Fluidization of Group B particles with a rotating distributor

A novel rotating distributor fluidized bed is presented. The distributor is a rotating perforated plate, with 1% open-area ratio. This work evaluates the performance of this new design, considering pressure drop, Δp, and quality of fluidization. Bed fluidization was easily achieved with the proposed device, improving the solid mixing and the quality of fluidization. In order to examine the effect of the rotational speed of the distributor plate on the hydrodynamic behavior of the bed, minimum fluidization velocity, Umf, and pressure fluctuations were analyzed. Experiments were conducted in the bubbling free regime in a 0.19 m i.d. fluidized bed, operating with Group B particles according to Geldart's classification. The pressure drop across the bed and the standard deviation of pressure fluctuations, σp, were used to find the minimum fluidization velocity, Umf. A decrease in Umf is observed when the rotational speed increases and a rise in the measured pressure drop was also found. Frequency analysis of pressure fluctuations shows that fluidization can be controlled by the adjustable rotational speed, at several excess gas velocities. Measurements with several initial static bed heights were taken, in order to analyze the influence of the initial bed mass inventory, over the effect of the distributor rotation on the bed hydrodynamics.Publicad

Thermal analysis and optimization of a heat regenerator composed of two coupled moving bed heat exchangers

This work presents a study to optimize the performance of a heat regenerator composed by two coupled moving bed heat exchangers (MBHE). A MBHE is used to recover heat, from a hot gas stream, and the other one is used to preheat an air stream. A direct application might be a gasifier. The heat exchangers performance was studied in two cases, considering or not the conduction heat transfer in the solid phase. When the solid conduction is taken into account, a numerical solution is obtained, while an analytical solution is possible when the conduction terms are neglected. In both cases, the optimum values of bed length (in the air flow direction) and particle diameter were obtained from an exergy point of view. Finally, an energy optimization of the heat regenerator was carried out, obtaining the optimal heat regenerator dimensions as a function of gas velocity and gas flow rate.Publicad

Modelling non-isothermal absorption of vapour into expanding liquid sheets

The problem of non isothermal absorption of vapour into freely expanding liquid sheets is addressed in this study. This is done in the context of four models that characterise the coupled heat and mass transfer in the liquid phase: a nonlinear model retaining the effect of sheet growth, an approximate model for slowly increasing mass flow rate in the sheet, a large Lewis number model and finally, a boundary layer model. These models have been numerically or analytically solved and applied to the comparative analysis of two different working pairs, LiBr H₂O and LiNO₃NH₃, under conditions representative of adiabatic absorption in refrigeration systems. The limits of applicability of each model have been assessed and the sensitivity of the results to the sheet aperture angle, heat of absorption and initial subcooling has also been tested. For equal initial mass fraction and subcooling, the models indicate that Sherwood number and the rate of absorption in laminar expanding sheets for the LiNO₃NH₃solution are always superior to those for the LiBr H₂O solutionThe authors wish to express their gratitude to Dr. M. Venegas for her useful comments. This work has been partially funded by the Spanish Government Research Grants DPI 2002 02439 and ENE 2005 08255 CO2 02, as well as by the Autonomous Community of Madrid & UC3M through CCG07 UC3M/AMB 3412 project. Their contribution is greatly appreciatedPublicad

Voidage distribution around bubbles in a fluidized bed: influence on throughflow

In this work, a new method for measuring void fraction distribution around endogenous bubbles in a 2D fluidized bed is presented. The technique is based on illuminating a transparent-wall 2-dimensional bed with diffuse light from the rear and recording the distribution of light that penetrates the bed. The recording is made with a high speed video-camera, which gives frames with grey level corresponding to the light penetration and from which the voidage distribution around the bubbles can be determined. In this way, voidage distribution in the region very close to the bubble contour (r/Rb≲1.2) is obtained, which was not possible in previous studies due to limitations in spatial resolution. A correlation is proposed for the voidage at the contour of the bubble, with the voidage depending on the radial position and the polar angle ε(r, θ). In addition, the effect of the voidage distribution on the throughflow crossing the bubbles was studied and an increase of 20% was determined for the average bubble geometry of the more than 100 bubbles analysedPublicad

Thermal and mechanical stresses in bayonet tubes of solar central receivers working with molten salt and liquid sodium

One of the most promising technologies for solar thermal power are solar power towers (SPTs), in which direct solar radiation is redirected by heliostats to a receiver located on top of a tower. The technology used by SPT allows obtaining high thermal efficiencies as well as a high number of hours of operation thanks to thermal storage. However, the high thermal gradients to which the receiver is subjected, in addition to the corrosion of the molten solar salt, can cause the rupture of the receiver and this limits the maximum irradiation the receiver can withstand. To overcome this problem there are different strategies, such as the use of working fluids that are less corrosive than molten salts or the development of new designs of the receiver to avoid overheating of the pipes. In this work we analyze the thermal and structural behavior of a new design of SPT receiver in which bayonet tubes are used instead of simple tubes. A bayonet tube consists of a tube inside another one. In the bayonet tube the working fluid first circulates through the inner tube and then through the annular section between the tubes. An eccentric bayonet tube, created by displacing the inner tube with regards the outer tube, reduces the overheating of the fluid and the outer tube wall as will be shown later. Besides, this work also assesses the effect of using either molten salt or liquid sodium as a working fluid on the thermal and structural behavior of the absorber tube. Since the extreme thermal conditions of central receivers preclude a detailed experimental analysis, the analyses of the present work are performed through multi-physics (CFD – FEM) simulations of the working fluid flow in the annular section and the stresses in the outer wall of the bayonet tube, which are the most critical elements of the receiver. In particular, to perform the hydrodynamic and thermal analysis of the fluid section and the outer wall of the tube, the RANS equations of the fluid together with the turbulent RSM model and the head diffusion equation of the wall were solved using ANSYS Fluent v18 CFD code. Boundary conditions of temperature and non-uniform irradiation were selected to represent typical operative conditions of receivers. Subsequently, using the temperature profiles obtained from the CFD simulations for each working fluid, ANSYS Workbench v18 was employed to obtain the thermal and mechanical stresses in the outer tube as a function of its different constraints, including the attachment of the tube. The results obtained with the CFD – FEM simulations show that, regardless of the working fluid, the eccentricity of the bayonet tube decreases local peaks of temperature in the flow and temperature gradients in the outer tube wall, which leads to a reduction of the wall stresses of the SPT receiver. Furthermore, thanks to its high conductivity, liquid sodium is able to yield lower temperature gradients and stresses in the wall, independently of the kind of tube, compared to molten salt

Coherent structures and bubble-particle velocity in 2-D fluidized beds

This work presents an experimental study to characterize ascending bubbles and granular velocity in the dense phase of a 2-D fluidized bed. Three different non-intrusive techniques based on images obtained with a high speed camera are developed, and applied to the images. First the bubble paths are characterized with time-average concentration maps and the bubble velocities are obtained, using a tracking algorithm over the mass centers of the bubbles. Finally, a PIV (particle image velocimetry) method is used to characterize the particle velocity vectors. This procedure is repeated for different bed aspect ratios, and different superficial gas velocities. This study analyzes the superficial gas velocity influence on the bed behavior, and how the bubble path configuration depends on the bed aspect ratio. The PIV measurements give us information on the location of the recirculation regions and the influence of the superficial gas velocity.Publicad

On the minimum fluidization velocity in 2D fluidized beds

In the present study, a new correlation for the determination of the minimum fluidization velocity in 2D fluidized beds was developed. The proposed correlation was based on the experimental results obtained in 2D fluidized beds with different particle sizes, bed thicknesses and bed heights. Thus, the proposed correlation depends only on the nondimensional variable t/dp, where t is the bed thickness and dp is the particle size. The proposed correlation was compared with other experimental results that can be found in the literature, and two different trends were observed. Namely, one set of experimental results was in accordance with the proposed correlation, while the other set deviated from the theoretical results. In particular, the minimum fluidization velocities of the experimental results were greater than the predicted values of the proposed correlation. In view of the differences in the experimental conditions, the observed discrepancies may be attributed to the effects of electrostatic charge and particle shape. In addition, the experimental fluidization– defluidization curves were compared to the theoretical results of Jackson's model, and the parameters were fitted to the experimental data. However, Jackson's model is based on a 1D bed; thus, general parameters could not be obtained for a bed with a fixed particle size and thickness due to the two dimensional voidage distribution in the bed and bed cohesion effects, which are a result of electrostatic forces and are not considered in Jackson's modelThis work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660). Their supports are greatly appreciatedPublicad

Characteristic lengths and maximum entropy estimation from probe signals in the ellipsoidal bubble regime

The bubble size, surface and volume distributions in two and three phase flows are essential to determine energy and mass transfer processes. The traditional approaches commonly use a conditional probability density function of chordlengths to calculate the bubble size distribution, when the bubble size, shape and velocity are known. However, the approach used in this paper obtains the above distributions from statistical relations, requiring only the moments inferred from the measurements given by a sampling probe. Using image analysis of bubbles injected in a water tank, and placing an ideal probe on the image, a sample of bubble diameter, shape factor and velocity angle are obtained. The samples of the bubble chord-length are synthetically generated from these variables. Thus, we propose a semi-parametric approach based on the maximum entropy (MaxEnt) distribution estimation subjected to a number of moment constraints avoiding the use of the complex backward transformation. Therefore, the method allows us to obtain the distributions in close form. The probability density functions of the most important length scales (D,D20,D30,D32), obtained applying the semi-parametric approach proposed here in the ellipsoidal bubble regime, are compared with experimental measurementsPublicad

Characterization of flow-induced vibrations in gas-solid fluidized beds: elements of the theory

This paper revisits the basic hypothesis underlying the measurement of flow-induced vibration in fluidized beds. A novel theoretical approach based on the standing pressure field characterizing the bed dynamics is proposed to link the pressure fluctuations to the measured accelerometer signals. The model provides a reliable prediction of the carrying frequency band and helps in designing the accelerometer measurement process. The model was tested with previous results reported in the literature as well as with piezoelectric accelerometer measurements collected from a lab-scale experimental facility. A study on accelerometer measurements was conducted to identify the main limitations expected for measuring flow-induced vibrations in a gas-solid fluidized bed. The structural response of the vessel to flow-induced vibration was mostly determined by the "bed acoustics" that can be dominated by either elastic or compression waves. Finally, the survival of an envelope process on the measured accelerometer signal guaranteed the quality of the flow dynamical information collected during the measurement process.Financial support from projects DPI2009-10518 (MICINN) and CARDENER-CM (S2009ENE-1660).Publicad