SESSION 6
FLAME DYNAMICS AND TURBULENCE
VALIDATION OF MODEL BASED ACTIVE CONTROL OF COIVIBUSTION INSTABILITY
Z. Gboneim^{l}, M. Fleifil^{2}, A. F. Ghoniem^{2}
^{1}Faculty of Engineering, Ain Shams University, Cairo, Egypt
^{2}Massachusetts Institute of Technology, Cambridge, MA 021394307
ABSTRACT
The demand for efficient, compact and clean combustion systems have spurred
research into the fundamental mechanisms governing their performance and means
of interactively changing their performance characteristics. Thermoacoustic
instability which is frequently observed in combustion systems with high power
density, when burning close to the lean flammability limit, or using exhaust gas
recirculation to meet more stringent emission regulations, etc. Its Occurrence
and/or means to mitigate them passively lead to performance degradation such as
reduced combustion efficiency, high local heat transfer rates, increase in the
mixture equivalence ratio or system failure due to structural damage. This paper
reports on our study of the origin of thermoacoustic instability, its dependence
on system parameters and the means of actively controlling it.
We have developed an analytical model of thermoacoustic instability in premixed
combustors. The model combines a heat release dynamics model constructed using
the kinematics of a premixed flame stabilized behind a perforated plate with the
linearized conservation equations governing the system acoustics. This
formulation allows model based controller design. In order to test the
performance of the analytical model, a numerical solution of the partial
differential equations governing the system has been carried out using the
principle of harmonic separation and focusing on the dominant unstable mode.
This leads to a system of ODEs governing the thermo fluid variables: Analytical
predictions of the frequency and growth rate of the unstable mode are shown to
he in good agreement with the numerical simulations as well as with those
obtained using experimental identification techniques when applied to a
laboratory combustor. We use these results to confirm the validity of the
assumptions used in formulating the analytical model. A controller based on the
minimization of a cost function using the LQR technique has been designed using
the analytical model and implemented on a bench top laboratory combustor. We
show that the controller is capable of suppressing the pressure oscillations in
the combustor with a settling time much shorter than what had been attained
before and without exciting secondary peaks.
LEVELSET FLAMELET LIBRARY APPROACH FOR PREMIXED
TURBULENT COMBUSTION
P. Nilsson and X. S. Bai
Division of Fluid Mechanics, Lund University, S221 00 Lund, Sweden
email: pen@ms.vok.lth.se, bai@ms.vok.lth.se
ABSTRACT
Modeling of a lean premixed propane/air turbulent flame stabilized by a bluff
body is presented. The laminar flamelet library approach, which has been
successfully used for modeling of nonpremixed turbulent flames, is applied to
premixed combustion. In this approach the mean flame location and mean flame
thickness are modeled by a levelset Gequation and the variance of G, along
with empirical expressions. The detailed species, temperature and density are
calculated using a presumed probability density function together with the
laminar flamelet library. The sensitivity of the results to the model components
is illustrated. Comparison of calculations and experimental data is presented.
TURBULENT FLAMES IN LEAN H_{2}AIRCO_{2} MIXTURES
B. E. Gelfand, V. P. Karpov, and O. E. Popov
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow,
Russia
ABSTRACT
Turbulent combustion rates in lean hydrogenair mixtures (f=0.39, 0.26, and
0.21) diluted with carbon dioxide were measured in a closed vessel with central
ignition at room temperature and pressures ranging from 0.1 to 0.5 MPa.
Turbulence was produced by four identical fans with rotational speeds up to 6000
r.p.m. The turbulent intensity ranged up to about 10 m/s and measured by a
thermoanemometer. Inhomogeneity of the turbulent intensity did not exceed 20%.
The methods of highspeed schlieren photography and pressure transient
measurements were used for the studies. Results show that turbulent premixed
flames in the mixtures diluted with CO_{2} exhibit turbulent quenching. The flame
speed increases initially with turbulence up to a point where this speed
decreases just prior to quenching of the flame. The extinction limits in
turbulent flames of hydrogenaircarbon dioxide mixtures at the concentrations
of hydrogen and carbon dioxide close to the limiting concentrations obtained in
the experiments with laminar flames. The variation of initial pressure from 0.1
to 0.5 MPa does not affect the turbulent combustion rate.
EXPERIMENTAL CHARACTERISATION OF THE aPARAMETER IN TURBULENT SCALAR FLUX FOR
PREMIXED COMBUSTION
Peter A. M. Kalt and Robert W. Bilger
Department of Mechanical and Mechatronic
Engineering, University of Sydney,
Sydney, NSW 2006 Australia
ABSTRACT
The aparameter is a variable appearing in the formulation of the Bray number, a
dimensionless term that can be used to predict the behaviour of the turbulent
scalar flux as counter gradient or gradient in premixed combustion. However, the
successful application of the Bray number is hampered by poor characterisation
of the aparameter. The results of previous laserimaging studies of conditional
mean velocities in turbulent premixed methane/air and propane/air Bunsenflames
are used to formulate an empirical description of a. The resulting description
of a suggests that the behaviour of the scalar flux is dependent on the heat
release parameter and the lengthscale ratio, l_{I}/d_{L}, rather than the velocity
ratio, u'/S_{L}.
THE INFLUENCE OF BODYFORCE INSTABILITY ON THE TEMPORAL MOTION OF CELLULAR FLAME
FRONTS AT LOW LEWIS NUMBERS
Satoshi Kadowaki
Department of Mechanical Engineering, Nagoya Institute of Technology Gokisocho,
Showaku, Nagoya 4668555, Japan
ABSTRACT
We calculate the twodimensional unsteady reactive flow to study the influence
of bodyforce instability on the temporal motion of cellular flame fronts at low
Lewis numbers. The equation used is the compressible NavierStokes equation
including an exothermic onestep irreversible chemical reaction, where the
hydrodynamic effect caused by thermal expansion is taken into account. We
superimpose the disturbance with the peculiar wavelength on a stationary plane
flame and. simulate the evolution of the disturbed flame front. The disturbance
superimposed grows initially with time and then the flame front changes from a
sinusoidal to a cellular shape. After the formation of a cellular flame, cells
on the flame move laterally when the Lewis number is lower than unity. Because,
the diffusivethermal effect and the nonlinear effect of the flame front play a
primary role in the appearance of the lateral movement of cells. Bodyforce
instability has a great influence on the lateral velocity of cells and on the
structure of flame fronts. As the acceleration increases, the lateral velocity
becomes smaller and the cell depth becomes larger. The former is due to the
augmentation of the hightemperature region at a convex flame front toward the
unburned gas, and the latter is due to the increase in the instability level.
Jing Hu, Boris Rivin and Eran Sher
The Pearlstone Center for Aeronautical Engineering Studies Department of
Mechanical Engineering, BenGurion University of the Negev
BeerSheva, Israel
Fax: 97276472813, Email: sher@menix.bgu.ac.il
ABSTRACT
Key Words: Methaneair flames, Electric field effects on flames, Ionic wind
The effects of an electric field on the behavior of methaneair flames have been
studied. Coflowing diffusion flames and candletype flames under the electric
field effect have been observed experimentally. A numerical model, which
considers various physical and chemical phenomena associate with the flamefield
interaction process, has been developed to explain the experimental
observations. The model employs a twodimensional cylindrical coordinate system
and assumes axial symmetry. A simplified chemical reaction scheme for a methane
air mixture which contains 19 chemical species and 31 reactions is employed. The
mass, momentum, species and energy conservation equations are solved numerically
by an integrated version of the PHOENICS and CHEMKIN computer codes. It is
concluded that the effects of an electric field on the flame behavior are mainly
due to ionic wind effects.
BURNING VELOCITY AT STRONG TURBULENCE: ROLE OF FLAME GEOMETRY AND TRANSIENT
ETFECTS
Andrei Lipatnikov and Jerzy Chomiak
Department of Thermo and Fluid Dynamics, Chalmers University of Technology,
Gothenburg, Sweden
ABSTRACT
The reduction of the rate of the increase of burning velocity U_{t} with r.m.s
turbulent velocity u' or even the reduction of burning velocity with u' are
wellknown phenomena (the socalled bending effect) which are still badly
predicted by the premixed turbulent combustion theory. The goal of the paper is
to discuss certain geometrical and transient mechanisms which can contribute to
the phenomena side by side with the processes of flamelet stretching and
quenching, commonly invoked to explain them. For these purposes, the propagation
of planar onedimensional and statistically spherical, premixed turbulent flames
is simulated by employing the Turbulent Flame Speed Closure Model. The results
of the simulations reveal two effects. First, the reduction in the flame speed
by the flame curvature, well known but weak under laminar conditions, is
substantial in the turbulent case. The reduction increases with u' and this can
lead to the bending or even a decrease in U_{t}(u'). Second, when the ignition
energy provided by a spark is close to a critical value, a regime of kernel
development, characterized by reduced burning velocity, can occur even in
relatively large, spherical turbulent flames. The physical cause of this "memory
effect" consists in the formation of a highly dispersed kernel followed by slow
afterburning. When the spark energy is kept, constant, the increase of u'
increases the critical ignition energy and the aforementioned regime occurs. As
a result, a decrease in U_{t} with u' can be observed. The two mechanisms affect
statistically curved turbulent flames only.
LAMINAR BURNING VELOCITY OF HYDROGENAIR PREMIXED FLAYXYES AT ELEVATED PRESSURE
Xiao Qin, Hideaki Kobayashi and Takashi Niioka
Institute of Fluid Science, Tohoku University,
211 Katahira, Aobaku, Sendai, Miyagi 9808577, Japan
ABSTRACT
Experimental and numerical studies on the laminar burning velocity of hydrogen
air mixtures were performed. Measurements of laminar burning velocities were
conducted using a new technique based on a particle tracking velocimetry (PTV)
and image processing for burnerstabilized flames in a highpressure chamber.
Equivalence ratios of the mixtures were varried from 0.6 to 3.0 for the pressure
range from 0.1 MPa to 0.5 MPa. A numerical simulation was conducted by solving
NavierStokes equations considering detailed reaction mechanisms and transport
properties. At high pressures, the experimental and numerical results agreed
reasonably with each other for mixtures of equivalence ratios of 1.0 and 2.0,
while discrepancies were seen for equivalence ratio 3.0. These discrepancies can
be diminished effectively by modifying the ratecoefficient expressions of
recombination reactions.
Mohsen M.M. AbouEllail
Mechanical Engineering Department, The American University in Cairo, Cairo, Egypt
Karam R. Beshay and Mohy S. Mansour
Mechanical Power Engineering Department, Cairo University, Giza, Egypt
ABSTRACT
The structure of premixed methaneair flames is analyzed using the "laminar
flamelet concept". A new model based on onedimensional set of transformed
coupled second order differential conservation equations describing the species
concentrations of CO_{2}, CO, O_{2}, CH_{4}, H_{2}O, H_{2} and N_{2} and the sensible enthalpy are
presented in the present work. The equations are rigorously derived and solved
numerically. In these equations, a reaction progress variable (c) is taken as
the independent variable that varies from zero to one. A threestep chemical
kinetic mechanism is adopted. This was deduced in a systematic way from a
detailed chemical kinetic mechanism. The rates for the three steps are related
to the rates of the elementary reactions of the full reaction mechanism.
Calculations are made for different fixed values of the scalar dissipation rate
(c) until the flamelet eventually reaches the extinction limit at different
levels of pressure. Moreover, simultaneous and instantaneous 1D measurements of
CO_{2}, O_{2}, CO, N_{2}, CH_{4}, H_{2}O, H_{2}, OH and temperature have been carried out in a
premixed laminar methaneair flame. A onedimensional UV RamanRayleigh and
Laser Induced Predisposition (LIPF) technique has been applied in the present
work. The spatial and temporal resolutions are limited to the signaltonoise
ratio and the laser pulse duration. The results of the calculations are assessed
against the measurements and previous predictions based on the asymptotic
approach of Cl mechanism and a 4step reduced mechanism. The burning velocity at
different equivalence ratios was also deduced from the flamelet properties and
assessed against available data.
STATISTICS OF FLAME DISPLACEMENT SPEED FROM EXPERIMENTAL STUDY OF
FREELYPROPAGATING PREMIXED TURBULENT FLAMES AT VARIOUS LEWIS
NUMBERS
B. Renou, A. Boukhalfa, D. Puechberty and M. Trinite
CORIAUMR 6614 INSA et Universite de Rouen,
76801 Saint Etienne du Rouvray, Cedex, FRANCE
ABSTRACT
Local scalar flame properties of freelypropagating turbulent premixed flames
including the flame curvature h, and local displacement flame speed S_{d} have
been measured simultaneously for methane, propane and hydrogen/air flames at
Lewis numbers varying from 0.33 to 1.4 and u'/S_{L}° varying from 0 to 3.1. An
advanced field imaging technique based on high speed laser tomography was used
to measure the temporal evolution of local flame properties. Local flame
curvature and local displacement speed were calculated from flame front
contours. Local flame speed S_{d} is controlled by the mean flame stretch since the
temporal evolution of Sa repartition on flame contours indicates a strong
variability of mean and local flame speed with flame expansion. Flame response
in terms of displacement speed to curvature is found to be statistically depend
and a linear relationship is observed. For propane/air flames the local
displacement speed can be assumed to be independent of local flame curvature at
each stages of flame propagation, whereas very strong increasing of displacement
speed with positive curvatures can be observed along the wrinkled flame contour
for hydrogen/air flames. Spatially averaged values of S_{d} and h for low turbulent
flames, computed at each stages of flame propagation, indicate a good agreements
with the asymptotic relation, with signs and values Markstein length depending
of value of Lewis numbers.
EXPERIMENTAL AND NUMERICAL INVESTIGATION OF MULTIPLE RECTANGULAR JETS
A.A. Mostafa, M.M. Khalifa and E.A. Shabana
Mechanical Power Dept., Faculty of Engineering, Cairo University, Cairo, Egypt
ABSTRACT
An experimental and numerical study of the turbulent flow structure of three
rectangular jet flows was conducted. Measurements were performed using a hot
wire anemometer having an xtype probe. Predictions were obtained using a
finitedifference based computer program that solves the parabolic governing
equations of the mean flow along with the turbulence kinetic energy and its
dissipation rate. Measurements of the streamwise mean velocity, kinetic energy
and shear stress are presented and compared with the numerical results. The
achieved good agreement between the experimental and numerical results proves
the ability of the mathematical model to predict successfully the flow field of
multiple rectangular jet flows.
Keywords: Experimental data, numerical predictions, multiple rectangular jets
