Using Computation Fluid Dynamics to Determine Oil Droplet Breakup Parameters during Emulsion Atomization with Pressure Swirl Nozzles

Abstract

A wide range of commercial powdered products are manufactured by spray drying emulsions. Some product properties are dependent on the oil droplet size, which can be affected by fluid mechanics inside the spray nozzle. However, most of the key flow parameters inside the nozzles are difficult to measure experimentally, and theoretical estimations present deviations at high shear rates and viscosities. Therefore, the purpose of this study was to develop a computational model that could represent the multiphase flow in pressure swirl nozzles and could determine the deformation stresses and residence times that oil droplets experience. The multiphase flow was modelled using the Volume-of-Fluid method under a laminar regime. The model was validated with experimental data using the operating conditions and the spray angle. The numerically calculated shear stresses were found to provide a better prediction of the final oil droplet size than previous theoretical estimations. A two-step breakup mechanism inside of the nozzle was also proposed. Additionally, some of the assumptions used in the theoretical estimations could not be confirmed for the nozzles investigated: No complete air core developed inside of the nozzle during atomization, and the shear stress at the nozzle outlet is not the only stress that can affect oil droplet size. Elongation stresses cannot be neglected in all cases.

Influence of Nozzle Geometry and Scale-Up on Oil Droplet Breakup in the Atomization Step during Spray Drying of Emulsions

“…Firstly, sufficiently high stresses need to act for a sufficient time for breakup to occur. This was shown by Ballesteros and Gaukel [34] to be the case for pilot-scale nozzles. Secondly, the flow conditions responsible for oil droplet breakup have to be similar.…”

“…It has to be noted that the calculation of the capillary number assumes a spherical droplet in equilibrium conditions. This is likely not the case, as it was shown by Ballesteros and Gaukel [34] that oil droplets are already partially broken up in the inlet port of the nozzle. Another notable observation is in cases where larger oil droplets were observed despite larger estimated shear rates, both at pilotand industrial-scale (Figure 6A).…”

“…It is generally assumed that stresses in the outlet orifice are sufficiently high to override the impact of the rest of the nozzle. This assumption is also challenged by Ballesteros and Gaukel [34]. The authors investigated the local shear and elongation stress as well as stress histories in comparable geometries of the Mini SDX and SK by means of computational fluid dynamics simulation.…”

“…A possible explanation could be an insufficient residence time of the oil droplets in the high-stress zone of the nozzle orifice. Based on the results of Ballesteros and Gaukel [34], this is likely not the case, as the residence time in the nozzle orifice is expected to be much higher than the critical breakup time of oil droplets.…”