High-fidelity simulations of a rotary bell atomizer with electrohydrodynamic effects

Abstract

Atomizing flows involve the breakup of a liquid into a spray of droplets. These flows play a vital role in various industrial applications such as spray painting and fuel injection. In particular, these processes can have severe impacts especially in automotive paint shops - which can account for up to 50% of the total costs and 80% of the environmental concerns in an automobile manufacturing facility. A device commonly used for painting vehicles is called an electrostatic rotary bell atomizer (ERBA). ERBAs rotate at high speeds while electrically charging the liquid and operating in a background electric field to direct atomized charged droplets towards the target surface. The atomization process directly influences the transfer efficiency (TE) and surface finish quality. Optimal spray parameters used in industry are often obtained from expensive trial-and-error methods. To overcome these limitations, a computational tool has been developed to simulate three-dimensional near-bell ERBA atomization using a high-fidelity volume-of-fluid transport scheme. Additionally, the solver is equipped with physics modules including centrifugal, Coriolis, electrohydrodynamic (EHD), and shear-thinning viscous force models. The primary objective of this research is to investigate the influence of EHD parameters on near-bell atomization of paint and subsequently improve TE in ERBAs in a cost-effective manner. Using the tools developed, numerical simulations are performed to understand the physics of electrically assisted atomization. The influence of various operating parameters, such as liquid flow rate, bell rotation rate, liquid charge density, and bell electric potential, on atomization is examined. Results from a comparative study indicate that the electric field accelerates breakup processes and enhances secondary atomization. The droplet velocity, local Weber number and charge density statistics are also analyzed to understand the complex physics in electrically assisted breakup. Additionally, the effect of shear-thinning behavior of the liquid on atomization is also explored. High-fidelity simulations allow for the extraction of breakup statistics which are otherwise challenging to obtain experimentally. These findings have the potential to drive improvements in the design and operation of ERBAs, leading to enhanced TE and surface finish quality while reducing costs and environmental concerns in automotive paint shops.