【正文】 valve. The correlation between the critical cavitation index and the passage area ratio of the two throttles is investigated. Relevant validation experiments are conducted at a custommanufactured testing apparatus. The experimental results are consistent with the simulated ones. Further analyses indicate that (a) the large backpressure can improve not only the anticavitation capability but also the total load rigidity of the water hydraulic twostage throttle valve, (b) an appropriate passage area ratio will be beneficial for improving the anticavitation capability of the water hydraulic twostage throttle valve, and (c) the water hydraulic twostage throttle valve with a passage area ratio of would have the best anticavitation performance with the lowest risk of cavitation. Keywords: putational fluid dynamics simulation cavitation load rigidity passage area ratio twostage throttle valve water hydraulics 1 INTRODUCTION Water hydraulic systems are operated with raw water (pure tap water) substituting for mineral oil. They have advantages in terms of durability, reliability, safety, and cleanness. Such sys tems are being more and more popular, especially in fields of steel and glass production, coal and gold mining, food and medicine processing, nuclear power generation, ocean exploration, and underwater robotics [1–5]. Because the opening of a water hydraulic control valve is very small pared with that of oil valve, the water flow velocity through the water hydraulic control valve is larger under the same pressure condition。 they concluded that the twostage throttle had stronger anticavitation capability than the singlestage one, and that the shape of seats also affected the anticavitation capability of the throttles. The cavitation choking appeared only when cavitation index was less than . They also conducted several studies on the relationships between the cavitation and the configuration of control valve, and between the cavitation and the passage area ratio of two throttles. However, the pressure between two throttles was assumed to be constant, which may not be applicable for the twostage throttle with a medium chamber. As an extension of Liu et al. [15, 16], this research will focus on the investigation of cavitation characteristics in the water hydraulic twostage throttle poppet valve. This objective entails the following research tasks: (a) on the basis of the RNG k – 1 turbulent model, numerical simulations will be conducted to investigate the pressure distribution inside the medium chamber located between the two throttles, (b) the effect of the passage area ratio of the two throttles and pressure between the two throttles on the anticavitation capability will be studied, (c) the design criteria of the twostage throttle valve will be established for lowcavitation and noise, and (d) experimental validation will be performed through a custommanufactured testing apparatus, to demonstrate applicability of the developed simulation approach and design criteria. 2 SIMULATION OF PRESSURE BETWEEN TWO THROTTLES Statement of problems If water pressure decreases below the saturation vapour pressure pv (absolute pressure) of water under a given temperature, the vapour or gas will spill out of the water, and then cavitation will occur. For water, pv . MPa when the temperature is 20 8C. Generally, the likelihood of the cavitation for the throttle valve can be measured by a cavitation index (K ), which can be expressed as follows P o u tP i nP o u tK ??? Pi nPvPo u t The critical cavitation index (Kc) is the minimum cavitation index for the throttle under noncavitating flow in throttle valve. For a singlestage throttle valve, Ksc . [14, 17–20]. It denotes that, if the cavitation index K is less than , the cavitation will occur. Therefore, the cavitation index (K ) should be larger than so as to avoid cavitation in the throttle valve. The larger the pressure drop across the throttle valve, the smaller the cavitation index (and thus greater probability of cavitation for the throttle valve). The twostage throttle valve is a practicable configuration to mitigate cavitation, being extensively used in water hydraulic pressure relief valves and throttle valves. Figure 1(a) is a scheme of water hydraulic twostage throttle valve, which consists of a poppet and a seat with a step shape bore. There are two tandem throttles to take the total pressure drop (pin pout). Therefore, the pressure drop across each stage throttle is less than the total drop of the valve, which is differed from a singlestage throttle. Hence, the cavitation index of each stage throttle should be larger than that of the singlestage throttle valve, leading to a reduced possibility of cavitation and erosion. For the twostage throttle valve, the definition of cavitation index Kt can be analogously expressed as follows P ou tPin