@inproceedings{RakhiSchmidt, author = {Rakhi, Rakhi and Schmidt, Heiko}, title = {A stochastic approach to investigate the incompressible temporally developing turbulent boundary layer}, series = {Conference on Modelling Fluid Flow (CMFF'18), The 17th International Conference on Fluid Flow Technologies Budapest, Hungary, September 4-7, 2018}, booktitle = {Conference on Modelling Fluid Flow (CMFF'18), The 17th International Conference on Fluid Flow Technologies Budapest, Hungary, September 4-7, 2018}, pages = {8}, abstract = {In the present study, we focus on a new application of the One-Dimensional Turbulence (ODT) Medina M{\´e}ndez model to a temporally developing turbulent boundary layer. Due to dimensionality reduction in ODT, this model achieves major cost reductions as compared to full 3D simulations and is, thus, able to explore large parameter regimes. The model is fully and deterministically resolved for the diffsion effcts along a 1-Medina M{\´e}ndez D domain, while the turbulent advection effects are represented by means of mapping events. Here, we apply the model for the first time to incompressible temporally developing turbulent boundary layers and compare our results to DNS [1]. We use no-slip and impermeable boundary conditions at the top and the bottom wall, respectively. To trigger the transition in the present study, uniform velocity is provided at the bottom wall for different bulk Reynolds numbers ( for Reb = 500, 1500, 1000, 2000). We compare the velocity statistics i.e. mean, root mean square and cross stresses to the recent DNS data. The comparison suggests that ODT is capable to reproduce several DNS velocity statistics which makes it an interesting tool to investigate higher Reynolds numbers as well as passive and active scalars in the future.}, language = {en} } @misc{RakhiSchmidt, author = {Rakhi, Rakhi and Schmidt, Heiko}, title = {Investigating incompressible temporally developing turbulent boundary layers using One-Dimensional Turbulence}, series = {Proceedings in applied mathematics and mechanics}, volume = {18}, journal = {Proceedings in applied mathematics and mechanics}, number = {1}, issn = {1617-7061}, doi = {10.1002/pamm.201800214}, pages = {2}, language = {en} } @misc{RakhiKleinMedinaMendezetal., author = {Rakhi, Rakhi and Klein, Marten and Medina M{\´e}ndez, Juan Ali and Schmidt, Heiko}, title = {One-dimensional turbulence modelling of incompressible temporally developing turbulent boundary layers with comparison to DNS}, series = {Journal of Turbulence}, volume = {20}, journal = {Journal of Turbulence}, number = {8}, issn = {1468-5248}, doi = {10.1080/14685248.2019.1674859}, pages = {506 -- 543}, abstract = {The incompressible temporally developing turbulent boundary layer (TBL) is analysed using the map-based stochastic one-dimensional turbulence (ODT) model. The TBL is a canonical flow problem, which is, in the present study, formed by a planar moving wall and a free stream at rest. An understanding of this idealised flow is of fundamental relevance for the numerical analysis of turbulent boundary-layer-type flows. In the present ODT simulations, the flow variables are resolved on all scales along a wall-normal, one-dimensional domain. These variables are evolved by a deterministic and a stochastic process. The latter models the effect of turbulent advection and pressure fluctuations, whereas the former represents molecular diffusion. The model is appropriate for high Reynolds numbers for which the turbulence field exhibits a broad range of scales and is notionally featureless. We show that ODT is able to capture salient features of the TBL by comparing the various statistics with available reference direct numerical simulation (DNS) results for different bulk Reynolds numbers in the range 250 ≤ Reb ≤ 2000 using fixed model parameters. The influence of the model parameters is analysed for Reb = 1000 and optimal parameter values are provided. The results discussed in this paper suggest that ODT is an economical and reasonably accurate approach for the simulation of transient turbulent boundary-layer-type flows.}, language = {en} } @misc{RakhiSchmidt, author = {Rakhi, Rakhi and Schmidt, Heiko}, title = {Using a Stochastic One-Dimensional Turbulence model to study Incomporessible Spatially Developing Turbulent Boundary Layers}, series = {Proc. of the 11th International Symposium on Turbulence and Shear Flow Phenomena (TSFP11), Southampton, UK, July 30 to August 2, 2019}, journal = {Proc. of the 11th International Symposium on Turbulence and Shear Flow Phenomena (TSFP11), Southampton, UK, July 30 to August 2, 2019}, pages = {6}, language = {en} } @misc{RakhiSchmidt, author = {Rakhi, Rakhi and Schmidt, Heiko}, title = {One-dimensional turbulence: application to incompressible spatially developing turbulent boundary layers}, series = {Journal of Heat and Fluid Flow}, journal = {Journal of Heat and Fluid Flow}, number = {85}, issn = {0142-727X}, doi = {10.1016/j.ijheatfluidflow.2020.108626}, pages = {16}, language = {en} } @misc{RakhiGuentherRichteretal., author = {Rakhi, Rakhi and G{\"u}nther, Vivien and Richter, Jana and Mauß, Fabian}, title = {Steam reforming of methane over nickel catalyst using a one-dimensional model}, series = {International Journal of Environmental Sciences}, volume = {5}, journal = {International Journal of Environmental Sciences}, number = {1}, issn = {2519-5549}, doi = {10.47604/ijes.1520}, pages = {1 -- 32}, abstract = {Steam reforming of hydrocarbons is a well established chemical process which provides synthesis gas (H2 and CO). These synthesis products can hence be converted to numerous valuable basic chemicals. For the industrial application of steam reforming, a detailed understanding of the process is a prerequisite. Models that capture the detailed homogeneous and heterogeneous reaction kinetics and the comprehensive transport processes as well as their interaction have the potential to optimize the catalytic process without expensive experimental campaigns. In this paper, a detailed investigation has been done using a multi-step reaction mechanism for modeling steam reforming of methane over nickel-based catalyst using a one-dimensional (1D) model, LOGEcat [1]. The model is applicable to the simulation of all standard after-treatment catalytic processes of combustion exhaust gas along with other chemical processes involving heterogeneous catalysis, such as, the Sabatier process [27]. It is a 1D tool, thus is computationally cost effective and is based on a series of perfectly stirred reactors (PSR). The model is used to perform the simulations for various reactor conditions in terms of temperature, pressure, flow rates and steam-to-carbon (S/C) ratio. Several chemical reaction terms, such as, selectivity, yield, conversion, and mole fraction have been shown with respect to the varied parameters and the results are compared with 2D simulations and experimental reference data. We report a very good agreement of the various profiles produced with 1D model as compared to the reference data. Note that the main aim of this study is to check how far the 1D model can capture the basic chemistry for modeling steam reforming of methane over nickel-based catalysts. It is interesting to note that the cost effective reduced order model is capable to capture the physics and chemistry involved with a multi-step reaction mechanism showing the predictive capability of the model. This study forms the basis for further analysis towards the thermochemistry of the species to develop a kinetically consistent reaction mechanism.}, language = {en} } @misc{RakhiShresthaGuentheretal., author = {Rakhi, Rakhi and Shrestha, Krishna Prasad and G{\"u}nther, Vivien and Mauß, Fabian}, title = {Thermodynamic analysis to develop a detailed surface reaction mechanism}, series = {Fuel Science - From Production to Propulsion, Aachen, Germany, May 2022}, journal = {Fuel Science - From Production to Propulsion, Aachen, Germany, May 2022}, pages = {2}, abstract = {The reduction of greenhouse gasses such as CO2 and CH4 is becoming necessary due to global environmental problems. The reforming of light hydrocarbons is a particularly efficient process for producing synthesis gas, H2 and CO, from greenhouse gasses [1]. The steam reforming of methane is the most important method to produce syngas in industry by using a nickel catalyst. Nickel-based catalysts are the conventional catalysts in industrial applications due to their fast turnover rates, good availability, and low costs, however, limited by their tendency towards coke formation. In this study, a detailed surface reaction mechanism is developed for steam reform-ing of methane over nickel and results are compared with the reference data.}, language = {en} } @misc{RakhiGiriGuentheretal., author = {Rakhi, Rakhi and Giri, Binod Raj and G{\"u}nther, Vivien and Mauss, Fabian}, title = {Investigation dry reforming of methane over nickel using a one-dimensional model}, series = {PAMM}, volume = {23}, journal = {PAMM}, number = {4}, doi = {10.1002/pamm.202300266}, pages = {8}, abstract = {In the field of catalysis, dry reforming, that is, methane reforming with CO2, is in the focus due to growing environmental concerns about oil depletion and global warming with a desire to produce synthesis gas. However, this process can lead to the formation of carbon, which can cause catalyst deactivation, especially at industrial conditions. Nevertheless, the key to develop a more coke-resistant catalyst is a better comprehension of the reforming process at a molecular level. Regardless of all the investigations available in literature, the detailed path for the conversion of methane to syngas and carbon remains a controversial issue. Another problem in setting up a reaction mechanism is the difficulty to define the thermodynamic data for intermediate surface species and this leads to the development of thermodynamic consistent surface reaction mechanisms in literature where the thermodynamic data are not used to calculate the rate coefficients of the reverse reactions. Rather the Arrhenius parameters for the forward as well as backward reactions are explicitly given in the reaction mechanism to establish thermodynamic equilibrium. In this investigation, a kinetically consistent detailed surface reaction mechanism is developed which consists of 26 reversible reactions with the help of a one-dimensional model, LOGEcat. Our previous work constructs the basis of the present investigation. Further, a detailed sensitivity analysis of reversible reactions and reaction pathways is performed to understand the mechanism better. The mechanism is validated for dry reforming of methane over nickel catalyst, however, it can also be used for other processes, such as, steam reforming and partial oxidation. The mechanism is tested by comparing the simulation results with the literature experiments and simulations in a wide range of temperature. The new developed kinetically consistent surface reaction mechanism is able to accurately express the dry reforming of methane over the nickel catalyst for complete range of temperature and also provide a useful insight into the key rate determining steps.}, language = {en} } @misc{RakhiGiriGuentheretal., author = {Rakhi, Rakhi and Giri, Binod Raj and G{\"u}nther, Vivien and Mauss, Fabian}, title = {Insight into the thermodynamic model for reforming of methane over nickel catalyst}, series = {Proceedings of the 64th International Conference of Scandinavian Simulation Society, SIMS 2023 V{\"a}ster{\aa}s, Sweden, September 25-28, 2023}, journal = {Proceedings of the 64th International Conference of Scandinavian Simulation Society, SIMS 2023 V{\"a}ster{\aa}s, Sweden, September 25-28, 2023}, doi = {10.3384/ecp200025}, pages = {192 -- 197}, abstract = {The reforming of light hydrocarbons to produce synthesis gas, H2 and CO, is an important intermediate for manufacturing valuable basic chemicals and synthesis fuels. In order to understand these reforming processes better, elementary step reaction mechanisms are developed. In the available literature, the surface reaction mechanisms are usually achieved with the help of reaction kinetic parameters without using the thermochemistry of the species referred to kinetic models due to the unavailability of the thermochemistry of the intermediate species involved in the multi-step reaction mechanism. In this work, investigations are made to obtain the thermochemistry of the intermediate species to establish thermodynamic equilibrium in order to develop a thermodynamic model for steam reforming of methane over nickel. The thermochemistry of the surface bound species is taken from different sources available in the literature and after that a detailed sensitivity analysis is performed to match the results with experiments. The simulation set up is adapted from the literature experiments given in [1]. The results produced with the one-dimensional tool using the thermodynamic model developed in the present investigation consisting of 21 reversible reactions are compared with the kinetic scheme with 42 irreversible reactions from reference simulation along with their experimental results. Both the models show some major differences in the reaction pathways which provides a useful insight into the key rate determining steps and needs further investigations.}, language = {en} } @misc{RakhiGuentherMauss, author = {Rakhi, Rakhi and G{\"u}nther, Vivien and Mauß, Fabian}, title = {A detailed surface reaction mechanism to investigate oxidation of methane over nickel catalyst}, series = {Proceedings in Applied Mathematics \& Mechanics : PAMM}, volume = {22}, journal = {Proceedings in Applied Mathematics \& Mechanics : PAMM}, number = {1}, issn = {1617-7061}, doi = {10.1002/pamm.202200055}, abstract = {We have developed a kinetically consistent detailed surface reaction mechanism for modeling the oxidation of methane over a nickel-based catalyst. A one-dimensional model, LOGEcat based on the single-channel 1D catalyst model, is used to perform the simulations. The original multi-step reaction mechanism is thermodynamically consistent and consists of 52 reactions. By thermodynamic consistency, we mean that the equilibrium is achieved with the support of the Arrhenius parameters and does not depend on the thermochemistry of the species involved in the considered reactions. The detailed mechanism developed in this investigation contains 26 reversible reactions. These reactions are obtained with the use of the thermochemistry of the species. The study focuses on ensuring kinetic consistency and this is done with the help of thermodynamic analysis by bringing the thermochemistry of the species in play in order to develop a surface reaction mechanism. The new mechanism can be used to understand the other processes, for example, steam- and dry-reforming of methane over nickel, however, the main focus of the paper is to check the performance of the detailed mechanism for catalytic partial oxidation of methane. The applicability of the mechanism is checked for various reactor conditions in terms of parameters such as temperature and pressure by comparing the results with the available reference data. The detailed mechanism developed in this study is able to accurately express oxidation of methane over the nickel catalyst for the considered reactor conditions.}, language = {en} } @misc{RakhiGuentherMauss, author = {Rakhi, Rakhi and G{\"u}nther, Vivien and Mauß, Fabian}, title = {Insights into dry reforming of methane over nickel catalyst using a thermodynamic model}, series = {Reaction Kinetics, Mechanisms and Catalysis}, journal = {Reaction Kinetics, Mechanisms and Catalysis}, issn = {1878-5204}, doi = {10.1007/s11144-023-02426-8}, pages = {14}, abstract = {A thermodynamic model is developed using a one-dimensional model, LOGEcat to understand the dry reforming of methane over nickel-based catalysts. To do so, we have extended our previously developed mechanism (Rakhi and Shrestha in React Kinet, Mech Catal 135:3059-3083, 2022) which contains 21 reversible reactions by adding 5 more reversible reactions and updating the thermochemistry of one intermediate species. The adjusted mechanism contains 26 reversible reactions obtained with the help of thermodynamic analysis. This study focuses on using the thermodynamic model for dry reforming of methane and insights into the reaction pathways and sensitivity analysis for the kinetically consistent surface reaction mechanism. The applicability of the mechanism is examined for reactor conditions in terms of parameters such as temperature by comparing the results with the available reference data. The mechanism is able to accurately express the reforming conditions of methane over the nickel catalyst for complete range of temperature and also provide useful insights into the reaction pathways established with the thermodynamic model.}, language = {en} } @misc{MaussRakhi, author = {Mauss, Fabian and Rakhi, Rakhi}, title = {Numerical study of catalytic methanation reactions using a kinetic model}, series = {SAE Technical Paper}, journal = {SAE Technical Paper}, number = {2024-24-0022}, issn = {0148-7191}, doi = {10.4271/2024-24-0022}, pages = {5}, abstract = {Even if huge efforts are made to push alternative mobility concepts, such as, electric cars (BEV) and fuel cell powered cars, the importance and use of liquid fuels is anticipated to stay high during the 2030s. The biomethane and synthetic natural gas (SNG) might play a major role in this context as they are raw material for chemical industry, easy to be stored via existing infrastructure, easy to distribute via existing infrastructure, and versatile energy carrier for power generation and mobile applications. Hence, biomethane and synthetic natural gas might play a major role as they are suitable for power generation as well as for mobile applications and can replace natural gas without any infrastructure changes. In this paper, we aim to understand the direct production of synthetic natural gas from CO2 and H2 in a Sabatier process based on a thermodynamic analysis as well as a multi-step kinetic approach. For this purpose, we thoroughly discuss CO2 methanation to control emission in order to maximize the methane formation and minimize the CO formation and to understand the complex methanation process. We have considered an equilibrium and kinetic modelling study on the NiO-SiO2 catalyst for methanation focusing on CO2 derived SNG. In this work, a comprehensive thermodynamic analysis of CO2 hydrogenation is preformed to define the optimum process parameters followed by the kinetic simulations. Further, the simulations can be performed at various conditions, for example, catalyst mass, mass flow, pressure, temperature etc. to optimize the methanation process.}, language = {en} }