Flash-boiling and cavitating jets

In cases where a liquid, stored in a high-pressure vessel, flows towards a low-pressure environment, some interesting phenomena might be triggered that can change the morphology of the flow. If the liquid’s local pressure drops below the local saturated vapour pressure, boiling process, generally termed flashing, occurs. This phenomenon has been exploited in clean combustion technologies to improve mixing and combustion processes, and multiple aerosol industrial applications where small droplets in the vicinity of the nozzle exit are desired. Flashing is known to produce fine sprays and like cavitation has an impact on the atomisation and spray dynamics. The phase change is manifested by bubbles forming within the liquid, changing progressively the regime of the flow. The exact time and position of the bubble generation is not a trivial task especially in the presence of turbulence. The flow is strongly dependent on the initial pressure and superheat or subcooling degree. The flow characteristics dominantly affect the subsequent atomisation upstream of the nozzle exit. The regime of the jet varies depending on the local topology including the length of the tubes, size and shape of the orifice. In flashing, extensive bubble nucleation occurs in the liquid. Thus, multiphase CFD codes dealing with flash boiling need to be able to account for the impact of the phase change throughout the liquid
dealing with flash boiling need to be able to account for the impact of the phase change.
cryoFoam incorporparates a novel numerical approach for simulating the atomisation of flashing liquids accounting for the distinct stages, from primary atomisation to secondary break-up to small droplets using the Eulerian-Lagrangian-Spray-Atomisation model coupled with the homogeneous relaxation model. The surface density equation (Σ-equation) is implemented in the code and solved in a fully Eulerian approach for tracking liquid structures of any shape, and computes the spray characteristics. A modified version for the transport equation of the surface density is used and new source terms accounting for the changes in Σ due to evaporation in both dense and dilute spray regions are added. This approach has the advantage of avoiding the unrealistic common assumption of pure liquid rather than a mixture 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.