Final Report Summary - ALIAS (Acoustic Liners for air conditioning system) Executive Summary:On modern aircraft, passengers and crew breathe a mixture of fresh and recirculated air. This combination rather than fresh only allows the regulation of temperature, pressure and humidity. The air is bled from the engines and supplied to air conditioning units. It is then ducted into the cabin, circulated and eventually drawn into the lower fuselage where is sucked out by the pressurization outflow valve for the cycle to begin again. Besides creating a safe and comfortable environment, the aircraft air conditioning systems generate noise. The noise radiated from the aircrafts' air conditioning systems is reduced thanks to acoustic liners. These liners present a major design challenge because of the need to address a wide range of conflicting requirements. Acoustic liners must provide high levels of noise reduction over a wide range of operating conditions. They should also be light and flexible to meet strict weight and tight space restrictions.Until now, acoustic liners for air conditioning systems are made of porous materials, very efficient for sound absorption in the high-frequency range. However, the air conditioning systems manufacturer who is the Topic Manager of the JTI-CS-2012-2-SGO-02-036 CFP, is developing an electrically driven air system composed of an air pump which generates low- and mid-frequency noise. New kind of acoustic liners are therefore required to mitigate this noise source.The ALIAS project (Acoustic Liners for Air conditioning Systems) consists in designing optimized locally reacting treatments for mid-frequency acoustic attenuation in air conditioning systems. The French Aerospace Lab (ONERA) and the SME ATECA, specialized in Aeronautics, Advanced Materials & Manufacturing, combined their research and technological capabilities to propose a solution of TRL5 that meets the industrial requirements. This was achieved by implementing a simulation-based design process, during a two-year long project between 01/01/2013 and 31/05/2015. Project Context and Objectives:The ALIAS project aimed at demonstrating that an acoustic liner based on Helmholtz-resonator-like concepts could mitigate the noise emitted by a jet pump used in air conditioning systems. Indeed, contrary to fan-based air-conditioning systems, this device generates noise in the mid-frequency range, so that classical lining solutions made of porous material cannot damp the noise source, under the industrial constraints of tight space restrictions. An alternative solution, which has been used for a long time in turbofan engines nacelles, consists in implementing perforate-over-honeycomb liners, which are very effective to damp acoustic waves in the mid-frequency range when the liner thickness is of the order of a few inches. For reaching this objective, ONERA and ATECA relied on their long know-how for designing and manufacturing acoustic liners for turbofan engines inlets. However, many challenges had to be tackled. The optimal liner acoustic impedance had to be assessed for the specific environment of the air-conditioning system duct. Contrary to nacelle inlets, the shape of the lined section is a cone of small radius, in which the flow profile is far from uniform. ONERA had therefore to include these effects into its numerical tools, so that a numerical assessment of the industrial geometry and flow field impact on the noise attenuation within the duct could be performed. Another objective of the project was to propose a liner concept that could achieve the expected acoustic attenuation while complying with the industrial requirements on thickness, weight and cost. ATECA, as an SME specialized in liner manufacturing, drove the investigations to ensure that the concept solutions proposed by ONERA could indeed be acceptable on an industrial point-of-view. This design process had to be de-risked by a step-by-step investigation, including small samples manufacturing and laboratory tests in configurations as representative as possible of the industrial application. The final objective of the project was to demonstrate the relevance of the chosen solution in an industrially-relevant environment, with the realistic noise source. ATECA had to develop new manufacturing process and tooling so that a cone-shaped scale 1 acoustic liner could be built. Testing it in the industrially-relevant environment could only be feasible through a close cooperation between the CfP partners and the Topic Manager, who owned the jet pump setup and the acoustic facility in which the liner efficiency could be assessed. At the end of the project, all these objectives stated in the Annex I have been fully achieved. The only deviation from the Annex I was a slight delay in the production schedule of the demonstrators and the subsequent tests in the Topic Manager’s acoustic facility, which required a three-month extension of the project duration.Project Results:First, technical specifications were defined with the Topic Manager. Then, ONERA in-house simulation tools (a fast modal analysis code as well as a more complex solver of the linearized Euler equations) were used to estimate the acoustic liner impedance that maximizes noise damping of the set of cut-on modes in the target range of frequency. A sheared flow profile in a cylindrical duct was considered. The next step was to design locally reacting acoustic liners that best fit the target impedance. For this purpose, semi-empirical models were reproduced from literature to link geometrical properties to the acoustic impedance attained. This allowed the selection of five concepts of acoustic treatments.Laboratory samples of these concepts were then manufactured by ATECA and tested in ONERA facilities, both in an impedance tube and in the B2A flow duct with a grazing flow up to M=0.3. In the flow duct, the acoustic impedance was measured in-situ, except for a specific advanced concept, the impedance of which was educed from pressure measurements at the wall opposite the liner. The accuracy of theoretical predictions has been confirmed by comparing them with the acoustic impedance measurements. Then, the latter were used as an input for the ONERA acoustic propagation code, which predicted the expected attenuation in the industrial configuration. These numerical predictions allowed the selection of the best two concepts. The selection criteria were not only the reached acoustic attenuation, but also manufacturing, costs and weight considerations. These issues were indeed specifically studied by ATECA, who performed an analysis for integration of proposals solutions in an industrial context.Finally, large-scale prototypes of the two selected designs were manufactured by ATECA and tested in industrial settings in the acoustic facility of the Topic Manager. The measured acoustic attenuation proved to be as high as expected in the target mid-frequency range.Potential Impact:Large-scale prototypes of the two selected designs were manufactured by ATECA and tested in industrial settings in the acoustic facility of the Topic Manager, which corresponds to a TRL 5 solution.The performance of the designed acoustic liners has been proved in the target frequency range and operating conditions. An acoustic liner concept is now available for the Topic Manager, so that the mid-frequency noise source of jet pumps can be mitigated, which is one of the necessary steps for the implementation of such innovative devices in an air-conditioning system. The know-how gained by ALIAS partners could now be used to expand the developed design-based process to various applications in aeronautics, or to other operating conditions of air conditioning systems. Especially, low-frequency noise attenuation under tight thickness restrictions is still an issue to be tackled. One of the innovative design studied during ALIAS project, but not yet manufactured at large-scale, could be a good candidate. Moreover, the liner design process would strongly benefit from the knowledge of the source modal content and noise level within the duct, which remains an open issue. ALIAS results were directly exploited by partners, since ATECA has increased its product range with those developed in ALIAS project, as planned at the beginning of the project (see the Annex I). Indeed, in June 2015, two prototypes of acoustic liners were bought to ATECA by an air-conditioning systems manufacturer. This could be done through the know-how developed during the ALIAS project, especially regarding the assembly techniques and associated procedures. Moreover, ONERA is currently developing an acoustic liner for damping noise in its large wind tunnels, based on one of the concepts tested in ALIAS. This was made possible thanks to the improved capabilities on liner design gained during the project. Equipping ONERA large wind tunnels with such acoustic liners could yield a huge increase of competitiveness, since this could allow a combination of aerodynamics and aeroacoustics test campaigns at the same time, even at very high flow speed. Regarding dissemination activities, due to the short time-scale, no publication has been finalized during the project. However, a communication on the project results has been submitted to the 22th AIAA/CEAS Aeroacoustics Conference that will be held in June 2016. The pdf file of this paper is attached to this report. In Annex I, communications activities within the Iroqua and X-Noise networks was planned. However, during the project duration, X-Noise technical events were out of the scope of the project topic: in September 2013 X-Noise workshop was dedicated to atmospheric and ground effects on aircraft noise, and in September 2014 to aircraft noise reduction by flow control and active techniques. A similar mismatch occurred between project progress and Iroqua annual meeting agenda; in addition to Topic Manager’s request for limiting the presentation of confidential data, this prevented the dissemination of the project activities to Iroqua network. That is why ALIAS partners waited the end of the project and the gathering of final results to prepare the communication towards the acoustics community, through the AIAA/CEAS conference. Moreover, dissemination was done to the aeronautics industry through the participation of ATECA to the Paris Air Show from June 19th to June 25th 2015. ATECA had a stand in one of the Halls and could present its activities related to the prototypes’ manufacturing (including ALIAS activities).List of Websites:Contact details for ONERA: estelle.piot@onera.fr; Contact details for ATECA: v.fascio@ateca-fr.com Related documents final1-alias_publishable-summary_revised.pdf
