A critical analysis of the thermodynamic advantages of reheat in gas turbines

Abstract

Apparently conflicting ideas on the effects of reheat on gas turbine performance can be obtained when comparing thermodynamics textbook treatments with recent literature on the new generation of reheat machines. These are resolved through the use of models with varying degrees of realism. The models are built with ‘live mathematics’ software which makes it straightforward to incorporate additional irreversibilities and less-simplified gas property relationships, and can be more effective than algebraic analyses for demonstrating the effect of cycle parameters. It is shown how the sign of the change in overall efficiency due to reheat depends on pressure and temperature ratios and on compressor and turbine efficiencies, and why a high compressor pressure ratio is beneficial. The trade-off between gains in specific net work and efficiency, by selection of the reheating pressure, is illustrated, the current relevance of the value that maximizes net work, traditionally termed ‘optimum’, is questioned and it is demonstrated that an exhaust temperature suitable for combined cycle operation requires a higher reheating pressure. Further results show how fuel consumption and gross power are divided between high- and low-pressure combustors and turbines respectively.