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The impact of internal heat transfer on the performance of air breathing cycles

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This presentation considers the effect of heat transfer between fluid streams on the efficiency of air breathing cycles, such as gas turbines. To correctly characterize the effect of heat transfer requires the definition of a new property, ‘mechanical work potential’, which is a measure of the maximum useful work that can be extracted from a fluid by an isentropic turbine exhausting to a fixed exit static pressure. A balance equation for the property is developed. The equation shows the surprising result that entropy creation through thermal mixing has no effect on cycle efficiency. It does, however, show that a second heat transfer term, ‘thermal creation’, does alter cycle efficiency. Thermal creation occurs in regions of the turbine where heat transfer occurs across a finite pressure difference. The term is the non-linear version of the acoustic energy creation term proposed by Lord Rayleigh in 1878, and formalized mathematically by Putnam and Dennis in 1954, in their thermo-acoustic criterion. The new method is used to examine the impact of heat transfer within a high pressure turbine, of a gas turbine cycle, on the cycles overall efficiency. It is shown heat transfer within the freestream, between coolant and combustion products, causes a negligible change in efficiency and therefore can be ignored in the design process. The method also shows that heat transfer from the freestream, through the walls of the turbine blade and into the blade coolant passages, results in ~0.5% rise in stage efficiency. This is a significant and should be accounted for in the design process. The new property and balance equation offers a method of analysing a broader range of air breathing problems.

This talk is part of the Institute for Energy and Environmental Flows (IEEF) series.

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