Experimental study of a bubbling fluidized bed with a rotating distributor

Esta tesis consiste en la caracterización experimental de la hidrodinámica de un nuevo lecho fluido de distribuidor rotatorio. Existen numerosas referencias en la literatura en las que se analizan los factores que influyen en la calidad de la fluidización en lechos burbujeantes, como son la tasa de mezcla, el tamaño de burbuja y la heterogeneidad en el lecho. Entre estos factores se encuentran la geometría del lecho, el caudal de gas empleado en la fluidización y el tipo de distribuidor. Las heterogeneidades que se producen con frecuencia en lechos industriales han hecho que numerosos investigadores hayan incorporado modificaciones de distinta índole sobre los lechos convencionales, como por ejemplo alterar el sistema de suministro de aire o usar diseños innovadores. El nuevo diseño de distribuidor que se estudia en esta tesis intenta, mediante la introducción del giro del distribuidor, lograr mayores tasas de mezcla del gas y aumentar la dispersión de las partículas, al tiempo que se consigue una fluidización más uniforme. La posibilidad de controlar la velocidad de giro del distribuidor permite operar en un amplio rango de condiciones de operación sin perder la calidad de fluidización. Los experimentos fueron realizados en un lecho constituido por un cilindro transparente de diámetro 192 mm y altura 0.8 m lleno de partículas de arena del tipo B de acuerdo con la clasificación de Geldart. El distribuidor rotatorio es una placa perforada acoplada en su eje al eje de un motor eléctrico. La velocidad de giro se controla mediante un inversor de frecuencia que permite trabajar con un rango de velocidades que en los experimentos se varía de 0 a 100 rpm. La descripción completa de la instalación experimental se encuentra en el Capítulo 2. La caracterización experimental de la hidrodinámica del lecho realizada en la tesis incluye, por un lado, la descripción global del lecho sin giro y con giro en el distribuidor, mediante medidas absolutas de presión (Capítulo 4) y por otro, el estudio de las propiedades de las burbujas que se forman en el lecho usando sondas ópticas específicamente diseñadas y construidas para esta tesis (Capítulo 6). Con el fin de interpretar de manera adecuada las señales de presión que se utilizan para la caracterización del lecho, el Capítulo 3 contiene un estudio de los valores de la desviación estándar de las señales de presión en lechos fluidos burbujeantes. Se ha obtenido una función semi-empírica, que depende de la velocidad del gas, que permite predecir dichas fluctuaciones de presión en lechos con partículas del tipo B. Este modelo permite explicar las diferencias en las medidas cuando se emplean sensores de presión en modo diferencial o absoluto, obteniéndose una buena correspondencia entre los valores teóricos y las medidas experimentales para diferentes tamaños de lechos, posición de los sensores y propiedades de las partículas. En el Capítulo 4 se estudia el efecto del giro del distribuidor en el comportamiento hidrodinámico global del lecho, analizándose el cambio en la mínima velocidad de fluidización y en las fluctuaciones de presión. Se ha observado una disminución en el valor de la mínima velocidad de fluidización a medida que aumenta la velocidad de giro. Además se analizaron los espectros de frecuencia y la desviación estándar de las fluctuaciones de presión. Las medidas se repitieron a distintas alturas iniciales del lecho para ver como afectaba esta altura a la magnitud del efecto provocado por el giro. Se ha comprobado que la rotación del distribuidor permite fluidizar lechos con poca altura, que en ausencia de giro presentan una estructura de chorros y no se consiguen fluidizar. Por otro lado, conforme aumenta la altura inicial del lecho, el efecto de la rotación sobre la velocidad de mínima fluidización tiende a disminuir. Se demuestra por tanto que mediante el ajuste de la velocidad de giro en el distribuidor, se puede cambiar la velocidad del aire necesario para fluidizar el lecho, lo que permite mantener unas condiciones uniformes de fluidización en un rango mayor de caudales. Una vez realizado el análisis global, se estudiaron las características locales del lecho. Para ello, se usaron sensores de presión diferencial y sondas ópticas con las que se midieron las cuerdas de las burbujas que se forman en el lecho y su velocidad. Los resultados obtenidos usando las dos sondas se encuentran en el Capítulo 5. Las funciones de densidad de probabilidad de la cuerda y de la velocidad se calcularon aplicando el Método de la Máxima Entropía. Existe un tamaño mínimo de cuerda que es posible medir usando sondas intrusivas para que el error sea tolerable. Este límite inferior se ha tenido en cuenta en la formulación de las ecuaciones para la obtención de las funciones de distribución de probabilidad. La función de densidad de probabilidad de los diámetros se ha deducido a partir de las medidas experimentales de las cuerdas, aplicando herramientas estadísticas. Los resultados de las sondas de presión y de las sondas ópticas son bastante parecidos, aunque las sondas ópticas proporcionan información más local, y pueden utilizarse en posiciones muy próximas al distribuidor. Se ha comprobado que el método de la Máxima Entropía es un método simple que ofrece varias ventajas frente a otros métodos aplicados hasta la fecha para la obtención de las distribuciones de tamaño en lechos fluidos: no es necesario suponer a priori la forma de la distribución, el número de muestras requeridas es menor que en otros métodos y se evita la transformación inversa, que es un cálculo complejo. Una vez desarrollado y validado el método de transformación de cuerdas en diámetros se estudió el efecto del giro del distribuidor en el tamaño de las burbujas y su frecuencia de paso a distintas posiciones en el lecho. Los resultados se presentan en el capítulo 6. Primero se ha obtenido un modelo simple para analizar la influencia de la aceleración centrifuga que actúa sobre la burbuja en el momento en que se desprende del distribuidor una vez formada. Este análisis indica que el giro hace que el diámetro inicial de la burbuja sea menor que si el distribuidor estuviera parado. Los resultados experimentales muestran que, a igualdad en el exceso de aire, el tamaño de las burbujas es menor cuando el distribuidor gira. Los tamaños medidos de burbuja en distintas posiciones radiales confirman la tendencia puesta de manifiesto por el modelo: para el distribuidor rotatorio el diámetro medio disminuye a distancias mayores del eje del lecho, donde la aceleración centrífuga es mayor. La rotación del distribuidor también hace que la distribución de burbujas en la sección radial del lecho sea más homogénea. Además, para el distribuidor rotatorio se observa que el aumento del tamaño de las burbujas a medida que aumenta la altura es menos acusado que con ausencia de giro. Esto puede deberse a una disminución de la coalescencia lograda por la ruptura de los caminos preferenciales de ascensión de las burbujas gracias al giro. _________________________________________________This thesis presents the experimental fluid dynamic characterization of a new fluidized bed with a rotating distributor. Many works in the literature analyze the factors that influence the quality of fluidization in bubbling beds, e.g. the rate of solids mixing, the size of the bubbles and the extent of heterogeneity in the bed. These factors include among other, the bed geometry, the gas flow rate and the type of gas distributor. The non heterogenous structures often found in industrial fluidization processes have led many investigators to modify the conventional fluidized bed devices, alter the air supply system or try innovative designs to avoid these heterogeneities. The novel distributor design studied in this thesis tries to solve some of these difficulties; the aim of the distributor rotation being to overcome low radial gas mixing and particle dispersion, and to achieve a more uniform fluidization. The possibility to control and adjust the rotational speed of the distributor plate offers a wide range of operating conditions while maintaining the quality of fluidization. The fluidized bed is a transparent cylinder with 192 mm ID and a height of 0.8 m filled with Geldart B silica particles. The distributor is a perforated plate that is coupled to the shaft of an AC electric motor. It can rotates around the bed axis and the rotational speed can be varied using a frequency inverter. In the experiments this speed was varied between 0 and 100 rpm. A complete description of the experimental set-up can be found in Chapter 2. The experimental fluid dynamic characterization presented in this thesis includes a global description of the bed behavior with and without rotation of the distributor, using pressure measurements (Chapter 4). In addition, the differences in the characteristics of the generated bubbles are studied by means of in house made optical probes (Chapter 6). To better understand the pressure signal recorded for the bed characterization, the behavior of the standard deviation of pressure fluctuations in fluidized beds for group B particles in the bubbling regime is studied in Chapter 3. An empirical-theoretical function, which depends on the gas velocity, is proposed for predicting the pressure signal fluctuations. The differences in the standard deviation of pressure fluctuations obtained from absolute or differential sensors are analyzed and compared to experimental values corresponding to different bed sizes, pressure probe positions and particle properties. In chapter 4 the effect of the rotational speed of the distributor plate on the global hydrodynamic behavior of the bed is studied. Minimum fluidization velocity and pressure fluctuations were first analyzed. A decrease in the minimum fluidization velocity is observed when the rotational speed increases. The standard deviation and the power spectra of the pressure fluctuations are also discussed. 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. The rotation of the distributor allows to fluidize very shallow beds which had a jet structure when the static distributor is used; the effect of the rotation becomes globally less important for deeper beds. These characteristics show that adjusting the rotational speed it is possible to change the gas velocity needed to fluidize the bed, facilitating the fluidization and maintaining a uniform fluidization. Once the global fluid dynamic behavior is known, local characteristics are analyzed. Differential pressure and optical probe measurements were carried out in order to obtain the size and velocity of the bubbles rising in the bed. Results obtained from the two types of probes were compared in Chapter 5. The probability distributions of bubble pierced length and velocity were obtained applying the Maximum Entropy Method. The minimum bubble pierced length that it is possible to measure using intrusive probes, due to their finite size, has been introduced as a constraint in the derivation of the size distribution equations. The probability density function of bubble diameter was inferred applying statistical tools to the pierced length experimental data. Results on bubble size obtained from pressure and optical probes have been found to be very similar, although optical probes provide more local measurements and can be used even at very low heights in the bed, near the distributor. The Maximum Entropy Method has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds: the distribution shape does not have to be pre-established, the number of samples required is lower than in other methods and the backward transformation procedure is avoided. The effect of the distributor rotation on the bubble size, bubble passage frequency and bubbles distribution at different radial and axial positions in the bed was studied with the optical probes and the results are presented in Chapter 6. A simple theoretical expression is obtained in order to analyze the centrifugal acceleration influence on the bubble when it detaches from the distributor. This analysis points out that the centrifugal acceleration imparted by the distributor rotation causes the decrease of the initial bubble radius. The experimental results show that the bubbles are smaller when the rotating distributor is used if the excess gas for the static and rotating configuration is similar. The bubble size radial profile indicates that when the distributor rotates, the diameter of the bubbles close to the bed walls is smaller due to the effect of a higher centrifugal acceleration. The distributor rotation also promotes the more homogenous distribution of the bubbles over the bed surface. At higher axial positions even smaller bubbles are found for the rotating case. This may be due to a lower coalescence rate of the bubbles when the distributor rotates as the rotation may break the channeling in the bed

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, Uc, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor platesPublicad

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

The water cost effect of hybrid-parallel condensing systems in the thermo-economical performance of solar tower plants

The importance of considering the water price in the analysis of the impact of dry versus hybrid condensing systems in the thermo economical performance of solar tower plants was demonstrated in this work. The dry condensing system consists of several induced-draft air-cooled condenser cells (ACCs) and the hybrid system consists of a parallel system where the condensing steam is split between the ACCs and a surface steam condenser where circulating water is cooled in a wet mechanical-draft cooling tower. The influence of the operating parameters of either the dry or wet cooling systems on the cooling load and fan power consumption were studied. Then, for a given condensing system (a system with a defined number of installed ACCs units and cooling tower units) and given the dry-air and wet-bulb air temperatures, the operating parameters were optimized to maximize the revenues of the power plant. This optimization depends on the water-to-electricity price ratio , showing that at low ambient temperature when this ratio increases it is not profitable to turn on the cooling towers since the water cost is not counterbalanced by the higher cycle efficiency obtained with the lower condensation temperature. Finally, the annual operation and the LCOE and NPV of the CSP plant located in Dunhuang were analyzed for both dry and hybrid condensing systems with different number of ACCs and wet towers, showing that the most cost-effective configuration is the 16 ACCs with 3 wet cooling towers for water-to-electricity price ratio = 4 ($/m3)/($/kWhe) and = 5 ($/m3)/($/kWhe), but for = 10($/m3)/($/kWhe), the best option is with only 2 wet towers.This research is partially funded by the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE)

Modeling the heat transfer coefficient between a surface and fixed and fluidized feds with phase change material

The objective of this work is to model the heat transfer coefficient between an immersed surface and fixed and bubbling fluidized beds of granular phase change material (PCM). The model consists of a two-region model with two different voidages in which steady and transient conduction problems are solved for the fixed and fluidized bed cases, respectively. The model is validated with experimental data obtained under fixed and fluidized conditions for sand, a common material used in fixed and fluidized beds for sensible heat storage, and for a granular PCM with a phase change temperature of approximately 50 degrees C. The superficial gas velocity is varied to quantify its influence on the convective heat transfer coefficient for both the materials. The model proposed for the PCM properly predicts the experimental results, except for high flow rates, which cause the contact times between the surface and particles to be very small and lead the model to overpredict the results.This work was partially funded by the Spanish Government (Project No. ENE2010-15403), the regional Government of Castilla-La Mancha (Project No. PPIC10-0055-4054), and Castilla-La Mancha University (Project No. GE20101662)

Modeling and experiments of energy storage in a packed bed with PCM

This work presents a numerical and experimental study of the transient response of a packed bed filled with a granular phase change material (PCM). The proposed numerical model accounts for the progressive evolution of the enthalpy with temperature during the phase change rather than using a constant phase change temperature. This temperature-dependent enthalpy is included in the model as an apparent specific heat that is dependent on temperature according to the measurements obtained by differential scanning calorimetry (DSC). The model also includes the energy, stored in the wall, which has been shown to have a non-negligible effect in several experimental facilities. The equations presented are non-dimensionalized, which results in the same differential equation system regardless of whether a granular PCM or sensible heat storage material is used. In this manner, the same numerical method can be used in cases with or without a granular PCM. Numerical and experimental results are obtained for a conventional granular material (sand) and two commercial granular PCMs with different phase change temperatures. The numerical and experimental heating results exhibit good agreement, and the energy stored in the wall of the bed represents between 8 and 16% of the energy stored in the granular material. (C) 2016 Elsevier Ltd. All rights reserved.This work presents a numerical and experimental study of the transient response of a packed bed filled with a granular phase change material (PCM). The proposed numerical model accounts for the progressive evolution of the enthalpy with temperature during the phase change rather than using a constant phase change temperature.This work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662

Experimental study of fixed and fluidized beds of PCM with an internal heat exchanger

This work presents the results of experiments performed in a thermal energy storage tank filled with particles, which was heated using hot air and then discharged with a cold water stream circulating inside a heat exchanger immersed in the bed. Both fixed- and fluidized-bed configurations were studied. The materials used were sand, which is a material commonly employed in sensible heat storage, and a granular phase change material (PCM), in which the phase change occurs over the temperature range of 4050 degrees C. Three different heat exchangers with different helical coil geometries were tested by measuring the temperatures in the bed and at the water inlet and outlet. Higher heat transfer coefficients between the bed and the water flow and higher heat exchanger effectiveness were observed for the heat exchanger with the greatest distance between coils, as it allows better contact between the bed particles and the heat exchanger surface when the particles are fluidized. (C) 2016 Elsevier Ltd. All rights reserved.This work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662)

Economic and thermo-mechanical design of tubular sCO2 central-receivers

Supercritical CO2 central-receivers must withstand high temperatures and pressures combined with cyclic operation, which makes the solar receiver susceptible to creep-fatigue failure. In this work, a creep-fatigue analysis of a sCO2 Inconel 740H tubular receiver of a 2 MWe solar tower plant has been accomplished to study the influence of the tube size on the receiver and solar field design. A 2D numerical model of the tubular receiver that accounts for the thermal conduction in both radial and circumferential directions was developed to determine the sCO2 and wall temperature profile, which is crucial for the creep-fatigue calculations. The receiver flux distribution, which is an input to the model, was obtained with SolarPILOT, while a conventional recompression model was used to calculate the cycle efficiency and inlet temperature to the receiver. Comparison of the results of the 2D model with those of a 1D model showed that the 1D model overestimates the creep fatigue rupture time by two orders of magnitude. Furthermore, the efficiency and costs of the heliostat field and receiver were calculated for different receiver tube sizes. Smaller tubes allowed a higher maximum heat flux leading to smaller receiver and heliostat field designs, which resulted in higher overall efficiency of the power plant and lower material costs. For a design ensuring 25 year receiver lifetime the minimum sCO2 solar receiver cost, 345 €/kWth, was obtained for the smallest pipe diameter.This research is partially funded by the Spanish government under the projects RTI2018-096664-B-C21 (MICINN/FEDER, UE) and RED2018-102431-T (AEI, MICINN) and the fellowship “Programa de apoyo a la realizaci on de proyectos interdisciplinares de I+D para j ovenes investigadores de la Universidad Carlos III de Madrid 2019e2020” under the project ZEROGASPAIN-CM-UC3M (2020/00033/001), funded on the frame of “Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid 2019e202”

Experimental and computational study on the bubble behavior in a 3-D fluidized bed

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 0.193 m internal diameter and 0.8 m height, and it is filled with silica sand particles, reaching a settle height of 0.22 m. 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 studiedThis work has been partially funded by the Spanish Govern ment (ProjectDPI200910518) and the Autonomous Community of Madrid (ProjectS2009/ENE1660). The authors gratefully appreciate their supportPublicad

Standard deviation of absolute and differential pressure fluctuations in fluidized beds of group B particles

This work describes the behaviour of the standard deviation of pressure fluctuations 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.This work has been supported by the National Energy Programme of the Spanish Ministry of Education under the project number ENE2006-01401 and by the Universidad Carlos III de Madrid (CCG06-UC3M/ENE-0764).Publicad