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

2019 ◽  
Vol 874 ◽  
pp. 455-482 ◽  
Author(s):  
Abin Krishnan ◽  
R. I. Sujith ◽  
Norbert Marwan ◽  
Jürgen Kurths
Keyword(s):  
High Speed ◽  
Power Production ◽  
Acoustic Power ◽  
Spatiotemporal Dynamics ◽  
Combustion Noise ◽  
Pressure Oscillations ◽  
Power Sources ◽  
Thermoacoustic Instability ◽  
Stable Combustion ◽  
Large Clusters

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.

scholarly journals Spatiotemporal dynamics during the transition to thermoacoustic instability: Effect of varying turbulence intensities

2018 ◽  
Vol 10 (4) ◽  
pp. 337-350 ◽  
Author(s):  
Nitin B George ◽  
Vishnu R Unni ◽  
Manikandan Raghunathan ◽  
RI Sujith
Keyword(s):  
High Speed ◽  
Equivalence Ratio ◽  
Pressure Fluctuations ◽  
Spatial Dynamics ◽  
Local Heat ◽  
Spatiotemporal Dynamics ◽  
Combustion Noise ◽  
Pressure Oscillations ◽  
Thermoacoustic Instability ◽  
Turbulence Generator

An experimental study on a turbulent, swirl-stabilized backward facing step combustor is conducted to understand the spatiotemporal dynamics during the transition from combustion noise to thermoacoustic instability. By using a turbulence generator, we investigate the change in the spatiotemporal dynamics during this transition for added turbulence intensities. High-speed CH* images of the flame (representative of the field of local heat release rate fluctuations ([Formula: see text]( x, y, t))) and simultaneous unsteady pressure fluctuations ([Formula: see text]( t)) are acquired for different equivalence ratios. In the study, without the turbulence generator, as the equivalence ratio is reduced from near stoichiometric values, we observe an emergence of coherence in the spatial dynamics during the occurrence of intermittency, enroute to thermoacoustic instability. As the turbulence intensity is increased using the turbulence generator, we find that there is an advanced onset of thermoacoustic instability. Spatial statistics and the instantaneous fields of [Formula: see text] show that during the transition from combustion noise to thermoacoustic instability, the emergence of coherent spatial structures in the instantaneous fields of [Formula: see text] for the experiments with higher turbulence intensities is advanced. However, as the equivalence ratio is reduced further, we notice that higher turbulence intensities result in the reduction of the strength of the pressure oscillations during the state of thermoacoustic instability. We find that, at these low equivalence ratios, there is a decrease in the coherence due to the dispersal of [Formula: see text], which explains the reduction in the strength of the pressure oscillations.

Pattern formation during transition from combustion noise to thermoacoustic instability via intermittency

2018 ◽  
Vol 849 ◽  
pp. 615-644 ◽  
Author(s):  
Nitin B. George ◽  
Vishnu R. Unni ◽  
Manikandan Raghunathan ◽  
R. I. Sujith
Keyword(s):  
Pattern Formation ◽  
Flow Field ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
High Speed ◽  
Mie Scattering ◽  
Combustion Noise ◽  
Small Scale ◽  
Thermoacoustic Instability

Gas turbine engines are prone to the phenomenon of thermoacoustic instability, which is highly detrimental to their components. Recently, in turbulent combustors, it was observed that the transition to thermoacoustic instability occurs through an intermediate state, known as intermittency, where the system exhibits epochs of ordered behaviour, randomly appearing amidst disordered dynamics. We investigate the onset of intermittency and the ensuing self-organization in the reactive flow field, which, under certain conditions, could result in the transition to thermoacoustic instability. We characterize this transition from a state of disordered and incoherent dynamics to a state of ordered and coherent dynamics as pattern formation in the turbulent combustor, utilizing high-speed flame images representing the distribution of the local heat release rate fluctuations, flow field measurements (two-dimensional particle image velocimetry), unsteady pressure and global heat release rate signals. Separately, through planar Mie scattering images using oil droplets, the collective behaviour of small scale vortices interacting and resulting in the emergence of large scale coherent structures is illustrated. We show the emergence of spatial patterns using statistical tools used to study transitions in other pattern forming systems. In this paper, we propose that the intertwined and highly intricate interactions between the wide spatio-temporal scales in the flame, the flow and the acoustics are through pattern formation.

scholarly journals Simulation of 2-Way Fluid Structure Interaction in a 3D Model Combustor

2012 ◽  
Author(s):  
Mina Shahi ◽  
Jim B. W. Kok ◽  
P. R. Alemela
Keyword(s):  
Flue Gas ◽  
Fluid Structure Interaction ◽  
Operating Conditions ◽  
Limit Cycle Oscillation ◽  
Structural Domain ◽  
Pressure Oscillations ◽  
Fluid Structure ◽  
Thermoacoustic Instability ◽  
Structure Interaction ◽  
Computational Fluid Dynamics Analysis

The liner of a gas turbine combustor is a very flexible structure that is exposed to the pressure oscillations that occur in the combustor. These pressure oscillations can be of very high amplitude due to thermoacoustic instability, when the fluctuations of the rate of heat release and the acoustic pressure waves amplify each other. The liner structure is a dynamic mechanical system that vibrates at its eigenfrequencies and at the frequencies by which it is forced by the pressure oscillations to which it is exposed. On the other hand the liner vibrations force a displacement of the flue gas near the wall in the combustor. The displacement is very small but this acts like a distributed acoustic source which is proportional to the liner wall acceleration. Hence liner and combustor are a coupled elasto-acoustic system. When this is exposed to a limit cycle oscillation the liner may fail due to fatigue. In this paper the method and the results will be presented of the partitioned simulation of the coupled acousto-elastic system composed of the liner and the flue gas domain in the combustor. The partitioned simulation uses separate solvers for the flow domain and the structural domain, that operate in a coupled way. In this work 2-way fluid structure interaction is studied for the case of a model combustor for the operating conditions 40–60 kW with equivalence ratio of 0.625. This is done in the framework of the LIMOUSINE project. Computational fluid dynamics analysis is performed to obtain the thermal loading of the combustor liner and finite element analysis renders the temperature, stress distribution and deformation in the liner. The software used is ANSYS workbench V13.0 software, in which the information (pressure and displacement) is also exchanged between fluid and structural domain transiently.

Modal Decomposition Analysis of Combustion Instability Due to External Perturbation in a Mesoscale Burner Array

2022 ◽  
Author(s):  
Jeongan Choi ◽  
Rajavasanth Rajasegar ◽  
Qili Liu ◽  
Tonghun Lee ◽  
Jihyung Yoo
Keyword(s):  
Growth Rates ◽  
High Speed ◽  
Combustion Instability ◽  
Decomposition Analysis ◽  
External Perturbation ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Combustion Instabilities ◽  
Thermoacoustic Instability ◽  
Growth Regime

Abstract In this work, the growth regime of combustion instability was studied by analyzing 10 kHz OH planar laser induced fluorescence (PLIF) images through a combination of dynamic mode decomposition (DMD) and spectral proper orthogonal decomposition (SPOD) methods. Combustion instabilities were induced in a mesoscale burner array through an external speaker at an imposed perturbation frequency of 210 Hz. During the transient onset of combustion instabilities, 10 kHz OH PLIF imaging was employed to capture spatially and temporally resolved flame images. Increased acoustic perturbations prevented flame reignition in the central recirculation zone and eventually led to the flame being extinguished inwards from the outer burner array elements. Coherent modes and their growth rates were obtained from DMD spectral analyses of high-speed OH PLIF images. Positive growth rates were observed at the forcing frequency during the growth regime. Coherent structures, closely associated with thermoacoustic instability, were extracted using an appropriate SPOD filter operation to identify mode structures that correlate to physical phenomena such as shear layer instability and flame response to longitudinal acoustic forcing. Overall, a combination of DMD and SPOD was shown to be effective at analyzing the onset and propagation of combustion instabilities, particularly under transient burner operations.

scholarly journals Time Series Analysis for Predicting Hydroelectric Power Production: The Ecuador Case

2019 ◽  
Vol 11 (23) ◽  
pp. 6539 ◽  
Author(s):  
Julio Barzola-Monteses ◽  
Mónica Mite-León ◽  
Mayken Espinoza-Andaluz ◽  
Juan Gómez-Romero ◽  
Waldo Fajardo
Keyword(s):  
Time Series ◽  
Power Production ◽  
Hydroelectric Power ◽  
Power Sources ◽  
Regression Methods ◽  
Production Values ◽  
Starting Point ◽  
Electrical Faults ◽  
Electrical Generation ◽  
Variable Precipitation

Electrical generation in Ecuador mainly comes from hydroelectric and thermo-fossil sources, with the former amounting to almost half of the national production. Even though hydroelectric power sources are highly stable, there is a threat of droughts and floods affecting Ecuadorian water reservoirs and producing electrical faults, as highlighted by the 2009 Ecuador electricity crisis. Therefore, predicting the behavior of the hydroelectric system is crucial to develop appropriate planning strategies and a good starting point for energy policy decisions. In this paper, we developed a time series predictive model of hydroelectric power production in Ecuador. To this aim, we used production and precipitation data from 2000 to 2015 and compared the Box-Jenkins (ARIMA) and the Box-Tiao (ARIMAX) regression methods. The results showed that the best model is the ARIMAX (1,1,1) (1,0,0)12, which considers an exogenous variable precipitation in the Napo River basin and can accurately predict monthly production values up to a year in advance. This model can provide valuable insights to Ecuadorian energy managers and policymakers.

scholarly journals Analysis of Diesel Knock for High-Altitude Heavy-Duty Engines Using Optical Rapid Compression Machines

Energies ◽  
2020 ◽  
Vol 13 (12) ◽  
pp. 3080
Author(s):  
Xiangting Wang ◽  
Haiqiao Wei ◽  
Jiaying Pan ◽  
Zhen Hu ◽  
Zeyuan Zheng ◽  
...  
Keyword(s):  
High Altitude ◽  
High Speed ◽  
Ambient Pressure ◽  
Critical Conditions ◽  
Heavy Duty ◽  
Pressure Oscillations ◽  
Spray Impingement ◽  
Rapid Compression ◽  
The Given ◽  
Injection Pressures

In high altitude regions, affected by the low-pressure and low-temperature atmosphere, diesel knock is likely to be encountered in heavy-duty engines operating at low-speed and high-load conditions. Pressure oscillations during diesel knock are commonly captured by pressure transducers, while there is a lack of direct evidence and visualization images, such that its fundamental formation mechanism is still unclear. In this study, optical experiments on diesel knock with destructive pressure oscillations were investigated in an optical rapid compression machine. High-speed direct photography and simultaneous pressure acquisition were synchronically performed, and different injection pressures and ambient pressures were considered. The results show that for the given ambient temperature and pressure, diesel knock becomes prevalent at higher injection pressures where fuel spray impingement becomes enhanced. Higher ambient pressure can reduce the tendency to diesel knock under critical conditions. For the given injection pressure satisfying knocking combustion, knock intensity is decreased as ambient pressure is increased. Further analysis of visualization images shows diesel knock is closely associated with the prolonged ignition delay time due to diesel spray impingement. High-frequency pressure oscillation is caused by the propagation of supersonic reaction-front originating from the second-stage autoignition of mixture. In addition, the oscillation frequencies are obtained through the fast Fourier transform (FFT) analysis.

Effects of B20 on Combustion, Emissions and Performance of a Light-Duty Diesel Engine

2009 ◽  
Author(s):  
Amy M. Peterson ◽  
Po-I Lee ◽  
Ming-Chia Lai ◽  
Ming-Cheng Wu ◽  
Craig L. DiMaggio
Keyword(s):  
Diesel Engine ◽  
High Speed ◽  
Combustion Noise ◽  
Low Temperature Combustion ◽  
Performance And Emission ◽  
Ultra Low Sulfur Diesel ◽  
White Grease ◽  
Light Duty ◽  
Di Diesel Engine ◽  
And Performance

This paper compares 20% bio-diesel (B20-choice white grease) fuel with baseline ultra low sulfur diesel (ULSD) fuel on the performance of combustion and emissions of a light-duty 4-cylinder 2.8-liter common-rail DI diesel engine. The results show that operating the engine in the Low Temperature Combustion (LTC) regime produces lower PM and NOx with a slight penalty in fuel consumption, THC, and CO emissions. B20, in general, produces less soot. A slight increase in NOx emissions is shown with B20 compared to ULSD, with an exception at the high speed point where B20 has lower NOx values. In addition, the performance and emission characteristics are investigated as a function of the ECU injection strategy. The addition of pilot injections is found to effectively reduce combustion noise and extends the injection retard window to reach LTC combustion regimes with acceptable noise level for LD diesel engines.

Multifractal characteristics of combustor dynamics close to lean blowout

2015 ◽  
Vol 784 ◽  
pp. 30-50 ◽  
Author(s):  
Vishnu R. Unni ◽  
R. I. Sujith
Keyword(s):  
Stability Margin ◽  
Static Stability ◽  
Combustion Noise ◽  
Stable Operation ◽  
Nonlinear Behaviour ◽  
Classical Literature ◽  
Unified Framework ◽  
Simple Description ◽  
Thermoacoustic Instability ◽  
Lean Blowout

In classical literature, blowout is described as loss of static stability of the combustion system whereas thermoacoustic instability is seen as loss of dynamic stability of the system. At blowout, the system transitions from a stable reacting state to a non-reacting state, indicating loss of static stability of the reaction. However, this simple description of stability margin is inadequate since recent studies have shown that combustors exhibit complex nonlinear behaviour prior to blowout. Recently, it was shown that combustion noise that characterizes the regime of stable operation is itself dynamically complex and exhibits multifractal characteristics. Researchers have already described the transition from combustion noise to combustion instability as a loss of multifractality. In this work, we provide a multifractal description for lean blowout in combustors with turbulent flow and thus introduce a unified framework within which both thermoacoustic instability and blowout can be described. Further, we introduce a method for predicting blowout based on the multifractal description of blowout.

Measurements of Periodic Reynolds Stress Oscillations in a Forced Turbulent Premixed Swirling Flame

2018 ◽  
Author(s):  
Christopher Douglas ◽  
Jamie Lim ◽  
Travis Smith ◽  
Benjamin Emerson ◽  
Timothy Lieuwen ◽  
...  
Keyword(s):  
Reynolds Stress ◽  
High Speed ◽  
Large Scale ◽  
Turbulent Transport ◽  
Stereoscopic Particle Image Velocimetry ◽  
Thermoacoustic Instability ◽  
Image Velocimetry ◽  
Planar Laser ◽  
Swirling Flame ◽  
Dynamics Simulations

This work is motivated by the thermoacoustic instability challenges associated with ultra-low emissions gas turbine combustors. It demonstrates the first use of high-speed dual-plane orthogonally-polarized stereoscopic-particle image velocimetry and synchronized OH planar laser-induced fluorescence in a premixed swirling flame. We use this technique to explore the effects of combustion and longitudinal acoustic forcing on the time- and phase-averaged flow field — particularly focusing on the behavior of the Reynolds stress in the presence of harmonic forcing. We observe significant differences between ensemble averaged and time averaged Reynolds stress. This implies that the large-scale motions are non-ergodic, due to coherent oscillations in Reynolds stress associated with the convection of periodic vortical structures. This result has important implications on hydrodynamic stability models and reduced order computational fluid dynamics simulations, which do show the importance of turbulent transport on the problem, but do not capture these coherent oscillations in their models.

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