ABSTRACT

The development and validation of a three-stage freezing model for polymorphous materials, with applications to food sprays, is presented. In the first stage, the cooling of the droplet down to the freezing temperature is described as a convective heat transfer process in turbulent flow. In the second stage, when the droplet has reached the freezing temperature, the solidification process is initiated, which results in the release of latent heat. The amount of latent heat released is related to the amount of heat convected away from the droplet. The solidification process is monitored with a progress variable that is used to determine the specific heat capacity of the semisolid droplet. After completion of the crystallization process, in stage three, the cooling of the solidified particle is described again by a convective heat transfer process until the particle temperature is close to that of the gaseous environment. The freezing model has been validated with experimental data of a single cocoa butter droplet in an air flow. Subsequently, the model has been implemented into the computational fluid dynamics code KIVA-3 and has been validated with experimental data of a cocoa butter spray. In addition, the sensitivity of drop sizes with respect to variations in material and processing parameters has been investigated.