Computational Fluid Dynamics study of a vortex chamber spray dryer

The reduction of energy consumption in industrial processes has become a priority of the 21st century. As a widespread technique, the production of powders by spray drying is among themost energy costly industrial unit operations. The development of vortex chambers since the late ’60s has shown great potential in the intensification of numerous industrial processes, although a proper understanding of the flow inside a vortex chamber has not yet been fully achieved. This thesis addresses the CFD study of a vortex chamber spray dryer for milk powder production. In the current 6-wheel technology, milk droplets are first injected in a hot zone for a fast preliminary drying before reaching the periphery of the chamber where the establishment of a rotating fluidized bed allows to increase the drying efficiency and the particle residence time. The accurate prediction of the 3D multiphase turbulent swirling flow which takes places in the chamber is not an easy task. In particular, this CFD study confirmed the difficulty for RANS turbulence models to accurately predict the free vortex zone observed in vortex chambers. Numerical simulations highlighted the importance of the cold air reflux towards the back of the chamber, entraining the smallest particles. They also confirmed the existence of a centrifugal force barrier of about 600 g, hence preventing particles to escape the rotating fluidized bed, as well as a so-called ’dead zone’ at the center of the chamber where the centripetal acceleration given to the particles is very low. Multiphase simulations were performed with two types of atomizers. Hollow cone injection evidenced the difficulty for small droplets (30 microns) to form a stable rotating fluidized bed, whereas larger droplets undergo multiple wall collisions, possibly resulting in undesired wall deposits. Full cone injection turned out not to be appropriate with the current 6-wheel geometry. Because of the absence of centrifugal force along the axis and due to the limited space available in the chamber, the droplets do not manage to reach the periphery of the chamber and are immediately entrained towards the chimney. The limitations encountered by the 6-wheel geometry lead us to propose a new extended geometry with 3 wheels at the back and an additional wheel at the very front of the chamber in order to increases the particle residence time. First simulations show promising results in terms of the expected gas flow pattern.