The primary purpose of the compressor is to increase the pressure of the air through the gas turbine core. It then delivers this compressed air to the combustion system. The compressor comprises the fan and alternating stages of rotating blades and static vanes.
Rotors and stators embody an aerodynamic design to maximise the efficiency of the compression process. Aerodynamic design has evolved from a design process carried out primarily in two dimensions (2D) by hand, and based on extensive rig-based correlations, to an entirely computer-based process carried out in 3D with a precise definition and control of the whole aerofoil shape. This has been made possible by the continued development of computational fluid dynamics allowing accurate modelling of the airflow, coupled with the growth in computing capacity.
Mechanical design is focused around delivering a low-weight, high-integrity and balanced solution for the compressor based on the aerodynamic performance and blade shape required. Both the material selection and component design must meet the life requirements taking into account both steady and unsteady loads. Detailed vibration analysis is carried out to ensure potential resonances are avoided or, if present, are within controlled limits. These are then confirmed via telemetry tests on engines over the whole operating range. Parts are also designed taking into account manufacturing capability to ensure a seamless transition from design to manufacture.
Validating the compressor performance and capabilities as part of the engine development programme is a vital part of the design process. Aerodynamic rig tests are carried out to confirm the efficiency and operability of the compressor over the range of operating conditions. Mechanical testing is performed on key elements of the design including telemetry engine testing with strain gauges, testing to validate the blade vibration responses and rub tests to corroborate the blade and sealing fin performance. Greater use of advanced computational fluid dynamics and mechanical models is reducing the amount of physical testing required to validate designs.
Compressor discs are machined mostly by turning and milling, while a drum is manufactured by joining discs in either a welded or bolted assembly. Welding is more commonly used because it results in reduced weight and increased stress benefits. The latest technology is inertia friction welding, where one of the pieces is held stationary while its complementary part is rotated, linked to a flywheel with enormous inertia. The pieces are driven together and the inertia is converted into heat by friction. As the material melts it is driven out of the join, giving a high-quality weld. In disc manufacture novel high-speed grinding processes are being developed that dramatically speed up the creation of disc slot features. These processes rely on automated continuous dressing and detailed 3D simulation of the process to optimise the wheel profiles and approach paths.
Compressor blades are manufactured primarily by precision hot forging of an extruded billet, followed by machining of the root features and finishing operations. After forging, the blade surface is polished to achieve the highly smooth finish essential to achieving good aerodynamic performance. As blade shapes become more complex with optimised leading edge profiles, manual techniques for finishing are gradually being replaced by automated processes. These technologies will achieve much greater quality and repeatability of these features while allowing the scaling up of production volumes. The success of such techniques is built upon advances in the capability to model the forging/manufacturing process, as well as on-condition process monitoring whereby, for instance, the polishing adapts to wear rates in the wheel and allows for self-dressing.
Manufacturing blades and discs as a single structure (blisk) removes the need for leakage paths and blade fixings, resulting in weight savings of up to 30 per cent. Bladed rings (blings) can save even more weight. Manufacture and repair techniques also contribute to making blisks the best life-cycle value solution. While smaller blisks are machined usually from a solid block, larger blisks are made by friction welding a forged aerofoil onto the disc. This joining technology produces a very high quality weld and maintains the material strengths across the join. Further weight loss is possible by removing the disc inner with the remaining ring carrying all the centrifugal loads. Such blings are being developed utilising a titanium metal matrix composite with embedded silicon carbide fibres to carry the resulting stresses (the matrix can be up to 100 per cent stiffer and 50 per cent stronger than unreinforced titanium).
Material developments need to keep pace with the drive for increased thermal efficiency cycles that results in ever higher pressures and temperatures. Development of high-temperature materials for compressors drums and discs is typically shared with turbines, but specific blade alloy development is necessary since compressors do not incorporate cooling technology. As well as advanced nickel-based super alloy developments, new manufacturing process control allows the microscopic structure of existing alloys within a component to be tailored to give specific attributes, further extending the overall temperature capability of the part. The dual microstructure disc is such an example, where the sections close to the blade have optimised creep resistance while those in high stress areas are optimised for strength.
Damage from ingested objects (Foreign Object Damage, known as FOD) is a hazard for compressors. If damage is outside acceptable limits, this can result in increased inspections, monitoring or even engine removal. Intrascope blending is carried out on wing through inspection ports to remove such damage and avoid cost and operational disruption. Intrascope blending can also be used for returned lease engines or engine resale and may provide an alternative to a check and repair.
The Trent 1000 anti-icing function is designed to prevent “classic” ice formation due to water freezing as it hits cold metal. Modern high-bypass ratio engines have lower fan pressure ratios, so temperatures at the core inlet are slightly lower than on earlier engines, which can increase the possibility of ice build-up. The core engine section stators (ESS) warm-air, anti-icing bleed (delivered from the Intermediate Pressure compressor) is switched on automatically only when needed, such as during descent – controlled by rating, altitude, ambient temperature, and forward speed – and on the ground during taxi in cold conditions such as freezing fog. The heated ESS removes the requirement for most fan anti-icing procedures.
Maintaining the compressor’s stability and range of operability becomes more challenging as the demands increase for ever higher pressure ratios and smaller cores. Bleeding air from the compressor can increase part-speed stability, but wastes energy and can increase noise. Variable angle stator vanes at the front of the compressor improve low-power performance, but add mechanical complexity. Other methods aim to control the regions of the flow that are prone to aerodynamic instability and breakdown that can lead to compressor surge. One example is casing treatments, where specially designed cavities over the blade tips can re-energize the flow. More active flow injection systems are also being researched.
Centrifugal compressors – employing a rotating impeller and static diffusion system – tend to be more compact and robust than purely axial systems and are often used in smaller engines for helicopters and small jets. A centrifugal compressor comprises an inlet, a rotating impeller, a static radial diffuser that turns kinetic energy into pressure energy, and an exit system. Impellers operate at high speed and basically impart kinetic energy as well as diffusing the air to attain a static pressure rise. The downstream radial diffuser incorporates passages forming divergent nozzles that convert the kinetic energy to pressure energy; its radial geometry also naturally removes swirl imparted to the flow by the impeller. The exit duct minimises exit pressure loss as well as performing further diffusion and aligning the air direction required for downstream engine components.