| 1. |
- Bergström, Christer
(author)
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Numerical Modelling of Fuel Sprays
- 1999
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Doctoral thesis (other academic/artistic)abstract
- The way the fuel is introduced into the combustion chamber is one of the most important parameters for the power output and the generation of emissions in the combustion of liquid fuels. The interaction beween the turbulent gas flow field and the liquid fuel droplets, the vaporisation of them and the mixing of the gaseous fuel with the ambient air that are vital parameters in the combustion process. The use of numerical calculations is an important tool to better understand these complex interacting phenomena. This thesis reports on the numerical modelling of fuel sprays in non-reacting cases using an own developed spray module. The spray module uses the stochastic parcel method to represent the spray. The module was made in such manner that it could by coupled with different gas flow solver. Results obtained from four different gas flow solver are presented in the thesis, including the use of two different kinds of turbulence models. In the first part the spray module is coupled with a k-epsilon based 2-D cylindrical gas flow solver. A thorough sensitivity analysis was performed on the spray and gas flow solver parameters, such as grid size dependence and sensitivity to initial values of k-epsilon. The results of the spray module were also compared to results from other spray codes, e.g. the well known KIVA code. In the second part of this thesis the spray was injected into a turbulent and fully developed crossflow studied. The spray module was attached to a LES (Large Eddy Simulation) based flow solvers enabling the study of the complex structures and time dependent phenomena involved in spray in crossflows. It was found that the spray performs an oscillatory motion and that the Strouhal number in the wake was about 0.1. Different spray breakup models were evaluated by comparing with experimental results.
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| 2. |
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| 3. |
- Bergström, John, et al.
(author)
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Estimation of numerical accuracy for the flow field in a draft tube
- 1999
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In: International journal of numerical methods for heat & fluid flow. - : Emerald. - 0961-5539 .- 1758-6585. ; 9:4, s. 472-486
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Journal article (peer-reviewed)abstract
- The potential for overall efficiency improvements of modern hydro power turbines is a few percent. A significant part of the losses occurs in the draft tube. To improve the efficiency by analysing the flow in the draft tube, it is therefore necessary to do this accurately, i.e. one must know how large the iterative and the grid errors are. This was done by comparing three different methods to estimate errors. Four grids (122,976 to 4,592 cells) and two numerical schemes (hybrid differencing and CCCT) were used in the comparison. To assess the iterative error, the convergence history and the final value of the residuals were used. The grid error estimates were based on Richardson extrapolation and least square curve fitting. Using these methods we could, apart from estimate the error, also calculate the apparent order of the numerical schemes. The effects of using double or single precision and changing the under relaxation factors were also investigated. To check the grid error the pressure recovery factor was used. The iterative error based on the pressure recovery factor was very small for all grids (of the order 10-4 percent for the CCCT scheme and 10-10percent for the hybrid scheme). The grid error was about 10 percent for the finest grid and the apparent order of the numerical schemes were 1.6 for CCCT (formally second order) and 1.4 for hybrid differencing (formally first order). The conclusion is that there are several methods available that can be used in practical simulations to estimate numerical errors and that in this particular case, the errors were too large. The methods for estimating the errors also allowed us to compute the necessary grid size for a target value of the grid error. For a target value of 1 percent, the necessary grid size for this case was computed to 2 million cells.
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| 4. |
- Bergström, John, et al.
(author)
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Time-phase averaging for the approximate solution of the flow in a hydraulic turbine
- 1999
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In: Proceedings of ASME/JSME FEDSM'99. - : American Society of Mechanical Engineers.
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Conference paper (peer-reviewed)abstract
- The refurbishment of old hydropower installations and the continuos development of new installations has increased the interest for better design tools to improve their efficiency. Computational fluid dynamics has been used with great success to improve the design of the runner. However, extensive model testing has been necessary to improve the design of the surrounding waterways. Even after testing, some uncertainty has remained concerning the difference between the model scale and the full scale turbine system. The current trend is therefore to include as much as possible of the water conduits with a simultaneous solution of the flow in the turbine runner in an effort to reduce the need for model testing. However, if high numerical accuracy is required the number of mesh points for a complete model of the turbine system has to be at least 10^7. The mesh size together with the need for a time dependent mesh in the runner makes it unlikely that a full simulation with a rotating runner and advanced turbulence modeling will be possible within the next several years, even if the most optimistic estimate of future computer capacity are taken into account. It is therefore of great interest to find new approximations that will make a more refined analysis of the waterways external to the runner possible.In this paper we present a model for the runner that preserves any flow non-uniformity existing at the inlet of the runner in a realistic way through the runner. This has enabled a complete analysis of the interaction of the flow through the penstock, spiral casing and guide vanes with the flow in the draft tube. The mesh requirement and the computational time is considerably reduced compared to a full simulation with a sliding mesh model for the runner. The main drawback with the new model is believed to be that the blade wakes are averaged out of the problem.The model we propose is based on a time-phase averaging technique. The essence of the model is similar to the time averaging technique used by Adamczyk (Adamczyk, 1985), but with different averaging time and different mathematical notation that makes it possible to use the model in a general case, i.e. both for axial and radial machines. A phase function is central to the technique and is introduced for weighting in the averaging procedure. The phase function makes it possible to time average the flow inside a runner. It is constructed with generalised functions and a geometrical description of the suction and pressure side of a runner blade at a reference position. Exact equations for the time-phase averaged variables are derived by a formal time-phase averaging of the governing equations. Some of the terms are accounted for in an approximate way in the present simulation but it is possible to calculate better approximations with a simulation of an isolated runner in a rotating coordinate system. However, even with the crude approximations that we have used the simulation produces realistic results for the particle paths through the runner.
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| 5. |
- Engström, T. Fredrik, et al.
(author)
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Gyroscopic design of swirling flow diffusers
- 1999
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In: Proceedings of ASME/JSME FEDSM'99. - : American Society of Mechanical Engineers.
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Conference paper (peer-reviewed)abstract
- The flow in the draft tube of a hydro power plant is often swirling when the turbine is operating outside its best efficiency point. The swirl gives rise to gyroscopic effects when the flow is forced through the bend in the draft tube. The resulting complex flow field causes losses.The idea in this paper is to investigate the possibility of using a simple model to calculate a new geometry of the draft tube that avoids distortion of the vortex core. Simulations are carried out using the CFD code CFX. A Reynolds stress model, with wall functions, is used to model turbulence.A loss factor is calculated and it was found that the new design draft tube shows approximately the same loss as a non-modified draft tube. The explanation to the somewhat surprising result is that the flow through and after the bend is dominated by the centrifugal effects from streamline curvature. It is therefore concluded that the most important loss mechanism appears to be triggered by streamline curvature.
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| 6. |
- Gebart, Rikard, et al.
(author)
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Squeeze flow rheology in large tools
- 1999
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In: Fifth International Conference on Flow Processes in Composite Materials. - : University of Plymouth Press. - 1870918010 ; , s. 365-372
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Conference paper (peer-reviewed)
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| 7. |
- Lundström, T. Staffan, et al.
(author)
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In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique
- 1999
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In: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 20:1, s. 146-154
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Journal article (peer-reviewed)abstract
- This report discusses the advantages and drawbacks of the multi-cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably- between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.
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| 8. |
- Runnemalm, Anna, et al.
(author)
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Sound intensity measurements of an open organ pipe
- 1997
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In: ISMA 97 conference. - : Institute of Acoustics. - 1901656047
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Conference paper (peer-reviewed)abstract
- What part of an open organ pipe emits most of the sound and what is the spatial sound distribution for the harmonic tones? The sound intensity from a blown open organ pipe, tuned to C4 at 260 Hz, is measured in an anechoic room using a 3D sound intensity probe. The measurements are performed both in the vertical and in the horizontal plane in front of the pipe. The two dimensional sound intensity vector field for different harmonic tones are studied. The study focuses on the active mean intensity field and the results point out the main sources of acoustic power and their contribution to the far field. Pipes made of different material and tooled in different ways are compared. Interference phenomena between sources are also visualised. First it can be noted that the top and the mouth are the main sound radiators compared to the walls. They are of about the same strength for the first four harmonics. For the fifth and sixth harmonics the top is the main sound radiator. For odd numbered harmonic tones the mouth and the top emit sound in phase and for even numbers, out of phase. The material choice and tooling method affect the sound spectra.
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| 9. |
- Göransson, Peter
(author)
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Numerical Modelling of Dynamics of Light Porous Materials
- 1998
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Doctoral thesis (other academic/artistic)abstract
- Porous materials are among the most commonly used materials for noise and vibration reduction in modern transportation vehicles. To design, industrially relevant, weight and cost effective noise and vibration measures, there is a need for general prediction models capable of representing the elasto-acoustic behaviour of such materials. The objective of the present work, is to contribute to the modelling of the inherent fluid-structure interaction phenomena related to porous materials. The modelling approach chosen allows for solution of problems having multiple layers of materials with complicated geometrical shapes and including effects of different boundary conditions along the interfaces to other fluid and solid materials. To solve general three dimensional dynamic problems involving porous materials, a finite element formulation of Biot's equations, describing the fluid-structure interaction in porous materials is proposed. The resulting discrete equation systems, including coupling matrices to other fluid and solid materials, have symmetric matrices and are thus readily implemented into standard finite element software packages. Effects of viscous dissipation, thermal interaction, solid frame disspation and inertial coupling are taken into account. In addition, a finite element formulation of a simplified equivalent fluid model for low stiffness porous materials is proposed.
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| 10. |
- Gullbrand, Jessica
(author)
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Large Eddy Simulation of Turbulent Flows in Combustor Related Geometries
- 1999
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Doctoral thesis (other academic/artistic)abstract
- Numerical simulations using Large Eddy Simulation (LES) are applied to turbulent swirling flow fields. The swirling motion is often introduced into combustors to act as flame holders or enhance the mixing between species. Different turbulence models capture the swirling motion more or less accurately. LES is well suited for understanding details of swirling flows. It resolves all the large scales in the flow field and only the small scales have to be modeled. The small unresolved scales are the Subgrid Scales (SGS) and the model must take into account the interaction between the small scales and their influence on the resolved scales. In order to separate the effects from the SGS models and the numerical scheme, the problem must be well resolved and be of high order. SGS models have been applied, investigated and compared in swirling flow fields. Four SGS models are considered: an implicit, a stress similarity, a dynamic divergence and an exact differential model. The implicit model uses no SGS model. For the stress similarity model, similar behaviour between the resolved and unresolved stresses is assumed. The model parameter in the dynamic divergence model are depending upon both space and time and it is recalculated during the whole simulation. If a particular form of differential filter function is applied, an explicit expression of the SGS stress tensor can be received. This is the exact differential model. In the simulations, the stress similarity model is shown to have the largest effect on the results. Otherwise, the SGS models only show minor effects on both mean velocities and turbulence intensities. A high order Cartesian grid method have been proposed and employed in the simulations. Cartesian grids have features that are very suitable for LES. The grid generation is simple and fast, it does not require a lot of computational storage and the discretized governing equations can be easily extended to higher orders. The drawback of Cartesian grids is that it does not represent complex geometries correctly. The boundary conditions can be misplaced by as much as a cell size and this reduces the order of the solution. A high order wall treatment is proposed to handle the low order wall problem and it is incorporated into the Cartesian grid method. The high order Cartesian grid method is shown to maintain the order of the discretization.
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