Research On Flow Characteristics Of Typical Throttles In Water Hydraulic Servo Valve

Water hydraulics operated by fresh water or seawater instead of mineral oil is a hydraulic technology, which has become more and more attractive owing to the great advantages such as environmental friendliness, non-flammability, cheapness, cleanliness and easy disposal. However, the applications of water hydraulics should be based on the outstanding water components. The flow characteristics of throttles have great effects on the performances of water hydraulic components, and are the bases of developing them. By comparison with oil medium, the flow of water hydraulic throttle is of high Reynolds Number and cavitation is inevitable to occur. Therefore, some important design parameters in oil hydraulics, such as flow coefficient, cannot be utilized directly in the design of water hydraulic components. As the key element of water hydraulic servo system, water hydraulic servo valve includes a series of throttles, such as flapper-nozzle valve, annular gap damper, spool valve. In order to develop sound water hydraulic servo valve and other relevant water control components, the flow characteristics of such typical throttles, including flow-pressure characteriscs and cavitation phenomenon, should be investigated. The flow characteristics of the typical throttles and hydraulic half bridge consisted of a fixed orifice and a flapper-nozzle, are numerically studied using RNG k ?εturbulent model combined with mass transfer equation, volume fraction equation and two-layer zonal model through commercial CFD software package FLUENT. The distribution figures of velocity, pressure, steam line and vapor volume in the whole flow field are obtained, and great emphases are given to the shrinkage and attachment to walls of stream line. In addition, the distribution zone and intensity of cavitation are also predicted and analyzed. The simulations results show that: (1) the increase of clearance between flapper and nozzle can restrain cavitation; (2) the augment of overlap between spool and sleeve can decrease the angle of incidence, and the inlet chamfer can lead to the occurrence of cavitation for the annular gap damper; (3) the cavitation becomes more serious with the increase of opening. However, the inlet bevel can restrain cavitation for the spool valve; (4) the back pressure of flapper-nozzle valve combined with the aptotic orifice can restrain cavitaion apparently. While the experimental system is technologically reconstructed, a series of coupled elements, including flapper-nozzle, annular gap damper and spool valve, are designed and manufactured. Under the conditions of various throttle clearances, inlet and outlet pressure levels, experimental test are conducted to obtain the flow characteristics of these typical throttles. And emphasis is exerted to study the change trend of flow coefficient. The experimental results exhibit that: (1) the decrease of orifice diameter can be beneficial to the stability of flow coefficient for the flapper-nozzle throttle; (2) the chamfer of spool can lead to the increase of flow coefficient for the annular gap damper; (3) the increase of the opening of the spool valve can result in the augment of the flow coefficient. These conclusions are roughly consistent with those corresponding simulation results.