| 61. |
- Abou-Taouk, Abdallah, 1982
(författare)
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CFD Modeling of Combustion in Flexi-Fuel Burners at Gas Turbine Conditions
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main source of energy in the nearest future will come from fossil fuels. Increasing demands on environmentally friendly energy conversion implies that pollutants, such as CO and NOx, need to be reduced. This project aims to investigate recent developments in combustion modeling and turbulence modeling in the context of engineering type Computational Fluid Dynamics (CFD) analysis tools, applied to the main problem area of swirl-stabilized flexi-fuel flames. To perform a CFD simulation of reacting flows, kinetic information needs to be provided to the CFD code. Fully detailed kinetic mechanisms are expensive in terms of computer time when coupled with CFD and hence the use of global reaction mechanisms is preferable. Global mechanismsconsist of a reduced number of reactions mostly one-way reactions which are governed by tuned Arrhenius rate expressions. The optimization at each equivalence ratio is here done by comparing results from Perfectly Stirred Reactor (PSR) calculations for both a detailed reference mechanism and the chosen multi-step global reaction mechanisms.Steady-state Reynolds Averaged Navier Stokes (RANS), hybrid Unsteady RANS/Large Eddy Simulation (URANS/LES) and LES turbulence models have been used in the CFD work. This thesis includes the development and derivation of new 3-step global reaction mechanism for methane-air flames, which has been implemented and evaluated in the CFD analysis for two different burner configurations. In papers I and II an atmospheric burner test rig at Lund University (LTH) has been modeled and the results have been compared to high quality experimental data (emission and Particle Image Velocimetry (PIV) data). There is good agreement between the CFD simulations and measurements of emissions, velocity field and flame visualization. The second configuration that has been modeled and compared with experimental data is the Sandia Flame D test case, which is reported in paper III.Keywords: CFD, Chemistry, Combustion, RANS, LES, SAS-SST, PSR, Global reaction mechanism
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| 62. |
- Abou-Taouk, Abdallah, 1982
(författare)
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Optimization of Chemical Kinetic Mechanisms and Numerical Simulations of Industrial Gas Turbine Burners
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing demands on environmentally friendly energy conversion indicate that pollutants need to be reduced. Computational Fluid Dynamics is efficiently used today in turbulent reaction flow simulations to gain a deeper understanding of the combustion process and consequently be able to improve combustor design and reduce emissions. This thesis aims to investigate recent developments in turbulent reacting flow in the context of engineering type numerical analysis tools, applied to the main problem area of swirl-stabilized flexi-fuel flames. Combustion simulations involve a strong coupling between kinetics, transport and turbulence and their interactions. Fully detailed kinetic mechanisms are expensive in terms of computer time when coupled with Computational Fluid Dynamics, and the complexity of combustion chemistry must hence be downsized. Chemistry look-up tables and reduced reaction mechanisms are two well-known methods used today. Compared to chemical look-up tables, the use of a global mechanism involving a limited number of species is of obvious practical interest, specifically when dealing with complex burner geometries in the industrial applications featuring multiple inlets, dilution with burnt gases in recirculation zones and heat loss. The proposed optimization methodology for global schemes is based on the 1D balance equations for species mass fractions and temperature, and also laminar flame speed involved in the reduced mechanisms correctly reproducing the detailed chemistry solution. The optimizations are performed using a set-up of different tools, such as CANTERA, CHEMKIN and modeFRONTIER. A new approach to sub-grid scale modeling is also suggested, which relies on the idea that effects and properties of the spatial filtering, inherent to Large Eddy Simulation solvers, could be directly accounted for when reducing chemistry and molecular transport properties. The optimized global schemes have been implemented in the CFD toolbox Ansys CFX and used in numerical simulations (RANS, URANS/LES and LES turbulence models) of two different partially premixed industrial gas turbine combustors and the test case of Sandia Flame D. The focus has been on grid generation and modelling of the CFD domain. Measured and predicted flow and flame dynamics, averaged temperature and species concentrations compare well for the different set-ups.
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| 63. |
- Abou-Taouk, Abdallah, 1982
(författare)
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Prediction of Gas Turbine Combustor Performance Based on Optimal Reduced Kinetics and Dynamic Mode Decomposition
- 2015
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Ingår i: High-Pressure and High Reynolds Combustion Workshop, March 24-26, 2015 Clean Combustion Research Center King Abdullah University of Science and Technology, Thuwal, Saudi Arabia.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In the gas turbine community the development of combustor technology is currently following the general trend towards fuel-flexibility. Fully detailed mechanisms for these fuels are expensive in terms of computer time when coupled with CFD, and the complexity of combustion chemistry must hence be downsized. The research field hence may be described as CFD modeling of unsteady combustor flows in the context of finite-rate kinetics and the use of such a tool for eigenmode extraction and analysis. The research includes refinement of optimized global multi-step mechanisms for several different fuels as well as improvement and evaluation of the Dynamic Mode Decomposition in practical combustor settings.
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| 64. |
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| 65. |
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| 66. |
- Abrahamsson, Christoffer, 1984
(författare)
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Microstructure and liquid mass transport control in nanocomposite materials
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Some of the biggest problems currently facing the world are closely tied to unsolved technological challenges in the material sciences. Many materials have a porous microstructure that controls their overall properties. In the case of porous materials their properties often relate to how liquids and dissolved substances move (liquid mass transport) through the pores of the material and how these substances interact with the pore walls. Challenges related to such processes can be found in applications related to energy storage, oil well engineering, food, chromatography, and drug release. It is not a trivial matter to design a material synthesis method that reproducibly produces a robust material with the correct pore-structure and surface properties and in the end, the intended function. An added difficulty is that the material should maintain its function over the intended usage period. These generic difficulties summarizes why some technological problems related to porous materials still remains unsolved. The research community is therefore trying to acquire a better understanding of the mechanisms that governs how the synthesis process affects the microstructure and the resultant liquid mass transport properties.The focus of this work has been to investigate the nanoparticle organization in dispersions and in aggregated microporous materials, and how this organization affects the liquid diffusion and permeability through the material. To study these processes several model material synthesis methods, characterization techniques, and theoretical models were developed. Specifically the work investigated how the particle concentration, shape and aggregation conditions affected the formed microstructure. The role of microstructure anisotropy was investigated by aligning plate-shaped particles in magnetic fields during the material synthesis. In addition, the effect of several different additives on the magnetic alignment process was explored. Furthermore, a responsive nanocomposite material was synthesized in which temperature could be used to reversibly adjust the pore size of the material. The findings showed that particle concentration, aggregation conditions, magnetic fields and temperature responsive microgels can be used to control the liquid mass transport through colloidal dispersions and gels. In some cases the experimental results together with simulations were used to derive microstructure and mass transport correlations for different particle aggregation conditions. These correlations are of general application when predicting the pore size and liquid mass transport in aggregated nanoparticle materials.
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| 67. |
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| 68. |
- Abrahamsson, Hans, 1966
(författare)
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On Turbulent Wall Jets
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An experimental investigation of the two- and three-dimensional turbulent wall jets has been conducted. The measurements have been performed using different hot-wire techniques in large scale wall jet facilities, and the measurements were focused on the fully-developed region of the flow. The two-dimensional wall jet is found to be self-preserving, and the streamwise development of the maximum mean velocity and the half-width is independent of the inlet Reynolds number when the viscosity and inlet momentum flux are used as scaling parameters. The near wall region is characterised by a short universal mean velocity profile and a high turbulence level caused by the influence of the outer region. The lateral spreading rate of the three-dimensional wall jet is found to be five times larger than the normal spreading rate, and this is connected to a secondary mean motion in the lateral plane which shows a negative normal mean velocity and a sharp twist of the velocity vector away from the symmetry plane near the wall. The turbulent kinetic energy budget indicates a transport of turbulent energy by advection and turbulent diffusion towards the wall region, where the turbulence level is enhanced and the near wall peak in the streamwise turbulence intensity is attenuated. A similarity analysis of the equations governing the 2DWJ show that the shear stress in the outer region scales with the friction velocity instead of the commonly used maximum mean velocity, and thus the outer region is governed by two velocity scales. The similarity analysis also shows that the growth of the wall jet (i.e. the half-width) is not linear but depends on the local Reynolds number. The velocity profiles in the overlap region are not logarithmic (as commonly believed) but show a power law behaviour. The similarity theory is in good agreement with experimental observations. Pulsed hot-wire anemometry measurements (PHWA) show good agreement with LDA data in the outer region of the 2DWJ, and these techniques give a significantly higher turbulence level (e.g. the shear stress) than the HWA. This difference can be explained by the high local turbulence levels, which are so high that local flow reversals occur. Even larger differences between PHWA and HWA are found in the 3DWJ because the local turbulence levels are higher and negative velocity samples are more frequent in the 3DWJ than the 2DWJ.
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| 69. |
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| 70. |
- Abrahamsson, Oskar, 1986
(författare)
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A Verified Theorem Prover for Higher-Order Logic
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is about mechanically establishing the correctness of computer programs. In particular, we are interested in establishing the correctness of tools used in computer-aided mathematics. We build on tools for proof-producing program synthesis, and verified compilation, and a verified theorem proving kernel. With these, we have produced an interactive theorem prover for higher-order logic, called Candle, that is verified to accept only true theorems. To the best of our knowledge, Candle is the only interactive theorem prover for higher-order logic that has been verified to this degree. Candle and all technology that underpins it is developed using the HOL4 theorem prover. We use proof-producing synthesis and the verified CakeML compiler to obtain a machine code executable for the Candle theorem prover. Because the CakeML compiler is verified to preserve program semantics, we are able to obtain a soundness result about the machine code which implements the Candle theorem prover.
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