Chairman: Y. Yener


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.


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


  1. 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.


    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.


    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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