An optical investigation of air particle flows.
McCluskey, Denise R
This thesis is a fundamental study of air-particle flow fields where the experimental parameters are characteristics of coal-fired electricity generating stations. The optical flow field measurement technique Particle Image Velocimetry (PIV) was adapted to study the particle flow fields and, in addition to the velocity vector map, particle concentration information was obtained. On phenomenon under investigation was the formation of ropes (high density ribbons of pulverised coal) in a small scale model of the pneumatically driven pulverised fuel transport lines of coal-fired power staions. The main findings of the study were that ropes form in bends and, when in contact with the bend wall, ropes are slowed by frictional forces. After they leave the bend, ropes fall throught he main airflow, maintaining their coherence. If the length of horizontal pipework is sufficiently long, the ropes will form a deposit. The Froude number is the relevant scaling parameter for the deposit's equilibrium position since the dominant influence on this position is the distance, L(f,s), for the rope to slow to zero when it is travelling along the bottom of the duct. The equilibrium position of the deposit, for a given Froude number is dependant upon the air-to-particle ratio: the higher this ratio then the further downstream the deposit. The factors influencing particle jet dispersal were investigated in view of their relevance to the coherence of the ropes. This study is also of relevance to the behavior of coal burner systems where an air-coal mixture is injected into a complex, usually swirling, airflow. The broad conclusions of the experimental results are as follows: 1. For a given particle loading and background velocity, the behavior trends of air-particle jets issued into a background airflow at velocities comparable to the mainstream values are similar to the theoretical description of singlee-phase jets which was derived by Squire and Troucer (1944). These general trends are as follows: (a)As the issuing velocity of the jet increases, its concentration half-width increases. (b)As the issuing veloccity of the jet increases, the rate of decrease of the centre-line velocity decreases, almost linearly with injection velocity. 2. For a given jet velocity, the higher the air-particle loading of the jet, the less the jet disperses and the less rapid the acceleration of the jet. 3. For a given relative velocity ratio between the jet and the background airflow, the higher the background airflow velocity, the more the jet disperses, the greater the rate odd acceleration of the jet and the more uniform the cross-sectional velocity profile.