IX CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIAS TÉRMICAS. 9th BRAZILIAN CONGRESS OF THERMAL ENGINEERING AND SCIENCES

Transcription

1 IX CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIAS TÉRICAS 9th BRAZILIAN CONGRESS OF THERAL ENGINEERING AND SCIENCES NACELLE DESIGN FOR AERONAUTICAL ENGINES Cláudio Sade Brodt EBRAER Empresa Brasileira de Aeronáutia Av. Brigadeiro Faria Lima, 170, PC 191, São José dos Campos - SP, Brazil laudio.brodt@embraer.om.br Guilherme Lara Oliveira EBRAER Empresa Brasileira de Aeronáutia Av. Brigadeiro Faria Lima, 170, PC 191, São José dos Campos - SP, Brazil guilherme.oliveira@embraer.om.br João Roberto Barbosa Instituto Tenológio de Aeronáutia CTA/ITA/IE/IEE São José dos Campos - SP, Brazil barbosa@me.ita.br Paper CIT Abstrat. The naelle design and integration with the airrat is a key point in the overall airrat design. The naelle inlet must deliver the mass low required by the to develop the neessary thrust at all light envelope. inimum variations in the inlet apture area, the inlet proile and the nozzles exit areas may have a great inluene in the low properties, ompromising the perormane. Although some theoretial and empirial methods are used nowadays to design naelles with reasonable perormane, they are not usually available in the open literature, due to its tehnologial importane in the aerospae industry. So, the main motivation o this paper is to evaluate the proposed methodology or subsoni naelles design and hek the basi perormane or speii s. The method is divided in three steps: the hardpoints estimation, whih satisy the speii onditions required by the, the aerodynami proile design, whih must be smooth and suitable, and the CFD analysis to hek and improve. Key Words: Airrat, Naelle, Design, CFD. 1. Introdution The is responsible or delivering the neessary thrust to the airplane at all operational onditions. To ahieve this, it requires dierent amounts o air mass low at eah light ondition, whih is delivered by a suitable naelle. It must also yield enough room or all s build up unit (EBU) and protet the turbomahinery rom oreign objets ingestion (FOI) and impat. An appropriate method to naelle design is very important in order to assure requirements and so overall airrat perormane. The naelle design may be divided into our main steps: the inlet apture-area estimation, the inlet proile deinition, the exit areas determination and the exhaust proile loting deinition. inimum variations in any o these parts may have a great inluene in the low properties, ompromising the perormane. Despite its tehnologial importane or the aerospae industry, the open literature laks a lear, omplete and detailed naelle design method. Some empirial and statistial ormulae are known but their generality are not suitable or speii design appliations, limiting their appliation to hek analysis. This paper summarizes the work done by Brodt, 001, whose main motivation was to develop a new design methodology or subsoni ivil naelles equipped with speii s to assure the design airrat perormane at all light onditions. The EBRAER new regional jet ERJ-170 was hosen as a ase study to evaluate the methodology. The results were ompared to existing naelle and with others obtained rom statistial approahes. The purposed method is divided basially into three steps. Firstly, to determine all naelle geometry points, alled hardpoints, to math requirements. Inlet and exit areas dimensions are determined rom momentum onservation theorem and ontinuity. Engine dimensions and urther assumptions will give the intermediate points. Then, a smooth and onvenient aerodynami proile will be itted to those points using CAD tools. Finally, a CFD analysis will be iteratively arried out in order to improve the naelle perormane.. Theoretial Approah Typially, the propulsion system is divided into three main parts, the, the naelle inlet system and the naelle exhaust system, whih will be analyzed separately in this setion, details o whih may be seen in attingly, The naelle is onsidered as isolated and the low axis-symmetri. 1

2 .1 Engine The EBRAER ERJ-170 is equipped with two underwing GE short dut high bypass turboan s, CF34-8 model (3.5m length, 1.17m an diameter, bypass ratio 5, 63165N maximum thrust). Sometimes, it is possible that the main harateristis are not known at the beginning o the naelle design. Engine design inormation suh as types, positioning and dimensioning may be ound, or instane, in Oates, 1989a, Oates, 1989b, Roskam, 1997 and Raymer, Inlet System The is surrounded by owls, whih protet its omponents and provide smooth suraes or lean airlow, internally and externally. The inlet system is part o the naelle. Subsoni installations are easier to design beause a simple ixed geometry is able to apture a wide range o low. The inlet system most important parameter, the area, must satisy the maximum low requirement. It must be minimum to minimize drag, weight and ost and onsistent with the speiied internal perormane (pressure reovery) and stability (pressure distortion). In pratie, the ree stream low must be irst slowed down by the inlet system to a speed o about ah 0,4 to 0,7 to keep the ompressor blades tip speed below soni speed. Pressure loss and distortion o the air supplied to the aet the installed thrust and the uel onsumption. 1% redution in the inlet pressure reovery, the ratio between and ree stream total pressures, may redue the thrust up to 1,3%. The Pitot, or normal shok inlet (NSI), is suitable or high subsoni appliations. The inlet radius or a given maximum operational ah number is alulated rom Eq. (1) and it an be ompared to initial layout estimates rom Fig. (1) (Raymer, 1999). A A A A A A 1 1 γ γ + 1 γ γ + 1 ( γ + 1) ( γ 1). ( γ + 1) ( γ 1) ( γ 1 ). ( γ 1 ). ( γ + 1) ( γ 1). (1) where is the airrat light envelope maximum operational ah number at the indiated position, γ is the speii heats ratio, A * is the area. Figure 1. Capture area estimation Another important parameter onerning inlet design is the length o the diuser, whih is the internal part o an inlet, where the ree stream low is slowed down to the speed required by the an. For long diusers, it is important to veriy that the ross setional area is smoothly inreasing rom the to the. For subsoni airrats, the diuser should be as short as possible, without exeeding an internal angle o about 10 o, and this usually points out or a PITOT inlet with a length approximately equal to its ront ae diameter. In general, pratie shows that the naelle maximum diameter tends to be roughly 10% greater than the bare to aommodate many systems. The

3 inlet itsel extends about 60% o the diameter in ront o the an ae and the inlet area is about 70% o the maximum area, although depending on the type. Important parameters to hek diusers eiieny are the stagnation pressure ratio r d and the isentropi or kineti eiieny η kd, shown in Eq. (a, b) and desribed in details in Hill, PT, an r and d η PT, kd r d γ γ 1. γ 1 γ 1 (a, b) where P T, is the total pressure o the ree stream low. The model proposed or the naelle theoretial analysis is based on an aerodynami dut in an air stream low. The main stations o the low are shown in Fig. (): the irst station,, is at the undisturbed low stream, station is the dut entry, station is the -ae and station e is the dut exit. The internal ross-setional area at station, A, is known rom the an. Capture area A is determined irst. It ontrols how muh low goes through the dut. Further details are ound in Seddon, Captured stream tube F p+1/.dp Internal low ENTRY External low A V p dl A+dA V+dV p+dp e EXIT Figure. Aerodynami dut in an air stream Assuming uniorm onditions o pressure and veloity at eah station, a very small stream tube boundary inlination to its axis, total pressure loss aused essentially by rition on the walls o the dut and no ompressibility eet in kineti pressure on approah, the momentum onservation equation, Eq. (3), gives an approximate expression, Eq. (4), or the total pressure loss along the tube. 1 ρ. A. V.( V + dv V ) PS. A + PS +. dps. da ( PS + dps ).( A + da) df (3) where ρ is the density, A the loal area, V the volume and P S the stati pressure. df g P q. CF.. dl (4) A A where df is the rition ore on an element o the boundary, g.dl is an element o the boundary area, q is the dynami pressure, g the loal perimeter along whih the rition ore is applied and C the rition oeiient. Dividing Eq. (4) by the entry dynami pressure q, using ontinuity and reerring the loss in total pressure to a untion o both the approah loss P a and the dut loss P d Eq. (5a) is obtained. A omparison o the measured loss oeiients with those given by Eq. (5) would reveal the extent o any lip separation at low speeds and that o pre-entry separations at high speeds. The pressure reovery η σi, Eq. (5b), represents the dut eiieny in terms o pressure loss. P q l l A g A g. CFa.. dl + CFd A A... A A 0 l Pa dl q Pd + q P and η σi 1 (5a, b) q where C Fa is the approah eetive rition oeiient and C Fd is the dut eetive rition oeiient 3

4 EBRAER ERJ-170 installations are podded, so it will be assumed that there is negligible approah loss in ruise, implying that the area in Eq. (1) is equivalent to the entry area A. The wetted area, or the approah surae, is zero. Figure (3) (Squire, 1947), based on inlet dut onial experiments onsidering arbitrary setion shape diusers and area variation, gives C Fd. The equivalent one angle dut is given by Eq. (6a). The drag oeiient C F depends on the low harateristis and. is alulated rom Eq. (6b, ) or ompressible turbulent lows with Reynolds number not greater than 10 9, known as the Shlihting empirial equation with the Prandt-Glauert orretion or ompressibility eets (Fox, 1995). Figure 3 Eetive rition in dut.( g.. ) 1 A g A α.tan, C g. g.( l l ) F [log ( R ) e ( ) ],58 0,65 and 4. A. V R e (6a, b, ) g υ. where R e is the Reynolds eetive number or the equivalent lat plate and υ is the luid visosity. The diuser length, (-) in Fig. (), is estimated iteratively rom Eq. (5) and (6) to obtain the best perormane relation omprising minimum total length and high pressure reovery..3 Exhaust System The most important problem onerning the exhaust system design is to math the nozzles exit areas at dierent altitudes to the neessary thrust. In some ases, it may be required to use nozzle variable geometry. However, or most subsoni ivil appliations, a simple onvergent nozzle an aelerate the exhaust gases eiiently. An initial naelle layout is drawn based on the assumption that the exit area is approximately 0.5 to 0.7 times the aptured area or a subsoni onvergent nozzle. The nozzle arrangement may have a substantial eet on the naelle external drag, so it is desirable to have an exit one angle less than 15 o and the outside angles o the nozzle be less than 0 o in the nozzle-losed position. Nozzle areas are estimated rom data perormane requirements and its dek. ass low was alulated assuming ideal isentropi 1-D low gas. The nozzle ideal mass low is given by Eq. (7) as untion o the low properties. However, the atual mass low is not usually equal to the ideal one, as shown in Eq. (8). 1+ γ! P γ γ 1. ( 1 γ ) T (7) m ideal A T R T (. V A) m! C. m! C. ρ (8) real d ideal d. where C d is the nozzle disharge oeiient γ 1 P γ T. 1 (9) γ 1 PS 4

5 The uninstalled net thrust is the exit gross thrust minus the inlet ram drag, as shown in Eq. (1). The eetive veloity, or the ully expanded isentropi veloity, takes into aount loal ompressibility eets, as shown in Eq. (13). The oeiient C g is the nozzle gross thrust oeiient, whih relates the ideal to the atual thrust. C d and C g oeiients urves as untion o nozzle pressure ratio (NPR) are intrinsially related to the nozzle aerodynami quality. Further details onerning nozzle oeiients are ound in Oates, 1989 and GEAE, 1999a. FN ( C. m. V ) ( m!, ) FG F! v (10) total total ram g nozzle e nozzle total. A m! ( P P ) V e V +. S, nozzle S, amb (11) The nozzle exit areas are alulated iteratively rom the nozzle bypass exit area and the resulting gross thrust ompared to the required thrust taken rom the dek to the speii light onditions. Detailed onsiderations on yles and aerothermodynamis are ound in Oates, Naelle hardpoints To start a subsoni naelle design, it is suggested to set the lip radius in the range o 6 to 10% o the inlet ront ae radius. To minimize distortion, two dierent radii are used: 8% or the inner part o the lip and 4% or the outer. Inner lip radius up to 50% greater than the outer radius would redue the eet o the angle o attak during take-o and landing. In this study, 0 hardpoints were alulated as indiated in Fig. (4) and desribed below. DIAETER () COPLETE NACELLE DESIGN , , LENGTH () Figure 4. Hardpoints numbering onvention The spinner oordinates, points 1 and, are both known rom dimensions. Point 3 y oordinate is known rom the an radius; its x oordinate is estimated by Eq. (5). Point 4 y oordinate is estimated by Eq. (1). The maximum radius oordinates, point 6, depends on the aessories pakage maximum radius and was set to 10% greater than this value. Equations (9) to (13) give the y oordinates dierene between points 8-13 and (nozzle areas). The outlet guide vanes (OGV) at the bypass entry (points 10 and 11) and the low-pressure turbine (LPT) exit oordinates (points 16 and 17) are dimensions. Thrust reverser owl trailing edge thikness and ater ore owl trailing edge are both set to 0,005m. Flow was onsidered as axial at the high-pressure ompressor (HPC) and the LPT. The bypass dut was onsidered onvergent. Points 18, 19 and 0 are determined having in mind that the ater plug angle must be between 10 o and 15 o (the maximum is desirable to minimize the total naelle length and the minimum to provide minimum loss). The one angle rom the LPT to the plug is usually in the range o 0 o to 7 o. The end plug diameter, a manuaturing requirement, was set to 0.076m. To alulate points 7, 8, 13, 14, 15 and 18, it was onsidered that the thrust reverser owl angle rom point 6 (the maximum diameter position), is not greater than 10 o (greater values applied to low subsoni lows), the at owl angle is 5

6 between 10 o and 15 o and the nozzle areas and trailing edges thikness are already alulated. The EBU maximum volume riteria design determines, together with the nozzle area restritions, points 9 and 1 oordinates. The minimum diameter position was set as 15% o the naelle hord (rom point 5 to points 7, 8) and the leading edge radius as an arithmeti average o the min and max radius to determine point 5. Naelle optimization requires the perormane analysis at several light operating onditions. In this work, only a ew extreme onditions, like takeo/top limbing (high thrust, high mass low, very high angle o attak (AOA), low airrat speed) and maximum ruise (low thrust, low mass low, low angle o attak, high airrat speed), were hosen to alulate the hardpoints..5 Naelle aerodynami design The naelle suraes were drawn smoothly rom the alulated hardpoints. Aerodynami itting urves were used to redue drag. The naelle was divided into two parts: the ront owl, whih omprises the inlet, the an and the thrust reverser owls and the at ore owl. For both parts, a maximum radius o urvature was set to prevent large adverse gradient pressures and smooth transitions were applied. The inlet ront owl has an airoil proile. The XFOIL sotware (see Drela, 001) is an iterative design and analysis tool or subsoni isolated airoils based on the panel method applied or integral -D boundary layers. Although not applied or axis-symmetri problems and ompressible lows, it was useul to deine the initial proile through points 3, 4, 5, 6, 7 and 8 to be iteratively tested. The hosen proile was the one that supported the highest ree low ah number beore boundary layer separation, attempting to minimize drag. The at geometry proile was obtained with CAD tools onsidering smooth polynomial itting urves through the hardpoints. Naelle perormane was analyzed using Fluent CFD ommerial ode (Fluent, 1998), whih uses inite volume ormulation, ell-entered, to solve the Navier-Stokes equations or unstrutured meshes. Seond order upwind and expliit time integration were set. Fluent validation and appliation or aeronautial problems an be ound in Fluent, 1998, attos, 1999 and Damian, Results 3.1 Inlet apture radius The EBRAER ERJ-170 maximum ruise speed is ah 0.8. The maximum ah number at the an stage was hosen to be The an diameter is 1.160m. The inlet apture diameter, alulated rom Eq. (1), was D (A m ). Compared to the data shown in Fig. (1), the results are up to 17% higher. It was also neessary to estimate the diuser length. Figure (5) shows the an ah number inluene on the inlet pressure reovery, η σi. The inlet pressure loss, whih diretly inluenes the pressure reovery, has two dierent soures: the irst one is due to the wetted area and its visous eet on surae; the seond is due to the diuser angle eet (Squire, 1947). So, the pressure loss is diretly proportional to both the wetted area and the diuser angle. However, while the wetted area is diretly proportional to the dut length, the diuser angle is inversely proportional to it and a ompromise solution must be ound. Hene, or a an ah number o 0.65 the diuser length would be about 0.56m. It is also important to notie the strong inluene o the an maximum ah number on the pressure reovery. It is lear that any turbomahine improvement in the an allowing to get higher inlet ah numbers will provide beneits in terms o η σi, thrust and uel onsumption. The high-pressure reovery oeiient obtained or the designed inlet (η σi 99,35%) onirms its good perormane. Table (1) resumes the main inlet diuser results % 99.50% 99.40% Inlet Pressure Reovery an0,60 an0,65 an0,70 nsi 99.30% 99.0% 99.10% 99.00% Inlet Diuser Length (m) Figure 5. Fan ah number inluene in the inlet diuser length 6

7 Table 1. Results obtained or the inlet radius sizing 3.. Nozzle areas Parameter English Units SI Units Inlet Radius 1.75in m Pressure Loss psig 66.49Pa η σi (%) η kd (%) η d (%) There are two exit areas in the ERJ-170 : the bypass and the ore. This work will ous on two ritial onditions, one at take-o, and another at ruise, based on GEAE, 1999b, although it is reommended to test or several other light onditions. Both o them are reerred to the ISA (Ideal Standard Atmosphere) ondition (15 o C and 101,35Pa at sea level) and detailed in Table (). Table. Critial onditions to alulate the nozzle exit areas (ISA onditions) Condition ah Alt. (m) Ala ( o ) Beta ( o ) Desription , Long range ruise, max ontinuous Climb one inoperative Bypass and ore exit areas were alulated rom the two onditions above using the dek inormation. The results are shown in Table (3) and are in agreement with experimental data taken rom Raymer, 1999 (0.48 to 0.67m ). Table 3. Results obtained or nozzle exit areas (ISA onditions) 3.3. Naelle Design Position Area (m ) Thrust argin (%) BPR Bypass Core Total The 0 naelle points ould be ixed ater alulating the inlet radius, the outlet areas and the dimensions and ompared with existing data (Fig. (6)) NACELLE DESIGN COPARISON GEAE STATISTICAL THEORETICAL DIAETER () Figure 6. Results omparison LENGTH () The inlet proile, omprising points 3 to 8, was drawn using beta-spline itting urves with maximum urvature radius, similar to an airoil. The urves were then aerodynamially analyzed and optimized in the XFOIL sotware or 7

8 ah number 0.4 and angle o attak 4.8 o onditions. The results show that the pressure distribution was reasonable and did not overpass C P, limit The pressure peaks notied in the region rom 0.3 to 0.5% o the mean hord, as shown in Fig. (7), were removed with a slightly and non-pereptive hange in the initial proile, to avoid very high pressure gradients or shok waves, to maintain the external low around the naelle and to keep the alulated hardpoints positioning to assure the orret properties. The resulting proile is shown in Fig. (8). 3.5 Initial design XFOIL orretion Cp ean Chord (%) Figure 7. Charateristi o the inlet proile Figure 8. Initial inlet proile Beta-spline itting urves with maximum urvature radius were also used to draw the exhaust proile. The loting urve was assumed to be orthogonal to the bypass and to the ore exit duts (points and 16-17, respetively) to obtain smooth suraes or the exhaust duts (the bypass and the ore duts). Finally, the ull naelle proile, omprising the inlet and the exhaust systems, is generated. Some hardpoints oordinates previously alulated had to be slightly hanged or the sake o onvergene the aerodynami design. However, these hanges were marginal, as shown in Table 4, and did not aet the main harateristis o the naelle. The inal naelle oniguration is shown in Figure 9. Points 5, 6 and 9 had larger hanges. Table 4. Naelle adjusted hardpoints Point L adj (mm) Delta L (mm) R adj (mm) Delta R (mm)

9 Figure 9. Complete Naelle Designed 3.5 CFD Analysis Two ERJ-170 naelle models were analyzed: an axis-symmetri one or ondition 1 (high speed) and a bidimensional one or ondition (high AOA). A 00,000 quadrilateral elements axis-symmetri mesh was built. The realizable k-ε turbulene model (Fluent, 1998) with wall untion treatment was used. Boundary layer reinement was obtained with about 0 nodes, the irst one being at Y + ~6 (low-reynolds number is not suitable to this problem). The results in Fig. (10) to (13) show that ondition 1 is more ritial than ondition. The naelle simulated is a little dierent rom Fig. (9), beause at the time the simulation was done, points 9 and 1 in the bypass dut were not set. Problems were deteted like strong shok waves in the outer inlet owl, with a diamond oniguration (ompression and expansion) ormed in the ore owl. Thereore, external drag inrease and boundary layer thikness raise, whih may ause separation low, already deteted in the thrust reverser owl trailing edge, were expeted. Inlet distortion in the internal low is also a problem to pay attention. Bypass dut exit nozzle is hoked and thik mixing layers in both trailing edges (thrust reverser and at ore owls) may ause heat losses and noise. Some hanges are proposed or uture iterations: to derease the relative inlet proile thikness, keeping the main hardpoints at the same position, and to redue the thrust reverser owl, the at ore owl and the plug angles. It is expeted that the improvement o the designed naelle in Fig. (9) would reah the atual results or the ah number distribution, shown in Fig. (14). Figure 10. Naelle ah number distribution or ruise 9

10 Figure 11. Naelle ah number distribution or ruise Figure 1. Naelle exhaust system ah number distribution or ruise Figure 13. Naelle ah number distribution or take o 10

11 Figure 14. EBRAER ERJ-170 naelle ah number distribution 4. Conlusion The new methodology to design subsoni naelles proposed in this paper was applied to the EBRAER ERJ-170 airrat. The results, when ompared to the existing naelle, indiate that the methodology is aeptable and may lead to a shorter naelle. The inlet and the outlet areas hardpoints were determined rom the momentum and mass onservation equations. Further assumptions onerning harateristis determined the remaining hardpoints. The naelle proile was drawn in CAD using smooth proiles itted to the alulated hardpoints. A suitable aerodynami analysis was done in the XFOIL or the initial inlet proile to prevent pressure peaks. A CFD analysis ompletes the work, heking the methodology and improving the naelle perormane. The methodology is more aurate than the statistial ones beause it uses more realisti data. It may be applied to draw more suitable naelle proiles and to maintain expeted perormane, without undesired harateristis that would disturb the low delivered to the and the external low around the supporting struture. Pressure distribution has been alulated and used to optimize naelle perormane through inlet proile redrawing, whih means to avoid separation and to derease the drag. From the CFD analysis, it is possible to oresee problems and to orret them. The naelle proile design is an iterative and time-onsuming proess. As a ontinuation o this work, additional light onditions may be tested. Next iterations should ous on trying to redraw thinner leading edges and smaller thrust reverser, at ore and plug owl angles to minimize loss o perormane at limb angles o attak. 5. Reerenes Brodt, C.S., 001, Design and Integration o Naelles or Aeronautial Engines, ITA Under-Graduation Projet, São José dos Campos, SP, Brazil, 118p Damian, R.B., 00, Estudo Comparativo de Códigos Comeriais de CFD na Solução de Problemas Simples de Esoamentos Compressíveis e Inompressíveis, ENCIT Paper number , Brazil, 1p Drela,., Youngren, H., 001, XFOIL 6.9 User Guide, 33p Fluent In., 1998, Fluent 5 User s Guide, 4 Volumes Fox, R.W., Donald, A.T., 1995, Introdução à eânia dos Fluidos, 4 a Edição, Ed. Guanabara Koogan S.A., 66p GEAE, 1999a, ERJ-170/CF34-8E Core Nozzle Thrust Coeiient, EBRAER GEAE, 1999b, ERJ-170/CF34-8E Isolated Naelle Perormane, EBRAER GEAE, 001, ERJ-170/CF34-8E Propulsion System Familiarization Training, EBRAER Hill, P.G., Peterson, C.R., 1970, ehanis and Thermodynamis o Propulsion, 3 rd print, Addison-Wesley attingly, J.D., Heiser, W.H., Daley, D.H., 1987, Airrat Engine Design, AIAA Eduation Series attos, B. S. de, et all: Transoni Euler Flow Calulation Around a Transport Coniguration with Powered Engine Eets, AIAA Paper number , USA Oates, G.C., 1984a, Aerothermodynamis o Airrat Engine Components, USA Oates, G.C., 1984b, Aerothermodynamis o Gas Turbine and Roket Propulsion, USA Oates, G.C., 1989, Airrat Propulsion Systems Tehnology and Design, AIAA, Washington DC, USA, 58p Raymer, D.P., 1999, A oneptual approah, 3 rd Ed., Washington D.C., USA Roskam, J., 1997, Airplane Design Vol. II: Preliminary oniguration and integration o the propulsion system, USA. Seddon, J., Goldsmith, E.L., 1985, Intake Aerodynamis, AIAA, Washington, D.C., 44p Squire, H.B., 1947, Experiments on Conial Diusers, RAE Aero 16 11