Design Overview of a Three Kilowatt Recuperated Ceramic Turboshaft Engine

A three kilowatt turboshaft engine with a ceramic recuperator and turbine has been designed for small unmanned air vehicle (UAV) propulsion and portable power generation. Compared with internal combustion (IC) engines, gas turbines offer superior reliability, engine life, noise and vibration characteristics, and compatibility with military fuels. However, the efficiency of miniature gas turbines must be improved substantially, without severely compromising weight and cost, if they are to compete effectively with small IC engines for long-endurance UAV propulsion. This paper presents a design overview and supporting analytical results for an engine that could meet this goal. The system architecture was chosen to accommodate the limitations of mature, cost-effective ceramic materials: silicon nitride for the turbine rotors and toughened mullite for the heat exchanger and turbine stators. An engine with a cycle pressure ratio below 2:1, a multistage turbine, and a highly effective recuperator is shown to have numerous advantages in this context. A key benefit is a very low water vapor-induced surface recession rate for silicon nitride, due to an extremely low partial pressure of water in the combustion products. Others include reduced sensitivity to internal flaws, creep, and foreign object damage; an output shaft speed low enough for grease-lubricated bearings; and the potential viability of a novel premixed heat-recirculating combustor.

Design Overview of a Three-Kilowatt Recuperated Ceramic Turboshaft Engine

A three-kilowatt turboshaft engine with a ceramic recuperator and turbine has been designed for small unmanned air vehicle (UAV) propulsion and portable power generation. Compared with internal combustion (IC) engines, gas turbines offer superior reliability, engine life, noise and vibration characteristics, and compatibility with military fuels. However, the efficiency of miniature gas turbines must be improved substantially, without severely compromising weight and cost, if they are to compete effectively with small IC engines for long-endurance UAV propulsion. This paper presents a design overview and supporting analytical results for an engine that could meet this goal. The system architecture was chosen to accommodate the limitations of mature, cost-effective ceramic materials: silicon nitride for the turbine rotors, and toughened mullite for the heat exchanger and turbine stators. An engine with a cycle pressure ratio below 2:1, a multistage turbine, and a highly effective recuperator is shown to have numerous advantages in this context. A key benefit is a very low water-vapor-induced surface recession rate for silicon nitride, due to an extremely low partial pressure of water in the combustion products. Others include reduced sensitivity to internal flaws, creep, and foreign object damage; an output shaft speed low enough for grease-lubricated bearings; and the potential viability of a novel premixed heat-recirculating combustor.

Ceramic Recuperator and Turbine: The Key to Achieving a 40 Percent Efficient Microturbine

Based on the use of state-of-the-art component technologies and the use of existing metallic materials, achieving an electrical efficiency anywhere near 40 percent in low pressure ratio recuperated microturbines is proving elusive. Current microturbines, rated at say 100 kW, operate with efficiencies approaching 30 percent. Advancing this to an upper level of about 35 percent is projected based on the ability to operate at turbine inlet temperatures greater than 1100C, and the utilization of a higher cost superalloy recuperator. This paper puts into perspective the challenge of trying to achieve 40 percent efficiency for small recuperated turbogenerator designs with radial flow components; the major constraints being associated with stress limitations in both the turbine and recuperator. Various publications (issued by both industry and the Government) often mention an efficiency goal of 40 percent for small gas turbines of this configuration, however, it needs to be recognized that the means to achieve this are beyond current high temperature metallic component capabilities. To achieve this “goal” necessitates increasing the operating temperature of the turbine and recuperator above 1100C and 800C respectively. Such advancements are projected to be technically and cost-effectively achievable by utilizing ceramic components, which with a dedicated development program, could perhaps become a reality in less than a decade to meet both future distributed power generation needs and defense applications, and be in concert with ever-demanding conservation goals and reduced emissions.

Heat and Mass Transfer Design of a Silicon Carbide Micro-Channel Heat Exchanger

Efficiency and emissions of advanced gas turbine power cycles can be improved by incorporating high-temperature ceramic heat exchangers. In cooperation with the DOE, a highly effective microchannel ceramic recuperator for a microturbine is under development. In this recuperator, the use of microchannel architecture will improve heat transfer and provide a more uniform temperature distribution. This will result in overall higher productivity per unit volume compared to conventional hardware. The use of ceramic for the recuperator will allow higher temperature operation than available in conventional microturbines. Based on a model for a typical microturbine, these changes may improve the overall system efficiency from about 27% to over 40%.

Emergence of Recuperated Gas Turbines for Power Generation

In the emerging deployment of microturbines (25–75Kw), a recuperator is mandatory to achieve thermal efficiencies of 30 percent and higher, this being important if they are to successfully penentrate the market currently dominated by Diesel generator sets. This will be the first application of gas turbines for electrical power generation, where recuperators will be used in significant quantities. The experience gained with these machines will give users’ confidence that recuperated engines will meet performance and reliability goals. The latter point is particularly important, since recuperated gas turbines have not been widely deployed for power generation, and early variants were a disappointment. Recuperator technology transfer to larger engines will see the introduction of advanced heat exchanged industrial gas turbines for power generation in the 3–15 Mw range. After many decades of development, existing recuperators of both primary surface and plate-fin types, have demonstrated acceptable thermal performance and integrity in the cyclic gas turbine environment, but their capital costs are high. A near-term challenge to recuperator design and manufacturing engineers is to establish lower cost metallic heat exchangers that can be manufactured using high volume production methods. A longer term goal will be the development and utilization of a ceramic recuperator, since this is the key component to realize the full performance potential of very small and medium size gas turbines.

Progress on the AGATA Project: A European Ceramic Gas Turbine for Hybrid Vehicles

The European EUREKA project EU 209 or AGATA - Advanced Gas Turbine for Automobiles is a program dedicated to the development of three critical ceramic components; i. catalytic combustor, ii. radial turbine wheel, iii. static heat exchanger, designed for a 60 kW turbogenerator hybrid electric vehicle. The objective is to develop and test the three components as a full scale feasibility study with an industrial perspective. The AGATA partners represent car manufacturers as well as companies and research institutes in the turbine, catalyst and ceramic material fields in France and Sweden. The program has been running since early 1993 with good progress in all three sub-projects. The turbine wheel design is now completed. FEM calculations indicate that the maximum stress occur during cold start and is below 300 MPa. Extensive mechanical testing of the Si3N4 materials from AC Cerama and C&C has been performed. The catalytic combustor operates uncooled at 1350°C. This means a severe environment for both the active catalyst and the ceramic honeycomb substrates. Catalysts with high activity even after aging at 1350°C have been developed. Ceramic honeycomb substrates that survive this temperature have also been defined. The catalytic combustor final design is ready and the configurations which will be full scale tested have been selected. The heat exchanger will be a ceramic recuperator with 90 % efficiency. Both a tube concept and a plate concept have been studied. The plate concept has been chosen for further work. Sub-scale plate recuperators made of either cordierite or SiC have been manufactured by C&C and tested.

Highly Compact Ceramic Recuperator for Engine Applications

Metallic plate-and-fin recuperators for heat engine applications are one of the most expensive components of the engine because of the high temperature alloys required and the inherent expense of brazing or welding the individual plates. A highly compact, all-prime surface, internally manifolded, plate-and-fin ceramic recuperator has been developed to deliver equivalent high performance at reduced cost. Performance and economics of this unique recuperator are presented.

First Experimental Results on a Silicon-Nitride Recuperator With Six Heat Exchanger Elements

A prototype ceramic recuperator consisting of six heat exchanger elements of Si3N4, being arranged side by side in a self-sealing and self-supporting configuration, was investigated experimentally with respect to the heat transfer-, friction- and mechanical-properties as well as to the sealing performance. The heat exchanger elements are developed in collaboration by Kernforschungsanlage Jülich GmbH (KFA) and Rosenthal Technik AG (RTAG). The recuperator arrangement corresponds well to an unique design principle for small automotive “ceramic” gas turbines. The thermodynamic measurements were carried out with air, electrically heated up to temperatures of about 100 C, and at pressures on the HP-side up to about 2 bar. The self-sealing and self-supporting heat exchanger arrangement promises to be satisfying for vehicular application purposes. Partial rupture of heat exchanger walls in the prototype-elements began at a pressure difference of about 3 bar. On the basis of the experimental results, a design of a vehicular gas turbine for 70-Kw shaft power and with heat exchanger elements for advanced fabrication technique is presented. A planned test facility for gas temperatures up to about 1200 C and with a new set of heat exchanger elements from Si3N4 or SiC is described briefly.