LibertyWorks®, a subsidiary of Rolls-Royce Corporation, first studied CMC (ceramic matrix composite) exhaust mixers for potential weight benefits in 2008. Oxide CMC potentially offered weight reduction, higher temperature capability, and the ability to fabricate complex-shapes for increased mixing and noise suppression.
In 2010, NASA was pursuing the reduction of NOx emissions, fuel burn, and noise from turbine engines in Phase I of the Environmentally Responsible Aviation (ERA) Project (within the Integrated Systems Research Program). ERA subtasks, including those focused on CMC components, were being formulated with the goal of maturing technology from Proof of Concept Validation (Technology Readiness Level 3 (TRL 3)) to System/Subsystem or Prototype Demonstration in a Relevant Environment (TRL 6).
In April 2010, the NASA Glenn Research Center (GRC) and Rolls-Royce (RR) jointly initiated a CMC Exhaust System Validation Program within the ERA Project, teaming on CMC exhaust mixers for subsonic jet engines. The initial objective was to fabricate and characterize the performance of a 0.25 scale low bypass exhaust system that was based on a RR advanced design, with a 16-lobe oxide/oxide CMC mixer and tail cone (center body). Support Services, LLC (Allendale, MI) and COI Ceramics, Inc. (COIC) supported the design of a mixer assembly that consisted of the following oxide/oxide CMC components mounted on separate metallic attachment flanges: a) a lobed mixer and outer fan shrouds, and b) a tail cone. TRL 4 (Component/Subscale Component Validation in a Laboratory Environment) was achieved in a cost-effective manner through subscale rig validation of the aerodynamic and acoustic performance via testing at ASE FluiDyne (Plymouth, MN) and at NASA GRC, respectively. This encouraged the NASA/ RR/COIC team to move to the next phase of component development; full scale CMC mixer design for a RR AE3007 engine. COIC fabricated the full scale CMC mixer, which was vibration tested at GRC under conditions simulating the structural and dynamic environment of a mixer. Air Force Research Laboratory (AFRL, Wright-Patterson Air Force Base (WPAFB)) provided test support by assisting with instrumentation and performing 3D laser vibrometry to identify the mixer mode shapes and modal frequencies over the engine operating range.
Successful vibration testing demonstrated COIC’s new process for fabricating full scale CMC mixers and the durability of the Oxide CMC component at both room and elevated temperatures. A TRL≈5 (Component Validation in a Relevant Environment) was attained and the CMC mixer was cleared for ground testing on a Rolls-Royce AE3007 engine for performance evaluation to achieve TRL 6.