Air flow patterns in an industrial milk powder spray dryer

The air flow patterns in an industrial milk powder spray dryer have been investigated. Isothermal three-dimensional transient simulations in the absence of atomised liquid droplets have been carried out using the commercial CFD code (CFX10.0) in which the transient Navier–Stokes equations are solved. The shear stress transport (SST) turbulence model was implemented to model the effects of turbulence.These simulations are possibly the first to include the following approximations to industrial spray dryer practice: (a) introduction of an internal fluid bed, (b) outlet ducts located near the top rather than the bottom of the dryer and (c) placing the constant atmospheric pressure condition downstream of the dust collector rather than at the dryer exit.As expected, the simulations showed that the main air jet oscillated and precessed about the central axis with no apparent distinct frequency. In turn, the recirculation zones between the main jet and the chamber walls fluctuated in size. Good agreement was found between the movements of the main jet via simulations and from telltale tufts installed in the plant dryer. This supported other indications that the simulations were an accurate representation of the actual flows. The different outlet boundary condition (including a flow resistance representing the baghouse) appeared to have little influence on the overall flow field. In the gas-only simulations, different fluid bed flows within the range used industrially had only a local influence by reducing the length of the main jet. This may have an effect on the particle capture by the fluid bed.