Thermal Stresses in Gas Turbine Combustion Chamber Panels made of Ceramic Matrix Composite

In a goal of improving fuel efficiency and environment friendliness of aircraft, engine manufacturers are pushing development and introduction of ceramic materials into their hot section components. In particular, the use of... [ view full abstract ]

In a goal of improving fuel efficiency and environment friendliness of aircraft, engine manufacturers are pushing development and introduction of ceramic materials into their hot section components. In particular, the use of ceramic matrix composites (CMC’s) has been demonstrated in combustion chambers, where they allow reducing emissions of air pollutants like NOx gases. Requiring less cooling air flow than current metal alloys, they enable better optimization of the combustion chamber reaction and flow, without sacrificing on operability and durability.

In such an application, materials are subjected to large temperature gradients both through their thickness and within the surfaces that are exposed to hot gases. Nature of the resulting thermal stress distributions may vary significantly depending on the combustor configuration. Considering certain limitations of the CMC’s, for example their interlaminar strengths, it appears essential to clearly understand these stress states in order to make good design decisions.

Within this aim, this paper describes numerical heat transfer and stress analysis of potential CMC sub-components in an annular combustor subjected to realistic gas turbine conditions. Material modeled is the A-N720 oxide-oxide CMC from COI Ceramics. Flat and curved combustor panels are analyzed with convective and radiative heat load on one side, and convective cooling on the other side. Heat load is applied either on the concave side or convex side of the panels, to represent the outer liner or the inner liner of an annular combustor, respectively. Panels cover either the full circumference (360 degrees) of the combustor or only a portion of it. An orthotropic, damage-free model of the CMC is used to calculate the temperature and thermal stress distributions.

Both compressive and tensile stresses are obtained at the center of the panels, mainly compressive stress on the hot side, and tensile stress on the cool side. Only compressive stress is observed when heat is applied on the concave side of a segmented panel. The 360 degree panels show the largest stress. Per analysis of the results, potential failure modes include delamination at temperature transitions and panel edges, tensile rupture on the cool side, and compressive damage accompanied with fiber buckling on the hot side and panel edges. In all cases, adding an insulation layer on top of the CMC shows appreciable stress reductions.