Numerical probing of suspended lactose droplet drying experiment

Single droplet drying (SDD) device has been extensively used to measure drying kinetics of droplets of 0.5–2 mm initial diameter. Despite the large size, it has deemed to be the most practical for such purposes in the past 40 years. There is a surplus of numerical investigations of SDD in the literature. However, this is the first time detailed computational fluid dynamics (CFD) simulations were carried out on the actual fluid behaviour inside and around a lactose solution droplet in SDD by glass suspension experiment. The full set of the governing equations were solved and the fluid flow was investigated. A lactose concentration gradient was developed within the droplet due to momentum transfer and water removal. The CFD predictions of lactose concentrations could be used to predict the formation of crust or crystallization in the droplet. Furthermore, accumulation of solute within the droplet indicated the tendency to form solid structure. The simulated overall drying rate curve compared well with the available laboratory data. The predicted temperature results have also been benchmarked against the experimental measurements generated from the thermal history of lactose droplet drying. More importantly, CFD simulations have been extended to similar small droplet sizes occurring during spray drying, which are experimentally inaccessible with SDD. These analyses are relevant to spray drying to validate simpler 1D models.