The transport of a flow from a static system into a rotating system can be realized by means of orifices in the rotating wall. In this paper the experimental study of a liquid flowing through a radial, sharp-edged orifice with a l/d-ratio of 1.56 in a rotating shaft is presented. The discharge coefficient cd for (circumferential) orifice speeds of up to 24 m s−1 and Reynolds numbers ranging from Red = 1.2 × 104 to 3.4 × 104 is evaluated for an oil with a density of 920 kg m−3 and a kinematic viscosity of 5.3 × 10−6 m s−2.
A modular test rig was designed, consisting of two concentric rotating shafts forming an annular duct. The outer shaft is fitted with the orifices through which the liquid passes from the static into the rotating system. The modularity allows the exchange of the shaft element containing the orifices. For this study two shaft elements with 5 or 12 radial, cylindrical, sharp-edged orifices were used. Thus, a wider range of flow velocities through a single orifice was achieved.
This study is the first to illustrate the effect of cavitation in a rotating orifice. Outside the cavitation regime a change of the approaching flow represented by the velocity ratio causes a change of the discharge coefficient while within the cavitation regime cd additionally depends on the cavitation parameter. A relationship for the flow contraction in the cavitation regime depending on the orifice velocity and the pressures upstream and downstream of the orifice is derived.
For a second set of orifices, where the liquid exits the rotor into the surrounding air, a significant regime change depending on the ratio of orifice rotational speed and flow velocity occurs. For higher flow velocities through the orifice this change occurs for lower orifice speeds. A likely cause is the onset of cavitation.