Cavitating Injector Flow Simulations Considering Longitudinal and Lateral Needle Displacement

Transcription

1 Technical Paper Caviaing Injecor Flow Simulaions Considering Longiudinal and Laeral Needle Displacemen David Greif 1) Peer Sampl ) Wilfried Edelbauer 3) 1) AVL-AST d.o.o., Trg Leona Sulja, 000 Maribor, Slovenia ( david.greif@avl.com) )-3) AVL Lis GmbH, Hans Lis Plaz 1, 800 Graz, Ausria Received on June 11, 013 Presened a he JSAE Annual Congress on May, 013 ABSTRACT: Presen paper oulines he simulaion echnique for dynamic analysis of caviaing Diesel injecor flow, considering longiudinal and laeral needle displacemens. The curren research is performed wih he goal o evenually enable direc coupling of 1D hydraulic ools, providing exac needle posiions in hree coordinaes, wih 3D CFD ool AVL FIRE. A his sage he needle displacemens in longiudinal and laeral direcions have been predefined. In realiy he needle will be pushed aside by he flow for differen reasons; differen radii a he hole inle and non-equi-angular hole posiioning. Prediced flow shows visible non-uniformiy resuling from non-symmeric naure of he injecor domain. KEY WORDS: hea engine, numerical calculaion, modeling, fuel injecion, caviaion, caviaing flow, injecor flow, nozzle flow, mass ransfer, needle displacemen [A1] 1. Inroducion Numerical simulaions of he caviaing injecor flows are of grea imporance in engine developmen process. For emissions and performance reasons rail pressures are consanly increasing and he flow aggressiveness is consequenly increasing, oo. A high pressures caviaion aggressiveness is also higher, hence accurae flow predicion is very imporan. Due o he naure of he nozzle producion process, he nozzle ip domains are exremely difficul o produce o be fully symmeric. The consequences of he producion inconsisency may be differen spray hole inle radius, displaced spray hole axis and spray hole surface finish. As he final resisance of he flow occurs a he inles ino he spray holes, he forces acing ono he needle ip may become asymmeric. Such condiions may displace he needle in is radial direcion, which will furher enhance he asymmeric naure of he flow hrough he nozzle ino he combusion chamber. Furhermore, here is a small bu no negligible radial moion due o he bearing clearance of he needle guide. As he flow wihin he injecion holes is differen from hole o hole, he sprays developing inside he engine may be affeced as well. This may have a combined effec on mixure formaion, argeing and finally on combusion. Presen paper demonsraes he capabiliy of he commercial Compuaional Fluid Dynamics (CFD) program AVL FIRE v013.1, o predic he effecs of he above oulined laeral needle displacemens. The program has wo possibiliies in erms of modeling of caviaing flows inside injecion nozzles; sandard model is discussed by Alajbegovic e al (1) and polydispersed model is described in deail by Wang e al (). The models have been validaed agains measured daa as discussed by Alajbegovic e al (1), Wang e al (), Wang and Greif (3), as well as for esimaion of caviaion erosion inside injecion equipmen by Greif and Srinivasan (4). The presened mehodology feaures simulaneous longiudinal and laeral needle moion, which mimics he plunger movemen under realisic condiions. Acual needle posiion will be deermined by combining he desired needle lif, longiudinal forces acing ono he needle ip and he sum of laeral forces, which push he needle ip sideways. In presened sudy boh, longiudinal and laeral needle moion has been prescribed. The needle moves wih hree degrees of freedom, bu in pre-defined fashion. Curren furher mehodology developmens deal wih coupling of he 3D flow prediced hrough CFD wih 1D hydraulic sofware ools as described by Caia e al (5) and Jelovic e al (6). As a resul he informaion from 1D ool abou 3D needle displacemen will be direcly refleced in an auomaic modificaion of he compuaional grid of he injecor. Copyrigh 014 Sociey of Auomoive Engineers of Japan, Inc. All righs reserved 85

2 David Greif e al./inernaional Journal of Auomoive Engineering 5 (014) Theoreical Bacground.1. Muli-fluid model In framewor of he muli-fluid mehod, each fluid is considered as a coninuum media and he conservaion laws apply. An ensemble averaging is effeced o remove he microscopic inerfaces, resuling in macroscopic conservaion equaions which are analogous o heir single-phase counerpars bu differ in ha a new variable, volume fracion, and new erms, he inerfacial exchange erms, are inroduced, see for example Ishii (7), and Lahey and Drew (8). The averaged coninuiy and momenum equaions presened below follow from he heoreical wor of Drew and Passman (9). Coninuiy equaion: v (1) Momenum conservaion: v v v p g M v in where,, v and p are respecively he averaged volume fracion, densiy, velociy and pressure; he subscrip is a phase indicaor (=liquid, or =vapor, or =air); v in is he inerfacial velociy; is he phase change rae and M is he inerfacial momenum ransfer erm, wih he drag being he mos imporan force... Inerfacial mass exchange One of he ey elemens of caviaion modeling is vaporizaion rae, or he inerfacial mass exchange,. In he curren sudy he Rayleigh-Plesse equaion wihou viscous and surface ension erms for he dynamics of a single spherical bubble serves as he saring poin: rr 3 p r (3) c where r is he bubble radius and c is he densiy of he liquid (couninuous) phase. The single bubble mass change rae is r (4) 4r d For a populaion of bubbles wih disribuion funcion f, defined as he number of bubbles wihin he radius span of (r,r+dr), he oal mass ransfer rae, in space (x,) will be c f ( r) 4r 0 drdr d (5) () where subscrips c and d denoe he coninuous and dispersed phases respecivelly. For mono-disribuion sysem of bubbles, he above formulaion reduces o: (6) N c d 4 r r where N is he bubble number densiy represening he number of bubbles per uni volume. I is calculaed according o he assumed diminishing linear ramp: N 0 d 0.5 (7) N N0 1 1 d 1 d 0.5 This is a raher heurisic formula and is used o model he coalescence effecs. The iniial number densiy, N 0, depends on he characerisics of he liquid phase. The values repored for waer vary beween 10 9 and sies per uni volume (10). The qualiy of waer affecs he iniial number densiy considerably. The difference beween fresh and diry waer can differ several orders of magniude. The same can be expeced for diesel fuel. The iniial number densiy used mosly is There are differen models for he soluion of equaion (3) o obain he bubble radius change rae. In he presen wor linearized version has been solved by neglecing he inerial erm. Therefore, he mass ransfer rae becomes: 1 1 d 3 3 c sign( p)3.95 N d p (8) c and he effecive pressure difference, p, equals: p p sa (p CE c c ) (9) 3 The effecive pressure difference includes he effecs of pressure flucuaions. The closure coefficien C E (Egler coefficien), depends on local urbulence level and varies beween 1 and Turbulence modeling Following he same ensemble averaging procedure, here will be urbulen ranspor equaions for each phase. Sandard urbulence model in AVL FIRE is --f model (Basara (11), Hanjalic e al. (1) ), which is paricularly suied for predicing near wall urbulence. I has been exended o muliphase case. The equaions for he urbulence closure are ( v ) P (10) N K N l l1, l l1 Copyrigh 014 Sociey of Auomoive Engineers of Japan, Inc. All righs reserved 86

3 N N ( v ) Dl l1, l l1 ( C P C ) C v 1 4 (11) v (1) ( f P ) N ( ) l1 where f and are defined by 1 P f L f ( C 1 C )( ) 3 1 T (13) C T (14) and T MAX (,6 ) (15) Fig. 1 Compuaional grid of he 8-hole injecor. As he needle is displaced in radial direcion, inernal cells need o be smoohed o mainain sufficien cell qualiy. This problem is especially pronounced a very low needle lifs, where he aspec raios of cells in he gap region are quie high. L MAX (, C ) 3/ 3/ 4 1/ 4 (16) Deailed modeling informaion is available in AVL FIRE User s Manual (13). 3. Geomerical Modeling Presen sudy considers a single geomery variaion. Figure 1 shows he compuaional grid used in he CFD simulaion. For consideraion of needle moion wih hree degrees of freedom, an enire mesh needs o be modeled. Noe ha reducion of he compuaional complexiy o a cae segmen of he enire domain is herefore no possible. The whole domain consiss of he hollow cylinder around he needle shaf, needle sea area wih nozzle sac and he injecion holes (commonly referred o as spray holes). The compuaional mesh of he nozzle (shown in Figure 1) conained cells. Needle was allowed o be displaced in longiudinal and laeral direcions. Figure shows an example of he needle displaced in laeral direcion. Noe ha he lef gap beween he needle and he sea is considerably larger han he gap on he righ side. This means ha he cross-secional area available for he fuel flow is noably non-uniform around he perimeer of he needle ip. Such condiions will affec he flow ino he spray holes. Fig. Laeral needle displacemen Needle movemen and operaing condiions Presen sudy assumes predefined needle moion wih needle having hree degrees of freedom. In realiy he needle will be displaced due o he pressure forces acing on he needle ip surface, resuling from he asymmeric naure of he injecor domain, as a consequence of he producion process. I will undergo sideways displacemens wihin he olerance of he specific injecor. Figure 3 shows he predefined needle moion in axial direcion (red line) and in wo radial direcions (blue and green lines). For purposes of demonsraing he capabiliy of he applied CFD ool, simplified movemen has been applied. Duraion of injecion was 0.5 ms, which is in he order of pilo injecion evens, so was he maximum needle lif of 50 m. In principle, such informaion can be an inpu from a 1D hydraulic Copyrigh 014 Sociey of Auomoive Engineers of Japan, Inc. All righs reserved 87

4 ool as well, such as AVL BOOST as discussed by Caia e al (5) and Jelovic a al (6). I is imporan o noe ha in presen mehodology here is no need o generae a moving mesh, only he saring mesh is required. The res of he posiions are generaed auomaically by applying he so called mesh deformaion by formula. Displacemen coordinaes are read according o Figure 3 and he compuaional grid is adjused before each ime-sep fully auomaically. Fig. 4 Pressure field afer 150 s. Fig. 3 Axial and Laeral needle moion. Pressure boundary condiion a he inle of he injecor was in he order of 50 MPa, oule pressure level was defined a MPa. 4. Resuls 3D simulaion resuls are presened nex, focusing on he flow along he needle ip ino he sac area and finally ino he spray holes. These are he areas which feaure he mos dynamic physics and are also very exposed o caviaion phenomena. The resuls are ploed a differen needle lifs, wo planar cus are shown simulaneously o illusrae radial needle displacemen. The needle is moving in axial direcion along Z coordinae axis, herefore X-Z and Y-Z planar cus are used o illusrae radial needle moion effecs. Pressure disribuion afer 150 s is shown in Figure 4. This is a ime insan afer maximum needle lif as he needle sars o close. One can clearly see non-symmeric pressure field developmen along he needle sea owards he spray hole inles. Even a increased needle lif lower pressure level is observed in he righ gap along he needle of he Y-Z cu mared wih a red circle. Figure 5 shows he liquid phase velociy magniude a he same ime insan. Very asymmeric flow ino he righ spray hole is idenified in boh depiced planar cus. Asymmery occurs inside he gap beween he needle and he sea, as well as inside he spray holes. Fig. 5 Liquid phase velociy magniude afer 150 s. Fig. 6 Phase disribuion afer 0 s. Asymmeric effecs are he mos pronounced in phase disribuion resuls shown in Figure 6. Presened ime is near needle closing afer 0 s. No only is he phase disribuion differen along he needle sea, vapor srucures inside he spray hole form on opposie sides; upper and lower spray hole side. If he needle was posiioned in he middle of he nozzle volume, he vapor formaion would be symmeric. Copyrigh 014 Sociey of Auomoive Engineers of Japan, Inc. All righs reserved 88

5 Nex, he resuls a differen heighs (Z coordinae) a he end of he needle sea and wihin he sac are invesigaed. The locaions of hree planar cus normal o he Z axis are depiced in Figure 7. They show asymmeric flow ino he sac and ino he spray holes due o laeral needle displacemens, above all a he lower sac cu. x y [m/s] Fig. 9 Liquid velociy a upper sac cu afer 150 s. Fig. 7 Planar cu posiions. Figures 8 o 10 show he liquid velociy resuls on he hree planar cus shown in Figure 7. Asymmeric effecs are very pronounced a all locaions. Figure 8 shows he resuls a he end of he sea. The velociy pea of 50 m/s is clearly pushed o he boom end of he depiced view. Figure 10 shows he resuls in he lower sac area, where enrance ino he spray hole is also visible. Very nonhomogeneous and asymmeric velociy disribuion is found wihin he sac. y x x y [m/s] Fig. 10 Liquid velociy a lower sac cu afer 150 s [m/s] Fig. 8 Liquid velociy a he sea cu afer 150 s. Figure 9 shows he same ype of he resuls a he upper sac locaion. The velociy magniude pea is again pushed o he lower side. Figure 11 shows he mass flow raes for each spray hole separaely for he enire duraion of injecion. As during he needle opening phase he flow raes are sill raher comparable, one can noe ha during needle closing period he flow rae differs considerably from hole o hole. Two direcional laeral movemen has large influence on mass flow disribuion hrough each hole. Especially a maximum needle lif posiion and during needle closing phase he effec is pronounced. Copyrigh 014 Sociey of Auomoive Engineers of Japan, Inc. All righs reserved 89

6 References Fig. 11 Mass flow rae by hole. Presened resuls can be inerfaced wih spray simulaions o predic mixure formaion. The inerface plane would be he oule of he spray hole and for each spray hole (we have 8 oule planes in he presen case) he mehod will produce a separae inerface file, called nozzle file. Transien condiions wrien in he inerface files are hen used as inpu for spray simulaions and inroduce physical asymmeric effecs from he nozzle ino he engine. Such deails are imporan o model combusion and emissions effecs inside he engine accuraely and are he opic of ongoing research. 5. Conclusion The paper describes he capabiliy o predic asymmeric flow effecs as a consequence of laeral and axial needle displacemen. Despie he fac ha he moion of he needle was predefined, he mehodology represens a promising basis for coupling of 3D caviaing flow wih 1D hydraulic model. In such case he informaion abou he needle posiion, feauring hree degrees of freedom, would be provided as an inpu o he 3D model a he beginning of each ime sep. The applied CFD program can auomaically adjus he compuaional grid and reproduce asymmeric flow effecs presen along he needle and inside he spray holes. Such resuls are of grea engineering value for subsequen spray simulaions, used for predicion of mixure formaion inside he cylinder. As he flow differs from hole o hole, he condiions a he oule of each hole will also be differen and he spray shape and argeing are very liely o be affeced. Spray developmen resuling from non-axial needle movemen and coupling wih 1D hydraulic ools are he opics of ongoing research. (1) Alajbegovic A., Greif D., Basara B., Iben U., Caviaion Calculaion wih he Two-Fluid Model, 3rd European-Japanese Two-Phase Flow Group Meeing, Cerosa di Ponignano, Sepember 003 () Wang D.M., J. Han J., Greif D., Zun I. and Perpar M., Inerfacial Area And Number Densiy Transpor Equaions For Modeling Muliphase Flows wih Caviaion, Proceedings of ASME FEDSM 05, 9h Inernaional Symposium On Gas-Liquid Two-Phase Flow, Houson, Texas, USA, June 005. (3) Wang D.M. and Greif D., Progress in Modeling Injecor Caviaing Flows wih a Muli-fluid Mehod. FEDSM , ASME Forum on Caviaion and Muliphase Flow, Miami, FL, USA, July 006. (4) Greif, D., Srinivasan, V., Numerical Predicion of Erosive Caviaing Flows in Injecion Equipmen, , 10h Inernaional Conference on Engines & Vehicles Capri, Napoli (Ialy), 011. (5) Caia, V., Sampl, P., Taschl, R., Krammer, J., Greif, D., Coupled 1D-3D Simulaion of Common Rail Injecor Flow Using AVL HYDSIM and FIRE, ICE 009, Paper reference (6) Jelovic, M., Caia, V., Sampl, P., Greif, D., Modeling of common rail injecor dynamics and caviaion wih coupled 1D- 3D simulaion ools, JSAE Spring Convenion Proceedings, No.93-10, pp.3-6, Yoohama, Japan, May 010. (7) Ishii M., Thermo Fluid Dynamic Theory of Two-Phase Flow, Eyrolles, Paris, (8) Lahey, R.T., Jr. and Drew, D.A., An Analysis of Two-Phase Flow and Hea Transfer Using a Mulidimensional, Muli-Field, Two-Fluid Compuaional Fluid Dynamics (CFD) Model, Japan/US Seminar on Two-Phase Flow Dynamics, Sana Barbara, California, 000. (9) Drew D.A. and Passman S.L., Theory of Muli-componen Fluids, Springer, New Yor, (10) Franlin, R.E., McMillan, J. and Rozewicz, J. "Proc. ASME Caviaion", (1990). (11) Basara, B., An Eddy Viscosiy Transpor Model Based on Ellipic Relaxaion Approach, AIAA Journal, vol. 44, issue 7, pp , 006. (1) Hanjalic, K., Popovac, M. and Hadziabodic, M., A robus near-wall ellipic-relaxaion eddy-viscosiy urbulence model for CFD, In. J. Hea and Fluid Flow, 5, , 004. (13) AVL FIRE, User s Manual, Graz 013. Copyrigh 014 Sociey of Auomoive Engineers of Japan, Inc. All righs reserved 90