Abstract
This work presents an investigation of the performance of three recuperative cycles for gas turbines, with a particular interest for aero engine applications. The first configuration under investigation is the conventional recuperative cycle, in which a heat exchanger placed after the last turbine (low pressure or power turbine). In the second configuration, referred to as alternative recuperative cycle, a heat exchanger is placed between the high pressure and low pressure turbine, while in the third configuration, referred to as staged heat recovery cycle, two heat exchangers are employed, the primary one between the high and low pressure turbines and the secondary downstream the last turbine. At the first part of the present work, a parametric conceptual analysis was conducted using available literature data in order to investigate the impact of heat exchanger effectiveness and overall pressure ratio on cycle performance. The results show that the conventional recuperative cycle presents superior performance in relation to the alternative recuperative cycle for low overall pressure ratio values, while for higher values the alternative recuperative cycle outperforms. In addition, for the staged heat recovery cycle, the selection and combination of the effectiveness of the primary and secondary heat exchangers affects significantly the cycle efficiency. The second part of this work was focused on the assessment of practical issues regarding the implementation feasibility of the alternative recuperative and the staged heat recovery concepts in a recuperative aero engine. For the analysis, the advanced MTU-developed and designed intercooled recuperated thermodynamic cycle was used. The heat exchangers of the recuperation system in the intercooled recuperative cycle consist of specially profiled elliptic tubes placed in a 4/3/4 staggered arrangement. For the sizing of the recuperators, the GasTurb11 aero engines geometrical data was used as reference in order to design a recuperator which would be mountable in the limited available space between the intermediate pressure turbine and the low pressure turbine. In the analysis various recuperator scenarios were examined taking into consideration different axial lengths and tube core arrangements (5/4/5, 6/5/6 etc.) keeping always as basis the MTU-heat exchanger core geometry. For the determination of the recuperator inner/outer pressure losses and effectiveness, results from previously performed CFD computations, experimental measurements and from the e-NTU method were used. The recuperator effectiveness and pressure losses for each scenario were included and assessed with the use of thermodynamic cycle models of the recuperative aero engine, which were developed in CAPE-OPEN/COFE software. The performance analysis of the recuperative aero engine cycles showed the existence of significant optimization potential which can be further increased when combined with more flexible aero engine geometry architectures and supported by the improvement of the endurance of recuperator candidate materials and alloys.
