An Off-Design Analysis of an Inverse Brayton Cycle Based UAV Propulsion System

Abstract

An investigation into the off-design performance of an Inverse Brayton Cycle (IBC) based unmanned aerial vehicle (UAV) propulsion system was performed using Numerical Propulsion System Simulator (NPSS). Results of this analysis were then compared with the regenerated Brayton cycle (RBC). Turbomachinery performance characteristics from literature sources were scaled for use in the analysis. Comparisons focused on part-throttle specific fuel consumption (SFC) and operating range. Results showed that in a single spool configuration, running at constant speed, the IBC part load thermal efficiency was similar to the RBC. Also, it was observed that with an approximate propeller load applied to the single spool configuration the IBC had greater part load fuel consumption but a wider operating range than the RBC. A single spool gas generator with free power turbine configuration was also investigated. The free power turbine greatly restricted the operating range of the IBC with the originally specified design points on the compressor and turbine performance maps. By adjusting the component performance map design points, the free power turbine IBC operating range was extended and a significant improvement in part-throttle SFC over the RBC was observed.