scholarly journals Seeds of phase transition to thermoacoustic instability

2021 ◽  
Author(s):  
Raghunathan Manikandan ◽  
Nitin Babu George ◽  
Vishnu Unni ◽  
R. Sujith ◽  
Jürgen Kurths ◽  
...  
Keyword(s):  
Power Plants ◽  
High Amplitude ◽  
Spatiotemporal Patterns ◽  
Critical Threshold ◽  
Reacting Flow ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Stable Operating ◽  
Operating Regimes ◽  
Early Manifestation

Abstract Tackling the problem of emissions is at the forefront of scientific research today. While industrial engines designed to operate in stable regimes produce emissions, attempts to operate them at "greener" conditions often fail due to thermoacoustic instability. During thermoacoustic instability, hazardous high amplitude periodic oscillations lead to failure of these engines in power plants, aircrafts and rockets. Yet, identifying the onset of thermoacoustic instability remains elusive due to spatial variability and the continuous evolution of spatiotemporal patterns in the reacting flow field. Here, we show experimental evidence of early manifestation of the onset of thermoacoustic instability at certain zones. Our findings allow us to identify a critical threshold that enables us to distinguish stable operating regimes from hazardous operations. This opens new perspectives for predicting the onset of thermoacoustic instability and could be a step forward to "greener" operations. The developed methodology is applicable for other systems exhibiting phase transitions.

Intermittency route to thermoacoustic instability in turbulent combustors

2014 ◽  
Vol 756 ◽  
pp. 470-487 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Bluff Body ◽  
Combustion Instability ◽  
Signal Amplitude ◽  
High Amplitude ◽  
Operating Conditions ◽  
Combustion Noise ◽  
Continuous Measure ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Amplitude Fluctuations

AbstractThe 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.

Variability in Thermal Performance and Measured Heat Rate in Fossil-Fuelled Power Plants

1992 ◽  
Vol 206 (2) ◽  
pp. 109-123 ◽  
Author(s):  
F L Carvalho ◽  
F H D Conradie ◽  
H Kuerten ◽  
F J McDyer
Keyword(s):  
Thermal Performance ◽  
Power Plants ◽  
Performance Monitoring ◽  
Unit Commitment ◽  
Thermal Power ◽  
Heat Rate ◽  
Plant Performance ◽  
Plant Management ◽  
Fuel Quality ◽  
Stable Operating

The paper examines the variability of key parameters in the operation of ten thermal power plants in various commercial grid environments with a view to assessing the viability of ‘on-demand’ plant performance monitoring for heat rate declaration. The plants of various types are limited to coal- and oil-fired units in the capacity range of 305–690 MW generated output. The paper illustrates the influence of control system configuration on effective and flexible power plant management. The analysis of variability indicates that there is a reasonable probability of achieving adequately stable operating periods within the normal operating envelope of grid dispatch instructions when thermal performance monitoring and display can be undertaken with a high confidence level. The levels of variability in fuel quality, which were measured during nominally constant levels of fuel input and generated output, range from about +1 per cent for oil-fired plants to about ±5 per cent for coal-fired power plants. The implications of adopting on-line monitoring of unit heat rate as an input to the generation ordering and unit commitment process are potentially significant cost and energy conservation benefits for utilities having a high proportion of coal- and oil-fired generation.

Predicting the Amplitude of Thermoacoustic Instability Using Universal Scaling Behaviour

2021 ◽  
Author(s):  
Induja Pavithran ◽  
Vishnu R. Unni ◽  
Abhishek Saha ◽  
Alan J. Varghese ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Amplitude Spectrum ◽  
High Amplitude ◽  
Dominant Mode ◽  
Turbine Engines ◽  
Stable Operation ◽  
Scaling Relation ◽  
Thermoacoustic Instability ◽  
Universal Scaling ◽  
Engine Components ◽  
Accuracy Of Prediction

Abstract The complex interaction between the turbulent flow, combustion and the acoustic field in gas turbine engines often results in thermoacoustic instability that produces ruinously high-amplitude pressure oscillations. These self-sustained periodic oscillations may result in a sudden failure of engine components and associated electronics, and increased thermal and vibra-tional loads. Estimating the amplitude of the limit cycle oscillations (LCO) that are expected during thermoacoustic instability helps in devising strategies to mitigate and to limit the possible damages due to thermoacoustic instability. We propose two methodologies to estimate the amplitude using only the pressure measurements acquired during stable operation. First, we use the universal scaling relation of the amplitude of the dominant mode of oscillations with the Hurst exponent to predict the amplitude of the LCO. We also present a methodology to estimate the amplitudes of different modes of oscillations separately using ‘spectral measures’ which quantify the sharpening of peaks in the amplitude spectrum. The scaling relation enables us to predict the peak amplitude at thermoacoustic instability, given the data during the safe operating condition. The accuracy of prediction is tested for both methods, using the data acquired from a laboratory-scale turbulent combustor. The estimates are in good agreement with the actual amplitudes.

scholarly journals Stability and Limit Cycles of a Nonlinear Damper Acting on a Linearly Unstable Thermoacoustic Mode

2018 ◽  
Vol 141 (5) ◽  
Author(s):  
Claire Bourquard ◽  
Nicolas Noiray
Keyword(s):  
Growth Rate ◽  
Combustion Chamber ◽  
Limit Cycle ◽  
Nonlinear Effects ◽  
Coupled System ◽  
High Amplitude ◽  
Van Der Pol Oscillator ◽  
Mean Velocity ◽  
Thermoacoustic Instability ◽  
Numerical Predictions

The resonant coupling between flames and acoustics is a growing issue for gas turbine manufacturers, which can be reduced by adding acoustic dampers on the combustion chamber walls. Nonetheless, if the engine is operated out of the stable window, the damper is exposed to high-amplitude acoustic levels, which trigger unwanted nonlinear effects. This work provides an overview of the dynamics of this coupled system using a simple analytical model, where a perfectly tuned damper is coupled to the combustion chamber. The damper, crossed by a purge flow in order to prevent hot gas ingestion, is modeled as a nonlinearly damped harmonic oscillator. The combustion chamber featuring a linearly unstable thermoacoustic mode is modeled as a Van der Pol oscillator. Analyzing the averaged amplitude equations gives the limit cycle amplitudes as function of the growth rate of the unstable mode and the mean velocity through the damper neck. Experiments are also performed on a simple rectangular cavity, where the thermoacoustic instability is mimicked by an electro-acoustic instability. A feedback loop is built, through which the growth rate of the instability can be controlled. A Helmholtz damper is added to the cavity and tuned to the mode of interest. The stabilization capabilities of the damper and the amplitude of the limit cycle in the unstable cases are in good agreement between the experiments and the analytical and numerical predictions, underlining the potentially dangerous behavior of the system, which should be taken into account for real engine cases.

Intermittency in the Dynamics of Turbulent Combustors

2014 ◽  
Author(s):  
Vineeth Nair ◽  
R. I. Sujith
Keyword(s):  
Reynolds Number ◽  
High Speed ◽  
Bluff Body ◽  
Combustion Instability ◽  
Homoclinic Orbits ◽  
High Amplitude ◽  
Operating Conditions ◽  
Periodic Oscillations ◽  
Lean Blowout ◽  
Amplitude Fluctuations

The dynamic transitions preceding combustion instability and lean blowout were investigated experimentally in a laboratory scale turbulent combustor 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. The onset of high-amplitude, combustion-driven oscillations in turbulent combustors thus corresponds to a transition in dynamics from chaos to limit cycle oscillations through a state characterized as intermittency in dynamical systems theory. These excursions to periodic oscillations become last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. Such intermittent oscillations emerge through the establishment of homoclinic orbits in the phase space of the global system which is composed of hydrodynamic and acoustic subsystems that operate over different time scales. Such intermittent burst oscillations are also observed in the combustor on increasing the Reynolds number further past conditions of combustion instability towards the lean blowout limit. High-speed flame images reveal that the intermittent states observed prior to lean blowout correspond to aperiodic detachment of the flame from the bluff-body lip. These intermittent oscillations are thus of prognostic value and can be utilized to provide early warning signals to combustion instability as well as lean blowout.

scholarly journals Linear Taylor–Couette stability of a transversely isotropic fluid

2015 ◽  
Vol 471 (2178) ◽  
pp. 20150141 ◽  
Author(s):  
C. R. Holloway ◽  
R. J. Dyson ◽  
D. J. Smith
Keyword(s):  
Viscous Fluid ◽  
Shear Viscosity ◽  
Outer Cylinder ◽  
Transversely Isotropic ◽  
Biological Molecules ◽  
Preferred Direction ◽  
Stable Operating ◽  
Gap Width ◽  
Operating Regimes ◽  
Isotropic Fluid

Fibre-laden fluids are found in a variety of situations, while Couette devices are used for flow spectroscopy of long biological molecules, such as DNA and proteins in suspension. The presence of these fibres can significantly alter the rheology of the fluid, and hence must be incorporated in any modelling undertaken. A transversely isotropic fluid treats these suspensions as a continuum with an evolving preferred direction, through a modified stress tensor incorporating four viscosity-like parameters. We consider the axisymmetric linear stability of a transversely isotropic viscous fluid, contained between two rotating co-axial cylinders, and determine the critical wave and Taylor numbers for varying gap width and inner cylinder velocity (assuming the outer cylinder is fixed). Through the inclusion of transversely isotropic effects, the onset of instability is delayed, increasing the range of stable operating regimes. This effect is felt most strongly through incorporation of the anisotropic shear viscosity, although the anisotropic extensional viscosity also contributes. The changes to the rheology induced by the presence of the fibres therefore significantly alter the dynamics of the system, and hence should not be neglected.

Intermittency, Secondary Bifurcation and Mixed-Mode Oscillations in a Swirl-Stabilized Annular Combustor: Experiments and Modeling

2020 ◽  
Author(s):  
Samarjeet Singh ◽  
Amitesh Roy ◽  
Reeja K. V. ◽  
Asalatha Nair ◽  
Swetaprovo Chaudhuri ◽  
...  
Keyword(s):  
Mixed Mode ◽  
Equivalence Ratio ◽  
High Amplitude ◽  
The State ◽  
Turbulent Flames ◽  
Bulk Velocity ◽  
Thermoacoustic Instability ◽  
Mixed Mode Oscillations ◽  
Annular Combustor ◽  
Low Amplitude

Abstract We experimentally study thermoacoustic transitions in an annular combustor consisting of sixteen premixed, swirl-stabilized turbulent flames. We show the changes in the characteristics of bifurcations leading to the state of longitudinal thermoacoustic instability (TAI) when equivalence ratio and bulk velocity are systematically varied. Depending upon the bulk velocity, we observe different states of combustor operation when the equivalence ratio is varied. These states include combustion noise, intermittency, low-amplitude TAI, mixed-mode oscillations (MMO), and high-amplitude TAI. We closely examine the special case of MMO that is encountered during the transition from low-amplitude TAI to high-amplitude TAI. We also discuss the global and local flame dynamics observed during the state of MMO. We find that during epochs of low-amplitude oscillations of MMO, all the flames are partially synchronized, while during epochs of high-amplitude oscillations, all the flames are perfectly synchronized. Finally, we replicate the criticalities of bifurcation of the annular combustor in a phenomenological model containing sixth-order nonlinearities.

Effects of Combustor Acoustics on Fuel Spray Dynamics

2004 ◽  
Author(s):  
Wajid A. Chishty ◽  
Uri Vandsburger ◽  
William R. Saunders ◽  
William T. Baumann
Keyword(s):  
Control Systems ◽  
Liquid Fuel ◽  
Separated Flow ◽  
Fuel Spray ◽  
Reacting Flow ◽  
Thermoacoustic Instability ◽  
Response Characteristics ◽  
Frequency Response Characteristics ◽  
System Characteristics ◽  
Insight Into

An experimental liquid fuel LDI combustor, developed to study thermoacoustic instability processes and to test active combustion control systems, was found to demonstrate three distinct stability regimes, with system characteristics not reported in earlier literature. These observations led to a series of further investigations, both in reactive and non-reactive conditions, to gain an insight into effects of combustor acoustics on fuel spray dynamics. This paper presents only the non-reacting flow results, from both experimental and modeling investigations. The experimental setup and construction details of an isothermal acoustic rig are presented. Phase-locked PDA measurements of droplet velocities and diameters from a simplex atomizer spray were acquired, with and without combustor swirl co-airflow, under varying acoustic forcing conditions and spray feed pressures. Measurements made at four locations in the spray are related, in the paper, to these variations in mean and unsteady inputs. The dynamic behavior of the spray is then presented in terms of frequency response characteristics related to acoustic fields imposed on the spray. Finally, results from non-reacting spray modeling, predicting droplet trajectories, are reported. The modeling was done using the deterministic separated flow approach. These trajectories are compared to the reported experimental results to support preliminary explanations for the unique experimental observations of the swirl-stabilized kerosene flame in a single can combustor geometry.

Periodic Oscillations in Natural Circulation Boiling System

2017 ◽  
Author(s):  
Xianbing Chen ◽  
Puzhen Gao ◽  
Qiang Wang ◽  
Yinxing Zhang ◽  
Jiawei Liu
Keyword(s):  
Mass Flux ◽  
Nuclear Power ◽  
Power Plants ◽  
Two Phase Flow ◽  
Flow Instability ◽  
Natural Circulation ◽  
Flow Reversal ◽  
Phase Flow ◽  
Two Phase ◽  
Periodic Oscillations

Natural circulation has been widely used in some evolutionary and innovative nuclear power plants. Natural circulation systems are susceptible to flow instabilities which are undesirable in the nuclear power devices. An experimentally investigation of two phase flow instability in up-flow boing channel under natural circulation is presented in this paper. Flow instability with and without flow reversal have been found. A pulse signal of water temperature at the inlet of the test section can be detected when the channel suffers from flow reversal. Single phase and two phase flow alternate in the channel regardless of the occurrence of flow reversal. Periodic oscillations with multiple high-order harmonic waves are confirmed by applying Fast Fourier Transform to the time traces of flow rates. Period of flow instability which is the reciprocal of the frequency with the largest amplitude in the amplitude-frequency plane are obtained. Period of flow oscillation presents a nonlinear change with the increase of mass flux. Period of flow instability increases rapidly with the increase of mass flux and decreases slowly when it reaches the maximum value.

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