently different.

Early Detection of Thermoacoustic Instabilities Using Hidden Markov Models

ABSTRACTThis paper presents a dynamic data-driven method for early detection of thermoacoustic instabilities in combustors based on short-length time series of sensor data, where the objective is near-real-time monitoring and active control of pressure oscillations. The main idea is to use the available data at different regimes of the combustion process to train respective hidden-variable models using the concept of Hidden Markov Modeling (HMM) as a statistical learning tool; here, (short-length) time-series data of pressure oscillations are used to infer a Markov chain with unobserved (hidden) states. The proposed HMM-based method has been validated on experimental data collected from an electrically heated Rijke tube apparatus for predicting onset of thermoacoustic instabilities. The results have been compared with those of the current state-of-the-art measurement techniques for instability growth rate and associated computational complexity. The applicability of the proposed method has been demonstrated with respect to anomaly detection and regime identification with limited data requirements, making it a potential candidate for monitoring and active control of thermoacoustic instabilities in commercial-scale combustors.

On the emergence of critical regions at the onset of thermoacoustic instability in a turbulent combustor.

We use complex network theory to investigate the dynamical transition from stable operation to thermoacoustic instability via intermittency in a turbulent combustor with a bluff body stabilized flame. A spatial network is constructed, representing each of these three dynamical regimes of combustor operation, based on the correlation between time series of local velocity obtained from particle image velocimetry. Network centrality measures enable us to identify critical regions of the flow field during combustion noise, intermittency, and thermoacoustic instability. We find that during combustion noise, the bluff body wake turns out to be the critical region that determines the dynamics of the combustor. As the turbulent combustor transitions to thermoacoustic instability, during intermittency, the wake of the bluff body loses its significance in determining the flow dynamics and the region on top of the bluff body emerges as the most critical region in determining the flow dynamics during thermoacoustic instability. The knowledge about this critical region of the reactive flow field can help us devise optimal control strategies to evade thermoacoustic instability.

Thermoacoustic instability as mutual synchronization between the acoustic field of the confinement and turbulent reactive flow

Thermoacoustic instability is the result of a positive coupling between the acoustic field in the duct and the heat release rate fluctuations from the flame. Recently, in several turbulent combustors, it has been observed that the onset of thermoacoustic instability is preceded by intermittent oscillations, which consist of bursts of periodic oscillations amidst regions of aperiodic oscillations. Quantitative analysis of the intermittency route to thermoacoustic instability has been performed hitherto using the pressure oscillations alone. We perform experiments on a laboratory-scale bluff-body-stabilized turbulent combustor with a backward-facing step at the inlet to obtain simultaneous data of acoustic pressure and heat release rate fluctuations. With this, we show that the onset of thermoacoustic instability is a phenomenon of mutual synchronization between the acoustic pressure and the heat release rate signals, thus emphasizing the importance of the coupling between these non-identical oscillators. We demonstrate that the stable operation corresponds to desynchronized aperiodic oscillations, which, with an increase in the mean velocity of the flow, transition to synchronized periodic oscillations. In between these states, there exists a state of intermittent phase synchronized oscillations, wherein the two oscillators are synchronized during the periodic epochs and desynchronized during the aperiodic epochs of their oscillations. Furthermore, we discover two different types of limit cycle oscillations in our system. We notice a significant increase in the linear correlation between the acoustic pressure and the heat release rate oscillations during the transition from a lower-amplitude limit cycle to a higher-amplitude limit cycle. Further, we present a phenomenological model that qualitatively captures all of the dynamical states of synchronization observed in the experiment. Our analysis shows that the times at which vortices that are shed from the inlet step reach the bluff body play a dominant role in determining the behaviour of the limit cycle oscillations.

On the characteristic flow and flame times for scaling oxy and air flame stabilization modes in premixed swirl combustion

We compare the conditions leading to the stabilization of turbulent methane air and oxy-flames in the outer recirculation zone (ORZ) of a lean premixed acoustically decoupled swirl combustor. The appearance of a flame in the ORZ is an important flame macrostructure transition that was previously shown to be associated with the onset of thermo-acoustic instability under acoustically coupled conditions. We find that, when similar bulk flow conditions are imposed in the ORZ, the transition is governed by the extinction strain rate and can occur at different adiabatic flame temperature and unstretched laminar burning velocity. First, we show that an important non-dimensional parameter characterizing the flow in the ORZ, that is the Strouhal number associated with the azimuthal ORZ spinning frequency, is independent of the Reynolds number and has the same constant value in air and oxy-combustion (St=fORZ.DinUin,bulk≈0.12). This has the important implication that the inlet velocity is a more relevant parameter choice than the inlet Reynolds number in order to maintain similar flow conditions in the ORZ. Next, by comparing the extinction strain rates – computed at the measured ORZ temperature – we show the existence of a single correlation between the inverse of the ORZ spinning frequency (taken as a characteristic ORZ flow time) and the inverse of the extinction strain rate (taken as a characteristic flame time) valid for both air and oxy flames and delimiting the regions of existence of different flame macrostructures.

Detecting the Onset of an Impending Thermoacoustic Instability Using Complex Networks

Recently, M. Murugesan and R. I. Sujith (“Combustion Noise is Scale-Free: Transition from Scale-Free to Order at the Onset of Thermoacoustic Instability,” Journal of Fluid Mechanics, Vol. 32, June 2015, pp. 225–245) showed that the transition from combustion noise to thermoacoustic instability can be represented as a change from a scale-free to a regular structure in the topology of complex networks. These topological changes of the complex networks during this transition can be quantified by calculating the network properties. In this paper, the variation of network properties, namely, clustering coefficient, characteristic path length, network diameter, and global efficiency is presented as the system dynamics undergoes transition from combustion noise to thermoacoustic instability. These network properties capture the change in system dynamics well before the rise in pressure amplitude levels in the combustors. These network properties can be used as early warning signals to detect the onset of impending thermoacoustic instabilities.

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

Active control of thermoacoustic amplification in an annular engine

In this paper, a new method is proposed to control the thermoacoustic amplification in thermoacoustic engines. This method, based on the active control of the spatial distribution of the acoustic field by means of auxiliary acoustic sources, is applied here to an annular thermoacoustic engine. Two auxiliary acoustic sources are used to tune the spatial distribution of the sound field in the engine in such a way that the thermal-to-acoustic energy conversion occurring into the thermoacoustic core is maximized. An experimental study of this device is proposed, which should be considered as a proof-of-concept study, aiming at demonstrating that the addition of auxiliary acoustic sources can be used advantageously to improve the efficiency of thermoacoustic engines. The overall device is characterized below and above the onset of thermoacoustic instability. It is demonstrated that below the onset of thermoacoustic instability, there exists an optimum phase shift between the auxiliary sources which maximizes the acoustic power available in the annular waveguide. When the device is operated above the onset of thermoacoustic instability, it is demonstrated that the appropriate tuning of the two auxiliary sources enables to improve significantly the acoustic work produced into the engine (compared to the case without active control), that the additional output acoustic power is significantly larger than the input electric power supplied to the acoustic sources, and that the overall efficiency of the engine is thus significantly increased. A discussion about the applicability of this new method for the improvement of actual, high power thermoacoustic engines is also provided.