High efficiency recuperated ceramic gas turbine engines for small unmanned air vehicle propulsion

Abstract

To perform long missions, small unmanned air vehicles (UAVs) need efficient, lightweight propulsion systems that can operate on energy dense fuels. Gas turbines offer better reliability, life, fuel flexibility, noise, and vibration than internal combustion (IC) engines, but they are uncompetitive due to fuel efficiencies around 6%. At this scale, conventional efficiency improvement approaches such as high pressure ratios and cooled metal turbines are impractical. Ceramic turbines could withstand high temperatures without cooling, but their life and reliability have been inadequate. This work explores the hypothesis that a low pressure ratio, highly recuperated ceramic engine design could overcome these problems. First, an accepted water vapor erosion model is extended to correctly account for the effects of recuperation, fuel type, and atmospheric humidity on the burned gas water vapor content. The results show that ceramic turbines without environmental barrier coatings can last 10,000 hours or more in highly recuperated engines, even at temperatures exceeding 1200[degrees]C. Next, a new design for a small recuperated ceramic engine is developed and analyzed, in which blade speeds are limited to 270 m/s – about half the typical value. A CARES slow crack growth analysis indicates this will lead to vastly improved life and reliability. The literature on foreign object damage and production costs suggests likely improvements in those areas, as well. Finally, an original ceramic recuperator is developed to fit the proposed engine design. Tradeoffs between fabrication constraints, weight, volume, effectiveness, pressure losses, and other considerations are explored through analysis, simulations, and experiments. For one design, these predict a thermal effectiveness in the 84-87% range at a specific weight of 44 grams per gram/second of airflow, surpassing the current state of the art by a factor of 1.25-1.5. A prototype designed for 1100[degrees]C operation was tested at 675[degrees]C exhaust inlet temperature. It did not crack or leak, and the performance roughly matched analytical predictions. With a heat exchanger of this type, a small, low pressure ratio turboshaft engine could achieve an efficiency of 23%, making it highly competitive with other state of the art propulsion systems by almost all performance metrics. In sum, this work contributes a novel ceramic recuperator that can enable low pressure ratio gas turbines to achieve high fuel efficiencies, and provides a significant extension of ceramic turbine life and reliability theory that shows such engines could achieve long service lives.