Acoustic-Convective Interference in Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

2021 ◽  
Author(s):  
Eirik Æs⊘y ◽  
José G. Aguilar ◽  
Mirko R. Bothien ◽  
Nicholas Worth ◽  
James Dawson
Keyword(s):  
Bluff Body ◽  
Transfer Functions ◽  
Pure Hydrogen ◽  
Mean Velocity ◽  
Flow Disturbances ◽  
Flame Response ◽  
Vortex Shedding Frequency ◽  
Flame Imaging ◽  
Velocity Spectra

Abstract We investigate the occurrence of modulations in the gain and phase of flame transfer functions (FTF) measured in CH4/H2 and pure H2 flames. These are shown to be caused by flow disturbances originating from the screws used to centre the bluff body indicative of a more generalised phenomenon of convective wave propagation. Velocity measurements are performed around the injector dump plane, inside the injector pipe, and in the wake of the bluff body to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the screws appear in the unforced velocity spectra and the magnitude of these convective modes depends on the screws’ location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear axisymmetric. However, the rms fields capture a strong asymmetry due to convective disturbances. To quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. For acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cut-off frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cut-off frequency.

Acoustic-Convective Interference In Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

2021 ◽  
Author(s):  
Eirik Æsøy ◽  
José G. Aguilar ◽  
Mirko R. Bothien ◽  
Nicholas A. Worth ◽  
James R. Dawson
Keyword(s):  
Bluff Body ◽  
Transfer Functions ◽  
Pure Hydrogen ◽  
Mean Velocity ◽  
Flow Disturbances ◽  
Flame Response ◽  
Vortex Shedding Frequency ◽  
Flame Imaging ◽  
Velocity Spectra

Abstract We investigate the occurrence and source of modulations in the gain and phase of flame transfer functions (FTF) measured in perfectly premixed, bluff body stabilised CH4/H2 and pure H2 flames. The modulations are shown to be caused by flow disturbances originating from the upstream geometry, in particular the grub screws used to centre the bluffbody, indicative of a more generalised phenomenon of convective wave propagation. Velocity measurements are performed at various locations around the injector dump plane, inside the injector pipe, and in the wake of the bluffbody to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the grub screws appear in the unforced velocity spectra and it is found that the magnitude of these convective modes depends on their location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear approximately axisymmetric. However, the urms and vrms fields capture a strong asymmetry due to convective disturbances. To further quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. The reason behind the excitation of the first mode is due to a condition that states that for acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cut-off frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cut-off frequency.

Flame Response to Transverse Acoustic Forcing With Minimal Axial Coupling

2017 ◽  
Author(s):  
Travis Smith ◽  
Benjamin Emerson ◽  
William Proscia ◽  
Tim Lieuwen
Keyword(s):  
Heat Release ◽  
Gas Turbines ◽  
Axial Flow ◽  
Release Mechanism ◽  
Direct Role ◽  
Flow Disturbances ◽  
Transverse Excitation ◽  
Transverse Acoustic ◽  
Flame Response

Instabilities associated with transverse acoustic modes are an important problem in gas turbines. A number of studies have reported results on the response of flames to transverse excitation, in order to understand the acoustic-velocity-heat release mechanism associated with combustion instabilities. However, all forced and self-excited transverse studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that the actual transverse disturbances play a negligible direct role in generating spatially integrated oscillatory heat release. Rather, they suggest that it is the induced axial disturbances that control the bulk of the heat release response. As such, there is a need to control the relative amplitudes of the axial and transverse disturbances exciting the flame, and determine their relative roles in the overall heat release response. This paper presents experimental results to address this issue. The flow field and flame edge were measured using 5kHz simultaneous sPIV and OH-PLIF, and the relative heat release fluctuations were measured through OH* chemiluminescence. The flame was forced with both strong transverse and axial oscillations, with various degrees of coupling between them, showing quite consistently that it is the axial flow disturbances that excite heat release oscillations. These observations demonstrate that the key role of the transverse motions is to set the “clock” for the frequency of the oscillations, but have negligible effect on the actual heat release disturbances exciting the instability. Rather, it is the axial disturbances, induced by inherent multi-dimensional effects that lead to the actual heat release oscillations.

Experimental Investigation Into the Role of Mean Flame Stabilization On the Combustion Dynamics of High-Hydrogen Fuels in a Turbulent Combustor

2021 ◽  
Author(s):  
Nikhil Ashokbhai Baraiya ◽  
Vikram Ramanan ◽  
Baladandayuthapani Nagarajan ◽  
Chetankumar S Vegad ◽  
S. R. Chakravarthy
Keyword(s):  
Bluff Body ◽  
Flame Structure ◽  
High Amplitude ◽  
Acoustic Mode ◽  
Flame Stabilization ◽  
Pure Hydrogen ◽  
Airflow Rate ◽  
High Hydrogen ◽  
Combustion Dynamics

Abstract A bluff-body turbulent combustor is mapped for its thermo-acoustic stability across variation in airflow rate, non-dimensionalized as the Reynolds number (Re) and fuel composition. The combustor stability is evaluated for three fuels, namely, pure hydrogen (PH), synthesis natural gas (SNG), and syngas (SG). The combustion dynamics display markedly different behavior across the fuels, in the extent of the unstable region, as well as the observed dominant Eigenvalues. At low Re, SNG displays stable combustion, while SG exhibits high amplitude oscillations at the fundamental duct acoustic mode. As the Re is increased, SNG displays very high amplitude oscillations at the duct acoustic mode, while SG exhibit relatively low amplitude oscillations at the third harmonic. In the case of PH, high amplitude oscillations observed at higher Re at the first harmonic. These peculiarities are investigated in light of the role of mean flame stabilization. The combustion dynamics of fuels is influenced by the global equivalence ratio, as well as the jet momentum ratio. These effects significantly demarcates the dynamics of SNG and SG combustion. This is seen manifested in mean flame structure of flame at high amplitude oscillations, whereby result in SNG flame to be present in the wake, while the SG flame resides in the shear layer. The driving by the flame because of their mean stabilization quantified by a spatial Rayleigh index. It confirms the presence of large driving regions for SNG compared to that of SG, results in the observed differences in amplitude of the oscillations.

Thermodynamic and Kinetic Investigation of the Solvent Effect on the Oxidation of [Co(en)2SCH2COO]+ with S2O82- In Water-Methanol and Water-tert-Butyl Alcohol Mixtures

1993 ◽  
Vol 58 (5) ◽  
pp. 1001-1006 ◽  
Author(s):  
Oľga Vollárová ◽  
Ján Benko
Keyword(s):  
Transfer Functions ◽  
Activation Parameters ◽  
Butyl Alcohol ◽  
Oxidation Of Co ◽  
Kinetics Of Oxidation ◽  
Tert Butyl ◽  
Tert Butyl Alcohol ◽  
Alcohol Mixtures ◽  
Kinetics Of

The kinetics of oxidation of [Co(en)2SCH2COO]+ with S2O82- was studied in water-methanol and water-tert-butyl alcohol mixtures. Changes in the reaction activation parameters ∆H≠ and ∆S≠ with varying concentration of the co-solvent depend on the kind of the latter, which points to a significant role of salvation effects. The solvation effect on the reaction is discussed based on a comparison of the transfer functions ∆Ht0, ∆St0 and ∆Gt0 for the initial and transition states with the changes in the activation parameters accompanying changes in the CO-solvent concentration. The transfer enthalpies of the reactant were obtained from calorimetric measurements.

Experimental investigation on the momentumless wake using trailing edge blowing

2008 ◽  
Vol 222 (8) ◽  
pp. 1477-1486 ◽  
Author(s):  
Y Wu ◽  
X Zhu ◽  
Z Du
Keyword(s):  
Three Dimensional ◽  
Velocity Profiles ◽  
Axial Fan ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Piv Measurement ◽  
Image Velocimetry ◽  
Velocity Spectra ◽  
Momentumless Wake ◽  
Wake Characteristics

A developed plate stator model with and without trailing edge blowing (TEB) is studied using experimental methods. Wake characteristics of flow over the stator in the three-dimensional wake regimes are studied using hot-wire anemometry (HWA) and particle image velocimetry (PIV) techniques. First, the mean velocity profiles have been measured in the wake of the stator using HWA. Four wake characteristics have been obtained through momentum thickness judgments: pure wake, weak wake, momentumless wake, and jet. These velocity profiles show some differences in momentum deficit for the four cases. Then, the velocity spectra of the pure wake and momentumless wake obtained through the HWA measurements showed that TEB can eliminate the shedding vortex of the stator. Characteristic length scales based on the wake turbulent intensity profiles showed that the momentumless wake can reduce the wake width and depth. PIV measurement is carried out to measure the flow field of the four wakes. Finally, the application of TEB approaching momentumless wake status is used on an industrial ventilation low-pressure axial fan to assess noise reduction. The results show that TEB can make the outlet of the stator uniform, reduce velocity fluctuation, destroy the vorticity structure downstream of the stator, and reduce interaction noise level of the stator and rotor.

Role of Plaque Morphology in Altering the Flow Characteristics in Diseased Coronary Artery

2012 ◽  
Author(s):  
Carlos Moreno ◽  
Kiran Bhaganagar
Keyword(s):  
Coronary Artery ◽  
Nonlinear Response ◽  
Flow Characteristics ◽  
Proximal Region ◽  
Patient Specific ◽  
Plaque Morphology ◽  
Mean Velocity ◽  
Peak Location ◽  
The Mean

Patient specific simulations of a single patient based on an accurate representation of the plaque in a diseased coronary artery with 35% stenosis are performed to understand the effect of inlet forcing frequency and amplitude on the wall shear stress (WSS). Numerical simulations are performed with unsteady flow conditions in a laminar regime. The results have revealed that at low amplitudes, WSS is insensitive to forcing frequency and is it in phase with Q. The maximum WSS is observed at the proximal region of the stenosis, and WSS has highest negative values at the peak location of the stenosis. For higher pulsatile amplitude (a > 1.0), WSS exhibits a strong sensitivity with forcing frequencies. At higher forcing frequency the WSS exhibits nonlinear response to the inlet forcing frequency. Furthermore, significant differences in the mean velocity profile are observed during maximum and minimum volumetric flow rates.

scholarly journals Dynamic responses of anchored ducted flames to harmonic velocity forcing

2017 ◽  
Vol 10 (1) ◽  
pp. 72-85
Author(s):  
Ze-tian Ren ◽  
Su-hui Li ◽  
Min Zhu
Keyword(s):  
Large Scale ◽  
Bluff Body ◽  
Flow Instability ◽  
Low Frequency ◽  
Vortex Method ◽  
Vortex Structures ◽  
Dynamic Responses ◽  
Discrete Vortex ◽  
Flame Response ◽  
Complicated Geometries

This paper aims at developing a computationally inexpensive method to investigate the premixed flame instabilities. The kinematic G-equation is combined with a two-dimensional discrete vortex method, and the conformal mapping is applied to make calculations for complicated geometries more efficiently. The vortex dynamics and flame response to harmonic velocity forcing of an anchored ducted V-flame are investigated, and the effects of harmonic forcing, Reynolds number, and bluff body geometry are examined. Results show that the vortex structures, flow instability, and flame response are closely coupled with each other. The unsteady vortex structures generate instabilities at the flame base, and the convection of the flame wrinkles then influences the flame dynamics downstream. The flame heat release fluctuates with larger amplitude under low-frequency forcings, while the phase of the flame transfer function is quasi-linear with increasing forcing frequency. Both higher inflow velocity and sharper bluff body corners can result in more unsteady large-scale vortex structures and hence influence the flame responses.

Measurement of Flame Frequency Response Functions Under Exhaust Gas Recirculation Conditions

2012 ◽  
Vol 134 (9) ◽  
Author(s):  
Joseph Ranalli ◽  
Don Ferguson
Keyword(s):  
Transfer Function ◽  
Carbon Capture ◽  
Transfer Functions ◽  
Combustion Process ◽  
Exhaust Gas Recirculation ◽  
Exhaust Gas ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Flame Response ◽  
Gas Recirculation

Exhaust gas recirculation has been proposed as a potential strategy for reducing the cost and efficiency penalty associated with postcombustion carbon capture. However, this approach may cause as-yet unresolved effects on the combustion process, including additional potential for the occurrence of thermoacoustic instabilities. Flame dynamics, characterized by the flame transfer function, were measured in traditional swirl stabilized and low-swirl injector combustor configurations, subject to exhaust gas circulation simulated by N2 and CO2 dilution. The flame transfer functions exhibited behavior consistent with a low-pass filter and showed phase dominated by delay. Flame transfer function frequencies were nondimensionalized using Strouhal number to highlight the convective nature of this delay. Dilution was observed to influence the dynamics primarily through its role in changing the size of the flame, indicating that it plays a similar role in determining the dynamics as changes in the equivalence ratio. Notchlike features in the flame transfer function were shown to be related to interference behaviors associated with the convective nature of the flame response. Some similarities between the two stabilization configurations proved limiting and generalization of the physical behaviors will require additional investigation.

Response of Non-Axisymmetric Premixed, Swirl Flames to Helical Disturbances

2014 ◽  
Author(s):  
Vishal Acharya ◽  
Timothy Lieuwen
Keyword(s):  
Heat Release ◽  
Surface Area ◽  
Local Area ◽  
Flame Surface ◽  
Flow Disturbances ◽  
Flame Response ◽  
Flow Oscillations ◽  
Phase Variations ◽  
The Mean ◽  
Flame Shape

Flow oscillations associated with hydrodynamic instabilities comprise a key element of the feedback loop during self-excited combustion driven oscillations. This work is motivated in particular by the question of how to scale thermoacoustic stability results from single nozzle or sector combustors to full scale systems. Specifically, this paper considers the response of non-axisymmetric flames to helical flow disturbances of the form u^i′∝expimθ where m denotes the helical mode number. This work closely follows prior studies of the response of axisymmetric flames to helical disturbances. In that case, helical modes induce strong flame wrinkling, but only the axisymmetric, m = 0 mode, leads to fluctuations in overall flame surface area and, therefore, heat release. All other helical modes induce local area/heat release fluctuations with azimuthal phase variations that cancel each other out when integrated over all azimuthal angles. However, in the case of mean flame non-axisymmetries, the azimuthal deviations on the mean flame surface inhibit such cancellations and the asymmetric helical modes (m ≠ 0) cause a finite global flame response. In this paper, a theoretical framework for non-axisymmetric flames is developed and used to illustrate how the flame shape influences which helical modes lead to net flame surface area fluctuations. Example results are presented which illustrate the contributions made by these asymmetric helical modes to the global flame response and how this varies with different control parameters such as degree of asymmetry in the mean flame shape or Strouhal number. Thus, significantly different sensitivities may be observed in single and multi-nozzle flames in otherwise identical hardware in flows with strong helical disturbances, if there are significant deviations in time averaged flame shape between the two, particularly if one of the cases is nearly axisymmetric.

Significance of the Direct Excitation Mechanism for High-Frequency Response of Premixed Flames to Flow Oscillations

2020 ◽  
Author(s):  
Vishal Acharya ◽  
Tim Lieuwen
Keyword(s):  
Heat Release ◽  
Acoustic Waves ◽  
Premixed Flames ◽  
Axial Flow ◽  
Flame Length ◽  
Excitation Mechanism ◽  
Direct Mechanism ◽  
Direct Excitation ◽  
Flow Disturbances ◽  
Flame Response

Abstract Premixed flames are sensitive to flow disturbances, which can arise from acoustic or vortical fluctuations. For transverse instabilities, it is known that a dominant mechanism for flame response is “injector coupling”, whereby pressure oscillations associated with transverse waves excite axial flow disturbances. These axial flow disturbances then excite heat release oscillations. The objective of this paper is to consider another mechanism — the direct sensitivity of the unsteady heat release to transverse acoustic waves, and to compare its significance relative to the induced axial disturbances, in a linear framework. The rate at which the flame adds energy to the disturbance field is quantified using the Rayleigh criterion and evaluated over a range of control parameters, such as flame length and swirl number. The results show that radial modes induce heat release fluctuations that always add energy to the acoustic field, whereas heat release fluctuations induced by mixed radial-azimuthal modes can add or remove energy. These amplification rates are then compared to the flame response from induced axial fluctuations. For combustor centered flames, these results show that the direct excitation mechanism has negligible amplification rates relative to the induced axial mechanism for radial modes. For transverse modes, the fact that the nozzle is located at a pressure node indicates that negligible induced axial velocity disturbances are excited; as such, the direct mechanism dominates. For flames that are not centered on pressure nodes, the direct mechanism for mixed-modes, dominates for certain nozzle locations and flame angles.

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