DITF - ISE - Artículos de Revistas Científicas
http://hdl.handle.net/10016/963
2014-11-24T17:54:00ZA novel methodology for simulating vibrated fluidized beds using two-fluid models
http://hdl.handle.net/10016/18766
A novel methodology for simulating vibrated fluidized beds using two-fluid models
Acosta-Iborra, Antonio; Hernández-Jiménez, Fernando; Vega, Mercedes de; Villa Briongos, Javier
The present work considers the use of the two-fluid (Euler-Euler) CFD approach for the continuum description of vibrated fluidized beds as a less computationally demanding alternative to the discrete description given by Lagrangian-Eulerian methods such as DEM. In particular, a novel simulation strategy consisting on solving the two-fluid model equations in a coordinate reference system that moves with the vibrating walls of a gas-solid fluidized bed is proposed. By this way, vibration is transformed into simple alternating acceleration terms that are introduced through body forces in both the gas and the particle phase equations. The results of a series of two-fluid model simulations compare well with discrete particle simulations as well as with experimental data reported for beds containing Geldart group B particles. In general, the results of a series of two-fluid model simulations show similar trends to those seen in discrete particle simulations as well as in experimental data reported for beds containing Geldart group B particles. Exception of that is the velocity of bubbles, for which the two-fluid simulations compare less satisfactorily with the available experimental data. The two-fluid model simulations are also able to reproduce expected phenomena like the bubble growth with the vibration amplitude and the dependence of the pressure drop fluctuation on the vibration strength. In view of these promising results, the proposed two-fluid model formulation opens the possibility of increasing the scale of the vibrated fluidized beds currently simulated.
2012-08-01T00:00:00ZSolid conduction effects and design criteria in moving bed heat exchangers
http://hdl.handle.net/10016/13921
Solid conduction effects and design criteria in moving bed heat exchangers
Almendros-Ibáñez, José Antonio; Soria-Verdugo, Antonio; Ruiz-Rivas, Ulpiano; Santana, D.
This work presents a theoretical study of the energetic performance of a moving bed heat exchanger (MBHE), which consists of a flow of solid particles moving down that recovers heat from a gas flow percolating the solids in cross-flow. In order to define the solid conduction effects, two solutions for the MBHE energy equations have been studied: an analytical solution considering only convection heat transfer (and neglecting solid conduction) and a numerical solution with the solid conductivity retained in the equations. In a second part, the power requirements of a MBHE (to pump the gas and to raise the down-flowing particles) are confronted with the heat transferred considering the variation of design parameters, such as gas and solids’ velocities, solids particle diameter or MBHE dimensions. The numerical results show that solid conductivity reduces the global efficiency of the heat exchanger. Therefore, a selection criterion for the solids can be established, in which their thermal conductivity should be minimized to avoid conduction through the solid phase, but to a limit in order to ensure that temperature differences inside an individual solid particle remain small. Regarding the other energy interactions involved in the system, these are at least one order of magnitude lower than the heat exchanged. Nevertheless, for a proper analysis of the system the efficiency of the devices used to pump the gas and to raise the particles and the relative costs of the different energy forms present in the system should be taken into account.
2011-05-01T00:00:00ZCirculation of an object immersed in a bubbling fluidized bed
http://hdl.handle.net/10016/13854
Circulation of an object immersed in a bubbling fluidized bed
Soria-Verdugo, Antonio; García-Gutiérrez, L.M.; Sánchez-Delgado, Sergio; Ruiz-Rivas, Ulpiano
The motion of a large object in a bubbling fluidized bed was experimentally studied using digital image analysis (DIA). The experiments were performed in a 2 D bubbling fluidized bed with glass spheres as bed material. The object motion was measured using non intrusive tracking techniques, while independent measurements of the dense phase velocity (using Particle Image Velocimetry (PIV)) and bubble velocity (using DIA) were carried out. The effect of the dimensionless gas velocity on the object motion was also analyzed. This work characterizes the circulation patterns of an object with a density similar to that of the bed, but much larger in size. Object size and density remained constant throughout the experiments. A comparison between the motion of sinking objects and the motion of the dense phase provided evidence of the feeble effect of buoyant forces on the motion of sinking objects. In contrast, the motion of rising objects is linked to the motion of bubbles. It was found that objects may be raised to the surface of the bed either by the action of a single bubble (one jump) or by several passing bubbles (multiple jumps). Based on these results, the circulation time of objects throughout the bed is a function of two parameters: the maximumdepth attained by an object and the number of jumps during its rising path. This relationship is presented along and the multiple jumps phenomenon is studied in detail. Finally, an estimate of the circulation time of an object based on semi empirical expressions is presented for different dimensionless gas velocities. The probability density function of the circulation time shows two different modes as the object was less prone to be raised atmoderate depths. The estimate of the circulation time was found to be in good agreement with our experimental data
2011-01-01T00:00:00ZBuoyancy effects on objects moving in a bubbling fluidized bed
http://hdl.handle.net/10016/13853
Buoyancy effects on objects moving in a bubbling fluidized bed
Soria-Verdugo, Antonio; García-Gutiérrez, L.M.; García-Hernando, Néstor; Ruiz-Rivas, Ulpiano
The effect of buoyant forces on the motion of a large object immersed in a bubbling fluidized bed (BFB) was experimentally studied using digital image analysis. The experiments were performed in a 2 D bubbling fluidized bed with glass spheres as bed material and cylindrical objects with different densities and sizes. The object motion was measured using non intrusive tracking techniques. The effect of gas velocity was also analyzed. The circulation of an object in a BFB is defined by several parameters. The object might be able to circulate homogeneously throughout the bed or stay in preferred regions, such as the splash zone or the bottom zone. While circulating, the object moves back and forth between the surface of the bed and the inner regions, performing a series of cycles. Each cycle is composed by sinking and rising paths, which can be one or several, depending on whether a passing bubble is able to lift the object to the surface or the object is detached from it or its drift at an intermediate depth. Therefore, the number of rising paths or number of jumps that the object undergo in a cycle, interleaved with sinking paths, and the maximum attained depth characterize each cycle, together with the mean sinking and rising velocities of the object. In this work, experimental measurements of the probability distributions of the number of jumps and the maximum attained depth, the axial homogeneity of object motion and rising and sinking object velocities are presented for objects with different sizes and densities. The results show a coherent behavior, independent of density and size, for the probability distributions of the number of jumps. This is also true for the maximum attained depth, but only when a proper circulation throughout the bed is ensured. Such a proper circulation and axial homogeneity is, on the other hand, much affected by object density, size and gas velocity. Rising and sinking velocities are highly dependent on gas velocity, as established in well known models of bubble and dense phase velocities. Nevertheless, rising velocities are practically unaffected by object density or size, while sinking velocities show a low dependence on density and a steeper one on size. These results suggest that buoyant forces are relevant during the sinking process, and almost neutral during the rising path
2011-06-15T00:00:00Z