Numerical and Experimental Investigations on Rotary Bell Atomizers with predominant Air Flow Rates

For high-quality spray painting of small parts, a rotary bell atomizer with a narrow spray pattern is used in the automotive industry. The required unusual high shaping air flow rate yields in an atomization process predominated by a pneumatic atomization and rather than by a rotary atomization, called hybrid bell atomizer in this article. Numerical and experimental investigation on typical high-speed rotary bell atomizers, with rotation type of high rotational speed 40000-60000 rpm of the bell, were already successful demonstrated. For these high-speed rotary bell atomizer for painting bigger areas the ratio between tangential velocity at the bell edge and axial shaping air velocity at the bell edge is in the range of 0.8 and 4, depending on the process parameter. At the hybrid bell atomizer (10000-20000 rpm), this ratio is between 0.2 and 0.4. The first step of the present study includes the theoretical characterization of spray cone velocity profile using two definitions of swirl-number compared to experimental measurements of particle velocities using Laser-Doppler-Velocimetry (LDV). This study was carried out on varying shaping air settings and rotational speeds. The results show that the the swirl of the main airflow field is dominated by the secondary airflow, which is induced coaxial in an angle of 45 degrees. The influence of the circumferential speed of the bell cup on the swirl of the main airflow field plays a subordinate role, so the resulting spray pattern is only weakly influenced by the number of revolutions of the bell-cup. In the second step, the hybrid bell atomizer was examined numerically. In order to implement the hybrid atomization concept in the simulation correctly, methods for creating droplet initial conditions in the trajectory calculation was developed. The simulation results were verified through comparisons of calculated and measured velocity profiles inside the spray cone and calculated and measured film thickness distributions on the work piece. In the present investigations of the atomizer, it has been demonstrated numerically and experimentally that the airflow field of this hybrid bell atomizer is strongly impacted by the secondary shaping air and both the circumferential speed of the bell cup and the direct electrostatic charge on the bell have only a minor effect on the generated spray pattern and the resulting transfer efficiency.