Exergy Loss Considerations in Education for a Turbofan Power Cycle

Abstract

The conversion of heat into mechanical shaft power or electrical energy is still the worldwide most used electric and propulsion energy source. Whether coal, liquid hydrocarbons, natural gas, nuclear or solar heat are used as primary energy source such cycles are continuously improved aiming at better efficiency, at flexible operating conditions or at reducing manufacturing or maintenance cost. The classic thermal power cycle design uses detailed simulation of the cycle fluid’s changes of state (typically represented by averaged pressure and temperature or specific enthalpy) along the flow paths. Components of such a cycle may be ducts, filters, mixers, compressors, combustors, heat exchangers, nozzles, turbine stages etc. The notations of exergy and anergy were invented in the nineteen-fifties but they still play an astonishingly silent role in education. Knowing already the Carnot factor a student can understand the second law of thermodynamics by the simple fact that the exergy part of energy can be converted into any other form of energy while the anergy part cannot be converted into any exergy form of energy. The most used approach via the saying “that the state variable entropy can only grow” is for most students a mental detour because they do not understand the tie of energy conversion and entropy. The exergy loss breakdown of a cycle gives an instructive view to opportunities and challenges. This paper explains methods to calculate a breakdown of the exergy losses with the example of a typical but notional turbofan using commercially confirmed component performance data. The considerations cover exergy losses by heat exchange, by combustion, by heat loss to ambient, by mixing different fluids, by internal pressure losses, by friction and dissipation in turbomachinery and by the bottoming heat discharge as well as in the propulsive process. Quantified exergy losses for such effects make the limitations of technical improvements visible without going into detailed design work. The purpose of this paper is to motivate for teaching and using exergy-based considerations especially in the basic performance simulation. Additionally the context is occasion for an extra plea on replacing the colloquial expression “energy consumption” by “exergy consumption”. The reader of this paper should have basic knowledge of thermodynamic cycle performance simulation methods and of turbomachinery.