Efficiency optimization of advanced gas turbine recuperative-cycles R. Bontempo and M. Manna Dipartimento di Ingegneria Industriale, Universit` a degli Studi di Napoli Federico II, Via Claudio 21, 80125 Naples, Italy Abstract The paper presents a theoretical analysis of three advanced gas turbine recuperative-cycles, that is the intercooled, the reheat, and the intercooled and reheat cycles. The internal irreversibilities, which characterise the compression and expansion processes, are taken into account through the polytropic efficiencies of the compressors and turbines. As customary in simplified analytical approaches, the study is carried out for an uncooled closed-circuit gas turbine without pressure losses in the heat exchangers and using a calorically perfect gas as working fluid. Although the accurate performance prediction of a real gas turbine is prevented by these simplifying assumptions, this analysis provides a fast and simple approach which can be used to theoretically explain the main features of the three advanced cycles and to compare them highlighting pros and contra. The effect of the heat recuperation is investigated comparing the thermal efficiency of a given cycle type with those of two reference cycles, namely the non-recuperative version of the analysed cycle and the simple cycle. As a result, the ranges of the intermediate pressure ratios returning a benefit in the thermal efficiency in comparison with the two reference cycles have been obtained for the first time. Finally, for the sole intercooled and reheat recuperative-cycle, a novel analytical expression for the intermediate pressure ratios yielding the maximum thermal efficiency is also given. Keywords: gas turbine; intercooling; reheating; efficiency optimization; recuperative-cycles. 1 Introduction In the last decades gas turbines have experienced a great success both in the aeronautical and power generation tech- nological fields, so that a large number of theoretical, numerical and experimental studies have recently appeared in the open literature. Moreover, in order to increase the power density and the efficiency of this kind of engine, several variants of the classical Brayton cycle have been developed and successfully applied, such as: gas-turbine/fuel-cell systems 1 , combined cycles 2,3 , cogeneration plants 4,5 , hybrid solar gas turbine 6 , chemical recuperation cycles 7,8 , steam and water injection gas turbines 9,10 , etc. Among others, the more classical variants are the intercooled, the reheat and the recuperative cycles which frequently experience a practical application. To quote some examples, consider the GE LMS100 11 , the ABB GT-24 and GT-26 12 , and the Rolls-Royce WR-21 13 , where the aforementioned techniques are adopted alone or together. Innovative nuclear plants are also a very important application of these advanced tech- niques. For instance, the intercooled recuperative closed circuit gas turbine plant is used in both the gas-cooled and lead-cooled fast reactors. In the standard recuperative cycle, the hot gas at the turbine outlet is used to pre-heat the air from the compressor delivery. By doing so, there is room for a reduction in the overall heat added to the fluid without changing in the net specific-work, with an obvious increase in the thermal efficiency. Nowadays, there are several numerical models and commercial tools for the accurate evaluation of the aerother- mal performance of a recuperative gas turbine. Many of these works can handle most of the technological aspects significantly affecting the performance of a gas turbine plant, such as the real-gas effects 14,15 , the blade cooling 16–19 , the pressure losses in the heat-exchangers 20,21 , etc. However, because of the great relevance of the heat recuperation, this procedure has been widely studied also from a theoretical and purely analytical point of view, as reported in all classical gas-turbine textbooks 9,22–24 , as well as in many research papers. In order to tackle the analysis through an analytical approach, some simplifying assumptions have to be introduced in the model. In fact, the theoretical analysis of the recuperative-cycles is generally carried out in the simplified framework of an uncooled closed-circuit 1 Bontempo R., Manna M. Efficiency optimisation of advanced gas turbine recuperative-cycles Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy, 2019 DOI: 10.1177/0957650919875909