Search Results (12)

In aerospace applications, engine parts, especially those around the rotor blade tips, are coated with an abradable seal, a specific material layer. Its design produces a tighter seal without harming the blades by allowing it to wear down or “abrade” somewhat when the blade tips come into contact. In turbines and compressors, this reduces gas leakage between high- and low-pressure zones, increasing engine efficiency. Abradable seals are crucial to contemporary jet engines because they enhance performance and lower fuel consumption. The materials selected for these seals are designed to balance durability and abrasion resistance under high temperatures and speeds. Metal matrix, oxide particles, and porosity are the three most prevalent phases. An ideal mix of characteristics, such as hardness and erosion resistance, determines how effective a seal is, and this is accomplished by keeping the right proportions of elements in place throughout production. The primary objective of this research is to optimize abradability by utilizing various FEM tools to simulate the rub rig test and modify testing parameters, including Young’s modulus, yield stress, and tangent modulus, to analyze their impact on the wear behavior of the abradable seal and blade. Two microstructure models (CoNiCrAlY–BN–polyester coating) were found to perform optimally at porosity levels of 56% and 46%, corresponding to hardness values of 48 HR15Y and 71 HR15Y, respectively. Changing factors like yield stress and tangent modulus makes the seal more abrasive while keeping its hardness, porosity, and Young’s modulus the same. Furthermore, altering the Young’s modulus of the shroud material achieves optimal abradability when tangent modulus and yield stress remain constant. These findings provide valuable insights for improving material performance in engineering applications. To improve abradability and forecast characteristics, this procedure entails evaluating the effects of every single parameter setting, culminating in the creation of the best abradable materials. This modeling technique seems to provide reliable findings, providing a solid basis for coating design in the future. Full article

Understanding cycle-to-cycle variations (CCV) is of practical importance for the combustion of fossil and renewable fuels, as increasingly stringent emission regulations require reductions in the negative effects of such variations. The subject of this study is the flow around inlet valves, since oscillations of such inlet flows affect the flow structure in the cylinder and are thus one of the causes of CCV. To this end, a parametric analysis of the influences of the mass flow rate and valve lift of two parallel engine intake valves on the flow structures is performed. This follows on from an earlier similar study where the flow around a single intake valve was investigated. To analyse the flow behaviour and, in particular, the interactions of the flow leaving these two valves, an optical test rig for 2D particle image velocimetry (PIV) and a large eddy simulation (LES) are used. Proper orthogonal decomposition (POD), together with a quadruple decomposition and the Reynolds stress transport equations, are used to study the turbulence phenomena. The PIV and LES results are in good agreement with each other. The detailed LES analysis of the flow structures shows that, for small valve lifts, the flow separates along the whole perimeter of the intake valve, and for larger valve lifts, the flow escapes only to one side. This is, for combustion engines with the tumble concept, the stage at which the tumble movement develops. Moreover, the flow structures are strongly influenced by the valve lift, while they are unaffected by the variation in the mass flow. The turbulent kinetic energy in the flow field increases quadratically with a decreasing valve lift and increasing mass flow. The large, high-energetic flow structures are particularly dominant near the jet, and the small, low-energetic structures are homogeneously distributed within the flow field. The specific Reynolds stress transport equation shows the limitations of two-dimensionality and large timesteps in the PIV results and the limitations of the LES model. Full article

Due to stringent emission regulations, it is of practical significance to understand cycle-to-cycle variations in the combustion of fossil or renewable fuels to reach future emission regulations. The present study aims to conduct a parametric investigation to analyse the influence of the valve lift and different mass flows of an inlet valve of the test engine “Flex-OeCoS” on the flow structures. To gain a deeper understanding of the flow behaviour, an optical test bench for 2D Particle Image Velocimetry (PIV) and a Large Eddy Simulation (LES) are used. Turbulence phenomena are investigated using Proper Orthogonal Decomposition (POD) with a quadruple decomposition and the Reynolds stress transport equation. The results show good agreement between the PIV and LES. Moreover, the main flow structures are primarily affected by valve lift while being unaffected by mass flow variation. The turbulent kinetic energy within the flow field increases quadratically to the mass flow and to the decreasing valve lift, where large high-energetic flow structures are observed in the vicinity of the jet and small low-energetic structures are homogeneously distributed within the flow field. Furthermore, the convective flux, the turbulent diffusive flux, the rate of change, and the production of specific Reynolds stress are the dominant terms within the specific Reynolds stress transport equation. Full article

With the development of hypersonic air-breathing propulsion systems, such as the supersonic combustion ramjet (Scramjet) and rocket-based combined cycle (RBCC) engines, the mixing process of supersonic airstream with fuel in the engine combustor has been drawing more and more attention. Due to the compressibility effects, the mixing process in a supersonic condition is significantly inhibited. In the present paper, the novel strategy of wall-jet induced shock waves (WJISW) is put forward to realize mixing enhancement. The interaction process between WJISW and the supersonic mixing layer is researched and the enhanced-mixing mechanism is revealed, employing large eddy simulation (LES) methods. The fine vortex structures of the flow field are well captured and presented, utilizing the numerical schlieren technique. Detailed visualization results indicate that WJISW in a low frequency condition can result in the ‘region action mode’ (RAM) never reported before. The drastic dynamic behaviors including growth, deformation, and distortion in the interaction region can undoubtedly promote the mixing of upper and lower streams. The Reynolds stress distributions along the streamwise x-direction suggest that more intense fluctuations can be achieved with a low frequency WJISW. Moreover, a sharp increase in mixing layer thickness can be realized in the interaction region. The dynamic mode decomposition (DMD) analysis results show that the mixing layer evolution process is dominated by the mode induced by WJISW, which leads to the coexistence of both large- and small-scale structures in the flow field. The entrainment process corresponding to large-scale vortices and the nibbling process corresponding to small-scale vortices can obviously promote mixing enhancement. It is suggested that the present proposed strategy is a good candidate for enhanced-mixing with application to Scramjet and RBCC combustors. Full article

Effective testing of ceramic matrix composites (CMCs) and CMC/coating systems for high temperature, high stress, high velocity and/or severe oxidation/corrosion environments is a critical need in materials/coatings evaluation for extreme environments of hot section parts in jet engine and hypersonic applications. Most current technology can evaluate two or three of the extreme conditions for a given application; however, incorporating as many of the extreme thermo-mechanical-environmental factors is highly advantageous to understand combinatorial effects. A high velocity oxygen fuel (HVOF) burner rig offers an excellent platform to evaluate many of these extreme conditions. In this work, the following three different thermo-mechanical-environmental test conditions using an HVOF rig on SiC-based CMCs are highlighted: (1) fatigue at temperature for >Mach 1 velocity and high temperature compared to typical stagnant air test environment, (2) high temperature hard particle erosion at temperature for ≤Mach 1 conditions and (3) ~Mach 5 near-hypersonic velocity conditions at very high temperature exposure. Full article

Gas turbines are expected to play a key role in the energy production scenario in the future, and the introduction of carbon-free fuels is fundamental for the development of a sustainable energy mix. The development of a reliable numerical model is thus fundamental in order to support the design changes required for the burners. This paper presents the results of a numerical investigation on a turbulent, diffusive, combustion test case, with the purpose of identifying the best compromise between accuracy and computational cost, in the perspective of the model application in real, more complex, geometries. Referring to a test case has two main advantages. First, a rather simple geometry can be considered, still retaining a few peculiar flow features, such as recirculation vortices and shear layers, which are typical of real applications. Second, the experimental setup is much more detailed than in the case of real turbines, allowing a thorough model validation to be performed. In this paper, the Standard 2-equations k-ε model and the Speziale-Sarkar-Gatski Reynolds Stress Model are considered. Moreover, both the FGM combustion model and the detailed chemistry model are used, coupled with two chemical reaction mechanisms, and their results are compared. Finally, a standard and an enhanced near-wall approach are employed to solve the transport equations close to the walls. The results show a good agreement in the temperature distribution at the axial positions corresponding to the experimental measurements. Overall, the standard wall function approach for describing the near-wall flow proved to be more effective at increasingly higher distances from the jet centre. Such differences are related to the formulations employed by the two near-wall approaches, which led to changes in the predicted flow field around the fuel jet. Finally, the adoption of a reaction mechanism describing in detail the species concentration is mandatory whenever the reliable prediction of the NOx formation is of primary importance. The conclusion reached in this paper can be helpful for the development of reliable and cost-effective CFD models of turbine combustors. Full article

Nickel-based superalloys contain various elements which are added in order to make the alloys more resistant to thermal and mechanical stress and to the adverse operating environments in jet engines. In particular, higher combustion temperatures in the gas turbine are important, since they result in higher fuel efficiency and thus in lower CO₂ emissions. In this paper, a semi-quantitative assessment scheme is used to evaluate the relative supply risks associated with elements contained in various Ni-based superalloys: aluminium, titanium, chromium, iron, cobalt, niobium, molybdenum, ruthenium, tantalum, tungsten, and rhenium. Twelve indicators on the elemental level and four aggregation methods are applied in order to obtain the supply risk at the alloy level. The supply risks for the elements rhenium, molybdenum and cobalt are found to be the highest. For three of the aggregation schemes, the spread in supply risk values for the different alloy types (as characterized by chemical composition and the endurance temperature) is generally narrow. The fourth, namely the cost-share’ aggregation scheme, gives rise to a broader distribution of supply risk values. This is mainly due to the introduction of rhenium as a component starting with second-generation single crystal alloys. The resulting higher supply risk appears, however, to be acceptable for jet engine applications due to the higher temperatures these alloys can endure. Full article

Supersonic impinging jet flows always occur when aircrafts start short takeoff and vertical landing from the ground. Supersonic flows with residues produced by chemical reaction of fuel mixture have the potential of reducing aircraft performance and landing ground. The adverse flow conditions such as impinging force, high noise spectrum, and high shear stress always take place. Due to rare data on particle-gas impinging jet flows to date, three-dimensional numerical simulations were carried out to investigate supersonic impinging jet flows of particle-gas two phases in the present studies. A convergent sonic nozzle and a convergent-divergent supersonic nozzle were used to induce supersonic impinging jet flows. Discrete phase model (DPM), where interaction with continuous phase and two-way turbulence coupling model were considered, was used to simulate particle-gas flows. Effects of different particle diameter and Stokes number were investigated. Particle mass loading of 10% were considered for all simulations. Gas and particle velocity contours, wall shear stress, and impinging force on the ground surface were obtained to describe different phenomena inside impinging and wall jet flows of single gas phase and gas-particle two phases. Full article

A combustion facility which includes uniaxial mechanical loading was implemented that enables environmental conditions more akin to jet engine environments compared to conventional static environment tests. Two types of woven SiC/SiC ceramic matrix composites (CMCs), melt-infiltrated (MI) and chemical vapor infiltrated (CVI), were subjected to fatigue loading in the combustion facility and under isothermal furnace conditions. Some CVI test coupons were coated with a multilayer environmental barrier coating (EBC) of mullite + ytterbium monosilicate using slurry infiltration process to demonstrate the performance with a coating. Combustion conditions were applied using a high velocity oxy fuel gun on the front side of the specimen and mechanical loading was applied using a horizontal hydraulic MTS machine. All the specimens considered were subjected to tension-tension fatigue loading at 100 MPa, stress ratio of 0.1 and specimen front-side surface temperature of 1200 ± 20 °C. Nondestructive evaluation (NDE) methods, such as electrical resistance (ER), was used as an in-situ health monitoring technique. Similar fatigue tests were performed in an isothermal furnace for comparison. A much lower fatigue life was observed for the uncoated specimens tested under combustion conditions in comparison to isothermal furnace condition. This difference in fatigue life was attributed to damage associated with added thermal stress due to the thermal gradient and higher rate of oxidative embrittlement due to the presence of high velocity combustion gases in the combustion environment. EBC coating increased the fatigue life in combustion environment. However, EBC coated specimens experienced spallation in the high-velocity flame due to the presence of micro cracks in the coating surface. Fracture surfaces of the failed specimens were investigated under the scanning electron microscope (SEM) to determine the extent of oxidation and damage. Full article

The rapid growth of the aviation sector around the globe has witnessed an overwhelming impact on fossil fuel resources. With the implementation of stricter environmental laws over emissions by conventional jet fuels, growing demand for research on alternative fuels has become imperative. One-hundred percent Synthetic Paraffinic Kerosene (SPK) and Fully Formulated Synthetic Jet Fuel have surfaced as viable alternatives for gas turbine engines due to their similar properties as that of Jet Fuel. This paper presents results from an experimental study performed on a small gas turbine engine, comparing emissions performance and vibrations for conventional Jet A-1 Fuel, thermally stressed 100% SPK and Fully Formulated Synthetic Jet Fuel. Different vibration frequencies, power spectra were observed for different fuels. Gaseous emissions observed were nearly the same, whereas, significant changes in particulates emissions were observed. Full article

Jatropha curcas is a promising renewable feedstock for biodiesel and bio-jet fuel production. To study gene expression in Jatropha in different tissues throughout development and under stress conditions, we examined a total of 11 typical candidate reference genes using real-time quantitative polymerase chain reaction (RT-qPCR) analysis, which is widely used for validating transcript levels in gene expression studies. The expression stability of these candidate reference genes was assessed across a total of 20 samples, including various tissues at vegetative and reproductive stages and under desiccation and cold stress treatments. The results obtained using software qBase^PLUS showed that the top-ranked reference genes differed across the sample subsets. The combination of actin, GAPDH, and EF1α would be appropriate as a reference panel for normalizing gene expression data across samples at different developmental stages; the combination of actin, GAPDH, and TUB5 should be used as a reference panel for normalizing gene expression data across samples under various abiotic stress treatments. With regard to different developmental stages, we recommend the use of actin and TUB8 for normalization at the vegetative stage and GAPDH and EF1α for normalization at the reproductive stage. For abiotic stress treatments, we recommend the use of TUB5 and TUB8 for normalization under desiccation stress and GAPDH and actin for normalization under cold stress. These results are valuable for future research on gene expression during development or under abiotic stress in Jatropha. To our knowledge, this is the first report on the stability of reference genes in Jatropha. Full article

The increasing demand for high energy density fuels and the concern for their safety have propelled research in the field of gelled propellants, where understanding the combustion of single gelled fuel droplets is the first stage to predict the spray combustion characteristics. The experiments utilized single-isolated freely falling gelled unsymmetrical dimethylhydrazine (UDMH) droplets instead of the conventional suspended droplet approach, in order to eliminate the perturbation associated with the suspension mechanism. Morphological transformations of the gelled droplet involved in the combustion processes were monitored by employing a high-speed digital camera, while the effects of ambient pressure and oxygen concentration on burning rate constants were also studied. The experimental results show that four main phenomena (droplet deformation, bubble formation and growth, vapor jetting and luminous jetting flame with “horn” shape) and three distinct phases were identified in the droplet combustion process; the high yield stress and polymer chain structure of polymer gellant are responsible for the appearance of bubbles with almost the same order of magnitude as the droplets. Increasing the ambient pressure can increase the burning rate, postpone the appearance of microexplosions, and weaken microexplosion intensity; while increasing the ambient oxygen concentration can promote the appearance of microexplosions, strengthen microexplosion intensity and increase the burning rate. Full article