FloEFD simulation of micro-turbine engine

Transcription

1 FloEFD simulaion of micro-urbine engine T.V. Trebunskikh, A.V. Ivanov, G.E. Dumnov Menor Graphics, Moscow, Russia Absrac Keywords: micro-urbine engine, CFD, combusion, compressor, urbine Turboje engines have complex geomery and physics of processes occurring in hem. Undersanding hese processes is very imporan for designing of a high-performance produc. I is very difficul o invesigae some processes inside of an engine during he ess. In such condiions CFD mehods become an exremely useful ool. A compuaional sudy of inernal flow in KJ 66 micro-urbine engine calculaed as one uni is presened. The calculaions are made wih mulicad-embedded CFD ool FloEFD a several roaional speeds (roaion of air in he compressor and he urbine, conjugae hea ransfer, air/kerosene combusion are aken ino accoun). The main resuls of simulaion are presened. Some parameers are compared wih experimenal daa. Thereby i is shown ha prediced emperaure a he oule of he compressor and he combusion chamber, mass flow a he inle of he engine and hrus of he engine have good agreemen wih experimenal daa. Raher nonuniform disribuion of fluid emperaure a he oule of he combusion chamber is observed ha can negaively affec he urbine performance. Nomenclaure c c p specific hea specific hea a consan pressure C, C μ, C 1 urbulen consans d minimum disance from a wall e c T, specific inernal energy g vecor of graviy h h 0 m H I k K m M ref r r Q H r s S T Sc Sc T c p T, enhalpy individual hermal enhalpy of m h componen oal enhalpy uni ensor urbulen kineic energy reacion rae parameer molar mass pseudo molecules saic pressure effecive pressure reference value of pressure randl number urbulen randl number specific hea release (or absorpion) per uni volume disance from a poin o he roaional axis in he roaional reference frame shear sress ensor volume hea source Schmid number urbulen Schmid number ime emperaure 1

2 U x y y m y, y O F Y F Z i vecor of velociy radius vecor concenraion concenraion of m h componen mass fracion of oxidizer and fuel residue respecively mass fracion of he fuel mass fracion of he combusion produc urbulen dissipaion rae dynamic viscosiy urbulen viscosiy eigenvalues of he hermal conduciviy ensor densiy ρ reference value of densiy ref σ, σ k R τ τ Ω Subscrips F O fuel oxidizer produc 1. Inroducion urbulen consans Reynolds sress ensor summarized sress ensor vecor of angular velociy Micro-urbine engines have been developed for specific fligh applicaions. They have been used in unmanned aerial vehicles (UAVs), which were designed for shor fligh duraion. A lo of differen kinds of UAVs are now operaing worldwide. Anoher common applicaion for small gas urbines is an auxiliary power uni (AU) supplemening aircraf engines which provide addiional power when required (1,). Because of he small size micro-urbine engines show small mass flow raes of air, low pressure raios, bu very high roaional speeds. A micro-urbine engine chosen for he sudy is KJ 66 (Fig.1), which is one of he mos robus small engines wih available design. Figure 1. The scheme of KJ 66 micro-urbine engine. Complex geomery and small size of his kind of engines limi he access of convenional insrumens for he measuremen of flow parameers which is needed for beer undersanding of a complex flow srucure. Also choosing opimal design of single pars of an engine during ess is raher expensive procedure. In such condiions CFD mehods become an exremely useful ool. Resuls demonsraed in his paper are obained in mulicad-embedded full-feaured general purpose CFD ool FloEFD serving he needs of popular MCAD sysems. Several auhors invesigaed KJ 66 engine bu hey calculaed i as separae pars. Thereby nonuniformiy of parameers beween main pars of he engine (e.g. velociy and pressure disribuion a he inle of he combusion chamber) is no aken ino accoun in ha approach. In

3 his paper a compuaional sudy of inernal flow in KJ 66 micro-urbine engine calculaed as a whole uni is presened. According o his approach more accurae resuls are obained. In he firs par of he paper, he saemen of he problem including descripion of boundary condiions and mesh is presened. In he nex secion modelling approach is described including governing equaions, roaional and combusion models. In he hird secion main resuls of calculaion of all engine par are presened and compared o experimenal daa (1-3). Major conclusions are lised in he las secion.. Saemen of he problem KJ 66 micro-urbine engine is a high-performance model je engine ha has been well known since is inroducion. The compressor is a urbocharger wheel wih he diameer of m. The compressor s diffuser is made of aluminum which blades have he form of fa wedges. The axial urbine wheel has 3 blades ha can sand exremely high roaional speeds. The combusion chamber is compac, so ha he shor shaf can be used. KJ 66 engine weighs around 0.95 kg depending on he version and offers a very good hrus o weigh raio (1). The combusion chamber feaures direc injecion of fuel hrough six vaporizing sicks for achieving complee combusion before he urbine. The model of he engine was buil in SolidWorks CAD sysem and demonsraed in Fig. and he real prooype is shown in Fig. 3. This engine is calculaed as one uni (360 degrees wihou ransferred, symmerical or periodic condiions). Figure. The model of KJ 66 engine in FloEFD. Figure 3. The real prooype. Several mesh varians wih he oal cells number of ~600000, ~ , ~ are examined. The local iniial mesh was creaed on all main pars of he engine such as he compressor, he combusion chamber, he urbine and so on. The addiional local mesh on all blade s surfaces was specified (Fig. 4). Figure 4. The calculaion mesh of KJ 66 engine model wih ~ cells. Five cases wih roaional speeds of 40000, 60000, 80000, (he nominal mode) and rpm are considered here. Specifying of local roaional zones in FloEFD is shown in Fig. 5. 3

4 Figure 5. The roaional regions of KJ 66 engine. The oal pressure and he saic emperaure of air are a and K respecively a he inle of he engine. A he oule he same condiions are reaed as amospheric ones. The saic emperaure of kerosene is 300 K a he oules of he fuel sicks and mass flow depends on he roaional speed. Kerosene is specified as a gas phase. The air fuel raio is ~65. The solid pars are specified as aluminum, seel and inconel for consideraion of conjugae hea ransfer. Iniial emperaure of solid pars, which was obained in he preliminary calculaions, is specified. The radiaion in he combusion chamber is no aken ino accoun because of invesigaion of general characerisics of he engine. The calculaion is provided in he ransien regime wih he ime sep s. In he solids ime sep equals 0.01 s unil esablishing of he flow. The combusion is calculaed using equilibrium approach wih wo models he model of non premixed combusion and he model of premixed combusion wih he limied combusion rae. 3. Modelling approach 3.1. Governing equaions FloEFD solves he Favre-averaged Navier-Sokes equaions, which are formulaions of mass, momenum and energy conservaion laws for fluid flows wih modified k-ε urbulen model wih he damping funcions proposed by Lam and Bremhors (4) and wih he laminar/ urbulen ransiion. The governing equaions for all dependan variables can be wrien in he general form: U S. (1) Table 1. The dependan variables wih he corresponding ranspor coefficiens and he source erms. S U 1 Ω r ρ g Ω U ref H U τ T h y k S m m T m r r σ k y k τ y r r R k τ U σ k R σ f C τ U f C 1 1 k ρ g x k ref ref 3 () R τ s, τ s ki 3 (3) 0 4

5 T U UI s U, (4) 3 U H h Ω r k h m y m, (5) 3 m k C f, (6) μ 7.5 f, 1 exp R d 1 Re f 3 f, f exp Re, (7) 1 kd k R d, Re (8) C 0.09, C , C 9 1., r 0. 9, 1, (9) k To describe urbulen boundary layers wih approach Two-Scale wall funcions (5) Van Dries s universal profiles are employed in FloEFD: he near wall funcion and he subgrid model of boundary layer. This approach allows FloEFD o overcome a radiional CFD code resricion of having a very fine mesh densiy near walls in a calculaion domain. 3.. Roaional modelling approach The local roaing zones are used for analysis of he fluid flow in he model (4). In accordance wih he employed approach, each roaing solid componen is surrounded by an axisymmerical roaing zone, which has is own coordinae sysem roaing ogeher wih he componen. The fluid flow equaions in he non-roaing zones of he compuaional domain are solved in he non-roaing Caresian Global Coordinae Sysem. To connec soluions obained wihin he roaing zones and in he non-roaing par of he compuaional domain, special inernal boundary condiions are se auomaically a he fluid boundaries of he roaing zones Combusion modelling FloEFD can consider he hermal effecs of chemical reacions relaed o he combusion of gas-phase mixures (4). The model implemened in FloEFD refers o he equilibrium approach o consideraion of combusion producs, where several subsances mixed up o he molecular level reac insanly unil he chemical equilibrium is achieved. Thermodynamical properies of individual subsances which occur in his sysem are deermined from he maximum enropy condiion of he isolaed sysem a he implemenaion of he conservaion laws of aoms, energy and elecrically neural saemen (6). This approach does no provide abiliy o calculae he combusion of he premixed mixures. The limied combusion rae opion exends he equilibrium model in case of premixed combusion. The convenional single reacion irreversible mechanism is inroduced for his approach: K M M M, (10) O F where K f T is a reacion rae parameer, which depends on emperaure. The produc ha appears in he reacion is ransferred over a fluid region in accordance wih he following equaion: Z Z Z Z 1 U m Y Y K F F 1. (11) Sc Sc m m O F In his model he equaion of sae has he following form: h T,, Y, Z y h y h y h, (1) F O O F F 1 y y y O F T,, Y, Z F, (13) O F y 1 Y 1 Z, y Y 1 Z, y Z. (14) O F F F 3.4. Conjugae Hea Transfer FloEFD allows predicing simulaneous hea ransfer in solid and fluid media wih energy exchange beween hem. Hea ransfer in fluids is described by he energy conservaion 5

6 equaion (4). The phenomenon of anisoropic hea conduciviy in solid media is described by he following equaion: e T Q. (15) i H Media considered here are isoropic so ha 4. Resuls. 1 3 In his secion resuls of KJ 66 micro-urbine engine calculaion are presened. The resuls of his simulaion are compared wih experimenal daa. Despie he calculaion of he whole engine i is more convenien o invesigae FloEFD resuls by dividing hem ino four groups: he compressor, he combusion chamber, he urbine calculaions and general resuls Compressor KJ 66 micro-urbine engine uses a compressor wheel from a car urbocharger (7). This wheel is coupled o a wedge diffuser in he engine as i is shown in Fig. 6. Figure 6. The compressor and he diffuser of KJ 66 engine. Figure 7 shows flow rajecories colored by velociy magniude and pressure disribuion on surfaces of he compressor and he diffuser a he normal mode. ressure on he compressor s blades can be lower han a and can reach a on he diffuser s blades. ressure disribuion in compressor s secions a he normal mode can be seen in Fig. 8. Figure 7. Flow rajecories colored by velociy magniude (lef) and pressure disribuion on surfaces of he compressor and he diffuser (righ) a normal mode. 6

7 Figure 8. ressure disribuion a he longiudinal secion (lef) and he cross secion (righ) a normal mode. In Fig. 9 air mass flow a he inle of he engine a various roaional speeds of he compressor can be seen. The FloEFD resuls are compared wih experimenal daa from Kamps T. (1) The values of mass flow mach good experimenal daa and almos do no depend on cells number. As for efficiency of he compressor i was obained ha his par has raher low efficiency for such ype of compressors of micro-urbine engines. The large source on inefficiency is locaed in he wedge diffuser. 4.. Combusion chamber Figure 9. Air mass flow a he inle of KJ 66 engine. The combusion chamber of KJ 66 engine feaures direc fuel injecion hrough 6 vaporizing sicks o ensure complee combusion inside he chamber. The combusion chamber s model is shown in Fig. 10. Figure 10. The combusion chamber of KJ 66 engine. 7

8 I should be noed ha boh approaches of he combusion model (see p.3.3) used in his simulaion do no show significan disincions. Thereby he resuls indicaed here relae o he non premixed combusion approach. Figure 11 shows flow rajecories colored by velociy magniude and pressure disribuion on inernal and blade s surfaces of he NGV a he normal mode. Fig. 1 displays fluid emperaure and velociy disribuions a wo longiudinal secions of he combusion chamber wih flow vecors a he normal mode. Temperaure in he combusion chamber reaches ~400 K. Increasing of velociy in he zones of holes of he combusion chamber is observed especially on he rear wall of i. Raher nonuniform fluid emperaure disribuion a he oule of he combusion chamber can be seen in Fig. 13. Temperaure gradiens a his secion have a negaive influence on he performance of he urbine and, as a resul, his fac can lead o lifeime reducion of i. Figure 11. Flow rajecories colored by velociy magniude and pressure disribuion on inernal and blade s surfaces of he NGV a he normal mode. Figure 1. Fluid emperaure (lef) and velociy (righ) disribuions a wo longiudinal secions of he combusion chamber wih flow vecors a he normal mode. Figure 13. Fluid emperaure disribuion a he oule of he combusion chamber. 8

9 Figures 14 and 15 show comparison of emperaure and fuel mass fracion disribuion in he combusion chamber a rpm obained in FloEFD and Fluen presened by C.A. Gonzales, K.C. Wong and S. Armfield (). The FloEFD model is simplified as well as he Fluen model (examining no all pars of he engine) bu all feaures of he combusion chamber are aken ino accoun. The symmery condiions are no used in FloEFD model as opposed of Fluen model. So here are some differences in disribuion of parameers can be seen in hese figures. Basically FloEFD and Fluen resuls have a good agreemen aking ino accoun poins lised above. I is clearly visible in Fig. 14 ha primary combusion zone is locaed in he cenral par of he chamber. I can be seen in Fig. 15 ha a he oule of he combusion chamber fuel mass fracion equals ~0.0. Figure 14. Temperaure disribuion in he combusion chamber a rpm obained in FloEFD (lef) and Fluen () (righ). Figure 15. Fuel mass fracion disribuion in he combusion chamber a rpm obained in FloEFD (lef) and Fluen () (righ). Figure 16 shows emperaure and fuel mass fracion disribuion in he combusion chamber a rpm obained in FloEFD where all feaures of whole KJ 66 micro-urbine engine are examined. In his figure i can be seen ha disribuions of emperaure and fuel mass fracion have some difference from previous picures due o aking ino accoun he NGV (nozzle guide vanes) ha leads o increase of speed a he oule of he combusion chamber. Figure 16. Temperaure (lef) and fuel mass fracion (righ) disribuions in he combusion chamber a rpm obained in FloEFD (he NGV is aken ino accoun). 9

10 FloEFD allows exporing some parameers as loads for srucural and hermal analyses o Creo Elemens/ro Mechanica. Surface emperaure was expored for hermal calculaion. Then he srucural analysis was provided using emperaure which was a resul of ha calculaion and pressure which was expored from FloEFD. Figure 17 shows displacemen disribuion of srucural analysis. The combusion chamber is deformed under loads and displacemen can reach m NGV and urbine Figure 17. Displacemen disribuion on surfaces of he combusion chamber in Creo Elemens/ro Mechanica (scaling 0%). The urbine wheel is made of m hick sainless seel or inconel depending on modificaion. The blades are wised and profiled manually. The models of he NGV and he urbine are shown in Fig. 18. Figure 18. The NGV (lef) and he urbine (righ) of KJ 66 engine. ressure disribuion a a secion of he NGV and he urbine and emperaure disribuion on he urbine s blades a he normal mode are presened in Fig. 19. During he passage of flow hrough he NGV and urbine pressure drops from a o a. Spread of emperaure values reaches 100 K on he urbine s blades ha can lead o raher srong deformaion of hem. Figure 19. ressure disribuion a a secion of he NGV and he urbine (lef) and emperaure disribuion on he urbine s blades a he normal mode. 10

11 4.4. General resuls General resuls of FloEFD predicion are provided in his secion. Figure 0 shows flow rajecories colored by velociy magniude. ressure disribuion on surfaces of he engine and velociy disribuion near surfaces of he engine are presened in Fig. 1. Figure 0. Flow rajecories colored by velociy magniude. Figure 1. ressure (lef) and velociy (righ) disribuions. In Fig. prediced and experimenal emperaure magniudes a he oule of he diffuser, a he oule of he combusion chamber and a he oule of he engine a differen modes can be seen. The prediced and measured (1,) emperaures a he oule of he compressor a rpm have very close values. A he oule of he combusion chamber a modes up o rpm value of mean emperaure equals ~930 K and a he normal mode and rpm ~990 K. A rpm prediced mean emperaure has 1.6% discrepancy wih experimenal daa. Figure. Temperaure a he oule of he diffuser, he combusion chamber and he engine. Figure 3 shows comparisons of measured (1) and prediced values of hrus of KJ 66 engine a differen modes. I can be seen ha experimenal and prediced values have a good agreemen up o rpm and a rpm some discrepancy from experimenal daa is observed. ossible 11

12 reason of his divergence can be deformaion of engine s pars a high roaional speeds. This deformaion is no aken ino accoun in his invesigaion. Figure 3. Thrus of KJ 66 engine. Comparison of prediced values of orque of he compressor and he urbine of KJ 66 engine wih differen cell numbers is presened in Fig. 4 where a good agreemen beween each oher is observed. As he compressor and he urbine are on he same shaf i means ha momen produced by he urbine is enough for supporing specified roaional speed of he compressor s wheel. Thereby he engine is in he operaion condiion. I can be seen a normal mode ha he higher cell number, he beer agreemen beween orques, e.g. discrepancy is ~ 4% for ~ cells and ~0.8% for ~ cells. I can be noed ha here is mesh convergence in his ask. All parameers are similar in all considered cases of cell numbers. Thereby mesh of ~ cells is enough for definiion almos all inegral parameers here. 5. Conclusions Figure 4. Torques of he compressor and he urbine of KJ 66 engine. 1. The CFD analysis of KJ 66 micro-urbine engine, which is calculaed as one uni wihou any ransferred, symmerical and periodical condiions beween is pars in FloEFD sofware is presened in his paper. Also i is shown how o provide hermal and srucural analyses in Creo Elemens/ro Mechanica using CFD resuls as emperaure and pressure loads. As he whole engine in ransien regime is calculaed so unseady effecs of a saor-roor ineracion, unseady hea ransfer of he urbine s blades can be aken ino accoun and he qualiy of he simulaion can be improved due o unseady combusion.. Comparisons of measured and prediced values of he main inegral parameers such as air mass flow a he inle of he engine, hrus, emperaures a he oule of he diffuser and a he oule of he combusion chamber show a good agreemen. Also i can be seen a good 1

13 agreemen beween calculaions in FloEFD and Fluen for appropriae model of he engine (some pars of he engine are no aken ino accoun). Moreover change of he flow paern due o aking ino accoun all pars of he KJ 66 engine is shown here. Mesh convergence is presened for his calculaion. All parameers are similar in all considered cases of cell numbers so ha mesh of ~ is enough in his ask for definiion almos all inegral parameers. Thereby FloEFD allows providing series of wha-if CFD analyses wih raher small mesh and expor daa for srucural and hermal analyses. 3. Complicaed zones of KJ 66 engine are shown and he causes of heir occurrence are examined: 3.1. The compressor of his engine has he raher low efficiency for such ype of compressors of micro-urbine engines. The large source on inefficiency is locaed in he wedge diffuser. 3.. Raher nonuniform disribuion of fluid emperaure a he oule of he combusion chamber is observed. I can negaively affec on he performance of he urbine Maximum value of emperaure on he inernal rear wall of he housing of he engine can reach 750 K. If hese poins are aken ino accoun he engine can be modified for beer performance. References 1. Kamps, T. Model je engines, UK, Gonzalez, C.A., Wong, K.C., Armfield S. Compuaional sudy of a micro-urbine engine combusor using large eddy simulaion and Reynolds average urbulence models, Ausral Mahemaical Soc, Ausralia, Schreckling, K. Home buil model urbines, UK, FloEFD Technical Reference, Menor Graphics Corporaion, Enhanced urbulence modeling in FloEFD, Menor Graphics Corporaion, Volkov, V.A., Ivanov, A.V., Srelsov, V.U., Khokhlov, A.V. Using of equilibrium models for calculaion of gas combusion, 5h Russian Naional Conf of Hea Exchange, Moscow, Russia, Versraee, D., Hendrick,., Djanali, V., Gonzalez, C., Ling, J., Wong, K.C., Armfield, S. Micro propulsion aciviies a he universiy of Sydney, Ausralia. 8. Gonzalez, C.A., Wong, K.C., Armfield, S. A compuaional sudy of he influence of he injecion characerisics on micro-urbine combusion, 16h Ausral. Fluid Mechanics Conf, Ausralia, Ling, J., Wong, K.C., Armfield, S. Numerical Invesigaion of a Small Gas Turbine Compressor, 16h Ausral. Fluid Mechanics Conf, Ausralia,