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DEVELOPMENT OF ADVANCED ACTUATION CONCEPTS TO PROVIDE A STEP CHANGE IN TECHNOLOGY USED IN FUTURE AERO-ENGINE CONTROL SYSTEMS (ADVACT)

Final Report Summary - ADVACT (Development of advanced actuation concepts to provide a step change in technology used in future aero-engine control systems)

The prime objective of the ADVACT project was to provide the technical background to enable the achievement of improvements in operation, availability, costs and environmental impact of gas turbines by the provision of extended in-flight actuation and control of engine parameters. The work has investigated the applications and technologies to the stage where laboratory demonstrations have been completed and the requirements of the applications are understood.

Two potential applications for gas turbines have been investigated within ADVACT. These are for flow control in intakes and on aerofoil cascades. Intakes would benefit from improved control during the infrequent events of high incidence operation such as cross winds, rotation and spillage during windmill. This would allow a thinner intake lip to be used which should lead to greater efficiency at all other times. Aerofoil control should give improved efficiency or operating envelope and may provide an alternative for mechanically actuated variable guide vanes (VGVs).

The project conducted research on the following technologies.

-Boundary layer control and MEMS devices
The state of a boundary layer can have a significant influence on the ability for an air flow to remain attached to the surface. Control of the boundary layer can maintain attachment for longer, cause detachment or increase the amount of turning achieved by an aerodynamic device. Continuous sucking or blowing of air through the surface to affect the layer has been known for many years.

The ONERA work has concentrated on the fundamental flow understanding and the effect of vortex generators and jets in gas turbine flows. Significant improvements in separation were achieved with both fixed vortex generators and modulated jets. The ONERA and MTU models showed that from an industrial point of view, combinations of several methods to model the small and large scale elements within the flow and to use local mesh refinement methods gave a good compromise between accuracy and computational effort.

The aero modelling work suggested that full frequency control was unnecessary and that higher frequencies were desirable. Tests on a cascade rig showed a drag reduction in the order of 50% relative to a blade without control. Under the measured conditions, the use of modulated blowing reduced the mass flow requirements by 40% compared to continuous blowing. Major progress was achieved in simulating and controlling boundary layers within an intake. Experimental techniques are well advanced and a macroscopic simulation of the modulating microvalves was successfully deployed. The microvalves unfortunately arrived too late in the programme to be evaluated within the rig.

-Shape memory alloys (SMAs)
For most practical actuator applications, the SMA is loaded by an external spring element. When heated, the SMA will move towards a pre-set shape, when cold, the SMA can be deformed to a different shape by the external spring. In this way a repeated two-way actuator can be produced. Partially due to domination by the medical market, but also due to metallurgical limitations and the need to maintain very accurate composition control, production had previously been limited to small sections up to around 10 mm wide strip or 2 mm diameter wire.

During the project, large sections of improved temperature (Mf>50 degrees Celsius) alloys have been developed. Also, development of large area actuator coating methods has been achieved, with very robust coatings formed. New methods have been developed within the programme which allow straightforward measurements to be used and enabled major advances in practical engineering design methods. The original concept of a structural element pre-stressed in bending balanced by an SMA element in tension has proved to be very successful. Simple test pieces have shown very good agreement to validate the methods.

-High temperature electromagnetic (EM) actuators
Conventional electromagnetic (EM) actuators are highly versatile, but are typically restricted to around 200 degrees Celsius, primarily by the insulation. This is a severe limitation for gas turbine applications, where the area just outside of the core engine is typically up to 360 degrees Celsius. All types of EM actuators including solenoids, motors etc. are of course used in cooler areas for applications ranging to small valves to thrust reverser actuation. Numerous potential applications have been identified ranging from turbine tip clearance to fluid system controls.

The work has progressed in the two complimentary areas of coil and system design. Early reviews identified the available technology and applications. The results reported from work package 5 developed a technology to provide coils with a practical environment temperature limit of 400 degrees Celsius, a major improvement over previous capabilities. It is a fundamental enabling technology and will have a significant impact on many applications outside of the two investigated here. Design of coil heat transfer has also seen significant advances. a novel grooved armature and stator design has been developed which greatly extends the useful range of force actuation.

A practical device was built and demonstrated for compressor tip clearance control at elevated temperatures. Although currently too heavy for aircraft applications, the approach has been further developed in NEWAC and considered for turbine tip clearance control in the Environmentally Friendly Engine (EFE) UK programme. Although it was not possible to produce a high temperature fuel valve within the programme, a thorough application and component design study was completed.

-Vibration control systems
Control of engine rotor vibration remains a major reliability facet of engine design and is a significant cost in engine manufacture and operation. Low vibration is essential to ensure long life of many engine components such as the rotor bearings and the engine external systems. At higher levels of response, excessive rotor vibration causes significant wear of the casing liners, increasing the blade tip to casing clearances leading to loss of engine efficiency and increase in CO2 emissions. Transmission of vibration into the airframe is also increasingly becoming a concern as airframers strive to improve the cabin environment for the comfort of the passengers.

The concept developed by Polytechnic University of Turin, requires modifications near the rotor bearings and supports only, and as these are the low radius parts of the rotor, the system can be relatively compact and light in weight. The few new mechanical features are similar to those in existing engines with squeeze-film bearings, so the new concepts and their effects are easily understood. The system is tuneable, both electrically and mechanically. Some compromise on rotor support stiffness and hence rotor critical speeds may be involved, but again this is typical of squeeze-film damper design too.

The effectiveness of the Turin concept was well demonstrated in ADVACT WP6 by means of theoretical studies and a laboratory scale rotordynamics test rig (RTR). The test rig also enabled good demonstration that the controlling electronics is effective and readily feasible to make.

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