Numerical modelling of superheated jet atomisation

Lyras, Konstantinos (2018) Numerical modelling of superheated jet atomisation. (PhD thesis), Kingston University, .


The aim of this research project is to provide the academic and industrial community with a numerical tool that can be used for describing extreme flow cavitation scenarios and the atomisation process of these multiphase jets in a low-pressure environment. The research lies in the intersection of Numerical Analysis, Applied Physics and programming. From the physical point of view, the project has two different strands: The first is developing a methodology for channel flows due to a rapid pressure drop which is possible to result into various flow regimes inside the channel. The second step is to track the liquid fragmentation of the liquid jet downstream the channel exit and describing the atomisation process to liquid ligaments and blobs to droplets. Using a fully Eulerian approach, this research aims towards a holistic approach that addresses some of the major challenges that govern superheated jets atomisation. The finite volumes method in a compressible framework is used utilising various models for modelling the underpinning physics of flashing jets. Flashing occurs either if a liquid follows an isothermal depressurisation or isobaric heating. In both cases, the fluid fails to adjust to the local changes in pressure and temperature admitting a metastable state which makes the process more challenging to understand. The Homogeneous-Relaxation-Model (HRM) is used for modelling the heat transfer under sudden depressurisation conditions accounting for the non-equilibrium vapour generation. A new pressure equation is proposed which employs the continuity equation indirectly. The pressure responds to compressibility and density changes due to the rapid phase change and includes the surface tension contribution in the pressure-velocity coupling algorithm. The coupling of the continuity and momentum equation with the HRM and the interface tracking method is thoroughly described. The result of this coupling is a conserved numerical method that is capable of characterising the flow regimes and the impact of bubble nucleation on the mass flow rate. The present study presents a numerical approach for simulating the atomisation of flashing liquids accounting for the distinct stages, from primary atomisation to secondary break-up to small droplets Following the Eulerian-Lagrangian-Spray-Atomisation approach, the concept of the surface density Σ is introduced into the methodology for the spray dynamics. The proposed approach has the advantage of avoiding the unrealistic common assumption of pure liquid at the nozzle exit. It models the change in the regime inside the nozzle treating flashing in a unified approach simulating the metastable jet both inside and outside the nozzle. Important mechanisms such as thermal non-equilibrium, aerodynamic break-up, droplet collisions and evaporation are modelled in a novel atomisation model. The modified Σ- equation employed a new source term proposed for cryogenic jets. A wide range of numerical tests is presented for validation and obtaining insights for the underlying physics. Short and long nozzle geometries are tested for both low and high-pressure releases for flashing water, R134A, liquid nitrogen and LNG. Results for turbulent flows for both sub-cooled and superheated liquids are presented showing that the proposed approach can accurately simulate the primary atomisation.

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