Special Topics in Multiphase Mixing - Cloud Height

The goal of the study was to determine the effect of particle size and density on the cloud height of particles. A Lightnin 8.5 centimeter diameter stainless steel model A310® impeller was used to suspend the particles. Aluminum, urea formaldehyde, and polyoxymethylene (acetal) spheres were used as testing material with specific gravity values of 2.7, 1.2, and 1.4 respectively.  A literature review was conducted in order to form a basis for experiments. Assessments could be drawn from previous literature findings. A Plexiglas, flat bottom tank 28.3 centimeters in diameter was used. The tank was filled with water to a height of 56 centimeters for each test. The tank contained 3 centimeter baffles. The impeller height off the bottom of the tank was 9.5 centimeters, about one third the tank diameter. The cloud height was measured at different impeller speeds (measured in RPM) ranging from nearly just suspended speed (NJS) to 2355 RPM, the maximum speed of the equipment. The particle concentrations used within the tank were 1%, 2.5%, and 5% by weight. Particle mixtures as well as pure materials were tested. It was determined that cloud height is dependent on speed, particle size, and particle density. Concentration also has a small effect. A notable observation is certain mixtures produce homogenous clouds, such as a mixture of aluminum and acetal. Other mixtures produce a stratified cloud, in which one particle forms a distinctly higher cloud height. This is seen in a mixture of acetal and urea. It is hypothesized that large differences in particle size produce this stratified cloud.