Effect of Spray Temperature Conditions on Droplet Size Distribution by a Rotating Disk

The size and morphology of particles generated by a spray dryer strongly depend on the atomization process because it is the initial stage of the process. In a practical apparatus, a rotating disk atomizer is exposed to the hot dry air, while liquid raw material is usually kept at room temperature. In this study, the effect of the temperature condition on the droplet size distributions is, therefore, investigated using ion-exchanged water and castor oil. Not only the temperature of liquid raw material, but also the temperature of the rotating disk are individually controlled in the experiment. As a result, the droplet size distribution was strongly influenced by the difference between liquid and disk temperatures. When the temperature difference was large, bimodal droplet size distributions were observed. On the contrary, mono-modal size distributions were obtained for the smaller temperature differences. Furthermore, when temperature of disk was much higher than that of the liquid raw material, film flow locally fluctuated on the rotating disk was observed. According to these results, droplet size distributions could be affected by the stability of liquid film flow on the rotating disk. When the temperature difference between liquid and disk is large, the physical properties of the liquid raw material near the disk surface would be changed. In particular, when the temperature of the disk is higher than that of liquid, local density and viscosity near the disk surface are always smaller than those of the upper bulk liquid, respectively. Such two-layer flow could lead to unstable film flow. Thus, the distributions of droplet size could be characterized by the temperature dependence of physical properties of the liquid raw material.