The Challenges for Aero-Engine CFD

Transcription

1 Title - Arial 28pt The Challenges for Aero-Engine CFD Leigh Lapworth, Rolls-Royce plc., Derby, UK. The information in this document is the property of Rolls-Royce plc and may not be copied or communicated to a third party, or used for any purpose other than that for which it is supplied without the express written consent of Rolls-Royce plc. This information is given in good faith based upon the latest information available to Rolls-Royce plc, no warranty or representation is given concerning such information, which must not be taken as establishing any contractual or other commitment binding upon Rolls-Royce plc or any of its subsidiary or associated companies.

2 The Product 2 Trent 900 cutaway (powers Airbus A380) Trent 800 on Singapore Airlines Boeing 777

3 3 Some Facts and Figures* The power a Trent engine generates at take off is about 90,000 horsepower - equivalent to the power of 1,200 family-sized cars. There are 92 high pressure turbine blades in a Trent 800 engine. Each of these generates about 800 horsepower - equivalent to a Formula 1 racing car. While generating its 92,000lb thrust, the Trent sucks in more than 1 ton of air per second at about 350 miles per hour. Equivalent to emptying a squash court of air in less than one second. By the time the air leaves the nozzle at the back of the engine, it has been accelerated to a speed of 1050 miles per hour. Fuel burns in the Trent engine's combustion chamber at temperatures up to 2,000 C, which is well above the 1,300 C at which some component metals used would start to melt. The heat transfer rate achieved by the cooling air system in each High Pressure Turbine blade is equivalent to a domestic central-heating boiler or air-conditioning unit. The Boeing 777, which is powered by two Trent 800 engines, carries around 330 passengers and gives about 120 passenger-miles to the gallon. The tip speed of the Trent fan blades and first stage turbine blades is over 1,000 miles per hour. *

4 4 Civil Aerospace Drivers The ACARE * Environmental Goals for 2020 Background: In 2000, the European Union Commissioner Philippe Busquin asked a distinguished group of representatives from the European aviation industries to set out their vision for the future of aviation in the medium and long term. ACARE was set up with the objective of realising the goals. In 2002 the Strategic Research Agenda was published which set out four goals aimed at meeting the environmental challenge for The Goals To reduce fuel consumption and CO2 emissions by 50 per cent, To reduce perceived external noise by 50 per cent, To reduce NOx by 80 per cent, To make substantial progress in reducing the environmental impact of the manufacture, maintenance and disposal of aircraft and related products. * Advisory Council for Aeronautical Research in Europe

5 5 The ACARE Challenge Reductions in emissions from aviation can be gained from three main sources: Target: Airframe plus Engine plus Operations can deliver 50 per cent reduction in CO2 emissions per passenger kilometre. *

6 6 The ACARE Challenge CCAEP = Committee on Aviation Environmental Protection Nox and Noise reductions remain difficult goals to achieve leading a range of innovative new concepts. *

7 Radical new concepts 7 easyjet ecojet* Target EIS in 2015, concepts: Rear mounted open-rotor engine with high propulsive efficiency Lower design cruise speed to reduce drag and a shorter design range to reduce weight Noise reductions from shielding and subsonic rotor tip speeds (via gearbox) New light-weight materials * Photograph: Frank Baron ** Silent Aircraft Initiative** Target EIS in 2030 onwards, SAX-40 concept aircraft designed by Cambridge MIT-Institute with industrial support A noise of 63 dba outside airport perimeter. This is some 25dB quieter than current aircraft

8 8 The Challenge of Turbomachinery Aerodynamic performance of the turbomachinery is the critical factor in engine efficiency, thrust and operability Adjacent rows of rotating and stationary blades, Inherently unsteady flow field, Transitional and turbulent flow fields with complicated secondary flows and leakage effects. Stringent levels of conservation needed

9 25 years of Turbomachinery CFD Turbomachinery CFD has a long pedigree: 1952: C-H Wu - S1-S2 stream surface method Coupled through-flow and blade-to-blade CFD - still mainstay of design 1979: Denton - single blade row CFD based on sheared H-meshes and finite volume time-marching scheme 1983: Single blade row CFD using single block meshes and simple mixing length turbulence models. Bespoke and academic codes Steady Multistage CFD 1992: Denton - steady mixing planes using circumferential averaging 1985: Adamcyzk passage averaging and deterministic stresses 1995: Le Jambre overlapping meshes and networked workstations Unsteady Multistage CFD 1996: Denton simple H-meshes with sliding planes between blade rows 1992: Giles linear unsteady single blade row with prescribed unsteady b.c.s 1993: Dawes unstructured meshes with spatial & temporal adaptation 1998: He Multistage with phase lagging to reduce blade counts 2000: Hall Multi-frequency linear analysis using harmonic balance 2005: Vahdati Whole annulus 17 blade row compressor simulation 2006: Schluter 20 o sector of whole engine including LES of combustor 9

10 Multistage Design by CFD Multistage CFD allows compressors to exploit the 3D aerodynamic design space Unsophisticated sheared H-mesh codes with mixing planes, 2D design 3D blading gives better efficiency and stall range enabling: Lower blade counts and higher loadings Trent 900 datum compressor at design point red regions indicate flow separation 10 3D design Trent 900 3D re-design at design point

11 Simulating Operability stage research compressor Pressure Rise Coefficient, y EXP UNSTEADY STEADY EXP UNSTEADY STEADY Efficiency, h Simulating the stability boundary: Vx/Umid Steady multistage CFD performs well at the design point, but cannot predict stability boundary where wakes and corner separations are more pronounced leading to higher levels of blade row interaction Unsteady multistage CFD with sliding planes performs much better near the stall boundary. HYDRA simulation (Montomoli, Cambridge U) 60

12 12 HYDRA CFD Solver HYDRA Hybrid unstructured CFD capability, Parallel on shared and distributed memory machines, Convergence acceleration using pre-conditioning and multigrid, Steady and unsteady flow, Mixing and sliding planes for turbomachinery, 1 and 2 equation turbulence models, transition and LES capability, Moving mesh, Linearised unsteady and adjoint CFD capabilities Development network Initial code developed by Prof. Mike Giles at Oxford UTC in CFD, Ongoing development by Aerothermal Methods Group and network of UTCs Oxford, Cambridge, Loughborough, Surrey, DLR, etc.

13 13 HYDRA Applications Air Systems Turbines Fans Full aircraft Compressors Noise Energy Exhausts Installations

14 14 Improving Efficiency, Reducing SFC

15 15 Open Rotor Simulations Open rotor rig 140 tested in ARA wind tunnel Mesh generated by PADRAM extending to large radius, typically 4-5 rotor heights. CFD simulations using HYDRA Simulations performed at the Whittle Laboratory (Hall & Zachariadis)

16 Rig Take-Off Conditions (M 8 = 0.20) Cruise Conditions (M 8 = 0.75) RIG DATA HYDRA RIG DATA HYDRA Overall Propulsive Efficiency (?) Overall Propulsive Efficiency (?) (Zachariadis & Hall, Cambridge University)

17 Ground Effects 17 Engines on the ground can ingest a ground vortex Can influence engine operability, CFD used to design intake lines to meet operability criteria CFD also used for crosswind and incidence effects HYDRA simulation (West, RR)

18 18 Design Using Adjoint CFD Engine Section Stator optimisation 86 design parameters skew, lean, sweep, LE & TE recamber and endwall profiling Unconstrained SQP optimisation using adjoint gradients from HYDRA (Duta, Oxford University)

19 ESS Optimisation Using Adjoint CFD 19 Original Optimum Contours of axial velocity near ESS Trailing Edge (Duta, Oxford University)

20 Coupling CFD & CAA for Noise 20 CFD Solution at plane U is decomposed into radial modes. CAA applied as a transfer function. Intake Liner CAA used to compute sound pressure level (SPL) on fuselage exterior. Used to estimate cabin noise CAA U CFD S Fan CFD/CAA Simulation (RR & Boeing, AIAA )

21 HYDRA Buzz-Saw Noise 21 CAA solution (HYDRA Linearised Euler) Blades skewed individually to match measured blade tip stagger angles Full Annulus CFD solution Non-Linear HYDRA Complex geometry (eg tip gap) Includes intake acoustic liner 55 Million nodes 40 dual processor PC cluster nodes 4-5 days run time using multi-grid Surface static pressure CFD/CAA Simulation (RR & Boeing, AIAA )

22 Buzz-saw source radiation and transmission into Cabin 22 Measurement Prediction SPL [db] 10dB Engine Order CFD + CAA transmission Characteristics calibrated against exterior measurements CFD/CAA Simulation (RR & Boeing, AIAA )

23 Combustion CFD modelling 23 l Prediction targets: Combustor internal flows: Velocities, temperatures, pressures and emissions at both steady and unsteady state conditions l Diffuser flow and external aerodynamics: Velocity profiles and pressure loss l Flows for small-scale components: Fuel injectors Cooling devices Port flows l Metal temperatures: Boundary conditions for thermal analysis l Main challenges: l Fluid mechanics: Recirculations, high turbulence, complex geometry l Two-phase flows: Liquid fuel modelling l Combustion: Varying fuel properties, chemistry-turbulence interaction l Heat transfer: Radiation and convection to walls l Pollutant formation: NOx, smoke, CO, UHC l geometry mesh solution

24 Cooling the Turbine 24 Turbine entry temperatures (TET) are well above the melting point of the metal RB211-22C (1971) TET=1500K, Trent 800 (1996) TET>1800K Higher TET increases the propulsive efficiency of the engine Turbine is cooled via Thermal barrier coating Film cooling Conduction from internal cooling passages which feed film cooling holes

25 25 Hot Gas Ingestion Modelling l On vertical take-off/landing aircraft (Harrier, JSF), when the engines are vectored downwards, it is possible for the hot exhaust gases to re-enter the engine intake l This is a potential source of engine stall l HYDRA CFD simulations are being used in place of wind tunnel tests. (Ricardson, Cambridge University)

26 26 Hot Gas Ingestion Modelling Experimental facility HYDRA simulation contours of temperature (Ricardson, Cambridge University)

27 27 Future directions High geometric and physical fidelity Increasing use of unsteady CFD and LES Component coupling, leading to: System level design optimisation Virtual engine simulations Continued use of HPC E.g. Trent engines have O(5000) blades LES simulation of compressor stator (McMullan, Loughborough) Whole Engine CFD (e.g. Stanford) LES simulation of a fan rotor for broadband noise (Ray, Cambridge)