Akhilesh Sahu - Academia.edu (original) (raw)
Papers by Akhilesh Sahu
Korean Journal of Chemical Engineering, 2020
The influence of frictional packing limit ( FPL ) on prediction of hydrodynamics and performance ... more The influence of frictional packing limit ( FPL ) on prediction of hydrodynamics and performance of fluidized bed reactors was studied. Dense gas-solid flows in non-reactive (under isothermal cold and at elevated temperatures) and reactive atmospheres (fluidized bed gasifier) were simulated using Eulerian-Eulerian methodology considering a range of values for FPL . Simulations under cold flow conditions were conducted to establish a range of FPL values that provides physically realistic predictions. It is noticed that bed pressure drop increases with increasing value of FPL when superficial gas velocity ( U ) is less than or equal to the minimum fluidization velocity. For larger values of U , predicted pressure drop is unaffected by the choice of value of FPL . However, in these cases, the distribution of particles, their velocities and bubbling behavior are significantly affected by FPL . Effect of FPL at elevated temperatures is similar to the one observed at cold flow conditions. It is further noticed that FPL not only affects the predictions on bed hydrodynamics but also has profound influence on reactive flow characteristics such as bed temperature and product gas composition. Sensitivity analysis under cold flow conditions could reveal better predictions when the ratio of FPL to close packing limit is chosen between 0.9 and 0.97.
International Journal of Thermal Sciences, 2018
In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flo... more In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flow inside the fluidized bed reactor are investigated systematically. To this end, 3D simulations have been carried out by incorporating the appropriate model parameters and using the well-known Euler-Euler two fluid methodology in ANSYS FLUENT. The methodology is validated against the experimental results available in the literature. Temperature, air velocity and particle sizes are varied systematically. Results show that the variation of minimum fluidization velocity with temperature depends upon the particle size. For small particles (Geldart's group B), the minimum fluidization velocity decreases with an increase in temperature and the trend is reversed when large particles (Geldart's group D) are fluidized. This behavior is explained by analyzing the modification in inter-phase momentum exchange coefficient due to temperature variation. Voidage profiles, particle velocity and bubble characteristics are also seen to be influenced by the thermal conditions of the fluidized bed reactor. Particle axial velocity tends to decrease with an increase in the operating temperature. For group B particles, temperature has negligible effect on bubble size and expanded bed height. On the other hand, bubble size and expanded bed height decrease with an increase in temperature for group D particles.
Advanced Powder Technology, 2019
A numerical model for simulating a fluidized bed gasifier should include appropriate parameters t... more A numerical model for simulating a fluidized bed gasifier should include appropriate parameters to capture the dynamics of gas-solid flows, gasification kinetics and the interaction between these two. The focus of the present study is to analyze the effects of coal gasification chemistry models reported in literature on the prediction of product gas composition in a fluidized bed gasification reactor. Numerical results are validated against the experimental data available in literature. The validated model is used to examine the available chemical kinetics schemes for water gas shift reaction, steam methane reforming reaction and char heterogeneous reactions. It is also used to assess the effects of hydrodynamic models parameters such as drag model, particle-particle restitution coefficient and specularity coefficient on exit gas composition. Results show that the predictions of product gas composition are notably affected by the choices of the kinetics schemes for water gas shift and steam methane reforming reactions. Systematic analysis using the available choices to simulate initial processes such as moisture removal, volatile and tar cracking is reported. Drag models and the value of specularity coefficient are shown to have no effect on product gas composition, and the particle-particle restitution coefficient slightly influences the predicted gas composition.
Proceedings of the Combustion Institute, 2017
Experiments have been conducted to study the effect of ullage height on steady mass burning rates... more Experiments have been conducted to study the effect of ullage height on steady mass burning rates in methanol pool flames in a cavity. Two burner diameters are used. At low ullages, the flame dynamics are found to be effective in altering the mass burning rates. From baseline case with almost zero ullage, as the ullage is increased, mass burning rate decreases. It produces a local minimum at a given ullage based on the burner internal diameter. After this point, the mass burning rate increases with increasing ullage and reaches an almost uniform value. Numerical simulations are used to complement the results of the experimental study. Low ullage cases have been simulated using a validated numerical model that uses global single step chemistry, partial equilibrium for carbon-dioxide oxidation and optically thin approximation based radiation model. An axisymmetric domain has been employed. Even though the mass burning rates have been over-predicted by the numerical model, the variation trend has been captured quite well. Results from the numerical model reveal that for very low ullage, flame is phenomenally steady and mass burning rate is higher as the diffusion flame anchors around the rim. As the ullage is increased, a transient flame is seen to anchor around the rim and due to increased flame stand-off, the mass burning rate decreases. When the ullage is further increased, due to axial flapping of the flame that partially covers the burner, oxygen is transported into the burner, causing a recirculation pattern within the burner and partial premixing of fuel vapor and oxygen. As a result, the mass burning rate increases.
Progress in Computational Fluid Dynamics, An International Journal, 2016
Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodol... more Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodology. The method used is primarily based on the kinetic theory of granular flow (KTGF) and additionally by incorporating the features of frictional pressure models (FPM). Frictional stresses are accounted when solid volume fraction reaches the frictional packing limit (FPL). Investigations on the effects of bed pressure drop and other gas-solid flow characteristics have revealed that a value for frictional packing limit around 0.61 yields better results. It is also found that the FPM affects the bed hydrodynamics up to a superficial gas velocity of around 1.5 times the minimum fluidisation velocity. The numerical results of bed pressure drop and bed expansion ratio are validated against the corresponding experimental data available in literature. Detailed velocity and voidage profiles are reported along with the contours of solid volume fraction, and velocity vectors of gas and solid phases.
Proceedings of the Combustion Institute, 2015
Ignition of coal dusts deposited over sufficiently hot surfaces represents a common industrial ha... more Ignition of coal dusts deposited over sufficiently hot surfaces represents a common industrial hazard. If these surfaces are above a minimum threshold temperature, then the heat transfer from them and the associated chemical heat release can cause spontaneous ignition. This ignition front can be an ignition site for nearby combustibles as well, which depends on the location of ignition and the corresponding surface temperature attained by the coal dust. A comprehensive numerical study of ignition process of coal dust layers in different configurations, by solving the coupled Navier-Stokes and energy equations, provides a clear insight of the thermal field in the coal layer as well as the flow field over the coal surface. Such study will also provide quantitative information about the convective heat transfer coefficient, which varies for different cases. In this study, ignition phenomena of coal dusts deposited over a flat plate, wedges of different angles and a 3D corner, have been analyzed using a comprehensive numerical model. The governing equations are solved using Ansys FLUENT and user defined functions. The model has been validated using experimental results and is used to study the effect of wedge angles and the direction of gravity vector on the ignition process.
Korean Journal of Chemical Engineering, 2020
The influence of frictional packing limit ( FPL ) on prediction of hydrodynamics and performance ... more The influence of frictional packing limit ( FPL ) on prediction of hydrodynamics and performance of fluidized bed reactors was studied. Dense gas-solid flows in non-reactive (under isothermal cold and at elevated temperatures) and reactive atmospheres (fluidized bed gasifier) were simulated using Eulerian-Eulerian methodology considering a range of values for FPL . Simulations under cold flow conditions were conducted to establish a range of FPL values that provides physically realistic predictions. It is noticed that bed pressure drop increases with increasing value of FPL when superficial gas velocity ( U ) is less than or equal to the minimum fluidization velocity. For larger values of U , predicted pressure drop is unaffected by the choice of value of FPL . However, in these cases, the distribution of particles, their velocities and bubbling behavior are significantly affected by FPL . Effect of FPL at elevated temperatures is similar to the one observed at cold flow conditions. It is further noticed that FPL not only affects the predictions on bed hydrodynamics but also has profound influence on reactive flow characteristics such as bed temperature and product gas composition. Sensitivity analysis under cold flow conditions could reveal better predictions when the ratio of FPL to close packing limit is chosen between 0.9 and 0.97.
International Journal of Thermal Sciences, 2018
In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flo... more In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flow inside the fluidized bed reactor are investigated systematically. To this end, 3D simulations have been carried out by incorporating the appropriate model parameters and using the well-known Euler-Euler two fluid methodology in ANSYS FLUENT. The methodology is validated against the experimental results available in the literature. Temperature, air velocity and particle sizes are varied systematically. Results show that the variation of minimum fluidization velocity with temperature depends upon the particle size. For small particles (Geldart's group B), the minimum fluidization velocity decreases with an increase in temperature and the trend is reversed when large particles (Geldart's group D) are fluidized. This behavior is explained by analyzing the modification in inter-phase momentum exchange coefficient due to temperature variation. Voidage profiles, particle velocity and bubble characteristics are also seen to be influenced by the thermal conditions of the fluidized bed reactor. Particle axial velocity tends to decrease with an increase in the operating temperature. For group B particles, temperature has negligible effect on bubble size and expanded bed height. On the other hand, bubble size and expanded bed height decrease with an increase in temperature for group D particles.
Advanced Powder Technology, 2019
A numerical model for simulating a fluidized bed gasifier should include appropriate parameters t... more A numerical model for simulating a fluidized bed gasifier should include appropriate parameters to capture the dynamics of gas-solid flows, gasification kinetics and the interaction between these two. The focus of the present study is to analyze the effects of coal gasification chemistry models reported in literature on the prediction of product gas composition in a fluidized bed gasification reactor. Numerical results are validated against the experimental data available in literature. The validated model is used to examine the available chemical kinetics schemes for water gas shift reaction, steam methane reforming reaction and char heterogeneous reactions. It is also used to assess the effects of hydrodynamic models parameters such as drag model, particle-particle restitution coefficient and specularity coefficient on exit gas composition. Results show that the predictions of product gas composition are notably affected by the choices of the kinetics schemes for water gas shift and steam methane reforming reactions. Systematic analysis using the available choices to simulate initial processes such as moisture removal, volatile and tar cracking is reported. Drag models and the value of specularity coefficient are shown to have no effect on product gas composition, and the particle-particle restitution coefficient slightly influences the predicted gas composition.
Proceedings of the Combustion Institute, 2017
Experiments have been conducted to study the effect of ullage height on steady mass burning rates... more Experiments have been conducted to study the effect of ullage height on steady mass burning rates in methanol pool flames in a cavity. Two burner diameters are used. At low ullages, the flame dynamics are found to be effective in altering the mass burning rates. From baseline case with almost zero ullage, as the ullage is increased, mass burning rate decreases. It produces a local minimum at a given ullage based on the burner internal diameter. After this point, the mass burning rate increases with increasing ullage and reaches an almost uniform value. Numerical simulations are used to complement the results of the experimental study. Low ullage cases have been simulated using a validated numerical model that uses global single step chemistry, partial equilibrium for carbon-dioxide oxidation and optically thin approximation based radiation model. An axisymmetric domain has been employed. Even though the mass burning rates have been over-predicted by the numerical model, the variation trend has been captured quite well. Results from the numerical model reveal that for very low ullage, flame is phenomenally steady and mass burning rate is higher as the diffusion flame anchors around the rim. As the ullage is increased, a transient flame is seen to anchor around the rim and due to increased flame stand-off, the mass burning rate decreases. When the ullage is further increased, due to axial flapping of the flame that partially covers the burner, oxygen is transported into the burner, causing a recirculation pattern within the burner and partial premixing of fuel vapor and oxygen. As a result, the mass burning rate increases.
Progress in Computational Fluid Dynamics, An International Journal, 2016
Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodol... more Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodology. The method used is primarily based on the kinetic theory of granular flow (KTGF) and additionally by incorporating the features of frictional pressure models (FPM). Frictional stresses are accounted when solid volume fraction reaches the frictional packing limit (FPL). Investigations on the effects of bed pressure drop and other gas-solid flow characteristics have revealed that a value for frictional packing limit around 0.61 yields better results. It is also found that the FPM affects the bed hydrodynamics up to a superficial gas velocity of around 1.5 times the minimum fluidisation velocity. The numerical results of bed pressure drop and bed expansion ratio are validated against the corresponding experimental data available in literature. Detailed velocity and voidage profiles are reported along with the contours of solid volume fraction, and velocity vectors of gas and solid phases.
Proceedings of the Combustion Institute, 2015
Ignition of coal dusts deposited over sufficiently hot surfaces represents a common industrial ha... more Ignition of coal dusts deposited over sufficiently hot surfaces represents a common industrial hazard. If these surfaces are above a minimum threshold temperature, then the heat transfer from them and the associated chemical heat release can cause spontaneous ignition. This ignition front can be an ignition site for nearby combustibles as well, which depends on the location of ignition and the corresponding surface temperature attained by the coal dust. A comprehensive numerical study of ignition process of coal dust layers in different configurations, by solving the coupled Navier-Stokes and energy equations, provides a clear insight of the thermal field in the coal layer as well as the flow field over the coal surface. Such study will also provide quantitative information about the convective heat transfer coefficient, which varies for different cases. In this study, ignition phenomena of coal dusts deposited over a flat plate, wedges of different angles and a 3D corner, have been analyzed using a comprehensive numerical model. The governing equations are solved using Ansys FLUENT and user defined functions. The model has been validated using experimental results and is used to study the effect of wedge angles and the direction of gravity vector on the ignition process.