Shaping Air Flow Characteristics of a High-Speed Rotary-Bell Sprayer for Automotive Painting Processes

Abstract

International audienceElectrostatic rotary bell sprayers (ERBS) are widely used in automotive painting applications. These processes involve complex airflows to shape paint sprays and transport droplets toward automotive parts to be coated. Despite the importance of shaping airflow on global spray characteristics, a detailed characterization of this aerodynamic flow is still missing. For this purpose, an experimental study was conducted on the influence of some ERBS operating parameters on the development and characteristics of shaping airflow. Results show that, for low swirl numbers, the flow behavior is close to that of annular swirling jets and a good agreement is found between ERBS flow characteristics and data available in literature. When rotational speed of the bell cup is sufficiently fast, a change of regime is observed with a shift in the longitudinal flow development and an increase of recirculation zone length. This change of regime is attributed to vortex breakdown instability, known to occur when high swirl strength is beyond a critical value. Experimental results obtained in this study put forward a clear link between the shaping air flow rate and the rotation frequency on the aerodynamics and also provide valuable leads to design shaping air flow in modern ERBS

Schlieren Visualization of Shaping Air during Operation of an Electrostatic Rotary Bell Sprayer: Impact of Shaping Air on Droplet Atomization and Transport

“…The IRFV data for 20,000 and 50,000 RPM in Figure 10 show even less radial divergence of the liquid spray at 250 LPM SA air flow rate in comparison to when 150 LPM SA was used; this result is in agreement with the trend of decreasing pattern angles as the SA flow rates were increased. Furthermore, air entrainment regions showing recirculation mentioned by Stevenin et al [22] can be seen as red portions of low liquid concentrations in the high rotational speed, high SA flow case. These regions of high mixing contribute further to the atomization of the droplets in the secondary atomization region.…”

“…The high-speed rotary bell atomizer was operated without liquid flow at three different rotational speeds of 0, 20,000, and 50,000 RPM while using a SA flow rate of 50 LPM (Reynolds = 1.7 × 10 3 calculated based on velocity in the longitudinal direction [22]). Schlieren videos were captured at 4400 FPS; images left to right shown in Figure 3a are snapshots taken from videos S1, S2, and S3 respectively; which are uploaded as supplementary material.…”

“…Wilson et al [2] applied shadowgraph high-speed imaging to visualize the edge of a rotating cup and droplet formation, while infrared thermography has been used to qualitatively assess effects of rotational velocities and paint flow rates on spray shapes [21]. Despite these research efforts, the aerodynamics of SA remains unknown [22], and precise interactions between droplets and the SA are not understood.In this research, Schlieren imaging was used as the method for visualizing SA flows. The degree of the Schlieren effect is proportional to the first spatial derivative of the refractive index of the visualized gas [23].…”

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

“…In order to investigate the influence of the swirl generation, two swirl-numbers are defined in this study. First of all, the swirl-number from the rotating bell-cup is examined, whereby the swirl-number Scup is defined according to Stevenin et al [1]. The swirl-number Scup is defined as the ratio of the bell edge tangential velocity Utan,cup to a characteristic value of mean velocity in the axial direction Uax,mean [1].…”

“…First of all, the swirl-number from the rotating bell-cup is examined, whereby the swirl-number Scup is defined according to Stevenin et al [1]. The swirl-number Scup is defined as the ratio of the bell edge tangential velocity Utan,cup to a characteristic value of mean velocity in the axial direction Uax,mean [1]. Based on the results from numerical study, the characteristic mean velocity in axial direction was calculated over an area-weighted average in a annular plane at z = 0 with an approximated flow thickness of 1 mm at the bell edge.…”