【文章內(nèi)容簡介】 d 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 P ou tP ou tPin PvP ou tK ??? ??t Actually, for each stage throttle, the critical conditions for avoiding cavitation in a singlestage throttle should be satisfied. Then u t1??????K scP o u tinP P o u tK sco u tPP inP The pressure distribution inside the medium chamber located between the two throttles should be obtained to calculate cavitation indices for both stages. CFD simulation will be undertaken to explore the pressure distribution of the medium chamber. Mathematical model In view of the axisymmetric structure of the twostage throttle valve, the putational domain is simplified to a twodimensional axisymmetric geometrical model (ABCDE), as shown in Fig. 1(a)。 its grids are generated (Fig. 1(b)) through an advanced grid preprocessor (Gambit of software package FLUENT) [21, 22]. In Fig. 1(a), AB and BC denote fixed wall of seat, ED is fixed wall of poppet, CD the inlet, and AE the outlet. The water is assumed to be inpressible viscous fluid with ρ= kg/m3, and μ =106m2/s。 the effects of pressure and temperature on the density and viscidity of water are neglectable。 all of the walls are assumed adiabatic with a zero slip velocity. The inlet and outlet boundaries can be specified as pressure levels. The RNG k –1 equation model (turbulence kiic energy equation k and dissipation equation 1) and the multiphase model are used in this research. The Boussinesq hypothesis is employed to relate Reynolds stresses to the mean velocity gradients The turbulence kiic energy (k) and its rate of dissipation (1) are obtained from transport equations (6) and (7), respectively In this research, the multiphase model involves liquid and gas phases that are consid