Numerical simulations of primary break-up in two-phase flows

Abstract

Liquid-Gas interactions and break-up processes are found in many technological and environmental applications, from Internal Combustion and Gas Turbine engines to food processing and manufacturing. Their complete characterisation at realistic Weber and Reynolds numbers is not possible, due to the vast range of scales integrated and the requirement of a ’minimum’ computational mesh size to capture these scales. To this day, a number of questions remain unanswered, with relative research still ongoing. It is crucial to understand such phenomena so that any technological applications can be optimised and the environmental impact can be reduced. Currently, there is a high need to develop appropriate numerical modelling tools that provide both mass conservation and accurate interface topological properties. Two common interface modelling approaches are the Volume of Fluid and the Level Set, typically coupled into CLSVOF methods to ensure improved surface representation and good mass properties. In this work, a novel in-house Mass Conservative Level Set (CMLS) method is developed and validated extensively. The CMLS novelty is in the Level Set coupling with the Volume of Fluid, being processed only when necessary, providing a faster and more robust approach. Doing so, some numerically imposed limitations due to the ’physics’ and ’stability’, are overcome. The novel CMLS is employed for primary break-up investigations, in a single liquid droplet and jets. Single droplet break-up remains a benchmark test case, as it provides good foundations for liquid jet break-up and spray atomisation modelling. In such processes, the main effective parameters considered are the Weber and Reynolds numbers, along with the Ohneshorge (droplets) and Dynamic Pressure ratio (jets). Contrary to most studies, this work employs the surface density evolution using the Σ − Y model. The droplet break-up cases, show a strong correlation between the break-up initiation time and the Ohneshorge number, whilst as the Weber increases so does the droplet complete break-up time. This is of particular interest as at higher Weber numbers, surface density effects be- come negligible and thus by definition the complete break-up time should in fact decrease. However, similar behaviours were noted in previous studies. The droplets surface density evolution shows a ’quasi-independent’ relationship with the gas Weber. In the jets, a strong correlation between the surface density and ligament formation exists. However, the surface density is ’quasi-independent’ of the liquid Reynolds and the gas Weber. The gas boundary layer presence in jets, shows to both reduce and delay any liquid/gas inter- face perturbations and the potential break-up. To summarise, the present investigations are generally in good agreement with previous studies, with minimal contradictions in cases. The novel CMLS capabilities show promising results both in the two- and three- dimensional space. This work provides good foundations for a slightly alternative research approach in two-phase flows modelling.