The need for more efficient aircraft has led to the conception of innovative engine configurations, such as the combined-cycle engine proposed by Delft University of Technology or the Water-enhanced turbofan developed by MTU and Pratt & Whitney. The evaluation of the potential benefits of these novel engine concepts requires detailed performance studies considering weight and drag penalties as figures of merit. At present, no weight estimation tools with sufficient level of accuracy and flexibility are publicly available, apart from WATE++, a tool developed by NASA. However, this software is subject to export control restrictions and cannot be used outside the USA. For this reason, the development of a new component-based preliminary engine design tool, "Weight EStimation of gas Turbine engines" (WEST), was started. The goal of WEST is to predict the weight of novel engine architectures with a reasonable level of accuracy, accounting for design parameters like turbine inlet temperature, overall pressure ratio, mass flow rate, and turbomachinery configuration, with a minimal set of geometry inputs specified by the user. A previous work demonstrated that WEST can effectively predict the weight of turbofan gas generators. As only the main components are modeled, the WEST estimates account for approximately 70-90% of the actual engine weight.

The capabilities of WEST were expanded in this study to allow for the design of small-scale turboshaft engines. To this purpose, a methodology to design radial compressor disks was implemented, and successfully verified against FEM results. Regarding the modeling of the complete turboshaft, it was found that the predictions of the tool account for only 60-70% of the actual engine weight. Such result was, however, expected, as WEST does not take into account the particle separator and the integrated gearbox, which may account for up to $30\%$ of the engine's total weight.

The world is moving towards sustainability and there is immense pressure on Aerospace Industry to reduce its emissions to contribute to a carbon-neutral world. However, maturing gas turbine technology is a big bottleneck towards this goal and hence, this project focuses on the technical and economic feasibility of a new type of propulsion system, called Solid-Oxide Fuel Cell- Gas Turbine or SOFC-GT hybrid propulsion system. SOFC- GT, even though being a low TRL technology, has the potential to reduce fuel consumption, emissions, and operation costs, making it a suitable candidate for a future propulsion system.

This research concludes that in order to achieve the full potential of the SOFC-GT hybrid, engine parameters like BPR, FPR and cooling air requirements need to be changed. The Thrust-Specific Fuel Consumption (TSFC) for the propulsion system has decreased by 6.03% and 19.89% for 1 MW and 4 MW fuel cell power output in Off-Design (Cruise) condition. Along with this, core mass flow rate or size, cooling air requirement and Turbine Inlet Temperature for Cruise condition has decreased. The emission results show that the NOx emissions have been reduced by 29.11% and 78.05% respectively. Sensitivity analysis shows that the thermodynamic efficiency is most sensitive to engine parameters but the impact of fuel cell parameters is increasing as the fuel cell power output is increased.

The economic analysis done in this study shows that the SOFC-GT hybrid is not feasible for the commercially available fuel cell because of the substantial increase in weight of the propulsion system. However, the propulsion system will become feasible at the fuel cell system specific power of 2.30 kW/kg and 2.10 kW/kg for no emission tax and highest emission tax scenario respectively at a fuel price of $ 6/kg. Along with this, increasing the fuel cell power output leads to the increase in required specific power for fuel cell or has a negative impact on the overall economics. The overall economics of the aircraft is most sensitive to aircraft parameters but increasing the fuel cell power output decreases sensitivity substantially. In the end, the emission has a low impact on the overall economics of the aircraft.

This research shows that it is possible to integrate a SOFC with the turbofan if the fuel cell technology improves in the future. Along with this, the research provides multiple novel methodologies for technical and economic analysis of the SOFC-GT hybrid.

The need for more efficient aircraft has spawned the conception of a variety of novel engine and aircraft architectures, such as the combined-cycle engine proposed by researchers at Delft University of Technology. To evaluate the potential benefits of these concepts comprehensively, in-depth performance simulations are required. Furthermore, penalties with regard to weight and drag are critical considerations, thus new engine technologies must be evaluated within their full airframe-level integration and mission profile. Accurate weight estimations of a variety of design alternatives are therefore necessary in order to properly assess the validity of such technologies and to reach an optimal design with respect to the tradeoffs between weight, drag, and efficiency. No weight estimation tools with sufficient accuracy are publically available at present, thus a new, component-based preliminary engine design tool, named ‘Weight Estimation of Aeronautical Gas Turbine Engines’ (or WEST), was developed. WEST can be used to predict the weight of novel engine architectures to a reasonable degree of accuracy, all the while accounting for sensitivity to design parameters such as turbine inlet temperature, overall pressure ratio, mass flow rate, power, and choice of turbomachinery configuration. Similar methodologies are able to design components worth between 60 and 110% of the actual weight of an existing engine, whereas WEST was able to account for about 70-90%, thus exceeding initial expectations and improving the reliability of the estimations. This tool can therefore be used to estimate the weight of a variety of existing engines, and is sufficiently flexible to model novel architectures as well, since the results are based on the application-specific design of real engine components. Among the potential uses of WEST, a promising one would be to evaluate the weight of a large set of designs for a particular application. Upon these estimates, single-equation surrogate models could be developed using statistical regression, allowing for these equations to be used as a more computationally-efficient weight estimation methodology in a wide range of aircraft-level design and optimization studies. These equations would build upon the work described in this paper and could be used to accelerate the development and introduction of new aircraft-related technologies.

The constant pressure on the maritime sector to reduce Greenhouse Gas (GHG) emissions has led the shipping industry to search for alternatives, such as zero-emissions propulsion systems, which would allow meeting the 2050 target imposed by International Maritime Organization (IMO) regulation. Fuel cells have demonstrated to substantially contribute to the greening of energy conversion technologies. Specifically, Solid Oxide Fuel Cells have proven to be a reliable technology to produce energy from Liquid Natural Gas (LNG). However, the limited understanding of the effect of the components around the stack (also known as Balance of Plant, BoP) in SOFC power generation system represents one of the reasons of the slow development of this technology. The main objective of this research is to gain insight in the performance of the BoP components and their influence on the SOFC power generation system for maritime applications. A dynamic model describing a complete SOFC power generation system is developed in this work. The chosen system configuration consists of three blowers, two heat exchangers, a mixer, an external pre-reformer, an SOFC stack and an afterburner. Specifically, each BoP component is modelled dynamically using a 0-D approach and verified individually by using the software Cycle-Tempo. Then, the BoP models are integrated with an existing 1-D SOFC stack model. A dedicated control system is implemented and the load following capabilities of the complete system are studied. The model developed is able to simulate the time variation of all the BoP component characteristics and provides insights in the system efficiency when varying operating parameters such as stack current, anode recirculating ratio and fuel utilization. In particular, it is proven that working at low current enables higher cell voltage and, thus, higher system and stack efficiency. System fuel utilization significantly contributes to the system efficiency, which reaches the highest value for the highest fuel utilization. Additionally, the effect of the fuel utilization rate on the stack and system is the highest at lower currents. System and stack efficiency of respectively 58 % and 66 % are possible with the chosen system configuration. Anode recirculating contributes more to the system efficiency than the stack efficiency. The highest system efficiency is obtained for low current values and high recirculating ratio. Moreover, significant CO2 emission reduction is obtained for high recirculating ratio. The chosen control strategy succeeds on ensuring thermal safe operation, but does not guarantee fast response to load changes. In particular, a system response within 2 hours is achieved with the controller developed when the stack current is changed from 27 A to 23 A . Moreover, a load ramp of the stack current is a better choice in terms of thermal safe operation than a stepped change. The developed model represents a solid base for future development and research in the modelling of SOFC power generation systems for maritime applications. Nevertheless, future investigations on model validation, control system, start-up operations and system optimization are recommended.

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 vast number of pioneering theoretical developments inthe area of non-classical gas dynamics together with the rising number ofapplications of organic Rankine cycle (ORC) technology have shaped a new branchof fluid mechanics called non-ideal compressible fluid dynamics (NICFD). Thisfield of fluid mechanics is concerned with flows of dense vapors, whoseproperties do not comply with the ideal gas model. These types of flows occurin dense vapors, supercritical fluids and liquid-vapor mixtures. NICFD is encountered in a variety of industrial processes,the most relevant in the power and propulsion sector, and is of interest forfundamental research.  Chapter 2documents an extensive literature review which covers the main developments inthe numerical, theoretical and experimental areas. In particular, the review ofcurrent and past experiments was aimed at summarizing the lessons learned.Despite the relevance of NICFD applications, experimental information regardingthis type of flows is scarce, due to the challenges inherent with the operatingconditions of such experiments. The main objective of the research documented in thisdissertation was to perform accurate measurements of NICFD flows which can thenbe used to assess the predictive capabilities of state-of-the art numericaltools. To this end, the design and commissioning of suitable and fullyinstrumented facilities capable of generating NICFD flows in a multitude ofsteady, controlled conditions is necessary in order to provide high-quality andwell characterised data. Arguably, state-of-the-art optical techniques are mostsuited for this goal, together with more conventional temperature, pressure andmass flow rate measurements.Advanced laser diagnostic techniques such as particle imagevelocimetry (PIV) are possibly the measurement technique of choice whenaccurate measurements with a high spatial and temporal resolution are needed.However, the use of PIV or other optical techniques capable of providing localand instantaneous information within the flow is not documented. Therefore, afeasibility study was conducted by means of a simpler experiment:  the planar PIV technique was applied tocharacterise the dense vapor of an organic fluid (D4, a siloxane) stirred withina transparent container. Chapter 3 documents the successful results of thisexperiment. The optical properties of the dense vapor make PIV possible.Titanium dioxide (TiO2) seeding particles were used to track the low-speed motionof the fluid around a rotating disk. Vector fields of the natural convectionflow and of the superposition of natural convection and rotating flow wereacquired and studied as exemplary cases. The particles adequately trace theflow since the calculated Stokes number is 6.5×10-5. The quality ofthe experimental data was assessed by means of particle seeding density andparticle image Signal to Noise ratio (S/N). The results are deemed acceptablein view of envisaged high-speed flow experiments. In order to obtain measurement data of high-speed vaporflows in the NICFD regime, new experimental facilities must be conceived,designed, realized and tested. Chapter 4 presents the organic Rankine cyclehybrid integrated device (ORCHID), which was designed, built and commissionedat the Delft University of Technology. The facility can operate continuouslyand with a wide range of operating conditions. The maximum operating pressureand temperature are 25 bara and 350 °C. The facility has been designedto operate with siloxane MM as the working fluid, but it was numericallyverified that it may also be operated with other working fluids such as MDM, MD2M,D4, D5, D6, pentane, cyclopentane, NOVEC649, PP2, PP80, PP90, and toluene. Twotest sections allow to operate the ORCHID either as a supersonic/transonicvapor tunnel or as an ORC turbine test bed. Currently, a supersonic nozzlefeaturing a throat of 150 mm2 with optical access allows to perform gasdynamic experiments for the validation of numerical simulation codes. A secondtest section, a test-bench for mini-ORC expanders,  is being designed and will accommodate afully instrumented 10 kWe machine, however machines of any configuration andwith a rated power of up to approximately 80 kWe can be tested. Chapter 5 documents the achievements reached during thecommissioning of the ORCHID. The successful commissioning of the setup with MMas the working fluid is detailed and discussed based on the recordings ofseveral test runs, including the start up and shut down of the facility.  Data were acquired during the operation atsteady state at the two main operating conditions typical of supersonic nozzleand ORC turbine tests. The operation of the facility is characterized withregards to the process stability, moreover process variables are assessed fortheir uncertainties. The correct operation of the nozzle test section wasverified with a mass flow rate of fluid of m = 1.15 kg / s, and at a thermodynamic state at the nozzle inletcorresponding to T = 252 °C and P =18.36 bara. The test sectionconditions typical of a turbine experiment were T = 275 °C, P =20.8 bara, with a mass flow rate of m = 0.17 kg / s. All therelevant process variables of the test section are affected by a relativeuncertainty that is lower than 0.6 %.Chapter 6 reports the results of the first supersonic nozzleexperiments. Schlieren images of the MM flow through the two-dimensionalconverging-diverging nozzle with the inlet in the NICFD regime were recorded,together with the static pressure profile along the nozzle. A series ofschlieren photographs displaying Mach waves in the supersonic flow wereobtained and are documented at two operating conditions, namely, for inletconditions corresponding to a stagnation temperature and pressure of T0  = 252 °C and P0 = 18.4 bara, and to a back pressure of 2.1bara. Furthermore, static pressure values were measured along the expansionpath for operating conditions given by T0 = 252 °C and P0 = 11.2 bara at the nozzle inletand by a back pressure of 1.2 bara. The two inlet conditions of the fluidcorrespond to a compressibility factor of Z0 = 0.58 and Z0 = 0.79.These Mach number values together with the values of thestatic pressure along the top and the bottom profile of the nozzle were usedfor a first assessment of the capability of evaluating NICFD effects occurringin dense organic vapor flows by comparison with the results of CFD simulations.The outcome of this initial comparison was deemed satisfactory.Chapter 7 introduces the first steps towards the validationof a CFD solver for non-ideal compressible flows. In particular, anindustry-standard validation method was used together with syntheticexperimental data (experimental data were not available yet) to illustrate, asan exercise, how the uncertainty of NICFD software can be quantified. Theassessment is limited to determining how the uncertainties in model inputs,e.g., fluctuations in boundary conditions and thermodynamic property modelsinfluence the overall accuracy of NICFD simulations. The assessment of theuncertainty of the other sub-models is left for a successive phase of thisresearch program. The validation exercise confirmed the applicability of theproposed method, but also pointed out that the adopted validation metricsshould be complemented with additional statistical indicators. The errorsources in the designed experiment are identified and all the uncertainties areadequately quantified.The final chapter summarizes the answers to the researchquestions listed in the first chapter, above all that the ORCHID facility wassuccessfully commissioned and can generate stable dense vapor flows of organicfluids for both fundamental gas dynamic experiments and ORC turbine testing.The first experiments demonstrate that it can be used to obtain accurateoptical and non-optical measurements of supersonic nozzle flows for thevalidation of CFD codes, including flows in the NICFD regime.  An overview of the next phases of theresearch program is also provided.@en

Hydrogen Polymer Electrolyte Membrane (PEM) fuel cell technology is becoming increasingly popular in the transportation sector, especially the automotive industry. It can power electric drives by converting hydrogen into electricity in a chemical reaction with oxygen, whose products consist only of water. Therefore, hydrogen PEM fuel cell-based powertrains are free of any harmful emissions, enabling clean propulsion technology. An important part of an automotive fuel cell system is the air processing subsystem, which usually features an electrical air compressor (E-compressor) to supply the necessary oxygen for the chemical reaction. This E-compressor can absorb between 10-30% of the fuel cell gross power. One way to decrease this share is to expand the fuel cell air exhaust flow in a turbine that contributes to power the E-compressor, essentially realizing an electric turbocharger (E-turbocharger). E-turbochargers for fuel cells are still in the development phase, therefore there is a lack of experience and knowledge on their benefit. In this thesis, a model of an existing automotive PEM fuel cell air processing system has been developed using the AVL Cruise M software. In addition, two prototype PEM fuel cell E-turbochargers (ETC1 and ETC2, respectively) have been modeled and integrated into the main air processing system model. Steady-state simulations have been performed for three different power levels. Moreover, the turbine inlet temperature of the ETC1-based model was varied in the range of 60°C to 150°C to quantify what benefits exhaust heating with waste heat brings in addition to the inherent turbocharging performance gains. The results show that compared to the baseline system, the model featuring ETC1 achieved an increase in net power between 3.04-6.78% or a decrease in fuel consumption between 3.19- 7.67%, depending on the power level. For the ETC2-based model, from low to high load, the net power increased between 1.88-2.04%, or fuel consumption decreased between 2.07-3.17% compared to the baseline system. When increasing the turbine inlet temperature to 150°C, the model showed an additional decrease in fuel consumption of 2.1 percentage points at full power. The models could be improved by implementing a first-principle-based model of the E-compressor and E-turbocharger, which could have been developed in this project if more detailed information on the heat flows within these components was available.

A significant trend in aero engine design has been the rise in turbine inlet temperatures, as well as the drive to produce raise efficiency. Since the 1960s, turbine inlet temperatures have exceeded turbine material limits, with turbine cooling systems being used to bridge the gap. Modern engines require substantial amounts of cooling air, prompting a need to understand further the impact of turbine cooling on turbine and engine performance in cycle calculations.

Current models employed for performance analysis of cooled turbines are based on technical assumptions that are several decades old. This raises questions about the possibility of adapting models to more accurately represent modern engine technology. As such, this thesis aims to develop a cooled turbine model (CTM) for use in PyCycle, an open-source engine cycle analysis platform. The CTM is based on the cooled turbine blade row model defined by Young and Wilcox, which employs empirical constants to estimate cooling mass flow rates. The CTM can estimate the cooling flow requirements and the associated entropy rise for the turbine, accounting for the irreversibility in the turbine cooling process.

The implemented CTM models a complete turbine stage and multi-stage turbines based on three key aspects: the thermodynamics of a cooled turbine row, the work extraction in an equivalent uncooled turbine stage, and the conversion of thermodynamic properties between an absolute and rotating frame of reference. The CTM was verified and validated using three other cases, and it was found to accurately capture the effects of turbine cooling on bulk flow properties.

Following the implementation of the CTM, a study into the (semi-)empirical parameters and constants was performed to update existing parameters for modern engines. The limited availability of data relating to flow velocities and Mach numbers in aero-engine turbines forms a significant obstacle to the accurate specification of some empirical parameters. An estimation for the average Stanton number over turbine blades based on the gas temperature and Reynold’s number was derived and validated with the Von Karman Institute’s LS-89 turbine cascade results. The impact of updating the empirical parameters used in the CTM
has been assessed by formulating the cooling flowestimation as an optimization problem.
Using the updated ranges for the empirical parameters, the optimization study showed the potential for a significant reduction in the estimated cooling fraction.

A significant trend in aero engine design has been the rise in turbine inlet temperatures, as well as the drive to produce raise efficiency. Since the 1960s, turbine inlet temperatures have exceeded turbine material limits, with turbine cooling systems being used to bridge the gap. Modern engines require substantial amounts of cooling air, prompting a need to understand further the impact of turbine cooling on turbine and engine performance in cycle calculations.

Current models employed for performance analysis of cooled turbines are based on technical assumptions that are several decades old. This raises questions about the possibility of adapting models to more accurately represent modern engine technology. As such, this thesis aims to develop a cooled turbine model (CTM) for use in PyCycle, an open-source engine cycle analysis platform. The CTM is based on the cooled turbine blade row model defined by Young and Wilcox, which employs empirical constants to estimate cooling mass flow rates. The CTM can estimate the cooling flow requirements and the associated entropy rise for the turbine, accounting for the irreversibility in the turbine cooling process.

The implemented CTM models a complete turbine stage and multi-stage turbines based on three key aspects: the thermodynamics of a cooled turbine row, the work extraction in an equivalent uncooled turbine stage, and the conversion of thermodynamic properties between an absolute and rotating frame of reference. The CTM was verified and validated using three other cases, and it was found to accurately capture the effects of turbine cooling on bulk flow properties.

Following the implementation of the CTM, a study into the (semi-)empirical parameters and constants was performed to update existing parameters for modern engines. The limited availability of data relating to flow velocities and Mach numbers in aero-engine turbines forms a significant obstacle to the accurate specification of some empirical parameters. An estimation for the average Stanton number over turbine blades based on the gas temperature and Reynold’s number was derived and validated with the Von Karman Institute’s LS-89 turbine cascade results. The impact of updating the empirical parameters used in the CTM
has been assessed by formulating the cooling flowestimation as an optimization problem.
Using the updated ranges for the empirical parameters, the optimization study showed the potential for a significant reduction in the estimated cooling fraction.

Climate change is a pressing issue. Hydrogen aircraft are an excellent alternative to current kerosene aircraft, and appear to be the most viable long-term solution for net-zero flight. One of the main challenges for hydrogen aircraft is to sustain a low boil-off rate. This research explores the solution of active cooling of the LH2 mixture in the fuel tank. It focusses on the construction of a thermodynamic model for cryogenic liquid hydrogen fuel storage in aircraft, a system model for single-stage Reverse Turbo-Brayton Cryocoolers (RTBC), and the conceptual design of the RTBC’s miniature high-speed compressor. This is used for the integrated modelling of the RTBC, compressor and LH2 fuel tank for an exploration study of the carbon neutral hydrogen concept of the long-range Flying-V aircraft. Results show that the RTBC might offer a valuable addition to the Flying-V design space for boil-off control in addition to careful design of the insulation and tank shape.

Climate change is a pressing issue. Hydrogen aircraft are an excellent alternative to current kerosene aircraft, and appear to be the most viable long-term solution for net-zero flight. One of the main challenges for hydrogen aircraft is to sustain a low boil-off rate. This research explores the solution of active cooling of the LH2 mixture in the fuel tank. It focusses on the construction of a thermodynamic model for cryogenic liquid hydrogen fuel storage in aircraft, a system model for single-stage Reverse Turbo-Brayton Cryocoolers (RTBC), and the conceptual design of the RTBC’s miniature high-speed compressor. This is used for the integrated modelling of the RTBC, compressor and LH2 fuel tank for an exploration study of the carbon neutral hydrogen concept of the long-range Flying-V aircraft. Results show that the RTBC might offer a valuable addition to the Flying-V design space for boil-off control in addition to careful design of the insulation and tank shape.

With climate change posing increasing risks, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) aims to reduce CO2 emissions by 75% and NOx emissions by 90% per passenger kilometer by 2050, compared to a baseline aircraft from 2000. The ARENA project addresses this by developing a waste heat recovery system using aircraft engine exhaust gases, improving fuel efficiency. This thesis focuses on integrating the condenser of such a system into the propulsion unit, aiming to minimize drag from the ram air cooling duct by evaluating different heat exchanger topologies and duct designs.

The study uses the IMOTHEP Distributed fans Research Aircraft with electric Generators by ONERA (DRAGON) concept, a hybrid electric aircraft with two tail-mounted turbogenerators. The research proceeds in several stages. Initially, a multipass-condenser's potential to reduce pressure drop was examined by adjusting the heat exchanger blockage factor per pass, but results showed no reduction in pressure drop.

Next, a lumped parameter model was developed to analyze drag, pressure drop, temperature increase, and ram air duct length. This model evaluated the sensitivity of duct geometrical parameters on the drag recovery factor—a dimensionless number indicating net thrust. Findings revealed that inclining the heat exchanger efficiently increases the drag recovery factor, while the diffuser area ratio has a similar effect but is less space-efficient. The mass flow rate ratio showed less sensitivity, and fin height or pitch had the smallest effect on drag recovery.

Using the lumped parameter model, an optimal preliminary ram air duct design was identified for different spatial constraints. The study found that inline plain tube bundle and flat tube offset strip fin heat exchangers provided more compact solutions with higher drag recovery factors. For large diffuser-blocked area fractions, optimal duct geometry remained independent of heat exchanger type, characterized by a 70-degree maximum inclination angle, a 0.7 mass flow rate ratio, and maximized diffuser area ratio within spatial constraints.

A verification study of the lumped parameter model was conducted using a two-dimensional Reynolds Averaging Navier Stokes (RANS) computational fluid dynamics (CFD) analysis with k-ω SST turbulence and a porous zone to mimic the heat exchanger. The CFD analysis confirmed the lumped parameter model's predictions, with a 0.8% difference in drag recovery factor for optimal duct geometry. Across various geometries, the mean absolute difference in drag recovery factor between models was 1.082%, with a standard deviation of 0.584%.

In conclusion, integrating the condenser in the propulsion unit within spatial constraints results in positive thrust, thus supporting Meredith's 1935[2] claim. This integration reduces net drag and improves overall efficiency, contributing to significant emission reductions in line with ACARE's targets.