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Publication Title | Performance Enhancement of Microturbine Topped With Wave Rotors

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Pezhman Akbari

Department of Mechanical Engineering, Michigan State University, 2500 Engineering Building, East Lansing, MI 48824-1226 e-mail: akbari@egr.msu.edu

Razi Nalim

Department of Mechanical Engineering, Indiana University–Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202-5132 e-mail: mnalim@iupui.edu

Norbert Müller

Department of Mechanical Engineering, Michigan State University, 2455 Engineering Building, East Lansing, MI 48824-1226 e-mail: Mueller@egr.msu.edu

Introduction

A growing market for distributed power generation and propul- sion of small vehicles has motivated a strong interest in design of small gas turbine systems in the range of 30–300 kW. Known as microturbines, they are now widely used in the US for distributed power generation, shaving peak loads, and providing backup power for critical needs. They propel small commercial aircraft, unmanned air vehicles UAV , and terrestrial vehicles. Microtur- bines are often the preferred alternative to IC engines, because of their higher power density and robustness. They present several advantageous features such as compact size, simple operability, ease of installation, low maintenance, fuel flexibility, and low NOX emissions. Furthermore, recent electric-power crises and en- vironmental concerns have stimulated a strong interest in the re- search, development, and application of microturbines.

Despite their attractive features, compared with larger gas tur- bines, microturbines suffer from lower thermal efficiency and their relative output power, due to their lower component effi- ciencies, limited cycle pressure ratio, and peak cycle temperature. For example, experimental and theoretical research has shown that microturbines with pressure ratios of 3–5 without recupera- tion systems achieve only about 15%–20% efficiency 1,2 . For many applications improvement of their performance is desirable to enhance advantages over competing technologies. To achieve such improvements, current efforts are mainly focused on utilizing heat recovery devices, developing new high-strength, high- temperature materials for turbine blades, and improving the aero- dynamic quality of turbomachinery components 3 . The aerody- namics of turbomachinery has already yielded very high component efficiencies up to around 90% 4 . Further improve- ment is possible, but huge gains seem unlikely especially for small compressors with unavoidable tip leakage. Geometries of microturbines make blade cooling very difficult and internal cool- ing methods applied to larger engines are often impractical for smaller turbines. Hence, their lifetimes are shorter when using materials typical of larger gas turbines 5 . Therefore, there is significant research toward developing advanced metallic alloys and ceramics for high-thermal-resistance turbine wheels used in

Contributed by the IGTI Microturbines and Small Turbomachinery Committee of the ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received March 12, 2004; final manuscript received December 13, 2004. Committee Chair: D. Haught.

microturbines 6,7 . Utilizing conventional recuperators based on the use of existing materials can improve the overall efficiency of microturbines up to 30% 2,8–10 . Despite the attractive feature of the recuperator concept, a recuperator adds about 25%–30% to the overall engine manufacturing cost, which is a challenge for commercialization of microturbines 11–13 . The current trend of the microturbine market is to reduce the investment cost. There- fore, alternative devices need to be considered to achieve higher performance at lower component costs. Topping a microturbine with a wave rotor device is an appropriate solution.

In a wave-rotor-topped cycle, the combustion can take place at a higher temperature while the turbine inlet temperature can be equal to that of the baseline cycle. Also, a pressure gain additional to that provided by the compressor is obtained by the wave rotor. Thus, the wave rotor can increase the overall pressure ratio and peak cycle temperature beyond the limits of ordinary turboma- chinery. The performance enhancement is achieved by increasing both thermal efficiency and output work, hence reducing specific fuel consumption rate considerably. This enhancement is espe- cially favorable for smaller gas turbines often used for distributed power generation or propulsion of small vehicles 14–17 .

Wave Rotor History. The first successful wave rotor was tested by Brown Boveri Company BBC , later Asea Brown Boveri ABB , in Switzerland in the beginning of the 1940s 18 as a topping stage for a locomotive gas turbine engine 19–22 , based on the patents of Seippel 23–26 . This first unit suffered from inefficient design and crude integration 21 . Later, BBC focused on the development of pressure wave superchargers for diesel engines, anticipating greater payoff 27 . By 1987, their Comprex® supercharger appeared in the Mazda 626 Capella pas- senger car 28,29 . Since then, the Comprex® has been commer- cialized for heavy diesel engines, and also tested successfully on vehicles such as Mercedes-Benz 30 , Peugeot, and Ferrari 27 . Such a pressure-wave supercharger offers rapid load response and scale-independent efficiency, making its light weight and compact size attractive for supercharging small engines below about 75 kW or 100 hp 31,32 .

Propulsion applications resumed in the 1960s, with General Electric Company GE , General Power Corporation GPC , Mathematical Science Northwest MSNW , and Rolls Royce de- veloping prototypes 27,33 . In the 1980s, US agencies like the Defense Advanced Research Projects Agency DARPA and the Navy sponsored research programs on wave rotor science and

Performance Enhancement of Microturbine Engines Topped With Wave Rotors

Significant performance enhancement of microturbines is predicted by implementing vari- ous wave-rotor-topping cycles. Five different advantageous cases are considered for implementation of a four-port wave rotor into two given baseline engines. In these ther- modynamic analyses, the compressor and turbine pressure ratios and the turbine inlet temperatures are varied, according to the anticipated design objectives of the cases. Advantages and disadvantages are discussed. Comparison between the theoretic perfor- mance of wave-rotor-topped and baseline engines shows a performance enhancement up to 34%. General design maps are generated for the small gas turbines, showing the design space and optima for baseline and topped engines. Also, the impact of ambient temperature on the performance of both baseline and topped engines is investigated. It is shown that the wave-rotor-topped engines are less prone to performance degradation under hot-weather conditions than the baseline engines. DOI: 10.1115/1.1924484

190 / Vol. 128, JANUARY 2006 Copyright © 2006 by ASME Transactions of the ASME

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