Bell-cup serrations and their effect on atomization in electrostatic rotating bell atomizers

Abstract

In an Electrostatic Rotating Bell atomizer, fluid is driven from a central orifice over the bell-cup’s surface toward its edge by the centrifugal force. After the fluid is ejected, it breaks-up into droplets a short distance from the bell-cup’s edge due to fluid instability and pressurized air forced normal to the fluid. Images of the bell-cup’s edge were captured as a 20 wt% aqueous glycerol solution was atomized using serrated and non-serrated bell-cups. It was observed that serrations at the bell-cup’s edge dictated the atomization mechanism of the fluid; their presence caused fluid to be ejected as thin ligaments, while their absence caused fluid to be ejected as sheets. The SMD (Sauter mean diameter) of each bell-cup was determined by capturing (and processing) images at a location 7 mm downstream of the bell-cup at several angular frequencies, ω. The SMD decreased as the angular frequency of the bell-cup was increased. The finite difference method was used to solve the Navier–Stokes equation for the velocity, U, and film thickness of a fluid flowing in channels created by serrations just prior to ejection. It was shown that the rate of change of the SMD based on the angular frequency was proportional to that of the film thickness. Furthermore, a stretching rate $\dot{\gamma } = \omega^{2} R/\overline{U}$ was derived for serrated bell-cups based on the stretching of ejected ligaments and it was shown that the SMD $\propto \dot{\gamma }^{ - 0.5}$, where R is the distance from the axis of rotation. The SMD of sprays generated using different bell-cups operating at various angular frequencies were equal when plotted against the stretching rate $\dot{\gamma }$.

Graphic abstract