Quieting air intake in jet engines

Modern turbofan engines, also called bypass engines, have replaced turbojets in most commercial and military applications while retaining much of the original architecture. The turbofan sucks in air and pressurises it as in the past. In contrast to older turbojets, however, a portion of the compressed air bypasses the combustion chamber and is ejected out the back to provide additional thrust without fuel consumption, increasing efficiency.

By enhancing energy efficiency, the fan and bypass system also facilitate reductions in carbon dioxide emissions. They are also critical targets when it comes to decreasing noise. Novel acoustic liners that reduce both weight and noise are on the agenda to meet Clean Sky Joint Technology Initiative (JTI) requirements. The EU-funded project 'Large 3-shaft demonstrator - Aeroengine intake acoustic liner technology development' (ALTD) is playing an important part in this endeavour.

Scientists plan to bring a promising concept up to a technology-readiness level of six (TRL 6). This will be achieved with demonstration of an actual system prototype in an operational environment, namely an inflight-representative aeroengine ground test.

The intake acoustic liner under development relies on micro-perforation in which small mechanically drilled holes are placed in the material to absorb sound, reducing its intensity. Through extensive data collection and modelling, the acoustic liner and exact micro-perforation characteristics have been designed and optimised to satisfy noise reduction targets.

Scientists are also evaluating the materials and effects on the inner surface of the acoustic liner barrel. Full rain erosion testing has been completed. Hard particle erosion trials are underway, and fatigue analyses and effects of ultraviolet light are being considered.

A preliminary design review has led to the ordering of some items and definition of tooling requirements. The fully configured design is nearing completion and will be subjected to a critical design review in the next phase of the mechanical design process.

ALTD's pioneering aeroengine inlet acoustic liner and novel manufacturing processes will reduce the weight, noise and cost associated with acoustic attenuation technology. The project will thus make an important contribution to the EU's Clean Sky JTI for eco-friendly air transport.