Analysis of Three-Dimensional Flow Field Inside the Pilot Stage of the Deflector Jet Servo Valve

Current two-dimensional flow field model has some defects in describing the pilot stage’s flow field and static characteristics of the deflector jet servo valve(DJSV) because the three-dimensional(3D) jet of the flow field is ignored. In order to overcome the shortcomings, a new flow field model is proposed and the energy transfer process of the pilot stage is obtained. In this model, the flow field is divided into five regions: pressure jet region, free jet region, mixed collision region, secondary jet region and pressure recovery region. Especially, three-dimensional turbulent jet is adopted in the free jet region for the first time to describe the structure of the flow field, and the jet entrainment model is proposed in pressure recovery region to describe the coupling relationship between the pressure in the receiving chamber and the jet flow which has never been considered before. The static characteristics of the pilot stage, such as pressure-flow characteristics, pressure characteristics and flow characteristics are obtained, and the relationship between zero-position valve coefficients and the key structural parameters of the pilot stage is analyzed. The results show that main structural parameters that affect the pressure gain include the length of receiving chamber, the width of guide groove outlet and the width of the wedge; The thickness of jet-pan has the most significant influence on flow gain. The flow field structure and the static characteristics are verified respectively by finite element analysis(FEA) and experimental results, and the results show that the pilot stage mathematical model has good reliability which is beneficial to understand the working mechanism of the pilot stage provide theoretical basis for parameter optimization.

Model Construction and Performance Degradation Characteristics of a Deflector Jet Pressure Servo Valve under the Condition of Oil Contamination

The deflector jet pressure servo valve is a kind of high-precision hydraulic component that can be widely used in the antiskid braking system of an aircraft. In actual service, it will be faced with extreme working conditions of gradual oil contamination, which will cause performance degradation and function maladjustment of the whole valve. To this end, the paper proposes a performance degradation characteristic analysis method. In which, firstly, the structural characteristics and working principle of the deflector jet pressure valve are analyzed; then, the entire dynamics model of the pressure valve is built using the braking cavity as the load blind cavity. Secondly, the two main failure modes induced by oil contamination such as erosion wear of pilot stage and stuck of slide valve stage’s valve core are determined based on the engineering experience, aimed at which the failure mechanism is analyzed; then, the sensitivity simulation model of the servo valve’s output pressure with respect to key degradation parameters is established and the sensitivity analysis is performed. Finally, combining the theoretical analysis with multiphysics simulation correction methods, the performance degradation model of the typical failure modes are established, and then, the performance degradation characteristics under dynamic contamination conditions are analyzed, which is combined with the failure threshold determined by the dynamics simulation to finish the service life prediction of the deflector jet servo valve.

Numerical Investigation of Erosion Wear in the Hydraulic Amplifier of the Deflector Jet Servo Valve

Although possessing a remarkable anti-contamination capacity, the deflector jet valve is still confronted with erosion wear brought by solid particles in hydraulic oil. To describe the erosion wear mechanism in the hydraulic amplifier of servo valves, a RANS-based CFD simulation is conducted to obtain its internal wear distribution following the Oka erosion model, which shows the erosion wear in servo valves can be divided into four levels and the major wear happens on the shunt wedge, characterized by a regular and fluctuant distribution. Further, there exist multiple relative maximums of erosion rates, whose locations deviate from the jet center. On this basis, the correlation between the contamination level of hydraulic oil and the degree of erosion wear is established. Moreover, according to the working mechanism of the hydraulic amplifier, a new failure criterion of the deflector jet valve is proposed to carry out valve lifespan analyses. Then, a lifespan prediction formula is obtained, and calculations show that hydraulic oil must have a contamination level superior to NAS 5 if a lifespan of over 20,000 h is expected.