The Effect of Reference-Environment Models on the Accuracy of Exergy Analyses of Aerospace Engines

An exergy analysis is applied to a turbojet engine over a range of flight altitudes ranging from sea level to 15,000 m (~ 50,000 ft) to examine the effects of using different reference-environment models. The results of this analysis using a variable reference environment (equal to the operating environment at all times) are compared to the results obtained using two constant reference environments (sea level and 15,000 m). The actual rational efficiency of the turbojet decreases with increasing altitude, ranging from a value of 16.9% at sea level to 15.3% at 15,000 m. The percentage of the total exergy loss due to the exhaust emissions increases from 65% at sea level to 70% at 15,000 m, while the composition of this loss remains nearly constant with altitude. In the most extreme cases considered, the rational efficiency value calculated using a constant reference environment varies by approximately 2% from the variable reference environment value. However, the accuracy of the loss analysis provided by an exergy analysis is shown to depend strongly on the choice of reference environment. The magnitude of the predicted exhaust loss (which is the single major source of exergy loss in a turbojet) varies by as much as 18% from the actual value when using a constant reference environment. The breakdown of this loss when using a constant reference environment is shown to be dependent on the altitude chosen and can be in error by as much as 28%.