Convective Heat Transfer Coefficient in a Bubbling Fluidized Bed with PCM

Proceedings of: 14th International Conference on Fluidization: From Fundamentals to Products. Noordwijkerhout, Netherlands, 26-31 may 2013.This work presents an experimental study to determine the capacity of a Phase Change Material (PCM) in granular form to be used in a bubbling fluidized bed for thermal energy storage. The experimental measurements are focused on the determination of the heat transfer coefficient between a heated surface and the granular PCM in fluidized state. The results obtained indicates that the heat transfer coefficient notably increases (up to values three times higher) when the granular PCM is in solid form because it changes its phase when touches the heated surfaceThis work was partially founded 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).publicad

Procedimiento para la evaluación de la seguridad de un frenómetro

En este estudio se va a analizar el marco normativo y seguridad de una máquina concreta: un frenómetro utilizado en la Inspección Técnica de Vehículos (ITV). Existen diferentes modelos de frenómetros en el mercado, pero en el presente proyecto se pretende analizar el proceso necesario para evaluar la conformidad de cada máquina con la normativa que le es aplicable. Para ello, en primer lugar se realizará una descripción del panorama actual legislativo relacionado con las máquinas y lugares de trabajo, para así poder aplicar esta normativa al caso en estudio. Por todo lo citado anteriormente es objeto de este proyecto fin de carrera el estandarizar un proceso de chequeo del frenómetro, para así mantener un riguroso control de calidad y conformidad de los aparatos con la reglamentación de aplicación.Ingeniería Industria

Heat transfer and thermal storage in fixed and fluidized beds of phase change materials

Mención Internacional en el título de doctorThermal energy storage is a key technology for energy conservation since many energy sources are intermittent in nature. Latent heat storage is considered one of the most efficient ways of storing thermal energy because, unlike sensible heat storage, it provides a high-energy storage density with a small temperature swing. There are available many storage techniques, including sensible and latent heat storage or a combination of both. Fixed and fluidized beds may be feasible technologies when the storing materials may be encapsulated in cans, spheres or microencapsulated in highly porous structures with protecting envelopes. This PhD thesis deals with thermal storage and heat transfer in fixed and fluidized beds with phase change materials (PCMs). The behavior of a bed with granular PCM as a thermal storage system is studied. Charging and discharging experiments are carried out and models for the transient response of the bed are developed for fixed and fluidized bed configurations. Moreover, a model for the heat transfer coefficient between the bed of PCM and an immersed surface is presented and validated with experimental measurements. The experimental studies are conducted in a cylindrical bed filled with granular PCM and with air as the working fluid. The bed has an internal diameter of 200 mm. The granular PCMs used consist of paraffin, which is the material that changes its phase, bounded within a secondary supporting structure of SiO2, which ensures that the paraffin does not leak from the granulate when in its liquid form. The material is commercialized by Rubithermr and is available in two sizes involving particle diameters of 1-3 mm and 0.2-0.6 mm. The finer grade is used in fluidized bed because the particle size is appropriate for obtaining a bubbling fluidization, whereas the coarser grade is employed in the fixed bed conditions to achieve high gas velocities without exceeding the minimum fluidization velocity. The study of the storage behavior of a fluidized bed filled with PCM includes the comparison of its performance to that of well-known storage methods such as fluidized beds with sand and packed beds with sand and PCM. To accomplish this, heating and cooling experiments are conducted in the cylindrical bed mentioned before. For the fluidized bed with PCM the bed height and flow rate are varied to study their influence on the storage and recovery efficiencies. In addition, the stability of the PCM during several charging-discharging cycles is examined. The transient response of a packed and fluidized bed with PCM is modeled taking into account the progressive evolution of the enthalpy with the temperature during the phase change and the energy stored in the wall of the bed. The equations presented for each model are non-dimensionalized, which result in the same differential equation system regardless of whether a granular PCM or a conventional material is used. The models are positively verified against experimental data for the granular PCM and the conventional storage material, sand. The experimental determination of the heat transfer coefficient between a heated surface immersed in a fixed or fluidized bed and granular PCM is performed employing a heat transfer probe which consists of a cylindrical variable resistance of small diameter, 6 mm. The flow rate is varied to quantify its influence on the heat transfer coefficient. The results obtained for the granular PCM are compared with the heat transfer coefficients measured for the sand. The heat transfer coefficient for the particles when they undergo a phase change process is twice the heat transfer coefficient when there is no phase change of the PCM. These experimental data are used to validate a model proposed to calculate the heat transfer coefficient between an immersed surface and fixed and bubbling fluidized beds of granular PCM. The model consists of a two-region model with two different voidages, where a steady conduction problem is solved for the fixed bed case and a transient one for the fluidized bed case.El almacenamiento térmico es una tecnología clave para la conservación de energía ya que muchas fuentes energéticas son por naturaleza intermitentes en el tiempo. El almacenamiento de calor latente se considera una de las formas más efectivas de almacenar energía térmica ya que proporciona una alta densidad de almacenamiento energético con pequeñas variaciones en la temperatura, a diferencia del almacenamiento mediante calor sensible. Existen diversas tecnologías para almacenar calor sensible, latente o la combinación de ambos. Entre ellas, los lechos fijos y fluidizados son una opción válida cuando los materiales están encapsulados o microencapsulados en estructuras porosas protegidas por algún tipo de carcasa. Esta tesis doctoral estudia el almacenamiento térmico y la transferencia de calor en lechos fijos y fluidizados con materiales de cambio de fase (MCF). Con el fin de analizar el comportamiento de estos lechos con MCF como sistema de almacenamiento térmico, se han realizado ciclos de carga y descarga y se ha modelado la respuesta transitoria del lecho en condiciones de lecho fijo y de lecho fluidizado. Además, se incluye un modelo para el cálculo del coeficiente de transferencia de calor entre el lecho con MCF (ya sea fijo o fluidizado) y una superficie sumergida en el mismo. Tanto los modelos de almacenamiento de calor en el lecho como el del coeficiente de transferencia han sido validados con medidas experimentales. Los ensayos experimentales se han realizado en un lecho cilíndrico con MCF y aire como fluido de trabajo. El MCF granular utilizado consiste en una parafina, que es el material que cambia de fase, inmersa en una estructura secundaria de sílice que la soporta. Este tipo de recubrimiento asegura que no haya pérdidas de parafina cuando ésta esté en estado líquido. Este material lo comercializa la empresa Rubithermr y está disponible en dos tamaños de partícula: 1-3 mm y 0.2-0.6 mm. Las partículas más finas se utilizan en el lecho fluidizado ya que su menor tamaño permite conseguir un lecho burbujeante sin necesidad de altos caudales de aire, en cambio las partículas más gruesas se usan en el lecho fijo para poder aumentar el caudal sin llegar a sobrepasar la velocidad de mínima fluidización del material. El estudio del comportamiento de un lecho fluidizado con MCF incluye la comparación de su rendimiento con el de otros métodos de almacenamiento conocidos como son los lechos fluidizados con arena y los lechos fijos con arena y MCF. Con esta finalidad se han realizado ensayos de calentamiento en el lecho cilíndrico descrito anteriormente. Para el caso del lecho fluidizado con MCF también se ha estudiado la influencia que tiene la altura del lecho y el caudal utilizado en el almacenamiento y rendimiento de recuperación de energía. Además, se ha comprobado la estabilidad del MCF tras varios ciclos de carga y descarga. Para los modelos de respuesta transitoria de un lecho fijo y fluidizado con MCF se ha tenido en cuenta la variación de la entalpía con la temperatura durante el cambio de fase así como la energía almacenada en la pared del lecho. Las ecuaciones se presentan en forma adimensional y son válidas para cualquier tipo de material granular, indistintamente de si contiene MCF en su interior o no. Los modelos se han validado con los datos experimentales tanto del MCF como de la arena. La determinación experimental del coeficiente de transferencia de calor entre una superficie sumergida en un lecho fijo o fluidizado y el MCF granular se ha realizado utilizando un sensor de transferencia de calor que consiste en una resistencia cilíndrica de potencia variable de 6 mm de diámetro. El flujo de aire se ha modificado para cuantificar su influencia en el coeficiente de transferencia de calor. Los resultados experimentales obtenidos para el MCF se han comparado con los de la arena para los dos tipos de tecnología, lecho fijo o fluidizado. Estos datos experimentales se han usado para validar un modelo que calcula el coeficiente de transferencia de calor entre una superficie y un lecho fijo o fluidizado de MCF granular. El modelo diferencia entre dos regiones del lecho con distintas propiedades, una region cercana a la superficie de intercambio de calor y otra alejada de ésta. El modelo es apto para lecho fijo y fluidizado aunque para el primer caso se resuelve un problema de conducción estacionaria y para el segundo uno de conducción transitoria.Programa Oficial de Doctorado en Ingeniería Mecánica y de Organización IndustrialPresidente: Bo Leckner.- Secretario: Sergio Sánchez Delgado.- Vocal: Luisa Fernanda Cabeza Fabr

Thermal energy storage in a fluidized bed of PCM

The objective of the present work was to research the storage behavior of a fluidized bed filled with a granular phase change material (PCM) with a small particle diameter (d(p) = 0.54 mm). The performance of the fluidized bed was compared to that of well-known storage methods such as fluidized beds with sand and packed beds based of sand and PCM. For this purpose, heating experiments were conducted in a cylindrical bed with air as the working fluid. The influence of the bed height and flow rate on the storage and recovery efficiencies of the fluidized bed of PCM was analyzed. Additionally, the stability of the PCM during various charging-discharging cycles was studied. The results indicate that this PCM is an alternative material that can be used in fluidized bed systems to increase the efficiency of storing thermal energy in the form of latent heat. Under the experimental conditions tested in this study, higher charging efficiencies were observed for fixed and fluidized beds based on PCM than those of sand. High gas velocity and low bed height shorten the charging time but also reduce the charging efficiency. The cycling test shows that the PCM is stable under bubbling conditions up to 15 cycles, which corresponds to approximately 75 h of continuous operation.This work was founded partially 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).Publicad

Experimental heat transfer coefficients between a surface and fixed and fluidized beds with PCM

This work presents an experimental study to determine the capacity of a phase change material (PCM) in granular form to be used in fixed and bubbling fluidized beds for thermal energy storage. The experimental measurements are focused on determination of the heat transfer coefficient between a heated surface immersed in the bed and the granular PCM. The flow rate is varied to quantify its influence on the heat transfer coefficient. The PCM used is Rubitherm GR50 with a phase change temperature of approximately 50° C. The PCM is available in two different particle sizes, 0.54 mm and 1.64 mm, of which the finer is used in the fluidized bed and the coarser is used in the fixed bed. In addition, the results obtained for the PCM are compared with the heat transfer coefficients measured for sand, a material commonly used for thermal storage. In comparing the heat transfer coefficients for fixed and fluidized beds, the heat transfer coefficients in the fluidized bed with PCM are nearly three times higher than those for the fixed bed at the same gas flow rate. This increase in the heat transfer is a result of two main factors: first, the continuous renewal of PCM particles from the heated surface when they are fluidized, and second, the large quantities of energy in latent form absorbed by the PCM. In the fixed bed there is no renovation of particles, consequently only a small percentage of particles are able to change its phase. Hence, there is no increase in the heat transfer coefficient due to this fact.This work was partially funded by the Spanish Government (Project ENE2010-15403), theRegional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662).Publicad

Energy storage with PCM in fluidized beds: Modeling and experiments

In recent years, the development of phase change materials (PCMs) has introduced new ways to increase the energy storage capacity of a system 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 PCMs 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 and also estimates the energy stored in the wall of the bed and in the distributor plate. The energy equation of the dense phase is numerically solved in enthalpy form, considering the dependence of enthalpy on temperature for phase changes occurring over a range of temperatures. The model's validity is verified against experimental data for two granular materials: sand, a typical material used in fluidized beds, and a granular PCM with a mean particle diameter of 0.54 mm and a phase change temperature of approximately 50 °C. For the sand, the temperature profiles obtained numerically perfectly agree with the values measured experimentally. In the case of the granular PCM, the fitting of the curves is improved when slow and similar heating rates are selected for the experiments and for the DSC measurements used to determine the PCM enthalpy&-temperature curve.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).Publicad

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

Three-dimensional two-fluid modeling of a cylindrical fluidized bed and validation of the Maximum Entropy method to determine bubble properties

Diameter and velocity of bubbles from a three-dimensional two-fluid model simulation of a cylindrical fluidized bed are presented. Two methods for obtaining the bubble size and velocity are compared: (i) estimation from the chord lengths and velocities of the detected bubbles using information from two virtual voidage probes (pierced bubble method) and (ii) calculation from the bubble volume and velocity directly obtained from the instantaneous 3D voidage field (tomography method). The Maximum Entropy method (MaxEnt) is employed to convert probability density functions of chord lengths into the corresponding diameter distributions. The algorithm for the direct evaluation of the bubble volume and velocity, based on the tomography reconstruction of the 3D field, is explicitly explained and used to evaluate the results obtained from the virtual void probe signals. Results show a good agreement between the bubble sizes obtained using the MaxEnt treatment of the chord lengths and the directly obtained bubble sizes, which confirms the robustness of the MaxEnt.method to infer bubble behavior in 3D bubbling beds. In particular, the mean bubble diameter obtained with the MaxEnt method applied to chord lengths was less than 4.5% different to the result from the tomography reconstruction. It was found that the bubble velocities obtained from virtual voidage probes are higher than the bubble velocities calculated with the tomography method, but the differences were not greater than 17% in the worst case. The probability density functions of bubble size and velocity obtained with the two methods were similar in terms of the location of the most probable values and the variation of the distribution with the distance to the distributor.The present work has been funded by the Spanish Ministerio de Ciencia e Innovación through the Project DPI2009-10518. The authors gratefully appreciate this support.Publicad

Experimental Studies of Phase Change Materials in a Bubbling Fluidized Bed

Proceedings of: 14th International Conference on Fluidization: From Fundamentals to Products. Noordwijkerhout, Netherlands, 26-31 may 2013.The aim of this work is to experimentally study the behaviour of three microencapsulated PCM in a bubbling fluidized bed for thermal energy storage. Different experiments, heating and cooling the granular PCM with fluidizing air, are carried out with different superficial gas velocities. When achieving their phase change temperature two of the three materials present agglomeration. For this reason, the material flowability and wear resistance are studied by measuring the angle of repose and attrition, respectively. Nevertheless, the angle of repose does not seem to be influenced by the temperature of the material and the particle size distributions after the attrition tests indicate that the bed particles are just slightly smaller than the original ones.This work was partially founded 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).Publicad