Department of Fluid Control and Automation Harbin Institute of Technology, Harbin, China
Cavitation in the flapper-nozzle pilot stage is an important source of the noise, performance deterioration and even failure of electrohydraulic servo valves. Experimental and numerical investigations of cavitation phenomenon appearing in the flow field between the flapper and nozzle of an electrohydraulic servo valve, as shown in Fig. 1, are carried out. Experimental observations are conducted with a variation of Reynolds number ranging from 630 to 2500 based on the nozzle inlet velocity and diameter. Images of cavitation phenomenon in the flow field are recorded and compared with CFD simulation results to confirm the occurrence and locations of cavitation sources. The computed numerical results show a good agreement with experimental observations. From both types of results, the nozzle inner wall tip, nozzle outer wall tip and flapper leading edge are shown as the locations of cavitation sources. At flow conditions with lower Reynolds numbers, onset cavitation and inception are found at the nozzle outer wall and the flapper leading edge. Further increasing of Reynolds numbers creates a separated flow and then jet flow. Attached cavity is found on the flapper curved surface together with the separated flow and cloud cavitation comes with the jet flow
Fig. 1 - Electrohydraulic servo valve.
To give the understanding of flow field distribution and cavitation phenomenon in the pilot stage of an electrohydraulic servo-valve, an assembly that represents flapper-nozzle pilot stage is produced. It contains a flapper, a nozzle, a house, a front cover and a back cover, as shown in Fig. 2.
To visualize the observed flow phenomenon, a high-speed video camera (PHANTOM-2009B) is used. It is placed 0.5 m in front of the test section and connected to a computer so that the recorded images can be directly displayed.
The circulating hydraulic fluid is MIL-H-5606 provided from CAESTRO. It is supplied to the test section by a hydraulic power source with a pump and other necessary components. In experiments, several flow conditions are created by varying inlet pressure and correspondingly, the inlet velocity of the nozzle in the range of 12.6~50 m/s.
Fig. 2 - Experimental system.
To compute the cavitation phenomena at different inlet velocity, the set of following equations are solved in simulations. Flow equations for a two-phase mixture are obtained from the ensemble averaging of the Navier–Stokes equations. The equations for mixture turbulent kinetic energy and its dissipation rate are obtained by the summation of the mixture as a single phase.
The detailed geometry and meshed model of flapper-nozzle are shown in Fig.3 (a)-(c).
The flapper has flattened surfaces facing to nozzle orifices. The model is discretized by using a systematic mesh to achieve clear information around the flapper and nozzle and reduce computing time.
Fig. 3 - Numerical models.
For comparison, the computational results are taken at mid-plane of the 3D computed domain (section B-B at the plane of Z=1.25mm). To obtain comparable information with experimental results, velocity and vapor fraction distribution are presented, as shown in Fig. 4..
Evolution of flow phenomenon between the flapper and nozzle can be seen from the computational velocity and vapor fraction distribution, as well as the experimental observations with the increasing of inlet velocity and Reynolds number correspondingly.
For the condition shown in Fig. 4, both experimental images and computed results show the cloud cavitation or cavitation shedding along jet flow between the housing wall and the flapper surface. Both of the experimental and numerical results also show that bubble shedding leading to the other side becomes longer and longer with the increase of Reynolds number.
- Cavitation in the pilot stage is confirmed as one significant mechanism for noise, vibration and performance deterioration of electrohydraulic servo valves
- Nozzle tip, flapper curved surface, and turbulent radial jet are main sources of cavitation in the pilot stage
- Attached cavity is formed at the separated flow region on the curved surface of the flapper
- The cloud-like cavitation occurs along the radial jet at higher Reynolds numbers