Impact of Hydrogen Addition On the Thermoacoustic Instability and Precessing Vortex Core Dynamics in a CH4/H2/Air Technically Premixed Combustor

Abstract We study the impact of H2 enrichment on the unsteady flow dynamics and thermoacoustic instability in PRECCINSTA swirl combustor. The experiments were performed at atmospheric conditions with H2/CH4 fuel mixtures at a global equivalence ratio of 0.65 and a constant thermal power of 20 kW. We analyze data with three fuel compositions: 0%, 20% and 50% H2 in two operating modes, premixed (PM) and technically premixed (TPM). A new multi-resolution modal decomposition method, using a combination of wavelet transforms and proper orthogonal decomposition (WPOD) is performed on time resolved flow velocity and OHPLIF measurements. Thermoacoustic oscillations are observed in the TPM operating mode alone, indicating that the primary heat release driving mechanism is due to fuel-air ratio oscillations. WPOD results for the 0% H2 TPM case reveals intermittent helical PVC oscillations along with axisymmetric hydrodynamic flow oscillations due to the thermoacoustic oscillations. These oscillations cause local flame extinction near the nozzle centrebody resulting in liftoff. A precessing vortex core (PVC) then develops in the flow and enables intermittent flame reattachment. In the 0% H2 premixed case, the flame remains lifted off the centrebody despite the presence of PVC oscillations. H2 enrichment results in the suppression of flame lift-off and the PVC in both operating modes. We show from flow strain rate statistics and extinction strain rate calculations that the increase of the latter with H2 addition, allows the flame to stabilize in the region near the centrebody where the pure CH4 cases show lift off.

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Impact of Hydrogen Addition on the Thermoacoustic Instability and Precessing Vortex Core Dynamics in a CH4/H2/air Technically Premixed Combustor

10.1115/gt2021-58794 ◽

2021 ◽

Author(s):

Anindya Datta ◽

Saarthak Gupta ◽

Santosh Hemchandra ◽

Ianko Chterev ◽

Isaac Boxx

Keyword(s):

Strain Rate ◽

Vortex Core ◽

Operating Mode ◽

Flow Dynamics ◽

Thermoacoustic Instability ◽

Operating Modes ◽

Precessing Vortex Core ◽

Lift Off ◽

H2 Addition ◽

Thermoacoustic Oscillations

Abstract We study the impact of H2 enrichment on the unsteady flow dynamics and thermoacoustic instability in the single nozzle PRECCINSTA swirl combustor. We analyze data from two operating modes, premixed (PM) and technically premixed (TPM). The experiments were performed at atmospheric conditions with H2/CH4 fuel mixtures at a global equivalence ratio of 0.65 while maintaining a constant thermal power of 20 kW. We examine the effect of H2 addition on the flow dynamics by analyzing cases with three fuel compositions: 0% H2, 20% H2 and 50% H2 in both operating modes. A new multi resolution modal decomposition method, using a combination of wavelet transforms and proper orthogonal decomposition (WPOD) of the experimental time resolved high speed flow velocity and OH-PLIF measurements is performed. Thermoacoustic oscillations are observed in the TPM operating mode alone. WPOD results for the 0% H2 TPM operating mode case reveals intermittent helical PVC oscillations along with axi-symmetric hydrodynamic flow oscillations due to the thermoacoustic oscillation. These oscillations cause local flame extinction near the nozzle centrebody resulting in liftoff. A precessing vortex core (PVC) oscillation develops in the flow that enables intermittent flame reattachment and results in intermittent thermoacoustic oscillations in this case. In the 0% H2 PM case, the flame remains lifted off of the centrebody despite the presence of PVC oscillations in this case as well. H2 enrichment results in the suppression of flame lift-off and the PVC in both operating modes. We show from flow strain rate statistics and extinction strain rate calculations that the increase of the latter with H2 addition, allows the flame to stabilize in the region near the centrebody where the pure CH4 cases show lift off. The lack of thermoacoustic oscillations in the PM operating mode shows that the primary heat release driving mechanism is due to fuel-air ratio oscillation that the thermoacoustic oscillation generates. The time averaged flow fields and the emergence of the PVC when the flame is lifted off, together suggest that PVC oscillations are caused by the separation between the vortex breakdown bubble and the wake behind the centrebody, as suggested by prior computational studies.

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Extinction strain rate suppression of the precessing vortex core in a swirl stabilised combustor and consequences for thermoacoustic oscillations

Combustion and Flame ◽

10.1016/j.combustflame.2019.09.031 ◽

2020 ◽

Vol 211 ◽

pp. 229-252 ◽

Cited By ~ 4

Author(s):

Efstathios Karlis ◽

Yushuai Liu ◽

Yannis Hardalupas ◽

Alexander M.K.P. Taylor

Keyword(s):

Strain Rate ◽

Vortex Core ◽

Precessing Vortex Core ◽

Extinction Strain Rate ◽

Thermoacoustic Oscillations ◽

Rate Suppression

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Open-Loop Control of the Precessing Vortex Core in a Swirl-Stabilized Combustor: Impact on Flame Shape and Flame Stability

Volume 4A: Combustion, Fuels, and Emissions ◽

10.1115/gt2018-75472 ◽

2018 ◽

Author(s):

Finn Lückoff ◽

Moritz Sieber ◽

Kilian Oberleithner

Keyword(s):

Vortex Core ◽

Open Loop ◽

Time Resolved ◽

Loop Control ◽

Precessing Vortex Core ◽

Flame Dynamics ◽

Combustion Properties ◽

Flame Shape ◽

Lift Off ◽

The Impact

In this study, we apply periodic flow excitation of the PVC at the centerbody of a generic swirl-stabilized combustor to investigate the impact of the precessing vortex core (PVC) on flame shape and flame dynamics. Previous studies revealed considerable influence of the PVC on combustion properties such as flame dynamics and fuel/air mixing. We employ time-resolved OH*-chemiluminescence and pressure measurements to investigate the influence of the PVC on flame dynamics and flame shape transition. The PVC is typically present in flames which are detached from the burner outlet. This lift-off is observed for increasingly lean mixtures in this study. With the help of the PVC actuation, studied in this work, the transition point between attached and detached flame is shifted towards richer mixtures. Moreover, the dynamics of heat release rate fluctuations that are related to PVC and thermoacoustic instabilities are extracted from the OH*-chemiluminescence data. This reveals a considerable damping of the thermoacoustic oscillations due to the PVC actuation under technically premixed conditions and the rise of additional modes due to the interaction of both dynamics.

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The Effects of Exit Boundary Condition on Precessing Vortex Core Dynamics

Volume 4B: Combustion, Fuels, and Emissions ◽

10.1115/gt2019-91079 ◽

2019 ◽

Author(s):

Danielle Mason ◽

Sean Clees ◽

Mark Frederick ◽

Jacqueline O’Connor

Keyword(s):

Boundary Condition ◽

Vortex Core ◽

Gas Turbines ◽

Vortex Breakdown ◽

Base Flow ◽

Eigenvalue Decomposition ◽

Precessing Vortex Core ◽

Swirling Jet ◽

Exit Boundary ◽

The Impact

Abstract Many industrial combustion systems, especially power generation gas turbines, use fuel-lean combustion to reduce NOx emissions. However, these systems are highly susceptible to combustion instability, the coupling between combustor acoustics and heat release rate oscillations of the flame. It has been shown in previous work by the authors that a precessing vortex core (PVC) can suppress shear layer receptivity to external perturbations, reducing the potential for thermoacoustic coupling. The goal of this study is to understand the effect of combustor exit boundary condition on the flow structure of a swirling jet to increase fundamental understanding of how combustor design impacts PVC dynamics. The swirling jet is generated with a radial-entry, variable-angle swirler, and a quartz cylinder is fixed on the dump plane for confinement. Combustor exit constriction plates of different diameters are used to determine the impact of exit boundary condition on the flow field. Particle image velocimetry (PIV) is used to capture the velocity field inside the combustor. Spectral proper orthogonal decomposition, a frequency-resolved eigenvalue decomposition that can identify energetic structures in the flow, is implemented to identify the PVC at each condition in both energy and frequency space. We find that exit boundary diameter affects both the structure of the flow and the dynamics of the PVC. Higher levels of constriction (smaller diameters) force the downstream stagnation point of the vortex breakdown bubble upstream, resulting in greater divergence of the swirling jet. Further, as the exit diameter decreases, the PVC becomes less energetic and less spatially defined. Despite these changes in the base flow and PVC coherence, the PVC frequency is not altered by the exit boundary constriction. These trends will help inform our understanding of the impact of boundary conditions on both static and dynamic flame stability.

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Onset of flame-intrinsic thermoacoustic instabilities in partially premixed turbulent combustors

International Journal of Spray and Combustion Dynamics ◽

10.1177/1756827718758511 ◽

2018 ◽

Vol 10 (3) ◽

pp. 171-184 ◽

Cited By ~ 3

Author(s):

Meenatchidevi Murugesan ◽

Balasubramanian Singaravelu ◽

Abhijit K Kushwaha ◽

Sathesh Mariappan

Keyword(s):

Network Model ◽

Model Analysis ◽

Flow Speed ◽

Dominant Mode ◽

Combustion Noise ◽

Driving Mechanism ◽

Thermoacoustic Instability ◽

Thermoacoustic Instabilities ◽

Partially Premixed ◽

Thermoacoustic Oscillations

We investigate the onset of thermoacoustic instabilities in a turbulent combustor terminated with an area contraction. Flow speed is varied in a swirl-stabilized, partially premixed combustor and the system is observed to undergo a dynamical transition from combustion noise to instability via intermittency. We find that the frequency of thermoacoustic oscillations does not lock-on to any of the acoustic modes. Instead, we observe that the dominant mode in the dynamics of combustion noise, intermittency and thermoacoustic instability is a function of the flow speed. We also find that the observed mode is insensitive to the changes in acoustic field of the combustor, but it varies as a function of upstream flow time scale. This new kind of thermoacoustic instability was independently discovered in the recent theoretical analysis of premixed flames. They are known as intrinsic thermoacoustic modes. In this paper, we report the experimental observation and the route to flame intrinsic thermoacoustic instabilities in partially premixed flame combustors. A simplified low-order network model analysis is performed to examine the driving mechanism. Frequencies predicted by the network model analysis match well with the experimentally observed dominant frequencies. Intrinsic flame-acoustic coupling between the unsteady heat release rate and equivalence ratio fluctuations occurring at the location of fuel injection is found to play a key role. Further, we observe intrinsic thermoacoustic modes to occur only when the acoustic reflection co-efficients at the exit are low. This result indicates that thermoacoustic systems with increased acoustic losses at the boundaries have to consider the possibility of flame intrinsic thermoacoustic oscillations.

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Phase-Opposition Control of the Precessing Vortex Core in Turbulent Swirl Flames for Investigation of Mixing and Flame Stability

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4044469 ◽

2019 ◽

Vol 141 (11) ◽

Cited By ~ 3

Author(s):

Finn Lückoff ◽

Moritz Sieber ◽

Christian Oliver Paschereit ◽

Kilian Oberleithner

Keyword(s):

Control System ◽

Flow Control ◽

Turbulent Mixing ◽

Vortex Core ◽

Small Scale ◽

Precessing Vortex Core ◽

Flame Dynamics ◽

Flow Control System ◽

The Impact ◽

Opposition Control

Abstract The precessing vortex core (PVC) is a helically shaped coherent flow structure that occurs in reacting and nonreacting swirling flows undergoing vortex breakdown. In swirl-stabilized combustors, the PVC affects important phenomena, such as turbulent mixing and thermoacoustic oscillations. In this work, a closed-loop flow control system is developed, which allows for phase-opposition control of the PVC, to achieve appropriate conditions to systematically investigate the influence of the PVC on turbulent flames. The control consists of a zero-net-mass-flux actuator placed in the mixing section of the combustor, where the PVC is most receptive to periodic forcing. The flow control system is characterized from pressure measurements and particle image velocimetry (PIV) and the impact on flame dynamics is extracted from OH*-chemiluminescence measurements. The data reveal that the PVC amplitude is considerably suppressed by the phase-opposition control without changing the overall characteristics of flow and flame, which is crucial to study the exclusive effect of the PVC on combustion processes. Moreover, the control allows the PVC amplitude to be adjusted gradually to investigate the PVC impact on turbulent mixing and flame dynamics. It is revealed that the PVC-induced flow fluctuations mainly affect the large-scale mixing, while the small scale mixing remains unchanged. This is because the suppression of the PVC allows other modes to become more dominant and the overall turbulent kinetic energy (TKE) budget remains unchanged. The destabilization of other modes, such as the axisymmetric mode, may have some implications on thermoacoustic instability.

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The Research of the Swirling Flow of the Power Fluid during its Passing through the Hydrodynamic Cavitator

Prospecting and Development of Oil and Gas Fields ◽

10.31471/1993-9973-2020-1(74)-53-71 ◽

2020 ◽

pp. 53-71

Author(s):

Ya. Ya. Yakymechko ◽

Ya. М. Femiak

Keyword(s):

Flow Rate ◽

Vibration Frequency ◽

Vortex Core ◽

Swirling Flow ◽

Theoretical Research ◽

Theoretical Dependence ◽

Flow Swirl ◽

Precessing Vortex Core ◽

Speed Fluctuation ◽

The Impact

The article presents the theoretical research of the use of swirling flows with reverse jets and with developed precessing vortex core in cavitators and other devices. While describing the motion of the vortex core in the free swirling jet of the fluid it is necessary to take into account that according to the experimental data the vortex core can swirl along the length of the jet and moves around the jet axis in the zone between the area of reverse flows and the boundary outer layer. In this case, it is the vortex core which is under the influence of the basic swirling flow. Herewith, it is necessary to take into account that due to commensurate sizes of the vortex core and the jet, the impact on the core will be different owing to non-uniform distribution of speeds in the jet itself. On the basis of the known formulas, the authors have deduced the improved theoretical dependence of the degree of flow swirl on the flow rate, the vortex core vibration frequency and structural parameters under the conditions of the consistency of swirling flow itself. The theoretical dependence shows that the degree of flow swirl is directly proportional to the precessing vortex core vibration frequency and inversely proportional to the square of mass flow rate. Thus, ensuring the consistency of the swirling flow with varying flow-rate requires the corresponding change of the swirl degree or the influence on the frequency of vibrations of the precessing vortex core. On the basis of the deduced theoretical dependences, the authors have developed and implemented in the computer programs the following calculations: the dependence of the coefficient of the flow swirl on the vortex core vibration frequency; the simulation of the precession of the vortex core in the swirling flow; the research of speed fluctuation in the swirling flow; speed fluctuation during the interaction of swirling jets.

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Excitation of the precessing vortex core by active flow control to suppress thermoacoustic instabilities in swirl flames

International Journal of Spray and Combustion Dynamics ◽

10.1177/1756827719856237 ◽

2019 ◽

Vol 11 ◽

pp. 175682771985623 ◽

Cited By ~ 1

Author(s):

Finn Lückoff ◽

Kilian Oberleithner

Keyword(s):

Vortex Core ◽

Oscillation Amplitude ◽

Premixed Flames ◽

Current Control ◽

Minor Effect ◽

Continuous Increase ◽

Combustion Dynamics ◽

Thermoacoustic Instabilities ◽

Precessing Vortex Core ◽

The Impact

In this study, we apply periodic flow excitation of the precessing vortex core at the centerbody of a swirl-stabilized combustor to investigate the impact of the precessing vortex core on flame shape, flame dynamics, and especially thermoacoustic instabilities. The current control scheme is based on results from linear stability theory that determine the precessing vortex core as a global hydrodynamic instability with its maximum receptivity to open-loop actuation located near the center of the combustor inlet. The control concept is first validated at isothermal conditions. This is of utmost importance for the proceeding studies that focus on the exclusive impact of the precessing vortex core on the combustion dynamics. Subsequently, the control is applied to reacting conditions considering lean premixed turbulent swirl flames. Considering thermoacoustically stable flames first, it is shown that the actuation locks onto the precessing vortex core when it is naturally present in the flame, which allows the precessing vortex core frequency to be controlled. Moreover, the control allows the precessing vortex core to be excited in conditions where it is naturally suppressed by the flame, which yields a very effective possibility to control the precessing vortex core amplitude. The control is then applied to thermoacoustically unstable conditions. Considering perfectly premixed flames first, it is shown that the precessing vortex core actuation has only a minor effect on the thermoacoustic oscillation amplitude. However, we observe a continuous increase of the thermoacoustic frequency with increasing precessing vortex core amplitude due to an upstream displacement of the mean flame and resulting reduction of the convective time delay. Considering partially premixed flames, the precessing vortex core actuation shows a dramatic reduction of the thermoacoustic oscillation amplitude. In consideration of the perfectly premixed cases, we suspect that this is caused by the precessing vortex core-enhanced mixing of equivalence ratio fluctuations at the flame root and due to a reduction of time delays due to mean flame displacement.

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