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Longitudinal supports shape influence on deflection and stresses in solar receiver tubes
(European Association for the Development of Renewable Energy, Environment and Power Quality (EA4EPQ), 2022-09) Montoya Sancha, Andrés; López Puente, Jorge; Santana Santana, Domingo José; Ministerio de Ciencia e Innovación (España)
Longitudinal supports have a key role in solar central receivers, preventing tubes from excessive deflection. In this work, its influence on tube deflection and thermo-mechanical stresses has been studied. In exiting literature, the longitudinal supports are not modelled when the behaviour of solar receiver tubes is studied. In this research, they have been considered developing numerical models using a finite element analysis software. To carry out the analysis, a new geometry of the metal sheet attached to the receiver tubes has been proposed. With this new shape, different cases have been studied, varying the separation between supports and their size to study its impact on stress and deflection in the tubes. Results have shown that extremely rigid supports may induce additional stress in the receiver tube. With the geometry considered in this research, supports do not cause additional mechanical stress in tubes. Small supports are preferred to bigger ones due to the stresses arisen in the own support.
Experimental determination of the forced convection heat transfer coefficient of an aluminum cooling plate with a channel shape inspired by nature
(HEFAT, 2022-08-08) Guil Pedrosa, José Félix; Coll Franck, Anne Maren; García Gutiérrez, Luis Miguel; Soria Verdugo, Antonio; Comunidad de Madrid; Universidad Carlos III de Madrid
Cooling is a crucial aspect in numerous applications where the optimal operation of electric, electronic, or electrochemical devices requires a controlled operating temperature. In this sense, metallic cooling plates are a suitable solution to dissipate heat from the surface of these equipment. The refrigeration capacity of cooling plates can be improved by circulating cold fluid along channels drilled in the metallic plate. The shape of these channels plays a critical role on the performance of the cooling plate since they affect both the distribution of temperature across the plate and the pressure drop required to pump the cooling fluid along the channel. The channel shape of a cooling plate can be optimized considering the Constructal law, which proposes the use of configurations found in nature to improve the performance in industrial applications. Following the Constructal law, a cooling plate made of aluminum, inside of which there is a channel with a shape resembling the outline of a flower, was built by a 3D printer. The performance of the plate was experimentally evaluated refrigerating the plate with various flow rates of cold water. To that end, an experimental facility was specifically designed and built to test the cooling capacity of the plate. The experimental setup consists of an enclosure inside of which the temperature of the atmosphere is controlled by a PID system connected to a thermoresistance and a heater, and a thermostatic bath to control the temperature of the cooling water at the inlet of the plate. The temperature of the plate was measured by an IR camera and the heat transfer coefficient by forced convection to the fluid were derived from the tests for both laminar and turbulent flow regimes of the fluid, obtaining values of 1703 and 3639 W/m2K, respectively, with maximum variations of 1 % for three replicates of each test, proving the high repetitiveness of the experimental procedure proposed. The average characteristic cooling time of the plate was measured to be 34.9 and 16.5 s for Reynolds numbers of the cooling flow of 1249 and 4918, respectively. Thus, an increase on the flow rate by 4 times results in a reduction of the characteristic cooling time by approximately 50 %.
Elliptical tubes receivers efficiency analysis in solar power towers
(American Institute of Physics (AIP), 2019-07-26) Laporte Azcué, Marta; Rodríguez Sánchez, María de los Reyes; Santana Santana, Domingo José; Ministerio de Economía y Competitividad (España)
The high costs of the heliostat field, as well as the issues related with the receiver operation in solar power towers are some of the main reasons to study different tube geometry shapes for the receiver. The new geometries intend to improve its thermal efficiency and increase its lifetime, such as the elliptical tubes presented in this research. A thermal model considering circumferential and longitudinal divisions of the tubes has been used to study the thermal efficiency of elliptical tubes receiver. For this analysis, the tube's geometry varies according to three different considerations when its major axis increases, and the resulting efficiencies are compared with the ones obtained for circular tubes of a fixed diameter. The results show that there is an improvement in the efficiency with the new tubes, because of the radiative exchange with the environment using the elliptical tubes, resulting in lesser radiative heat losses.
Allowable solar flux densities for molten-salt receivers: Input to the aiming strategy
(Elsevier, B.V., 2020-03) Sánchez González, Alberto; Rodríguez Sánchez, María de los Reyes; Santana Santana, Domingo José
Solar Power Tower technology requires accurate models and tools to assist in design and operation stages. The heliostat field aiming strategy seeks the maximization of the thermal output from the receiver, while preventing its permanent damage because of thermal stress and corrosion in molten salt receivers. These two limitations are translated into Allowable Flux Densities (AFD), which can be handled by the aiming strategy. This paper explains the methodology to determine AFDs, and analyzes the influence of tube geometry and material. AFD by corrosion is slightly lower in Haynes 230 than Inconel 625 and austenitic alloys. On the contrary, HA230 has better performance than In625 under thermal stress. Increment of tube wall thickness diminishes the AFD: slightly by corrosion, but significantly by thermal stress. The generated AFD databases feed the aiming model, herein applied to Gemasolar case study. In the cylindrical receiver, first northern panels are limited by thermal stress, while the last ones by corrosion. Under optimized aiming, HA230 receiver tubes produce equivalent thermal output than In625.
Reversal of gulf stream circulation in a vertically vibrated triangular fluidized bed
(Elsevier B.V., 2017-07-01) Cano Pleite, Eduardo; Hernández Jiménez, Fernando; Acosta Iborra, Antonio; Müller, Christoph R.; Ministerio de Economía y Competitividad (España); Universidad Carlos III de Madrid
The present work experimentally assesses the effect of vibration on the dynamics of particles in a fluidized bed of triangular shape. The base of the bed is composed of two inclined walls, each one forming an angle of 45 with the horizontal. The bed has 0.206 m span and 0.01 m thickness. The bed vessel is made of antistatic PMMA in order to allow optical access with a high-speed camera. The bed is mounted on an electrodynamic shaker which produces vertical vibration. The bed material is ballotini particles with a mean diameter of 1.15 mm up to the top of the inclined walls. Air was injected through the inclined bed walls to fluidize the bed to explore whether vibration of the bed vessel together with gas injection can make the dynamics of this bed different to that found when no gas is injected. A high speed camera was used to record the motion of particles in the bed. The velocity of the particles in the bed was obtained via Particle Image Velocimetry (PIV). The results show that several circulation patterns are observed as a function of vibration amplitude and frequency when the fluidization velocity is just below and above the minimum fluidization velocity. Noticeably, for zero gas velocity, particles ascend close to the side walls and descend in the center of the bed. By injecting fluidization gas, the circulation pattern of the bed can be reversed (i.e. particles descending near the inclined walls and ascending in the center of the bed). Conditions for which this reversal of the gulf stream circulation of particles appears in the triangular bed are explored in this work and these include gas superficial velocities higher than the minimum fluidization velocity and sufficiently high values of the vibration strength.
Segregation of equal-sized particles of different densities in a vertically vibrated fluidized bed
(Elsevier, B.V., 2017-07-01) Cano Pleite, Eduardo; Hernández Jiménez, Fernando; Acosta Iborra, Antonio; Tsuji, T.; Müller, Christoph R.; Ministerio de Economía y Competitividad (España); Universidad Carlos III de Madrid
The present work experimentally studies the influence of vibration and gas velocity on the density-induced segregation of particles in a pseudo-2D vibrated fluidized bed. One half of the particles of the bed are ballotini spheres of density 2500 kg/m(3) and the other half are heavier ceramic particles of density 4100 kg/m(3) or 6000 kg/m(3). Digital Image Analysis is used to characterize the rate and extent of particle mixing with time for different gas velocities, vibration amplitudes and frequencies. The results of the experiments indicate that the vibration strength and the gas velocity have an important effect on both the evolution and the final extent of density-induced particle segregation. It was observed that by introducing vertical vibration to a bed that is fluidized close to minimum fluidization conditions the rate of segregation and the final segregation index of a mixture of light and dense particles is enhanced. However, for vibration strengths greater than a critical value around 3-4, the degree of segregation decreases due to a more vigorous three dimensional mixing of particles in the bed.
Comparison of the heat transfer characteristics of molten salt, liquid sodium and supercritical CO2 in bayonet tubes of solar tower receivers
(American Institute of Physics (AIP), 2019-07-25) Pérez Álvarez, Rafael; Marugán Cruz, Carolina; Santana Santana, Domingo José; Acosta Iborra, Antonio; Ministerio de Economía y Competitividad (España)
The solar tower receivers tend to experience rupture problems due to the high thermal gradients and the corrosion produced by the working fluid, typically solar salt. In this work we have developed a series of CFD simulations to study a new receiver design composed of bayonet tubes aimed to reduce the overheating the receiver in the most thermally demanded area. These simulations evaluate the thermal behavior of the tubes for different working fluids, i.e. molten salt, liquid sodium and supercritical CO2. The simulations show that, for all the working fluids analyzed, it is possible to reduce the high temperatures of the tube thanks to the asymmetries created when the bayonet tube has an eccentric configuration. Besides, the greatest reduction of temperature in bayonet tubes is achieved when the working fluid is liquid sodium due to its higher thermal conductivity.
Performance evaluation of H2O-LiBr absorber operating with microporous membrane technology
(American Society of Thermal and Fluids Engineers (ASTFE), 2015) Vega Blázquez, Mercedes de; Venegas Bernal, María Carmen; García Hernando, Néstor; Ruiz-Rivas Hernando, Ulpiano
With the aim of reducing the size and increasing the energy efficiency of absorption chillers, the use of microporous membrane technology in these systems is at present under study. In particular, the simulation of a H2O-BrLi absorber using porous fibers for the heat and mass transfer between the solution and the vapor phase is considered in the present work. Heat and mass transfer process are modeled by means of selected correlations and data gathered from the open literature. Using the model developed, a simulation of the absorber is performed using typical operating conditions of absorption cooling chillers.
Heat pump for radiant cooled and heated floor driven by a microphotovoltaic system
(2013-04-15) Izquierdo Millán, Marcelo; Agustín Camacho, Pablo de; Martín, E.
This paper reports a solar trigeneration system installed at the Solar Energy Experimental Plant owned by the Spanish National Research Council (CSIC), located in Arganda del Rey, 20km east from Madrid.
Design of a statistical strategy to the control of a fluidized bed equipped with a rotating distributor
(Engineering Conferences International, 2013-05) Gómez Hernández, Jesús; Soria Verdugo, Antonio; Villa Briongos, Javier; Santana Santana, Domingo José
The present work proposes the use of the so-called "moving mean control charts" for the continuous monitoring of the fluidized bed. A rotating distributor was used as a counteracting measurement against the defluidization phenomena. The results showed the utility of the proposed strategy to identify the defluidization and recuperation processes.
Analysis of biomass and sewage sludge devolatilization using the distributed activation energy model
(Elsevier, 2011-04) Soria Verdugo, Antonio; García Hernando, Néstor; García-Gutiérrez, L.M.; Ruiz-Rivas Hernando, Ulpiano
The thermal decomposition of biomass (pine pellets) and sewage sludge was studied using thermogravimetric analysis under an inert atmosphere and the Distributed Activation Energy Model (DAEM) was employed. The activation energy and the frequency factor that characterize the kinetics were determined for both samples. A simplification of the process for prediction of devolatilization curves was proposed, evaluating its validity for both cases. The simplified method was found to combine both simplicity and low deviations with experimental data.
Experimental studies of phase change materials in a bubbling fluidized bed
(Engineering Conferences International, 2013) Izquierdo Barrientos, María Asunción; Sobrino Fernández, Celia; Almendros Ibáñez, José Antonio; Ellis, Noko; Bi, X.T.; Epstein, N.
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.
Convective heat transfer coefficient in a bubbling fluidized bed with PCM
(Engineering Conferences International, 2013) Izquierdo-Barrientos, María Asunción; Sobrino, Celia; Almendros Ibáñez, José Antonio
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 surface
Thermal analysis and optimization of a heat regenerator composed of two coupled moving bed heat exchangers
(ICHMT Digital Library Online; Graham de Vahl Davis and Eddie Leonardi, 2008-05-11) Almendros Ibáñez, José Antonio; Soria Verdugo, Antonio; Ruiz-Rivas Hernando, Ulpiano; Santana Santana, Domingo José
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.
Coherent structures and bubble-particle velocity in 2-D fluidized beds
(J. Werther, W. Nowak, K.E. Wirth and E.U. Hartge, 2008-05-13) Sánchez Delgado, Sergio; Almendros Ibáñez, José Antonio; Soria Verdugo, Antonio; Santana Santana, Domingo José; Ruiz-Rivas Hernando, Ulpiano
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.
Exergy optimization in a steady moving bed heat exchanger
(Blackwell Publishing, 2007) Soria Verdugo, Antonio; Almendros Ibáñez, José Antonio; Ruiz-Rivas Hernando, Ulpiano; Santana Santana, Domingo José
This work provides an exergy analysis of a moving bed heat exchanger to obtain for a range of incoming fluid flow rates the operational optimum and the incidence on it of the relevant parameters such as the dimensions of the exchanger, the particle diameter and the flow rate of the fluid. The MBHE proposed can be analyzed as a cross flow heat exchanger where one of the phases is a moving granular medium. In the present work the exergy analysis of the MBHE is carried out over operation data of the exchanger obtained in two ways: a numerical simulation of the steady state problem and the analytical solution of the simplified (avoiding conduction terms) equations. The numerical simulation is carried over the two steady energy equations (fluid and solid), involving for the solid the convection heat transfer to the fluid and the diffusion term in both directions, and for the fluid only the convection heat transfer to the solid. The analytical solution is the wellknown solution of the simplified problem neglecting conduction effects.
Throughflow velocity crossing the dome of erupting bubbles in 2-D fluidized beds
(Engineering Conferences International (ECI), 2007) Almendros Ibáñez, José Antonio; Sobrino Fernández, Celia; Sánchez Delgado, Sergio; Santana Santana, Domingo José; Vega Blázquez, Mercedes de; Ruiz-Rivas Hernando, Ulpiano
A new non-intrusive method for measuring the throughflow velocity crossing the dome of erupting bubbles in freely bubbling 2-D fluidized beds is presented. Using a high speed video-camera, the dome acceleration, drag force and throughflow velocity profiles are obtained for different experiments, varying the superficial gas velocity. The acceleration profiles show greater values in the dome zone where the gravity component is negligible. The drag force and the throughflow velocity profiles show a uniform value in the central region of the dome and the total throughflow increases with the superficial gas velocity.
Exergy Optimization of a Moving Bed Heat Exchanger
(2007-07-03) Almendros Ibáñez, José Antonio; Soria Verdugo, Antonio; Sánchez Delgado, Sergio; Sobrino Fernández, Celia; Ruiz-Rivas Hernando, Ulpiano; Macías-Machín, Agustín; Santana Santana, Domingo José
The MBHE proposed can be analyzed as a crossflow heat exchanger where one of the phases is a moving granular medium. In the present work the exergy analysis of the MBHE is carried out over operation data of the exchanger obtained in two ways: a numerical simulation of the stationary problem and a simplified analysis. The numerical simulation is carried over the two steady state energy equations (fluid and solid), involving (for the fluid) the convection heat transfer to the solid and the diffusion term in the flow direction, and (for the solid) only the convection heat transfer to the fluid. The simplified analysis followed the well-known e-NTU method, taking the equipment as a crossflow heat exchanger with both fluids unmixed.
Plastic fiber-optic probes for characterizing fluidized beds in bubbling regime
(16th International Conference on Plastic Optical Fibers, 2007) Vázquez García, María Carmen; Nombela, José Luis; Vega Blázquez, Mercedes de; Zubía, Joseba; Sánchez Montero, David Ricardo
Bubble measurements on a fluidized bed in bubbling regime using optical fibre probes (OFP) are reported. Comparisons between commercial pressure transducers (PT) measurements and OFP have also been carried out. OFP are able to detect smaller bubbles than the PT and reflect more localized phenomena in the bed.
Hydrodynamic Characteristics of a Fluidized Bed with Rotating Distributor
(Engineering Conferences International, 2007) Sobrino Fernández, Celia; Sánchez Delgado, Sergio; Almendros Ibáñez, José Antonio; Vega Blázquez, Mercedes de; Ruiz-Rivas Hernando, Ulpiano; Santana Santana, Domingo José
The performance of a novel rotating distributor fluidized bed is presented. The pressure drop and the standard deviation of pressure fluctuations were used to find the minimum fluidization velocity, Umf, and to characterize the quality of fluidization at different rotational speeds of the distributor plate. Experiments were conducted in the freely bubbling regime in a 0.19 m i.d. fluidized bed, operating with Group B particles according to Geldart’s classification. A decrease in Umf is observed when the rotational speed increases. Frequency analysis of pressure fluctuations shows that fluidization can be controlled by the adjustable rotational speed, at several excess gas velocities.

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