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.

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.

scholarly journals Combustion stratification study of partially premixed combustion using Fourier transform analysis of OH* chemiluminescence images

2017 ◽  
Vol 19 (10) ◽  
pp. 1024-1035 ◽  
Author(s):  
Mohammad Izadi Najafabadi ◽  
Bart Somers ◽  
Bengt Johansson ◽  
Nico Dam
Keyword(s):  
Heat Release ◽  
High Speed ◽  
Spatial Information ◽  
Fourier Transforms ◽  
Combustion Regime ◽  
Premixed Combustion ◽  
Two Dimensional ◽  
Spatial Frequencies ◽  
Partially Premixed Combustion ◽  
Partially Premixed

A relatively high level of stratification (qualitatively: lack of homogeneity) is one of the main advantages of partially premixed combustion over the homogeneous charge compression ignition concept. Stratification can smooth the heat release rate and improve the controllability of combustion. In order to compare stratification levels of different partially premixed combustion strategies or other combustion concepts, an objective and meaningful definition of “stratification level” is required. Such a definition is currently lacking; qualitative/quantitative definitions in the literature cannot properly distinguish various levels of stratification. The main purpose of this study is to objectively define combustion stratification (not to be confused with fuel stratification) based on high-speed OH* chemiluminescence imaging, which is assumed to provide spatial information regarding heat release. Stratification essentially being equivalent to spatial structure, we base our definition on two-dimensional Fourier transforms of photographs of OH* chemiluminescence. A light-duty optical diesel engine has been used to perform the OH* bandpass imaging on. Four experimental points are evaluated, with injection timings in the homogeneous regime as well as in the stratified partially premixed combustion regime. Two-dimensional Fourier transforms translate these chemiluminescence images into a range of spatial frequencies. The frequency information is used to define combustion stratification, using a novel normalization procedure. The results indicate that this new definition, based on Fourier analysis of OH* bandpass images, overcomes the drawbacks of previous definitions used in the literature and is a promising method to compare the level of combustion stratification between different experiments.

Simulation of Turbulent Lifted Flames Using a Partially-Premixed Coherent Flame Model (PCFM)

2008 ◽  
Author(s):  
Yongzhe Zhang ◽  
Rajesh Rawat
Keyword(s):  
Turbulent Combustion ◽  
Practical Interest ◽  
Flame Stabilization ◽  
Premixed Combustion ◽  
Pollutant Emissions ◽  
Jet Flame ◽  
Premixed Turbulent Combustion ◽  
Partially Premixed Combustion ◽  
Lifted Flames ◽  
Partially Premixed

Partially-premixed combustion occurs in many combustion devices of practical interest, such as gas-turbine combustors. Development of corresponding turbulent combustion models is important to improve the design of these systems in efforts to reduce fuel consumption and pollutant emissions. Turbulent lifted flames have been a canonical problem for testing models designed for partially-premixed turbulent combustion. In this paper we propose modifications to the coherent flame model (CFM) so that it can be brought to the simulation of partially-premixed combustion. For the primary premixed flame, a transport equation for flame area density is solved in which the wrinkling effects of the flame stretch and flame annihilation are considered. For the subsequent non-premixed zone, a laminar flamelet PPDF methodology, which accounts for the non-equilibrium and finiterate chemistry effects, is adopted. The model is validated against the experimental data on a lifted H2/N2 jet flame issuing into a vitiated coflow. In general there is fairly good agreement between the calculations and measurements both in profile shapes and peak values. Based on the simulation results the flame stabilization mechanism for lifted flames is investigated.

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).

Simulation of Turbulent Lifted Flames Using a Partially Premixed Coherent Flame Model

2009 ◽  
Vol 131 (3) ◽  
Author(s):  
Yongzhe Zhang ◽  
Rajesh Rawat
Keyword(s):  
Turbulent Combustion ◽  
Practical Interest ◽  
Flame Stabilization ◽  
Premixed Combustion ◽  
Pollutant Emissions ◽  
Jet Flame ◽  
Premixed Turbulent Combustion ◽  
Partially Premixed Combustion ◽  
Lifted Flames ◽  
Partially Premixed

Partially premixed combustion occurs in many combustion devices of practical interest, such as gas-turbine combustors. Development of corresponding turbulent combustion models is important to improve the design of these systems in efforts to reduce fuel consumption and pollutant emissions. Turbulent lifted flames have been a canonical problem for testing models designed for partially premixed turbulent combustion. In this paper we propose modifications to the coherent flame model so that it can be brought to the simulation of partially premixed combustion. For the primary premixed flame, a transport equation for flame area density is solved in which the wrinkling effects of the flame stretch and flame annihilation are considered. For the subsequent nonpremixed zone, a laminar flamelet presumed probability density function (PPDF) methodology, which accounts for the nonequilibrium and finite-rate chemistry effects, is adopted. The model is validated against the experimental data on a lifted H2∕N2 jet flame issuing into a vitiated coflow. In general there is fairly good agreement between the calculations and measurements both in profile shapes and peak values. Based on the simulation results, the flame stabilization mechanism for lifted flames is investigated.

Effects of porous media on partially premixed combustion and heat transfer in meso-scale burners fuelled with ethanol

Energy ◽  
2021 ◽  
pp. 120191
Author(s):  
Xinjian Chen ◽  
Junwei Li ◽  
Dan Zhao ◽  
Muhammad Tahir Rashid ◽  
Xinyuan Zhou ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Porous Media ◽  
Premixed Combustion ◽  
Partially Premixed Combustion ◽  
Partially Premixed ◽  
Meso Scale
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