Simulation of the Cooling Circuit with an Electrically Operated Water Pump

Transcription

1 Smulaton of the Coolng Crcut wth an Electrcally Operated Water Pump Dragan Smc, Chrstan Kral, Franz Prker Arsenal Research Faradaygasse 3, Object 1 A 1030 Venna, Austra phone: + 43 (0) fax: + 43 (0) e-mal: dragan.smc@arsenal.ac.at, Abstract In ths paper a coolng crcut wth an electrcally operated water pump was examned and smulated. The presented smulaton results were performed wth Dymola. Ths smulaton tool s based on the modellng language Modelca. The model of the water pump was realzed wth tme doman dfferental equatons. The parameters of the dfferental equatons are defned by the geometrcal data of the water pump. The water pump was drven by a controlled electrc motor. A battery was used as power source for the electrc motor. In the proposed applcaton the thermal behavour of all components n the coolng crcut (engne, ppes and cooler) s modelled. For the nternal combuston engne and the cooler the temperature dstrbuton s presented. The smulated and the theoretcal characterstcs of the water pump wll be compared. I. INTRODUCTION Each of the presented models of the coolng crcut consders flow equatons. These flow equatons represent the mass flow balance of the operatng flud from the flud nlet and flud outlet of the model. In all coolng models the thermal conducton of the operatng flud s consdered only n the states n whch the mass flow of the operatng flud s zero. The frcton losses of the operatng flud n the ppe components are modelled by means of characterstc curves. The ppe components are taken from the Modelca standard lbrary Modelca.Thermal.FludHeatFlow [1]. The water pump model s mplemented as a physcal model. The electrc drve components of the water pump are taken from the Modelca SmartDrves lbrary, whch s currently n the development phase. II. WATER PUMP MODEL The model of the water pump s represented by dfferental equatons. The parameters of the dfferental equatons are determned by geometrcal parameters of the water pump. All flow losses ncludng hydraulc losses of the operatng flud, hydraulc losses of the mpeller wheel and mpact losses of the operatng flud flowng through the dstrbutor were consdered. The mechancal losses of the bearng and the seal were modeled as mechancal frcton losses. A. Basc Desgn Bascally, the water pump can be seen as a flud flow machne. However, n flud flow machnes the acton prncple of the water pump s based on the energy theorem. The drvng torque on the mpeller wheel gets transmtted by the clutch and the nput shaft whle the curved shovels of the mpeller wheel exert a pressure on the operatng flud. Due to the centrfugal effect, the operatng flud s hurled outward n radal drecton of the mpeller wheel and therefore, leaves the mpeller wheel at the outsde extent wth hgher speed. Furthermore, because of the dffuser effect n the shovel channels of the mpeller wheel, the operatng flud leaves the mpeller wheel wth ncreased pressure, as well. The mass movement outward causes a negatve pressure at the nlet of the mpeller wheel whch draws further operatng flud. The drvng power s converted completely nto potental energy of the operatng flud. B. Man Equaton of the Water Pump An deal water pump s a flud flow machne wth an nfnte number of shovels. The converted energy n the mpeller wheel of the water pump s the knetc energy of the operatng flud. Therefore the man equaton of the water pump represents the proporton between mechancal energy of the nput shaft wth respect to the mpeller wheel of the water pump and the specfc energy of the operatng flud []. The man equaton of the water pump s: ( β ) ( r Y ) m& ω = π b ρ tan ω (1) In ths equaton r s the outer radus of the mpeller wheel of the water pump, b s the outer wdth of the mpeller wheel, ρ s the densty of the operatng flud and β s the outlet angle of the shovels of the mpeller wheel. Furthermore, m& represents the mass flow of the operatng flud n the water pump and Y represents the specfc energy of the mpeller wheel for an nfnte number of mpeller shovels. Fgure 1 shows the desgn of the mpeller wheel of a water pump.

2 the shovel channels of the mpeller wheel s presented n (5). The frcton factor, ξ, can be calculated usng the flow equaton whle takng the geometry of the water pump and the mpeller wheel nto account. Alternatvely, ths factor can be consdered to be constant f computaton s to complex. Y h c = ξ (5) Fg. 1. Impeller wheel desgn and geometrcal parameters C. Decrease of the Specfc Energy The rotatonal energy of the mpeller wheel gets reduced by decreasng twst n the outlet area of the mpeller wheel. Ths energy reducton equals the energy loss between the release energy of the mpeller wheel and the absorpton energy of the operatng flud. Wth a loss factor, k, t s possble to calculate ths energy loss, Y k, of the water pump [3] usng (). Wth an emprcal number, known as Pfleders number, p, the loss factor, k, can be found through (3). For the calculaton of Pfleders number, n practce, one equaton for sngle curved and one for double curved mpeller shovels exst. The usual water pumps n automotve ndustry have sngle curved mpeller shovels. The calculaton of Pfleders number s presented n (4). ( k) Y k = 1 Y () 1 k = 1 + p a 3 p = 1+ z π (3) β r (4) r r1 In these equatons z s the number of the mpeller shovels of the water pump and r 1 ndcates the radus of the nlet area of the mpeller wheel. The constant, a, s an expermental parameter, wth a value between 1. and.0, dependent on the desgn of the mpeller wheel and the dffuser []. D. Hydraulc Losses n Shovel Channels The entre amount of energy stored n the operatng flud between the shovels of the mpeller wheel gets reduced by flow resstances outsde of the shovel channels. These hydraulc losses are caused by frcton, change of flow drecton and change of the flow cross sectons. The calculaton of the hydraulc losses s dffcult. The relaton of the hydraulc losses, Y h, and the average velocty, c, of the operatng flud n E. Impact Losses Impact losses occur only f the operatng state of the water pump devates from the desgn state. In the devatng case the flud flow s not tangental wth respect to the shoveled channels of the mpeller wheel and the control devce. For an overloaded mpeller wheel, the devaton of the flow drecton from the shovel curve ncreases wth speed and load. Under these condtons, the addtonal mpact losses occur. Furthermore, the mpact losses sgnfcantly ncrease n underload operaton. The mpact losses also become larger n the low speed range of the water pump. However, the mpact losses of the water pump can be derved accordng to Y = a r ω r m& m& 1 A r ω + + p r3 m& A, (6) β a = ( ). (7) π In (6) r 3 s the nternal radus of the dffuser and factor a s an expermental value accordng to (7). F. Frcton Loss of the Impeller Wheel Between the faces of the mpeller wheel and the housng of the water pump a flud flm exsts. In ths thn flm frcton losses occur. These losses are called mpeller wheel frcton losses. Wth (8), t s possble to calculate these frcton losses of the mpeller wheel, P r, as stated n [3]. P r = r r ρ ω ω ν G. Characterstc Curve of the Water Pump A smple crculaton model wth a water pump and a valve s presented n Fg.. The valve creates varable frcton losses n the crculaton. Whle the water pump s drven at constant speed, the frcton losses of the valve are vared from zero to nfnty. A comparson between smulatons and [3] shows good coherence. In Fg. 3 the calculated characterstcs of the water pump are represented. The sold lnes represent pressure ncrease versus mass flow for constant speed operaton. The short dashed lnes represent the water pump loaded wth a constant hydraulc resstance. The long dashed lnes are curves for constant power operaton. (8)

3 Fg.. The model of the water pump ncludng a valve wth varable frcton loss Fg. 4. Temperature dstrbuton of the cooler IV. COOLING MODEL Fg. 3. Smulated characterstc curve of the water pump III. COOLER MODEL The cooler was modeled wth Matlab usng Smulnk. The cooler model was mplemented by means of dscrete volume elements [4, 5]. Frst, the coeffcents of convecton for the coolant, ar, and the steel tubes of the cooler were determned. Afterwards, the coeffcents of convecton of each volume element were calculated. Partcularly, the cooler fan was modeled through ncreased coeffcents of convecton wth respect to the transton from the cooler to ar. Fgure 4 shows the smulated temperature dstrbuton of a usual vehcle cooler. The thermal conductvty coeffcents of the smulaton wth Matlab Smulnk were taken nto account to create a cooler model n Modelca. Fgure 5 represents the man elements of the coolng crcut and the water pump. A battery feeds the electrc motor ncludng the control unt drvng the water pump. The electrc motor and the water pump are drectly connected by a shaft. The speed of the electrc motor and the water pump are adjusted wth the PumpSpeed model. Wth such an electrcal drve concept of the water pump t s possble to control the speed ndependently of the speed of the nternal combuston engne. The operatng condtons of the coolng crcuts can be adjusted by means of the appled water pump speed control. The nternal combuston engne s modeled as thermal source n ths model. In case of a fuel cell, such a thermal source can be used, as well. The thermal losses are consdered n each component. Fgure 5 llustrates a thermal management of an nternal combuston engne. The major dfference to a conventonal coolng crcut s the electrcally operated water pump. The desred speed of the water pump was calculated by the PumpSpeed model usng the temperatures of the operatng flud of the engne nlet and outlet. In the Electrc model of Fg. 5 an electrcal machne ncludng battery and generator s modeled. In a drve cycle smulaton, the generator flange (flange_b) has to be connected wth the belt drve of the nternal combuston engne. Both the battery and the machne model are part of the Modelca SmartDrves lbrary. All other components of the coolng crcut (cooler, ppes and thermostat) are the same as n a conventonal vehcle, taken from the Modelca.Thermal.FludHeatFlow lbrary.

4 Fg. 5. Model of coolng crcut V. TEMPERATURE DISTRIBUTION The New European Drvng Cycle (NEDC) defnes vehcle speed profles for one urban drve cycle (UDC) as well as one extra-urban drve cycles (EUDC), as shown as n Fg. 6(a). In order to demonstrate the functon of the proposed thermal management of the engne, a NEDC cycle of a vehcle s smulated. In Fg. 6(b) the top sold lne wthout marker represents the temperature of the engne cylnders and the sold lne wth the crcle marker shows the temperature of the engne block. The two lnes wth the quadrangle markers are the coolant temperatures of the engne nlet and the engne outlet. The temperature of the coolant of the engne nlet also equals the temperature at the water pump outlet. It can be seen clearly, that the coolant temperatures are contnuous and that the dfference between engne nlet and outlet remans almost constant. Wth ths thermal management concept an undesred thermal shock can be avoded and the heatng perod of the nternal combuston engne can be shortened. In Fg. 6(c) the sold lne wthout marker shows the temperature of the cooler. The sold lnes wth the quadrangle markers are the coolant temperatures of the nlet and outlet of the cooler. The sold lnes wth crcle markers are the ar temperatures of the nlet and the outlet of the cooler.

5 Fg. 6. (a) NEDC cycle; (b) engne and coolant temperature dstrbuton; (c) cooler and coolant temperature dstrbuton VI. ADVANTAGE An electrcally operated water pump s hghly effcent. Partcularly addtonal energy losses of conventonal coolng crcuts can be avoded. These losses are caused by frcton of the belt or the chan drve. Furthermore, the pump can be operated at an effcent speed wthout beng constraned to the speed of the nternal combuston engne. An addtonal advantage of the electrcally operated pump s the reducton of acoustc noses. Due to the mechancal decouplng the water pump can be mounted ndependently of the belt or chan. VII. CONCLUSIONS The energy consumpton of a vehcle can be reduced by means of an electrcally operated water pump. Wth that approach the water pump s only operated when t s needed. Due to the controlled drve a thermal shock of the coolng crcut can be avoded. Addtonal advantages of the proposed coolng crcut are the reduced mantenance and nstallaton tmes. The concept of electrcally drven water pumps gves rse to usng a sngle type of water pump for dfferent engnes and vehcles, unlke n conventonal water pump drves. REFERENCES [1] C. Kral, A. Haumer, M. Planer, Smulaton of a thermal model of a surface cooled squrrel cage nducton machne by means of the SmpleFlow-lbrary, Modelca 005 [] J.F. Gülch, Kreselpumpen, Sprnger Verlag Berln Hederlberg New York 004 [3] W. Kalde, Energeumwandlung n Kraft- und Arbetsmaschnen, Carl Hanser Verlag München Wen 198 [4] G. Merker and C. Baumgarten, Flud und Wärmetransport Wärmeübertragung, Stuttgart, Lepzg, B.G. Teubner, 1999 [5] G. Merker and C. Eglmeer, Flud und Wärmetransport Strömungslehre, Stuttgart, Wesbaden, B.G. Teubner, 000