An important aspect influencing the economic advantage of reusable launch vehicles over their disposable counterparts is the cost associated with the reuse itself. These costs are optimized through design for inspection and maintenance, as well as effective condition monitoring and predictive maintenance. In this context, the fouling of rocket cooling channels due to the thermal decomposition of hydrocarbon fuels, known as pyrolysis, is particularly challenging as cooling channels are difficult to inspect and clean. Carbon depositions resulting from the pyrolysis process act as thermal insulation, due to which wall temperatures can rise and the thermo-mechanical damage to the wall can increase. The current work proposes an approach to the monitoring of cooling channel fouling based on the sensing of decomposition products in the fuel flow. In addition to acting in real-time, this approach provides an early warning of damage that will occur, as it is based on reaction products that precede the deposition of carbon. It is inherently faster than the alternative approach of sensing increases in the wall temperature. The decomposition products are sensed by thermal conductivity gauges and, using a simplified theoretical model, the data is used to infer the mass of deposited carbon in the channel. The approach is tested experimentally for methane-based fuels on Inconel 600, Inconel 625 and Haynes 230. The model is shown to match carbon deposition mass measurements to within approximately the uncertainty of the scale used. Although these results are promising, further testing under more arduous conditions needs to be conducted in future work.

Additive manufacturing (AM) enables the fabrication of complex internal cooling channels for high-temperatureaerospace applications, but the rough as-built surfaces strongly influence flow. Understanding and predicting theseeffects is critical for accurate thermal–hydraulic design of AM-based cooling components. This study investigates therole of surface roughness on pressure drop and friction factor in an AM rectangular channel fabricated from Haynes230, a nickel-based superalloy. The cooling channel, with a hydraulic diameter of 1.995 mm, was tested in a closed-loop nitrogen flow facility at different mass flow rates, corresponding to Reynolds numbers in the range of 104 - 105.The test facility was developed under the MERiT+ project funded by Swedish National Space Agency at KTH Royal Institute of Technology, with industry partners from Siemens Energy and GKN Aerospace. Pressure drops wererecorded using multiple taps located along axial direction, and the Darcy friction factor was determined from localpressure drops. Complementary optical measurements of cross-sections at different streamwise locations wereperformed to assess variations in hydraulic diameter, while white light interferometry was employed to quantify surface roughness parameters, including Sa, Sq ,Ssk, and Sku. Numerical simulations were carried out in ANSYS Fluent usingthe pressure-based steady solver. Simulations were conducted with equivalent sand-grain roughness height, ks, derivedfrom characteristic parameters. Comparison between experimental and numerical results demonstrated consistenttrends in pressure drop and validated the approach for implementing hydraulic roughness in CFD. The data was furtheranalyzed to establish a predictive correlation between measured roughness parameters and equivalent sand-grainroughness. Several log-linear model forms were evaluated, and fitting was performed by minimizing errors in predictedfriction factors using the Haaland equation. The final correlation expresses (ks/Dh) as a function of normalized RMS roughness, and skewness, providing a compact, physically interpretable model. This work demonstrates a combinedexperimental, metrological, and numerical methodology for quantifying and predicting friction factor in AM channels.The proposed correlation, calibrated for Haynes 230 LPBF channels in the turbulent regime, bridges advanced surface characterization with classical fluid mechanics and offers a foundation for more reliable hydraulic design of AM-based cooling systems.

 The centrifugal compressor design involves trade-offs involving several geometric parameters. The goals of the design are generally multi-disciplinary and multi-dimensional. Recent legislative demands on the heavy-duty truck industry push the limits for efficiency and noise. Acoustic measurements indicate that the exit noise generated from the rotor is the major contributor to the tonal BPF noise levels. Advanced CFD techniques such as URANS and LES are employed to quantify noise levels. However, these methods demand more resources and time. The objective of this study is to identify a simple parameter that can indicate the impeller exit tonal Blade Pass Frequency (BPF) noise of a centrifugal compressor using RANS simulations and validate the same with experimental measurements. This work introduces a new parameter called acoustic crest factor that can indicate the tonal BPF noise from the impeller exit using the circumferential variation of static pressure at the impeller exit. Acoustic crest factor can predict the trends observed in normalized sound pressure level values measured at the impeller exit for both full and splitter-blade wheels. This is further validated by using acoustic crest factor to rank four different compressor impellers tested in an anechoic chamber with sound power level measurements. This shows that the identified parameter ‘Acoustic Crest Factor’ could be used as a design tool to rank designs based on acoustic signatures and also as an objective function for optimization. 

Cavity resonances in ducts is a classical problem that has involved researchers from very different fields over time. More recently the aerospace community became engaged again due to resonances found within the flow path of aero-engines, for example in bleed cavities in the low pressure compressor section. These resonances can lead to problems with the structural integrity of upstream components, and thus warrant investigation. The paper compares the results of the same cavity geometry within two wind tunnels of different dimensions and operational setups. The intention of the study is to isolate the Rossiter modes from any other geometric modes that are due to the wind tunnel's test section geometry. This isolation allows the modification of the model of Rossiter and the cavity depth model of East to improve the predictive capability over a larger range of Mach numbers and for deeper cavities. The experiments were performed for an operating range from low subsonic to transonic Mach numbers. The analysis focuses on modifying the constants in both the Rossiter model and the cavity depth model proposed by East along with investigating the phenomenon of mode switching for Rossiter modes. The analyses show a good repeatability of the data set between the two wind tunnels displaying strong resonances at similar operating points. An independence from Reynolds number of the acoustic frequency generation is demonstrated within the operating range. A mode switching behavior is identified with the shallowest and deepest cavity showing multiple mode transitions within the operating range.

Methane pyrolysis has been a topic of research in both energy technology and aerospace propulsion. Variousaspects such as heat transfer, catalytic materials and chemical kinetics as well as carbon deposition behaviourhave been in focus. However, the temporal behaviour of pyrolysis and carbon deposition has received relativelylittle attention. Considering the effect of maintenance on programme costs, it is important to have sensors withgood time resolution both in the laboratory and in operational systems. Therefore, it is of interest to investigatethe efficacy of real-time sensors that have the potential for integration into systems with real-world applications.Speed of Sound (SoS) transducers and Thermal Conductivity Gauges (TCG) have the potential to produce high-quality measurements at relatively low cost and in a compact form factor; however, they have not yet beendemonstrated in this research area. Therefore, the current work assesses their performance compared to gaschromatography (GC), using a newly developed experimental setup. It is shown, for the conditions tested, thatreal-time and in-flow SoS and TCG sensors can attain measurement quality comparable to that of the GC used;however, this performance is implementation dependent. Specifically, PCB based TCG and Time-of-Flight basedSoS measurements are found to be highly effective in capturing the dynamic characteristics of the methanepyrolysis process. Although the current indication is promising, further work is necessary to test the approachunder representative operational conditions specific to, for example, aerospace propulsion applications.

The effects of surface roughness, e.g. due to additive manufacturing or various wear mechanisms, is an active field of research, in particular for aerospace propulsion. With respect to rocket propulsion, such effects include performance changes of turbopump components and increasing pressure drop in cooling channels. An area that has not yet received significant attention is the effect of surface roughness on the thermo-catalytic decomposition of hydrocarbon fuels in cooling channels of, e.g. rocket engines or hypersonic aero engines. Although the decomposition of methane on rocket engine candidate materials and the resulting carbon depositions has been under active investigation in recent years, research mentioning the effect of surface roughness is sparse at best. The thermal decomposition of fuel, also known as pyrolysis, is a complex chemical process that can be catalyzed by materials such as nickel, iron and copper, which are common elements used in alloys for rocket engines and nozzles. Carbon deposited on, e.g. cooling channel walls has a thermal conductivity of a factor 100-1000 lower than that of copper and therefore effectively acts as a thermal insulator. Due to this, it has the potential to increase wall temperatures and thereby thermo-mechanical damage. The pyrolysis process depends on temperature, flow rate (or residence time), pressure, catalytic material, the area of the material exposed to the flow as well as the history of this material. High surface roughness materials may influence catalytic activity by increasing the number of active sites via surface area as well as the activity of the available catalytic sites. This is especially relevant given the increasing use of additive manufacturing in aerospace industry. The aim of this work is therefore to provide new data on the effects of surface roughness on fuel pyrolysis in components of aerospace propulsion systems. In this work, we focus on the experimental investigation of the effect of surface roughness on methane pyrolysis and the resulting carbon depositions. The candidate materials considered are nickel alloys Haynes 230 and Inconel 625 and are prepared by both additive and subtractive manufacturing methods resulting in a surface roughness range from Ra=0.40μm to Ra=11.24μm. This work is carried out as part of the MERiT+ project, a cooperation between KTH Royal Institute of Technology, GKN Aerospace Sweden and Siemens Energy, which is funded by the Swedish National Space Agency.

This paper thoroughly investigates the impact of tip clearance on the performance of a highly loaded unshrouded gas turbine rotor, specifically focusing on three different configurations: 0.98%S (of blade span), 1.35%S, and 1.92%S tip gaps. The study aims to quantify the performance parameters of the turbine stage at both the design point and at varying rotational speeds. Experimental measurements were conducted using the test turbine facility at the unit of Heat and Power Technology, KTH-Royal Institute of Technology, simulating actual flow conditions to assess leakage flow and its effects on turbine performance. Additionally, computational RANS models using the SST k − ω turbulence model were developed and validated with experimental data to visualise complex flow behaviour. The results reveal that increasing the size of the tip gap decreases the stage efficiency in line with the literature, as larger gaps induce stronger tip leakage vortices and a decrease in pressure drop across the rotor, with effects extending down to 50% span. A reduction in tip gap size improves efficiency by minimising leakage flow, leading to a performance gain of approximately 1.5%-pts for a tip gap change of 0.5% blade span. Both experimental and computational analyses show a high level of agreement validating the numerical models in predicting flow characteristics and turbine performance. Furthermore, comparisons with loss prediction models show that while minor discrepancies exist, the models provide a robust starting point for understanding tip leakage effects. Overall, this study provides valuable outcomes into the aerodynamic losses associated with tip leakage in high-pressure turbines and emphasise the importance of precise tip gap management for optimising turbine performance.

The rapid development of reusable launch vehicles (RLVs) has shifted attention towards methane as a rocket fuel, motivated by its balance of storability, performance, and the possibility of in-situ resource utilisation. Compared to kerosene-based fuel (e.g. RP-1), methane burns more cleanly and generates less soot. Compared to liquid hydrogen, it is easier to handle and store while still offering favourable propulsive efficiency and cooling performance compared to RP-1. However, impurities in methane derived from natural gas or biogas—such as ethane, propane, and carbon dioxide—may reduce this fuel’s thermal stability. Under rocket-like thermal conditions, methane can break down and deposit carbon (“coke”) on cooling channel walls. This reduces heat transfer, increases wall temperature, and shortens the lifetime of engines. If non-purified methane is to be used, its stability must be fully understood. This paper presents results from the MERiT project, a collaboration between KTH Royal Institute of Technology and GKN Aerospace. A series of controlled experiments examined the catalytic pyrolysis stability of methane with various impurity contents. Nickel 201 and Inconel 600 samples were heated to 800 °C at 200 kPa(A) while gas mixtures containing methane and different proportions of ethane, propane, and CO₂ were tested at a flow rate of 50 ml/min. Hydrogen production, coke deposition, and visual inspection were used to assess pyrolysis stability. The results demonstrate that propane, with concentrations as low as 2%, leads to a visually observable increase in cooking. Ethane also increases pyrolysis rates, though its effect is more moderate compared to propane. The effects of ethane and propane are both exponential in character across the 0–10% range. CO₂ demonstrated varying effects based on the samples used; however, regardless of the sample material, its effects were limited in increasing coke formation compared to ethane and propane. These findings suggest that while some level of ethane and CO₂ may be tolerated, propane must be removed if methane from natural gas is to be used in regenerative rocket nozzles.

Flow over open cavities can give rise to resonances where the acoustic response in the cavity couples with the shear layer oscillations. In turbomachinery, there are several cavities in which such resonance may occur, for example the bleed cavities in the intercompressor duct. This work is a study of flow induced cavity resonance in a rectangular Helmholtz type cavity with variable depth and at a range of Mach numbers. The phenomenon is studied by means of experimental test data, computational fluid dynamics and analytical models. It is identified that the cavity during resonance can respond with both plane modes as well as higher order antisymmetric modes in the cavity. It is shown that the shear layer locks in to the cavity response with integer periods when the cavity responds with antisymmetric modes and half-integer periods when the cavity responds with symmetric modes. It is also observed that there are specific ranges in cavity resonance frequencies which strongly interact with the shear layer instability and that the resonance may switch between different resonance modes even at steady flow condition.

 Cavity resonances in ducts is a classical problem that has involved researchers from very different fields over time. More recently the aerospace community became engaged again due to resonances found within the flow path of aero-engines, for example in bleed cavities in the low pressure compressor section. These resonances can lead to problems with the structural integrity of upstream components, and thus warrant investigation. 

This study expands the previous data set of an open box cavity with variable depth (D) and a rectangular opening with length (L), effectively testing L/D = [4, 2, ½] ratios. This parameter was indicated by Rossiter to be a strong predictive parameter in frequency generation. 

The paper compares the results of the same cavity geometry within two wind tunnels of different dimensions and operational setups. The intention of the study is to isolate the Rossiter modes from any other geometric modes that are due to the wind tunnel’s test section geometry. This isolation allows the modification of the model of Rossiter and the cavity depth model of East in order to improve the predictive capability over a larger range of Mach numbers and for deeper cavities. 

One wind tunnel has an open loop continuous flow operation where the Mach number is set by changing the outlet static pressure. The second wind tunnel has a blow-down operation where both Mach and Reynolds numbers can be set independently by pulling a downstream vacuum and setting the upstream total pressure. The experiments were performed for an operating range from low subsonic to transonic Mach numbers. The analysis focuses on modifying the constants in both the Rossiter model and the cavity depth model proposed by East along with investigating the phenomenon of mode switching for Rossiter modes. 

The analyses show a good repeatability of the data set between the two wind tunnels displaying strong resonances at similar operating points. The new proposed constants for the Rossiter model when using an L/D = 2 are 𝛾𝛾 = 0.15, 𝜅𝜅 = 0.58. For the deeper cavity with L/D = 0.5 the constants proposed are 𝛾𝛾 = 0.15, 𝜅𝜅 = 0.9. The modification of the cavity depth model by East to better predict the mode above Mach 0.3 is proposed by adjusting the constant B = 2.5 and leaving A = 0.65. An independence from Reynolds number of the acoustic frequency generation is demonstrated within the operating range. A modeswitching behavior is identified with the shallowest and deepest cavity showing multiple mode transitions within the operating range and a near constant Rossiter mode 1 for L/D=2.