Abstract
A three-stage solidification model for food droplets has been implemented in a computational fluid dynamics code. It comprises of an initial cooling stage that is based on the principles of convective heat transfer. This is followed by the solidification period that is initiated once the droplet cools to a phase change temperature. Finally, when the droplet is completely solidified, the tempering phase begins where the droplet cools to the temperature of the ambient air. The model has been validated with respect to the experimental data for cocoa butter. Additional simulations were made in which the crystallization behavior of the cocoa butter droplets in relation to the droplet size, ambient air temperature and the relative drop-gas velocity was investigated. It was found that the crystallization time is exponentially related to the droplet size. Further, it increased with the ambient temperature, but decreased with the relative drop-gas velocity. Overall, the results suggest operating at the extreme values of the process parameters, requiring high amount of energy, to minimize the crystallization time. It was concluded that there is a need for optimizing the operating conditions of the powder production process to minimize the energy requirement of the system while maintaining a reasonable crystallization time.
Highlights
Solidification is a multi-stage process, which can be divided into three distinct periods, viz., a pre-cooling period, a phase change period and a tempering period
After the pre-cooling and solidification stage when the droplet reaches a temperature of 273 K, it is completely solid. Beyond this point the tempering stage of the model starts in which the solid droplet cools to the ambient temperature
The circle and square symbols represent the ambient temperatures of 270 K and 220 K, respectively
Summary
Solidification is a multi-stage process, which can be divided into three distinct periods, viz., a pre-cooling period, a phase change period and a tempering period. In order to investigate the intricate details of the crystallization process a more advanced numerical approach based on Fourier’s heat conduction principles is essential This will aid in understanding the phase change characteristics such as the propagation of the solidification front and the internal temperature distribution in the material. Coupling this numerical approach with crystallization models, physical processes such as dendrite growth can be studied. If there is a high degree of impurity in the solution, i.e., many nucleating agents, crystallization is initiated as soon as the phase change temperature is reached, e.g., cocoa butter Further details are presented in the section titled Parametric Study of the Crystallization Process
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