| 1. |
- Wadekar, Sandip, 1989
(författare)
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Large-eddy simulation on the effects of fuel injection pressure on gasoline spray characteristics
- 2019
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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. |
- Persson, Urban, 1961-
(författare)
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Realise the Potential! : Cost Effective and Energy Efficient District Heating in European Urban Areas
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Member States of EU27 need to accelerate the integration of energy efficient technology solutions to reach the 20% energy efficiency target set for 2020. At current pace, projections indicate that only half of expected primary energy reductions will be reached. To meet the energy demands of growing populations and a vibrant economy, while simultaneously reducing primary energy supplies, the European continent faces a new kind of challenge never previously encountered. The identification and application of feasible, competitive, and comprehensive solutions to this problem are of highest priority if the remaining gap is to be closed in time. How is this multi-dimensional and complex dilemma to be dissolved? In this work, expanded use of district heating technology is conceived as a possible solution to substantially reduce future primary energy demands in Europe. By extended recovery and utilisation of vast volumes of currently disregarded excess heat from energy and industry sector fuel transformation processes, district heating systems and combined generation of heat and power can improve the general efficiency of the European energy balance. To investigate the possible range of this solution, this thesis introduces a set of methodologies, theoretical concepts, and model tools, by which a plausible future excess heat utilisation potential, by means of district heat deliveries to residential and service sectors, is estimated. At current conditions and compared to current levels, this potential correspond to a threefold expansion possibility for directly feasible district heating systems in European urban areas and a fourfold increase of European excess heat utilisation.
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| 3. |
- Korkmaz, Kadir Burak, 1989
(författare)
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Improved Power Predictions of Ships Using Combined CFD/EFD Methods for the Form Factor
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Performance prediction of a ship is one of the most important tasks during the design phase. Once the design is finalized, the speed attained at a certain power consumption has to be verified with the most accurate prediction as it is specified at the contract of a new ship order and also required by the legal authorities. Considering the current commercial tendencies and the requirements enforced by legal authorities, towing tank testing and the extrapolation methods recommended by the International Towing Tank Conference (ITTC) are used and regarded as a highly accurate power prediction methodology for common cargo vessels. However, some aspects of this methodology have been questioned such as the scale effects on the form factor and its determination method. It is argued in this thesis that if a part of the Experimental Fluid Dynamics (EFD) based measure or the extrapolation procedure causes higher uncertainty than the numerical uncertainty and modelling errors of a Computational Fluid Dynamics (CFD) application, the corresponding part of the performance prediction method can be replaced or supplemented by CFD. In this study, the possibility to improve the power predictions by the introduction of a combined CFD/EFD Method was investigated by replacing the experimental determination of the form factor with double body computations based on the Reynolds-Averaged Navier-Stokes (RANS) equations, i.e. CFD based form factors. As a result of a joint, study where the double body simulations performed with seven different CFD codes, the CFD based form factors compared well with the experimentally determined form factors. Additionally, the standard deviations of the CFD based form factors were similar to the experimental uncertainty of the form factors even though the abundance of unsystematically varied methods and grids. Following the Quality Assurance Procedure proposed by the ITTC, a best practice guideline has been derived for the CFD based form factor determination method by applying systematic variations to the CFD set-ups. After the verification and validation of the CFD based form factor method in model scale, the full scale speed-power-rpm relations between large number of speed trials and full scale predictions were investigated using the CFD based form factors in combination to the ITTC-57 line and the numerical friction lines. It is observed that the usage of CFD based form factors improves the predictions in general and no deterioration in the prediction accuracy is noted within the limits of this study. Therefore, the combination of EFD and CFD is expected to provide immediate improvements to the 1978 ITTC Performance Prediction Method.
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| 4. |
- Okhovat, Reza, 1984
(författare)
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Dynamic Equations foe Spherical and Cylindrical Shells Using Power Series Method
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Shells are commonly used in many branches of engineeringand have therefore beeninvestigated for a number of different types of shells.A shell can be considered as a curved plate having small thickness compared to the other geometrical dimensions as well as to the wavelengths of importance. The most important superiority of shells in comparison to plates is that shell structures can provide high strength and low weight because of their membrane stiffness.Spherical and cylindrical shells appear in manyapplicationsand some dynamic shell theories have thus been developed for these cases.All these theories seem to depend on more or less ad hoc kinematical assumptions and/orother approximations.In this thesis, dynamic equations for an isotropic spherical and an anisotropic cylindrical shell are derived by using a method developed during the last decade for bars, plates, and beams.The main advantage with the method is that it is very systematic and can be developed to any order. It also seems that the resulting structural equations are asymptotically correct to any order.First, dynamic equations are derived for a spherical shell made of a homogeneous, isotropic material.The starting point is a power series expansion of the displacement components in the thickness coordinate relative to the mid-surface of the shell. By using these expansions, the three-dimensional elastodynamic equations yield a set of recursion relations among the expansion functions.Applying the boundary conditions on the surfaces of the spherical shell and eliminating all but the six lowest order expansion functions give the shell equations as a power series in the shell thickness.In principle, the equations can be truncated to any order in the shell thickness, leading to very complicated expressions. Surface differential operators are introduced to decrease the length of the shell equations and tackle this complexity.the displacement field is split into a scalar (radial) part and a vector (tangential) part, and the shell equations are given in terms of the surface operators. After some manipulations, the final four shell equations are obtained in a more compact form which can be presented explicitly.Dynamic equations for an anisotropic cylindrical shell are also derived using the same technique. As a special case a 2D circular ring is considered and the eigenfrequencies are computed and are compared with the exact solution which is obtained by expressing the displacements in terms of Bessel and Neumann functions. Graphite epoxy is considered as an anisotropic material.For all cases, results are compared to exact three-dimensional theory. The computations for eigenfrequencies from the power series approximation are in good correspondence with results from the exact solution.
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| 5. |
- Ottersten, Martin, 1981
(författare)
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Numerical investigation of tonal noise sources from centrifugal fan
- 2020
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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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| 6. |
- Bergman, Martin, 1985
(författare)
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Affective Surface Engineering for Product Design
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Design research, sensation and perception, hard metrology, emotional functions, semantics, surface roughness, product interaction, core values, total appearance… the list of scientific phrases never ends. Yet, what do they mean and how shall we use it when we are communicating with the industry and our end users? Is it possible to link the product experience to process parameters, put a number onto it? When you can measure spoken needs or even better, implied needs, of a product, and relate that data to the production, it is possible do create advanced products with high interaction stimuli. By joining engineering sciences (hard metrology) with design science (soft metrology) correlations between customer’s product experience (emotional functions) and surface properties (technical functions) can be established.The research briefly handles an optimization process where the framework from Kansei Engineering (KE) is used to evaluate the semantics issues primarily regarding materials and functional surfaces of products. The basic idea is that; the stakeholder’s experience shall be observed already at the phase of ideation in the product development process, which then facilitates the project (in regard to the total appearance) later on when a concept reaches the production stage. The results presented in this thesis are carried out through a number of case studies together with the industry. The main result and aim of the research is a developed robust approach that links emotional functions with technical functions, which in the next step facilitates the improvement of the total appearance of a product. Nevertheless, it should be recognized that research is not yet complete. It is an iterative process, which confirms that the loop of the method needs to be complete. The developed method is a toolbox with the fundamental tools and workshops to facilitate the correlation process mentioned above, however the packaging of the final step in the method is not yet complete. The future research outlook will focus on the “independent industry implementation” where the method is used by the industry by guidelines only, thus without researchers support.
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| 7. |
- Finnsgård, Christian, 1974
(författare)
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Assembly processes and materials supply systems design
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lean production has over the last decades emerged as the most important paradigm for production, with the assembly processes in focus, including the objective to minimize non-value-adding work. Thus, much attention has been on the work of the assembly operator. However, little focus has been devoted to how the new requirements from lean production have been transformed into materials supply systems supporting the assembly processes. Hence, the purpose of this thesis is to contribute to increased knowledge about the relation between assembly processes and materials supply systems design.A framework is introduced in this thesis, describing how requirements from the assembly processes are transformed by the materials supply system into the actual materials flow. In the results of this thesis, a model structuring requirements in materials supply and assembly processes is proposed, tested, and subsequently different requirements were identified. Materials exposure is introduced as the transition from the materials supply system to the assembly system. Results show that that the materials exposure has a large impact on assembly workstations performance in terms of space needed, non-value-adding work and ergonomics. Further, an experiment showed that the materials exposure factors with the largest impact on picking time were packaging, angle of packaging and height above floor.Major contributions in this thesis are the identification and structuring of requirements in assembly processes and materials supply systems, and explaining how the materials exposure affects the assembly processes. How this knowledge can be applied in the design of new, or redesign of existing, production systems and materials supply processes in particular, is a managerial contribution.
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| 8. |
- Pandian Muthuramalingam, Vignesh, 1987
(författare)
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MULTICOMPONENT SPRAY-TURBULENCE INTERACTION
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Blended fuels are gaining importance in automotive industry owing to stringent legislations for emissions. It is important to understand the fuel spray formed as a consequence of injecting blended fuels into the engine (for direct injection engines) and also the influence of fuel spray on combustion properties. Fuel sprays are sought to be understood by formulating and modelling the physical processes involved in its formation and testing the predictions obtained from models using computer simulations. Complementing this procedure, experiments are performed under predefined boundary conditions either in single or multi cylinder engines or in constant volume spray chambers when deeper insight into sprays is required. The experiments are used to validate the models and also report any newly observed physical phenomenon which can be then investigated using the models. This work presents the computaional and modelling efforts for multicomponent fuel sprays whose behavior is studied in constant volume combustion vessel. Lagrangian-Eulerian framework is followed where the liquid fuel is modelled using Lagrangian approach and the gas phase is modelled using Eulerian approach. The focus of this work is on Lagrangian liquid phase modelling and it's interactions with the gas phase. The spray modelling is done using VSB2 stochastic blob and bubble (VSB2) model which is developed with the aim of minimising tuning parameters by treating spray and it's submodels as one entity. The VSB2 model also removes overshoot or undershoot in predicted quantities by using relaxation equations based on thermodynamic equilibrium. The methods for modelling secondary breakup, evaporation and momentum transfer of liquid droplets are outlined in this work. Specifically computational method for differential evaporation in multicomponent fuel sprays is discussed. The VSB2 model is validated against experiments performed in constant volume combustion vessels for multicomponent fuel sprays. Differential evaporation was predicted corrrectly by the model within acceptable limits when compared to experiments on component gasoline-diesel fuel blend. Effects of non-ideal vapor liquid equilibrium on multicomponent fuel evaporation of ethanol and iso-octane blend was also studied, and the predictions showed reasonable agreement with experiment. Ethanol was observed to have a strong influence on iso-octane and deviation from ideal behavior was strong for higher ethanol percentage and in these cases ideal vapor liquid equilibrium was seen to predict incorrect results.
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| 9. |
- Etikyala, Sreelekha, 1991
(författare)
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Particulate Formation in Gasoline Direct Injection Engines
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gasoline direct injection (GDI) engines are facing a great challenge because of the need to comply with increasingly stringent emission regulations while improving fuel economy. GDI engines are popularly known for their high fuel efficiency (by the standards of gasoline engines) and low emissions, enabling higher compression ratios and thus increased volumetric efficiency. Unfortunately, GDI engines tend to produce higher particulate number (PN) emissions than conventional port fuel injection (PFI) engines, mainly due to the challenges of in-cylinder liquid fuel injection. Cold starts, transients, and high loads account for a disproportionately high share of all PN emissions from GDI engines over a certification cycle. Understanding the mechanisms of PN formation during these stages is necessary for the further market penetration of GDI under the constraint of tighter emission standards. This knowledge becomes especially important when in future particles with sizes smaller than 10 nm are measured and legislated. This work presents experimental investigation of particulate emissions from a naturally aspirated single cylinder metal gasoline engine operated in a homogeneous configuration. The engine was modified to be capable of operating using DI, PFI, or both simultaneously. PFI was configured with a custom inlet manifold to inject about 50 cm upstream of cylinder head, forming a more homogeneous fuel-air mixture than would otherwise be possible. The experimental campaigns were structured to systematically isolate and study different PN formation mechanisms. Mixing quality was improved ubstantially by using a small amount of upstream injection together with direct injection and could be controlled by varying the mass split between the direct and upstream injectors. It was found that using a small upstream injection when operating in GDI mode could reduce PN emissions by up to a factor of 10 while only modestly increasing fuel consumption. The chemical composition of the fuel could also strongly affect particulate emissions. Therefore, to find alternative ways of reducing PN emissions, experiments were conducted using a gasoline engine with fuel blends containing renewable oxygenates – either 10% (v/v) ethanol (EtOH) or 22% (v/v) ethyl tert-butyl ether (ETBE). It was observed that PN emissions was reduced using oxygenated fuels at low load for both PFI and DI operation, but not at higher loads where PN increased instead. Measurements of solid PN (SPN) emissions revealed that more soot was formed at high load along with an increase in emissions of volatile organic compounds (VOC). PN measurements were conducted using a DMS500 fast particle spectrometer supplied by Cambustion. In addition, solid particulate measurements were performed by passing exhaust samples through a thermodenuder and a catalyst to remove most of the volatile organic compounds (VOCs) from the raw emissions. The results indicated that wall-wetting is the dominant particulate formation mechanism inside the cylinder: fuel-wall interactions with the piston, cylinder walls, and valves during the fuel injection period account for a significant fraction of the PN content of raw exhaust.
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| 10. |
- Matrisciano, Andrea, 1986
(författare)
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Development of an efficient solver for detailed kinetics in reactive flows
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of chemical kinetic mechanisms in CAE tools for reactive flow simulations is of high importance for studying and predicting pollutant formation. However, usage of complex reaction schemes is accompanied by high computational cost in both 1D and 3D-CFD frameworks. The combustion research community has addressed such challenge via two main approaches: 1) tailor made mechanism reduction strategies; 2) pre-tabulation of the chemistry process and look-up during run-time. The present work covers both topics, although much of the methodology development and validation efforts focused on tabulation. In the first eight months of the PhD work, an isomer lumping strategy based on thermodynamic data was developed and applied to a detailed three component reaction mechanism for n-decane, alpha-methylnaphthalene and methyl decanoate comprising 807 species and 7807 reactions. A total of 74 isomer groups were identified within the oxidation of n-decane and methyl-decanoate via analysis of the Gibbs free energy of the isomers. The lumping procedure led to a mechanism of 463 species and 7600 reactions which was compared against the detailed version over several reactor conditions and over a broad range of temperature, pressure and equivalence ratio. In all cases, very good agreement between the predictions obtained using the lumped and the detailed mechanism has been observed with an overall absolute error below 12%. In the second phase of the PhD work, a tabulated chemistry approach was developed, implemented and validated against an on-the-fly chemistry solver across different simulation frameworks. As a first attempt, a flamelet-based tabulation method for soot source terms was coupled to the stochastic reactor model (SRM) and tested against a well stirred reactor-based approach under Diesel engine conditions. The main purpose was to assess and quantify benefits of tabulation within the 0D-SRM framework with respect to soot formation only. Subsequently, a chemical enthalpy (ℎ298) based approach was developed and implemented within the SRM model to predict both combustion and emission formation. This approach was widely validated against the detailed on-the-fly solver solutions under 0D reactor conditions as well as Diesel engine conditions for a wide range of operating points. Good agreement was found between the two solvers and a remarkable speed-up was obtained by means of computational costs of the simulation. As a last step, the same tabulated chemistry solver was coupled to a commercial CFD solver (CONVERGE v. 2.4) via user defined functions and performances were assessed against the built-in on-the fly chemistry solver (SAGE) under Diesel engine sector simulations. The tabulated chemistry solver proved to be within an acceptable level of accuracy for engineering studies and showed a consistent speed-up in comparison to the SAGE solver. Across all the investigated frameworks, the developed tabulated chemistry solver was found to be a valid solution to speed-up simulation time without compromising accuracy of the solution for combustion and emissions predictions for Diesel engine applications. In fact, the much-reduced CPU times allowed the SRM to be included in broader engine development campaigns where multi-objective optimization methods where efficiently used to explore new engine designs.
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