Modeling an Installation of Recuperative Heat Exchangers for an Aero Engine

Abstract

This work presents the complete effort to model the presence of an integrated system of heat exchangers mounted in the exhaust nozzle of an aero engine which uses an alternative but more efficient thermodynamic cycle. The heat exchangers are operating as heat recuperators exploiting part of the thermal energy of the turbine exhaust gas to preheat the compressor outlet air before combustion and to reduce pollutants and fuel consumption. The presence of the heat exchangers enforces a significant pressure drop in the exhaust gas flow which can affect the overall efficiency of the thermodynamic cycle and the potential benefit of this technology. For this reason it is important to optimize the operation of the system of heat exchangers. The main target of this optimization effort is the minimization of the pressure losses for the same amount of heat transfer achieved. The optimization is performed with the combined use of experimental measurements and CFD methods. Since the CFD modeling is taking into consideration the overall geometry of the exhaust nozzle of the aero engine where the heat exchangers are mounted, the presence of the latter is unavoidably modeled with the use of a porosity model for practical reasons, having to do with CPU and memory requirements. The porosity model is taking into account the pressure drop and heat transfer behaviour of the heat exchangers and was developed and validated with the use of detailed experimental measurements. For the validation of the CFD model, isothermal experimental measurements carried out for laboratory conditions in a 1:1 model of a quarter of the exhaust nozzle of the aero engine, including four full-scale heat exchangers, were used. The CFD results were in good agreement with the experimental measurements and the same flow structures and problematic regions were detected. Thus, a complete 3-D CFD model of the overall exhaust nozzle of the aero engine was created and validated which at the next step formed the basis for the optimization of the overall aero engine installation for real engine operating conditions. The improved design of the aero engine installation presented decreased pressure losses in relation to the initial design and a more balanced mass flow distribution, showing the applicability of the overall methodology and its advantages for producing efficient engineering solutions for similar setups.