Flow and cavitation characteristics of water hydraulic poppet valves

Journal of Harbin Instiute of Technology (New Series), Vol. 9, No. 4, 2002Flow and cavitation characteristics of water hydraulic poppet valvesLIAO Yi-de'2, LIU Yin-shui2 , HUANG Yan', L/ Zhuang -yun'廖义德，刘银水,黄艳,李壮云(1. Dept. of Mechanical Engineering, Wuhan Institute of Chemical Technology , Wuhan 430073 , China; 2. College of Mechanical Engineering and Sci-ance, Huzhong University of Science and Technology, Wuhan 430074 ,China)Abstract: Two types of poppet valves were tested, one is a poppet with a sharp -edged seals, and the other isthat with a chamfered seat. During the tests, the effects of backpressure and poppet lift on fow characteristicswere considered. Cavitation inception was detected by the appearance and rapid growth of a particular low fre-quency component of the oulet pressure fluctuation of valve when cavitation occurs. Experimental results showcaviaion, back pressure, valve opening and its geomerical shape have significant efects on the flow character-istics of valve. The flow cofficient of throtle with waler used as working medium is0. 85 ~0.95 when there isno cavitation. The pressure drop of Alow saturation decreases with the increasing of poppet lit. The sharp- edgedthrotle has stronger anti cavitation ability than the chamfered one.Key words: water hydraulics; poppet valve; Alow cofficient ; frequency spectra analysis; cavitation inceptionCLC nuober: TH137.1Document code: AArticle ID: 1005-91 13( 2002 )04-0415-040 INTRODUCTIONgiven to experimental research of flow characleristicsand the effects of cavitation on them.Water hydraulic system has many advantages suchas environmental friendliness, safety, etc. and altracts1 TEST APPARATUSmore and more attention now adaysPoppet valves,just as in oil hydraulic system, are commonly used inTest apparatus is shown in Fig. 1. Experimentswater hydraulic system. However, because of low vis-were accomplished in lwo types of poppet valves, one iscosity and high vapor pressure of water, cavitation is a poppet with a sharp-edged seat ( called Assemblymore likely to happen in water hydraulic components,A), and the other is that with a chanfered seat ( calledespecially valves. The formation and collapse of vaporAssembly B), the angle of the chamfer is 30°. The twobubbles will result in local high temperature and highvalves used the same poppet, whose conical angle is al-pressure, which is sometimes even higher thanso 30°. The diameter of inlet passaged is 16. 0 mm.GPal2]. Therefore there are great differences betweenThe width of chamfer of Assembly B (s) is 5.36 mm.the valves using oil and water as working fluid.Few bibliographies about flow characteristics ofwater hydraulic valve were reported. Johnson and hiscoworkers caried out experimental investigation for flowForce transducerand foree characteristics of poppet and disc valves usingwater a8 working fluids, but effects of cavitation onflow and force characteristics were not under considera-transducertion.n the research of Oshima and lchikawatheeffects of cavitaion on flow characteristics were consid-Poppe(ered, but the working medium was oil. Due to greatOutlephysicochemical differences between water and mineral. Pressureoil, those resuls can not be cited direcly in water hy-* transducerdraulic valves.SeatIn water hydraulic valves, Reynolds mumber is a小-ways very high and flow is completely turbulent. Hencelnletcavitation is unavoidable. In this paper, enphasis wasFig.1 Test apparatus中国煤化工Sponsored by the National Natural Science Foundation of China ( Grant No. 59975031 )Recived 2001 -04 -25.MYHCNMHG●415●Journal of Harbin Institute of Technology (New Series), Vol.9, No. 4, 2002Fig.2 is the schematic diagram of the throttles. sure drop in flow saturation is relatively high.The water was drained to the outside of system directlyand so it is easy to keep the water temperature at 20土0.9|I 9C. f1low rate Q was measured by flow -rate meter.0.85When the low is small, the flow-rate is measured by心0.7}measuring cup. The poppet lift x was surveyed by dis-placement transducer; Upstream and down- -stream0.5L-0.sL.52.3.0pressure, P: and Pz. were measured by two pressurepr-p/MPatransducers respectively. The pressure signal is sam-pled and input into computer throughi A/D conversion ，Fig.3 Fow characteristics whenx = 0.5 mmthen its power spectra were made by a computer-aidedtesting system.When. x is 1. 0 mm( Fig. 4), the flow coeffcient ofAssembly A is smaller than Assembly B, which is op-posite to the case whenx is 0. 5 mm. This shows that30_0°poppet lift exerts influences on flow charateristics.When flow coefficient decreases, the pressure drops inAssembly A and B are 2. 04 and 1. 36 MPa and lowerrespectively than the case when x is 0.5 mm. Thatmeans flow saturation is easier to form at larger poppetlif. This accords with the case when using oil4i orhigh water-based fluid as working media6l .1.P:0.(Assembly A)(Assembly B)? 0.8F -AImrdAronbly A) 7Fig2 Scbematic diagram of poppet valve2 RESULTS AND DISCUSSION012.0PrP/MPa2.1 Flow and Pressure CharateristicsFig.4 Flow characteristics whenx = 1 mmThe relationship between the flow and the pressuredrop across a valve may be described byExperiment was also executed to investigate the2(P-B2)effects of outlet pressure or backpressure on flow coeffi-?=CAa(1)cient. Fig.5 shows the flow coefficient of Assembly BwhereAo is area of inlet passage way; C。is flow coeffi-decreases with backpressure increasing whenx = 0. 5cient; ρ is density of water; Q is flow rate throughmm, if there is no flow saturation. The results of As-valve; P1 is inlet pressure and P2 is outlet pressure.sembly A are same as Assembly B. In the range ofThe flow cofficient can be calculated readily frompressure drop considered, the flow saturation appearsexperimental measurements.in the case of adding backpressure, but not in the caseIn the test, the inlet pressure was kept constantof no backpressure. This is because when there is noand the outlet pressure was changed from high to low.backpressure, vapor bubbles in the downstream cham-TheCq - 4,(0p = P: - P2) curves of two valves isber are difcult to collapse and flow away into the re-shown in Fig.3 whenx = 0. 5 mm. The flow cofficienttumn line, the flow characteristic is litle affected.of Assembly A is larger than Assembly B, this is be-When backpressure is added, bubbles collapse in thecause although there is pressure recovery in Assemblyoutlet chamber, so the cavitation choking or flow satu-B, which increases the fow coffcient in some degree,ration is more likely to happen.the viscous restriction is higher than in Assembly A.When flow cofficient falls, the pressure drops in As-sembly A and B are 2. 75 MPa and 2. 02 MPa respec-08tively. The descreasing of flow coefficient means cavi-心0.F twibour besk preurtation choking or flow saturation. Assembly A is asharp-edged throttle, the vena contracta of flow: is be-中国煤化工405060hind throtle and the formation of vaprous bubbles arein downstream chamber ，which hardly produces influ-PHCNMHGoences on the 1low through the throttle. Hence the pres-Fig.5 Effets of back pressure on flow coeltcient●416.Jourmal of Harbin Insitute of Technology (New Series), Vol.9, No.4, 20022.2 Cavitation InceptionAccording toa series ofTF - (P: -P2) curves andMany methods are used lo diagnose cavitation in-Eq. (3), the cavitation index k; can be obtained whenception. It is often taken as the point where the flowcavitation incepts. Fig. 8 is the limit of cavitation in-drops by 1 ~ 3 percent or case vibration increasesception. For each assembly, when h; is above the cor-sharply. Based on the authors’ researches ，onceresponding curve， there is no cavitation inception;cavitation occurs， a particular ]ow frequency compo-when h, is below the curve , cavitation will occur. It cannent of outlet pressure fluctuation appears and rapidlybe seen that cavitation incepts earlier in Assembly Bgrows. The low-frequency fluctuations associated withthan in Assembly Acavitaiton are a distinct fealure that may be used as analtemative method to diagnose cavitation inception.2The results are obtained by the research on statistical20e 15characteristics of outlet pressure fluctuations of differenthydraulic pumps. However， experiments testify thatvalves have the same phenomena6l ..0132.025 3.0 .Whenx is 0. 5 mm, the inlet pressure is 3. 0 MPaand outlet pressure is 0.5 MPa， frequency domainFig.7 Variations of TF with pressure dropcurves of Assembly B is as shown in Fig. 6.0.45[0.30.40No cavitation inceptions 0.350.25Cavitation inception0.20 0.01 0.020.030.00.06 0.070.1.Flg.8 Limit of cavitation inception3 CONCLUSIONS1000300400500600Frequency/HzFrom above the following conclusions can be ob-Fig.6 Power spectra of outlet pressuretained:(1) The flow coefficient of throtle is a value ofIn order to reflect the variation of power speclra0.85 ~0.95 as Reynolds number increases in conditionwith back pressure more directly, a statistic variable ，of no cavitation while using waler as working medium.TF, is introduced. It can be expressed by 6 :(2) When there is no cavitation, the flow coef-TF = Sf(?S(f)/ 2 S(f)(2)fient with back pressure is larger than that without backpressure at the same Reynolds number.(3) Flow saturation pressure drop decreases withwheref; is frequency and S(f) is amplitude value ofthe increasing of poppet lift. The larger the lift is, thepower spectrum corTesponding tof.If the main frequency band of the signals shifts toeasier cavitation happens.(4) The shap-edged throttle has stronger anti-low frequency sector, TF will decrease. Fig. 7 showscavitation ability than the charnfered one.the relation of TF and pressure drop for Assembly Aand B. It can be seen that TF curves take a suddenReferences:tum and cavitation inception appears in two assemblies[1] uzY, YUZ Y, HE X F, et al. The development andwhen pressur drop is about 1.5 MPa. Comparing withperspective of water hydraulics ( keymote lecture) [ A ].the flow saturation pressure drop, cavitation inceptionProc of 4th JHPS International Symposium on Fluid Powerpressure drop is lower. lt shows that only cavitation de-[C]. Tokyo:[sn], 1999. 335-342.velops to some extent can it exert influences on flow[2] PATELLA RF, REBOUD JL. A new approach to evalu-and pressure characteristics. For different poppet lifttate the cavitation erosion power{ J]。Joumal of FluidsEngineering，1998, 120.. 335-344.the same conclusions can be obtained. .The possibility of cavitation occurrence also can[3]. JOHNSTON D N, EDGE K A, VAUGHAN N D. Experi-中国煤化工orce characleristics of hy-be expressed by cavitaiton index k;I1.Pro Instn Mech En-h, _ P2-P。(3)[4] USHIMA s, ICHIKAWA T，Cavitation phenomena andCNMHGP1-Pz.where P. is vapor pressure of waler.performance of oil hydraulic valve ( Ist report, mecha-●417●Joumal of Harbin Instiute of Technology (New Series)，Vol.9, No.4, 2002nism of generaion of cavitation and flow performnance)quipment with fre-esisant fluids[J]. The Mining Engi-[J]. Trans of Japan Soe of Mech Engrs (Series B)neer, 1979(9): 235-243.1985.51(462) :427-435.[7] L1Z Y, HEXF, YUZ Y, et al. A new indicator of cav-[5] OSHIMA s, ICHIKAWA T Caviation phenomena anditation inception for hydraulic pumpASMEperformarce of oil hydraulic valve ( 2nd report, influenceFEDSM ' 98 CD-ROM Proceedings[cj. [s1]:[s n],of the chumfer length of the seal and the flow perform-1998. 391 -395.ance)[J]. Trans of Japan Soc of Mech Engrs ( Series[8] JIANGC H, LIZ Y. A new diagnostie method for cavita-B), 1985. 51(462) :628-636.tion inception of hydraulic components[ A]. Proc of the[6] KENNY P.YARDLEY ED, EEDY A R M, et al. Sudy1st Int Sym on Fluid Power Trans and Conlrol[C]. Bei-of erosion and corrosion of materials used in hydraulic ejing:[s n], 1991. 394-398.中国煤化工MYHCNMHG●418●