Numerical analysis of regime stability of a water mist pressure swirl atomizer

The two-phase internal flow in water mist injectors is predicted by 3D numerical simulations, where the gas/liquid interface is captured by the volume of fluid methodology and the effect of turbulence is accounted for by the large eddy simulation approach, assuming incompressible flow under isothermal conditions. The numerical model is validated against experimental data for two large-scale pressure swirl atomizers from the literature, showing a satisfactory agreement. The flow development in real size atomizers is studied under a wide range of swirling conditions, defined by different injection pressures and swirling channel inclinations. Three internal flow regimes are identified, depending on the behavior of the air core at the discharge hole: an "open" regime, where a stable air core is formed inside the nozzle inducing the so-called hollow-cone liquid jet, a "closed" regime, where the discharge hole is fully occupied by the liquid, and an "unstable" regime characterized by a random switching between the open and closed regimes. The mass flow rate, the pressure field, the liquid phase fraction, and the flux of angular momentum are monitored along the nozzle, and a relation between the swirl number, defined as a nondimensionalization of the flux of angular momentum, at nozzle exit and the air core diameter is found. The analysis of the conditions causing the transition between the open and closed regimes and vice versa suggests that there exists a threshold value of the swirling number at the nozzle exit that defines the working regimes independently on the geometric and operating conditions.