Reduction of undesired lateral forces acting on the flapper of a flapper–nozzle pilot valve by using an innovative flapper shape

Highlights: • The simulated flow rate and main flow force show a good agreement with experiments. • The innovative flapper has little influence on the flow-pressure characteristics. • The innovative flapper can greatly reduce the Y direction force on the upper part. • The innovative flapper reduces both the X and Z direction forces on the lower part. - Abstract: The stability and dynamic performance of a flapper–nozzle pilot valve significantly depend on the flow forces acting on the flapper. Due to the shape of the flapper and flow structure in the flapper–nozzle pilot valve there are undesired lateral forces acting on the flapper, which are very potential to interfere with the stability of the flapper. Aiming to reduce these undesired lateral forces, an innovative flapper shape is proposed and a comparative study of flow forces acting on the two different flapper shapes is conducted. A simple rectangle shape is selected as the innovative flapper shape. The flow forces acting on the traditional flapper shape and innovative flapper shape are evaluated by means of CFD (Computational Fluid Dynamics) simulations and verified with the results from the semi-experimental approach. The evaluation of the flow forces is performed for each flapper shape with two different flapper–nozzle clearances of 0.10 mm and 0.05 mm under seven different flow conditions with the variation of inlet pressures from 1 MPa to 7 MPa. A good agreement between CFD results and semi-experimental results shows that the proposed innovative flapper shape has no effect on flow control characteristics since it is giving approximately the same flow rate and main flow force as the traditional flapper shape at every flow condition. Meanwhile the innovative flapper shape effectively reduces the undesired lateral forces acting on the flapper by altering the flow structure and reducing the strength of the jet flow and cavitation occurred in the flow field of flapper–nozzle pilot valve. At the lower part of the flapper with clearance 0.05 mm, the ratio between the X-direction lateral force and main flow force of traditional flapper is around 1.24–11.14%, while it is reduced to 0.18–0.42% by the innovative flapper. Also, the ratio is reduced from 7.93–18.44% to 0.69–0.93% with clearance 0.10 mm. For the Z-direction forces at the lower part, the ratio decreases from 0.20–11.77% and 7.84–17.94% (traditional flapper) to 0.92–2.65% and 1.63–4.08% (innovative flapper) with clearances 0.05 mm and 0.10 mm respectively.