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M. El Abbassi
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This article reviews the linear solvers available in OpenFOAM and assesses their impact on the convergence behaviour of the SIMPLE algorithm. The discretisation of transport equations in CFD results in large and sparse linear systems, for which the choice of linear solver strongly influences the computational time. Although the solver does not change the final discrete solution, the difference in speed and robustness between the solvers can be more than one order of magnitude. A brief overview is given concerning how the velocity and pressure fields are decoupled in OpenFOAM, followed by a detailed review of the main linear solver families, including direct methods, basic iterative methods, multigrid methods and Krylov subspace methods, with attention to their practical strengths and weaknesses. The performance of the most advanced solvers is evaluated on a full-scale non-reacting kiln case consisting of 2.3 million cells. The pressure-corrector equation is identified as the main bottleneck in the SIMPLE algorithm. The conjugate gradient (CG) solver with a multigrid (MG) preconditioner is found to be the fastest and most stable method, achieving speed-ups of up to a factor of 7 compared to the slower advanced methods. Using MG as a preconditioner also improves the robustness of the Bi-CGStab method.
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This article reviews the linear solvers available in OpenFOAM and assesses their impact on the convergence behaviour of the SIMPLE algorithm. The discretisation of transport equations in CFD results in large and sparse linear systems, for which the choice of linear solver strongly influences the computational time. Although the solver does not change the final discrete solution, the difference in speed and robustness between the solvers can be more than one order of magnitude. A brief overview is given concerning how the velocity and pressure fields are decoupled in OpenFOAM, followed by a detailed review of the main linear solver families, including direct methods, basic iterative methods, multigrid methods and Krylov subspace methods, with attention to their practical strengths and weaknesses. The performance of the most advanced solvers is evaluated on a full-scale non-reacting kiln case consisting of 2.3 million cells. The pressure-corrector equation is identified as the main bottleneck in the SIMPLE algorithm. The conjugate gradient (CG) solver with a multigrid (MG) preconditioner is found to be the fastest and most stable method, achieving speed-ups of up to a factor of 7 compared to the slower advanced methods. Using MG as a preconditioner also improves the robustness of the Bi-CGStab method.
This dissertation presents the development, validation, and application of an integrated Computational Fluid Dynamics (CFD) solver, which is built upon the opensource OpenFOAM framework, to predict and ultimately reduce thermal nitric oxide (NO) formation in industrial rotary kilns without sacrificing process productivity. The work is structured into three parts: theoretical foundations, solver implementation and validation through increasingly complex case studies, and final conclusions and recommendations.
The theoretical part of the dissertation established a comprehensive mathematical framework for simulating the complex interactions within rotary kilns, focusing on turbulent combustion, conjugate heat transfer (CHT), and thermal NO formation. Each part of this multi-physics problem is highlighted with a discussion of one or more available models to resolve it, which is often a trade-off between computational speed and accuracy.... ...
The theoretical part of the dissertation established a comprehensive mathematical framework for simulating the complex interactions within rotary kilns, focusing on turbulent combustion, conjugate heat transfer (CHT), and thermal NO formation. Each part of this multi-physics problem is highlighted with a discussion of one or more available models to resolve it, which is often a trade-off between computational speed and accuracy.... ...
This dissertation presents the development, validation, and application of an integrated Computational Fluid Dynamics (CFD) solver, which is built upon the opensource OpenFOAM framework, to predict and ultimately reduce thermal nitric oxide (NO) formation in industrial rotary kilns without sacrificing process productivity. The work is structured into three parts: theoretical foundations, solver implementation and validation through increasingly complex case studies, and final conclusions and recommendations.
The theoretical part of the dissertation established a comprehensive mathematical framework for simulating the complex interactions within rotary kilns, focusing on turbulent combustion, conjugate heat transfer (CHT), and thermal NO formation. Each part of this multi-physics problem is highlighted with a discussion of one or more available models to resolve it, which is often a trade-off between computational speed and accuracy....
The theoretical part of the dissertation established a comprehensive mathematical framework for simulating the complex interactions within rotary kilns, focusing on turbulent combustion, conjugate heat transfer (CHT), and thermal NO formation. Each part of this multi-physics problem is highlighted with a discussion of one or more available models to resolve it, which is often a trade-off between computational speed and accuracy....
This paper verifies a mathematical model that is developed for the open source CFD-toolbox OpenFOAM, which couples turbulent combustion with conjugate heat transfer. This feature already exists in well-known commercial codes. It permits the prediction of the flame’s characteristics, its emissions, and the consequent heat transfer between fluids and solids via radiation, convection, and conduction. The verification is based on a simplified 2D axisymmetric cylindrical reactor. In the first step, the combustion part of the solver is compared against experimental data for an open turbulent flame. This shows good agreement when using the full GRI 3.0 reaction mechanism. Afterwards, the flame is confined by a cylindrical wall and simultaneously conjugate heat transfer is activated and analysed. It is shown that the combustion and conjugate heat transfer are successfully coupled.
...
This paper verifies a mathematical model that is developed for the open source CFD-toolbox OpenFOAM, which couples turbulent combustion with conjugate heat transfer. This feature already exists in well-known commercial codes. It permits the prediction of the flame’s characteristics, its emissions, and the consequent heat transfer between fluids and solids via radiation, convection, and conduction. The verification is based on a simplified 2D axisymmetric cylindrical reactor. In the first step, the combustion part of the solver is compared against experimental data for an open turbulent flame. This shows good agreement when using the full GRI 3.0 reaction mechanism. Afterwards, the flame is confined by a cylindrical wall and simultaneously conjugate heat transfer is activated and analysed. It is shown that the combustion and conjugate heat transfer are successfully coupled.
This work studies how non-premixed turbulent combustion in a rotary kiln depends on the
geometry of the secondary air inlet channel. We target a kiln in which temperatures can reach values above 1800 degrees Kelvin. Monitoring and possible mitigation of the thermal nitric-oxide (NOx) formation is of utmost importance. The performed reactive flow simulations result in detailed maps of the spatial distribution of the flow, thermodynamics and chemical conditions of the kiln. These maps provide valuable information to the operator of the kiln. The simulations show the difference between the existing and the newly proposed geometry of the secondary air inlet. In the existing configuration, the secondary air inlet is rectangular and located above the base of the burner pipe. The secondary air flows into the furnace from the top of the flame. The heat release by combustion is unevenly distributed throughout the flame. In the new geometry, the secondary air inlet is an annular ring placed around the burner pipe. The secondary air flows circumferentially around the burner pipe. The new secondary air inlet geometry is shown to result in a more homogeneous spatial distribution of the heat release throughout the flame. The peak temperatures of the flame and the production of thermal NOx are significantly reduced. Further research is required to resolve limitations of various choices in our modeling approach.
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geometry of the secondary air inlet channel. We target a kiln in which temperatures can reach values above 1800 degrees Kelvin. Monitoring and possible mitigation of the thermal nitric-oxide (NOx) formation is of utmost importance. The performed reactive flow simulations result in detailed maps of the spatial distribution of the flow, thermodynamics and chemical conditions of the kiln. These maps provide valuable information to the operator of the kiln. The simulations show the difference between the existing and the newly proposed geometry of the secondary air inlet. In the existing configuration, the secondary air inlet is rectangular and located above the base of the burner pipe. The secondary air flows into the furnace from the top of the flame. The heat release by combustion is unevenly distributed throughout the flame. In the new geometry, the secondary air inlet is an annular ring placed around the burner pipe. The secondary air flows circumferentially around the burner pipe. The new secondary air inlet geometry is shown to result in a more homogeneous spatial distribution of the heat release throughout the flame. The peak temperatures of the flame and the production of thermal NOx are significantly reduced. Further research is required to resolve limitations of various choices in our modeling approach.
...
This work studies how non-premixed turbulent combustion in a rotary kiln depends on the
geometry of the secondary air inlet channel. We target a kiln in which temperatures can reach values above 1800 degrees Kelvin. Monitoring and possible mitigation of the thermal nitric-oxide (NOx) formation is of utmost importance. The performed reactive flow simulations result in detailed maps of the spatial distribution of the flow, thermodynamics and chemical conditions of the kiln. These maps provide valuable information to the operator of the kiln. The simulations show the difference between the existing and the newly proposed geometry of the secondary air inlet. In the existing configuration, the secondary air inlet is rectangular and located above the base of the burner pipe. The secondary air flows into the furnace from the top of the flame. The heat release by combustion is unevenly distributed throughout the flame. In the new geometry, the secondary air inlet is an annular ring placed around the burner pipe. The secondary air flows circumferentially around the burner pipe. The new secondary air inlet geometry is shown to result in a more homogeneous spatial distribution of the heat release throughout the flame. The peak temperatures of the flame and the production of thermal NOx are significantly reduced. Further research is required to resolve limitations of various choices in our modeling approach.
geometry of the secondary air inlet channel. We target a kiln in which temperatures can reach values above 1800 degrees Kelvin. Monitoring and possible mitigation of the thermal nitric-oxide (NOx) formation is of utmost importance. The performed reactive flow simulations result in detailed maps of the spatial distribution of the flow, thermodynamics and chemical conditions of the kiln. These maps provide valuable information to the operator of the kiln. The simulations show the difference between the existing and the newly proposed geometry of the secondary air inlet. In the existing configuration, the secondary air inlet is rectangular and located above the base of the burner pipe. The secondary air flows into the furnace from the top of the flame. The heat release by combustion is unevenly distributed throughout the flame. In the new geometry, the secondary air inlet is an annular ring placed around the burner pipe. The secondary air flows circumferentially around the burner pipe. The new secondary air inlet geometry is shown to result in a more homogeneous spatial distribution of the heat release throughout the flame. The peak temperatures of the flame and the production of thermal NOx are significantly reduced. Further research is required to resolve limitations of various choices in our modeling approach.
One of the quickest ways to influence both the wall temperature and thermal NOx emissions in rotary kilns is to change the air–fuel ratio (AFR). The normalized counterpart of the AFR, the equivalence ratio, is usually associated with premixed flames and studies of its influence on diffusion flames are inconsistent, depending on the application. In this paper, the influence of the AFR is investigated numerically for rotary kilns by conducting steady-state simulations. We first conduct three-dimensional simulations where we encounter statistically unstable flow at high inflow conditions, which may be caused by vortex stretching. As vortex stretching vanishes in two-dimensional flow, the 2D simulations no longer encounter convergence problems. The impact of this simplification is shown to be acceptable for the thermal behaviour. It is shown that both the wall temperature and thermal NOx emissions peak at the fuel-rich and fuel-lean side of the stoichiometric AFR, respectively. If the AFR continues to increase, the wall temperature decreases significantly and thermal NOx emissions drop dramatically. The NOx validation, however, shows different results and indicates that the simulation model is simplified too much, as the measured NOx formation peaks at significantly fuel-lean conditions.
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One of the quickest ways to influence both the wall temperature and thermal NOx emissions in rotary kilns is to change the air–fuel ratio (AFR). The normalized counterpart of the AFR, the equivalence ratio, is usually associated with premixed flames and studies of its influence on diffusion flames are inconsistent, depending on the application. In this paper, the influence of the AFR is investigated numerically for rotary kilns by conducting steady-state simulations. We first conduct three-dimensional simulations where we encounter statistically unstable flow at high inflow conditions, which may be caused by vortex stretching. As vortex stretching vanishes in two-dimensional flow, the 2D simulations no longer encounter convergence problems. The impact of this simplification is shown to be acceptable for the thermal behaviour. It is shown that both the wall temperature and thermal NOx emissions peak at the fuel-rich and fuel-lean side of the stoichiometric AFR, respectively. If the AFR continues to increase, the wall temperature decreases significantly and thermal NOx emissions drop dramatically. The NOx validation, however, shows different results and indicates that the simulation model is simplified too much, as the measured NOx formation peaks at significantly fuel-lean conditions.
Conference paper
(2018)
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M. El Abbassi, Dimas Dimas Ramadhan Abdillah Fikri, D.J.P. Lahaye, Cornelis Vuik
The wish to reduce the environmental footprint and to enhance economic gains of rotary kilns pushes the numerical simulation of combustion to include conjugate heat transfer. In this paper we study the influence of the refractory wall and radiative heat loss to the ambient, on the combustion process of a reference kiln model. Numerical results show that the inclusion of the refractory lining and the external radiative heat loss allows the inner wall temperature distribution to vary, with 60 % difference between its minimum and maximum. This is in sharp contrast with models that assume a fixed temperature at the wall. Consequently the maximum inner wall temperature increases by more than 200 %, the maximum flame temperature by nearly 13 % and maximum freeboard gas temperature by up to 90 %. It is thus important to account for these effects when modeling rotary kilns.
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The wish to reduce the environmental footprint and to enhance economic gains of rotary kilns pushes the numerical simulation of combustion to include conjugate heat transfer. In this paper we study the influence of the refractory wall and radiative heat loss to the ambient, on the combustion process of a reference kiln model. Numerical results show that the inclusion of the refractory lining and the external radiative heat loss allows the inner wall temperature distribution to vary, with 60 % difference between its minimum and maximum. This is in sharp contrast with models that assume a fixed temperature at the wall. Consequently the maximum inner wall temperature increases by more than 200 %, the maximum flame temperature by nearly 13 % and maximum freeboard gas temperature by up to 90 %. It is thus important to account for these effects when modeling rotary kilns.
This paper verifies a mathematical model that is developed for the open source CFD-toolbox OpenFOAM, which couples turbulent combustion with conjugate heat transfer. This feature already exists in well-known commercial codes. It permits the prediction of the flame’s characteristics, its emissions, and the consequent heat transfer between fluids and solids via radiation,
convection, and conduction. The verification is based on simplified 2D axisymmetric cylindrical reactors. In the first step, the combustion part of the solver is compared against experimental data for an open turbulent flame. This shows good agreement when using the full GRI 3.0 mechanism. Afterwards, the flame is confined by a cylindrical wall and simultaneously conjugate heat
transfer is activated and analysed. Finally, a backward facing step is included to increase flow complexity and the results are compared with the commercial CFD code ANSYS Fluent. It is shown that the combustion and conjugate heat transfer are successfully coupled. When radiation is disabled, comparable results are achieved by both solvers, while enabling radiation leads to larger discrepancies.
...
This paper verifies a mathematical model that is developed for the open source CFD-toolbox OpenFOAM, which couples turbulent combustion with conjugate heat transfer. This feature already exists in well-known commercial codes. It permits the prediction of the flame’s characteristics, its emissions, and the consequent heat transfer between fluids and solids via radiation,
convection, and conduction. The verification is based on simplified 2D axisymmetric cylindrical reactors. In the first step, the combustion part of the solver is compared against experimental data for an open turbulent flame. This shows good agreement when using the full GRI 3.0 mechanism. Afterwards, the flame is confined by a cylindrical wall and simultaneously conjugate heat
transfer is activated and analysed. Finally, a backward facing step is included to increase flow complexity and the results are compared with the commercial CFD code ANSYS Fluent. It is shown that the combustion and conjugate heat transfer are successfully coupled. When radiation is disabled, comparable results are achieved by both solvers, while enabling radiation leads to larger discrepancies.