Fabrication Materials for a Closed Cycle Brayton Turbine Wheel

A multidisciplinary analysis of a radial inflow turbine rotor is presented. This work couples high-fidelity fluid, structural, and thermal simulations in a seamless multidisciplinary analysis to investigate the consequences of material selection. This analysis extends multidisciplinary techniques previously demonstrated on rocket turbopumps and hypersonic engines. Since no design information is available for the anticipated Brayton rotating machinery, an existing rotor design (the Brayton Rotating Unit (BRU)) was used in the analysis. Steady state analysis results of a notional turbine rotor indicate that stress levels are easily manageable at the turbine inlet temperature, and stress levels anticipated using either superalloys or ceramics. I. Introduction Because of their relatively high power density, advanced, closed cycle Brayton power units are candidates for future NASA Exploration missions. In combination with a nuclear reactor, they can provide in-space and surface power as well as power for electric propulsion. One of the future in-space power applications is the Prometheus nuclear power plant (200 KW power class), and a pro posed mission is a long duration deep-space craft whose missions will likely focus on voyages to the outer planets. The baseline configuration for the Brayton unit of this power plant uses a radial inflow turbine fabricated from a superalloy material. One trade study is a turbine material change from a superalloy to a Silicon Nitride ceramic. Since the cycle and design have not been finalized, a previous design, the NASA Brayton Rotating Unit (BRU), was selected for this preliminary study. This paper provides a detailed description of the resulting analysis. The consequences of a turbine material change are quantified in this paper through the use of high-fidelity, three-dimensional, and multidisciplinary simulation techniques. These simulation techniques provide detailed information about the test article including pressure and temperature distributions, centrifugal loads, surface deflections including blade tip clearance, and thermal expansion. Although, numerical simulations do not replace experimental testing, they do provide design insights that reduce the risk and cost of a development program. Numerical simulations do this by allowing design variations to be considered quickly and inexpensively. The Numerical Propulsion Systems Simulation (NPSS) project developed and demonstrated techniques and software that allow a closely coupled, three-dimensional, fluid-structural-thermal analysis. The approach uses well validated fluid and structural analysis codes coupled together so that the correct physical boundary conditions are satisfied at the fluid-solid interfaces. Demonstrations of this capability include a rocket engine turbopump (ref. 1) and a hypersonic engine (ref. 2). This paper describes the application of these techniques, using H3D and ANSYS, to analyze forces on a notional Prometheus turbine rotor and the resultant stress and deformation field. The following sections describe the Brayton radial turbine, the numerical methods and multidisciplinary coupling techniques, the considerations for superalloy and ceramic materials, and results.