Research Papers: Radiative Heat Transfer

Influence of Index of Refraction and Particle Size Distribution on Radiative Heat Transfer in a Pulverized Coal Combustion Furnace

[+] Author and Article Information
Robert Johansson

Department of Energy and Environment,
Chalmers University of Technology,
Göteborg 412 58, Sweden
e-mail: robert.johansson@chalmers.se

Tim Gronarz

Institute of Heat and Mass Transfer,
WSA, RWTH Aachen University,
Augustinerbach 6,
Aachen 52056, Germany
e-mail: gronarz@wsa.rwth-aachen.de

Reinhold Kneer

Institute of Heat and Mass Transfer,
WSA, RWTH Aachen University,
Augustinerbach 6,
Aachen 52056, Germany
e-mail: kneer@wsa.rwth-aachen.de

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 7, 2016; final manuscript received November 8, 2016; published online January 10, 2017. Assoc. Editor: Laurent Pilon.

J. Heat Transfer 139(4), 042702 (Jan 10, 2017) (8 pages) Paper No: HT-16-1444; doi: 10.1115/1.4035205 History: Received July 07, 2016; Revised November 08, 2016

In this work, the influence of the radiative properties of coal and ash particles on radiative heat transfer in combustion environments is investigated. Emphasis is placed on the impact on the impact of the complex index of refraction and the particle size on particle absorption and scattering efficiencies. Different data of the complex index of refraction available in the literature are compared, and their influence on predictions of the radiative wall flux and radiative source term in conditions relevant for pulverized coal combustion is investigated. The heat transfer calculations are performed with detailed spectral models. Particle radiative properties are obtained from Mie theory, and a narrow band model is applied for the gas radiation. The results show that, for the calculation of particle efficiencies, particle size is a more important parameter than the complex index of refraction. The influence of reported differences in the complex index of refraction of coal particles on radiative heat transfer is small for particle sizes and conditions of interest for pulverized coal combustion. For ash, the influence of variations in the literature data on the complex index of refraction is larger, here, differences between 10% and 40% are seen in the radiative source term and radiative heat fluxes to the walls. It is also shown that approximating a particle size distribution with a surface area weighted mean diameter, D32, for calculation of the particle efficiencies has a small influence on the radiative heat transfer.

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Grahic Jump Location
Fig. 1

Refractive index for ash (left) and coal (right) and efficiencies calculated by Mie theory for two different particle sizes and different refractive indices. Gray shaded area denotes the blackbody radiation according to Planck for T = 1700 K to highlight the spectral range of interest for PCC. Solid lines correspond to Dp=1 μm, dash-dotted lines to Dp=10 μm and dashed lines to Dp=40 μm.

Grahic Jump Location
Fig. 2

Influence of the complex and the real part of the refractive index m on the scattering (black lines) and absorption (gray lines) efficiencies. Ash particles on the left and coal particles on the right. Reference values are m = 1.6 − i0.3 (coal) m = 1.6 − i0.01 (ash).

Grahic Jump Location
Fig. 3

Radiative source term for cases of type 1 with a cylinder diameter of 12 m and four different complex indices of refraction. For ash, the combined data, Refs. [1719] (see Table 1), is used for the complex index of refraction.

Grahic Jump Location
Fig. 4

Wall fluxes for cases of type 1 with four different complex indices of refraction. For ash, the combined data, Refs. [17-19,ib1,ib1] (see Table 1), is used for the complex index of refraction.

Grahic Jump Location
Fig. 5

(a) Radiative source term for a type 2 case with a cylinder diameter of 12 m and (b) wall fluxes for type 2 cases. The applied indices of refraction are listed in Table 1.

Grahic Jump Location
Fig. 6

Sensitivity to the complex index of refraction of coal particles expressed as the relative difference between the maximum result and the minimum result for (a) type 1 cases and (b) type 2 cases. Black lines is the wall flux, and gray lines is the source term at the center of the cylinder for type 1 cases and the average source term for type 2 cases.

Grahic Jump Location
Fig. 7

(a) Radiative source term for a type 1 case considering a size distribution for the coal particles compared to the use of a single mean diameter, D32, Eq. (3). (b) Relative error caused by an approximation of a particle size distribution with a mean, D32, particle diameter.




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