Smoothed Particle Hydrodynamics Simulation of an Air-Assisted Atomizer Operating at High Pressure: Influence of Non-Newtonian Effects

Abstract

A twin-fluid atomizer configuration is predicted by means of the two-dimensional (2D) weakly compressible smooth particle hydrodynamics (SPH) method and compared to experiments. The setup consists of an axial liquid jet surrounded by a high-speed air stream (Ug ≈ 60 m/s) in a pressurized reactor, which is operated at up to 11 bar. Two types of liquid are investigated: a viscous Newtonian liquid (μl = 200 mPa·s) consisting of glycerol/water mixture and a viscous non-Newtonian liquid (μ1,apparent. ≈ 150 mPa·s), which is a carboxymethyl cellulose solution. Three-dimensional (3D) effects are taken into account in the 2D code by introducing: (i) a surface tension term, (ii) a cylindrical viscosity operator, and (iii) a modified velocity accounting for the divergence of the volume in the radial direction. The numerical results at high pressure show a good qualitative agreement with experiment, i.e., a correct transition of the different atomization regimes with regard to pressure, and similar dynamics and length scales of the generated ligaments. The propagation velocity of the Kelvin–Helmholtz (KH) instability is well predicted, but its frequency needs a correction factor to be globally well recovered for the Newtonian liquid. The Sauter mean diameter (SMD), calculated from the spray size distribution, shows similar trends of the reactor pressure dependency. The simulation of the non-Newtonian liquid at high pressure shows the same breakup regime with finer droplets compared to Newtonian liquids, and the simulation at atmospheric pressure shows an apparent viscosity similar to the experiment.