Influence of approaches in CFD Solvers on Performance Prediction in Screw Compressors

Transcription

1 Purdue University Purdue e-pus Interntionl Compressor Engineering Conference School of Mechnicl Engineering 2014 Influence of pproches in CFD Solvers on Performnce Prediction in Screw Compressors Ahmed Kovcevic City University London, United Shm Rne City University London, United Kingdom, Nikol Stosic City University London, United Kingdom, Yu Jing Simerics, Inc, United Sttes of Americ, Sm Lowry Simerics, Inc, United Sttes of Americ, See next pge for dditionl uthors Follow this nd dditionl works t: Kovcevic, Ahmed; Rne, Shm; Stosic, Nikol; Jing, Yu; Lowry, Sm; nd Furmnczyk, Michl, "Influence of pproches in CFD Solvers on Performnce Prediction in Screw Compressors" (2014). Interntionl Compressor Engineering Conference. Pper This document hs een mde ville through Purdue e-pus, service of the Purdue University Lirries. Plese contct for dditionl informtion. Complete proceedings my e cquired in print nd on CD-ROM directly from the Ry W. Herrick Lortories t Herrick/Events/orderlit.html

2 Authors Ahmed Kovcevic, Shm Rne, Nikol Stosic, Yu Jing, Sm Lowry, nd Michl Furmnczyk This rticle is ville t Purdue e-pus:

3 1124, Pge 1 Influence of pproches in CFD Solvers on Performnce Prediction in Screw Compressors Ahmed KOVACEVIC 1 *, Shm RANE 1, Nikol STOSIC 1, Yu JIANG 2, Michl FURMANCZYK 2 nd Sm LOWRY 2 1 Centre for Positive Displcement Compressor Technology, City University London, EC1V0HB, UK..kovcevic@city.c.uk 2 Simerics Inc., Huntsville, AL 35801, USA. yj@simerics.com * Corresponding Author ABSTRACT Computtionl Fluid Dynmics (CFD) offers insight into screw compressor designs eyond the cpilities of other conventionl methods. It llows evlution of locl flow ptterns which influence performnce ut re difficult or impossile to investigte experimentlly. Implementtion of CFD in these mchines is chllenging due to the physics of the flow, the properties of the working fluids nd the complexity of flow pssges which chnge size nd position. This is dditionlly chllenged y lck of methodologies ville to generte the meshes required for the full three dimensionl trnsient simultions. Commercilly ville CFD solvers need to fully interct with customized grid genertors to enle resolution of grid deformtion during flow solution. However, the fctors tht influence flow predictions re not only relted to grids ut lso to the pproch which CFD solvers use to clculte distriution of prmeters such s pressure, velocities, tempertures, etc. In this pper, two pproches most commonly used in commercil CFD softwre re compred nd nlysed. The first is segregted cell-centre sed solver nd the second is coupled vertex-centre sed solver. Both re pressure sed finite volume solvers. Customized grid genertion softwre is used for meshing of moving rotors nd flow domins round the rotors in n oil free ir screw compressor with N rotor profile of 3/5 loe comintion. The deforming rotor grid is mintined s identicl in oth solvers. The performnce predictions otined y clcultions with these two CFD models re compred with mesurements otined on the test compressor in the City University London test rig. The comprison includes pressure in the compressor chmer, mss flow rte, indicted power nd the volumetric efficiency. The study revels differences etween the results otined y two different solvers nd the experimentl results. Anlysis presented in this pper provides good sis for further considertion of differencing schemes nd other chrcteristics nd settings for different CFD solvers in order to chieve ccurte predictions of flows in positive displcement mchines. 1. INTRODUCTION Rotry Screw Compressors re positive displcement mchines widely used in refrigertion, oil nd gs nd other industries. The working chmer of these mchines consists of pir of helicl rotors tht tightly mesh with ech other nd rotte inside csing. Figure 1 shows typicl cycle of opertion with the vrition of chmer volume nd pressure. The cycle strts when the screw loes egin to form volume tht increses with rottion nd enles gs dmission in the interloes. With further rottion, these pockets disconnect from the suction port. This disconnection usully occurs t the position in which the interloe volume chieves its mximum vlue. Further rottion of the rotors cuses reduction of the chmer volume nd n increse in the gs pressure nd temperture.

4 1124, Pge 2 When the rotors rech the dischrge port, this volume connects to the dischrge domin. The mximum efficiency of the cycle is reched when the internl nd externl pressures re mtched t the position of opening the working chmer to the dischrge port. Thermodynmic chmer models re commonly used in the design nd nlysis of twin screw compressors (Kuder nd Ru, 1994, Hnjlic nd Stosic, 1997). These help in the erly design stges to predict performnce nd give generl chrcteristics of screw mchine. Within the pst decde, Computtionl Fluid Dynmics (CFD) hs een incresingly used s tool for design improvements in the screw compressors, prticulrly for compressor ports (Kovcevic et l., 2007, Voorde et l., 2005, Pscu et l., 2012). Figure 1. Typicl Pressure nd Volume vrition in Screw Compressor The chllenge in implementing CFD in positive displcement screw mchines is the physics of the flow, the properties of working fluids nd the complexity of the flow pssges which chnge size nd position in time. Commercilly ville CFD solvers need to fully interct with customized grid genertors to enle resolution of grid deformtion during flow solution. A rekthrough ws chieved in 1999 when the rck genertion method originlly descried y Stosic (1998) ws pplied to generte numericl grids for twin screw rotors y Kovcevic (1999). The first grid genertor for screw compressors ws sed on nlyticl grid genertion principles which llowed vriety of CFD solvers to clculte performnce of screw compressors, (Kovcevic et l., 2002). Since then severl ctivities hve een reported on CFD nlysis of twin screw compressors. Kovcevic et l., (2007) hve presented vrious grid genertion spects for twin screw compressors nd reported results from CFD simultions of twin screw mchines for prediction of flow, het trnsfer nd fluid-structure interction. Results otined with Comet nd StrCD softwre were vlidted y use of Lser Doppler Velocimetry (Kovcevic et l., 2009, Kethidi et l., 2011). Pscu et l., (2012) hve reported use of the sme grid genertion tools for design of the dischrge port in n oil-free screw compressor. Suls nd Brnch (2013) hve utilized dt from full scle CFD models to improve one dimensionl lekge formultion in their thermodynmic chmer models. Huser nd Beinert (2013) compred CFD models nd experimentl mesurements of pressure pulstions in the dischrge chmer of screw compressor to predict the effect of operting prmeters on gs pulstion. Rne et l. (2013) hve extended the sme grid genertion frmework towrds vrile geometry twin screw rotors nd reported n influence on the performnce of rotor led nd profile vritions. However, the fctors tht influence flow predictions re not relted to grids only ut lso to the pproch which CFD solvers use to clculte the distriution of prmeters such s pressure, velocities, tempertures, etc. Kovcevic nd Rne (2013) hve evluted dry twin screw expnder using CFD for rnge of operting conditions nd compred the results with mesured dt provided y Dortmund University. It ws found tht the CFD prediction etter lign to mesurements t higher speeds thn t lower speeds. Similrly higher devitions were reported t higher pressure rtios. Some other recent studies, not pulished, hve lso reported differences etween CFD predictions nd mesurements. It is therefore necessry to further investigte the influence of CFD model

5 1124, Pge 3 prmeters, nd the different solver formultions currently ville for solving flows in screw mchines. The ojective of this pper is to compre two commonly used solvers in commercil CFD softwre. The first is coupled vertex-centre sed solver which is implemented in ANSYS CFX (2011) nd hs een commonly used for screw compressor nlysis. The second is the segregted cell-centre sed solver used in PumpLinx (2014). Both re pressure sed finite volume solvers. The lter segregted solver hs een frequently used for modelling flow nd cvittion in pumps nd piston nd scroll compressors. Recent development in the solver llows it to e used for modelling of twin screw compressors. Jing et l., (2007) hve reported the nlysis of crescent oil pumps using this solver nd compred predictions with the experimentl results. Wng et l. (2012) hve modelled vne oil pump using the segregted solver nd coupled it with n ODE kinetics model of the control spring. This pump performnce ws lso compred with experimentl results. Both cses provided good reltionship etween the model nd mesurements. In this pper, the grid genertion softwre SCORG (Kovcevic, 2007) is used for meshing the moving rotor domins in the oil free ir screw compressor with N rotor profile of 3/5 loe comintion. The sme deforming rotor grid generted y SCORG is used for performnce clcultion with oth solvers. The performnce is lso mesured on the test compressor in the test rig t City University London. Performnce predictions otined from CFD models re compred with mesurement results. The compred performnce indictors include pressure vrition in the compressor chmer, mss flow rte, indicted power nd the volumetric efficiency. 2. TEST CASE AND GRID GENERATION The compressor used for this study is n oil-free twin screw compressor with 3/5 loe rrngement nd N rotor profile rotors. The operting speed on the mle rotor vries from 6000 to rpm. The mle rotor dimeter is mm; the femle rotor dimeter is mm while the centre distnce etween the two rotors is mm. The length to dimeter rtio of the rotors is 1.6 nd the mle rotor hs wrp ngle deg. The nominl interloe, rdil nd end lekge gps re 160 micro meters ech. The dischrge port is designed for the uilt-in volume index of 1.8. Figure 2 shows the compressor mounted on the test rig. Figure 2. 3/5 N Rotor Screw Compressor ) The Mchine nd ) Extrcted Flow Model The fluid domin is extrcted from the CAD model nd consists of three min prts: the suction, rotors nd the dischrge. Figure 2 shows the 3D CAD model of the compressor nd the extrcted fluid volumes. The suction nd dischrge ports re extended y circulr pipes connected to the domins y explicitly connected interfces. The grids of the rotor domins deform with the rottion of rotors. The proprietry, commercilly ville grid genertor SCORG ws used to generte set of grid files tht re supplied to the solver s the simultion progresses. For detils on the grid genertion procedure refer to (Kovcevic et l., 2007). During opertion, the rotors re suject to therml deformtion which in turn chnges the clernce. CFD models used in this reserch do not tke into considertion chnges in clernces. Therefore, it is estimted tht the clernces will reduce with the increse in temperture nd the grids for CFD re generted using reduced uniform clernces of 60 micro meters in the interloe nd rdil gps. The end clernces re not included in the CFD model. Figure 3 shows the structured

6 1124, Pge 4 numericl grid on the rotor surfces nd Figure 3 shows the grid in the cross section of the rotor domin. The working fluid is ir. A molr mss of kg/kmol, Specific Het Cpcity e 03 J/kg K, Dynmic Viscosity 1.831e -05 kg/m s nd Therml Conductivity 2.61e -02 W/m K were specified in oth solvers. A uniform pressure of 1.0r ws specified t the suction while two cses with dischrge pressures of 2.0r nd 3.0r were nlysed for vriety of speeds from 6000rpm to 14000rpm. Figure 3. Grids in the Rotor domin ) On the rotor surfce nd ) In the rotor cross section 3. PRESSURE BASED COUPLED SOLVER The pressure sed coupled solver (ANSYS CFX) will e referred to s Solver-1 nd the segregted solver used in PumpLinx will e referred to s Solver-2. Solver-1 uses n Element-sed finite volume method. Figure 4 presents flow chrt with n overview of the solution process used y generic coupled solver wheres Figure 4 is 2D illustrtion of the Element-sed construction of the control volume. Figure 4. Overview of the Pressure sed solver ) Coupled Approch, ) Element-sed control volume, c) Cell centred control volume nd d) Segregted Approch

7 1124, Pge 5 In the Element-sed method of Solver-1, the sptil domin is firstly discretized into mesh y using n externl grid genertor like SCORG. This mesh is then used to construct virtul control volumes within the solver. All solution vriles nd fluid properties re stored t the nodes. A coupled system of equtions comprising the momentum equtions nd the pressure-sed continuity eqution is solved in one step. The remining equtions, such s energy nd turulence, re solved in decoupled fshion. The rte of convergence is expected to improve with this pproch ut the memory requirement increses times compred to segregted solver since the momentum nd pressure-sed continuity equtions needs to e stored in the memory t the sme time. Tle 1 summrizes the importnt selection of Solver-1 prmeters for typicl screw compressor nlysis. Tle 1. Solver-1 Modelling Prmeters (Used y ANSYS CFX) Criteri Selection Remrk Mesh in Rotor Hexhedrl + Pyrmid Generted y Customized Grid Genertor Mesh Deformtion User Defined Vi Junction Box routines Conservtive Mesh in Ports Tetrhedrl with Boundry Generted y Solver-1 lyer refinements pre-processor Represented in Figure 5 Turulence Model SST k Omeg Flow regime is Turulent Inlet Boundry Specified Totl Pressure Allows for flow to go in Opening Condition nd Temperture nd out of the domin Outlet Boundry Specified Totl Pressure Allows for flow to go in Opening Condition nd Temperture nd out of the domin Control Volume Grdients Guss Divergence Theorem Shpe functions used to interpolte Advection Scheme Upwind High Resolution Pressure-Velocity Coupling Co-locted lyout Rhie nd Chow 4 th Order Turulence Scheme First Order Upwind Trnsient Scheme Second Order Bckwrd Euler Fully Implicit Trnsient Inner Loop Coefficients Up to 20 itertions per time step Convergence Criteri 1e-03 r.m.s residul level Relxtion Prmeters Solver relxtion fluids 0.1 or lower For Stility Figure 5. Solver-1 Tetrhedrl Mesh Figure 6. Solver-2 Body-fitted inry tree Mesh

8 1124, Pge 6 4. PRESSURE BASED SEGREGATED SOLVER The pressure sed segregted Solver-2 uses cell-centred finite volume pproch. Figure 4c is 2D illustrtion of the cell centre sed construction of the control volume. The cell-centre pproch directly uses the mesh generted y the externl grid genertor to form control volumes. Figure 4d presents flow chrt with n overview of the solution process used y generic segregted solver. The governing equtions re solved seprtely for ech vrile. Firstly, the momentum equtions re solved using the updted vlues of pressure nd fce mss fluxes. This is followed y the pressure correction eqution. Fce mss fluxes, pressure, nd the velocity field re then corrected using the pressure correction otined from pressure-velocity coupling solution. The solution is then otined itertively until the convergence criteri re met. An importnt step in the segregted pproch is the pressure-velocity coupling lgorithm. Mny lgorithms, Simple, SimpleC, PISO, SimpleS (Simerics proprietry) etc., hve een developed over yers to improve the roustness of this pproch. The segregted lgorithm is memory-efficient, since the discretized equtions need only to e stored in the memory one t time. Tle 2 summrizes the importnt Solver-2 prmeters nd their selections for typicl screw compressor nlysis. Tle 2. Solver-2 modelling prmeters (Used y PumpLinx) Criteri Selection Remrk Mesh in Rotor Hexhedrl + Pyrmid Generted y Customized Grid Genertor Mesh Deformtion Screw Compressor Templte Conservtive Mesh in Ports Body-fitted inry tree Generted y Solver-2 pre-processor Represented in Figure 6 Turulence Model k-epsilon RNG Flow regime is Turulent Inlet Boundry Condition Pressure Inlet or Totl Pressure Specified Sttic/Totl Pressure nd Sttic/Totl Temperture Outlet Boundry Specified Sttic Pressure Pressure Outlet Condition nd Temperture Advection Scheme Upwind First/Second Order Pressure-Velocity Coupling Co-locted Lyout SIMPLE S Simerics Proprietry Turulence Scheme First Order Upwind Trnsient Scheme First Order Bckwrd Euler Fully Implicit Trnsient Inner Loop Coefficients Up to 25 itertions per time step Convergence Criteri 1e-02 r.m.s residul level Relxtion Prmeters Pressure 0.5 For Stility 5. EXPERIMENTAL RESULTS Experimentl investigtion of the compressor performnce ws crried out in the ir compressor test rig t City University London. Figure 7 presents the lyout of the mesurement setup with the min components nd mesurement points. The compressor is driven y vrile speed 75kW motor nd hs n internl synchronizing ger ox with the ger rtio 7.197:1. The speed of the motor is djusted using vrile frequency drive. The torque meter is instlled on the motor shft while the digitl encoder for the speed mesurement is mounted on the mle rotor shft. Figure 2 shows the compressor in the test rig. The pressure nd temperture of the gs re mesured t the inlet, the dischrge nd upstrem of the orifice plte. In ddition, three pressure trnsducers re used for recording the interloe pressures nd re locted in the working chmer through the compressor csing on the mle rotor side. The flow through the compressor is mesured y use of n orifice plte instlled in the dischrge line of the system. The dischrge line contins control vlve for regultion of the dischrge pressure. The dt cquisition is crried out using CompctRIO from Ntionl Instruments nd Lview. The mesurements were tken for dischrge pressures up to 2.5r nd speeds from 6000 to 8000rpm.

9 1124, Pge 7 Figure 7. Experimentl setup for screw compressor performnce mesurement 6. COMPARISON OF RESULTS AND DISCUSSION The results otined from CFD modelling re compred with the experimentl results in the sections tht follow. 6.1 Pressure-Angle Digrm Figure 8 shows the vrition of the chmer pressure with the ngle of rottion of the mle rotor t two speeds, 6000 nd 8000rpm. The results from CFD clcultion re compred with experimentl dt. Figure 8. Pressure-Angle digrm comprison t 2.0r dischrge pressure for ) 6000 rpm, ) 8000 rpm The internl pressure clculted y oth solvers is greeing well with the mesured pressure curve t oth speeds. Some differences etween predictions nd mesurements re noticed ner the pek pressure t the moment of opening of the dischrge port. Solver 2 shows slightly etter greement with mesured dt.

10 1124, Pge Mss Flow rte Figure 9 shows the comprison of the flow predictions from the CFD clcultions with experimentl dt nd Figure 10 shows the comprison etween the two solvers over the full rnge of speeds nd pressures. Solver-2 is predicting higher mss flow rte s compred to Solver-1 nd is closer to the experimentl results. The predicted mss flow rte depends on the ssumption of clernces. A smller clernce would result in higher flow rtes nd vice vers. The ssumed clernce gp ws 60 micrometres. In order to determine the sensitivity of the results to the clernces, nother clcultion ws performed with Solver-2 with clernce of 120 micrometres. As shown in Figure 9, this resulted in flow rte lower thn the mesured. This indictes tht the verge physicl clernce is etween 60 nd 120 micrometres, proly round 100 micrometres in this cse. It is known tht the clernces in the rotor domin re vrying non-uniformly with operting conditions nd the ssumption of verge clernces my introduce inccurcy. Therefore, further studies re needed in order to confirm if the lower thn mesured flow predicted y Solver-1 is due to the coupled solution pproch or some other fctors. Figure 9. Comprison of Experimentl nd CFD Predictions of ) Mss Flow Rte nd ) Indicted Power Figure 10. Comprison of CFD Predictions with 60um clernce over full rnge of ) Flow nd ) Indicted Power 6.3 Indicted Power In the experiment, the power ws mesured on the motor shft nd constnt mechnicl efficiency of 95% ws ssumed for the integrl gerox t ll speeds. Figure 9 shows the comprison of the indicted power prediction from CFD clcultions with experimentl dt nd Figure 10 shows the comprison etween the two solvers over the full rnge. Both solvers re predicting similr indicted power which is very close to the experimentl results. The ssumption of the constnt mechnicl efficiency of the gerox is not the most ccurte since the efficiency of the gerox chnges with speed. Despite this, the greement in power prediction is good.

11 1124, Pge Specific Power nd Volumetric Efficiency Specific power is the rtio of the indicted power nd flow through the compressor. A lower specific power indictes etter mchine. Figure 11 compres the specific power prediction from the CFD clcultions with the experimentl dt nd Figure 12 shows the comprison etween the two solvers over the full rnge. Results otined from Solver-2 re closer to the experimentl results nd could e further improved y specifying clernces closer to the expected rel vlues. Figure 11 compres the volumetric efficiency prediction from the CFD clcultions with the experimentl dt nd Figure 12 shows the comprison etween the solvers over the full rnge. Efficiencies otined from Solver-2 re closer to the experimentl results nd could e further improved y specifying ccurte clernces. As the compressor speed increses the volumetric efficiency increses. Both solvers predict the sme trend. Figure 11. Comprison of Experimentl nd CFD Predictions of ) Specific Power nd ) Volumetric Efficiency Figure 12. Comprison of CFD Predictions with 60um clernce of ) Specific Power nd ) Volumetric Efficiency The study reveled differences in performnce predictions etween the two CFD solvers nd lso devitions compred with the experimentl results. The time required for the segregted cell-centre Solver-2 to rech cyclic solution is out one third of the coupled Solver-1; it is less memory intensive nd resulted in flow predictions closer to the experimentl vlues.

12 1124, Pge CONCLUSIONS Flow in twin screw compressor ws modelled using two different CFD solvers which hve different pproches to discretistion nd solution of the governing equtions. Performnce predictions were otined over rnge of speed nd pressure rtios of the oil free ir compressor. These performnce prmeters were lso mesured on test rig. The following conclusions cn e derived from comprison of the results in this study: Differences exist etween the two solvers in the prediction of flow rtes through the compressor. Comprison with experimentl dt suggests tht Solver-2 is giving more ccurte estimtion of flow rtes for the expected clernces. In the future, it will e required to crry out further studies in order to estlish the reson for under prediction of the flow y the coupled solver used in Solver-1. Both solvers re predicting indicted power close to the experimentl dt. Opertionl clernces re chnging due to the chnge in temperture for different operting conditions. In order to otin ccurte predictions, this chnge should e ccounted for in the CFD models in future. This my require employment of fluid solid interction modelling. This study provides good sis for further considertion of the ppliction of vrile lekge gps sed on empiricl or nlyticl correltions nd lso in improvements of differencing schemes nd other settings required for specific CFD solvers. Avilility of extended dt from the experimentl mesurements will e eneficil for comprison over wider rnge of operting conditions nd will help improve the CFD models. REFERENCES ANSYS 13.0, 2011, User help. Hnjlic K., Stosic N., 1997, Development nd Optimiztion of Screw mchines with simultion Model Prt II: Thermodynmic Performnce Simultion nd Design Optimiztion. Trnsctions of the ASME 664 / Vol. 119 Huser J. nd Beinert M., 2013, CFD nlysis of pressure pulstion in screw compressors - comine theory with prctice. 8th Int conf on compressors nd their systems, p Jing Y., Furmnczyk M., Lowry S., Zhng D., nd Perng Chin-Yun, 2007, A Three-Dimensionl Design tool for Crescent Oil Pumps. SAE p Kuder K., Ru B., 1994, Auslegungsverfhren fur Schruenkompressoren. (Design Procedure for screw compressors), VDI Berichte 1135, S , Düsseldorf: VDI-Verlg. Kovcevic A., 2002, Three-Dimensionl Numericl Anlysis for Flow Prediction in Positive Displcement Screw Mchines, Ph.D. Thesis, School of Engineering nd Mthemticl Sciences, City University London, UK. Kovcevic A., Stosic N. nd Smith I. K., 2007, Screw compressors - Three dimensionl computtionl fluid dynmics nd solid fluid interction, ISBN , Springer-Verlg Berlin Heidelerg New York. Kovcevic A., Stosic N., Smith I. K., Mujic E., nd Guerrto D., 2009, Extending the role of computtionl fluid dynmics in screw mchines, 6 th Int conf on compressors nd their systems, p. 41. Kethidi M., Kovcevic A., Stosic N. nd Smith I. K., 2011, Evlution of vrious turulence models in predicting screw compressor flow processes y CFD. 7 th Int conf on compressors nd their systems, p Kovcevic A. nd Rne S., 2013, 3D CFD nlysis of twin screw expnder, 8th Interntionl conference on compressors nd their systems, London, p Pscu M., Kovcevic A., nd Udo N., 2012, Performnce optimiztion of Screw Compressors sed on numericl investigtion of the flow ehviour in the dischrge chmer. Int Compressor Conf t Purdue, Purdue, p PumpLinx 3.2.2, 2014, User help. Rne S., Kovcevic A., Stosic N. nd Kethidi M., 2013, CFD grid genertion nd nlysis of screw compressor with vrile geometry rotors, 8 th Int conf on compressors nd their systems, p Suls J. nd Brnch S., 2013, Use of CFD to develop improved one-dimensionl thermodynmic nlysis of refrigernt screw compressors. 8th Int conf on compressors nd their systems, p Stosic N., 1998, On Gering of Helicl Screw Compressor Rotors, Proceeding of IMechE, J. Mech. Eng. Science, Vol.212, nd pp Voorde John Vnde nd Vierendeels Jn, 2005, A grid mnipultion lgorithm for ALE clcultions in screw compressors. 17th AIAA Computtionl Fluid Dynmics Conference, Cnd, AIAA Wng D., Ding H., Jing Y. nd Xin X., 2012, Numericl modeling of vne oil pump with vrile displcement. SAE Pper