As turbofan technology advances, periodic engine inspection and maintenance still remain a significant part of aircraft operational costs. Operators are thus looking to engine condition-based maintenance (ECBM), leveraging sensor data and numerical engine models for continuous diagnostics. KLM Engine Services developed a surrogate model based on the High Dimensional Model Representation approach, capable of processing a large volume of engine data at a lower computational cost to estimate engine component condition. This study proposes enhancements to expand the model's capabilities, incorporating additional engine parameters and extending the operational envelope. The augmented surrogate model was then combined with a Long Short Term Memory network capable of predicting component condition based on trends generated from the surrogate model. The framework proposed has demonstrated the potential advantages of combining the surrogate and prediction model for engine diagnostics and prognostics, serving as a valuable starting point for future ECBM projects at KLM ES.

The environmental emergency is one of the most critical challenges of modern times. The exponential increase of industrial activities is the primary cause of anthropogenic climate change, with negative consequences for the environment, society and economy. In the transport sector, decarbonization is the most significant challenge. In aviation, the environmental objectives are to halve international CO2 emissions by 2030, and to reach net-zero carbon emissions by 2050. The achievement of these goals implies a step-change in the traditional practice of aircraft design, with many resources invested in research for promoting the use of fossil fuel-free propulsion systems and electrified auxiliary systems.

In this framework, the research presented in this dissertation is on methods for automated design optimization with applications to a novel electrically-driven Environmental Control System (ECS) for aircraft cabin cooling. The ECS is the main consumer of non-propulsive power onboard aircraft. The founding idea of the project, which has been carried out in collaboration with several companies, is to replace the traditional ECS equipping airliners, which is based on Air Cycle Machine (ACM) technology, with Vapour Compression Cycle (VCC) systems powered by high-speed centrifugal compressors. Work performed within this project demonstrated that such a VCC-based ECS can be more efficient and lighter than an ACM-based ECS in the case of mainstream passenger airplanes. The main goal of the research documented in this dissertation is to provide design methods and guidelines for the optimal design of aircraft ECS whose core is a VCC system powered by novel high-speed electrically-driven compressors and using low-Global Warming Potential (GWP) working fluids in place of the conventional R-134a refrigerant. Additionally, the study encompasses the analysis of the impact of the selected working fluid on the optimal design of the main system components, i.e., the heat exchangers and the centrifugal compressor. For this purpose, a novel integrated design optimization framework has been developed: it allows to perform the multi-objective optimization of the aircraft ECS across different points of the aircraft operating envelope. This method enables the concurrent automated optimization of thermodynamic cycle, preliminary component sizing and working fluid selection. Moreover, the successful application of the method to this complex case demonstrates that the approach is generally applicable to any thermal energy conversion system.

The method was applied to the design of the ECS of two different aircraft to demonstrate its capabilities: a large passenger rotorcraft and a single-aisle short-haul aircraft, i.e., the A320. Results show that it is possible to design an efficient VCC system for aircraft ECS that is powered by an electrically-driven centrifugal compressor and uses low-GWP refrigerants as working fluids. In particular, in the case of a small-capacity ECS for large rotorcraft, it was demonstrated that the use of high-molecular complexity refrigerants, such as haloolefins, enables the design of lighter and more efficient VCC systems if compared to the state-of-the-art. The test case of the airliner ECS provided the specifications to further develop and test the methodology: a so-called physics-based equation of state model was adopted for the computation of the thermodynamic properties of the working fluid. Molecular parameters allow to define the fluid, therefore they can be optimized as part of the global optimization of the design of the system. Parameters are constrained so as to define a realistic molecule, though non-existing, called pseudo-fluid. Actual working fluids whose molecular parameters are similar to those of the optimal pseudo-fluid}are selected in a following step of the design procedure. Optimal working fluids are therefore natural refrigerants. The use of these working fluids with null GWP would reduce the environmental footprint of the considered environmental control systems, while enabling an (albeit small, in the considered case) reduction of specific fuel consumption.

Complementary to the numerical investigation, a novel experimental setup called IRIS (Inverse organic Rankine cycle Integrated System) was designed, realized and successfully commissioned at the Propulsion & Power laboratory of Delft University of Technology. The setup was conceived to enable testing and performance analysis of VCC-based aircraft ECS and to validate in-house software for system and components design. The setup hosts two main test sections: one to test compressors and another to test air-cooled condensers. The results of the commissioning show that it is possible to continuously operate the IRIS setup in steady-state conditions at temperature levels which are very close to those at the design point, thus achieving a Coefficient of Performance (COP) equal to 3.76±0.48. @en

Hydrogen fuel cells are set to play an important role in decarbonizing the aviation sector. They operate at higher efficiencies than turbine engines, but their low operating temperature requires a dedicated thermal management system. The added drag, weight and parasitic power of such a system has a high impact on aircraft performance and its design must be integrated with overall propulsion system development. Among cooling options, flow boiling has the potential of increasing the cooling system performance, but design methods are lacking in the literature. To address this problem, this thesis presents preliminary design methods able to size both conventional single-phase and novel two-phase cooling systems and tests them on a reference regional turboprop aircraft. Sizing results show that pumped multi-phase systems result in lower system mass and drag than conventional single-phase cooling while vapour compression systems are a less attractive option due to their high power demand.

This study analyses the feasibility of an engine architecture using bypass cooled cooling air as a method for reducing specific fuel consumption of current and future high bypass turbofan engines. As cooled cooling air reduces required turbine cooling massflow, a major source of loss in modern aeroengines, it is believed that the concept can improve engine efficiency. No extensive or explicit study on the merits of this concept has been performed, however.
Using the 0-D engine analysis software GSP, engine performance parameters at Take-Off and Cruise, such as, specific fuel consumption and (specific) thrust were evaluated against altering feed cooling air temperature by 0 to -300K and heat exchanger induced pressure losses of 0 to 8% in the bypass. This was done for two test cases; the Leap-1A engine (TOC OPR 50, CRZ BPR 11.1, TO FN 121kN) representing current conventional technology and GTF2050, a geared turbofan engine (TOC OPR 75, CRZ BPR 17.1, TO FN 174kN) with entry into service of 2050. For the Leap-1A engine the specific fuel consumption increased up to 1.9% at cruise and for GTF2050, the increase was up to 4.1%. It is shown that the change in fuel consumption is linear with the decrease in cooling fraction, as for both engines the specific fuel consumption increases by +0.34% per percent cooling air reduction.
With a reduction in cooling fraction, the exhaust pressure of the core increases, resulting in a higher thrust (up to 2.3%). It is shown that the optimal take-off bypass ratio for minimum fuel consumption shifts up, for the GTF2050 engine from 16.4 to 17.5.
Furthermore, scenario tests and an exergy analysis were performed to find that the impact of different mechanisms; the pressure loss in heat exchanger has minor impact on performance; change in turbine efficiency has small impact; the effect of changed turbine massflow and heat rejection into the bypass are the dominant phenomena.
It is concluded that as a method for reducing fuel consumption, cooled cooling air with the bypass as a heat sink is not feasible for conventional architectures even with extreme design parameters such as high OPR and BPR. Heat rejection to the bypass cannot be effectively utilised and no improvement in core efficiency from reduced cooling air can compensate. In the edge case where there is no heat transfer in cruise, there is an observed specific fuel consumption benefit of up to 4%. The achievable benefit will be lower, however, when accounting for installation penalties.
It is recommended to focus turbine cooling studies on the impact and feasibility of active cooling massflow control without heat rejection, investigate alternative heat sinks for cooled cooling air (e.g. cryogenic fuel), and only consider bypass cooled cooling air as last resort when up-flowing cooling
schemes is not permissible or ineffective.

The research presented regards the study of D4 using experimental speed of sound measurements to obtain the ideal gas isobaric heat capacity. The isobaric heat capacity is not a useful property for the generation of equations of state, in the presented research it is used for the validation of thermodynamic models. However, due to the observed sensitivity of the conventional method used for the analysis of the speed of sound data, an effort is undertaken to develop a new analysis method. The aim of this newly developed method is to determine the same properties as the conventional method, albeit less sensitive towards a fluctuation in speed of sound.

Over the years, aero engines have evolved into the efficient turbofans present on commercial airliners today. Although these engines are very reliable, they still experience degradations in performance and reductions in component health. The degradation affects the operation of the engine, which can be measured using the pressure, temperature, and rotational speed sensors. Using accurate engine models together with engine sensor measurements, the condition of individual components can be determined. Surrogate models of a non-linear Gas Path Analysis model have been developed for the GEnx-1B turbofan in the form of High Dimensional Model Representations. It is determined that the surrogate models developed are able to determine component conditions with a high accuracy and low computational complexity. The models are able to properly identify the effects of water-washes and turbine failures when applied to real-world on-wing data. Due to the low computational complexity of the models, they provide the possibility for fleet-wide continuous engine diagnostics.

Water accumulating in the fuel tanks of any aircraft presents an unacceptable safety hazard. This accumulation, which is due to a combination of physical phenomena, is difficult to assess and therefore not well known. The purpose of this project is the development of a lightweight computational prediction method for water accumulation in helicopter fuel tanks. This model delivers a better understanding of the physical mechanisms involved and allows prediction of the accumulated water quantity under given conditions.

In the near future the Royal Netherlands Air Force will replace their fleet of F-16’s with the F-35. In the past the NLR has aided the Air Force with life cycle and deterioration analysis work on the F100-PW-220 engine, which powers the F-16. Understanding the physical system of the engine allows for on-condition maintenance. The same is preferred for the F135-PW-100 engine powering the F-35. Therefore, a preliminary lifing analysis tool has been developed for the F135-PW-100 engine rotor blades, based on open source literature.