DNS characterisation and LES modelling of the primary and secondary breakup of structure-viscous liquid jets

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If a liquid jet is ejected from a round nozzle, the jet disintegrates. Jet breakup describes the process of disintegration of the liquid volume into a complex system of droplets. The primary jet breakup, occurring in the area close to the nozzle, represents an important and difficult to model sub-process and is followed by the secondary breakup. In this process, the dynamics of already formed droplets play an important role. Fluid atomisation is a fundamental technical process with applications as diverse as fuel injection in combustion systems, application of insecticides, medical sprays, reactor cooling, painting of components, spray drying, firefighting and much more. Non-Newtonian fluids are used in many applications (e.g. in painting or medical sprays). While the properties of these fluids are time-independent in simpler cases, e.g. shear-thinning fluids, in more complex cases, such as viscoelastic fluids, there is an additional dependence on time. In the context of this proposal, fundamental, detailed numerical investigations of the jet break-up of liquid jets with a non-Newtonian behavior are carried out. This work is dedicated to both shear-thinning and thixotropic liquids. The numerical investigations are divided into Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES). For the description of the multiphase flow the Volume of Fluid (VOF) method is used. With the help of DNS calculations, basic break-up mechanisms in the liquid jet are investigated in detail. The relevant influences e.g. viscosity on the jet morphology and the breakup process are identified, described and quantified. From this, models required for the LES are derived and integrated into the LES code. In the LES, single sub-grid droplets are treated as Lagrange points. With the help of the LES, the primary breakup, but also the secondary breakup of the liquid jet, can be investigated. Thus, a methodology is developed to simulate the jet breakup processes with non-Newtonian properties and evaporation, as it occurs e.g. during spray drying. The conditions under which evaporation changes the processes during jet breakup are also investigated in detail.This proposal is implemented by two applicants (Munich/Stuttgart) who complement each other very well with regard to the numerical methods used (DNS/LES). Thus, it is possible to investigate the primary as well as the secondary jet breakup of non-Newtonian liquid jets in detail, to model it using a multiscale LES approach and to further understand these complex processes.