Effect of taper, pressure and temperature on cavitation extent and dynamics in micro-channels

Abstract

The prevention of erosion caused by hydrodynamic cavitation plays an important role in the design of hydraulic components, such as fuel injection systems. A high pressure level and the corresponding flow speeds at a very small scale of several micrometers limit the optical accessibility in real fuel injectors. For this reason, flat micro-channels of different geometry, covered by sapphire windows at the front and the back side are used in this paper. Calibration fluid ISO 4113 and dodecane serve as flow media. The aim is to investigate the influence of pressure, temperature and the geometry of the micro-channels on the cavitation extent and to resolve the cavitation dynamics such as cloud shedding. Detached clouds will collapse if the static pressure exceeds the vapor pressure and may damage channel walls of hydraulic components. Shadowgraph images visualize the cavitation extent and flow instabilities, Computational Fluid Dynamics (CFD) results reveal detailed pressure and velocity distributions. Particle Image Velocimetry (PIV) measurements show the motion of the cavitating structures. With high-speed (HS) visualization, cloud detachment is investigated.It was found that the normalized cavitation length can be described as an S-shaped curve depending on the normalized cavitation number. The cavitation length significantly increases in the parallel and diverging channels when reaching the cavitation transition point. The optical setup reveals a deeper insight into the cavitation inception in the shear layer near the throttle inlet. Kelvin–Helmholtz vortices downstream the shear layers could be observed. The calculated pressure and velocity distributions help to validate the plausibility of the cavitation inception for different tapers.For a deeper understanding of the overall flow conditions, basic velocity measurements of the phase boundary in the diverging part of a converging-diverging flow channel were carried out. No seeding particles are used as cavitation structures serve as tracers for the image evaluation. It was found that the velocity of phase boundary between liquid and vapor is lower than the mean fluid velocity derived from the measured mass flow because of the unstable behavior of cloud shedding. A recirculation flow and eddy formation at the cavitation tip is visible.With the help of high-speed visualization, the re-entrant motion which occurs in connection with cloud shedding could be observed. The investigation of the shedding frequency reveals its dependency of pressure, temperature and geometry.