Visualization of spray dynamics in a pilot spray dryer by laser-initiated fluorescence

This dissertation examines spray dynamics in a spray dryer. Experiments reveal extensive and interesting dynamics that influence the accuracy of spray dryer models, prediction of drying rates and retention of volatile flavor and aroma components. Spray dynamics are visualized by Laser Initiated Fluorescence (LIF). Spatial characteristics of the spray dynamics are examined by spray intermittency maps developed from digitizing and processing video results. Temporal characteristics are examined by time series analysis of Haar wavelet transformed images. To gain additional insight into the system a numerical model, the PSI-cell model, is used. This model combines heat, mass and momentum transfer effects and a k-{dollar}epsilon{dollar} representation of turbulence.; The dynamics encountered are temporal variations observed in the spray. These variations are large seemingly random recirculations occurring with the dryer otherwise at steady state.; Some spray intermittency and time-variant flow patterns are found in all experiments. In some cases this intermittency expands the spray significantly. The extent of spray intermittency is a strong function of the water mass flow rate and velocity. The area fraction occupied by the spray is correlated (R{dollar}sp2{dollar} = 0.81) with the ratio of water momentum input to air momentum input. It is also strongly affected by variations in the gas flow pattern caused by leaks in the column. Large changes in spray dynamics encountered with small changes in flow suggest that the dynamics might be controllable by variation in the air flow pattern. Adjusting the dynamics could provide a means for the maximizing retention of volatile flavor and aroma components. The unique time series analysis approach reveals evidence of underlying frequencies around one hertz.; The model predicts the basic shape of the spray and approximate internal conditions. These predictions are best at low water flow rates where the dynamic effects are small. The presence of dynamics rationalizes the difference between experimental results and model predictions. The model reveals a region of high shear and high turbulent kinetic energy, and the development of a fast moving central air jet. Turbulent droplet dispersion and vortical structures at the edge of the jet are the likely causes of the spray intermittency.