Development and Testing of a Novel Centrifugal Gas Turbine Design Phase II – Combustor Development and Load Testing, 18-R9540

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Principal Investigator
Klaus Brun

Inclusive Dates:  04/01/05 – 10/01/06

Background - A single disk centrifugal gas turbine was developed based on a straight radial outward flow design (no 180-degree flow turning as in conventional radial gas turbines). This simple geometry contains only two major elements — a rotor disk and a stator shroud. The rotor consists of a centrifugal compressor and high impulse radial outward flow turbine connected directly to an electric generator, which also acts as the starter motor. The stator shroud contains the combustor, nozzles, and fuel lines. Fuel supply lines are attached to the shroud and ducted directly to the fuel nozzles in the combustor. The fundamental difference between the centrifugal gas turbine and conventional centrifugal gas turbines is that the compressor and turbine section are installed on the same side of the rotating wheel, while the combustor and diffuser are mounted on the stationary shroud. Thus, the entire gas turbine assembly consists of two relatively easy to manufacture components.

This very simple gas turbine concept has the following advantages:

  • A single rotating component mechanically compact gas turbine
  • Short axial span and very high power density
  • No internal bearings or gears
  • Modular two-piece construction
  • Ease of maintenance/repair access

Approach - This report concludes Phases I and II of this prototype development project, which included the conceptual design, structural and aerodynamic analysis, performance prediction modeling, detailed design, prototype fabrication, test rig instrumentation, compressor/turbine characterization, rotordynamic signature, combustor design, combustor testing, controls implementation, light-off testing, no-load testing, and performance testing of the gas turbine.

Analyses that were performed included 1-D thermodynamics, FE structural and thermal, rotordynamic, 2-D blade path optimization, and 3-D computational fluid dynamics. Testing of both individual gas turbine elements (compressor, combustor, and turbine) as well as the entire assembly was completed. Based on results from these analyses, a rotor and stator geometry was optimized, and engineering drawings were prepared. The gas turbine rotor was machined from Inconel 716 and the stator from stainless steel materials using 5-axis milling.

A variable speed (200 horsepower, 40,000 revolutions/minute) test rig was designed and built to be able to operate both as the gas turbine driver (for compressor testing) and as a load absorber/break for gas turbine operational tests. The instrumentation and data acquisition required for compressor, turbine, and full gas turbine tests were implemented and tested to allow for accurate component/gas turbine performance measurement and operational control.

Using the machined rotor and stator hardware on the high-speed test rig, the dynamic and thermodynamic performance of the gas turbine's compressor and turbine elements was individually assessed, mapped, and employed to verify the code predictions. Results from these tests were used to design the gas turbine nozzles and combustor. The combustor was tested and then the entire gas turbine assembly was mounted in the test stand for operational and performance testing. Light-off test, no-load test, and performance tests were performed using natural gas as a fuel with the complete gas turbine assembled. The above results and tasks were successfully completed with the relevant details provided in this report.

Accomplishments - Tests showed that stable gas turbine combustion was achieved up to 26,000 revolutions/minute. The gas turbine reached self-sustaining power at 21,000 revolutions/minute and 405°C firing temperature. Thus, the centrifugal gas turbine concept was demonstrated to function properly and to achieve positive power output. However, the design output power was not achieved because of combustion stability limitations at speeds above 20,000 revolutions/minute. Maximum output power achieved was 1.4 kW at 23,000 revolutions/minute. Recommendations are provided in this report to overcome these operational limitations in the next model centrifugal gas turbine. Funding for additional work on this device is currently being sought from various sources.
 

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