| 1. |
- Ansari, Danish, et al.
(författare)
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A gyroid TPMS heat sink for electronic cooling
- 2024
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Ingår i: Energy Conversion and Management. - : Elsevier BV. - 0196-8904 .- 1879-2227. ; 319
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Tidskriftsartikel (refereegranskat)abstract
- The trend toward thinner and lighter electronic devices necessitates developing advanced thermal management solutions to address modern microprocessors’ escalating heat dissipation demands. This study introduces an advanced thermal solution incorporating a gyroid TPMS structure with remarkable physical properties for microprocessor cooling. Detailed investigations were conducted using a full-scale heat sink model to understand the inner geometric structure, flow, and heat transfer characteristics within a gyroid heat sink (GHS). The thermo-hydraulic performance of the GHS design was systematically assessed against that of a pinfin heat sink (PHS) across different porosities and flow rates. Both heat sinks were evaluated under non-uniform heating conditions, considering three heating schemes, each with eleven randomly distributed hotspots. The thermohydraulic performance was assessed by calculating temperature non-uniformity, thermal resistance, and pumping power. A correlation was established using the cell size and cell wall thickness of a unit cell of gyroid TPMS to calculate its hydraulic diameter. The analysis revealed that the enhanced thermal performance of the GHS design can be attributed to its intricate and convoluted flow structure, along with a significantly large heat transfer surface area. However, these same factors contribute to a notably high-pressure drop. Compared to the PHS design, the GHS design showed better thermal performance at all the selected porosities and flow rates, albeit with higher pumping powers. The GHS design showed improvement in the thermal performance as the porosity decreased. Investigation under heterogeneous heating conditions showed substantially lower temperatures at the hotspots in the GHS design, along with reduced temperature variation among them. The study's findings provide valuable insight into the advantages and drawbacks of gyroid TPMS structure for their application in electronic cooling.
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| 2. |
- Attarzadeh, Reza, et al.
(författare)
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Design analysis of the "Schwartz D" based heat exchanger : A numerical study
- 2021
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 177
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Tidskriftsartikel (refereegranskat)abstract
- Triply Periodic Minimal Surfaces (TPMS) have promising thermophysical properties, which makes them a suitable candidate in the production of low-temperature waste heat recovery systems. A TPMS thermal performance is connected to the complex flow patterns inside the pores and their interactions with the walls. Unfortunately, the experimental study's design analysis and optimization of TPMS heat exchangers are complicated due to the flow pattern complexity and visual limitations inside the TPMS. In this study, three-dimensional steady-state, conjugate heat transfer (CHT) simulations for laminar incompressible flow were carried out to quantify the performance of a TPMS based heat exchanger. TPMS Lattices based on Schwartz D architecture was modeled to elucidate the design parameters and establishing relationships between gas velocity, heat transfer, and thermal performance of TPMS at different wall thicknesses. In this study, four types of lattices from the same architectures with varying wall thickness were examined for a range of the gas velocity, with one design found to be the optimized lattice providing the highest thermal performance. The results and methodology presented here can facilitate improvements in TPMSheat exchangers' fabrication for recycling the waste heat in low pitch thermal systems.
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| 3. |
- Attarzadeh, Reza, et al.
(författare)
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Multi-objective optimization of TPMS-based heat exchangers for low-temperature waste heat recovery
- 2022
-
Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 212, s. 118448-
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Tidskriftsartikel (refereegranskat)abstract
- The transformation to a truly sustainable energy system will require taking better advantage of the waste heat. Integrating heat exchangers with the triply periodic minimal surface (TPMS) is a promising and efficient way to build waste heat recovery systems that harness heat emissions from the low pitch thermal systems. This is mainly due to the low hydrodynamic resistance and pressure drop in the TPMS while securing good heat transfer at low-temperature gradient. This study establishes a computational design and analysis of heat and mass transfer inside a heat exchanger based on the TPMS structure and determine thermal effectiveness, heat transfer coefficient, and pressure drop inside the channel. The non-linearity dependence of results to several design variables makes obtaining the optimal design configuration solely using conventional CFD or experimental study nearly impossible. Hence, a multi-objective optimization workflow based on a Genetic Algorithm for laminar flow is employed to reveal the underlying relationships between design variables for the optimal configurations. The results illustrate the local sensitivity of important parameters such as the heat transfer coefficient, Nusselt number, and thermal performance of the heat exchanger against various design variables. It is shown that the pressure drop is directly affected by gas inlet velocity, viscosity, and density, from high to low, respectively. The Pareto frontiers for the optimal thermal performance are extracted, and the correlation between design objectives is determined. This methodology provides a promising framework for heat exchangers' design analysis, including multi-objective goals and design constraints.
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| 4. |
- Atzori, Marco, 1992-, et al.
(författare)
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LES and RANS calculations of particle dispersion behind a wall-mounted cubic obstacle
- 2022
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 151, s. 104037-
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Tidskriftsartikel (refereegranskat)abstract
- In the present paper, we evaluate the performances of three stochastic models for particle dispersion in the case of a three-dimensional turbulent flow. We consider the flow in a channel with a cubic wall-mounted obstacle, and perform large-eddy simulations (LESs) including passive particles injected behind the obstacle, for cases of low and strong inertial effects. We also perform Reynolds-averaged simulations of the same case, using standard turbulence models, and employ the two discrete stochastic models for particle dispersion implemented in the open-source code OpenFOAM and the continuous Lagrangian stochastic model proposed by Minier et al. (2004). The Lagrangian model is consistent with a Probability Density Function (PDF) model of the exact particle equations, and is based on the modelling of the fluid velocity seen by particles. This approach allows a consistent formulation which eliminates the spurious drifts flawing discrete models and to have the drag force in a closed form. The LES results are used as reference data both for the fluid RANS simulations and particle simulations with dispersion models. The present test case allows to evaluate the performance of dispersion models in highly non-homogeneous flow, and it used in this context for the first time. The continuous stochastic model generally shows a better agreement with the LES than the discrete stochastic models, in particular in the case of particles with higher inertia.
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| 5. |
- $$$Behzadi, Amirmohammad, et al.
(författare)
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Application to novel smart techniques for decarbonization of commercial building heating and cooling through optimal energy management
- 2024
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Ingår i: Applied Energy. - : Elsevier. - 0306-2619 .- 1872-9118. ; 376
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Tidskriftsartikel (refereegranskat)abstract
- The present article proposes a novel smart building energy system utilizing deep geothermal resources through naturally-driven borehole thermal energy storage interacting with the district heating network. It includes an intelligent control strategy for lowering operational costs, making better use of renewables, and avoiding CO2 emissions by eliminating heat pumps and cooling machines to address the heating and cooling demands of a commercial building in Uppsala, a city near Stockholm, Sweden. After comprehensively conducting techno-environmental and economic assessments, the system is fine-tuned using artificial neural networks (ANN) for optimization. The study aims to determine which ANN design and training procedure is the most efficient in terms of accuracy and computing speed. It also assesses well-known optimization algorithms using the TOPSIS decision-making technique to find the best trade-off among various indicators. According to the parametric results, deeper boreholes can collect more geothermal energy and reduce CO2 emissions. However, deep drilling becomes more expensive overall, suggesting the need for multi-objective optimization to balance costs and techno-environmental benefits. The results indicate that Levenberg-Marquardt algorithms offer the optimum trade-off between computation time and error minimization. From a TOPSIS perspective, while the dragonfly algorithm is not ideal for optimizing the suggested system, the non-dominated sorting genetic algorithm is the most efficient since it yields more ideal points rated below 100. The optimization yields a higher energy production of 120 kWh/m2, as well as a decreased levelized cost of energy of 57 $/MWh, a shorter payback period of two years, and a reduced CO2 index of 1.90 kg/MWh. The analysis reveals that despite the high investment costs of 382.50 USD/m2, the system is financially beneficial in the long run due to a short payback period of around eight years, which aligns with the goals of future smart energy systems: reduce pollution and increase cost-effectiveness.
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| 6. |
- Behzadi, Amirmohammad, et al.
(författare)
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Smart design and control of thermal energy storage in low-temperature heating and high-temperature cooling systems : A comprehensive review
- 2022
-
Ingår i: Renewable & sustainable energy reviews. - : Elsevier BV. - 1364-0321 .- 1879-0690. ; 166, s. 112625-
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Forskningsöversikt (refereegranskat)abstract
- Thermal energy storage (TES) is recognized as a well-established technology added to the smart energy systems to support the immediate increase in energy demand, flatten the rapid supply-side changes, and reduce energy costs through an efficient and sustainable integration. On the utilization side, low-temperature heating (LTH) and high-temperature cooling (HTC) systems have grown popular because of their excellent performance in terms of energy efficiency, cost-effectiveness, and ease of integration with renewable resources. This article presents the current state-of-the-art regarding the smart design of TES integrated with LTH and HTC systems. TES is first explained in basic concepts, classification, and design possibilities. Secondly, the literature on well-known existing control approaches, strategies, and optimization methods applied to thermal energy storage is reviewed. Thirdly, the specifications, types, benefits, and drawbacks of the LTH and HTC systems from the viewpoints of supply and demand sides are discussed. Fourthly, the smart design of TES integrated with the LTH and HTC systems based on the control approach/strategy, optimization method, building type, and energy supplier is investigated to find the newest technology, ideas, and features and detect the existing gaps. The present article will provide a realistically feasible solution for having a smart storage configuration with the maximum possible energy efficiency, reliability, and cost-effectiveness for the building owners and the energy suppliers.
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| 7. |
- Behzadi, Amirmohammad, et al.
(författare)
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Supply-demand side management of a building energy system driven by solar and biomass in Stockholm : A smart integration with minimal cost and emission
- 2023
-
Ingår i: Energy Conversion and Management. - : Elsevier Ltd. - 0196-8904 .- 1879-2227. ; 292
-
Tidskriftsartikel (refereegranskat)abstract
- As part of the transition to a sustainable future, energy-efficient buildings are needed to secure users' comfort and lower the built environment's energy footprint and associated emissions. This article presents a novel, realistic and affordable solution to minimize the footprint of smart building energy systems and enable higher renewable energy use in the building sector. For this, an intelligent system is being developed using a rule-based automation approach that considers thermal comfort, energy prices, meteorological data, and primary energy use. In order to lower the installation cost and part of the environmental footprint, batteries are not used, and the heat pump's size is decreased via component integration. Also, different renewable resources are effectively hybridized using photovoltaic thermal panels and an innovative biomass heater to increase the share of renewable energy, enhance reliability, and shave peak load. In order to secure feasibility, the suggested framework is assessed from the techno-economic and environmental standpoints for 100 residential apartments in Stockholm, Sweden. Our results show that 70.8 MWh of renewable electricity is transferred to the local grid, and the remaining 111.5 MWh is used to supply the building's needs and power the electrically-driven components. The biomass heater meets more than 65% of the space heating demand, mainly at low solar power and high electricity prices, illustrating the value of integration strategies to reduce the system's dependability on the local grid. The results further reveal that most energy purchases during the cloudy days and nights are repaid through the sale of excess renewable production during the warmer hours, with a bidirectional connection with the grid. The monthly energy cost is less than 140 $/MWh for most of the years. The cost can be held low due to the exclusion of batteries and minimizing the heat pump size. The proposed system has a low emission index of 11.9 kgCO2/MWh and can reduce carbon dioxide emissions by 70 TCO2/year compared to using the supply from the Swedish energy mix.
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| 8. |
- Cifuentes, Luis, et al.
(författare)
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Chemical Explosive Mode Analysis for a Jet-in-Hot-Coflow burner operating in MILD combustion
- 2018
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Ingår i: Fuel. - : Elsevier. - 0016-2361 .- 1873-7153. ; 232, s. 712-723
-
Tidskriftsartikel (refereegranskat)abstract
- Large Eddy Simulations (LES) of Moderate and Intense Low oxygen Dilution (MILD) combustion of a Jet-in-HotCoflow (JHC) burner were performed using detailed chemistry. On the contrary to traditional flames, where heat release is occurring in very thin fronts, MILD combustion occurs in the distributed reaction regime where the reaction zone is broad, thus, this paper applies a direct Arrhenius closure with detailed chemistry to resolve important details of the fuel oxidation reactions. Comparisons of LES results are in good agreement with experiments, demonstrating that the simulations capture the intermediate species and finite reaction rate effects. A Chemical Explosive Mode Analysis (CEMA) was used to determine the flame structure and to detect the pre-and post-ignition regions, including the contributions to the CEMs analyzing the Explosion Index (EI) and Participation Index (PI). To the best of our knowledge, a detailed study of CEMA on MILD or flameless regime has never been reported. The flame structure was clearly visualized with CEMA, as well as the lean and the rich flame fronts. Different flame zones close to the anchoring points of these turbulent lifted flames were selected and the analysis demonstrates the contributions of dominant chemical species, such as HO2 and O. The reactions related to the dominant local CEM were obtained to highlight the nature of the stabilization in these highly diluted operating conditions.
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| 9. |
- Duwig, Christophe
(författare)
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A filtered flame approach for simulation of unsteady laminar premixed flames
- 2009
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Ingår i: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 13:2, s. 251-268
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Tidskriftsartikel (refereegranskat)abstract
- The simulation of laminar flames consists of capturing the evolution of a very large number of species that may react within a broad range of time scales. It results in a highly non-linear stiff numerical problem that requires large computational resources. In the present paper, an alternative approach for the simulation of unsteady premixed flames is proposed. The approach focuses on describing the flame using a single scalar only (so-called progress variable) on relatively coarse grids. The unresolved details of the flame structure are analysed and described in term of spatial filtering. A model originally developed for large eddy simulation is discussed in the framework of unsteady laminar flames. The ability of the present approach to represent the filtered flame structure is validated against detailed numerical simulations of freely propagating laminar flames. Further, simulations of steady and unsteady laminar Bunsen flames are presented with a focus on the influence of the numerical grid and filter width upon the flame dynamics.
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| 10. |
- Duwig, Christophe
(författare)
-
A flamelet Model for LES
- 2002
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Ingår i: Advances in Turbulence IX. - 8495999072
-
Konferensbidrag (refereegranskat)
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| 11. |
- Duwig, Christophe, et al.
(författare)
-
Efficient operation of a gas turbine on methanol using chemical recuperation
- 2012
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Ingår i: ASME Turbo Expo 2012: Turbine Technical Conference and Exposition. ; 1, s. 615-623
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Konferensbidrag (refereegranskat)abstract
- Environmental and political concerns, together with new legislations, are pushing for a fuel shift in the power industry and more generally for many thermal applications. Adding to the coming decrease of oil and natural availability (or price increase), it opens avenues for new fuels. Among those, alcohols are strong candidates. In fact, short alcohols are easily produced and stored and require only moderate modifications of existing combustion systems. For example, operating an existing gas turbine (GT) on methanol requires moderate modifications (mainly in the combustion system). However, methanol can be used more efficiently. Unlike methane or other hydrocarbons that decompose at high temperature (1000K), methanol undergoes an endothermic decomposition at low temperatures (400K to 600K) to give CO and H2. It therefore opens avenue for coupling the GT with a chemical recuperation system. In other words, the methanol will be cracked using the waste heat of the flue gases with a gain in fuel heating value hence the original fuel is thermally upgraded. The present study will investigate the upgraded fuel combustion properties. The laminar flame speed of the upgraded fuel/air mixtures will be presented and compared to methane and methanol under conditions relevant to GT combustion. Several upgraded fuel compositions will be considered depending on the water content in the feed methanol. Further, we consider a recuperated micro GT (Turbec T100) based cycle fueled with methanol. The numerical study focuses on different thermodynamic cycles. Firstly, a reference case is considered assuming a direct fueled GT. Further, cycles including the cracker are studied keeping the power constant. The fuel efficiency gain due to the cracker will be investigated as function of the water content in the feed methanol. Finally, a case including CO2-removal will be presented and it will be shown that the cracker enables an efficient carbon capture and sequestration scheme.
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| 12. |
- Duwig, Christophe
(creator_code:cre_t)
-
Exhibition : Towards the energy of the future – the invisible revolution behind the electrical socket
- 2023
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Konstnärligt arbeteabstract
- Energy Crisis! Electricity Price drama! The threat of global energy poverty! Media are generous with spectacular titles. Yes, energy is important, and yes, nearly all societal challenges are connected to how we convert, distribute and use energy. Therefore, the KTH Energy Platform and KTH Library presented an exhibition with the theme Towards the energy of the future – the invisible revolution behind the electrical socket.The exhibition displayed showcase illustrations from the book made by Lotta Waesterberg Tomasson, as well as books related to energy and electricity from the KTH Library's collections. In parallell with the exhibition, a series of live popular science lunch seminars with presentations of selected chapters of the book took place. As part of the exhibition, students from KTH's Electrical Engineering program also showcased exciting projects that connect to the anthology’s contents, made with materials and equipment from the student-driven ELAB and “Studentverkstan”. Visitors were also invited to share their reflections and ideas on energy.
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| 13. |
- Duwig, Christophe, et al.
(författare)
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Experimental and numerical study of flameless combustion in a model gas turbine combustor
- 2008
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:2, s. 279-295
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Tidskriftsartikel (refereegranskat)abstract
- Flameless combustion is an attractive solution to address existing problems of emissions and stability when operating gas turbine combustors. Theoretical, numerical and experimental approaches were used to study the flameless gas turbine combustor. The emissions and combustion stability were measured and the limits of the flameless regime are discussed. Using experimental techniques and Large Eddy Simulation (LES), detailed knowledge of the flow field and the oxidation dynamics was obtained. In particular the relation between the turbulent coherent structures dynamics and the flameless oxidation was highlighted. A model for flameless combustion simulations including detailed chemistry was derived. The theoretical analysis of the flameless combustion provides 2 non-dimensional numbers that define the range of the flameless mode. It was determined that the mixture that is ignited and burnt is composed of similar to 50% of fresh gases and similar to 50% vitiated gases.
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| 14. |
- Duwig, Christophe, et al.
(författare)
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Extended Proper Orthogonal Decomposition for Analysis of Unsteady Flames
- 2010
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Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 84:1, s. 25-47
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this work is to present the use of proper orthogonal decomposition (POD) and extended proper orthogonal decomposition (EPOD) for revealing flame dynamics as a set of statistical quantities referred as modes. The flame fluctuations are used to derive an empirical functions base representing the most important features of the flame. The capabilities of the technique are exemplified in the case of an unsteady laminar flame. The flame is naturally unsteady and can be excited to amplify the fluctuations. The data base consists of synchronous Particle Image Velocimetry (PIV) and OH-Planar Laser Induced Fluorescence (OH-PLIF) measurements. It was found that the POD based upon the PIV vectors only reveals flow features when the excitation is added. On contrary, the OH-PLIF based POD performs well in any case and constitutes a suitable base for the EPOD analysis.
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| 15. |
- Duwig, Christophe, et al.
(författare)
-
High resolution imaging of flameless and distributed turbulent combustion
- 2012
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 159:1, s. 306-316
-
Tidskriftsartikel (refereegranskat)abstract
- Planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements were used for the study of turbulence/combustion interactions in distributed reaction regimes including flameless or MILD combustion. A novel laboratory scale burner (Distributed and Flameless Combustion Burner - DFCB) was used to reach uniquely high Karlovitz numbers, presently reported up to 14,400. It consists of a highly turbulent piloted high speed jet burner with a vitiated coflow. Six cases are reported whereas two of them (leaner cases) led to an invisible reacting zone, though still emitting light in the UV and near infrared range. Simultaneous OH/CH(2)O PLIF image with 50 mu m spatial resolution were achieved to capture the variation of intermediate species in the reaction layer. When complemented with temperature images obtained by Rayleigh scattering measurement, it provided insights of the reaction front structures as well as measures of the flame brush thicknesses. In particular, variations in the jet velocity highlighted the influence of turbulent mixing (hence turbulence/chemistry interaction) on the flame structures as depicted by the formation of relatively large pools of CH(2)O. Further, variations in the jet stoichiometry impacted on the reaction zone visibility but only marginally on the intensity and moderately on the overall shape of the OH and CH(2)O signals. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 16. |
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| 17. |
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| 18. |
- Duwig, Christophe, et al.
(författare)
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Large Eddy Simulation of a Gas Turbine Combustion chamber: effect of the subgrid scale flame thickness on the flame dynamics
- 2004
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Ingår i: Direct and Large-Eddy Simulation V. - 1402020325 ; ERCOFTAC series ; 9, s. 343-350
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Konferensbidrag (refereegranskat)abstract
- A analysis of the subgrid scale flame thickness in the LES framework is presented. The influence of the subgrid flame thickness has been tested numerically on a swirl stabilized flame using a flamelet formulation. The analysis introduces a non-dimensional number a, characteristic for flame and the filter. The values of this number are computed for different LES combustion models and lie in a wide range. The variations of the number a lead to completely different flame dynamics showing the importance of subgrid flame thickness modelling in the LES framework.
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| 19. |
- Duwig, Christophe, et al.
(författare)
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Large eddy simulation of a H-2/N-2 lifted flame in a vitiated co-flow
- 2008
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:3, s. 453-480
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Tidskriftsartikel (refereegranskat)abstract
- A lifted turbulent H-2/N-2 flame in a vitiated co-flow is studied using Large Eddy Simulation together with a closure based on perfectly stirred reactors. A part of the closure, chemical look-up tables, are generated to close the filtered temperature equations and to compute local radical concentrations throughout the computational domain. The approach has been used to simulate a lifted turbulent flame. The results have been found to be insensitive to the combustion model employed and to the grid resolution. However, the results are very sensitive to the temperature of the co-flow stream and this result is well in line with previous findings. The numerical predictions were further compared to detailed experimental data obtained by Cabra et al. (2002). The agreement between the two sets of data is very good, indicating that the present approach predicts successfully the combustion process including the OH mass fractions. Finally, the LES data were used to study the flame dynamics and stabilization mechanisms.
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| 20. |
- Duwig, Christophe, et al.
(författare)
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Large Eddy Simulation of a premixed jet flame stabilized by a vitiated co-flow: Evaluation of auto-ignition tabulated chemistry
- 2013
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 160:12, s. 2879-2895
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Tidskriftsartikel (refereegranskat)abstract
- A tabulation technique assuming an auto-ignition dominated reaction pathway for highly turbulent premixed combustion is presented, implemented in an LES framework and evaluated. The tabulation method enables the reduction of the chemical system dimension to two scalars, allowing a computationally efficient model implementation, yet still retaining a sufficiently accurate representation of the chemical kinetics. The sensitivity of the LES model to the grid, inflow conditions, subgrid model, tabulation method assumptions and the chemical mechanism used in the tabulation process is evaluated with reference to detailed experimental measurements. The particular chemical mechanism utilized for the tabulation is shown to have a significant effect on the CO and OH concentrations, whilst only a small influence on the temperature and mixing fields. Comparisons with laminar flame based tabulation explain the misprediction of CO concentration. However, both the auto-ignition and laminar flame based tabulations fail to capture the OH concentration. The ability of the two tabulation techniques to capture the non-flamelet structure is discussed and the predictive capability of the two approaches is established. The general utility of a global Karlovitz number for describing the combustion regime and hence the selection of an applicable combustion model is brought into question considering that the variation of the local Karlovitz number in the simulations varies by up to 2 orders of magnitude, indicating a broad range of accessed flame structures. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 21. |
- Duwig, Christophe, et al.
(författare)
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Large Eddy simulation of a swirling flame response to swirl modulation with impact on combustion stability
- 2005
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Ingår i: 43rd AIAA Aerospace Sciences Meeting and Exhibit - Meeting Papers. ; , s. 4929-4940
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Konferensbidrag (refereegranskat)abstract
- A swirl stabilized flame has been studied numerically using Large Eddy Simulation. The response of the flame to a large swirl number modulation has as been determined. Decreasing the swirl number moderately does not destabilize the flame. Decreasing further the swirl number induces a transition from stabilized to a jet flame with increase in the flame fluctuations. Increasing back the swirl restores the original configuration. However, the re-stabilization has been found to be considerably slower as compared to the burner aerodynamic time scale. Further studies, including identification of some characteristic turbulent structures, have been carried out. Both a helical and an axial mode have been identified. The helical mode arises in the central recirculation zone, whereas the axial mode arises in the outer shear-layer. Both modes are possible sources of thermo-acoustic instabilities.
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| 22. |
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| 23. |
- Duwig, Christophe, et al.
(författare)
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Large Eddy Simulation of a turbulent flow past a pair of symmetric backward-facing steps: A sensitivity analysis
- 2007
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Ingår i: Collection of Technical Papers - 45th AIAA Aerospace Sciences Meeting. - 9781563478901 ; 16, s. 11096-11114
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Konferensbidrag (refereegranskat)abstract
- Large Eddy Simulation is used to study the flow behind a pair of symmetric backward facing steps. As reported in the literature, the flow exhibits an asymmetric pattern characterized by the deflection of the jet toward of the walls. The LES results are compared to LDA measurements showing the ability of the present numerical tool to capture the complex features of the flow. Furthermore, a sensitivity study is conducted in order to assess the influence of the grid resolution, inflow boundary, channel width (aspect ratio A h) and step size (h) on the flow field. The flow was found to be weakly sensitive to the grid, assuring the quality of the simulation results. The inflow boundary influences the results only marginally unless low frequency fluctuations are applied. In this case, the flow field recovers symmetry with suppression of the jet bending. The jet bending has also been shown to increase with the step size h and to reduce with increasing Ah.
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| 24. |
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| 25. |
- Duwig, Christophe
(författare)
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Large Eddy Simulation of flame stabilisation dynamics and vortex control in a lifted H2/N2 jet flame
- 2011
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Ingår i: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 15:3, s. 325-346
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Tidskriftsartikel (refereegranskat)abstract
- Flame stabilisation in (highly) preheated mixture is common in several industrial applications. When the reactants are injected separately in the device (usually at high-speed), the flame is lifted so that the fuel and oxidant first mix to give an ignitable mixture. If the temperature of the mixture is adequate, it auto-ignites stabilizing the flame. Here we focus on an academic lifted jet flame and Large Eddy Simulation (LES) is used to capture the flame and auto-ignition dynamics. Comparisons with experimental data show that LES simulates accurately high OH fluctuation levels at the stabilisation location. The vortex dynamics linked to these fluctuations is analyzed and it is found that small scale coherent structures play a vital role in the auto-ignition process. These structures are axial vorticity tubes (braids) and are located relatively far (in the radial direction) from the shear-layer. As a consequence, the lift-off height varies dramatically in time leading to OH fluctuations of the order of the mean OH concentration. This scenario is monitored in the compositional space highlighting the simultaneous evolution of OH, HO2 and temperature. Further, different strategies for open-loop control of the flame lift-off height are tested. In order to anchor the flame at different positions downstream of the nozzle, the vortex dynamics in the shear-layer was modified. Promoting successively vortex ring and braids, the auto-ignition region was moved significantly. In particular, modified nozzle geometries impacted the formation of braids and ensured a good premixing very close to the nozzle. As a consequence, it was possible to reduce significantly the lift-off height and stabilise the flame few diameters downstream of the nozzle.
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