Confirmation on the effectiveness of rectangle-shaped flapper in reducing cavitation in flapper–nozzle pilot valve

Abstract The existence of undesired flow-induced phenomenon, cavitation, in the flapper–nozzle pilot valve of two-stage servo-valves is a critical issue in practical applications. Here, taking innovation on the flapper shape is one of possible approaches to reduce cavitation in the pilot valve. By means of CFD (Computational Fluid Dynamics) simulations, it has been proved by setting a simple rectangle shape as an innovative flapper shape in our previous attempt. Therefore, in this work, the effectiveness of rectangle-shaped flapper in reducing cavitation is experimentally confirmed by comparing with traditional shape and square shape. The experimental observations of cavitation phenomena in three different flapper shapes are conducted for two different flapper–nozzle null clearances (0.2 mm and 0.1 mm) under four different flow conditions with the variation of inlet pressure in the range of 3–6 MPa. To provide verification on experimental results and a comprehensive understanding, CFD simulations of cavitation phenomenon in each flapper shape are also performed. The results are qualitatively analyzed and compared. The results explain that the cavitation intensity in the flapper–nozzle pilot valve associates with the strength of the turbulent jets and it increases with the increment of flapper–nozzle null clearance and inlet pressure. According to the experimental observations and CFD simulated results, the curved surface of traditional flapper shape is attributed to the spread of turbulent jets and consequent massive cavitation. Compared to traditional shape, the square shape relatively reduces cavitation due to lack of curved boundary on the flapper. However, on the other hand, its shorter flat land and larger annulus are not much effective to control the spread of turbulent jet which is responsible for cavitation in annulus region. Compared to two other flapper shapes, the rectangle shape significantly suppresses the cavitation by attenuating the turbulent jets on its straight and relatively longer flat lands. Therefore, the effectiveness of rectangle shape in reducing cavitation in the flapper–nozzle pilot valve is confirmed in this work.