Search

Over the last years, the development of PCMs has opened new ways to increase the storage capacity and the energy stored due to the high latent heat and high storage density of these materials. The aim of this work is to model the charging process of a fluidized bed with PCM material operating as an energy storage device. The temperature in the bed during the charging process of the fluidized bed has been modeled using the two-phase theory of fluidization. The dense phase is taken to be perfectly mixed and the bubble phase is taken to be in plug flow. The numerical model presented takes into account the fact that the phase change process of the bed material occurs over a temperature range. The energy equation of the dense phase is numerically solved in enthalpy form considering the dependence of enthalpy on temperature for phase change occurring over a range of temperatures. The model validity is verified against experimental data for two granular materials: sand, a typical material used in fluidized beds, and a granular phase change material with a mean particle diameter of 0.54 mm and a phase change temperature around 50 °C. For both materials the temperature profiles obtained numerically agree with the ones measured experimentally. [ABSTRACT FROM AUTHOR]

Highlights: [•] Study the storage behavior of a fluidized bed filled with PCM. [•] Comparison of the fluidized bed with other storage methods and materials. [•] Study the influence of the bed height and the flow rate in the fluidized bed. [•] The use of PCM in fluidized beds increase the efficiency of stored thermal energy. [ABSTRACT FROM AUTHOR]

Abstract: The results from a two-fluid Eulerian–Eulerian three-dimensional (3-D) simulation of a cylindrical bed, filled with Geldart-B particles and fluidized with air in the bubbling regime, are compared with experimental data obtained from pressure and optical probe measurements in a real bed of similar dimensions and operative conditions. The main objectives of this comparison are to test the validity of the simulation results and to characterize the bubble behavior and bed dynamics. The fluidized bed is 0.193m internal diameter and 0.8m height, and it is filled with silica sand particles, reaching a settle height of 0.22m. A frequency domain analysis of absolute and differential pressure signals in both the measured and the simulated cases shows that the same principal phenomena are reproduced with similar distributions of peak frequencies in the power spectral density (PSD) and width of the spectrum. The local dynamic behavior is also studied in the present work by means of the PSD of the simulated particle fraction and the PSD of the measured optical signal, which reveals as well good agreement between both the spectra. This work also presents, for the first time, comparative results of the measured and the simulated bubble size and velocity in a fully 3-D bed configuration. The values of bubble pierced length and velocity retrieved from the experimental optical signals and from the simulated particle fraction compare fairly well in different radial and axial positions. Very similar values are obtained when these bubble parameters are deduced from either simulated pressure signals or simulated particle volume fraction. In addition, applying the maximum entropy method technique, bubble size probability density functions are also calculated. All these results indicate that the two-fluid model is able to reproduce the essential dynamics and interaction between bubbles and dense phase in the 3-D bed studied. [Copyright &y& Elsevier]

Abstract: In this paper, the effect of a novel rotating distributor for fluidized beds on the bubble size is studied. The distributor is a perforated plate that rotates around the vertical axis of the column. The formation of the bubbles on the rotating distributor is theoretically analyzed. The pierced length of the bubbles ascending in the bed were measured using optical probes. The probability distribution of bubble diameter was inferred from these experimental measurements using the maximum entropy method. The radial profile of the bubble diameter is presented for the static and rotating configurations at different gas velocities. The frequency of bubble passage and the distribution of bubbles in the cross section of the bed are also reported. Results were finally shown for different heights above the distributor. A radial decrease in the bubble size when the distributor rotates is found. The bubble growth with the bed height is also lower in the rotating case. [Copyright &y& Elsevier]

This work describes the behaviour of the standard deviation of pressure fluctuations, σ, in fluidized beds for group B particles in the bubbling regime. An empirical-theoretical function, which depends on the gas velocity, is proposed for predicting the pressure signal fluctuations, and the corresponding values of are calculated. The differences in the standard deviation of pressure fluctuations obtained for absolute or differential sensors are analyzed and compared to experimental values corresponding to different bed dimensions, pressure probe positions and particle properties. [ABSTRACT FROM AUTHOR]

Abstract: 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, U mf, 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, σ p, were used to find the minimum fluidization velocity, U mf. A decrease in U mf 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. [Copyright &y& Elsevier]

Abstract: A new model is proposed for obtaining the velocity profile of the particle ejected from the bubble dome in a freely bubbling 2-D fluidized bed. Its basis is the supposition that the initial velocity of the ejected particles, with a direction perpendicular to the dome contour, depends on bubble velocity and bubble growth velocity. This model differs from those previously appearing in the literature in that it is valid not only for vertical-ascent circular bubbles. Experiments were carried out in a freely bubbling 2-D fluidized bed using a high-speed video camera to measure the velocity profile. Upon comparing these results with the proposed model, it was established that, excepting some isolated cases, the model properly predicts the magnitude and direction of the maximum particle ejection velocity and the velocity profile. Using the work of Shen et al. (2004. Digital image analysis of hydrodynamics two-dimensional bubbling fluidized beds. Chemical Engineering Science 59, 2607–2617), we obtain two general equations for the bubble velocity and the bubble growth velocity in a 2-D fluidized bed. These expressions, together with the proposed model, can be used to calculate the initial velocity of the ejected particles. [Copyright &y& Elsevier]

This work studies the defluidization time and the agglomerate generation in a bubbling fluidized bed (BFB) reactor during Cynara cardunculus L. gasification using, separately, two different bed materials, silica sand and sepiolite (Mg 8 Si 12 O 30 (OH) 4 (OH 2 ) 4 8H 2 ). The high adsorption capacity and the elemental composition of the sepiolite make it suitable as an alternative bed material in order to reduce agglomeration. Experiments were performed on a stainless steel lab-scale BFB reactor operating with air as a gasifying agent at different air excess ratios (u/u mf ). A quartz reactor was alternatively used for the visualization of bed material and biomass during gasification, allowing one to observe the agglomerate formation process. Pressure signals were analyzed both in time and frequency domain to determine the defluidization time. Furthermore, the shape and size of the bed material after the experiments were evaluated. Higher defluidization times in the case of sepiolite were measured. Particle sizes were affected by the type of bed material and the air excess and agglomerates of different shapes were formed for sepiolite and silica sand. [ABSTRACT FROM AUTHOR]

Abstract: This work presents an experimental study of the ejection velocity for different mechanisms of solid ejection in fluidized beds. The experiments were carried out in a 2D fluidized bed, where the bubble eruptions were recorded with a frequency of 250 frames per second using a high speed video camera with a resolution of 1.3 Megapixels. The results show that in isolated bubble eruption, the dome velocity is significantly reduced by the effect of a group of raining particles in the form of stalactites within the bubble. Higher velocities are observed when bubble coalescence takes place. If bubbles coalesce before the leading bubble breaks, the momentum of the trailing bubble together with the increase in the throughflow accelerate the dome of the leading bubble. In contrast, when coalescence occurs after the breakage of the leading bubble, the wake of the trailing bubble is projected into the freeboard with a very high velocity (wake spike mechanism). The last observed mechanism, the jet spike mechanism, occurs when a stream of bubbles reaches the bed surface following the path opened by the previous bubbles. A cloud of particles moving upward is observed, although their velocities are not as high as in the wake spike mechanism due to the interchange of momentum during the collisions with other particles. Finally, an explanation for some of the patterns of gas release from erupting bubbles recently observed by Hartung et al. [G. Hartung, C.R. Müller, J. Hult, J.S. Dennis, C.F. Kaminski, Laser diagnostic investigation of the bubble eruption patterns in the freeboard of fluidized beds. 1. Optimization of acetone planer laser induced fluorescence measurements. Ind. Eng. Chem. Res. 47 (2008) 5686–5697] is proposed. [Copyright &y& Elsevier]