Application and comparative analysis of radiative heat transfer models for coal-fired furnace

Abstract

Thermal radiation is a crucial mode of heat transfer in the industrial furnace. The present article compares various radiative heat transfer models for non-gray gases environments with typical application to the pulverized coal-fired furnace. An open source CFD software OpenFOAM is used to implement a three-dimensional photon Monte Carlo (PMC) method for radiation from gray gases and first-order spherical harmonics (P₁-method), discrete ordinates method (DOM) and zonal method for radiation from non-gray gases. The non-gray radiation properties are calculated using the weighted-sum-of-gray-gases (WSGG) and full-spectrum k-distribution models. The PMC method is employed as a benchmark for comparing the accuracy of the P₁-method and the DOM for gray gas environment, and the zonal method is employed as a benchmark for comparing the accuracy of P₁-method and DOM for the non-gray gas environment. The zonal method and PMC are validated against the exact solution for plane-parallel medium filled with a gray gas. The accuracy of the P₁-method and DOM is analyzed and compared with the zonal method for non-gray gas radiation inside a cubical box of 1 m sides. The effect of coal and ash particle’s radiative properties on the radiative heat transfer has been analyzed. The k-distribution model along with radiation due to coal and ash particles is presented. The variation in absorption and scattering efficiency due to particle size and the complex index of refraction is analyzed for different types of coal and ash particles. Isotropic, forward scattering, and δ-Eddington approximation are compared with the anisotropic scattering using Mie-theory. The coal and ash particle’s radiative properties are calculated using the Mie-theory and are averaged using Planck intensity distribution. The particle size of the coal and ash is a more critical parameter compared to the complex index of refraction. The slight variation in projected area per unit coal and ash particle leads to a significant change in the radiative heat source and radiative heat flux at the wall of industrial furnace.

Keywords:

• Anisotropic scattering (Mie-theory); full-spectrum k-distribution
• photon Monte Carlo
• spherical harmonics, discrete ordinates method
• weighted-sum-of-gray-gases
• zonal method