Abstract

Atomization is a process where the bulk of liquid jet disintegrates into liquid sheets, ligaments and droplets. It has enormous applications in industries and processes such as combustion, heat transfer systems, transport, biological systems and particularly our interest, coating processes. The Effervescent nozzle is a type of twin-fluid atomizer and has shown a superior performance in handling and spraying different liquids without any clogging issues; which is particularly interesting in thermal spray. In spite of significant number of experimental works, a few numerical works have been carried out. That makes it crucial to conduct a comprehensive numerical study on Effervescent atomizer.
The complex internal and external behaviors of Effervescent atomizer are governing the behavior of the flow. The latter is a turbulent and compressible multiphase flow. It is studied numerically by employing a three-dimensional compressible Eulerian method along with Volume of Fluid (VOF) surface-tracking method coupled with the Large Eddy Simulation (LES) turbulence model. The numerical study is conducted by using OpenFoam library, an open-source package introduced by Open-CFD.
In this study, the effect of varying the gas to liquid ratio (GLR) and the suspension (i.e. effect of viscosity, density and surface tension), on the structure of internal flow and consequently, the external flow is studied numerically. It is observed that the increase in GLR is accompanied with an evolution of the internal flow from a complex bubbly flow to an annular flow. This reduces the liquid film thickness at the discharge orifice. Further studies on internal pressure illustrated the critical condition, choked flow and pressure oscillations at the discharge orifice. The examination of increasing the GLR and evolving of internal flow resulted in changing in primary atomization parameters such as shortening the breakup length and widening the spray cone angle. Furthermore, the existence of a slip velocity between the two phases in the external flow results in dominant aerodynamic forces at high GLRs. Moreover, alternation of the liquid properties illustrated the higher spray velocity and wider cone angle of the spray, which demonstrates the superior performance of the Effervescent atomizer.