Design and Analysis of an Ejector Variable -Area Nozzle for Micro-Turbojets

This paper presents the design and evaluation of a variable-area nozzle for micro turbojet engine with a stock 22.5 lbs. thrust. A micro turbo jet engine is an engine that has a thrust range between 10-500lbs and typically uses a centrifugal compressor. The engine used in this study was a Jet Cat P100. Variable-area nozzles allow for peak exhaust velocity over a range of operating conditions to maximize thrust and minimize thrust specific fuel consumption and are used to spoil the thrust of an engine. The goal is to develop a variable-area nozzle with minimal addition to weight and overall engine length and diameter, as well as minimal decrement to baseline performance, and be able to spoil the thrust without stalling the engine. Multiple concepts were considered and were narrowed down to three selections for further analysis. The three included a 2D variable area nozzle, ring rack and pinion actuated nozzle and a pushrod actuated nozzle. Detailed design included structural and fluid dynamic analysis. Final designs were fabricated, tested and actuated at three different throttle settings; 100%, 85%, and 70%. The evaluation criteria for all the three engines were be based on size, weight ease of manufacturing and installation, actuation response time, and baseline thrust performance improvement, spoiling the thrust, addition to the circular area of the engine. All the nozzle design hit the optimum area at all throttle speeds and were able to spoil the thrust. This study goes more in depth into the rack and pinon nozzle design and performance. The rack and pinion actuated nozzle had the best improvement out of all the three design choices. It consisted of 8 individual flaps with an addition of an opening at the base of each flap which allowed for ambient air to mix with the engines exhaust gases with the goal of improving the engine performance further and helping reduce noise. The material used for the nozzle was 316 stainless steel because it has good resistance oxidation and has a melting point of 2500 F. All the flaps were attached to a ring that was connected to two servos motors. The servos are controlled mechanically by the use of a remote controller using a rack and pinion system. Evaluation included measures of thrust and fuel consumption, as well as exhaust total temperature and dynamic pressure. Pressure sensors were used to evaluate Mass Flow Parameters of the engine as the area of the nozzle is changing. Pitot static probes were incorporated into the variable nozzle to measure the pressure changes. The nozzle weight increased by 10% from the original. The engine performance increased by 5%, and the nozzle area increased by 15% before stalling the engine. There was no noticeable change in noise from the engine. This study showed the advantages that a variable area plays in unmanned aerial systems by giving them the ability to operate in different location that might not have a runway. There is still need for more study in this field.