COMBUSTION / MARANGONI CONVECTION
FUNDAMENTAL ANALYSIS ON FLAME PROPAGATION
BEHAVIOR AT LEAN LIMIT MIXTURES USING MICROGRAVITY TECHNIQUE
S. Okajima and A.Okajima
Faculty of Engineering, Hosei University, J-184 8584 Tokyo, Japan
From the viewpoint of low air pollution and energy saving on the combustion
equipments such as international combustion engines, it is very important to
elucidate the combustion behaviour of lean limit mixtures near the flammability
limit.
Thus, experiments have been carried out to examine the characteristics of flame
propagation close to the lean limit mixtures in a closed bomb by using the microgravity
technique. The flame characteristics are studied by realizing the spherical
flame propagation in a closed bomb under microgravity. Of course, microgravity
techniques make it possible to obtain the fundamental combustion properties
even for very lean mixtures because the symmetrical flame behaviour, by eliminating
the buoyancy effect, can be achieved by a microgravity technique. Microgravity
environment is achieved in a free fall facility of 490 m drop shaft in JAMIC
(Japan Microgravity Center) located in Kamisunagawa, Hokkaido, Japan. The gravity
level of the combustion chamber inside the falling capsule is 0.00001g. The
falling assembly contains a cylindrical combustion bomb, an igniter, a schlieren
photographic system and a camera unit. The cylindrical combustion bomb has 116
mm in diameter and 120 mm in length, which is equipped with a concave mirror
and a Pyrex glass window on the opposite walls along the optical axis.
Experiments have been performed at room temperature and an initial pressure
of 0.1 MPa. The fuels used are methane and propane of 99.9% purity. The main
conclusions of the study are as follows: (1) microgravity techniques make it
possible to determine very slow burning velocities with high accuracy, (2) the
minimum value of burning velocity obtained for methane-air mixtures at 293 K
and 0.1 MPa has been evaluated as 3.58 cm/s in the vicinity of 0.48 of equivalence
ratio, (3) the cellular flame can be observed at very lean methane-air mixtures,
and(4) the irregular flame propagation occurs when the Lewis number is less
than the unity for propane-air mixtures only.
Key Words: Microgravity, Lean limit mixtures, Burning velocity, Lewis number, Irregular
flame behaviour.
EFFECTS OF WIND AND OXYGEN CONCENTRATION ON A LAMINAR DIFFUSION FLAME ESTABLISHED
OVER A HORIZONTAL FLAT PLATE IN MICRO-GRAVITY ENVIRONMENT
H. Y. Wang, P. Corderio, P. Joulain and J. Torero
Laboratoire de Combustion et de Détonique , C.N.R.S. UPR 9028 -E.N.S.M.A.,
Université de Poitiers, BP 109 - Site du Futuroscope, 86960, Futuroscope
Cedex, France
Fire propagation over a condensed fuel in a micro-gravity environment is of
particular interest in a human-occupied spacecraft. The condensed fuel vaporises
due to heat transfer from the flame to the surface through radiation and convection.
The fuel exothermically reacts with the oxidiser and provides the necessary
heat to sustain further pyrolysis.
An experimental and numerical investigation of a laminar diffusion flame
established over a horizontal flat plate in micro-gravity environment is described.
Fuel is injected through the burner and the oxidiser (air) is provided by a
forced flow parallel to the surface. The experiments were conducted in a range
of oxygen concentration from 23 to 30% at two different wind velocities of 2.6
and 4 cm/s in a 5 s drop tower. The flame experiments make use of a pilot wire
to ignite the sample in normal-gravity environment. It is observed that after
the imposition of the micro-gravity, a plume region is maintained momentarily
due to the dominant inertia. The transition time is about 2.5 s for reducing
the overall air flow velocity to that of the forced boundary layer flow.
Direct Numerical Simulation (DNS) is used to provide a detailed description
of the transition process of the flame in micro-gravity environment. The main
advantage lies in the identification of the delay time for reducing an oscillated
pool-like flame to a boundary layer flame type in micro-gravity condition. High
enough spatial and temporal resolution with an efficient flow solving technique
is needed for describing the flame structure during the transition period. The
computational grid is fine enough to resolve the diffusion of fuel and oxygen.
The starting point of the analysis is the set of three-dimensional, partial
differential equations that governs the phenomena of interest here. The combustion
process is described by a global one-step, finite-rate reaction given by the
Arrhenius expression. This set consists, in general, of the following equations:
the continuity equation, the three momentum equations that govern the conservation
of momentum per unit mass in each of the three space dimensions, the equations
for conservation of chemical species and of soot formation, and the radiative
transfer equation. The finite-difference method is used to discretize the appropriate
three-dimensional conservation equations. The parameters varied are the oxygen
concentration and oxidiser flow velocity. The model results reflect the qualitative
features of the experiments.
COMBUSTION STUDY ON SOLID WASTE FUELS IN HIGH TEMPERATURE ENVIRONMENT USING
MICROGRAVITY TECHNIQUE
A. Okajima and S. Okajima
Faculty of Engineering, Hosei University, J-184 8584 Tokyo, Japan
From the viewpoint of effective use in the combustion furnaces of the solid
waste fuels produced from an industrial field, it is very important to examine
the combustion behaviour of solid waste fuels in high temperature environments.
Thus, the authors have made an attempt at two kinds of combustion experiments
of solid waste fuels under microgravity achieved in a freely falling capsule.
One is for elucidating the combustion characteristics in high temperature environments
and other one is for studying the influence of oxygen concentration on flame
development in room temperature and in a very small air flow. The combustion
behaviour of solid waste fuels is observed by taking direct photographs with
an
8-mm video camera installed on the falling capsule.
The test facility employed in the study contains an electrical furnace, an 8-mm
video camera, an air cylinder, a temperature control equipment of electrical
furnace, a delay device and a back light system. The electrical furnace is a
rectangular shape of 110 x 110 mm having 150 mm in length. The shape of the
solid waste fuel is a sphere ranging from 2 to 3 mm in diameter.
Experiments have been conducted in high temperature environment ranging from
800K to 1100K, in ambient temperature and 0.1 MPa in ambient pressure. The microgravity
environment of 10 second is realized by free fall facilities of 490 m drop shaft
in JAMIC (Japan Microgravity Center). The main results show that,
(1) there are three categories for the combustion behaviour of solid waste fuels
in a high temperature environment, that is ignition, luminous diffusion flame
and smoldering process, (2) above 900K the ignition occurs with a very strong
luminous flame and the complete combustion process is achieved throughout the
smoldering process; on the contrary, below 850K it almost takes the smoldering
process (surface combustion), (3) the ignition delay decreases monotonically
with increasing ambient temperature, (4) even for very small air velocity it
is very effective to enhance the combustion rate, and (5) the decrease of oxygen
concentration at room temperature markedly makes a change for the worse in the
combustion process.
Key Words: Solid waste fuels, Ignition delay, Luminous diffusion flame, Smoldering.
FIRST MEASUREMENT OF GAS TEMPERATURES BASED ON LIF OF FORMALDEHYDE
1A. Burkert, 2J. König, 1W. Triebel
1 Institut
für Physikalische Hochtechnologie e.V., Winzerlaer Straße 10,
D-07745 Jena, Germany
2 ZARM, University of Bremen, Am Fallturm, D-28359 Bremen, Germany
The reduction of air pollution e.g. from internal combustion engines and gas
turbines is an important task. Drop tower facilities equipped with optical diagnostic
systems are an approved tool to derive an advanced knowledge and understanding
of the kinetics in combustion processes as cool flame phase and two-stage ignition
[1, 2]. The two-dimensional detection of different species as e.g. formaldehyde
(HCHO) [3] during the combustion process and the derivation of concentration
and temperature fields are necessary to verify and develop reduced reaction
kinetics of technical fuels. HCHO is a well-known tracer for knocking in internal
combustion engines and is mainly present in the cool flame stage of ignition
[4]. Nevertheless, the derivation of concentration fields of formaldehyde is
difficult, mainly attributed to the quenching of fluorescence and to the lacking
of non-intrusive techniques for temperature measurements under cool flame conditions.
In this paper we present a technique for temperature measurement under cool
flame conditions based on laser induced fluorescence (LIF) of formaldehyde.
The spectroscopic LIF and PLIF investigations of HCHO were performed with a
set-up consisting of a double-tube XeF excimer laser, an optical multi-channel
analyser, two intensified CCD cameras (ICCD) and hot calibration cells [1, 3].
A detailed LIF investigation of formaldehyde under different temperatures, pressures
and concentrations revealed a temperature dependence of the ratio of the spectral
regions from 390...400 nm to 390...418 nm. A system dependent calibration function
can be obtained by recording of LIF spectra of formaldehyde under controlled
temperature conditions in a high-pressure gas cell and determining the LIF-ratio
from 390...400 nm to 390...418 nm.
First temperature measurements based on LIF of formaldehyde were performed in
a pre-heated measurement cell including a small porous sphere filled with liquid
n-heptane. For this purpose a laser light sheet was formed to excite formaldehyde
in the cool flame phase of ignition. LIF images were recorded with two ICCD
cameras, which were equipped with specially designed filters.
In addition, we discuss modifications of the laser diagnostic system at the
drop tower of Bremen, which are necessary to apply this new method of measurement
of temperature fields and to investigate ignition processes in more detail under
microgravity conditions.
- König J., Eigenbrod Chr., Rath H.J., Grebner D., Hein J., Triebel W.,
"Formaldehyde-PLIF detection of cool-flame reactions during two stage ignition
of alkane droplets", 5th Intl. Microgravity Combustion Workshop,
Cleveland 18-20th May 1999
- König, J., Eigenbrod, Chr., Rath, H.J., Grebner, D. Hein, J., Triebel,
W., "Formaldehyde-PLIF Detection of Cool-Flame Reactions During Two Stage Ignition
of Alkane Droplets", 5th Intl. Microgravity Combustion Workshop, NASA/CP-1999-208917,
189, 1999
- Burkert A., Grebner D., Müller D., Triebel W., König J., "Single
shot imaging of formaldehyde in hydrocarbon flames by XeF excimer laser induced
fluorescence", Twenty-Eighth Symposium (International) on Combustion,
The Combustion Institute, Pittsburgh, PA, 2000, pp. 135-141
- Bäuerle B., Warnatz J., Behrend F., Twenty-Sixth Symposium (International)
on Combustion, The Combustion Institute, Pittsburgh, PA, 1996, pp. 2619-2626
SIMULATION OF GEOPHYSICAL FLUID FLOWS UNDER MICROGRAVITY
C. Egbers, P. Chossat and R. Hollerbach
University of Technology, LAS, Karl-Liebknecht-Strasse 102, D-03046 Cottbus,
Germany
The objective of this research programme is to study thermally-driven rotating
fluids, in order to investigate the stability, pattern formation, and transition
to turbulence of incompressible fluids trapped between concentric, co-axially
rotating spheres.
The physical mechanisms addressed in this proposal are important for a large
number of astrophysical and geophysical problems having spherical geometry flows
shaped by rotation and convection: for example, to explain the mantle convection
of the earth, the differential rotation effect on the sun, the zonal atmospheric
jets on the giant planets, or the flow in a planet’s interior (like the earth).
Understanding and controlling fluid flow in a spherical geometry under the influence
of rotation will also be useful in a variety of engineering applications, such
as improving spherical gyroscopes and bearings, and centrifugal pumps. Furthermore,
study of effects related to the electro-hydrodynamic force, which serves to
simulate the central gravity field in the space experiments, will find applications
in high-performance heat exchangers, and in the study of electro-viscous phenomena.
It will also help to understand the motion of liquids in several ground-based
industrial applications where injected ions are a source of charge, e.g. in
electrostatic precipitators and ion-drag pumps.
The experimental set-up is performed to investigate the problems of thermal
convection in the fluid shell between two concentric half-spheres with and without
rotation, and also with differentially rotating spheres (around the same axis).
A central symmetric force field similar to the gravity field acting on
planets can be produced by applying a high voltage potential between the inner
and outer sphere, using the effect of dielectrophoretic force field.
THERMOCAPILLARY FLOW STRUCTURES UNDER MICROGRAVITY
IN A LONG FLOATING ZONE (NEAR THE RAYLEIGH LIMIT)
D. Schwabe
Physical Institute of the University of Giessen, 35392 Giessen, Germany
The length L and the "ideal" shape of floating zones is limited to some mm under
normal gravity (1-g) due to hydrostatic pressure. Therefore, the hydrothermal
waves of oscillatory thermocapillary flow can only travel in azimuthal direction,
which allows for wavelengths l j up to » 2p R, with R the zone radius. Axial
components l z are suppressed or frustrated because L << 2p
R under 1-g. The theory for infinitely long zones by Xu and Davis, Phys. Fluids
27 (1984) 1102, predicts much smaller critical Marangoni numbers for the onset
of oscillations Mac = -¶ s /¶ T × h -1 × c -1
× R2 × ¶ T/¶ z and different oscillation periods from what is found
under normal gravity in short floating zones. This is most likely due to the
end effects in small – L floating zones which do not allow the most dangerous
states with axial components of the hydrothermal waves.
Under microgravity (m -g) non-deformed floating zones of a length up to the
Rayleigh limit Lmax = 2p R are possible, giving an aspect ratio Amax
= L/R = 2p » 6.28. In such a zone the axial and the
azimuthal space for the development of a hydrothermal wave is the same and one
can expect to observe and measure l z besides l j , different frequencies
and a smaller Mac of the most dangerous mode.
We have realized an A = 5 floating zone from silicone oil (2 cSt, Pr = 29) under
microgravity during the 14 min. ballistic phase of the sounding rocket MAXUS-4
(29.04.2001). We installed 4 different supercritical Marangoni numbers and came
to fully relaxed flow states because of the small zone dimensions (R = 3.0 mm,
L = 15.0 mm). The flow was observed by tracer motion in a vertical and in a
horizontal light sheet. The oscillation amplitude was measured with very fine
thermocouples placed 0.7 mm below the free surface (wire Æ =
25 m m); -5 thermocouples placed at different axial positions but the same azimuth,
4 placed in different azimuthal positions but the same z.
We report on the measured Mac, the frequencies at supercritical Ma,
the mode number and the axial and the radial component of the hydrothermal wave
in this unique object. The measurements are discussed in comparison to the predictions
of theory and the hydrothermal wave is visualized with a video-clip.
DYNAMIC FREE-SURFACE DEFORMATIONS DUE TO HYDROTHERMAL WAVES IN THERMOCAPILLARY
LIQUID BRIDGES
H. C. Kuhlmann and Ch. Nienhüser
ZARM - University of Bremen, Am Fallturm, D-28359 Bremen, Germany
The physical mechanism for the onset of oscillations in thermocapillary flows
has long been discussed controversially. It is now well established that the
transition to oscillatory flows in moderately high-Prandtl-number liquid bridges
is caused by the hydrothermal-wave instability mechanism, if the mean surface
tension is asymptotically large. According to another hypothesis, the dynamic
deformability of the liquid-gas interface is required, for oscillatory flow
to occur in high-Prandtl-number fluids. Some investigations are underway to
measure the dynamic free-surface deformations and to relate these to the flow
and temperature fluctuations. In this presentation dynamic surface deformations
are considered to
leading order in an expansion for small capillary numbers, i.e. for small relative
variation of the surface tension. It has been found that the dynamic surface
deformations associated with the critical oscillatory flow decouple, at leading
order, from the dynamic deformations of the axisymmetric basic flow. Therefore,
the dynamic deformability is not responsible for the instability for small capillary
numbers. Using a numerical analysis, we have predicted the shape of the deformations
caused by the critical oscillatory mode and determined their amplitude relative
to that of the associated surface-temperature fluctuations.
Acknowledgments:
This work has been supported by DLR and by NASDA. We are indebted to S. Yoda
for his continued encouragement.
