SESSION 9
COMBUSTION SYSTEMS 1
Chairman: Y. Yener
THE ROLE OF RADIATIVE TRANSFER IN COMBUSTION
S.H. Chan
Department of Mechanical Engineering
The University of Wisconsin-Milwaukee
P.O.Box 784
Milwaukee, Wisconsin 53201, U.S.A.
ABSTRACT. Radiative transfer being the dominant mode of heat transfer in high temperature
combustion systems, the classical studies concern mainly with the prediction of radiative heat transfer
from a given flame to the surroundings. This review focuses, however, beyond the prediction of
radiative flux from a flame; rather it aims to examine the active role of radiation in affecting combustion
and turbulent processes of flames. First, the effects of thermal radiation on the basic flame structure are
presented. They show that thermal radiation can induce a new extinction limit not achievable by other
means, and can greatly affect the reaction rates to alter the emission of NOx and other pollutants.
Second, the recent advances in turbulence-radiation interaction in radiating turbulent flames are
reviewed. The fluctuating scalars in the flames can now be simulated by a semicausal stochastic method
to account for all the correlated effects by their neighboring points in temporal and spatial spaces. The
simulation yields accurately the ensemble averaged radiation intensity and other radiation statistics.
Finally, a scheme is proposed for the integration of radiation, turbulence and reaction kinetics in
transport processes for a complete simulation of turbulent radiating flames.
EFFECTS OF PARTICLE CHARACTERISTICS ON RADIATIVE HEAT EXCHANGE BETWEEN FLUIDIZING PARTICLES AND A COOLED SURFACE IN A FLUIDIZED BED
Jun YAMADA*, Yasuo KUROSAKI**, Takanori NAGAI**
*Department of Mechanical System Engineering
Yamanashi University, Kofu, JAPAN
**Department of Mechanical and Intelligent Systems Engineering
Tokyo Institute of Technology, Tokyo, JAPAN
This study deals with the radiation heat transfer occurring in a gas-solid fluidized bed between the
fluidizing particles and a cooled heat transfer surface. Fluidized beds are conventionally used in high
temperature heat exchanger applications such as chemical reactors and coal combustors due to their
excellent heat transfer performance. In this type of application, the radiation transfer between fluidized
particles and the heat transfer surface is considered to be a significant heat transfer mechanism. However,
even though much effort has been directed at clarifying the radiation transfer mechanism in fluidized
beds, questions have remained concerning the effects of (1) the fluidizing velocities on the radiation
transfer, (2) the optical characteristics of particles, and (3) the particle diameter [1].
This is because the temperature of fluidizing particle adjacent to the heat transfer surface has not been
well known although this is considered to affect the radiation heat transfer strongly. When a bed is
operated at a high temperature and has cooled heat transfer surface, for example, the temperature of
fluidizing particles is lower than that of particles being in a deep region of the bed, since the particles
adjacent to the surface are cooled by conduction while in contact with the surface and/or by convection
in the thermal boundary layer on the surface. Consequently, the low temperature particles suppress the
radiation heat transfer between the cooled surface and high temperature particles fluidizing in the
depth.
In this study, in order to evaluate the heat transfer, radiation emitted from fluidizing particles to a
cooled surface was measured using an infrared (IR) camera (IRC-160ST, Nippon Avionics), which is
located external to the bed so as not to disturb the bed fluidization. Figure 1 shows a schematic diagram
of the experimental setup. The experimental fluidized bed has a cross section of 100 mm x 100 mm
and a height of 1200 mm. A heat transfer surface having a diameter of 50 mm is located 60 mm above
the felt sheet distributor settled at the bottom of the bed.
Since the measurements for the radiation energy are carried out through the heat transfer surface, the
heat transfer surface must be transparent. We use CaF2 windows, whose transparency is 94% in the
camera's range of detectable wavelength (3.0 - 5.4  ). The heat transfer surface was cooled by
convection of low temperature air that is generated by an air cooler (AC-70, Tohin) applying the
Joule-Thompson effect.
Figure 2 shows typical distribution of the radiation emitted by fluidizing particles against the heat
transfer surface, where the circle indicates the transparent heat transfer surface. The bright region in
Fig. 2, where the radiation energy is high, indicates a bubble rising along the surface. Bubbles appear
bright because high-temperature fluidizing particles can be seen through them. In the dark region
there are low-temperature particles. They emit little radiation, and obstruct the strong radiation from
the depth of the bed as well. This definitely proves that cooled fluidizing particles suppress the radiation
heat transfer.
In addition, such experimental results revealed that the radiation heat transfer is enhanced by (1) using
large particles, (2) operating at a high fluidizing velocity, and (3) using beds (particles) with high
emissivities.
Figure l. Schematic diagram of the experimental setup
Figure 2. Distribution of radiative energy emitted by fluidizing particles
REFERENCES
- Saxena, S. C., Srivastava, K. K. and Vadivel, R., Experimental Techniques for the Measurement of Radiative and Total Heat Transfer in Gas Fluidized Beds: A Review, Experimental Thermal and Fluid Science, Vol. 2, pp. 350-364,1989.
THERMAL RADIATION OF GAS-SOLID-DISPERSIONS AT HIGHER PARTICLE LOADINGS
Hans-Gerd BRUMMEL*, Dieter VORTMEYER**
* Siemens AG, Power Generation KWU
Department: ´Total Plant Thermodynamics, Steam Generators´
D-91050 Erlangen, Germany
** Technical University of Munich
Faculty of Mechanical Engineering
D-85747 Garching, Germany
ABSTRACT. The mechanisms of heat transfer between high temperature particle laden flows and
heating surfaces have been the subject of investigations for many years. An engineering approach for
the treatment of the radiative transfer between these gas-solid-dispersions and the surrounding walls
was developed by Biermann and Vortmeyer already in the late sixties. This method incorporates a
simplified particle radiation two-flux-model and coupling equations for the interaction with the gas
phase. The required absorption data for the particles were evaluated by Biermann from his emission
measurements for fly-ashes from a wide range of European coals. Theoretically this model is also
capable of taking into account the backscattering of radiation by the particles. Since no data for the
scattering characterization of fly-ashes were available, in the past the application of the model was
restricted exclusively to small particle loadings. For these conditions (optical thickness < 0.3) the
radiative transfer is dominated by absorption to an extent that scattering can be neglected. For this
special case the equations for the particle emittance can be transferred to a simple expression similar
to Bouguer´s law for a grey gas.
This paper extends the described model by generating data for the scattering characterization of fly-
ash particles. As a result of this work the former restrictions for the described radiation model do no
longer exist. This enables process engineers to design heating surfaces with improved accuracy for
technical applications that operate with higher concentrations of dispersed solids (e.g. entrained
pressurized coal gasification (EPCG) and circulating fluidized bed combustion (CFBC)).
While data for the mean absorption efficiency  were evaluated by Biermann from his
experimental work for a large number of fly-ashes, data for the mean backscattering efficiency 
could not be obtained analogously. In principle both efficiencies are a function of the wavelength of
the radiation and the size and the optical properties of the particle considered (i.e. refractive index
and absorption index). For homogeneous spheres these dependencies were formulated by Mie at the
beginning of this century.
To obtain data for the optical properties of fly-ashes a new determination procedure was developed.
Utilizing the particle diameters and the temperatures of Biermann´s emission experiments as input
parameters, the Mie-theory was applied iteratively varying the absorption index to obtain the
measured mean absorption efficiencies of the examined fly-ashes. The refraction index was held
constant during these calculations (for the mineral compositions of fly-ashes and the temperature
range of interest this parameter can be represented by the constant value  with fairly good
accuracy).
By using the developed Mie-program in the iterative manner, it was found that 40 % of the fly-ashes
considered had nearly identical absorption indices. Therefore it was possible to evaluate
representative data. With these optical properties the absorption, and in particular, the desired
backscattering efficiencies could be determined by further Mie-calculations for any particle size of
interest. Consequently the particle radiation model of Biermann and Vortmeyer could be fully
applied.
It should be noted, that the absorption indices obtained by this method are mean values, assumed
constant over the entire wavelength spectrum. But the spectral characteristic of the absorption index
is not neglected generally by this approach as in the first studies incorporating the Mie-theory into
particle radiation calculations. The wavelength dependency is indirectly taken into account. The
Planck averaging of the monochromatic absorption efficiencies in the Mie-program results in
different absorption indices for different temperatures. Therefore good correlation exists with
comparable data obtained from spectral measurements of optical properties carried out by Goodwin
for fly-ash compositions of American coals.
Numerous studies have been executed with the generated efficiencies to predict radiation fluxes for
high particle loadings and different particle sizes which can be summerized by the following results:
- The simplified engineering approach neglecting scattering leads to a significant overprediction of radiative heat fluxes for optical thicknesses > 1.0 and particle sizes < 30
 .
- At very high particle loadings where the particle emissivity becomes independent of the optical thickness (asymptotic range) the solid phase exclusively determines the radiation. Therefore the coupling equations for gas and particle radiation which have been formulated by Biermann and Vortmeyer for optically thin conditions had to be adjusted for higher optical thicknesses.
- Further analyses with the extended model resulted in a procedure to determine the maximum layer thickness for the radiative exchange between gas solid dispersions and surrounding walls. The knowledge of this thickness is important to avoid temperature stratifications (hot center flows) in heat exchangers where the temperature of the entire particle laden flow has to be decreased by radiative transfer (e.g. EPCG radiation coolers).
- Although the Mie-theory is only valid for particles of ideal spherical shape in a strictly scientific sense, the representative optical properties for fly-ashes, determined by this work, have been applied for verification calculations in comparison to emission measurements for clouds of CFBC ash particles. The shape of these ashes generally differs significantly from an ideal sphere. Nevertheless a good correlation between measured and calculated data could be observed, even for very high particle loadings.
Finally an emittance diagram, derived from calculations with the described radiation model, was
developed to facilitate the determination of the emittance of a fly-ash cloud in a similar way as the
emittance of a gas is obtained applying the diagrams developed by Hottel et al..
Although the complex Mie-theory was utilized in a large variety of calculations to evaluate data for
the absorption and backscattering characterization of fly-ashes, it was possible to incorporate the
results of these computations in an user friendly radiation model designed for engineering practice.
CFD CALCULATION OF COMBUSTION AND RADIATION FOR A SONIC SYNGAS JET FIRE USING A PREMIXED FLAMELET TURBULENT COMBUSTION MODEL
Aziz Üngüt* , Andy D. Johnson*, Lee Phillips*, and Arij I. van Berkel**
* Shell Research Limited. Shell Research and Technology Centre, Thornton
P.O. Box 1, Chester CH1 3SH, United Kingdom
** University of Twente
ABSTRACT. CFD simulation of radiative heat transfer from an under-expanded
syngas (10 % CO, 90% H2) jet fire is reported. The jet emerges from a 2 mm
diameter nozzle with a mass flow rate of 13.7 g/s. Turbulent transport equations are
solved using the CFX-F3D solver and, the k- turbulence model. The combustion
process is modelled using the premixed laminar flamelet approach proposed by Leeds
university. Radiation heat transfer is estimated using the CFX-RADIATION solver
that simulates volume heating, and cooling from the calculated flow grid using a Monte
Carlo method. A lookup table is created to estimate the local effective absorption
coefficients in a four-dimensional data matrix of local temperature, CO2 and H2O mass
fractions, and path length using a narrow band spectral radiation method. A method is
proposed to estimate the local CO2 and H2O mass fractions from the flamelet library by
assuming that the presence of these molecules outside the flammable region contribute
very little to the radiation emissions because of lower local temperatures. Calculated
flame size and shape are in reasonable agreement with the experimentally determined
values, but the CFD simulation overestimates the lift off height by a factor of two.
Calculated total radiation flux from the jet fire show good agreement with the
experiments establishing CFD simulation as a useful tool to assess radiation scenarios.
We plan to extend the CFD simulation to large scale syngas jet fire release scenarios
to estimate the impingement and radiation hazards associated with real situations.
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