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Performance Enhancement of Microturbine Engines Topped With Pezhman Akbari Performance Enhancement of Department of Mechanical Engineering, Michigan State University, 2500 Engineering Building, Microturbine Engines Topped East Lansing, MI 48824-1226 e-mail: [email protected] With Wave Rotors Razi Nalim Department of Mechanical Engineering, Significant performance enhancement of microturbines is predicted by implementing vari- Indiana University–Purdue University ous wave-rotor-topping cycles. Five different advantageous cases are considered for Indianapolis (IUPUI), implementation of a four-port wave rotor into two given baseline engines. In these ther- Indianapolis, IN 46202-5132 modynamic analyses, the compressor and turbine pressure ratios and the turbine inlet e-mail: [email protected] temperatures are varied, according to the anticipated design objectives of the cases. Advantages and disadvantages are discussed. Comparison between the theoretic perfor- Norbert Müller mance of wave-rotor-topped and baseline engines shows a performance enhancement up Department of Mechanical Engineering, to 34%. General design maps are generated for the small gas turbines, showing the Michigan State University, design space and optima for baseline and topped engines. Also, the impact of ambient 2455 Engineering Building, temperature on the performance of both baseline and topped engines is investigated. It is East Lansing, MI 48824-1226 shown that the wave-rotor-topped engines are less prone to performance degradation ͓ ͔ e-mail: [email protected] under hot-weather conditions than the baseline engines. DOI: 10.1115/1.1924484 Introduction microturbines ͓6,7͔. Utilizing conventional recuperators based on the use of existing materials can improve the overall efficiency of A growing market for distributed power generation and propul- microturbines up to 30% ͓2,8–10͔. Despite the attractive feature sion of small vehicles has motivated a strong interest in design of of the recuperator concept, a recuperator adds about 25%–30% to small gas turbine systems in the range of 30–300 kW. Known as the overall engine manufacturing cost, which is a challenge for microturbines, they are now widely used in the US for distributed commercialization of microturbines ͓11–13͔. The current trend of power generation, shaving peak loads, and providing backup the microturbine market is to reduce the investment cost. There- power for critical needs. They propel small commercial aircraft, fore, alternative devices need to be considered to achieve higher unmanned air vehicles ͑UAV͒, and terrestrial vehicles. Microtur- performance at lower component costs. Topping a microturbine bines are often the preferred alternative to IC engines, because of with a wave rotor device is an appropriate solution. their higher power density and robustness. They present several In a wave-rotor-topped cycle, the combustion can take place at advantageous features such as compact size, simple operability, a higher temperature while the turbine inlet temperature can be ease of installation, low maintenance, fuel flexibility, and low equal to that of the baseline cycle. Also, a pressure gain additional NO emissions. Furthermore, recent electric-power crises and en- X to that provided by the compressor is obtained by the wave rotor. vironmental concerns have stimulated a strong interest in the re- Thus, the wave rotor can increase the overall pressure ratio and search, development, and application of microturbines. peak cycle temperature beyond the limits of ordinary turboma- Despite their attractive features, compared with larger gas tur- chinery. The performance enhancement is achieved by increasing bines, microturbines suffer from lower thermal efficiency and both thermal efficiency and output work, hence reducing specific their relative output power, due to their ͑lower͒ component effi- fuel consumption rate considerably. This enhancement is espe- ciencies, limited cycle pressure ratio, and peak cycle temperature. cially favorable for smaller gas turbines often used for distributed For example, experimental and theoretical research has shown power generation or propulsion of small vehicles ͓14–17͔. that microturbines with pressure ratios of 3–5 without recupera- tion systems achieve only about 15%–20% efficiency ͓1,2͔. For Wave Rotor History. The first successful wave rotor was many applications improvement of their performance is desirable tested by Brown Boveri Company ͑BBC͒, later Asea Brown to enhance advantages over competing technologies. To achieve Boveri ͑ABB͒, in Switzerland in the beginning of the 1940s ͓18͔ such improvements, current efforts are mainly focused on utilizing as a topping stage for a locomotive gas turbine engine ͓19–22͔, heat recovery devices, developing new high-strength, high- based on the patents of Seippel ͓23–26͔. This first unit suffered temperature materials for turbine blades, and improving the aero- from inefficient design and crude integration ͓21͔. Later, BBC dynamic quality of turbomachinery components ͓3͔. The aerody- focused on the development of pressure wave superchargers for namics of turbomachinery has already yielded very high diesel engines, anticipating greater payoff ͓27͔. By 1987, their component efficiencies up to around 90% ͓4͔. Further improve- Comprex® supercharger appeared in the Mazda 626 Capella pas- ment is possible, but huge gains seem unlikely especially for senger car ͓28,29͔. Since then, the Comprex® has been commer- small compressors with unavoidable tip leakage. Geometries of cialized for heavy diesel engines, and also tested successfully on microturbines make blade cooling very difficult and internal cool- vehicles such as Mercedes-Benz ͓30͔, Peugeot, and Ferrari ͓27͔. ing methods applied to larger engines are often impractical for Such a pressure-wave supercharger offers rapid load response and smaller turbines. Hence, their lifetimes are shorter when using scale-independent efficiency, making its light weight and compact materials typical of larger gas turbines ͓5͔. Therefore, there is size attractive for supercharging small engines ͑below about 75 significant research toward developing advanced metallic alloys kW or 100 hp͓͒31,32͔. and ceramics for high-thermal-resistance turbine wheels used in Propulsion applications resumed in the 1960s, with General Electric Company ͑GE͒, General Power Corporation ͑GPC͒, Mathematical Science Northwest ͑MSNW͒, and Rolls Royce de- Contributed by the IGTI Microturbines and Small Turbomachinery Committee of veloping prototypes ͓27,33͔. In the 1980s, US agencies like the the ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND ͑ ͒ POWER. Manuscript received March 12, 2004; final manuscript received December Defense Advanced Research Projects Agency DARPA and the 13, 2004. Committee Chair: D. Haught. Navy sponsored research programs on wave rotor science and 190 / Vol. 128, JANUARY 2006 Copyright © 2006 by ASME Transactions of the ASME Fig. 1 Schematic configuration of a typical wave rotor technology. The 1985 ONR/NAVAIR Wave Rotor Research and ͓ ͔ Fig. 2 Schematic of a gas turbine topped by a four-port wave Technology Workshop 33 offered a comprehensive review of rotor prior work. Since the late 1980s, a sustained research program at NASA Glenn Research Center ͑GRC͒ collaborating with the US Army Research Laboratory ͑ARL͒ and Rolls-Royce Allison has aimed to and widths to generate and utilize wave processes, a significant develop and demonstrate the benefits of wave rotor technology for and efficient transfer of energy can be obtained between flows in future aircraft propulsion systems ͓34–40͔. Experimental studies the connected ducts. Through rotation, the channel ends are peri- at NASA on four-port ͓41͔ and three-port ͓42–44͔ wave rotors odically exposed to the ports located on the stationary end plates enabled the estimation of loss budgets ͓45,46͔ and simulation initiating compression and expansion waves within the wave rotor code validation ͓47͔. Initially, a simple three-port flow divider channels. Thus, pressure is exchanged dynamically between fluids wave rotor was built and tested to evaluate loss mechanisms and by utilizing unsteady pressure waves. Unlike a steady-flow turbo- calibrate the simulation code. Next, a four-port pressure ex- machine that either compresses or expands the fluid, the wave changer was built and was tested to evaluate the pressure-gain rotor accomplishes both compression and expansion within a performance for application to a small gas turbine ͑Allison 250͒. single component. Even though the wave rotor is internally un- Wave rotor cycles with lower thermal loads have been proposed steady, the flows in the ports are almost steady, aside from the ͓48,49͔, and mechanical design issues are being addressed in cur- fluctuations due to the opening and closing of the channels as they rent NASA-sponsored research. enter and exit port regions. Therefore, the wave rotor outflow The French National Aerospace Research Establishment shows very low pulsating behavior. To minimize leakage, the gap ͑ONERA͒ has also investigated wave rotor enhancement of gas between the end plates and the rotor has to be very small, but turbines in auxiliary power units, turboshaft, turbojet, and turbo- without contact under all operating and thermal expansion condi- fan engines ͓15͔. Consistent with NASA studies ͓50͔, ONERA tions. anticipates the largest gains and efficiency for engines with a low With axial channels and matched port flow alignment, the compressor pressure ratio and high turbine inlet temperature, such power required to keep the rotor
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