Heat Exchangers for Aero Engine Applications

Nowadays, the needs for safer, cleaner and more affordable civil aero engines are of increasing importance. To cover these needs, a technology for an advanced aero engine design, which uses an alternative thermodynamic cycle, has been presented. The basis of this cycle lies in the integration of a system of heat exchangers installed in the exhaust nozzle of the aircraft engine. The heat exchangers are operating as heat recuperators, exploiting the thermal energy of the turbine exhaust gas to pre-heat the compressor outlet air before combustion and, thus, resulting in reduced pollutants and decreased fuel consumption. In this work, a procedure for the optimization of this installation is presented. The minimization of the pressure losses on the exhaust gas side is chosen as the optimization criterion of this effort. The optimization is based on experimental measurements in laboratory conditions and 2D CFD modelling for the flow inside the exhaust duct and through the heat exchangers. Detailed measurements were carried out in order to derive the pressure drop law through one heat exchanger. This pressure drop law was used in a CFD approach where the heat exchangers were modelled as porous media with prescribed pressure drop. The accuracy of this pressure drop law was validated in comparison with experimental data for a wide range of inlet conditions. A 1:1 model of the quarter of the nozzle was constructed in a wind tunnel with four full-scale heat exchangers installed in the exhaust nozzle. The measurements proved the non-uniformity of the flow field, which resulted in increased pressure losses for the heat exchangers that were operating with increased mass flow. It was possible to make improvements in the nozzle installation in order to optimize the mass flow balance through the heat exchangers and minimize the pressure losses. The optimization was based on 2D CFD modelling for the flow inside the exhaust duct and through the heat exchangers. The adopted porous media model modelled the presence of the heat exchangers inside the nozzle. The CFD modelling proved sufficient in predicting the main flow field phenomena, which were responsible for the non-uniform distribution of the mass flow and the increased pressure losses. Then, modifications in the nozzle geometry and the heat exchangers orientations were implemented in the CFD modelling. These modifications proved that with a careful approach, a better arrangement of the heat exchangers could be achieved.