Session: 02-02 Ceramics and Composites II: Application

Paper Number: 101601

101601 - Performance and Emissions Benefits of Cooled Ceramic Matrix Composite Vanes for High Pressure Turbines 

Since the efficiency of today’s aero-engines is significantly degraded by large amounts of turbine cooling air, there is a strong need for heat-resistant materials that are suitable to replace the conventionally used nickel-based superalloys. One promising opportunity is the use of ceramic matrix composites (CMC), which offer a higher temperature capability and lower density than metals, and thus enabling the development of lighter and more efficient aero-engines. Although several hot gas components have the potential for CMC applications, especially CMC high pressure turbine vanes have been investigated and tested in recent decades. Moreover, it has already been announced that CMC vanes will be installed in a future engine. Taking these facts into account, it is important to assess what potentials can be achieved by CMC vanes and how to adapt the thermodynamic cycle in order to reach the full potential. Therefore, this publication focuses on the performance benefits of cooled CMC high pressure turbine vanes in terms of their aero-engine application.

 

A realistic design scenario of cooled CMC vanes is the so-called shell and spar concept. In this approach, the thermal load is carried by an impingement cooled CMC shell in the shape of an airfoil, and a metallic spar within the shell is used as structural support and jet plate. After impingement cooling, the air exits the shell through the trailing edge.

This publication focuses on the application of cooled CMC vanes as turbine inlet guide vanes (IGVs) downstream of the combustor. When replacing metallic IGVs with CMC vanes, it is important to keep the rotor inlet temperature (T41) constant in order to avoid an increase in rotor coolant mass flow. Otherwise, the efficiency of the thermodynamic cycle would decrease. Studies on modern highly cooled aero‑engines have shown that the efficiency gain from rotor cooling air reduction outweighs the thermodynamic potential of a higher T41. Furthermore, rotor cooling air is considered to perform no work within the turbine stage.

Using CMC IGVs at constant T41 leads to aerodynamic and thermodynamic benefits. In contrast to film-cooled metallic vanes, the cooling air is only discharged through the trailing edge, resulting in lower mixing losses between cooling air and hot gas. This improves the aerodynamics and thus the turbine efficiency. Additionally, the necessary coolant pressure decreases, since no cooling air has to be ejected at high static pressures along the leading edge. Consequently, the cooling air can be extracted at an earlier compressor stage, resulting in colder cooling air and less coolant compression work. Compared to metallic IGVs, CMC IGVs offer a significant reduction in coolant mass flow, leading to a decrease in turbine inlet temperature (T4) at constant T41. In terms of emissions, a lower T4 results in a reduced production of nitrogen oxides (NOx).

 

Based on a state-of-the-art ultra-high bypass ratio (UHBR) engine, the previously described performance benefits of cooled CMC IGVs are analyzed with DLR in-house programs. The 0D performance of the reference engine is modeled using the framework Gas Turbine Laboratory (GTlab). The turbine stage and blade row layout, including preliminary cooling air requirements, are designed by means of a 1D meanline tool. Afterwards, efficient cooling concepts for metallic and CMC IGVs are detailed using a 2D cooling design program. To ensure matching between engine and turbine design, the turbine results are directly linked to the performance model. Finally, the change in NOx emissions is estimated by means of a correlation-based p3-T3-method that was calibrated with certification data at sea level static for a rich-quench-lean (RQL) combustor concept.

Presenting Author: Patrick Wehrel German Aerospace Center (DLR)

Presenting Author Biography: The author studied mechanical engineering at the TU Dortmund in Germany and is working as a researcher at the German Aerospace Center (DLR) now. He focuses on preliminary aero-engine design, the coupling of engine and turbine design, and the potential of new materials, such as ceramic matrix composites (CMC) or titanium aluminide (TiAl), in the context of their engine application.

Authors:

Patrick Wehrel German Aerospace Center (DLR)
Robin Schöffler German Aerospace Center (DLR)
Clemens Grunwitz German Aerospace Center (DLR)
Francisco Carvalho German Aerospace Center (DLR)
Martin Plohr German Aerospace Center (DLR)
Jannik Häßy German Aerospace Center (DLR)
Anna Petersen German Aerospace Center (DLR)