Phase Locked PIV Measurements in Wake of an Automotive Fan Model

B3 - Measurement & Test

Particle Image velocimetry (PIV) measurements focus on the flow structure and turbulence in the wake behind an automotive fan model. The experiments have been performed in an optically index-matched visualization facility located at JHU. The fan, its shroud and the housing are made of acrylic, and the liquid is an aqueous solution of 62% NaI that has the same refractive index (1.49) as that of acrylic. Consequently, the fan blades become almost invisible, facilitating measurements using optical techniques at any desired region within the machine. The 30 cm diameter fan was designed by Bosch, and its geometry as well as tip Reynolds number have been matched with that of a fan operating in air, at both RAM and idle conditions. The flow in the closed loop facility is generated by a variable speed, 20 hp pump, while the fan is operated by a 0.5 hp motor. In addition to visualizations, the facility is equipped with a flowmeter, pressure taps across the shroud and a fan torque-meter, i.e. to determine its performance and efficiency. The orientation of the fan blades (phase) is monitored by a shaft encoder that feeds a signal to the PIV system, enabling us to record phase-locked data.
The present paper mostly describes measurements in the wake of this fan. The data has been acquired in nine 50x50 mm sample areas, which are patched together to provide a field of view of 105x125 mm in an axial-radial plane. The vector spacing is 0.4 mm. An overlap between neighboring sections is used for comparison, and accurate patching. Phase locked data has been recorded in the same axial-radial plane for 10 different rotor phase angles, separated equally by 7.2 degrees, covering an entire blade passage. Each of the 90 datasets consists of over 1250 instantaneous velocity distributions. Data analysis includes image enhancement, followed by cross correlations using in-house software, and compares favorably to calculations by commercial software (DaVis) implementing velocity gradient-based deformable windows. Phase averaged mean velocity and turbulence distributions reveal many features of the near wake. They capture three generations of blade wakes and tip vortices, providing a comprehensive view of the evolution of flow in the rotor wake. Included are e.g., the very turbulent tip leakage vortex that expands and diffuses with axial distance, the rotor wake with elevated turbulence and velocity deficit, and the high turbulence in the separated region behind the hub of the fan.