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.

Experimental Investigation of Combustion Dynamics in a Turbulent Syngas Combustor

2017 ◽  
Author(s):  
Nikhil Ashokbhai Baraiya ◽  
Baladandayuthapani Nagarajan ◽  
Satynarayanan R. Chakravarthy
Keyword(s):  
High Speed ◽  
Equivalence Ratio ◽  
Bluff Body ◽  
Dynamic Pressure ◽  
Fuel Mixture ◽  
High Amplitude ◽  
Acoustic Oscillations ◽  
Combustion Dynamics ◽  
Dynamic Pressure Measurement ◽  
Body Location

In the present work, the proportion of carbon monoxide to hydrogen is widely varied to simulate different compositions of synthesis gas and the potential of the fuel mixture to excite combustion oscillations in a laboratory-scale turbulent bluff body combustor is investigated. The effect of parameters such as the bluff body location and equivalence ratio on the self-excited acoustic oscillations of the combustor is studied. The flame oscillations are mapped by means of simultaneous high-speed CH* and OH* chemiluminescence imaging along with dynamic pressure measurement. Mode shifts are observed as the bluff body location or the air flow Reynolds number/overall equivalence ratio are varied for different fuel compositions. It is observed that the fuel mixtures that are hydrogen-rich excite high amplitude pressure oscillations as compared to other fuel composition cases. Higher H2 content in the mixture is also capable of exciting significantly higher natural acoustic modes of the combustor so long as CO is present, but not without the latter. The interchangeability factor Wobbe Index is not entirely sufficient to understand the unsteady flame response to the chemical composition.

The Role of Mean Flame Anchoring on the Stability Characteristics of Syngas, Synthesis Natural Gas, and Hydrogen Fuels in a Turbulent Non-Premixed Bluff-Body Combustor

2019 ◽  
Author(s):  
Nikhil Ashokbhai Baraiya ◽  
Satynarayanan R. Chakravarthy
Keyword(s):  
Natural Gas ◽  
Flow Rate ◽  
Momentum Flux ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Inlet Flow ◽  
Combustion Dynamics ◽  
Flow Rates ◽  
Inlet Flow Rate ◽  
The Difference

Abstract A lab-scale bluff body combustor is mapped for its stability and flame dynamics of non-premixed flames. The characteristics are observed across variations in the fuel composition, as well as in the inlet flow rate. The combustor is seen to exhibit markedly different dynamics for each of the varied fuel compositions. This behavior is explained on the basis of mean flame stabilization behavior and on the combined effects of the fuel-jet momentum flux and global equivalence ratio. It is seen that the H2 flames primarily act as a pilot source for secondary combustion of either CO or CH4. Further, it is seen that, the high momentum flux associated with H2-CO mixtures result in combustion near the wall and outside the bluff-body shear layers at low inlet flow rates. Whereas, at high inlet flow rates, the mean heat release rate is seen to stabilize closer to the injection holes as well as extend to near the bluff-body shear layer. This marked difference in flame stabilization is seen to have a drastic effect on the nature of oscillations inside the chamber. This is contrasted to H2-CH4 (synthesis natural gas) flames that exhibit stabilization inside the bluff-body wake at high inlet flow rate. The difference between H2-CH4 and H2-CO flames with regards to combustion dynamics is then explained as a result of the flame stabilization behavior, which is seen to be different across the varied fuel compositions. While H2-CH4 flame exhibits the well-known large wake structures responsible for combustion instability, H2-CO flame exhibits no such structures, owing to their stabilization point. Further analysis using pressure fixed phase instants reveal the difference in nature of combustion dynamics across the tested fuel compositions and are justified using the spatial Rayleigh index map.

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.

scholarly journals Flame Stabilization and Blow-off of Ultra-Lean errortypeceH2-Air Premixed Flames

Energies ◽  
2021 ◽  
Vol 14 (7) ◽  
pp. 1977
Author(s):  
Faizan Habib Vance ◽  
Yuriy Shoshin ◽  
Philip de Goey ◽  
Jeroen van Oijen
Keyword(s):  
Bluff Body ◽  
Flame Stabilization ◽  
Cfd Simulations ◽  
Preferential Diffusion ◽  
Previous Hypothesis ◽  
Hydrogen Flame ◽  
Efficient Combustion ◽  
Lean Limit ◽  
Displacement Speed

The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study, we use experiments and numerical simulations for pure errortypeceH2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (ϕ=0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck, flames with closed flame fronts are found to be stabilized inside a recirculation zone.

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.

scholarly journals Thermoacoustic Instability Considerations for High Hydrogen Combustion in Lean Premixed Gas Turbine Combustors: A Review

Hydrogen ◽  
2021 ◽  
Vol 2 (1) ◽  
pp. 33-57
Author(s):  
Jadeed Beita ◽  
Midhat Talibi ◽  
Suresh Sadasivuni ◽  
Ramanarayanan Balachandran
Keyword(s):  
Gas Turbine ◽  
Gas Turbines ◽  
Hydrogen Combustion ◽  
Dynamic State ◽  
Pure Hydrogen ◽  
High Hydrogen ◽  
Combustion Dynamics ◽  
Thermoacoustic Instability ◽  
Gas Turbine Combustors ◽  
Lean Premixed

Hydrogen is receiving increasing attention as a versatile energy vector to help accelerate the transition to a decarbonised energy future. Gas turbines will continue to play a critical role in providing grid stability and resilience in future low-carbon power systems; however, it is recognised that this role is contingent upon achieving increased thermal efficiencies and the ability to operate on carbon-neutral fuels such as hydrogen. An important consideration in the development of gas turbine combustors capable of operating with pure hydrogen or hydrogen-enriched natural gas are the significant changes in thermoacoustic instability characteristics associated with burning these fuels. This article provides a review of the effects of burning hydrogen on combustion dynamics with focus on swirl-stabilised lean-premixed combustors. Experimental and numerical evidence suggests hydrogen can have either a stabilising or destabilising impact on the dynamic state of a combustor through its influence particularly on flame structure and flame position. Other operational considerations such as the effect of elevated pressure and piloting on combustion dynamics as well as recent developments in micromix burner technology for 100% hydrogen combustion have also been discussed. The insights provided in this review will aid the development of instability mitigation strategies for high hydrogen combustion.

A Review of Mechanisms Controlling Bluff-Body Stabilized Flames With Closely-Coupled Fuel Injection

2011 ◽  
Author(s):  
Jeffery A. Lovett ◽  
Caleb Cross ◽  
Eugene Lubarsky ◽  
Ben T. Zinn
Keyword(s):  
Liquid Fuel ◽  
Bluff Body ◽  
Fuel Injection ◽  
Flame Structure ◽  
Flame Stabilization ◽  
Current Understanding ◽  
Two Phase ◽  
Combustion Region ◽  
Combustion Systems ◽  
And Performance

The processes controlling bluff-body stabilized combustion have been extensively studied over the years because such stabilization approaches are commonly used in many practical systems. Much of the current understanding of this problem was attained in experimental and analytical studies of premixed combustion systems where the complexities introduced by fuel atomization, vaporization and mixing could be neglected. Yet, practical considerations often require fuel injection just upstream of the bluff-body stabilized combustion region. Consequently, it’s necessary to develop understanding of the fundamental processes in such non-premixed systems. Supplying fuel via the injection of discrete liquid fuel jets requires understanding of the complex physics of two-phase sprays and the transport to various regions within the combustor. This paper describes current understanding of the manner in which these processes affect flame stabilization in bluff-body combustion systems that employ close-coupled, liquid fuel injection. Specifically, the paper compares findings of premixed bluff-body flames with recent results obtained in studies using close-coupled fueling at Georgia Tech to support postulates of the processes controlling flame stabilization and flame structure. These findings are also used to propose a set of parameters that can be used to describe the combustion behavior and performance of such combustion systems.

Effect of Chemical Composition of Syngas on Combustion Dynamics Inside Bluff-Body Type Turbulent Syngas Combustor

2018 ◽  
Author(s):  
Nikhil Ashokbhai Baraiya ◽  
Satynarayanan R. Chakravarthy
Keyword(s):  
Chemical Composition ◽  
High Speed ◽  
Bluff Body ◽  
Dynamic Pressure ◽  
Flame Structure ◽  
Acoustic Behavior ◽  
Body Type ◽  
Combustion Dynamics ◽  
Co2 Intensity ◽  
Dynamic Pressure Measurement

In the present work, the chemical composition of syngas is changed by varying the H2/CO ratio, to map the change in the acoustic behavior of a bluff-body combustor. It was observed that with increase in hydrogen concentration in the syngas mixture, the frequency shifts to higher modes and the flame structures changes. The flame oscillations are mapped by means of simultaneous high-speed OH* and CO2* chemiluminescence imaging along with dynamic pressure measurement. The plots of spatial distribution of OH* and CO2* intensity are used to understand change in flame structure with change in chemical composition and also to help in understanding the kinetics affecting acoustic behavior of the flame. The change in flow structures with change in chemical composition of fuel is studied by simultaneous high-speed PIV, OH* chemiluminescence and dynamic pressure measurements.

Flame Stabilization by Hot Products Gases Recirculation in a Trapped Vortex Combustor

2012 ◽  
Author(s):  
Joseph Burguburu ◽  
Gilles Cabot ◽  
Bruno Renou ◽  
Abdelkrim Mourad Boukhalfa ◽  
Michel Cazalens
Keyword(s):  
Flame Stabilization ◽  
Main Flow ◽  
Pollutant Emissions ◽  
Airflow Rate ◽  
Combustion Instabilities ◽  
Combustion Dynamics ◽  
Lean Combustion ◽  
Technology Strategies ◽  
Hot Products ◽  
Trapped Vortex

New regulations regarding NOx emissions are forcing manufacturers to develop advanced research and technology strategies. Ultra-lean combustion is considered as an attractive solution; however, it generally produces combustion instabilities in swirl-stabilized burners. This work provides experimental results for a new burner technology based on two concepts: the trapped vortex combustor (TVC) and the ultra-compact combustor (UCC). Methane/air flame stabilization was achieved by generating hot product recirculation, with a rich pilot flame located in an annular cavity, and by flame holders located in the main flow slightly upstream of the cavity. In addition, azimuthal gyration could be added to the main flow to reproduce the suppression of the last diffuser stage, which increased the velocity and modified the mixing between the cavity and the mainstream due to centrifugal forces. The combustor characterization was performed by coupling several optical diagnostics, pollutant emissions, and pressure measurements (for both cold and reactive conditions) at atmospheric pressure. An understanding of the combustion dynamics was achieved through phase averaged PIV/CH* images. The analysis highlighted the importance of the stabilization process of a double vortex structure inside the cavity and the presence of reactive gas close to the upstream cavity wall. These conditions were improved by a high cavity equivalence ratio and a high main airflow rate. The addition of swirl considerably increased the flame stability.

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