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Sökning: hsv:(TEKNIK OCH TEKNOLOGIER) hsv:(Maskinteknik) hsv:(Rymd och flygteknik) > Licentiatavhandling

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1.
  • Wadekar, Sandip, 1989 (författare)
  • Large-eddy simulation on the effects of fuel injection pressure on gasoline spray characteristics
  • 2019
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Increasing the injection pressure in gasoline direct injection engines has a substantial potential to reduce emissions while maintaining high efficiency in spark ignition engines. Present gasoline injectors operate at pressures of 20 to 30 MPa. However, the use of higher-pressure fuel injection (40 to 60 MPa or more) could potentially reduce emissions and increase fuel efficiency. To fully exploit the capabilities of high-pressure fuel injection technology, a fundamental understanding of gasoline spray characteristics and behavior at such high injection pressures is vital. Such an understanding could also be used to further model development and facilitate the integration of advanced injection systems into future gasoline engines. This work presents numerical simulation studies on gasoline sprays formed at fuel injection pressures between 40 and 150 MPa. Three nozzle hole shapes (divergent, convergent, and straight) with different configurations (6 or 10 holes) were considered in the simulation to determine how a nozzle geometry affects spray formation. The numerical calculations were performed in a constant volume spray chamber under non-vaporizing conditions to best match the experimental setup. The gas flow was modeled using a large-eddy simulation (LES) approach, while a standard Lagrangian model was utilized to describe the liquid fuel spray. Spray atomization was modeled using the Kelvin Helmholtz –Rayleigh Taylor (KH-RT) atomization model, with the droplet size distribution being assumed to follow a Rosin-Rammler distribution function. Simulation results for the spray liquid penetration length are validated with experimental findings under different fuel injection pressures. Afterwards, an arithmetic mean droplet diameter (D10) and a Sauter mean droplet diameter (D32) as a function of pressure are compared against the measured droplet diameters. Simulated drop size distributions are presented and compared with measured droplet sizes. The results indicate that high fuel injection pressures increase the liquid penetration length and significantly reduce droplet sizes, and that nozzle shape significantly affects spray characteristics and spray formation. In addition, raising the injection pressure from 40 to 150 MPa with a divergent nozzle was predicted to reduce the SMD from 13.4 to 7.5 μm while increasing the probability of observing droplet diameters of 5-10 μm from 40% to 72%. Similar results were obtained for the other nozzle shapes.
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2.
  • Ottersten, Martin, 1981 (författare)
  • Numerical investigation of tonal noise sources from centrifugal fan
  • 2020
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Heating, ventilating, and air conditioning systems (HVAC) are today an important part of many people's life. They provide a sufficient amount of airflow with the correct temperature, quality, and humidity. The negative side is the noise it produces. Many improvements have been made in building development to reduce noise from the environment. When so, the noise from the HVAC system becomes clearer. The dominant tonal noise in an HVAC system is produced by the fan. In this work tonal noises produced by a centrifugal fan is investigated to be able to understand the generation mechanism and identify their sources. The approach is to use the hybrid computational aeroacoustics  (CAA) method, that couples a computational fluid dynamics (CFD) method with the Ffowcs Williams and Hawkings (FW-H) acoustic analogy. Recirculating flows, which are responsible for reducing the fan efficiency and increasing the noise generation, are observed between the shroud and the blade trailing edges. It is found that the recirculating flows are associated with the gap between the shroud and the inlet duct. The recirculating flow causes large modeled turbulence kinetic energy (TKE). The TKE is unevenly distributed among the blades due to the unsteady recirculating flow. Moreover, the position of the largest TKE periodically varies among the blades. The period corresponds to approximately 4 times the fan rotation period, it was also found in acoustic measurements. Different pressure distributions among the blades are found and ascribed to the turbulence initializing from the inlet gap. The turbulence develops along the shroud wall and interacts with the blades at their leading edges. The interaction renders uneven surface pressure distributions among the blades as well as significant peak differences. As the distances to the inlet gap and the shroud increases, the difference of the pressure distributions among the blades decays. The wall-pressure fluctuations indicates that the locations of the tonal noise sources agree with the locations of the uneven surface pressure distributions and the significant pressure peaks, which are near the blade leading edges.
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3.
  • Capitao Patrao, Alexandre, 1988 (författare)
  • Simulation and Analysis of a Novel Open Rotor Propeller - the Boxprop
  • 2016
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Economic factors and environmental awareness is driving the evolution of aircraft engines towards increasingly higher efficiencies, reaching for lower fuel consumption and lower emissions. The Counter-Rotating Open Rotor (CROR) is actively being researched around the world, promising a significantly increased propulsion efficiency relative to existing turbofans by employing two, counter-rotating propeller blade rows, thereby increasing the bypass ratio of the engine. Historically, these engines have been plagued by very high noise levels, mainly due to the impingement of the front rotor tip vortices on the rear rotor. In modern designs, the noise levels have been significantly decreased by clipping the rear, counter-rotating propeller. Unfortunately, this comes at a cost of decreased efficiency.An alternative, potential solution lies with the Boxprop, which was invented by Richard Avellán and Anders Lundbladh. The Boxprop consist of blade pairs joined at the tip, and are conceptually similar to box wings. It is hypothesized that the Boxprop can eliminate the tip vortex found in conventional blades, consequently increasing the efficiency of the blades, and reducing their acoustic signature.The present work highlights advances done in the research surrounding the Boxprop. A validation of the deployed CFD methodology is presented, in which numerical and experimental results compare favourably. Performance results for a Boxprop (GP-X-701) designed for cruise conditions are presented and compared with a generic conventional propeller (GP-S-609). It is shown that the present Boxprop cruise design can reach the required thrust for replacing the front rotor of a modern CROR, but with increased swirl relative to the analyzed conventional propeller. This is mainly due to the effect of the blade passage unloading one of the Boxprop blade halves near the tip, forcing the blade to be more highly loaded closer to the hub. The swirl generated by the Boxprop could be partially recovered if it is used together with a rear counter-rotating propeller. A Wake Analysis Method (WAM) is presented in this work and is used to quantify the power flows inherent to the flow features of the propeller wake. The power flows can be characterized as propulsively beneficial, recoverable, or pure losses. It has the ability to distinguish the kinetic energies of the tip vortices, wakes, and other disturbances from the flow field. The Wake Analysis Method was applied on the two propellers mentioned earlier, and confirmed that the Boxprop produces 50\% more swirl than the conventional propeller. Additionally, the method very clearly shows the lack of tip vortex on the Boxprop, and the presence of it in the flow field of the conventional propeller.
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4.
  • Jonsson, Isak, 1990 (författare)
  • Experimental Aerothermal Study on Internal Jet Engine Structures
  • 2020
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • For commercial aviation, one potential gain in efficiency can be found in the jet engine auxiliary modules, such as internal jet engine components. These components have historically largely been overlooked, prioritising units such as turbines or compressors. The publically available information for these auxiliary units is therefore relatively sparse even though they can enable substantial weight reduction and novel synergistic integration. The continuously increased fidelity of modern numerical tools poses a dilemma to the experimentalist. Higher accuracy and resolution are sought, but the accuracy of the experimentalist tools has stagnated. This thesis summarises instrumentation implemented methods in the Turbine Rear Structure(TRS). For the multi-hole probe and heat transfer measurement via IR-thermography a comprehensive uncertainty analysis and error mitigations are presented. The work presents a relatively high accuracy of 4% to 6% for the performed heat transfer studies on the outlet guide vane in the TRS. The presented implementation of the multi-hole probe in the TRS provides up to twice as high accuracy compared to conventional installation. Both approaches are general with few geometrical limitations and can be implemented on studies with similar ambient conditions. Two different Reynolds numbers, several flow-coefficients and three different surface roughness numbers have been investigated and novel results regarding transition location, streamline, heat transfer and loss distribution are presented in the attached papers.
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5.
  • Nilsson, Stefan, 1985 (författare)
  • Advanced Fluid-Structure Interaction Modelling and Simulation for Aerospace Applications
  • 2022
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Numerical fluid-structure interaction (FSI) methods for the prediction of aeroelastic phenomena are important within aerospace. The continuous development of computer technologies has enabled the use of more advanced FSI methods. The use of advanced methods has the potential to provide more accurate predictions. It also enables simulation of applications for which engineers traditionally have relied upon wind tunnel testing and flight testing, and still do to a large extent. Hence, the use of more advanced FSI methods would limit the need for wind tunnel testing and flight testing, and in extension reduce the lead time and cost of aircraft development. High Reynolds number flows, involving separated flow, are very challenging to simulate. Hybrid Reynolds-averaged Navier-Stokes (RANS)-large-eddy simulation (LES) techniques provide the possibility to simulate such flows for industrial purposes. Hybrid RANS-LES methods are employed in this thesis for two applications which require turbulence-resolving techniques. First, the effects of elastic walls on the aeroacoustics in transonic cavity flow are investigated. The prediction of structural vibrations is also important since vibrations may endanger the structural integrity, additionally, vibrations may negatively affect other apparatuses. The features of cavity flow appear in weapon bays and landing gear bays in an aircraft. In a deep cavity, the flow constitutes of broadband and tonal noise, referred to as Rossiter modes. The cavity structure is simulated with a modal-based approach and with a non-modal approach where the equation of motion is solved for all degrees-of-freedom of a reduced order finite element model. The results evince that the aeroacoustic field is altered by the elastic walls. For the investigated case, the energy of the 4th Rossiter mode is depleted and a strong tone is induced at a frequency below the 4th Rossiter mode, which is absent in the rigid cavity; these observations are made with both the structural simulation methods. However, with the non-modal approach, a second strong tone is induced at a frequency above the 4th Rossiter frequency. The second investigated application is the aeroelastic prediction of a wing at Mach numbers ranging from subsonic to supersonic speeds. The viscous effects become significant at transonic speeds and may provoke shock induced flow separation. It is shown that the viscous effects play an important role under such circumstances and that both static and dynamic structural responses differ significantly depending on whether hybrid RANS-LES or unsteady RANS is employed for the flow simulation.
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6.
  • Samuelsson, Sebastian, 1987 (författare)
  • Challenges in aero engine performance modeling
  • 2016
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • There is a continuous drive for ever more efficient aero engines due to environmental as well as economical concerns. As the technology of conventional turbofan engines matures, there is a need for new aero engine concepts as well as incremental improvement of existing technologies. In order to improve existing turbofan architectures there is a trend towards integrating the design of the different components in the whole engine system. This creates new challenges both within engine manufacturing companies and between overall equipment manufactures (OEMs) and their suppliers. Methods need to be developed where different component requirements can be balanced against each other for the best performance of the system as a whole. Furthermore, there is a need for multidisciplinary design and optimization, coupling simulations involving several different computational disciplines. In this thesis, a method for consistent conceptual design is presented. In consistent design, the outcomes of the conceptual design are used to iteratively update the assumptions made in the initial thermodynamic cycle calculations until they are consistent. This enables the designer to balance different components against each other. In addition, a first coupling study of a turbine rear structure and whole engine performance is made, indicating the necessity of coupled simulations. Some considerations regarding modeling of engines at conditions far off-design are made. This is needed because some dimensioning mechanical load cases occur at these operating points. Finally, non-hierarchical analytical target cascading is introduced as a method that can be used for coupled optimization during the remainder of this research project.
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7.
  • Siggeirsson, Elias, 1989 (författare)
  • Integrated Duct Aerodynamics
  • 2018
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Air traffic is rapidly increasing, with no sign of slowing down, resulting in increased green-house gases emission. With emission criteria becoming stricter every year, the aircraft design is subjected to continuous improvements. The development of more efficient aircraft engines plays a central role in the strive towards lower emission levels. The engine is built up from different modules that are normally designed in isolation. However, as high-performance computational resources are becoming more accessible, a move towards a more integrated design can be considered. The integrated design lowers the need for modeling the interaction effects between modules, and thus produces a more realistic flow representation. To take further advantages of the increasing computational resources, higher fidelity models such as the Delayed Detached Eddy Simulation (DDES) model should be considered for future simulations. The steps towards applying those models for industrial relevant configurations is the subject of this thesis. An experimental test rig, representing the integrated intermediate compressor duct (ICD), is simulated using the Chalmers-developed code G3D::Flow. The ICD has not been studied to the same extent as the surrounding components but has great potential as a better design can lead to shorter and lighter engines. The simulations are performed using the Spalart-Allmaras (SA) one-equation turbulence model, which has been implemented as part of the work presented in this thesis and verified for simple test cases. The SA model was chosen as it is easily altered to a DDES model and has proven to be efficient for turbomachinery applications. Furthermore, to give confidence in the accuracy of the G3D:Flow solver, the results from simulating the ICD are compared to results obtained using the commercial code CFX. The ICD simulations are performed at two different off-design conditions, where different amount of mass-flow is extracted through a bleed-pipe upstream of the duct. In the two cases, 10% and 40% of the inlet mass-flow is extracted through the bleed pipe. The results from the two solvers agree well for the 10% bleed case with significant differences in the results obtained for the 40% bleed case. To further ensure the capabilities of G3D::Flow and to serve as a benchmark case for future unsteady simulations, steady state results from G3D::Flow were compared to experimental data. The simulated results compare well to the experimental data for the lower bleed fraction whereas there are strong pressure fluctuations present in the higher bleed fraction. Those effects where suspected to be caused by the short bleed pipe, affecting the boundary condition resulting in difficulties to get a converged solution. As a step towards analysing the ICD using the DDES model, a single blade module was simulated. This work was conducted to analyse the performance of the DDES model on a smaller scale, where the transition location between the RANS and LES modes was of great interest. A modification to the original DDES model was suggested by literature, resulting in improved performance.
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8.
  • Thulin, Oskar, 1987 (författare)
  • On Exergy and Aero Engine Applications
  • 2016
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Aero engine performance analysis is highly multidimensional using various measures of component performance such as turbomachinery and mechanical efficiencies, and pressure loss coefficients. Using conventional performance analysis, relying on only the first law of thermodynamics, it is possible to understand how the performance parameters affect the component performance, but not how the component performance relates to the system performance. A comprehensive framework has been detailed to analyze an aero engine in one common currency by complementing the analysis with the second law of thermodynamics. As it yields a measure of the lost work potential in every component it is used to relate the component performance to the system performance. The theory includes a more detailed layout of all the terms that apply to a propulsion unit than presented before and is here adopted to real gases to be used in state of the art performance codes. The theory is also extended upon by presenting the installed rational efficiency, a true measure of the propulsion subsystem performance, including the installation effects of the propulsion subsystem as it adds weight and drag that needs to be compensated for in the performance assessment.The exergy methodology is applied to a modern direct-drive two-spool turbofan, chosen for its dominating market share in modern commercial aviation. The loss sources during an aircraft mission are then assessed and yield the major contributors in the entropy generated during combustion, the thermal energy leaving the nozzle and the exhaust nozzle kinetic energy that is not contributing to the thrust. Radical technology that can be utilized to address each specific loss are thereafter detailed. This includes intercooled and recuperated cycles, reheated cycles, bottoming Rankine cycles, pulse detonation combustion, piston topped composite cycles, nutating disc combustion, and open rotor and other ultra high bypass architectures.
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9.
  • Dadfar, Reza, 1978- (författare)
  • Active Control and Reduced-Order Modeling of Transition in Shear Flows
  • 2013
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In this thesis direct numerical simulation is used to investigate the possibilityto delay the transition from laminar to turbulent in boundary layer flows.Furthermore, modal analysis is used to reveal the coherent structures in highdimensional dynamical systems arising in the flow problems.Among different transition scenarios, the classical transition scenario isanalysed. In this scenario, the laminar-turbulent transition occurs when Tollmien-Schlichting waves are triggered inside the boundary layer and grow exponentiallyas they move downstream in the domain. The aim is to attenuate the amplitudeof these waves using active control strategy based on a row of spatiallylocalised sensors and actuators distributed near the wall inside the boundarylayer. To avoid the high dimensional system arises from discretisation of theNavier Stokes equation, a reduced order model (ROM) based on EigensystemRealisation Algorithm (ERA) is obtained and a linear controller is designed.A plasma actuator is modelled and implemented as an external forcing on theflow. To account for the limitation of the plasma actuators and to further reducethe complexity of the controller several control strategies are examinedand compared. The outcomes reveal successful performance in mitigating theenergy of the disturbances inside the boundary layer.To extract coherent features of the wind turbine wakes, modal decompositiontechnique is employed where a large scale dynamical system is reduced toa fewer number of degrees of freedom. Two decomposition techniques are employed:proper orthogonal decomposition and dynamic mode decomposition.In the former procedure, the flow is decomposed into a set of uncorrelated structureswhich are rank according to their energy. In the latter, the eigenvaluesand eigenvectors of the underlying approximate linear operator is computedwhere each mode is associated with a specific frequency and growth rate. Theresults revealed the structures which are dynamically significant to the onsetof instability in the wind turbine wakes.
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10.
  • Martinsson Bonde, Julian, 1992 (författare)
  • Novel, yet similar: A similarity-assisted product family design approach for structural aero-engine components
  • 2023
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The aviation industry is in a state of transformation. The climate crisis has amplified the need to innovate, and consequently manufacturers in the aviation industry need to investigate new and more sustainable design concepts. This is challenging, because there is no obvious replacement for kerosene-fueled aero-engines, though there are multiple technologies in development that may potentially take its place. Examples of such technologies include electric or hybrid-electric propulsion, or combustion engines fueled by hydrogen or synthetic sustainable aviation fuels. This increases the challenge for manufacturers, who must deal with high technological uncertainty. At the same time, manufacturers need to assert that the cost of realization is feasible for new aero-engine component designs, while also fulfilling the requirements for safety and performance. There is therefore a need for methods and tools that will assist designers in making fast and efficient design evaluations, to enable the exploration of large design spaces at reduced costs and lead-times. To make design space exploration more efficient, a similarity-assisted design space exploration method is proposed. This method provides increased trustworthiness in design space exploration results, while also highlighting opportunities for reuse of knowledge and other assets from legacy designs. Additionally, a software tool for automatically generating aero-engine structural components has been developed. This software enriches all generated geometries with information used to facilitate automated manufacturability analysis, as well as evaluation of structural performance. By utilizing the automated geometry generation tool in conjunction with the proposed design space exploration method, designers can quickly and efficiently evaluate the manufacturability and structural performance of novel concepts.
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