Dynamics of Non-Premixed Bluff Body-Stabilized Flames in Heated Air Flow

2010 ◽  
Author(s):  
Caleb Cross ◽  
Aimee Fricker ◽  
Dmitriy Shcherbik ◽  
Eugene Lubarsky ◽  
Ben T. Zinn ◽  
...  
Keyword(s):  
Heat Release ◽  
Vortex Shedding ◽  
High Speed ◽  
Bluff Body ◽  
Combustion Process ◽  
Flame Stabilization ◽  
Inlet Temperature ◽  
Fuel Injectors ◽  
Flame Holder ◽  
Process Heat

This paper describes a study of the fundamental flame dynamic processes that control bluff body-stabilized combustion of liquid fuel with low dilatation. Specifically, flame oscillations due to asymmetric vortex shedding downstream of a bluff body (i.e., the Be´nard/von-Ka´rma´n vortex street) were characterized in an effort to identify the fundamental processes that most affect the intensity of these oscillations. For this purpose, the spatial and temporal distributions of the combustion process heat release were characterized over a range of inlet velocities, temperatures, and overall fuel-air ratios in a single flame holder combustion channel with full optical access to the flame. A stream of hot preheated air was supplied to the bluff body using a preburner, and Jet-A fuel was injected across the heated gas stream from discrete fuel injectors integrated within the bluff body. The relative amplitudes, frequencies, and phase of the sinusoidal flame oscillations were characterized by Fourier analysis of high-speed movies of the flame. The amplitudes of the flame oscillations were generally found to increase with global equivalence ratio, reaching a maximum just before rich blowout. Comparison of the flame dynamics to the time-averaged spatial heat release distribution revealed that the intensity of the vortex shedding decreased as a larger fraction of the combustion process heat release occurred in the shear layers surrounding the recirculation zone of the bluff body. Furthermore, a complete transition of the vortex shedding and consequent flame stabilization from asymmetric to symmetric modes was clearly observed when the inlet temperature was reduced from 850°C to 400°C (and hence, significantly increasing the flame dilatation ratio from Tb/Tu ∼ 2.3 to 3.7).

Determination of Equivalence Ratio and Oscillatory Heat Release Distributions in Non-Premixed Bluff Body-Stabilized Flames Using Chemiluminescence Imaging

2011 ◽  
Author(s):  
Caleb Cross ◽  
Eugene Lubarsky ◽  
Dmitriy Shcherbik ◽  
Keary Bonner ◽  
Alex Klusmeyer ◽  
...  
Keyword(s):  
Heat Release ◽  
Vortex Shedding ◽  
Equivalence Ratio ◽  
Bluff Body ◽  
Fuel Injection ◽  
Flame Stabilization ◽  
Operating Conditions ◽  
Flame Holder ◽  
Air Stream ◽  
Local Equivalence

In an effort to elucidate the fundamental processes controlling bluff body flame stabilization, the dependence of the spatial distribution of the local equivalence ratio and the heat release dynamics upon the mode of fuel injection was studied. Experiments were performed in a single flame holder combustion channel which was supplied with a high-temperature air stream. Jet-A fuel was injected across the incoming air stream from one of two locations: a cylindrical fuel bar installed 0.25 m upstream of the bluff body, or from fuel injectors integrated within the bluff body 2.5 cm upstream of the trailing edge (i.e., close-coupled injection). The time-averaged spatial distributions of the combustion heat release were characterized by CH* and C2* chemiluminescence imaging of the flame, and ratios of the C2* to CH* light emission were used to characterize the local equivalence ratio. The spatial average of the C2*/CH* value in the flame was found to increase linearly with increasing global equivalence ratio for fuel injection upstream of the bluff body, whereas this value was relatively constant for close-coupled injection. This constant value equaled the same average C2*/CH* value obtained for upstream fuel injection at globally stoichiometric conditions, suggesting that combustion resulting from close-coupled fuel injection took place, on average, in stoichiometric flamelets throughout the combustor. The heat release dynamics due to asymmetric (von Ka´rma´n) vortex shedding were also investigated for each operating condition by recording high-speed movies of the flame at 24 kHz. Upon processing of these movies, the amplitudes of heat release fluctuations due to von Ka´rma´n vortex shedding were found to be significantly higher for close-coupled injection than for injection well upstream of the flame holder for all operating conditions. This is attributed to an increase in span-wise fuel-air mixing and near-wake heat release for upstream fuel injection, resulting in a hotter recirculation zone which suppressed the von Ka´rma´n instability more than the close-coupled case.

The influence of the geometry of V-gutter bluff body on transient vortex shedding

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Luckachan K George ◽  
Raja Sekar K ◽  
Srikrishnan A R ◽  
Kannan R
Keyword(s):  
Vortex Shedding ◽  
High Speed ◽  
Bluff Body ◽  
Vortex Motion ◽  
Flame Stabilization ◽  
Cold Flow ◽  
Transient Simulations ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
The Impact

Abstract This study investigates the turbulent flow field downstream of V-gutters using unsteady numerical modelling. An important domain of application of the vortex shedding induced by the V-gutters is the flame stabilization in high speed combustion systems which find extensive applications in aerospace engineering. In view of this, the present study analyses the impact of the V-gutter geometry, as characterized by the included angle, on inducing vortex motion in the wake. Transient simulations are carried out for three values of the semi-span angle, α = 30°, 45° and 60°. Based on the analysis of the saddle point and the vortex shedding frequency, the study shows that an increase in span angle within this range, favours the effectiveness of the method in flame stabilization. Though the simulations are done for cold flow, the dominant mechanism of vortex shedding is adequately addressed in the analysis.

Dynamics of Flame Stabilized by Triangular Bluff Body in Partially Premixed Methane-Air Combustion

2011 ◽  
Author(s):  
T. V. Santosh Kumar ◽  
P. R. Alemela ◽  
J. B. W. Kok
Keyword(s):  
Heat Release ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Reaction Rates ◽  
Flame Stabilization ◽  
Operating Conditions ◽  
Premixed Combustion ◽  
Partially Premixed Combustion ◽  
Computational Fluid Dynamics Analysis ◽  
Partially Premixed

In the design and operational tuning of gas turbine combustors it is important to be able to predict the interaction of the flame stabilization recirculation area with the burner aerodynamics. In the present paper transient computational fluid dynamics analysis is used to study these effects. Vortex interactions with the flame play a key role in many practical combustion systems. The interactions drive a large class of combustion instabilities and are responsible for changing the reaction rates, shape of the flame and the global heat release rate. The evolution of vortex shedding in reactive flows and its effects on the dynamics of the flame are important to be predicted. The present study describes dynamics of bluff body stabilized flames in a partially premixed combustion system. The bluff body is an equilateral wedge that induces the flame recirculation zone. The wedge is positioned at one-third length of the duct, which, is acoustically closed at the bottom end and open at the top. Transient computational modeling of partially premixed combustion is carried out using the commercial ANSYS CFX code and the results show that the vortex shedding has a destabilizing effect on the combustion process. Scale Adaptive Simulation turbulence model is used to compare between non-reacting cases and combustion flows to show the effects of aerodynamics-combustion coupling. The transient data reveals that frequency peaks of pressure and temperature spectra and is consistent with the longitudinal natural frequencies and Kelvin-Helmholtz instability frequency for reactive flow simulations. The same phenomenon is observed at different operating conditions of varying power. It has also been shown that the pressure and heat release are in phase, satisfying the Rayleigh criterion and therefore indicating the presence of aerodynamic-combustion instability. The data are compared to the scarce data on experiments and simulations available in literature.

Aerodynamics of a two-dimensional bluff body with the cross-section of a train

2020 ◽  
Vol 23 (12) ◽  
pp. 2679-2693 ◽  
Author(s):  
Huan Li ◽  
Xuhui He ◽  
Hanfeng Wang ◽  
Si Peng ◽  
Shuwei Zhou ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Large Amplitude ◽  
High Speed ◽  
Bluff Body ◽  
Mean Amplitude ◽  
Two Dimensional ◽  
Aerodynamic Forces ◽  
Moderate Reynolds Number ◽  
Amplitude Mode

Experiments on the aerodynamics of a two-dimensional bluff body simplified from a China high-speed train in crosswinds were carried out in a wind tunnel. Effects of wind angle of attack α varying in [−20°, 20°] were investigated at a moderate Reynolds number Re = 9.35 × 104 (based on the height of the model). Four typical behaviors of aerodynamics were identified. These behaviors are attributed to the flow structure around the upper and lower halves of the model changing from full to intermittent reattachment, and to full separation with a variation in α. An alternate transition phenomenon, characterized by an alteration between large- and small-amplitude aerodynamic fluctuations, was detected. The frequency of this alteration is about 1/10 of the predominant vortex shedding. In the intervals of the large-amplitude behavior, aerodynamic forces fluctuate periodically with a strong span-wise coherence, which are caused by the anti-symmetric vortex shedding along the stream-wise direction. On the contrary, the aerodynamic forces fluctuating at small amplitudes correspond to a weak span-wise coherence, which are ascribed to the symmetric vortex shedding from the upper and lower halves of the model. Generally, the mean amplitude of the large-amplitude mode is 3 times larger than that of the small one. Finally, the effects of Reynolds number were examined within Re = [9.35 × 104, 2.49 × 105]. Strong Reynolds number dependence was observed on the model with two rounded upper corners.

scholarly journals Development of a Swirl and Bluff-Body Stabilized Burner for Low-NOx, Lean-Premixed Combustion

1995 ◽  
Author(s):  
Jeffery A. Lovett ◽  
Warren J. Mick
Keyword(s):  
Bluff Body ◽  
Fuel Injection ◽  
Swirling Flow ◽  
Cylindrical Tube ◽  
Inlet Temperature ◽  
Lean Premixed Combustion ◽  
Fuel Injectors ◽  
Air Mixing ◽  
Low Nox Combustion ◽  
Heavy Duty Gas Turbine

A burner configuration utilizing both swirl and bluff-body stabilization was developed and tested for dry low-NOx combustion of natural gas fuel. A multiple number of these burners can be used to make up a can combustor. The burner consisted of a central hub supporting an axial swirler and spoke-type fuel injectors mounted coaxially within a 100 mm diameter cylindrical tube. The swirl typically provided strong recirculation and mixing, while the flame was anchored physically to the center hub. Tests were conducted at typical heavy-duty gas turbine conditions of 620 K inlet temperature and 10 atmospheres pressure. Parametric studies were conducted with various configurations of the burner to determine the corresponding effects on fuel-air mixing, flame stability, and NOx and CO emissions. The results show that ultra-low NOx emissions can be obtained if the fuel injection is sufficiently well distributed. The compact flame produced by the highly mixed swirling flow results in very low CO emissions as well. The results suggest also that swirl-strength is reduced in an upstream swirler configuration.

Combustion Oscillations in Bluff Body Stabilized Diffusion Flames With Variable Length Inlet

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
M. Madanmohan ◽  
S. Pandey ◽  
A. Kushari ◽  
K. Ramamurthi
Keyword(s):  
Heat Release ◽  
Phase Angle ◽  
Bluff Body ◽  
Diffusion Flames ◽  
Combustion Process ◽  
Low Frequency ◽  
Pressure Oscillations ◽  
Pipe Length ◽  
Entry Length ◽  
Low Frequency Oscillations

This paper describes the results of an experimental study to understand the influence of inlet flow disturbances on the dynamics of combustion process in bluff body stabilized diffusion flames of liquid petroleum gas and air. The results show the influence of weak disturbances created by the change in incoming pipe length on the amplitude of pressure oscillations and the phase angle between pressure and heat release. It is seen that the phase delay increases as the entry length increases. The rms value of pressure, however, generally falls with the increase in length. The phase angle is seen to be in the second quadrant, showing that the heat release oscillations damp the pressure oscillations. Therefore, the decrease in the phase angle results in the reduction in damping and hence an increase in pressure fluctuations. The dominant frequencies of combustion oscillations are found to be the low frequency oscillations, and the frequency of oscillations increases with a decrease in the inlet pipe length and an increase in the flow Reynolds number. It is suggested that such low frequency oscillations are driven by vortex shedding at the wake of the bluff body, which energizes the diffusion and mixing process.

Effect of Air Pressure and Gutter Angle on Flame Stability and DeZubay Number for Methane-Air Combustion

2017 ◽  
Vol 34 (1) ◽  
Author(s):  
S. Boopathi ◽  
P. Maran
Keyword(s):  
High Speed ◽  
Pressure Range ◽  
Bluff Body ◽  
Apex Angle ◽  
Flame Stabilization ◽  
Air Pressure ◽  
Flame Stability ◽  
Stability Chart ◽  
Stable Flame

AbstractThe combustion at high speed reactants requires a flame holding characteristics to sustain the flame in the afterburner. The flame holding characteristics of the combustor is carried out by the bluff-body stabilizers. The range of conditions of parameters influencing the flame stabilization is to be identified and the effects on the flame sustainability have to be investigated. DeZubay used the concept of DeZubay number and flame stability envelope to determine the stabilization and blowout range. In the present work, the effect of air pressure and the angle of apex of the V-gutter on flame stabilization and blowout mechanism have been experimentally investigated for six different apex angles and four different air pressure conditions. The value of DeZubay number at each condition has been calculated and verified with DeZubay stability chart for flame stabilization. The results show that stable flame is obtained for the entire pressure range when the apex angle of the V gutter is in 60° and 90°.

Analysis of Combustion in the Cetane-Rating Engine Using High-Speed Photography

1993 ◽  
Vol 207 (4) ◽  
pp. 317-328 ◽  
Author(s):  
N Ladommatos ◽  
M Parsi ◽  
N McGrath ◽  
S Mayne
Keyword(s):  
Heat Release ◽  
High Speed ◽  
Combustion Process ◽  
Injection System ◽  
Exhaust Emission ◽  
High Speed Photography ◽  
Diesel Fuels ◽  
Single Hole ◽  
Release Analysis

Modifications have been made to the injection system of the engine used for cetane-rating diesel fuels. These involved the replacement of the standard pintle nozzle with a single-hole orifice-type nozzle. The aim of the modifications was to improve the combustion process and thereby increase the precision of the cetane-rating test. The modifications to the injection system have been assessed using heat release analysis, exhaust emission measurements and high-speed photography of the combustion flames.

Effects of combustor geometry on the combustion process of an RBCC combustor in high-speed ejector mode

2019 ◽  
Vol 33 (27) ◽  
pp. 1950330
Author(s):  
Taiyu Wang ◽  
Zhenguo Wang ◽  
Zun Cai ◽  
Jian Chen ◽  
Mingbo Sun ◽  
...  
Keyword(s):  
Supersonic Flow ◽  
Flow Field ◽  
Heat Release ◽  
Reaction Mechanisms ◽  
Total Pressure ◽  
High Speed ◽  
Combustion Process ◽  
Numerical Approach ◽  
Combined Cycle ◽  
Chemical Reaction Mechanisms

The combustion characteristics of high-speed ejector mode in a 2-dimensional strut-based RBCC (rocket-based combined cycle) combustor had been investigated numerically in a Mach 2.5 supersonic flow. The numerical approach had been validated by comparing numerical results with available experimental data. Besides, three different hydrogen-air chemical reaction mechanisms had also been compared. The effect of the combustor geometry on the combustion process was then discussed by analyzing the heat release distribution and flow field. It was found that the wall configuration, closeout angle of the converging location and converging ratio all have significant influences on the heat release distribution and flow field structures. It is demonstrated that a converging–diverging wall configuration is beneficial for the combustion process with significant heat release increase compared to the other wall configurations. In addition, the closeout angle of the converging location is also closely related to the combustion performance, and there exists an optimized closeout angle in a specific combustor geometry. It is also revealed that the major heat release region moves upstream obviously with increase in the converging ratio, leading to an enhanced combustion process. However, the converging ratio is still to be optimized to keep a balance between heat release increase and total pressure loss of the supersonic flow.

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