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Papers by Alessandro Parente
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Law of the wall for smooth and sand-grain roughened surfaces as a function of the dimensionless s... more Law of the wall for smooth and sand-grain roughened surfaces as a function of the dimensionless sand-grain roughness height k + S Blocken et al. [1].. .. 12 3 Turbulent kinetic energy profiles at the inlet and outlet sections of an empty computational domain (see Figure 2b), Dashes: cell value for turbulent dissipation rate and turbulent kinetic energy averaged over the first cell. Short dashes: cell value for turbulent dissipation rate and kinetic energy.
info:eu-repo/semantics/publishe
ABSTRACT The prediction of NO emissions from industrial burners represents a key goal of Computat... more ABSTRACT The prediction of NO emissions from industrial burners represents a key goal of Computational Fluid Dy- namics (CFD) aided design. Simplified NO formation mechanisms are usually desirable, to reduce the computational eort related to the numerical simulations; however, they must be able to capture the NO trends with acceptable accuracy. Simplified mechanisms for the thermal and prompt NO formation routes are generally available within the existing commercial CFD packages and they provide acceptable NO predic- tions at relatively high temperatures. However, when operating at lower temperatures and with hydrogen, other mechanisms can be relevant, such as those involving N2O and NNH intermediates. This can become particularly relevant in non traditional combustion regimes, such as flameless combustion, characterized by operating temperature far below the levels observed in traditional burners. The present work shows a nu- merical and experimental investigation of a flameless combustion burner operating with methane-hydrogen mixtures with a H2 content up to 50% by wt. The work is focused on the requirements of the CFD model for the accurate prediction of the NO emissions from the burner. In particular, the influence of the combustion model and kinetic mechanism on the temperature fields on which the NO prediction is based is thoroughly discussed, together with the simplified NO formation paths to be included in the model. The approach based on the direct coupling of simplified NO mechanisms to the CFD calculation is compared to a dierent methodology, based on the post-processing of the CFD results with detailed kinetic mechanism for the gas- phase combustion and pollutants formation. A validation methodology is also implemented to quantitatively assess the degree of agreement between the numerical results and the experiments and to guide the selection of the modeling parameters required for predicting NO emissions accurately.
Journal of Wind Engineering and Industrial Aerodynamics, 2012
The simulation of Atmospheric Boundary Layer (ABL) flows is commonly performed using commercial C... more The simulation of Atmospheric Boundary Layer (ABL) flows is commonly performed using commercial CFD codes with RANS turbulence modeling, applying the standard k-e model. However, when applied to the simulation of the homogenous ABL, this approach may result in an undesired decay of the velocity and turbulent fully-developed profiles specified at the inlet of the computational domain. This behavior is due to an inconsistency between turbulence model, inflow conditions and wall function formulation. An approach has been introduced recently to overcome this problem, which consists in the modification of the turbulence model and wall function formulation to retrieve an overall consistent treatment of the neutral ABL. Such methodology, previously applied to simulation of the atmospheric boundary layer over flat terrain and ground-mounted bluff bodies, is here applied to the simulation of the flow over complex terrains and hills, at wind tunnel and atmosphere scale. In a time of limited scientific funding, the availability of open source CFD software such as OpenFOAM is a very attractive option to investigate; therefore, a comparison between OpenFOAM and the commercial code FLUENT 13.0 has been carried out in the present paper. The potential of the proposed methodology and the satisfactory performances of OpenFOAM are demonstrated.
Volume 3: Coal, Biomass and Alternative Fuels; Cycle Innovations; Electric Power; Industrial and Cogeneration, 2015
Micro Gas Turbines (mGTs) have arisen as a promising technology for Combined Heat and Power (CHP)... more Micro Gas Turbines (mGTs) have arisen as a promising technology for Combined Heat and Power (CHP) thanks to their overall energy efficiencies of 80% (30% electrical + 50% thermal) and the advantages they offer with respect to internal combustion engines. The main limitation of mGTs lies in their rather low electrical efficiency: whenever there is no heat demand, the exhaust gases are directly blown off and the efficiency of the unit is reduced to 30%. Operation in such conditions is generally not economical and can eventually lead to shutdown of the machine. To address this issue, the mGT cycle can be modified so that in moments of low heat demand the heat in the exhaust gases is used to warm up water which is then re-injected in the cycle, thereby increasing the electrical efficiency. The introduction of a saturation tower allows for water injection in mGTs: the resulting cycle is known as a micro Humid Air Turbine (mHAT). The static performance of the mGT Turbec T100 working as an...
info:eu-repo/semantics/nonPublishe
ABSTRACT The oxy-coal combustion is emerging as the most likely low-cost " clean coal &a... more ABSTRACT The oxy-coal combustion is emerging as the most likely low-cost " clean coal " technology for both carbon capture and NOx and SOx emissions reduction. The use of CFD tools is crucial for cost-effective oxy-fuel technologies development and environmental concerns minimization. The coupling of detailed chemistry and CFD simulations is still prohibitive, especially for large-scale plants, because of the high computational effort required. So, the development of simple and reliable kinetic mechanisms is therefore necessary. This work presents a CFD study on coal pyrolysis, regarding implementation, validation and comparison of different devolatilization models, assessing their capacity to predict the total volatile yield, depending on heating rate, temperature and coal type.
info:eu-repo/semantics/nonPublishe
Applied Thermal Engineering, 2015
ABSTRACT Flameless combustion is able to provide high combustion efficiency with low NOx and soot... more ABSTRACT Flameless combustion is able to provide high combustion efficiency with low NOx and soot emissions. The present work aims at investigating the role of closure sub-models for the modelling of a flameless furnace, as well as the main NO formation paths. Among the different turbulence models that were tested, modified k–ɛ provides the best agreement with the experimental data, especially for temperature measurements. Reynolds stress model leads to smaller deviation for radial velocity predictions. Since in flameless combustion regime the turbulence–chemistry interaction as well as the kinetic mechanism play a fundamental role, the Eddy Dissipation Concept (EDC), coupled with four different kinetic schemes (JL, KEE58, GRI 2.11 and GRI 3.0) was considered. The GRI 2.11 and KEE58 mechanisms perform better, thus confirming the necessity of turbulence/chemistry interaction models accounting for finite-rate chemistry when flameless combustion is studied. As far as NO emissions are concerned, the N2O intermediate NO mechanism is found to play a major role, while thermal NO formation mechanism is not as relevant as in traditional combustion regime. An assessment of the uncertainty related to the choice of boundary conditions as well as to the choice of the parameters of the physical models is also performed. Finally the operation characteristics (such as the recirculation rate and the location of the reaction zone) of the furnace are evaluated.
International Journal of Hydrogen Energy, 2008
A numerical and experimental investigation of a burner operating in MILD combustion regime and fe... more A numerical and experimental investigation of a burner operating in MILD combustion regime and fed with methane and methane-hydrogen mixtures (with hydrogen content up to 20% by wt.) is presented. Numerical simulations are performed with two different combustion models, i.e. the ED/FR and EDC models, and three kinetic mechanisms, i.e. global, DRM-19 and GRI-3.0. Moreover, the influence of molecular diffusion on the predictions is assessed. Results evidence the need of a detailed chemistry approach, especially with H 2 , to capture the volumetric features of MILD combustion. The inclusion of molecular diffusion influences the prediction of H 2 distribution; however, the effects on the temperature field and on the major species are negligible for the present MILD combustion system. A simple NO formation mechanism based on the thermal and prompt routes is found to provide NO emissions in relatively good agreement with experimental observations only when applied on temperature fields obtained with the EDC model and detailed chemistry.
Energy & Fuels, 2013
Interaction between turbulent mixing and chemical kinetics is the key parameter which determines ... more Interaction between turbulent mixing and chemical kinetics is the key parameter which determines the combustion regime: only understanding such interaction may provide insight into the physics of the flame and support the choice and/or development of modeling tools. Turbulence/chemistry interaction may be evaluated through the analysis of the Damköhler number distribution, which represents the flow to chemical timescale ratio. Large Damköhler values indicate mixing controlled flames. On the other hand, low Damköhler values corresponds to slow chemical reactions: reactants and products are quickly mixed by turbulence so the system behaves like a perfect stirred reactor. The calculation of the Damköhler number requires the definition of proper flow and chemical timescales. For turbulent conditions, the flow timescale can be evaluated as the integral timescale , even though in literature other mixing scales
Over the past 30 years, the Eddy Dissipation Concept (EDC) has been widely applied in the industr... more Over the past 30 years, the Eddy Dissipation Concept (EDC) has been widely applied in the industry for the numerical simulations of turbulent combustion problems. The success of the EDC is mainly due to its ability to incorporate detailed chemical mechanisms at an affordable computational cost compared to some other models. Detailed kinetic schemes are necessary in order to capture turbulent flames where there is strong coupling between the turbulence and chemical kinetics. Such flames are found in Moderate and Intense Low-oxygen Dilution (MILD) combustion, where chemical time scales are increased compared with conventional combustion, mainly because of slower reactions (due to the dilution of reactants). Recent modelling studies have highlighted limitations of the standard EDC model when applied to the simulation of MILD systems, noticeably a significant overestimation of temperature levels. Modifications of the model coefficients were proposed to account for the specific features of MILD combustion, i.e. an extension of the reaction region and the reduction of maximum temperatures. The purpose of the present paper is to provide functional expressions showing the dependency of the EDC coefficients on dimensionless flow parameters such as the Reynolds and Damköhler numbers, taking into account the specific features of the MILD combustion regime, where the presence of hot diluent and its influence on the flow and mixing fields impacts on the reaction rate and thermal field. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet in Hot Co-flow (JHC) burner at different co-flow compositions (3%, 6% and 9% O2 mass fraction) and fuel-jet Reynolds numbers (5000, 10,000 and 20,000). Results show promising improvement with respect to the standard EDC formulation, especially at diluted conditions and medium to low Reynolds numbers.
info:eu-repo/semantics/nonPublishe
info:eu-repo/semantics/nonPublishe
Law of the wall for smooth and sand-grain roughened surfaces as a function of the dimensionless s... more Law of the wall for smooth and sand-grain roughened surfaces as a function of the dimensionless sand-grain roughness height k + S Blocken et al. [1].. .. 12 3 Turbulent kinetic energy profiles at the inlet and outlet sections of an empty computational domain (see Figure 2b), Dashes: cell value for turbulent dissipation rate and turbulent kinetic energy averaged over the first cell. Short dashes: cell value for turbulent dissipation rate and kinetic energy.
info:eu-repo/semantics/publishe
ABSTRACT The prediction of NO emissions from industrial burners represents a key goal of Computat... more ABSTRACT The prediction of NO emissions from industrial burners represents a key goal of Computational Fluid Dy- namics (CFD) aided design. Simplified NO formation mechanisms are usually desirable, to reduce the computational eort related to the numerical simulations; however, they must be able to capture the NO trends with acceptable accuracy. Simplified mechanisms for the thermal and prompt NO formation routes are generally available within the existing commercial CFD packages and they provide acceptable NO predic- tions at relatively high temperatures. However, when operating at lower temperatures and with hydrogen, other mechanisms can be relevant, such as those involving N2O and NNH intermediates. This can become particularly relevant in non traditional combustion regimes, such as flameless combustion, characterized by operating temperature far below the levels observed in traditional burners. The present work shows a nu- merical and experimental investigation of a flameless combustion burner operating with methane-hydrogen mixtures with a H2 content up to 50% by wt. The work is focused on the requirements of the CFD model for the accurate prediction of the NO emissions from the burner. In particular, the influence of the combustion model and kinetic mechanism on the temperature fields on which the NO prediction is based is thoroughly discussed, together with the simplified NO formation paths to be included in the model. The approach based on the direct coupling of simplified NO mechanisms to the CFD calculation is compared to a dierent methodology, based on the post-processing of the CFD results with detailed kinetic mechanism for the gas- phase combustion and pollutants formation. A validation methodology is also implemented to quantitatively assess the degree of agreement between the numerical results and the experiments and to guide the selection of the modeling parameters required for predicting NO emissions accurately.
Journal of Wind Engineering and Industrial Aerodynamics, 2012
The simulation of Atmospheric Boundary Layer (ABL) flows is commonly performed using commercial C... more The simulation of Atmospheric Boundary Layer (ABL) flows is commonly performed using commercial CFD codes with RANS turbulence modeling, applying the standard k-e model. However, when applied to the simulation of the homogenous ABL, this approach may result in an undesired decay of the velocity and turbulent fully-developed profiles specified at the inlet of the computational domain. This behavior is due to an inconsistency between turbulence model, inflow conditions and wall function formulation. An approach has been introduced recently to overcome this problem, which consists in the modification of the turbulence model and wall function formulation to retrieve an overall consistent treatment of the neutral ABL. Such methodology, previously applied to simulation of the atmospheric boundary layer over flat terrain and ground-mounted bluff bodies, is here applied to the simulation of the flow over complex terrains and hills, at wind tunnel and atmosphere scale. In a time of limited scientific funding, the availability of open source CFD software such as OpenFOAM is a very attractive option to investigate; therefore, a comparison between OpenFOAM and the commercial code FLUENT 13.0 has been carried out in the present paper. The potential of the proposed methodology and the satisfactory performances of OpenFOAM are demonstrated.
Volume 3: Coal, Biomass and Alternative Fuels; Cycle Innovations; Electric Power; Industrial and Cogeneration, 2015
Micro Gas Turbines (mGTs) have arisen as a promising technology for Combined Heat and Power (CHP)... more Micro Gas Turbines (mGTs) have arisen as a promising technology for Combined Heat and Power (CHP) thanks to their overall energy efficiencies of 80% (30% electrical + 50% thermal) and the advantages they offer with respect to internal combustion engines. The main limitation of mGTs lies in their rather low electrical efficiency: whenever there is no heat demand, the exhaust gases are directly blown off and the efficiency of the unit is reduced to 30%. Operation in such conditions is generally not economical and can eventually lead to shutdown of the machine. To address this issue, the mGT cycle can be modified so that in moments of low heat demand the heat in the exhaust gases is used to warm up water which is then re-injected in the cycle, thereby increasing the electrical efficiency. The introduction of a saturation tower allows for water injection in mGTs: the resulting cycle is known as a micro Humid Air Turbine (mHAT). The static performance of the mGT Turbec T100 working as an...
info:eu-repo/semantics/nonPublishe
ABSTRACT The oxy-coal combustion is emerging as the most likely low-cost " clean coal &a... more ABSTRACT The oxy-coal combustion is emerging as the most likely low-cost " clean coal " technology for both carbon capture and NOx and SOx emissions reduction. The use of CFD tools is crucial for cost-effective oxy-fuel technologies development and environmental concerns minimization. The coupling of detailed chemistry and CFD simulations is still prohibitive, especially for large-scale plants, because of the high computational effort required. So, the development of simple and reliable kinetic mechanisms is therefore necessary. This work presents a CFD study on coal pyrolysis, regarding implementation, validation and comparison of different devolatilization models, assessing their capacity to predict the total volatile yield, depending on heating rate, temperature and coal type.
info:eu-repo/semantics/nonPublishe
Applied Thermal Engineering, 2015
ABSTRACT Flameless combustion is able to provide high combustion efficiency with low NOx and soot... more ABSTRACT Flameless combustion is able to provide high combustion efficiency with low NOx and soot emissions. The present work aims at investigating the role of closure sub-models for the modelling of a flameless furnace, as well as the main NO formation paths. Among the different turbulence models that were tested, modified k–ɛ provides the best agreement with the experimental data, especially for temperature measurements. Reynolds stress model leads to smaller deviation for radial velocity predictions. Since in flameless combustion regime the turbulence–chemistry interaction as well as the kinetic mechanism play a fundamental role, the Eddy Dissipation Concept (EDC), coupled with four different kinetic schemes (JL, KEE58, GRI 2.11 and GRI 3.0) was considered. The GRI 2.11 and KEE58 mechanisms perform better, thus confirming the necessity of turbulence/chemistry interaction models accounting for finite-rate chemistry when flameless combustion is studied. As far as NO emissions are concerned, the N2O intermediate NO mechanism is found to play a major role, while thermal NO formation mechanism is not as relevant as in traditional combustion regime. An assessment of the uncertainty related to the choice of boundary conditions as well as to the choice of the parameters of the physical models is also performed. Finally the operation characteristics (such as the recirculation rate and the location of the reaction zone) of the furnace are evaluated.
International Journal of Hydrogen Energy, 2008
A numerical and experimental investigation of a burner operating in MILD combustion regime and fe... more A numerical and experimental investigation of a burner operating in MILD combustion regime and fed with methane and methane-hydrogen mixtures (with hydrogen content up to 20% by wt.) is presented. Numerical simulations are performed with two different combustion models, i.e. the ED/FR and EDC models, and three kinetic mechanisms, i.e. global, DRM-19 and GRI-3.0. Moreover, the influence of molecular diffusion on the predictions is assessed. Results evidence the need of a detailed chemistry approach, especially with H 2 , to capture the volumetric features of MILD combustion. The inclusion of molecular diffusion influences the prediction of H 2 distribution; however, the effects on the temperature field and on the major species are negligible for the present MILD combustion system. A simple NO formation mechanism based on the thermal and prompt routes is found to provide NO emissions in relatively good agreement with experimental observations only when applied on temperature fields obtained with the EDC model and detailed chemistry.
Energy & Fuels, 2013
Interaction between turbulent mixing and chemical kinetics is the key parameter which determines ... more Interaction between turbulent mixing and chemical kinetics is the key parameter which determines the combustion regime: only understanding such interaction may provide insight into the physics of the flame and support the choice and/or development of modeling tools. Turbulence/chemistry interaction may be evaluated through the analysis of the Damköhler number distribution, which represents the flow to chemical timescale ratio. Large Damköhler values indicate mixing controlled flames. On the other hand, low Damköhler values corresponds to slow chemical reactions: reactants and products are quickly mixed by turbulence so the system behaves like a perfect stirred reactor. The calculation of the Damköhler number requires the definition of proper flow and chemical timescales. For turbulent conditions, the flow timescale can be evaluated as the integral timescale , even though in literature other mixing scales
Over the past 30 years, the Eddy Dissipation Concept (EDC) has been widely applied in the industr... more Over the past 30 years, the Eddy Dissipation Concept (EDC) has been widely applied in the industry for the numerical simulations of turbulent combustion problems. The success of the EDC is mainly due to its ability to incorporate detailed chemical mechanisms at an affordable computational cost compared to some other models. Detailed kinetic schemes are necessary in order to capture turbulent flames where there is strong coupling between the turbulence and chemical kinetics. Such flames are found in Moderate and Intense Low-oxygen Dilution (MILD) combustion, where chemical time scales are increased compared with conventional combustion, mainly because of slower reactions (due to the dilution of reactants). Recent modelling studies have highlighted limitations of the standard EDC model when applied to the simulation of MILD systems, noticeably a significant overestimation of temperature levels. Modifications of the model coefficients were proposed to account for the specific features of MILD combustion, i.e. an extension of the reaction region and the reduction of maximum temperatures. The purpose of the present paper is to provide functional expressions showing the dependency of the EDC coefficients on dimensionless flow parameters such as the Reynolds and Damköhler numbers, taking into account the specific features of the MILD combustion regime, where the presence of hot diluent and its influence on the flow and mixing fields impacts on the reaction rate and thermal field. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet in Hot Co-flow (JHC) burner at different co-flow compositions (3%, 6% and 9% O2 mass fraction) and fuel-jet Reynolds numbers (5000, 10,000 and 20,000). Results show promising improvement with respect to the standard EDC formulation, especially at diluted conditions and medium to low Reynolds numbers.