A dimensional analysis approach for modelling the size of droplets formed by bi-fluid atomisation

• Droplet size varies in a complex way according to the distance to the nozzle.
• Droplet size was modelled by dimensional analysis of the atomisation process.
• Both the atomisation and coalescence mechanisms contribute to the droplet size.
• Air velocity prevail in the spray characteristics.

This paper presents a dimensional analysis (DA) § approach of the atomisation process using a bi-fluid nozzle, allowing to predict droplet sizes of model solutions and skimmed milk concentrates in large ranges of operating conditions. Experimental results confirmed the atomisation mechanism described in the literature, by underlining that the spraying operation is controlled by the coupling of liquid physicochemical properties (viscosity, surface tension, density) and operating conditions (air pressure and liquid flow rate). It was also highlighted that droplet coalescence occurs from a certain distance to the nozzle, counteracting the atomisation mechanism and leading to a reincrease in the droplet size when moving away from the nozzle. Consequently, the modelling of droplet size by DA was improved by adapting the model coefficients of the dimensionless process relationship to the involved mechanisms: either atomisation only close to the nozzle outlet or atomisation followed by droplet coalescence at longer distance to the nozzle.