Aerodynamic Optimisation of a Turbofan Bypass Duct with a Heat Exchanger Modelled as a Porous Zone

Abstract

The drive for improved fuel efficiency and decreased environmental impact has resulted in the development of advanced jet engine concepts that utilise waste heat recovery, including intercooled-recuperated engines and combined-cycle turbofans (CC-TF). A promising CC-TF variant employs an Organic Rankine Cycle (ORC) bottoming unit to recover waste heat, however it also requires extensive heat dissipation. Integrating a condenser into the bypass duct is a potential solution to meet this heat dissipation requirement. Axisymmetric RANS CFD simulations are employed to simulate this configuration and optimise aerodynamic performance, requiring simplified models for computational efficiency. This study quantifies the effect of the condenser integration on bypass stream net thrust and applies a gradient-based optimisation framework to refine the bypass duct geometry for improved thrust generation. The condenser air-side pressure drop and heat transfer performance are modelled through a porous zone, while the effect of the fan and OGV on the airflow pressure and velocity are predicted using actuator disk models. The integration of the CC-TF condenser results in a 13.7% net thrust penalty, however this penalty is reduced to 9.29% when simulating the heat added by the condenser to the airflow. The optimised bypass duct design further improves net thrust by 3.32% compared to the initial condenser integration approach. The actuator disk setup has been verified and validated by comparing the fan and OGV performance to predictions made by other CFD studies and to data collected in the wind tunnel by the NASA R4 engine. Verification studies also confirmed sufficient mesh refinement and correct porous zone calibration. Future studies can expand on this framework to incorporate the power generated by the ORC unit and assess potential fuel savings.