scholarly journals Role of Shear Layer Instability in Driving Pressure Oscillations in a Backward Facing Step Combustor

2016 ◽  
Author(s):  
Srinivas K. Kirthy ◽  
Santosh Hemchandra ◽  
Seunghyuck Hong ◽  
Santosh Shanbhogue ◽  
Ahmed F. Ghoniem
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Acoustic Pressure ◽  
Combustion Instability ◽  
High Amplitude ◽  
Open Loop ◽  
Hydrodynamic Modes ◽  
Pressure Oscillations ◽  
Layer Mode ◽  
Low Amplitude

This paper presents a global hydrodynamic stability analysis of flow fields in a backward facing step combustor, assuming weakly non-parallel flow. The baseline experiments in a ‘long’ combustor of length of 5.0 m shows the presence of two combustion instability states characterized by coherent low and high amplitude acoustic pressure oscillations. The analysis is performed for Propane-air mixtures at three values of ϕ = 0.63, 0.72 and 0.85 which correspond to quiet, low amplitude and high amplitude instability states in the long combustor experiments. Base flow velocity and density fields for the hydrodynamic stability analysis are determined from time averaged PIV measurements made after the length of the duct downstream of the step has been shortened to eliminate acoustic pressure oscillations. The analysis shows that the shear layer mode is self-excited for the ϕ = 0.72 case with an oscillation frequency close to that of the long combustor’s fundamental acoustic mode. We show from an analysis of the weakly forced, variable density Navier-Stokes equations that self-excited hydrodynamic modes can be weakly receptive to forcing — suggesting that the low amplitude instability in the long combustor is due to semi-open loop forcing of heat release oscillations by the shear layer mode. At ϕ = 0.85 the flow is hydrodynamically globally stable but locally convectively unstable. Spatial amplification of velocity disturbances by the convectively unstable flow causes high amplitude combustion instability in the long combustor case. These results show that combustion instability can be sustained by acoustic and hydrodynamic modes being either strongly coupled, resulting in fully closed loop forcing, or weakly coupled, resulting in semi-open loop forcing of the flame by a self-excited hydrodynamic mode.

Impact of Precessing Vortex Core Dynamics on Shear Layer Response in a Swirling Jet

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Mark Frederick ◽  
Kiran Manoharan ◽  
Joshua Dudash ◽  
Brian Brubaker ◽  
Santosh Hemchandra ◽  
...  
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Vortex Core ◽  
Combustion Instability ◽  
Forced Response ◽  
Longitudinal Acoustic ◽  
Precessing Vortex Core ◽  
Acoustic Forcing

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Multifractality in combustion noise: predicting an impending combustion instability

2014 ◽  
Vol 747 ◽  
pp. 635-655 ◽  
Author(s):  
Vineeth Nair ◽  
R. I. Sujith
Keyword(s):  
Scale Invariance ◽  
Prognostic Value ◽  
Combustion Instability ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Combustion Noise ◽  
Warning Signals ◽  
Stochastic Background ◽  
Turbulent Flow Field ◽  
Low Amplitude

AbstractThe 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.

Engineering Precursors to Forewarn the Onset of an Impending Combustion Instability

2014 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Time Scales ◽  
Combustion Instability ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Combustion Noise ◽  
Additional Measure ◽  
Statistical Measures ◽  
Random Manner ◽  
Low Amplitude ◽  
Measured Pressure

The present study aims at arming an operator of fielded turbulent combustors with a repertoire of mathematical measures for real-time monitoring to forewarn the onset of impending combustion instability. In turbulent combustors, the route to high-amplitude, periodic, combustion-driven oscillations from conditions of low-amplitude, chaotic, combustion noise happens through an intermediate regime in flow conditions where the measured pressure fluctuations display bursts of intermittent, high-amplitude, periodic oscillations that appear in a near-random manner amidst chaotic fluctuations. This loss of chaos from combustion noise to combustion instability can be quantified to serve as a precursor to impending instability. The recurrence properties of intermittent burst oscillations can be quantified using dynamical systems theory by tracking the distribution of the aperiodic segments in the measured signals. Several statistical measures may be constructed through such recurrence quantification that provide robust early warning signals to an impending instability. Further, the transition to combustion driven oscillations leads to a collapse of the number of relevant time scales involved in the problem. This collapse in time scales can be quantified using generalized Hurst exponents which serve as an additional measure that captures the onset of an impending combustion instability well in advance. The various patent pending measures illustrated in this paper serve as precursors due to the inevitable presence of an intermittent regime of burst oscillations in turbulent combustors.

Impact of PVC Dynamics on Shear Layer Response in a Swirling Jet

2017 ◽  
Author(s):  
Mark Frederick ◽  
Joshua Dudash ◽  
Jacqueline O’Connor ◽  
Kiran Manoharan ◽  
Santosh Hemchandra ◽  
...  
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Combustion Instability ◽  
Forced Response ◽  
Coupling Mechanism ◽  
Longitudinal Acoustic ◽  
Acoustic Forcing ◽  
The Impact

Combustion instability, or the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a non-reacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

scholarly journals Experimental Study on Dynamic Combustion Characteristics in Swirl-Stabilized Combustors

Energies ◽  
2021 ◽  
Vol 14 (6) ◽  
pp. 1609
Author(s):  
Donghyun Hwang ◽  
Kyubok Ahn
Keyword(s):  
Experimental Study ◽  
Heat Release ◽  
Dynamic Pressure ◽  
Combustion Instability ◽  
Flame Structure ◽  
Pressure Sensors ◽  
Necessary Condition ◽  
Rayleigh Criterion ◽  
Pressure Oscillations ◽  
Geometric Conditions

An experimental study was performed to investigate the combustion instability characteristics of swirl-stabilized combustors. A premixed gas composed of ethylene and air was burned under various flow and geometric conditions. Experiments were conducted by changing the inlet mean velocity, equivalence ratio, swirler vane angle, and combustor length. Two dynamic pressure sensors, a hot-wire anemometer, and a photomultiplier tube were installed to detect the pressure oscillations, velocity perturbations, and heat release fluctuations in the inlet and combustion chambers, respectively. An ICCD camera was used to capture the time-averaged flame structure. The objective was to understand the relationship between combustion instability and the Rayleigh criterion/the flame structure. When combustion instability occurred, the pressure oscillations were in-phase with the heat release oscillations. Even if the Rayleigh criterion between the pressure and heat release oscillations was satisfied, stable combustion with low pressure fluctuations was possible. This was explained by analyzing the dynamic flow and combustion data. The root-mean-square value of the heat release fluctuations was observed to predict the combustion instability region better than that of the inlet velocity fluctuations. The bifurcation of the flame structure was a necessary condition for combustion instability in this combustor. The results shed new insight into combustion instability in swirl-stabilized combustors.

Frequency Response Analysis of Piecewise Nonlinear Vibration Isolator

2005 ◽  
Author(s):  
Patrick Stahl ◽  
G. Nakhaie Jazar
Keyword(s):  
Low Frequency ◽  
High Amplitude ◽  
Relative Displacement ◽  
Response Analysis ◽  
Frequency Response Analysis ◽  
State Behavior ◽  
Vibration Isolators ◽  
Short Period ◽  
Secondary Suspension ◽  
Low Amplitude

Non-smooth piecewise functional isolators are smart passive vibration isolators that can provide effective isolation for high frequency/low amplitude excitation by introducing a soft primary suspension, and by preventing a high relative displacement in low frequency/high amplitude excitation by introducing a relatively damped secondary suspension. In this investigation a linear secondary suspension is attached to a nonlinear primary suspension. The primary is assumed to be nonlinear to model the inherent nonlinearities involved in real suspensions. However, the secondary suspension comes into action only during a short period of time, and in mall domain around resonance. Therefore, a linear assumption for the secondary suspension is reasonable. The dynamic behavior of the system subject to a harmonic base excitation has been analyzed utilizing the analytic results derived by applying the averaging method. The analytic results match very well in the transition between the two suspensions. A sensitivity analysis has shown the effect of varying dynamic parameters in the steady state behavior of the system.

Active Control of Combustion Instability Using Symmetric and Asymmetric Premix Fuel Modulation

2007 ◽  
Author(s):  
Daniel Guyot ◽  
Christian Oliver Paschereit
Keyword(s):  
Heat Release ◽  
Closed Loop ◽  
Modulation Frequency ◽  
Combustion Instability ◽  
Open Loop ◽  
Closed Loop Control ◽  
Nox Emissions ◽  
Loop Control ◽  
Asymmetric Modulation ◽  
Control Schemes

Active instability control was applied to an atmospheric swirl-stabilized premixed combustor using open loop and closed loop control schemes. Actuation was realised by two on-off valves allowing for symmetric and asymmetric modulation of the premix fuel flow while maintaining constant time averaged overall fuel mass flow. Pressure and heat release fluctuations in the combustor as well as NOx, CO and CO2 emissions in the exhaust were recorded. In the open loop circuit the heat release response of the flame was first investigated during stable combustion. For symmetric fuel modulation the dominant frequency in the heat release response was the modulation frequency, while for asymmetric modulation it was its first harmonic. In stable open loop control a reduction of NOx emissions due to fuel modulation of up to 19% was recorded. In the closed loop mode phase-shift control was applied while triggering the valves at the dominant oscillation frequency as well as at its second subharmonic. Both, open and closed loop control schemes were able to successfully control a low-frequency combustion instability, while showing only a small increase in NOx emissions compared to, for example, secondary fuel modulation. Using premixed open loop fuel modulation, attenuation was best when modulating the fuel at frequencies different from the dominant instability frequency and its subharmonic. The performance of asymmetric fuel modulation was generally slightly better than for symmetric modulation in terms of suppression levels as well as emissions. Suppression of the instability’s pressure rms level of up to 15.7 dB was recorded.

The Histogram of Pressure Oscillations Amplitudes, in Lean Premixed Combustions, Caused by Thermo-Acoustic Instabilities

2010 ◽  
Author(s):  
Nasser Seraj Mehdizadeh ◽  
Nozar Akbari
Keyword(s):  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Equivalence Ratio ◽  
Combustion Instability ◽  
Premixed Combustion ◽  
Space Distribution ◽  
Rayleigh Criterion ◽  
Pressure Oscillations ◽  
Lean Premixed

Lean premixed combustion is widely used in recent years as a method to achieve the environmental standards with regard to NOx emission. In spite of the mentioned advantage, premixed combustion systems, with equivalence ratios less than one, are susceptible to the combustion instability. To study the lean combustion instability, by experiments, one premixed combustion setup, equipped with reactant supplying system, is designed and manufactured in Amirkabir University of Technology. In this research, gaseous propane is introduced as fuel and several experiments are performed at nearly atmospheric pressure, with equivalence ratios within the range of 0.7 to 1.5. In this experiments fuel mass flow rate is varied between 2 and 4 gr/s. Unstable operating condition has been observed in combustion chamber when equivalence ratio is less than one. To distinguish the combustion instability for various operating conditions, probability density functions, spectral diagrams, and space distribution of pressure oscillations, along with Rayleigh Criterion, are utilized. Accordingly, effect of equivalence ratio on stabilizing the unstable combustion system is investigated. Moreover, convective delay time is calculated for all experiments and the results are compared with Rayleigh Criterion. This comparison has shown good agreement the experimental results and Rayleigh Criterion. Finally, stability limits are identified based on inlet mass flow rate and equivalence ratio.

Application of instantaneous rotations to S‐wave vertical seismic profiling

Geophysics ◽  
1997 ◽  
Vol 62 (5) ◽  
pp. 1365-1368
Author(s):  
M. Boulfoul ◽  
Doyle R. Watts
Keyword(s):  
High Amplitude ◽  
Seismic Profile ◽  
P Wave ◽  
Petroleum Exploration ◽  
S Wave ◽  
Amplitude Variation With Offset ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
Seismic Models ◽  
Low Amplitude

The petroleum exploration industry uses S‐wave vertical seismic profiling (VSP) to determine S‐wave velocities from downgoing direct arrivals, and S‐wave reflectivities from upgoing waves. Seismic models for quantitative calibration of amplitude variation with offset (AVO) data require S‐wave velocity profiles (Castagna et al., 1993). Vertical summations (Hardage, 1983) of the upgoing waves produce S‐wave composite traces and enable interpretation of S‐wave seismic profile sections. In the simplest application of amplitude anomalies, the coincidence of high amplitude P‐wave reflectivity and low amplitude S‐wave reflectivity is potentially a direct indicator of the presence of natural gas.

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