New Insights From Conceptual Design of an Additive Manufactured 300 W Micro Gas Turbine Towards UAV Applications

Battery lifetime is a crucial factor for many applications of Unmanned Aerial Vehicles (UAV). Owing to the high energy density of hydrocarbon fuels, Ultra Micro Gas Turbines (UMGT) with power outputs below 1 kW have a clear potential as battery replacement in drones. However, previous work on gas turbines of this scale revealed severe challenges due to air bearing failures, heat transfer from turbine to compressor, rotordynamic instability and manufacturing limitations. To overcome these obstacles, we propose a novel additively manufactured gas turbine architecture with hollow shaft, integrated cooling vanes and conventional roller bearing technology operating at 500,000 rpm. The development of the gas turbine rotor is based on an analytic engine model that highlights the effect of mass flow dependent turbomachinery efficiency and heat transfer effects on optimum pressure ratio and turbine inlet temperature. The results showed, that striving for highest feasible pressure ratios not only requires excessive rotational speeds but most likely results in reduced cycle efficiency. An optimum pressure ratio of 2.5 at turbine inlet temperature of 1200 K was defined as design operating point of the 300 W gas turbine. Following meanline turbomachinery design, the dimensions of the electric generator were matched based on an analytic model such that the required power output is reached and bending modes are avoided at operational frequency. Starting from this preliminary design geometry, the feasibility of an additively manufactured turbine rotor was investigated for different technologies. The geometrical constraints imposed on turbomachinery and rotor contour were defined and resulting turbomachinery geometry is presented. In addition, a novel radial inflow combustor concept is proposed based on porous inert media combustion. These efforts yielded a novel conceptional engine architecture that exploits the potential of additive manufacturing for UMGT development. To further detail the conceptional study, the effect of surface roughness on compressor performance was investigated by CFD analysis. Additionally, a conjugate heat transfer model was implemented to evaluate compressor and turbine heat flux for different rotor materials. The results showed, that depending on selected material, electric efficiency of up to 5 percent can be reached, while energy density is increased by factor 3.6 compared to Lithium Ion batteries.