|Definition:||Turbomachines are systems that consist in an assembly of several components. The components that have an influence on performances are mainly the compressor, the combustor and the turbine and eventually the heat exchanger, if present. The turbomachines performances study concerns either the performance of an elementary component or the overall performance of the machine that depends directly upon the performances of each component that are the data to be introduced in cycles studies to evaluate this overall performance. The efficiency of the system has a direct effect on the production of green house effect carbon dioxide.
Experimental measurements remain a major way to evaluate the real performances of a turbojet, at ground or high altitude conditions. Nevertheless numerical simulation, steady and unsteady, becomes more and more reliable and induces a noticeable reduction of development times even if an ultimate final experimental verification is still necessary.
Performances of every elementary components, compressors etc ..., tend to reach their limits and the quest of a still better performance, particularly to reduce the greenhouse effect by consumption reduction, needs to consider new and more complicated cycles, involving for instance heat exchangers.|
|Sub-domains:||1. compressor : pressure ratio, efficiency, resistance to distortion, stall margin, active control, number of stages.|
2. turbine : efficiency, cooling, number of stages.
3. combustion : efficiency, stability, ignition, extinction, instabilities, active control, gaseous emissions, soots.
4. the complete engine : thrust, specific thrust, weight, dimensions, specific consumption, emission indices, noise.
6. maintenance constraints.
Turbomachinery / Propulsion Aerodynamics
|Definition:||The studies of turbomachinery aerodynamics aim to reduce the number of stages and the number of blades in keeping good performances in the thermodynamic cycle. The improvements of compressors and turbines are obtained in taking in account aerodynamic requirements but also mechanical and thermal constraints. For the aerodynamic part, the interactions between the stages are studied in order to evaluate the efficiency of the engine in all the flight envelope and to predict the phenomena onset at the limits of this domain : surging, rotating separation.The optimisation of the blades needs the knowledge of the geometrical effects like tip clearance on performances.|
|Sub-domains:||1. Computational Fluid Dynamics.|
2. Wind Tunnel Testing.
3. Tip clearance effect.
4. Heating flux.
6. Rotating stall.
|Definition:||Combustion in turbomachines is the mean to introduce energy in the system. The quality of the combustion has a direct influence upon a great number of factors and particularly the pollution through gaseous (NOx, CO, unburned hydrocarbons, ...) and soot emissions. The behaviour of the condensed phase (liquid kerosene) has a major impact on the location and the development of combustion and emission.Combustion is of the turbulent type. The accurate prediction of multiphase turbulent combustion supposes the mastering of turbulence itself. In particular, radiation cannot be anymore ignored as it redistributes energy in the volume and thus change the chemical kinetics. So the problem is still very opened and remains strategic considering the particular focus that is made upon pollution. For this study, experimental investigation with sophisticated diagnostic techniques and numerical simulation, averaged or unsteady, remain mandatory. Nitric oxides production being maximum at stoechiometric conditions, combustion processes at lean or rich conditions induce a deep evolution of the combustors. The catalytic combustion is a possible route but the use in aeronautical engines presents some difficulties like lifetime of catalytic substrates.|
|Sub-domains:||1. combustion efficiency.|
2. injection : atomisation, vaporisation.
3. chemical kinetics.
4. turbulent combustion modelling.
5. radiative transfers.
7. ignition, extinction.
9. active control.
10. auto-ignition and flash-back.
11. gaseous emissions.
13. catalytic combustion.
|Definition:||In air intakes the kinetic energy is partially transformed into pressure. The distortion of the flow in front of the engine is due to the non uniformity of the flow in front of the lips, the shape of the diffusor, the development of boundary layer on the walls and sometimes the shock-boundary layer interaction. This distortion has to be reduced to keep a good efficiency of the turbojet. The mass flow needed by the engine has to be provided by the air intake for the overall flight envelope. Thus additional inlets or variable geometry are used for supersonic aircraft and fighter.For supersonic vehicles (aircraft and missile) external flow compression induces a penalty for the cowl drag, so mixed compression air intakes are interesting for high cruise Mach numbers. However in this case, some small perturbations can cause the buzz phenomenon by dynamic effect of the shock displacement near the throat. Some devices have to be developed to prevent this risk (internal diverter, porous wall,...).
For turboprops, air intakes are located behind the propeller or under the rotor and these strong interactions have to be taken in account in air intake design.At the take-off the presence of the propeller reduce locally the leading edge slat efficiency. The rotative movement of the airflow issued from the propeller is at the origin of a rolling moment to be controlled.
The design of air intake and its installation on the vehicle have to respect some constraints like noise reduction and minimisation of radar signature.
Others problems are the icing of the lips and the ingestion of sand, dust or birds.|
|Sub-domains:||1. Computational Fluid Dynamics.|
3. Propeller, propfans, turbojets, turbofan.
4. Wind Tunnel Testing.
5. Noise reduction.
6. Radar signature.
|Definition:||Heat transfers concern mainly the combustor, the turbine and the heat exchangers, if present.
In the combustor heat transfers are of two types : convective and radiative. The effect can be damages of the combustor wall through hot points or thermal fatigue due to accumulation of ignition and extinction in the successive flight cycles. The convective heat transfers are due to the direct contact of reacting hot flows with walls. The solutions go through the mastering of fluid dynamics and aerothermochemistry and more precisely of the techniques of wall cooling by film or impingement. Numerical simulation and analytic experimentation play a considerable role in this problem. The radiative heat transfers are due mainly to the emission of hot gases and soots ; the wall to wall radiative transfers are less important.
In the turbines, heat transfers are of the convective type. They concern mainly three problems:
exchanges between main flow and the external wall of the turbine blades with the associated cooling techniques, cooling of the blades by internal air circuits and the transfer of energy in internal rotating cavities in the core of the engine. In this last case, the flow rates being very low, the movements of the fluid in the cavity are completely driven by the friction on the walls, necessitating a great mastering of turbulence phenomena.
In the heat exchangers, heat transfers are the reason to use this component. New conceptions of these devices must be developed to meet the requirements of aeronautical constraints as compacity, weights, geometrical adaptation.In the three cases, investigations must be coupled to the study of conductive heat transfers inside the materials.|
2. rotating flows.
Nozzles, Vectored Thrust, Reheat
|Definition:||The performances of the nozzles are strongly linked to the afterbody aerodynamics for supersonic aircraft. On fighters, modifications of the geometry of the nozzle are needed for the adaptation at very different flight conditions. On the botttail boundary layer separation can occur due to jet expansion or flight in incidence or sideslip.
In order to reduce the jet noise, some shapes of nozzle can be used on transonic aircraft and solutions with ejector silencer are studied for supersonic aircraft.The reduction of IR signature is an important operational constraint and several techniques of mixing to reduce the jet temperature can be used.
Vectored thrust is a means of controlling the aircraft stability or improving the manoeuvrability of the vehicle. Several systems are used (deflectors in the jet, rotation of the nozzle, wall injection inside the nozzle,...).|
|Sub-domains:||1. Computational Fluid Dynamics.|
2. Wind Tunnel Testing.
3. Noise Reduction.
6. Flow separation.
7. Infrared Signature.
|Definition:||Engine control includes two branches:|
- Engine modelling.
- Engine control system architecture and its related equipment (sensors and actuators).
1. Engine modelling consists:
- in identifying a mathematical model with the engine actual operating data: such mathematical model is necessary to define the control strategy and loops of the engine during its operations.
- in defining the control algorithms that will be implemented in the control loops.
2. engine control systems include all sensors, actuators, and regulators or/and computers that determine the operating parameters of the operating engine. Sensors are temperature, pressure, rpm,?; actuators are variable surface (vanes, blade angle settling) control actuators , fuel pumps and metering units; digital computers or hydro-mechanical regulators may be used to build up the control loops. System architecture is influenced by engine architecture and mission profile, by the thermal and dynamic environment created in and around the engine, and available equipment technology.
|Sub-domains:||1. Engine modelling is evolving fast with the increase of on-board computing capability: these allow more sophisticated and efficient control strategies by implementation of model based schemes, with identification of model parameters on actual engine operating status, in order to set up operating points based on performance computed parameters instead of single sensor signals. Such innovative strategy allows to reduce operating margins coming from engine to engine dispersion, or ageing, or component wear, and therefore contributes to the increase of engine performance and/or life increase. This is possible thanks to the best advanced mathematics such as fuzzy logic, neuronal networks, Kalman filtering, genetic algorithms,?|
2. Engine control systems are influenced by advances in electronics , sensor and actuator technologies:
- high temperature ( more than 200C°) electronics allows to incorporate intelligence in the harsh environment of an operating engine, giving the way to smart sensors and actuators, and distributed architecture.
- power electronics allows to shift from hydraulic to electrical actuation with better reliability and maintainability as a benefit for the aircraft operator: more electrical systems allows easier health monitoring, and trouble shooting.
Infra-red and Radar Signature Control
|Definition:||The knowledge of physical phenomena which contribute to the infrared radiation emitted by an aircraft in flight. Different parts of the aircraft radiate: the airframe, the exhaust jet and the motor for few aspect angles (rear part of the engine and air intake). The first aim is to have an understanding of the physical process which emits radiation in order to reduce it, if it is technologically possible.The airframe signature depends on its temperature and the radiative properties of the material.
The temperature itself varies with the flight profile (altitude and speed), the material thermodynamic properties and the internal heat dissipation of electronic systems.
The jet infrared signature presents a spectral emission which is characteristic of the chemical species that can be found, mainly water vapour and carbon dioxide. The radiation level is also function of the hot gases temperatures.The motor radiation can be observed for rear angle of aspect, the emission of the nozzle hot parts being partly absorbed by the hot gases in the jet exhaust. Its emission can also be perceived for very specific angle of aspect, when the first stage of the compressor is in direct view from the air intake, or through its propagation along the airduct.
The control/reduction of the RCS (Radar Cross Section) of an aircraft is obtained by two main approaches: optimisation of the global or local shape and use of Radar Absorbing Materials (RAM). Modelling the RCS, by numerically solving Maxwell equations, is now a necessary way to define efficient shape modifications and materials.
The chosen solutions have to be assessed by ground or in-flight measurements.Radar materials development is an important topic, as any material has to fulfil several functions, needing, for example, thermal or mechanical properties.|
|Sub-domains:||1. Material thermodynamic and optical properties.|
2. Skin temperatures.
3. Internal heat dissipation.
4. Jet aerodynamic description, temperature, pressure and species concentration.
5. Spectroscopic data base of emitting species.
6. Radiative transfer computation in hot gases media.
7. Multi-reflection in cavities.
8. Radar Cross Section computation.
9. Radar Absorbing Materials.
10. Ground RCS measurements.
11. In-flight RCS measurements.
Auxiliary Power Unit
|Definition:||APU are subsystems that deliver power, either mechanical, electrical, hydraulic or pneumatic, for specific periods of the aircraft mission, when the main power source (driven by the engines) is no longer available or insufficient , on ground or in flight: on ground before and during engine start, in flight shut downs, special power needs for payload or armaments, more generally every power needs in emergency circumstances.
APU include a turbine , and an electric or hydraulic generator to convert mechanical power, and a system for derive compressed air for delivery to other systems such as engine starter or environmental control system (ECS). The generated power is in the range of a few kW up to several 100kW.|
|Sub-domains:||1. The evolutions have the same goals as those of propulsion turbines: increasing power per kilogram , decreasing fuel consumption, reducing costs through various ways (reduction of part counts, more efficient manufacturing process ,..).
Another trend is the integration of APU in the broader platform system optimisation including the main power generation, and the main power consumers: in this scope some effective concepts have evolved such as the Integral Power Unit which is a machine integrating on a single shaft the former APU , the electrical generator and the ECS compressor and turbine . Such an IPU has a much longer operating time, and a broader operating domain that may open design options to variable cycle concepts.|
Fuels and Lubricants
|Definition:||Aviation turbine fuel: propellant used in jet engines.
Jet fuel, or more commonly kerosene, is a refined petroleum distillate intermediate in volatility between gasoline and gasoil. Several grades can be found. In civil aviation, kerosene Jet A1 (or Jet A in the USA), is used world-wide. For military applications, kerosene is often used with additives to improve its properties. Link with "Emissions pollution".
Lubricant: substance interposed between rotating parts to limit friction and wear.Aviation lubricants are ester based compounds and are very stable at temperatures met in the engines.|
|Sub-domains:||In a jet engine, a lubricant has two main functions:|
1. lubricate rotating parts such as bearings and gears, to limit wear and friction.
2. evacuate the heat generated between these rotating partsSecondary functions can be mentioned, such as remove particles of wear or pollution and protect parts against corrosion.
Test Bench Calibration
|Definition:||The calibration of a test bench consists of determining all the losses due to the test cell configuration affecting the engine performances (thrust, ?).|
The calibration of the test cell must be determined for a given engine configuration.
|Sub-domains:||The resulting corrective factors are determined to take into account:|
1. the energy loss due to the boundary layers in the vicinity of the test cell walls.
2. the heterogeneity of the flow field upstream the engine inlet.
3. the aerodynamic forces on the mechanical system maintaining the engine.
Engine Health Monitoring
|Definition:||Engine health monitoring is the assessment of engine physical condition by monitoring and interpreting available engine instrumentation and operation cycles, in order to detect incipient trouble in advance of critical anomalies. Health monitoring techniques have evolved from flight engineer/pilot tasks through visual and tactile cues available on cockpit gages to automatically monitored data on on-board computer, transferred in real time or differed time to ground station for analysis.|
|Sub-domains:||Future trends are towards an increase in the sophistication of on-board and ground-based engine monitoring and maintenance systems including dedicated on-board diagnostic processors and algorithms, advanced diagnostics and prognostics instrumentation with fault accommodating logic. The ultimate vision is a combined monitoring system that applies prognostics within an engine health management system to allow aircraft operators to automatically track remaining life of engine component. In addition processing of signals such as vibrations and acoustic signature should be developed to identify and locate mechanical incipient failures ( disk or blade crack initiation, bearing wear,..).|
Experimental Facilities and Measurement Techniques
|Definition:||Characterisation of reactive flows requires the knowledge of several parameters: concentration and temperature of various species, as well as velocity and turbulence. These quantities are currently obtained by non intrusive optical techniques based on either molecular or particle scattering.|
|Sub-domains:||1. Measurements of temperatures and concentrations:|
CARS (Coherent Anti-Stokes Raman Scattering), LIF (Laser Induced Fluorescence), LII (Laser Induced Incandescence), DFWM (Degenerated Four Wave Mixing), REMPI (Resonantly Enhanced Multi-Photon Ionisation), DLAS (Diode Laser Absorption Spectroscopy), EBF (Electron Beam Fluorescence).
2. Velocity measurements: LDV (Laser Doppler Velocimetry), PIV (Particle Image Velocimetry), DGV (Doppler Global Velocimetry), PDA (Phase Doppler Anemometry: velocity and particle size), L2F (Laser Two-Focus Velocimetry).
|Definition:||Computational methods for propulsion consist in the development, validation, and use of software tools for the numerical simulation of physical phenomen taking place in propulsion devices, such as turbine engines, jet engines, rocket motors (liquid and solid propellants), missiles, ramjets, launchers. The numerical codes must provide capabilities for treating multi-physics situations implying multi-species, multi-phases, turbulent, chemically reacting flows with heat transfers from convective and radiative processes and strong fluid-structure couplings. This necessitates the knowledge of applied mathematics, numerical methods, computer sciences and physical modellings. Physical models and the codes where they are made available have to be validated and calibrated by comparison with experimental data. In particular physical, thermodynamic and chemical properties are of major importance, as well as input data for the models used in numerical simulations and may require dedicated experiments to be acquired. Unsteady flows are of major concern as instabilities, transient flows and noise generation are often critical aspects of propulsion devices. Emissions (pollutants, noise, radiation, ...) are often to be controlled and must receive special attention. Computations often include complex, hostile conditions and stiff mechanisms which require high computational power, robustness and efficiency. Grid tailoring and grid adaptation is often mandatory, as well as parallel computing and code coupling. The computer codes are run along the following steps: After the grid generation (or adaptation), the solver are run and data are eventually exchanged between solvers before post-processing and analysis of the results. Numerical simulations are used for understanding physics of complex situations for performance prediction and for flow control or system design studies.|
|Sub-domains:||1. Physical modelling (turbulent, heat transfer, reactive flows, two-phase flows, radiative medium?).|
2. Unsteady flows, vortex flows, aeroacoustics.
3. Development of numerical schemes and algorithms.
4. Code coupling, multi-physics, multi-scale simulations.
5. Development and production of software.
6. Validation of software, model characterisation.
7. Grid generation and adaptation.
8. High Performance computing (vector and parallel processing).
9. Complex applications, system analysis, control.
|Definition:||These topics are of the most importance in the actual context.The emissions of the turbojets are produced by the combustion. They are of two categories : minor and major species.
Minor species : NOx, CO, HC, particulates, are characterised by the fact that in certain conditions they can be quasi avoided. The art of the engineer consists in finding the best compromise to minimise these emissions, some, like NOx and CO, necessitating contradictory conditions. Their study requires a great knowledge in advanced chemical kinetics, particularly for soots : precursors, nucleation, growth and oxidation. The characteristic times are of the order of the millisecond.
Major species : CO2, H2O, have a production strictly proportional to the fuel consumption and cannot be avoided. They concern mainly the greenhouse effect. The only way to reduce them is to reduce the fuel consumption and the solution relies on overall engine, and aircraft, efficiencies.
The effect of these emissions on the environment involve very complex phenomena belonging to the physical chemistry of the atmosphere. Characteristic times are much longer, from one second to several days. The evolution of the effluents are not deterministic and depend of the local and instantaneous meteorological conditions, and particularly the eventual presence of the solar radiation. These phenomena include heterogeneous mechanisms due to the presence of condensed phases that constitute aerosols presenting important evolution in composition during their lifetime. The two characteristic situations are the local impact, in the vicinity of airports, and the global impact during the flight, the consequences being different for troposphere and stratosphere.
In all these problems, sophisticated diagnostics have the greatest importance.|
|Sub-domains:||1. chemical kinetics.|
2. turbulent combustion.
3. atmosphere physico-chemistry.
5. species transport and dispersion.