Design optimization of a mixed-flow compressor impeller for a small turbojet engine

Purpose - This paper aims to present the results of a design optimization study for the impeller of a small mixed-flow compressor. The objective of the optimization is to obtain an impeller geometry that could minimize a cost function based on the specific thrust and the thrust specific fuel consumption of a small turbojet engine. Design/methodology/approach - The design methodology is based on an optimization process that uses a configurational database for various compressor geometries. The database is constructed using design of experiments and the compressor configurations are generated using one-dimensional in-house design codes, as well as various tools and programs of the Agile Engineering Design System, which is a commercially available turbomachinery design system developed at Concepts NREC. The cost function variations within the design space are represented through a neural network. The optimum configuration that minimizes the cost function is obtained using a direct search optimization procedure. Findings - The optimization study generated a small 86 mm diameter mixed-flow impeller with a 500 meridional exit angle. The optimized compressor, as well as the engine that it is designed for, were shown to have improved performance characteristics. Research limitations/implications - Preliminary performance and flow analysis of the optimized impeller show shock structures and possible shock-boundary layer interactions within the blade passages indicating further geometrical fine tuning may be required based on more detailed computational studies or experimental tests. Practical implications - A further study including the effect of diffuser is required to carry the results to a more practical level. Originality/value - The originality and the value of the paper comes mainly from two different aspects: combining various in-house and commercial turbomachinery design codes in one robust methodology to obtain an optimum mixed-flow compressor impeller that will maximize the performance requirements of a small unmanned air vehicle (UAV) turbojet engine under restricted size and power conditions; and investigation of the design optimization and analysis of a mixed-flow compressor that could have potential applications in small jet engines to be used in high-performance UAV applications. Design optimization studies on this type of compressor are very limited in the open literature. For many years, these compressors have been disregarded because of their bulky design in large-scale engines. However, as mentioned above, they present a great potential for small-scale jet engines by supplying enough pressure rise, as well as high mass flow rate compared to their centrifugal counterparts.