A centrifugal compressor is a mechanical machine with purpose to convert kineticenergy from a rotating impeller wheel into the fluid medium by compressingit. One application involves supplying boost air pressure to downsized internalcombustion engines (ICE). This allows, for a given combustion chamber volume,more oxygen to the combustion process, which is key for an elevated energeticefficiency and reducing emissions. However, the centrifugal compressor is limitedat off-design operating conditions by the inception of flow instabilities causingrotating stall and/or surge. These instabilities appear at low flow rates andtypically leads to large vibrations and stress levels. Such instabilities affectthe operating life-time of the machine and are associated with significant noiselevels.The flow in centrifugal compressors is complex due to the presence of a widerange of temporal- and spatial-scales and flow instabilities. The success fromconverting basic technology into a working design depends on understandingthe flow instabilities at off-design operating conditions, which limit significantlythe performance of the compressor. Therefore, the thesis aims to elucidate theunderlying flow mechanisms leading to rotating stall and/or surge by means ofnumerical analysis. Such knowledge may allow improved centrifugal compressordesigns enabling them to operate more silent over a broader operating range.Centrifugal compressors may have complex shapes with a rotating partthat generate turbulent flow separation, shear-layers and wakes. These flowfeatures must be assessed if one wants to understand the interactions among theflow structures at different locations within the compressor. For high fidelityprediction of the complex flow field, the Large Eddy Simulation (LES) approachis employed, which enables capturing relevant flow-driven instabilities underoff-design conditions. The LES solution sensitivity to the grid resolution usedand to the time-step employed has been assessed. Available experimentaldata in terms of compressor performance parameters, time-averaged velocity,pressure data (time-averaged and spectra) were used for validation purposes.LES produces a substantial amount of temporal and spatial flow data. Thisnecessitates efficient post-processing and introduction of statistical averagingin order to extract useful information from the instantaneous chaotic data. Inthe thesis, flow mode decomposition techniques and statistical methods, suchas Fourier spectra analysis, Dynamic Mode Decomposition (DMD), ProperOrthogonal Decomposition (POD) and two-point correlations, respectively, areemployed. These methods allow quantifying large coherent flow structures atvfrequencies of interest. Among the main findings a dominant mode was foundassociated with surge, which is categorized as a filling and emptying processof the system as a whole. The computed LES data suggest that it is causedby substantial periodic oscillation of the impeller blade incidence flow angleleading to complete system flow reversal. The rotating stall flow mode occurringprior to surge and co-existing with it, was also captured. It shows rotating flowfeatures upstream of the impeller as well as in the diffuser. / <p>QC 20171117</p>

In order to reach climate and environmental goals, Sweden is increasing the implementation of intermittent renewable energy sources such as wind and solar power to the electricity grid. The increase of intermittent energy sources is rising power regulation requirement towards hydropower, which increasingly exposes the hydraulic turbines to high loads and fluctuating hydraulic forces. These conditions affect the turbine’s structural and rotor dynamic behavior, leading to fatigue in turbine components. Identifying the parameters that affect the dynamics of the water turbine is an essential part of analyzing and, if possible, avoiding these situations. Furthermore, accurate rotor dynamic models are necessary to design for a robust hydropower unit and improve the estimate of wear on turbine components. Added parameters (added mass, polar moment of inertia, and damping) are hydrodynamic effects occurring due to interaction between structural vibrations and surrounding fluid. Added parameters can modify the turbine’s natural frequencies and consequently its dynamic behavior. Therefore, it is of interest to study and quantify the impact of these parameters on the turbine for accurate rotor dynamic modeling and turbine design. The added parameters have been investigated by conducting experiments on a model Kaplan runner, for which the project has been divided into two consecutive parts. First, experiments were performed in a test rig, in which the runner was excited in a lateral movement to determine added mass and linear damping. Secondly, experiments were performed in a test rig similar to the first, except the runner was excited in a torsional movement to determine added polar moment of inertia and torsional damping. Force and displacement have been measured during both movements, with the runner placed in air and thereafter in quiescent water. The added parameters were quantified by comparing measurements conducted with the runner in air against those conducted in water. By varying the excitation frequency and amplitude, added parameters have been analyzed against excitation frequency, velocity, and acceleration to determine dependent variables. The dimensionless added mass ratio, γma, was investigated within a range of acceleration of 0.07m/s2 to 5.00 m/s2 and in an excitation frequency of 2-9 Hz. Results exhibited a frequency-dependent added mass ratio, leading to a mass addition variation of 1.00-1.49 times the test rig mass with a mean γma of 1.22. Similarly, the dimensionless added polar moment of inertia, γIp, was investigated within a range of angular acceleration between 2.4 rad/s2 to 29.6 rad/s2 and in an excitation frequency range of 2-10 Hz. The mean added polar inertia ratio, γIp, was obtained as 1.09 times the polar moment of inertia of the test rig, corresponding to an increase in polar inertia of about 9%, compared to the total dry polar inertia of the test rig. Results showed that the added polar inertia ratio varied by approximately 1.8% within the studied frequency range. Thus, no frequency dependence could be determined. Due to measurement uncertainties and limitations of the test rigs, added linear damping and torsional damping could not be determined in either of the existing test rigs (lateral and torsional movement).

Many industries have entered a new global phase which takes the environment in mind. The gas turbine industry is no exception, where the utilization of green fuels is the future to spare the environment from carbon dioxide and NOx emissions. Hydrogen has been identified as a fuel which can fulfil the global requirements set by governments worldwide. Combustion instabilities are not inevitable during gas turbine operations, especially when using a highly reactive and diffusive fuel as hydrogen. These thermoacoustics instabilities can damage mechanical components and have economic consequences in terms of maintenance and reparation. Understanding these thermoacoustic instabilities in gas turbine burners is of great interest. COMSOL Multiphysics offers a robust acoustic module compared to other available acoustic simulation programs. In this thesis, an Acoustic finite element model was built representing an atmospheric combustion rig (ACR), used to test the burners performance and NOx emissions. Complementary computational fluid dynamics (CFD) simulations were performed for 100 % hydrogen as fuel by using the Reynolds average Navier-Stokes (RANS) lag EB k - epsilon turbulence model. Necessary data was successfully imported to the Acoustic finite element model. Different techniques of building the mesh were used in COMSOL Multiphysics and NX. Similar results were obtained, proving that both mesh tools work well in acoustic simulations. Two different ways of solving the eigenvalue problem in acoustics were implemented, the classic Helmholtz equation and Linearized Navier-Stokes equations, both in the frequency domain. The Helmholtz equation proved to be efficient and detected multiple modes in the frequency range of interest. Critical modes which lived in the burner and the combustion chamber were identified. Defining a hard and soft wall boundary condition at the inlets and outlet of the atmospheric combustion rig gave similar eigenfrequencies when comparing the two boundary conditions. The soft wall boundary condition was defined with a characteristic impedance, giving a high uncertainty whether the results were trustworthy or not. A boundary condition study revealed that the boundary condition at the outlet was valid for modes living in the burner and combustion chamber. Solving the eigenvalue problem with the Linearized Navier-Stokes equations proved to be computationally demanding compared to the Helmholtz equation. Similar modes shapes were found at higher frequencies, but pressure perturbations were observed in the region where the turbulence was dominant. A prestudy for a stability analysis was established, where the ACR and the flame was represented as a generic model. Implementing a Flame Transfer Function (FTF), more specifically a linear n - tau model, showed that the time delay tau is most sensible for a parametric change and hence needs to be chosen cautiously

Hydro power plants are an essential part of the infrastructure in Sweden as they stand for a large amount of the produced electricity and are used to regulate supply and demand on the electricity grid. Other renewable energy sources, such as wind and solar power, have become more popular as they contribute to a fossil free society. However, wind and solar power are intermittent energy sources causing the demand for regulating power on the grid to increase. Hydro power turbines are designed to operate at a certain design point with a specific flow rate. The plants are operated away from the design point when used to regulate the supply and demand of electricity. This can cause a specific flow phenomenon to arise in the draft tube at part load conditions called a Rotating Vortex Rope (RVR) which causes dangerous pressure fluctuation able to damage blades and bearings. A solution to mitigate a RVR is to inject pulsating momentum into the draft tube by using an actuator operating at a certain frequency. A literature study was conducted and three techniques were numerically simulated using ANSYS Workbench 19.0 R3; a fluidic oscillator, a piston actuator and a synthetic jet actuator. A dynamic mesh was used to simulate the movement of the piston actuator and diaphragm of the synthetic actuator whilst the mesh of the fluidic oscillator was stationary. The relative errors of the three numerical models were all below 3 %. All devices showed promising results and could potentially be used to mitigate a RVR because they all have the ability to produce high energy jets. The fluidic oscillator had an external supply of water, whereas the other two did not, which means that it could inject the largest mass flow. The piston actuator required a driving motor to move the piston. The diaphragm of the synthetic jet actuator was moved by a Piezoelectric element. Advantages of the fluidic oscillator are that it has no moving parts, in contrary to the two other devices, it can directly be connected to the penstock or draft tube to obtain the required water supply and it is easy to install. It will most likely also be smaller compared to the other two for the same mass flow rate. It does however not generate a pulsating jet, but rather an oscillating jet. The other two devices generate pulsating jets, but have problems with low pressure areas during the intake stroke which can cause cavitation problems. These areas cause the formation of vortex rings close to the outlet. Simulations showed that a coned piston together with a coned cylinder outlet could decrease losses by almost 16 % compared to a normal piston and cylinder. It also decreased the risk for cavitation and the required force to move the piston. Otherwise, a shorter stroke length for a constant cylinder diameter or a longer stroke length for a constant volume displacement also decreased the risk for cavitation and required force. The gasket between the piston and cylinder is a potential risk for leakage. A solution to avoid critical low pressure areas is to install an auxiliary fluid inlet or valve which opens at a certain pressure for the piston actuator as well as the synthetic jet actuator. This will also allow larger mass flow rates and a higher injected momentum. Both devices are more complicated to install and require likely more maintenance compared to the fluidic oscillator. However, there exist many possible design options for the piston actuator. The design of the synthetic jet is more limited because of the diaphragm. The amplitude of the diaphragm also has a direct effect on the pressure levels. The losses increased proportional to the mass flow to the power of three which suggests that it is better to install many small actuators instead of a few large ones.

With increasing demands for lowering emissions from diesel engines, bio fuel has been introduced to the fuel mixture. This fuel is based on vegetable oil with a much smaller carbon footprint than fossil fuel. The chemical composition of bio fuel has lead to deposits forming inside the fuel injector in diesel engines, these deposits are usually denoted as Internal Diesel Injector Deposits (IDID). At Scania CV AB an injector test rig is designed with the goal of creating and investigating IDID. This project has made a theoretical investigation of how IDID are formed and how this affects the mechanics inside the injector. It has also analysed injector components from a worst case scenario perspective in order to find a testing method for creating IDID in the test rig. By analysing performance changes from a build-up perspective, where IDID decreases the tolerances inside the injector, as well as friction, formed when deposits cause injector mechanics to stick together, it has been found that injector performance does hardly change from build-up and that performance changes only occur when friction is introduced. From the injector component analysis it is found that the limiting factors in rig testing come from fuel system components rather than the injector itself. This is the base for a rig running test method presented.

Every computational fluid dynamics engineer deals with a never ending story – limitedcomputer resources. In computational fluid dynamics there is practically never enoughcomputer power. Limited computer resources lead to long calculation times which result inhigh costs and one of the main reasons is that large quantity of elements are needed in acomputational mesh in order to obtain accurate and reliable results.Although there exist established meshing approaches for the Siemens 4th generation DLEburner, mesh dependency has not been fully evaluated yet. The main goal of this work istherefore to better optimize accuracy versus cell count for this particular burner intended forsimulation of air/gas mixing where eddy-viscosity based turbulence models are employed.Ansys Fluent solver was used for all simulations in this work. For time effectivisationpurposes a 30° sector model of the burner was created and validated for the meshconvergence study. No steady state solutions were found for this case therefore timedependent simulations with time statistics sampling were employed. The mesh convergencestudy has shown that a coarse computational mesh in air casing of the burner does not affectflow conditions downstream where air/gas mixing process is taking place and that a majorpart of the combustion chamber is highly mesh independent. A large reduction of cell count inthose two parts is therefore allowed. On the other hand the RPL (Rich Pilot Lean) and thepilot burner turned out to be highly mesh density dependent. The RPL and the Pilot burnerneed to have significantly more refined mesh as it has been used so far with the establishedmeshing approaches. The mesh optimization has finally shown that at least as accurate resultsof air/gas mixing results may be obtained with 3x smaller cell count. Furthermore it has beenshown that significantly more accurate results may be obtained with 60% smaller cell count aswith the established meshing approaches.A short mesh study of the Siemens 3rd generation DLE burner in ignition stage of operationwas also performed in this work. This brief study has shown that the established meshingapproach for air/gas mixing purposes is sufficient for use with Ansys Fluent solver whilecertain differences were discovered when comparing the results obtained with Ansys Fluentagainst those obtained with Ansys CFX solver. Differences between Fluent and CFX solverwere briefly discussed in this work as identical simulation set up in both solvers producedslightly different results. Furthermore the obtained results suggest that Fluent solver is lessmesh dependent as CFX solver for this particular case.

The purpose of this work is to test the feasibility of using particle image velocimetry(PIV) for measurements of flow through a porous medium, more specifically in this casea rotating bed reactor S3. The results from experiments preformed can then be usedto validate and improve computational fluid dynamics models. The report presentsdifferent possible combinations of solids and fluids for refractive index matchingand tests some velocity limits of the optical equipment used. PIV appears to be apromising method for measuring flow through a porous medium. The theoreticallimit due to motion blur is an angular velocity of around 3800 RPM, and the actualtested lower bound for this limit is 453 RPM.

There is a strong competition among automotive manufacturers to reduce the radiated noise levels. One important source is the engine exhaust where the main noise control strategy is by using efficient mufflers. Stricter vehicle noise regulations combined with various exhaust gas cleaning devices, removing space for traditional mufflers, are also creating new challenges. Thus, it is crucial to have efficient models and tools to design vehicle exhaust systems. In addition the need to reduce CO2 emissions puts requirements on the losses and pressure drop in exhaust systems. In this thesis a number of problems relevant for the design of modern exhaust systems for vehicles are addressed. First the modelling of perforated mufflers is investigated. Fifteen different configurations were modeled and compared to measurements using 1D models. The limitations of using 1D models due to 3D or non-plane wave effects are investigated. It is found that for all the cases investigated the 1D model is valid at least up to half the plane wave region. But with flow present, i.e., as in the real application the 3D effects are much less important and then normally a 1D model works well. Another interesting area that is investigated is the acoustic performance of after treatment devices. Diesel engines produce harmful exhaust emissions and high exhaust noise levels. One way of mitigating both exhaust emissions and noise is via the use of after treatment devices such as Catalytic Converters (CC), Selective Catalytic Reducers (SCR), Diesel Oxidation Catalysts (DOC), and Diesel Particulate Filters (DPF). The objective of this investigation is to characterize and simulate the acoustic performance of different types of filters so that maximum benefit can be achieved. A number of after treatment device configurations for trucks were selected and investigated. Finally, addressing the muffler design constraints, i.e., concerning space and pressure drop, a muffler optimization problem is formulated achieving the optimum muffler design through calculating the acoustic properties using an optimization technique. A shape optimization approach is presented for different muffler configurations, and the acoustic results are compared against optimum designs from the literature obtained using different optimization methods as well as design targets. / <p>QC 20121016</p>

QC 20120613

Prediction of vehicle disc brake roughness noise is a non-trivial challenge. In fact, neither the source mechanisms, nor the transfer paths are so far well understood. Traditionally, disc brake noise problems are studied as part of the friction-induced noise field, where the source is considered to be a more or less local phenomenon related to the brake disc and brake pad. However, for the roughness noise of interest here this viewpoint is not adequate when attempting to solve the interior noise problem since the transfer of vibro-energy from the brake into the vehicle body is a crucial aspect and plays an important role in the understanding and solution to the problem. The vibroacoustic energy transfer associated with the brake roughness noise is a problem where geometrical complexity and material combinations, including rubber bushings, pose an intricate modelling problem. Additionally, system altering effects from moving parts and loadings are important, e.g. due to the steering or brake systems. In addition, the source mechanisms themselves must also be understood to be able to solve the problem. The current work constitutes a combined experimental and theoretical investigation, aiming at an increased understanding of the source, the transfer paths and how they are a affected by change in the operational state. The experimental study of the vehicle disc brake roughness noise, is based on measurements conducted in a laboratory using a complete passenger car. It is found that the interior noise is a structural-borne broadband noise event well correlated to vehicle speed and brake pressure. The results suggest that the friction source may be divided into vibrations created in the sliding direction and vibrations created normal to the contact plane, where the sliding direction levels appear to be proportional to brake pressure according to Coulomb’s friction law; the vibration level in the normal direction of the contact plane on the other hand has behaviour proportional to Hertz contact theory. The measurements also indicate that the brake force created carried by the suspension system when braking will also alter the vibro-acoustic response of the system. To facilitate the theoretical simulations, an approach for modelling of the suspension system is developed. The vibro-acoustic transfer path model developed is using a modal based on the Craig-Bampton method where a restriction on the coupling modes is suggested. The approach suggested uses undeformed coupling interfaces, to couple structures of fundamentally different stiffness such as may be the case in a vehicle suspension system where for instance rubber bushings are combined with steel linking arms. The approach show great potential inreducing computational cost compared to the classical Craig-Bampton method. / QC 20110913