Performance Prediction of a Microjet Engine Run on Alternative Fuels

Microjet engines have gone from being developed in private workshops meant for the RC model aircraft world to become a common feature in various military and commercial applications. Modern microjets emulates the full-sized engines in function and sophistication but have documented major fuel efficient problems. The engines are mostly run on aviation kerosene, which is rather expensive, but a swap to a cheaper or more environmental friendly fuel may result in costly redesigns of the entire fuel delivery system and a reduced power output. The solution is to blend the alternative fuel with the regular fuel to maintain engine functionality to a lower cost and/or increased fuel availability. This project aims to determine whether the performance of the Merlin VT80 microjet engine run on different blends may be predicted by using a gas turbine model entirely based on simple aero-thermodynamic equations. The fuels tested were blends of Jet A-1 and standard diesel and biodiesel. Performance parameters for each blend where collected during experimental runs for every 10th throttle setting using different monitors and readers. An intake horn with an elliptical profile and bell-mouth geometry was designed to enable air mass flow rate calculations using a pressure sensor attached to the intake to measure the dynamic pressure of the air flow which will give the flow velocity. Basic gas turbine thermodynamic equations where used to describe the different processes at each stage throughout the engine based on relevant input parameters. The equations were used to determine the unknown compressor pressure ratio at different speeds by testing different compressor pressure ratio values in the equations to generate a thrust output which was compared to the experimentally found result until a satisfying vale was reached. The compressor map of another microjet of similar size was researched to give reasonable values for the different efficiencies needed in the equations. The results show that most of the data collected at the experimental rig may be derived from the measured pump voltage which mathematical relationships are being used in the gas turbine model. When the compressor pressure ratios and efficiencies found are used as input parameters, the model can predict the microjet engine's thrust output with very small deviations from the true value regardless of what fuel blend is used. Impeller analysis suggested that the compressor geometry is too complicated to be described with simple equations which means the compressor pressure ratio cannot be derived and thus must remain an input parameter. This concludes that simple thermodynamic equations alone cannot completely predict the performance of the Merlin VT80.Validerat; 20131003 (global_studentproject_submitter