Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - IDEAS (Innovative DEsign of acoustic treatments for Air conditioning Systems: from laboratory to an industrially-relevant environment)
Teaser
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. In Clean Sky...
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. In Clean Sky 2, the Systems-ITD is developing several technological bricks to address needs for a future electrical Environmental Control System (ECS) for large or regional aircraft configurations. One promising brick of the future electrical ECS is an electrically driven air system composed of an air pump. This air pump generates low- (100-1000 Hz) and mid-frequency (1000-5000Hz) noise, on contrary to standard fan-based systems which generate high-frequency noise. This noise is conducted through several ducts to the exterior of the aircraft, thus contributing to ramp noise. This noise must be mitigated to comply with the local regulations on perceived noise for both the passengers and the airport workers when the aircraft is at the gate or is taxiing to the runway (in Europe the maximum daily noise exposure is 85 dB with an exposure limit value of 87 dB) . Acoustic liners are therefore required to mitigate the noise source. They must be light and compact to meet strict weight and tight space restrictions, they must handle harsh operating conditions and their manufacturing must be as cheap as possible so that the air conditioning systems be competitive. Until now, acoustic liners for fan-based systems are made of porous materials, very efficient for sound absorption in the high-frequency range while addressing the weight and space requirements. For the air pump system, the liner design is much more challenging because of the lower range of frequency to be dealt with, which is conflicting with the low thickness and low mass requirements. A breakthrough in the liner concept design and manufacturing is therefore necessary. This will rely on an accurate knowledge of the noise source (spectrum, modal content), which can only be obtained by advanced measurement technique such as in-duct modal detection. ONERA, the French Aerospace Lab, and the two SMEs ATECA and POLY-SHAPE will combine their research and technological capabilities to propose new ideas in the domain of acoustic liners and in-duct modal detection for air conditioning systems.
Work performed
During the first period of IDEAS project, several concepts of liners have been designed by ONERA in the first workpackage. They are made of architectured materials which combine low-frequency- and middle frequency-attenuation capabilities. The design took into account the tight constraints on weight, temperature resistance and space that were defined by the Topic Manager, Liebherr Aerospace. Samples of these concepts have then been manufactured by POLY-SHAPE and ATECA, both at the dimensions of ONERA impedance tube and flow duct (denoted as B2A, ie a thermo-aero-acoustics bench). Classical manufacturing techniques have been combined with additive manufacturing techniques. These samples are now being tested in ONERA laboratory facilities to assess the acoustic impedance of the samples, especially under flow conditions close to the industrial application. This will allow a down-selection of the two best concepts, as well as a checking and improvement of the impedance models used in the design phase.
In parallel, activities on in-duct modal detection were performed in WP2. ONERA numerical tools have been improved to take into account a shear flow profile in the modal detection process. The influence of the shear flow, compared to a simpler uniform flow assumption, was assessed with a shear flow profile representative of the industrial application. Several design of microphone rings were then investigated. The best design was selected with regards to constraints on the number of microphone available in Liebherr Aerospace acoustic facility and with regards on accuracy and robustness of the modal detection in the frequency range addressed in this project. Finally, a modal detection ring associated to this design was manufactured.
In the second period of the project, WP1 activities on the laboratory experiments dedicated to acoustic impedance measurements will be finished, and WP3 activities will be performed. The modal detection ring and the associated numerical tools will be used to assess the modal content of a realistic jet pump, in Liebherr Aerospace facility. These data will be used as inputs for numerical simulations of the acoustic attenuation that can be expected from the liner concepts selected at the end of WP1. Finally, two prototypes of liners will be manufactured at full scale, and their efficiency will be checked in a dedicated experimental campaign conducted in Liebherr Aerospace facility.
Final results
In the IDEAS project, two issues related to noise mitigation in aircraft air conditioning systems are addressed: the detection of the modal content of a jet pump noise source, and the design of an acoustic liner that can be efficient along a wide frequency range while being compliant with the stringent industrial requirements. The innovative solutions proposed to tackle these two issues will be matured during the project to increase their TRL level. The noise detection and acoustic liner technologies that will be matured in IDEAS may easily be leveraged towards addressing noise issues in other sectors of the transport industry.