关于二级液压节流锥阀的低汽蚀研究论文外文翻译-中英文文献对照翻译

中文共4400字第一部位外文原文标题：关于二级液压节流锥阀的低汽蚀研究Research on low cavitation in water hydraulic two-stagethrottle poppet valveAbstract: Cavitation has important effects on the performances and lifespan of water hydraulic control valve, such as degradingefficiency, intense noise, and severe vibration. Two-stage throttle valve is a practicable configuration to mitigate cavitation, which is extensively used in water hydraulic pressurerelief valves and throttle valves. The pressure distribution inside a medium chamber located between two throttles of a two-stage throttle valve is investigated through numerical simulations. The effects of the passage area ratio of the two throttles and the inlet and outlet pressures on the pressure inside the medium chamber are examined. The simulation results indicate that (a) the pressureinside the medium chamber is not constant, (b) the locations ofmaximum and minimum pressures inside the medium chamber are both fixed, which will not vary with the passage area ratio or the inlet and outlet pressures, and (c) the ratio of the pressure drop acrossthe front throttle to the total pressure drop across the two-stage throttle valve is nearly constant. The critical cavitation indexof the two-stage throttle valve is then established. A semiempirical design criterion is obtained for the water hydraulic two-stage throttle valve. The correlation between the critical cavitation index and the passage area ratio of the two throttlesis investigated. Relevant validation experiments are conducted ata custom-manufactured testing apparatus. The experimental results are consistent with the simulated ones. Further analyses indicatethat (a) the large backpressure can improve not only the anti-cavitation capability but also the total load rigidity of the water hydraulic two-stage throttle valve, (b) an appropriate passage area ratio will be beneficial for improving the anti-cavitation capability of the water hydraulic two-stage throttle valve, and (c) the water hydraulic two-stage throttlevalve with a passage area ratio of 0.6 would have the best anti-cavitation performance with the lowest risk of cavitation.Keywords: computational fluid dynamics simulation cavitation load rigidity passage area ratio two-stage throttle valve water hydraulics1 INTRODUCTIONWater hydraulic systems are operated with raw water (pure tapwater) substituting for mineral oil. They have advantages in terms of durability, reliability, safety, and cleanness. Such systems are becoming more and more popular, especially in fields of steel andglass 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 compared with that of oil valve, the water flow velocitythrough the water hydraulic control valve is larger under the same pressure condition; thus cavitation erosion may occur due to thehigh vapour pressure of water. Cavitation has an important effecton the performance and lifespan of water hydraulic control valve,such as degrading efficiency, intense noise, and severe vibration. Previously, a number of studies on the relationships between cavitation and discharge coefficient, thrust force and pressuredistributions in the valves, and mitigation of cavitation damagewere carried out through increasing outlet pressure of valves,modifying shapes of throttles, adding stages of valves, selectinganti-corrosion materials for parts, and controlling maximum fluid temperature and flow velocity. Tsukiji et al. [6] investigatedcavitation by flow visualization in hydraulic poppet-type holdingvalves to reduce cavitation. Some measures were employed to diminish noise and cavitation through controlling flowrate, upstream pressure, downstream pressure, and valve lift in case ofconverging flow.Aoyama et al. [7] studied experimentally the unsteady cavitation performance in an oil hydraulic poppet valve. It wasfound that, as absolute values of the variation rates of inlet and outlet pressure increased, the incipient cavitation index exhibited a tendency to decrease, whereas the final cavitationindex a tendency to increase under all geometrical parameters. Asthe absolute values of the variation rates of inlet and outletpressure further increased, the hysteresis between the incipientand final cavitation indices would become larger than ever for each combination of the valve and valve seat.Ishihara et al. [8] studied oil flow unsteadiness effect oncavitation phenomena a t sharp-edged orifices. The rate of pressure drop across the orifice was kept constant, and cavitation incipience and finale were recorded by using scattered laser beams showing that (a) there existed two types of cavitation, namely,gaseous cavitation and vapourous cavitation, and (b) cavitationincipience and finale varied with the initial condition, the temperature of hydraulic oil, and the rate of pressure drop.Johnston et al. [9] carried out an experimental investigationof flow and force characteristics of hydraulic poppet and discvalves using water as the working fluid. The axisymmetric valvehousing was constructed from clear perspex to facilitate flow visualization; tests were performed on a range of different poppet and disc valves operating under steady and non-cavitating conditions, for Reynolds numbers greater than 2500. Measured flow coefficients and force characteristics showed obvious differences depending on valve geometry and opening.Vaughan et al. [10] conducted computational fluid dynamics (CFD) analysis on flow through poppet valves. Simulations were comparedwith experimental measurements and visualized flow patterns. Aqualitative agreement between simulated and visualized flow patterns was identified. However, errors in the prediction of jetseparation and reattachment resulted in quantitative inaccuracies. These errors were due to the limitations of the upwind differencing scheme employed and the representation of turbulence by the k –1 model, which was known to be inaccurate when applied to recirculating flow.Ueno et al. [11] investigated experimentally and numericallythe oil flow in a pressure control valve under an assumption ofnon-cavitating conditions for various configurations of the valveson the basics of a finite difference method. They concluded thatthe main noise of the testing valves was generated from cavitation, and the noise was affected by the valve configuration. Pressure measurements and flow visualization at two locations in a valvechamber were also performed on the basis of two-dimensional models. Through comparisons of the measured and calculated results, several design criteria were set up for low-noise valves.Martin et al. [12] investigated cavitation in spool valves inorder to identify damage mechanisms of the related components.Tests were conducted in a representative metal spool valve as well as a model being three times larger. Data taken under non-cavitating conditions with both of these valves showed that the orientationof high-velocity angular jets would be shifted due to variationsin valve opening and Reynolds number. By means of high-frequency response pressure transducers strategically placed in the valvechamber, the cavitation could be sensed through the correlation of noise with a cavitation index. The onset of cavitation could bedetected by comparing energy spectra for a fixed valve opening with a constant discharge. The incipient cavitation index, as definedin this investigation, was correlated with the Reynolds number for both valves.Gao et al. [13] performed a simulation of cavitating flows inhydraulic poppet valves by means of an renormalization group (RNG) k –1 turbulence model, which was derived from the nstantaneousNavier–Stokes equations based on the RNG theory. Experiments were conducted to catch cavitation images around the seat of a poppetvalve from perpendicular directions, using a pair of industrialfibre scopes and a high-speed visualization system. The binarycavitating flowfield distributions obtained through digital processing of the original cavitation image showed satisfactoryagreement with numerical results; the vibrations of valve body and poppet induced by the cavitating flow were detected using a vortex displacement transducer, a laser displacement metre and a digitalstrain device. It was concluded that the opening and cone of aconical valve had significant effects on the intensity of cavitation. However, in this study, only downstream cavitationinception volume was analysed in estimating the anti-cavitationcapability, whereas the effects of outlet pressure were not considered.Oshima et al. [14] experimentally investigated the influencesof (a) chamfer length in the valve seat, (b) the poppet angle, and(c) the oil temperature on the flow characteristics and the cavitation phenomena, using water instead of oil as the workingmedium in water poppet throttles. The cavitation phenomena weredirectly observed and the pressure distribution between the valveseat and poppet surface was measured in water poppet throttles. Comparison analyses on the condition of critical cavitation wereconducted between oil and water. It was found that the dischargecoefficient and the critical cavitation number for water poppetthrottles were considerably different from those of oil ones, dueto the high density and low viscosity of water. Recently, Liu etal. [15, 16] investigated experimentally the flow and cavitationcharacteristics of a two-stage throttle in water hydraulic system; they concluded that the two-stage throttle had stronger anti-cavitation capability than the single-stage one, and that the shape of seats also affected the anti-cavitation capability of the throttles. The cavitation choking appeared only when cavitationindex was less than 0.4. 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 wasassumed to be constant, which may not be applicable for the two-stage 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 waterhydraulic two-stage throttle poppet valve. This objective entailsthe 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 chamberlocated between the two throttles, (b) the effect of the passagearea ratio of the two throttles and pressure between the two throttles on the anti-cavitation capability will be studied, (c)the design criteria of the two-stage throttle valve will be established for low-cavitation and noise, and (d) experimentalvalidation will be performed through a custom-manufactured testing apparatus, to demonstrate applicability of the developed simulation approach and design criteria.2SIMULATION OF PRESSURE BETWEEN TWO THROTTLES2.1Statement of problemsIf water pressure decreases below the saturationvapour 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 . 0.023 MPa when the temperature is 20 8C. Generally, the likelihood of the cavitationfor the throttle valve can be measured by a cavitation index (K ),which can be expressed as followsThe critical cavitation index (Kc) is the minimum cavitationindex for the throttle under non-cavitating flow in throttle valve. For a single-stage throttle valve, Ksc . 0.4 [14, 17–20]. It denotes that, if the cavitation index K is less than 0.4, thecavitation will occur. Therefore, the cavitation index (K ) should be larger than 0.4 so as to avoid cavitation in the throttle valve.The larger the pressure drop across the throttle valve, thesmaller the cavitation index (and thus greater probability of cavitation for the throttle valve). The two-stage throttle valveis a practicable configuration to mitigate cavitation, being extensively used in water hydraulic pressure relief valves andthrottle valves. Figure 1(a) is a scheme of water hydraulic two-stage 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 isdiffered from a single-stage throttle. Hence, the cavitation index of each stage throttle should be larger than that of thesingle-stage throttle valve, leading to a reduced possibility ofcavitation and erosion.For the two-stage throttle valve, the definition of cavitation index Kt can be analogously expressed as followsActually, for each stage throttle, the critical conditions foravoiding cavitation in a single-stage throttle should be satisfied. ThenThe pressure distribution inside the medium chamber locatedbetween 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.2.2Mathematical modelIn view of the axisymmetric structure of the two-stage throttle valve, the computational domain is simplified to a two-dimensionalaxisymmetric 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 a nd BC d enote fixed wall of seat, ED i s fixed wall of poppet, CD the inlet, and AE the outlet. The water is assumed to be incompressible viscous fluid with ρ= 998.2 kg/m3, and μ=1.007×10-6m2/s; the effects of pressure and temperature on the density and viscidity of water are neglectable; all of the wallsare assumed adiabatic with a zero slip velocity. The inlet andoutlet boundaries can be specified as pressure levels.The RNG k –1 equation model (turbulence kinetic energy equation k and dissipation equation 1) and the multiphase model are used in this research. The Boussinesq hypothesis is employed torelateReynolds stresses to the mean v elocity gradients The turbulence kinetic energy (k) and its rate of dissipation (1) are obtained from transport equations (6) and (7), respectivelyIn this research, the multiphase model involves liquid and gas phases that are considered as interpenetrating continua. The lawof conservation of mass and momentum is satisfied in each phase.The derivation of the conservation equations can be done by ensembling the local instantaneous balances for all of the phases.Volume fractions (ap) represent the space occupied by gas phase.The continuity equation for gas phase can yieldIn these equations, Gk represents the generation of turbulence kinetic energy due to the mean v elocity gradient; sk and s1 are the inverse effective turbulent Prandtl numbers for k and 1, respectively.2.3Simulation results Sensitivity analyses of the variations ofpressures inside the medium chamber along with the changing passage area ratio of the two throttles and the inlet and outlet pressures were performed. As a large number of numerical simulations were conducted, it is impossible to include all of the results here.Instead, representative outcomes are explicated to demonstrate important findings. The simulated contours of the volume fractionof water vapour are shown in Fig. 2. The passage area ratio (r) could significantly affect the cavitation occurrence and intensity under conditions of pin . 10 MPa and pout . 1 MPa. There are several key transferring points for passage area ratio including r . 0.2, 0.5, and 2.0. When r was less than 0.2, water vapour would be formedinside the medium chamber of the two-stage throttle valve, and the corresponding maximum volume fraction of water vapour would increase up to 85.3 per cent as shown in Fig. 2(a). This was ascribedto the high fluid velocity and the intense recirculation inside the medium chamber. The recirculation would cause the formation of alow-pressure zone and thus potential cavitation. With an increasingr level, the volume fraction of water vapour would be reduced. When r ranged from 0.5 to 0.8, the maximum volume fraction of water vapour would fall (to 0.162 per cent or below) and, consequently, thepossibility of cavitation would decrease. However, with furtherincrease of the passage area ratio (e.g. when r . 2.0), the maximum volume fraction of water vapour would increase up to 5.15–26.7 per cent as shown in Figs 2(c) and (d); thus, cavitation would occuragain.The simulated contours of pressure inside the medium chambersare presented in Fig. 3 when the passage area ratio varied from 0.1 to 6.0 under conditions of pin . 10 MPa and pout . 1 MPa. It canbe seen from Fig. 3 that, when r was less than 0.2 or larger than2.0, the pressure drop would be solely taken by the front or rearthrottle of the two-stage throttle valve. This looked like a single-stage throttle valve, with its anti-cavitation capabilitybeing similar to that of single-stage throttle valve. When r wasbetween 0.2 and 2.0, the total pressure drop would be shared by the two tandem throttles. Obviously, a sound r level would help suppresscavitation inception in the water hydraulic two-stage throttlepoppet valve.In order to clarify the correlations among pressure levelsinside the medium chamber and the related impact factors (r, pin,and pout), a virtual line (l ) was employed to locate the pressurelevels inside the medium chamber. They ranged from the midpoint of line AE to the midpoint of line CD. The virtual line was dividedinto eight parts, and the corresponding nodes were marked by ordinal numbers (from 0 to 8); the pressure levels on the virtual line were denoted as pl.The effects of the passage area ratio (r) on the pressure (pl) under condition of pin . 10 MPa and pout . 4 MPa are illustratedin Fig. 4. The pressure level would vary with the passage area ratio. The larger the passage area ratio r, the higher the pressure level on the virtual line. However, when r is less than 0.2, the pressures inside the medium chamber would become low. This was attributed to the small front throttle, which could be considered as a jet nozzle. The fluid would have less direction variations and jet contraction when passing through the front throttle. A high flow velocity through the front throttle would be generated, and thus the pressure would decrease rapidly. In such a case, the flow pattern would belike that of a single-stage throttle. From Fig. 4, the pressurelevels on the virtual line (l) would vary at different space positions, confirming that the pressure levels inside the mediumchamber were not constants. It was also very interesting that thelocations of the maximum and minimum pressures were fixed on thevirtual line, being indifferent of the variations of the passagearea ratio.第二部位中文翻译关于二级液压节流锥阀的低汽蚀研究摘要：汽蚀对于液压控制阀的性能和寿命有重要影响,如引起效率的降低,产生强烈的噪音和振动。