46 research outputs found
Effect of coefficient of restitution in Euler–Euler CFD simulation of fluidized-bed hydrodynamics
No full textCollision between particles plays an important role in determining the hydrodynamic characteristics of gas–solid flow in a fluidized bed. In the present work, earlier work (Loha, Chattopadhyay, & Chatterjee, 2013) was extended to study the effect of the elasticity of particle collision on the hydrodynamic behavior of a bubbling fluidized bed filled with 530-μm particles. The Eulerian–Eulerian two-fluid model was used to simulate the hydrodynamics of the bubbling fluidized bed, where the solid-phase properties were calculated by applying the kinetic theory of granular flow. To investigate the effect of the elasticity of particle collision, different values of the coefficient of restitution were applied in the simulation and their effects were studied in detail. Simulations were performed for two different solid-phase wall boundary conditions. No bubble formation was observed for perfectly elastic collision. The bubble formation started as soon as the coefficient of restitution was set below 1.0, and the space occupied by bubbles in the bed increased with a decrease in the coefficient of restitution. Simulation results were also compared with experimental data available in the literature, and good agreement was found for coefficients of restitution of 0.95 and 0.99
The Federal Approach to FiscalDecentralisation: Conceptual Contours for Policy Makers
No full textChanchal Kumar Sharma,in his paper demonstrates that in order for fiscal decentralisation to be effective, it must be approached federally. A federal approach is not a decentralised approach but a dynamically balanced approach; one that constantly keeps on adjusting the contrasting forces of centralisation and decentralisation to create a system that can ensure good governance in accordance with the rapidly changing global and local scenario. According to the author, the good governance of the present time has to be federally flexible and dynamically decentralised and institutions of fiscal federalism are crucial for achieving such a dynamic equilibrium. Fiscal decentralisation cannot be detached from the broader principles of fiscal federalism if it is to be successful, irrespective of the fact of whether it is being carried out in a federal or non-federal country. He argues that too much decentralisation or an overly strong central federal government precludes the survival of a constitutional federal state.Federalism; Fiscal Decentralization; centralization
The Federal Approach to FiscalDecentralisation: Conceptual Contours for Policy Makers
No full textChanchal Kumar Sharma,in his paper demonstrates that in order for fiscal decentralisation to be effective, it must be approached federally. A federal approach is not a decentralised approach but a dynamically balanced approach; one that constantly
keeps on adjusting the contrasting forces of centralisation and
decentralisation to create a system that can ensure good governance in
accordance with the rapidly changing global and local scenario.
According to the author, the good governance of the present time has to
be federally flexible and dynamically decentralised and institutions of
fiscal federalism are crucial for achieving such a dynamic equilibrium.
Fiscal decentralisation cannot be detached from the broader principles
of fiscal federalism if it is to be successful, irrespective of the fact of
whether it is being carried out in a federal or non-federal country. He
argues that too much decentralisation or an overly strong central federal
government precludes the survival of a constitutional federal state
Energy generation from fluidized bed gasification of rice husk
No full textThough gasification of biomass in fluidized bed system is an efficient way of biomass utilization, limited experimental data on the fluidized bed biomass gasification are available in open literature. Therefore, an experimental study of biomass gasification is conducted using a laboratory scale bubbling fluidized bed gasifier. Rice husk is used as the biomass material and air-steam mixture is used as the gasifying agent. As the non-granular nature of rice husk makes it difficult to fluidize, silica sand is used as the inert bed material to help in fluidization. Parametric studies are performed to determine the effects of reactor temperature, equivalence ratio, and steam-to-biomass ratio on the product gas composition and the heating value. The results show that both hydrogen percentage and the heating value of the product gas increase with increase in gasification temperature and steam-to-biomass ratio, but they decrease with increase in equivalence ratio. The maximum heating value (4.26MJ/Nm3) and hydrogen percentage (13.1%) are obtained at the gasification temperature of 850 oC, equivalence ratio of 0.35 and the steam-to-biomass ratio of 0.8
Assessment of drag models in simulating bubbling fluidized bed hydrodynamics
No full textThe hydrodynamic behavior of gas–solid flow is investigated in a 2-D bubbling fluidized bed reactor filled with 530 mm particles. The Computational Fluid Dynamics (CFD) is used to simulate the complex transient behavior of gas–solid flow. The CFD simulation of the bed hydrodynamics is based on the concept of Euler–Euler two-fluid model in combination with Kinetic Theory of Granular Flow (KTGF). In the present study, four different drag models are used to determine the drag force between the two phases and the results are compared. It is observed that the drag model has a significant effect on the simulation of gas–solid flow. The Gidaspow drag model and Syamlal–O’Brien model could predict the core-annulus structure of the bed very well. In comparison, the energy minimization multi-scale (EMMS) model and McKeen model cannot clearly predict the core-annulus structure of the flow. The Gisadpow model was found to provide better agreement with the experimental results of timeaveraged particle velocity. On the other hand, the Syamlal–O’Brien model and EMMS model predicted the time-averaged granular temperature comparatively well. The effect of turbulence modeling on the flow behavior is also studied by incorporating the RNG k–e turbulence model. The results showed that the effect of turbulence modeling on the bed hydrodynamics is not very significant
Thermodynamic analysis of hydrogen rich synthetic gas generation from fluidized bed gasification of rice husk
No full textThree dimensional kinetic modeling of fluidized bed biomass gasification
No full textBiomass gasification in fluidized bed system by using air–steam mixture as the gasifying agent is a promising way of utilizing biomass because it produces a gaseous fuel having relatively higher calorific value as well as higher hydrogen content with minimum or no heat addition to the gasifier. In the present work, a three dimensional numerical simulation of a bubbling fluidized bed biomass gasifier has been carried out. The numerical simulation is based on the Eulerian–Lagrangian approach where the
fluid phase is solved by using a continuum approach and the solid is modeled by using Lagrangian computational particle model. The chemical reactions are coupled with the complex hydrodynamic calculation of gas–solid fluidized bed. The simulations are performed by varying the gasification
temperature, equivalence ratio and steam-to-biomass ratio. Detail analyses of flow pattern, pressure distribution, and gascomposition distribution have been presented. The complex three dimensional flow structures are revealed by plotting the results in different planes. The results provide a detail insight into the gasifier's behavior including fluidization, thermal and chemical characteristics. Simulated outlet gas compositions are compared with our own experimental data and a very good resemblance is observed
Euler-Euler CFD modeling of fluidized bed: Influence of specularity coefficient on hydrodynamic behavior
No full textEuler-Euler two-fluid model is used to simulate the hydrodynamics of gas–solid flow in a bubbling fluidized bed with Geldert B particles where the solid property is calculated by applying the kinetic theory of granular flow (KTGF). Johnson and Jackson wall boundary condition is used for the particle phase, and different amount of slip between particle and wall is given by varying the specularity coefficient (ᶲ) from 0 to 1. The simulated particle velocity, granular temperature and particle volume fraction are compared to investigate the effect of different wall boundary conditions on the hydrodynamic behavior. Some of the results are also compared with the available experimental data from the literature. It was found that the model predictions are sensitive to the specularity coefficient. The hydrodynamic behavior deviated significantly for ᶲ = 0 and ᶲ = 0.01 with maximum deviation found at ᶲ = 0 i.e. free-slip condition. However, the overall bed height predicted by all the conditions is similar
