Modelling and Simulation of Thermal Radiation in Biomass Combustion

University dissertation from Div. Heat Transfer, P.O. Box 118, SE-22100 Lund, Sweden

Abstract: This thesis presents numerical investigations and simulations of fixed bed combustion of wood, models for predicting thermal radiative properties of particles and gases related to the combustion of wood, and a review of heat load calculations in gas turbines. All subjects have a common factor in thermal radiation. One 2D and one 3D model were considered for the fixed bed combustion. The introductory sensitivity analysis was performed in order to identify the significant parameters in a fixed bed combustor with updraft by using a two-dimensional CFD model with the computational domain treated as a one, i.e., bed and over fire air region were not separated. The numerical results of this 2D analysis using a global chemistry model showed that the parameters with the greatest influence were: bed porosity, initial wood particle diameter, thermal dispersion, selected solving algorithm, and inlet velocity and temperature. The 2D model showed promising for further development. The objectives of the 3D model was to improve the radiative model and radiative property model used in the 3D code, and to get more accurate predictions of the boundary conditions related to radiative heat transfer to the bed and the walls. The influence of the mean beam length was also of interest. In the 3D model the interaction between turbulence and chemistry was modelled by using the eddy dissipation concept, and the bed was treated with a one-dimensional FG-DVC (Functional Group Depolymerization Vaporization Crosslinking) approach. The P1-approximation proved to be a useful model when the mean beam length is uncertain, such in boilers. It was found that approximately 2 % of the energy input to the boiler was radiated back to the bed. The purpose of studying radiative property models for gases and soot particles was to produce more computationally efficient models, with no loss in accuracy, useful for furnace calculations. For the gases the exponential wide band model is considered due to its favourable balance between computational speed and accuracy. The Mie theory, the theory of infinite circular cylinders and the RDG-PFA (Rayleigh Debye Gans Polydisperse Fractal Aggregates) theory were used for the soot particles. For the exponential wide band model polynomials of the band strength and line overlap parameter are presented. For the soot particles are correlations given for the effective (transport) parameters. Both the polynomials and the correlations provide a vast improvement in computational speed with almost no loss in accuracy. The objective of the review of heat loads in gas turbine combustors is to identify the present status in this area and to stress the shortcomings of the present models. This was mainly done by a literature survey. Most of the published work neglect soot radiation. A detailed CFD analysis of the combustion chamber is an attractive area of research.

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