Di Martino, Massimiliano (2021) Three-dimensional computational fluid dynamics simulation of the hollow-cone spray process. [Tesi di dottorato]

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The characterization of atomization in small-scale applications, such as those typical of consumer goods industry, is not widely investigated, despite of its enormous interest as in the case of sanitation. In this field, the features of the atomizer are selected to achieve a wide spray pattern. This is the case of the pressure-swirl atomizer, where the swirl flow leads the liquid sheet to exhibit a distinctive hollow-cone shape. The configuration of the atomizer and the properties of the multiphase system (liquid-gas) affect the spray morphology and the droplets/ligaments distribution. Aim of the work is to investigate through CFD the stability of the gas-liquid interface produced by a swirling liquid injection at short and long distances from the nozzle outlet. By implementing the VOF and the VOF-to-DPM methods, we show transient simulations in which the liquid-gas interactions and the further propagation of droplets are resolved within and outside the nozzle, simultaneously. Depending on the different liquid properties and geometric features, we examine the hollow-cone spray performance in terms of cone angle and liquid sheet morphology. A stability analysis allows to determine whether spraying or jetting conditions are attained depending on Reynolds and Ohnesorge numbers, as the hollow-cone shape can degenerate into a straight jet under specific operating conditions. Viscosity is known to be a relevant parameter in fluid formulation, which impacts on both relevant dimensionless parameters. Newtonian and non-Newtonian rheologies are here considered for their ubiquitous presence in detergent or sanitation fluids. In both cases, we find a critical condition that marks the switch from spraying to jetting regime. We highlight the relevance of the non-Newtonian liquid properties in the primary atomization of hollow-cone sprays. By increasing the consistency index as well as the shear thinning index, both the liquid sheet sharpness and the aerodynamic interactions on the liquid-gas interface affect the spray performance. The static mesh refinement allows us to track the liquid-gas interface displacement at small distance from the exit orifice. Subsequently, a dynamic mesh adaption is implemented to improve the mesh quality and capture accurately the primary breakup at longer distance from the nozzle exit orifice. In this way, we compare the Newtonian and non-Newtonian primary atomization also in terms of spray pattern and droplet size distribution from the nozzle outlet to a wall placed further from the primary breakup region.

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