On Particle Transport and Turbulence Modification in Air-Particle Flows

Doctoral thesis, 2000

The transport of particles by airflow was studied for two different flow cases, the transport of pulp fibres in a mat-forming process and the transport of pulp fibres and spherical particles in a horizontal pipe. The transport of pulp fibres in the mat-forming process is first considered. Numerical calculations are made of the air-fibre flow through the forming head using an Eulerian two-fluid model. The calculations show that the flow field is governed mainly by the boundary conditions, which are rather complicated for the transport channel and forming head of the mat-forming device. Measurements of velocity and pressure are performed and used to verify the numerical results. To be able to measure turbulence in a dilute air-particle suspension, a measuring method is developed that uses a wedge-shaped hot-film probe in airflow. The poor frequency response of a wedge-shaped probe in airflow normally makes it necessary to make a dynamic calibration. To avoid this, a method that uses a correction function applied to the turbulent spectrum is developed and tested in a fully developed pipe flow. The second flow case investigated is the transport of pulp fibres and spherical particles through a horizontal pipe. Measurements of turbulence modification are made. The results show that the turbulence is damped close to the wall and enhanced in the centre region of the channel by the spherical (450 .my.m) particles. For the pulp fibres, the turbulence is damped over the entire cross section. Calculations are made of the transport of particles in a horizontal channel. The two-fluid model is extended to take into account turbulence, and a model for the turbulent dispersion of particles is suggested. The gas phase turbulence is calculated using a modified k-.epsilon. model that takes into account the influence of particles, both additional dissipation and additional production, on gas phase turbulence. The calculated results agree qualitatively with the experimental results for the horizontal pipe.