On the Preliminary Design of Aerospace-grade Compact Heat Exchangers

Abstract

The improvement of the efficiency of aircraft propulsion systems and the reduction of fuel consumption are key objectives in the development of next-generation propulsion systems. The feasibility and performance of promising concepts such as combined-cycle engines, hybrid-electric propulsion, and hydrogen-fueled systems often depend on complex thermal management or cooling systems, where compact heat exchangers (HXs) are critical components. More in detail, the integration of HXs in airborne systems is limited by strict space and volume constraints as well as by the impact of pressure losses and mass on the performance of the propulsion system. Therefore, sub-optimal HX designs can offset or even negate potential performance gains.
This dissertation documents research work performed as part of the ARENA project, which aims to develop methodologies for assessing the fuel-saving potential of airborne organic Rankine cycle thermal energy harvesting systems serving as bottoming units to gas turbine engines, leveraging unconventional propulsion system configurations. The first objective of the present research is to advance the state-of-the-art regarding methodologies for the optimal design and integration of aerospace-grade compact HXs into novel propulsion systems. The second objective is to investigate the potential of the Meredith effect, whereby the drag introduced by an air-cooled HX is offset by the acceleration of the heated airflow through a nozzle, for airborne ORC units, and to develop methods for the optimal design of the ramair cooling system.
To this end, an HX model for single- and multi-pass configurations, applicable for sizing and rating problems of components operating with sub- and supercritical working fluids, was implemented in the in-house software HeXacode and validated with experimental data. Preliminary design tools based on this HX model have been embedded within the system design framework for integrated system-and-component optimization of aero engines featuring an ORC bottoming unit. The system design framework was used to optimize the design of combined-cycle auxiliary power unit (CC-APU), turboshaft (CC-TS), and turbofan (CC-TF) configurations. The results demonstrate that achieving optimal performance requires balancing HX thermal effectiveness, pressure loss, and mass. However, this integrated optimization approach is computationally time-consuming. Therefore, a data-driven surrogate modeling methodology has been developed to predict the performance of optimized HXs under variable operating conditions and design specifications. This method reduces the computational cost associated with system optimization studies by more than half.
Results of optimal combined cycle engine configurations demonstrated that the thermal energy rejected by the condenser to the ramair stream can increase propulsive efficiency and specific thrust. This thrust gain is primarily influenced by the total pressure losses in the duct, ram-air temperature increase, and total to static nozzle pressure ratio. Therefore, the thrust that can be generated by the ram air cooling duct is much larger in the case of the CC-TF, whereby the condenser is positioned aft of the fan and outlet guide vane within the bypass duct, compared to the CC-TS configuration.
Due to the significant influence of the ram air cooling duct on the aerodynamic performance of the propulsion system, an accurate duct model is required for preliminary design studies. Therefore, a CFD model of the ram air duct has been developed, whereby the HX is modeled as a porous zone while a steady state RANS solver is used for the airflow within the duct. The CFD model has been used to develop a lumped parameter model of the ram air duct. The accuracy of the developed lumped parameter model is comparable to that of the CFD model, and enables fast evaluation of the duct performance at both design and off-design conditions. The lumped parameter model is also used to quantify the influence of HX and duct geometric design, heat rejection, and total pressure losses on drag recovery through the Meredith effect, providing new guidelines for ram air cooling systems for airborne applications.
The methodologies developed in this research work are directly applicable to waste heat recovery systems, fuel-cell-based electric propulsion or turbo-electric propulsion systems, and support early-stage preliminary design assessments. These results contribute to reducing the uncertainty in adopting combined-cycle and other novel propulsion systems, and to guiding future development of lightweight, high-performance HXs for low-emission aviation.