Jürgen Göring

A New Concept for Thermal Protection of All-Mullite Composites in Combustion Chambers

Abstract A new thermal protection concept for all-mullite composite shingles based on a thermally-sprayed mullite layer is described. Because of the insufficient thermal long-term stability of the Nextel™ 720 fibers in the 1273 plus regime, Nextel™ 720 fiber-based ceramic composites are protected by a flame-sprayed mullite coating in order to prevent the composite from thermal degradation in service. The protection layer is deposited on the front side of the ceramic shingle facing the hot gas stream. Front and back sides of the shingles are cooled through film and convection cooling, respectively. Reducing both the composite material and the protection layer to a single phase (mullite) system is a simple, but highly efficient approach to keep thermal and elastic misfit strains at the interface at reasonably low levels. Due to the porous grain texture intrinsic to thermally-sprayed materials, thermal conductivity of the protection layer is low, yielding a considerable thermal insulation effect depending on the layer thickness and the particular heat flow scenario of the combustion chamber. The microstructure/property relationship of the thermal protection layer and its interaction with the underlying composite are discussed focussing on the constraints of real combustion chamber operation conditions.

Development and Validation of Oxide/oxide CMC Combustors within the HiPOC Program

In the framework of the High Performance Oxide Ceramics program (HiPOC), three different oxide/oxide ceramic matrix composite (CMC) materials are studied for a combustion chamber application in continuation of the work reported in Gerendas et al. [1]. A variation in the micro-structural design of the three CMC materials in terms of different fiber architecture and matrix processing are considered in a first work stream. By modification of the matrix and the fiber-matrix interface as well as the application of an environmental barrier coating (EBC), the high temperature stability is enhanced. Furthermore, design concepts for the attachment of the CMC component to the metal structure of the engine are finalized in a second work stream. Issues like sealing of cooling leakage paths, allowance for the different thermal expansion and the mechanical fixation are addressed. An interim standard of the mechanical attachment scheme is studied on a shaker table. Also the friction coefficient between the metallic and ceramic components is analyzed in order to set the proper tightening torque. The manufacturing of the CMC combustor is improved in several iterations in order to achieve a high quality material with optimized fiber architecture. Afterwards, two CMC materials are selected for the combustion testing and the finalized design of the metallic and CMC components is manufactured. A fit check is performed prior to EBC application and laser drilling of the effusion holes in order to evaluate the impact of the manufacturing tolerances on the function of the sealing and attachment scheme and to correct small issues at this stage. First results from the validation testing in a high-pressure tubular combustion rig up to a Technology Readiness Level 4 (TRL4) are reported.Copyright