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Many thermodynamic texts imply that the entropy flux for thermal radiation (TR) is the same as that for heat conduction, the heat flux divided by the local temperature (q/T). However, for blackbody radiation (BR) emission a 4/3 coefficient occurs and recently it was shown that for non-blackbody radiation (NBR) the coefficient is greater than 4/3 [1]. Some of the fundamental equations that are used in thermodynamics express the entropy flux of heat transfer in a q/T type form. In this paper we address the use of the Clausius equality, and expressions extended from it for irreversible processes, when TR is involved. We find that the Clausius equality for reversible processes is applicable, while the statements extended for irreversible processes are not applicable. Also, we present an alternative derivation of the 4/3 coefficient that shows in a direct way that it follows from the observable relation between BR energy and emission temperature (i.e., energy is proportional to T4).

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.