Diffusion of solid fuelon a vibrating grate

This work is part of a long term project of developing a bed model, describing the combustion process of straw on a vibrating grate. For a vibrating grate, the mixing and transportation of the fuel are of great significance and the work presented in this report investigates how the effect of vibrations can be incorporated into a numerical model.

The chosen model approach has been to separate the gas and solid phases into two independent models related to each other through the bed porosity. By treating the bed as a porous media and using Ergun's equation for the gas flow, the numerical work is simplified and the computational time shortened. The vibrations are affecting the transport and mixing of the fuel and incorporated into the model through the diffusion coefficient in the conservation equation of the solid phase.

Experimental work has been carried out with the aim to study the behaviour of wood pellets on a vibrating grate and deriving the diffusion coefficient to be used in the numerical model. Three different grate designs are used and the particle trajectories have been captured by a camera placed above the grate. The diffusion coefficient is defined as the deviation from the mean movement of the particles. The results show that the diffusion of the particles increases with increasing vibration amplitude and frequency and decreasing particle layer thickness. There is a significant difference in the magnitude of the diffusion coefficients for the different test set-ups, which shows that the diffusion is strongly dependent on the grate design and a diffusion coefficient has to be determined for each type of grate to be modeled.

Different alternatives of how to represent the velocity and diffusion coefficients in the model have been investigated. It has been found that the vibrations give rise to both a diffusive and a convective contribution and that the velocity depends on the position of the grate. It is suggested that the mean velocity of the particles should be seen as a convective process whilst the deviation from the mean velocity should be treated as a diffusive process. In order to introduce a varying velocity depending on the position on the grate, a modification of the model is necessary where also the density will vary as a consequence of the continuity equation. The definition of the density will thereby change from being the particle density to be the cell density, i.e. a measure of how dense the particles are packed in each cell.