Quantifying Confidence Envelopes for Efficiency Values in the SR-30 Turbojet Engine

Abstract

Turbojet engines power most of the large military and commercial aircraft in production today. These types of engines are chosen over conventional piston-driven engines because of the turbojet’s superior fuel economy and thrust. To understand how turbojet engines can be compared and optimized, it is necessary to fully characterize their performance. This is generally achieved by calculating thermodynamic efficiency values for each component in the engine, and for the engine as a whole. For this research project, the Turbine Technologies SR-30 centrifugal flow turbojet engine was investigated. An adjustable coupling was designed to permit a single-point thermocouple to be moved and secured within the engine. From the data taken at multiple locations and throttle settings, temperature profiles of the compression and combustion chambers were created. A thermal/fluid dynamic equation routine was developed using Engineering Equation Solver (EES), in order to propagate these temperature profiles through efficiency and thrust calculations. The temperature profiles did not significantly affect theoretical thrust values. However, the dependence of component efficiency values on spatial temperature variation within the engine was significant. In the compression chamber, it was found that a 30°C variation in the temperature across the chamber resulted in a 15% variation in the calculated compressor efficiency. In the inner region of the combustion chamber, a variation in 20°C yielded a 20% variation in calculated turbine efficiency. In the outer region of the combustion chamber, where the temperature varied by almost 400 degrees Celsius, the turbine efficiency varied by about 600%. This work suggests optimal placement of the compression and combustion stage thermocouples when the SR-30 turbojet is to be used for undergraduate laboratories. It also highlights the risks posed by relying on single-point measurements to characterize complex flows.