Journal of KONES Powertran and Transport, Vol. 19, No. 1 1 FLOW ANALYSIS OF HYDRAULIC POPPET CONTROL VALVE BY MEANS OF COMPUTIONAL FLUID DYNAMICS Jacek azowsk SKA-Polska, Sp. z o.o. Jerozolmske Av. 15/17 p. 46-17 Warsaw, Poland tel./fax: +48 63 17 75 e-mal: j.lazowsk@ska-polska.pl Janusz Krasuck CIM-mes Projekt, Sp. z o.o. Jerozolmske Av. 15/17 p. 53-17 Warsaw, Poland tel.: +48 631 44 e-mal: j.krasuck@cm-mes.com.pl Abstract The poppet control valve s one of the most wdespread hydraulc components. The paper purpose s the evaluaton of flow characterstcs of poppet valve by means of a complete numercal analyss. The am of ths analyss s to evaluate the valve flud dynamc performance, explotng computatonal flud dynamcs (CFD) technques, n order to gve the relable ndcatons needed to defne the valve desgn crtera and avod expensve expermental tests. The numercal analyss was performed by usng the commercal code of CFD, and the numercal results show the complete flow feld nsde the valve. Axsymmetrc as well as three-dmensonal valve model was consdered and the smulaton results also are verfed wth expermental results. The results have confrmed the good accuracy of the smplfed CFD analyss based on D axsymmetrc valve model beng more effcent than full 3D approach and show ths method as very useful to forecast flow characterstcs of poppet control valves. Hydraulc control poppet valve, dstrbuton of the velocty n layers of the flud, computatonal grd n the modelled geometry, pressure dstrbuton for axsymmetrc and three-dmensonal mode for dfferent flow rate, flud velocty dstrbuton n valve meterng secton for axsymmetrc and three-dmensonal model for dfferent flow rates, the example of streamlnes, the comparson of flow characterstcs are contaned n the publcaton. Keywords: poppet control valve, computatonal flud dynamcs, flow characterstcs 1. Introducton The performance of a hydraulc control system s strongly nfluenced by the dynamc characterstcs of ts control valves. The functon of a control valve s to change the flow paths n hydraulc crcut, n partcularly to open or to close the flow path. The control valve s one of the most expensve and senstve parts of a hydraulc crcut and therefore must be desgned very carefully. In a classcal approach, a many expermental measurements on the specfc valve are needed. Alternatvely, a numercal analyss can be performed by means of computatonal flud dynamcs (CFD) codes. Computatonal flud dynamcs (CFD) s becomng a well establshed practce also n valve analyss and desgn [3], snce t can gve a clear nsght nto ts operaton mode, whch cannot be acheved by expermental tests and measurements. CFD also allows a reducton n the number of prototypes under test, as well as the tme and costs of the desgn and of the expermental phase. The dffcultes ntroduced by the use of CFD technques deal wth the necessty to reduce the computatonal doman to the most mportant regons of the flow where the flud dynamc phenomena are concentrated n order not to exceed the avalable computatonal resources. In fact, the tradtonal approach s axsymmetrc modellng of the valve. Ths approxmaton allows the use of a more refned grd and a more accurate analyss of the flow feld nsde the valve.
J. azowsk, J. Krasuck It requres assumng of some smplfcaton n partcular the geometry of set of crcular nlet holes placed n crcumference of valve housng should be replaced by one equvalent crcumference channel. In ths work, the nvestgatons have been carred out on the example of control ON-OFF valve URES6 by polsh manufacturer PONAR Wadowce. Wthn the work, the comparson of 3D CFD analyss based on three-dmensonal valve model and smplfed approach usng axsymmetrc model was conducted. The smulaton results were compared wth expermental flow characterstc of the valve[].. The hydraulc control poppet valve Two way control poppet valves are plot operated, low leakage solenod actuated valves. Two way poppet valves control the flow of a two way functon by blockng flow n one drecton (smlar to a check valve). They are generally selected due to ther low leakage and ablty to meet hgher flow requrements. Poppet valves are often used on sngle operaton actuators or n unloadng functons. They are avalable n normally closed and normally open types. Normally closed poppet valves act as a check valve when de-energzed, blockng flow from one drecton and allowng restrcted free flow n the reverse condton. The scheme of analyzed valve type URES6 s shown n Fg. 1. Inlet Outlet Axs Ol flow Inlet Fg. 1. The hydraulc control poppet valve URES6; 1-solenod, - sleeve, 3 - poppet, 4 plot, 5 sprng The valve plot 4 s held on ts seat by sprng force 5, blockng plot flow. Ths allows pressure at the nlet port to hold the poppet 4 on ts seat, thus, preventng flow through the valve. If the nose of the nlet port s pressurzed, the pressure wll overcome the sprng force 5, pushng the poppet 4 off ts seat, allowng free flow through the valve. When the col 1 s energzed, the valve plot 4 s pulled off ts seat. Ths vents the pressure nsde the poppet to nlet port, creatng a pressure mbalance across the man poppet. Ths dfferental lfts the poppet 4 allowng flow from the sde to nose. Snce poppet valves are ploted operated, a mnmum amount of pressure dfferental and flow between nlet and outlet ports must be present to overcome the sprng and lft the poppet. The flow through the valve orfce arses the pressure drop, whch s consdered as a dfference between nlet and outlet pressure. The pressure drop results from flud energy dsspaton as a result of change flow velocty and nternal frcton. That energy part can be dsspated n the process formaton of vortces and nternal frcton and can be expressed mathematcally as a coeffcent of the hydraulc losses. The change of drecton of flud velocty causes of hydrodynamc force actng on the poppet 3 n drecton of valve closng. 5
Flow Analyss of Hydraulc Poppet Control Valve by Means of Computonal Flud Dynamcs The flow through the valve s turbulent and the relatonshp between pressure drop and flow rate s nonlnear and has nature of parabolc functon [1, 6]. The one of the most mportant thngs, whch should be consdered by valve desgners, s to mnmze the pressure drops and to keep the hydrodynamc force n the lowest value, as t s possble. 3. Physcal model of the flow The choce of the physcal model should reflect the most phenomena n the object that allowng to attan posed the purpose of the work. In ths work, the man flow through the valve orfce between the man poppet and the valve seat was consdered. I was assumed the flow s unform and steady-state and workng flud s ncompressble [4]. Propertes of the workng medum were modelled as for the Newtonan flud n whch the dynamc vscosty s ndependent of the velocty of the flud. It assumes that the coeffcent of vscosty s also ndependent from the temperature and the pressure. A proper estmaton of turbulent phenomena has great mportance to determne the valve flow features. In partcular, the flow nsde a hydraulc valve s characterzed by the coexstence of free shear flows, due to the flow jet at the ext of the meterng secton, and wall bounded flows, whch are strongly nfluenced by the wall effects. The most sutable turbulence model for ths knd of problem appears to be the RNG-k-e model coupled wth the so called two layer zonal model [1, 4]. The most sutable turbulence model for ths knd. In the paper the k-. model of turbulence was assumed because s sutable for the flow analyss of ncompressble flud wth low velocty [1,4]. Ths turbulence model usng the mean component method and based on the Reynolds rules gves a relable estmaton of the turbulent quanttes upstream and downstream of the restrcted sectons and s able to estmate properly both the free jet and the wall bounded regon. 4. Mathematcal model of the flow The homogeneous, ncompressble and vscous flow of a workng medum through the control valve s descrbed by the Naver Stokes equatons (N-S) (1) for the steady-state flow. N-S equaton together wth the contnuty equaton () s the complete set of equatons that determne the flow pressure and velocty felds. Equaton N-S can be wrtten n the vector form [1,3]: 1 F grad p V (1) and the equaton of contnuty: dv ( V ), () where: flud densty [kg/m 3 ] p - pressure [Pa], - vector of the massve force [N], coeffcent of knematcal vscosty [m /s], - velocty vector of control volume [m/s]. Shear stress nsde the flud s expressed as: where: du, (3) dx 53
J. azowsk, J. Krasuck shearng stress n the lqud [Pa], coeffcent of dynamc vscosty [Pa s], du/dx shear speed [m/s]. The velocty dstrbuton across the flud layers s llustrated by Fg.. Fg.. Dstrbuton of the velocty n layers of the flud Velocty profle across the flud layers, shown n Fg., causes the momentum dfferences between a flud layers and n effect the shear stresses nsde the flud. The components of Reynolds stress tensor s expressed by equaton [1,3]: * j T ( uu, j u j, ) k j, T c k /, (4) 3 where: k - denote knetc energy [J], - denote dsspaton of knetc energy, c - constant. The k, parameters are related by followng equatons [1,4]: whle: (, (5) k u ), ( ju j ), ( k k, ), c ( u ), c1( ju j ), (, ),, (6) k / and constants value: k T / k, T c =.9, 1 = 145 155, c =19, k =1; = 13 5. Computatonal model The whole computatonal grd for two knds of consdered flow models s shown n Fg. 3. The boundary condtons are assumed as follows: INLET: on ths surface, the constant pump ow rate value has been enforced and n consequence a constant nlet velocty of the medum, OUTLET: on ths surface, the pressure value set by the pressure relef valve on the charge pump lne has been mposed; for smple nterpretaton of the results the p= value may be accepted too, SYMMETRY: these are the symmetry faces and allow the use of the smpled geometry as prevously descrbed, then axsymmetrc model turns to D model, and n three-dmensonal model the computatonal doman s lmted by two symmetry planes and ncludes two nlet 54
Flow Analyss of Hydraulc Poppet Control Valve by Means of Computonal Flud Dynamcs ports by means of that a quarter part of the valve may be consdered only. The axsymmetrc model FVM (Fnte Volume Method) s shown n Fg. 3a. The model was bult from 7968 two-dmensonal elements and has 869 nodes. The eght nlet holes were replaced by one equvalent nlet channel havng the same flow surface as the sum of all surface nlet holes of the valve. The three-dmensonal model shown n Fg. 3b represents a quarter part of the real valve and allows to model a real behavour of nlet flow. Inlets reflect the actual shape and locaton. A three-dmensonal model was assumed as reference model [7] for further analyss. Ths model s more complcated then axsymmetrc one and has been descrbed by 11 454 spatal elements and 34 437 nodes. Inlet Walls Outlet Openng x Symmetry axs a) Inlets Symmetryplane Symmetryplane Walls b) Fg. 3. The computatonal grd n the modelled geometry: a) axsymmetrc model D, b) 3D three-dmensonal model Outlet The nlet velocty of the workng medum s defned by the expresson: where: Q volumetrc flow rate [m 3 /s], V nlet flow velocty [m/s], S surface of nlet flow area [m ]. Q V, (7) S S L d, (8) As the workng medum, hydraulc ol was assumed. The flud densty and the knematcs vscosty have been set to = 87 [kg/m3] and to = 1.6 1-5 [m/s], respectvely. Smulaton analyss was carred out usng the commercal Advanced CFD package. For numercal solvng the problem, the Euler doman based approach was selected where the flud s 55
J. azowsk, J. Krasuck movng towards to fxed grd. The analyss was performed for a steady-state flow [4, 5], where the flow parameters such a velocty and pressure are tme ndependently n meterng sectons. 6. Numercal analyss and results Presented results allow to estmate dfferences between two consdered models of control valve. The analyss was performed for seven values of flow rates Q: 5, 1, 15, 9, 5, 3, 4 [l/mn] at the full valve openng (flow orfce x = mm). Fgure 5 and 6 show the maps of pressure dstrbuton and flow velocty for three-dmensonal and axsymmetrc model, n three cases of volumetrc flow rate: 1 l/mn, l/mn, 3 l/mn volumetrc. a) b) c) Axsymmetrcmodel Three dmensonalmodel Fg. 4. Pressure dstrbuton for axsymmetrc and three-dmensonal mode for dfferent flow rate: a) 1 l / mn, b) l / mn, c) 3 l / mn Comparng the maps n Fg. 4 t can be stated that the pressure dstrbuton nsde the valve resulted both axsymmetrc and three-dmensonal model s very smlar. The area wth negatve values of pressure s observed for both cases n the wall of valve body close to orfce edge. Of course t does not meet the real stuaton, because results from the assumed boundary condton OUTLET p=. For the great value of the outlet pressure, the pressure dstrbuton n meterng secton wll ncrease approprately. However, such sgnfcant local pressure drop ponts out to danger of the appearance of cavtatons, whch s fast growng along wth the growth of flow rate. Fgure 5 shows the velocty dstrbuton maps of the flud flow for the axsymmetrc and threedmensonal model. We observe the greatest speed streams n the central area of the streamlnes, and the lowest ones close to the valve walls. Lke to pressure dstrbuton, n ths case also we can state a good consstency between both models. The velocty dstrbuton on the outlet s a bt more homogeneous for three-dmensonal model. We can see n Fg. 6 that for three-dmensonal model the streamlnes are found more regular see the areas n whch flud flow s formed as the steady-state flow. The both models show a local flud vortexes appearng close to wall of poppet. For axsymmetrc model, the addtonal vortex close to wall of outlet channel s found, but t results of model smplfcatons regardng to nlet channel, because a such phenomena s not observed for three dmensonal model...15.1.5.5.1.15..6.4...4 1.6 1.5 1..5.5.9 bar bar bar 56
Flow Analyss of Hydraulc Poppet Control Valve by Means of Computonal Flud Dynamcs a) b) Axsymmetrcmodel Three dmensonalmodel 6 5 4 3 1 1 1 8 6 4 m/s m/s c) 18 16 14 1 1 8 6 4 m/s Fg. 5. Flud velocty dstrbuton n valve meterng secton for axsymmetrc and three-dmensonal model for dfferent flow rates: a) 1 l / mn, b) l / mn, c) 3 l / mn For selected flow rate Q = l/mn an example of streamlnes for both models s shown n Fg. 6. Axsymmetrcmodel Three dmensonalmodel Fg. 6. The example of streamlnes for volumetrc flow rate Q = l/mn On the bass of pressure, dstrbuton n nlet and outlet meterng sectons t s possble to calculate a correspondng mean pressure values n ths sectons and then the total pressure drop p d across the valve s expressed as: p d = p n p out, where: p n, p out mean pressure n the nlet and outlet meterng secton respectvely. A relatonshp between pressure drop and flow rate defnes a base flow characterstc of the valve p d = f(q). The comparson of flow characterstcs obtaned from smulaton results as well as expermental data s shown n Fg. 7. The both models gve good results compared to the real flow n the valve. For the consdered cases, the flow s turbulent and the flow curves are nonlnear. The largest pressure drop P d = 1,4 [bar] s observed for the largest volumetrc flow rate Q = 4 [l / mn]. For small values of volumetrc rate flow, from to 15 l/mn, practcally there are no dfferences between D and 3D characterstcs and they demonstrate the same varaton towards to expermental curve, showng the pressure drop n the axsymmetrc and three-dmensonal model. In ths case, the nfluence of the geometry of nlet meterng secton on the smulaton results s neglgble small. 57
J. azowsk, J. Krasuck PressuredropP d [bar] 1,6 1,4 1, 1,,8,6,4 Experment ModelD Model3D, 5 1 15 5 3 35 4 VolumetrcflowrateQ[l/mn] Fg. 7. The comparson of flow characterstcs for the axsymmetrc model, three-dmensonal model and experment For large values of volumetrc rate flow, over 15 l / mn, the gap between D and 3D characterstcs ncreases because of the growth of a vscous forces between flud layers and between flud and valve walls. Ths phenomenon s more accurately descrbed by 3D model. Maxmum dfference n pressure drop between 3D and axsymmetrc models s observed for maxmum flow rate but does not exceed 5% for flow rate 4 l/mn. It can be stated that for desgn practce the axsymmetrc model s enough accurate. 7. Summary and conclusons The paper presents a comparson the smulaton results obtaned from CDF analyss of on-off poppet control valve usng two dfferent numercal models - axsymmetrc and three-dmensonal. The valdaton of numercal models was based on the expermental flow characterstcs of the poppet control valve URES6 provded by the manufacturer []. The comparson of the pressure felds and flow velocty dstrbuton shows small dfferences between the both models, whch result from the modellng way of valve nlet channel. In the case of axsymmetrc model, the smplfcaton of the nlets geometry gves a good numercal effcacy wth computaton tme mn but mpacts on the flud streamlnes. In the case of three-dmensonal model, the computatons take 8 mn but the keepng count of real geometry of valve nlet holes cause that the streamlnes (flud element paths) nsde the valve seem to be more realstc. The comparson of the smulaton and expermental flow characterstcs confrmed a usefulness of the both models for subsequent analyss. On the bass of numercal analyss, results t can be stated that: the both models gve probable results compared to expermental data, numercal analyss based on axsymmetrc D model s much more effcent then 3D model, smplfcatons ntroduced n the D model gve a bt errors to be neglected, CFD analyss based on axsymmetrc model has suffcent accuracy for engneerng practce. The smulaton results have confrmed the good accuracy of the smplfed CFD analyss based on D axsymmetrc valve model beng more effcent than full 3D approach and show ths method as very useful to forecast n a desgn stage flow characterstcs of poppet control valves. 58
Acknowledgements Flow Analyss of Hydraulc Poppet Control Valve by Means of Computonal Flud Dynamcs The authors wsh to acknowledge the fnancal support of the European Commsson under the Grant AgreementNo. 356FP7-SME-8-1, Project HYDROCOAT, FP7 Framework Programme. Reference [1] Bukowsk, J., Mechanka pynów, PWN, Warszawa 1959. [] Karta katalogowa nabojowego zaworu rozdzelajcego URES6. Ponar Wadowce S.A. [3] Domagaa, M., Metodyka modelowana zaworów maksymalnych, Praca Doktorska, Poltechnka Krakowska, Instytut Informatyk Stosowanej, Kraków 7. [4] Yang, Y. S, Semn, C., Tsagaraks, N. G., Caldwell, Zhu, Y.,Water Hydraulcs A novel Desgn of Spool Valves for Enhanced Dynamc Performance, Proceedngs of the IEEE/ ASME, Internatonal Conference on Advanced Intellgent Mechatroncs, Xan, Chna 8. [5] Yuan, Q.-H., L, P.-Y., Usng steady flow force for unstable valve desgn: modellng and experments, Journal of Dynamc Systems, Measurement and Control, Vol. 17, No. 3, pp. 451-46, 5. [6] Gullon, M., Teora oblczane ukadów hydraulcznych, WNT Warszawa 1961. [7] Nowak, M. W., Metody dentyfkacj ukadów cgych z wykorzystywanem funkcj modulujcych sklejanych ch zastosowane w regulatorze adaptacyjnym, Praca Doktorska, Akadema Górnczo-Hutncza w Krakowe, 7. 59