The Power of Design
SwRI engineers develop a low-cost centrifugal gas turbine
By Klaus Brun, Ph.D. and Robert J. McKee
Approximately 50,000 industrial gas turbines are in use throughout the world, and these machines are steadily gaining market share against reciprocating engines and steam turbines in all industries. Gas turbines generally require less maintenance, have a higher availability, and can provide higher efficiency than reciprocating engines and steam turbines.
Industrial gas turbines are commonly used in applications where high power-to-weight ratio, low emissions and high availability requirements prohibit the use of reciprocating engines, despite their lower cost. In the oil and gas industry, small industrial gas turbines are used for pipeline compression, oil pumping, water injection, gas lift and offshore platform power generation - all applications where efficiency is of secondary concern to low weight and high reliability.
Most modern industrial gas turbines are technically complex machines consisting of multiple rotating parts, bearings, seals, lubricating oil systems and sophisticated electronic controls. They are so technically sophisticated that most users cannot perform basic repairs and maintenance.
Industry is demanding simple, low-cost gas turbines that can function under very rugged environmental conditions, are easy to repair or replace, can be operated by untrained staff and can be moved easily. At present there are few gas turbine products commercially available that satisfactorily meet these market requirements. To meet this growing demand, engineers at Southwest Research Institute (SwRI) have developed a novel, prototype centrifugal gas turbine using internal research funding.
Currently, most gas turbines use an axial flow compressor and an axial flow turbine. This design is a direct evolution of the jet engine and clearly provides the highest aerodynamic efficiency. However, the axial design does not typically provide the lowest weight, smallest dimensions or greatest ease of maintenance.
Simple, open-cycle gas turbines consist of three principal components: a compressor, a combustor and a turbine mounted on a single rotating shaft. The compressor ingests and compresses ambient air, the combustor heats this air by fuel combustion and the turbine expands the resulting hot air to drive a shaft that generates mechanical output power. The SwRI-designed centrifugal gas turbine is based on this simple, open cycle.
The fundamental difference between the SwRI centrifugal gas turbine and conventional gas turbines is that the compressor and turbine section are installed on the same side of the rotating wheel, while the combustor and nozzle are mounted on the stationary shroud. Thus, the entire gas turbine assembly consists of only two relatively easy-to-manufacture components: the rotating compressor turbine wheel, which is directly coupled to the generator/starter motor, and the combustor shroud, which holds the utility line connections. Unlike conventional gas turbines, no flow turning is required. Because the SwRI design incorporates only one rotating part, costs of manufacture, maintenance, repair and replacement are low.
The objective of the SwRI internal research project is to develop a 50-kilowatt centrifugal gas turbine design that can achieve simple cycle efficiency greater than 16 percent while maintaining rotordynamic and mechanical integrity and stability. During the last year, SwRI engineers designed the gas turbine's compressor, turbine and nozzle using in-house and commercial engineering codes, and built a prototype machine. The final hardware prototype was machined from Inconel(r) super-alloy using a five-axis milling machine.
A one-dimensional gas turbine thermodynamic cycle design code was written to predict the gas turbine performance and to serve as a preliminary design validation tool. For the compressor performance, the code uses empirical, characteristic-flow curves and a highly radial flow compressor design. For the turbine analysis, a high-impulse turbine design and experimental data were used.
A finite element stress analysis was performed to evaluate the maximum allowable rotational speed of the proposed centrifugal gas turbine. This design includes a 20-percent safety margin on speed to protect the machine from sudden overspeed excursions or operational upsets. Various possible rotor geometry options were evaluated. To maintain structural integrity and durability, the speed of the operational prototype is limited to less than 32,000 revolutions per minute (rpm).
A combustion system design must be appropriate to the unique radial-flow system. The system must stabilize the flame while allowing for cooling of the walls and dilution of the combustion gases to achieve a temperature that is compatible with the turbine inlet. The combustor development phase will be initiated in early 2005.
Completion of gas turbine hardware fabrication and final installation are under way. A rapid prototype of both the rotor and stator were also built and are being used for promotional activities. For prototype testing, the gas turbine will be mounted on a high-speed test rig driven by a 200-hp electric motor with a dynamic brake rather than being directly coupled with a high-frequency AC motor/generator. This will allow SwRI to test a wide range of prototype models rather than being limited to a single speed and load. In this rig, the AC motor/generator is coupled via an 11/1 rpm gearbox to the centrifugal gas turbine, and the gas turbine is fully supported on inboard and outboard high-speed bearings. A functioning prototype of the SwRI gas turbine will be available for demonstration by late 2005.
The SwRI gas turbine is compact, light and portable. These features make the SwRI gas turbine an ideal candidate for applications including military battlefield, oil production flare gas, and on-ship auxiliary power unit generation. Other possible applications include nanotechnology gas turbines, distributed power generation, combined heat and power, and hydrogen power generation.
SwRI has applied for
and received patent protection for its centrifugal gas
Comments about this article? Contact Brun at (210) 522-5449, or firstname.lastname@example.org.
Published in the Winter 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.