Turbines

Technology - turbines

The turbine is an assembly of discs with blades that are attached to the turbine shafts, nozzle guide vanes, casings and structures. The turbine extracts energy from the hot gas stream received from the combustor. This power is used to drive a fan, propeller, compressor or generator.

Rolls-Royce has a variety of turbine architectures to suit each application.

Design Technology

Rolls-Royce has invested in new technology to improve the design process, enabling the most advanced components to be designed and manufactured efficiently and reliably. This provides unprecedented amounts of information and data, earlier in the design process.

Unigraphics and Product Life Management have been introduced across the entire design, manufacture and assembly functions.

Design for Process Excellence is firmly embedded in the design process and encompasses all aspects of Dfx (Design for manufacture, assembly, service, aftermarket, etc), Quality Function Deployment and various optimisation tools.

These all ensure a holistically robust design.

The Turbine Blade

The turbine blade is the rotating component within the turbine which presents many challenges to the design and manufacturing communities. This component leads the way in terms of future technology.

Turbine blades are traditionally designed and analysed in isolation using Computational Fluid Dynamics (CFD), but multi-row CFD now considers the turbine as a whole and includes all the detailed shroud and cavity geometry.

Nozzle Guide Vane

The nozzle guide vane is a static engine component which has the main function of cleaning up flow from the upstream blade and is key to maximising downstream blade performance. Below are some of the key technologies to be implemented on to the Turbine Nozzle Guide Vanes:

Seal Segments

The Seal segment is crucial to reducing over tip leakage over the blade helping to maximise blade performance.

Turbine Aerothermal Analysis

The turbine environment is particularly harsh, making detailed measurements difficult to obtain.

Turbine rigs enable Rolls-Royce to obtain accurate data. They provide a vital means for analysing and understanding the complex external and internal air flows.

Rolls-Royce has developed techniques for taking detailed turbine measurements in a running engine. This enhances engine understanding and increases the store of reliable data for computer code validation.

Turbine blades are traditionally designed and analysed in isolation using Computational Fluid Dynamics (CFD). Rolls-Royce now uses multirow CFD which considers the turbine as a whole and includes all the detailed shroud and cavity geometry.

An increasingly detailed understanding of the turbine is necessary to produce more fuel efficient engines. Measurements from turbine rigs provide the accurate data that is vital for analysing the complex external and internal air flows in the harsh environment of the turbine. Techniques for taking detailed turbine measurements in a running engine enhance understanding and increase the store of reliable data for computer code validation.

Turbine cooling

High pressure turbine blades and nozzle guide vanes are designed with cooling passages and thermal barrier coatings, to ensure long life while operating at such high temperatures. Cooling air is taken from the compressor and is fed around the combustor into the blades to cool the aerofoils.

Design of cooled turbine components, to meet target metal temperatures, requires accurate understanding of thermal boundary conditions.

CFD is used to provide gas temperature profiles and component wall heat-transfer coefficients, and this is coupled with Finite Element Analysis (FEA) to predict metal temperatures accurately. Increasingly, conjugated CFD approaches are being used where both the fluid environment and the solid part of the Turbine aerofoil are being solved for using the same CFD solver.

Rolls-Royce and university research provides understanding, correlations and validation data for novel cooling techniques. 

In-engine validation of cooling designs is provided by thermal paints, which are used to assess both internal and external component surface temperatures. Laboratory interpretation of painted components taken from dedicated test engines is increasingly supplemented with in-engine boroscope paint assessment, thus providing additional validation data.

Control of Vibration

Turbine blading is subject to extremely high centrifugal loads and temperatures that reduce the fatigue strength of the material. It is therefore essential to minimise vibration amplitudes to remain below its endurance limit.

Very sophisticated computer simulations are required to quantify the sources of excitation in the gas stream and to modify the flow structure to mitigate the impact.

Detailed structural dynamic calculations are undertaken to optimise the modal characteristics and add friction damping to the system. Typical dampers convert relative movement between vibrating blades into energy extracted in the form of heat generated by friction rubbing of contact faces.

Turbine tip clearance control

Turbine tip clearances need to be kept at a minimum for optimum performance.

Tip clearances are generated by the difference in radial growth between the casing and rotor assemblies. To keep this difference below 0.235mm throughout the flight requires accurate thermal modelling and good control hardware. A 0.2mm gap, for example, costs about 0.3 per cent of fuel burn.

To maintain running gaps of this size throughout flight requires accurate thermal modelling and good control of turbine case temperatures via modulated, or semi-modulated, case cooling technology.

The main technology is turbine case cooling, and research is under way into the use of fully-modulated valves, allowing full flight cycle optimisation.

Liquid Crystal research also aids understanding of the impingement, enabling improved case cooling effectiveness. Through research programmes such as the Environmentally Friendly Engine (EFE), Rolls-Royce is researching mechanical and electrical tip clearance control actuation systems.

Shrouded vs Unshrouded Turbines

Rolls-Royce maintains world-leading technology and capability in the design and manufacture of shrouded and unshrouded turbine systems. The design style is optimised for each application.

Shrouded turbine blades reduce inefficiencies due to overtip leakage and minimise performance deterioration due to tip erosion. Unshrouded turbine blades are lighter, lower stressed and require smaller discs. These potential advantages have to be balanced against the complexity and weight of the tip clearance control system.

Rolls-Royce has unrivalled experience in the design of high performance shrouded (RB211/Trent/BR700 series) and unshrouded (AE3007/AE2100/EJ200/EFE) turbine systems.

The relative merits of shrouded and unshrouded turbine blades depend on the specific application, for which design styles are optimised.

Advanced Low Pressure Turbines

The Low Pressure Turbine (LPT) makes up approximately 20 per cent of the overall engine weight and its aerodynamic efficiency is an important driver on fuel consumption.

High lift aerodynamics are a key characteristic of modern LPTs. Doing more work than conventional airfoils, high lift aerofoils are designed with great care, utilising modern fully featured multi-row computational fluid dynamics. The high lift concept brings significant reduction in blade count, directly improving weight and cost.

Titanium Aluminide (TiAl) is a complex inter-metallic material offering a significant weight benefit for LPT blades. The 50 per cent reduction in material density enables additional weight savings in discs and casings. To address the significant casting and machining challenges presented by this material, Rolls-Royce have made substantial investments in Manufacturing Capability Acquisition.

High temperature materials

The increased demand for ever greater engine efficiency places increased focus on advanced material systems capable of withstanding elevated temperatures and mechanical loads.

Single crystal nickel “Super Alloys”, often containing up to 10 other exotic alloying elements, have provided the basis for recent material developments for turbine components. Successive generations of single crystal alloys have been developed with continuously improving temperature capability allied to high resistance to mechanical and chemical degradation.

Ceramic Matrix Composites (CMC) are being developed for various static applications with the aim of getting higher temperature capability and/or for reducing cooling requirements and weight. Rolls-Royce has over 20 years of experience in CMC technology and are currently investigating two types - Oxide/Oxide (Alumina) and SiC/SiC (Silicone Carbide).

Advanced surface treatments

Surface engineering plays an increasingly significant role in modern jet engines, providing improvements in component life, environmental performance and wear characteristics.

Surface treatments provide a step improvement in component performance across the entire jet engine, but nowhere is this more evident than in the turbine.

Tribomet abrasive coatings are used on rotating seal fins to provide protection against wear and prevent thermally induced cracking. Environmental coatings are applied to many turbine gas path components to provide increased resistance against oxidation and sulphidation attack, allowing extended service lives at elevated gas temperatures. Thermal barrier coatings allow increased turbine gas temperatures, thereby improving thermodynamic performance, without the penalty imposed by increased cooling air.

Thermal barrier coatings enable turbine gas temperatures to be increased, without the need for penalising increases in cooling air consumption.

Fleet operational experience is fed back into the design process to enable development of next generation coating systems.

Advanced manufacturing techniques

Manufacturing capability needs to keep pace with technology advances fully to realise improved component performance at an acceptable cost.

Soluble core technology enables complex internal cooling features to be incorporated which would otherwise be impossible with conventional casting techniques. Component lives can now be achieved at significantly reduced cooling flows.

The bi-cast process allows nozzle guide vanes to be manufactured with separate aerofoils and platforms. Molten metal is poured into mating grooves in the components to mechanically lock them together. This removes a number of design constraints and allows the use of different materials within the same component.

Direct laser deposition can be used to build up material on a substrate material in order to manufacture or repair parts.

Validation & Testing of Critical Parts

Lifing of critical rotating parts such as turbine discs is crucial to engine integrity and flight safety.

Rolls-Royce has invested in a multi-million pound facility to test blade attachment and disc life. The Critical Part validation programme uses both Development Engine and rig tests. Measurements are taken to confirm freedom from premature failures, high cycle fatigue lives, performance, internal air systems and metal temperatures.

Life limiting features are identified by analysis and assessed using various test programmes. They are exposed to the complex loading and temperature cycles they experience in an engine environment.

Technology demonstrators are being used to prove technologies prior to engine development programmes ensuring technologies are suitably ready prior to an engine development programme.

With safety and reliability come significant commercial benefits including fewer operational disruptions, improved maintenance scheduling, more economic spares holdings and increased customer satisfaction.

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