| Summary: | An assessment of the performance and operating attributes of a high-speed small gas turbine engine with power output approximately one kilowatt has been carried out. The design of an experimental test facility for such an engine is subsequently proposed. Cycle analyses and computations on a state-of-art small gas turbine engine suggest that its cycle efficiency can be improved from 4% to 9% through an achievable increase in compressor pressure ratio and polytropic efficiency or by incorporating a high performance recuperator. The compressor polytropic efficiency is improvable by 3 to 6 percentage points through effective clearance management. Previous experiments on the test engine demonstrated the risk of operability issues, particularly during the engine start-up transients. These operability challenges are tentatively attributed to shaft-bearing housing clearance variation differing from design intent due to differential radial thermal expansion of the shaft and bearing housing system. Parametric assessments of the clearance variation for varying imposed conditions by the primary flowpath are conducted using a reduced-order model. This model is derived from combined unsteady CFD and conjugate heat transfer computations and finite element analysis. The model shows that the size of the shaft-bearing housing clearance reduction can vary by an order of magnitude depending on the transient duration. The non-dimensional shaft-bearing housing clearance has a functional dependence on the Fourier number and is independent of the final normalized turbine inlet temperature. The engine characteristic timescales for the aero-thermal-structural interactions are identified. The rotor acceleration timescale is determined by the ratio of the rotational kinetic energy to the engine power output and is 0.15 seconds. This timescale scales linearly with the rotor radius 𝑟. The transient thermal timescale is determined by the ratio of the engine structure heat capacity to the estimated convective heat transfer rate and is 76 seconds. Results from cycle analyses, unsteady computations, the reduced-order model, and quantification of engine time scales are then used to formulate and design a small gas turbine engine research facility and the associated measurement system. In addition to experiments for characterizing the performance metrics, engine thermal condition and engine mechanical clearances, experiments are proposed to challenge the scaling for thermal-induced shaft-bearing housing clearance variation from engine start-up to steady state operation.
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