Abstract
Crab cavities are fundamental components of the HL-LHC upgrade project. These Radio Frequency cavities, operated at the appropriate frequency, ‘tilt’ the proton bunches to increase the ...luminosity at the collision points IP1 (ATLAS) and IP5 (CMS). Two different superconducting crab cavities were developed: RF Dipole (RFD) for horizontal deflection and Double Quarter Wave (DQW) for vertical deflection. During operation, the cavity walls are deformed due to the loading conditions. This deformation changes the electro-magnetic field inside the cavity and consequently its RF frequency. In the present study, the numerical evaluation of the Lorentz Force Detuning (LFD) and the Pressure Sensitivity (PS) of the DQW cavity, using COMSOL Multiphysics, is presented. The LFD is the fundamental frequency change of the cavity due to the electro-magnetic forces acting on its walls, while the PS is the frequency shift when the cavity is subjected to pressure fluctuations of the helium bath. Finally, a comparison with the results measured during the cold test of the manufactured cavities, and with the previous simulations results obtained for the RFD cavity is done.
The present work experimentally characterizes the behavior of the bed bulk and the solids velocity in a vertically vibrated pseudo-2D fluidized bed operated at minimum fluidization conditions. ...Measurements are undertaken combining Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV). Vibration at different amplitudes and frequencies is applied to the bed by the use of two vibro-motors symmetrically disposed at both sides of the bed vessel. The results show that both the center of mass of the bed and the bed surface oscillate with a frequency equal to that of the bed vessel. The bed surface oscillates in opposition of phase with the bed vessel, which reflects a cyclic compression and expansion of the bed bulk. The average solids velocity at each oscillation phase clearly shows that there exists a compression wave, produced by the impact of the bed bulk with the gas distributor, and an expansion wave, produced by the expansion of the bed bulk. Both waves travel upwards the bed bulk perturbing the velocity of particles along the bed height. The waves span all the bed width and separate the bed bulk into two clearly distinguishable regions with different relative velocities. When the particles belonging to the region under the wave move upwards, the particles in the region above the wave move downwards and vice versa. The results also reveal that the compression wave generated at the bottom of the bed propagates at a velocity similar to the reported velocity of sound inside a fluidized bed. Far from the distributor, this wave velocity resulted to be nearly independent of the vibration amplitude and frequency for the range of conditions tested. These results can be useful for the understanding of the behavior of particles and bubbles in vibrated fluidized beds.
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•Experiments of a fluidized pseudo-2D bed subject to vibration were performed.•Measurements were done at minimum fluidization conditions.•DIA and PIV were used to characterize the bulk and solids velocity oscillation.•The bed bulk cyclically expands and compresses.•Compression and expansion waves are observed in the solids velocity moving upwards.
One of the key devices of the HL-LHC project are SRF Crab Cavities. A cryomodule with two Double Quarter Wave (DQW) crab cavities has been fabricated at CERN in 2017 and successfully tested with beam ...in the Super Proton Synchrotron (SPS) in 2018. The aim of this study is to present and compare the estimation of the thermal budget for the different components of the cryomodule, performed with numerical and semi-analytical methods, with the experimental measurements carried out on the cryomodule before its installation in the SPS.
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•Isolated bubbles in a vibrated pseudo-2D bed are simulated with two-fluid models.•Oscillations and phase delays of bubble diameter and velocity are obtained.•The use of ...incompressible gas model yields unrealistic oscillations and delays.•Simulations with compressible gas and plenum compare well with experiments.•The phase delay of oscillations in vibrated beds is caused by the gas compressibility.
In this work the size and motion of isolated bubbles in a vertically vibrated fluidized bed are numerically investigated by means of two-fluid model simulations. The oscillations of the bed bulk and the bubble diameter and velocity are compared with experimental results of a pseudo-2D bed using an averaging of cycles method to account for the intrinsic unsteadiness caused by vibration. The effects of gas compressibility and the air plenum of the vibrated bed are also numerically investigated. The results show that the two-fluid model simulations resorting to a compressible gas model are able to reproduce both the cyclic compression and expansion of the bed bulk and the bubble oscillations observed in the experiments. In contrast, the simulations with the incompressible gas model fail to reproduce these effects. The presence of the air plenum in the numerical model diminishes the amplitude of the bed and bubble oscillations and improves their resemblance to the experiments. In the simulations with compressible gas, a phase delay is found between the bed displacement and the oscillation of bubble characteristics. In harmony with experiments, the phase delay is smaller in the lower half of the bed (i.e. close to the distributor) than in the upper half. This effect is not reproduced by the simulations with incompressible gas-phase. These results suggest that the phase delay in vibrated beds is caused by the compression of the gas phase, which leads to compression-expansion waves traveling through the bed. The simulations also confirm that the amplitude of vibration influences the magnitude of the bubble diameter and velocity oscillations, whereas the delay of the bubble characteristics is mainly affected by the bed vibration frequency.
•Fuel particle motion in a pseudo-2D bed is simulated.•A TFM–DEM hybrid model is used to describe the three phases in the bed.•A wall-friction model is also used to account for the wall ...effect.•Simulation results are compared with experiments for different fuel particle densities.•The inclusion of the wall-friction term improves the time-scale numerical results.
The mixing of fuel particles is a key issue on the performance of fluidized bed reactors. In this work, the motion of a non-reactive fuel particle in a pseudo-2D bubbling fluidized bed operated at ambient conditions is simulated employing a hybrid-model and introducing a new friction term that accounts for the effect of the bed vessel front and rear walls. The hybrid-model, implemented in the code MFIX, simulates the dense and gas phases using a Two-Fluid Model (TFM) whereas the fuel particles are modeled using a Discrete Element Method (DEM). The importance of the present hybrid-model is that the interaction of the continuum phases with the fuel particles behavior is fully coupled.
To improve the accuracy of the simulated fuel particle motion in a bubbling fluidized bed, a model accounting for the effect of the bed front and rear walls on the continuum solid phase is combined with the hybrid-model. The rising and sinking velocity of the fuel particles, the circulation time and statistical parameters associated to the location of the fuel particle in the bed were obtained from the simulations and compared with experimental measurements. According to the results, the prediction of these parameters is clearly improved when the friction term is included in the simulation.
In this work a numerical study of a pseudo-2D gas fluidized bed is carried out using the MFIX-DEM code with a twofold aim. The first aim is to check whether the DEM code reproduces the overall ...experimental value of the frictional force of the walls on the particles in the pseudo-2D bed in bubbling regime, previously measured by Hernández-Jiménez et al. (2013) by means of a global force balance in the bed. The second aim of this work is to perform a local study of the wall–particle frictional forces, using the results of the DEM simulations. The results showed that the force balance proposed by Hernández-Jiménez et al. (2013) is consistent with the DEM simulations, corroborating that the particle–wall overall force can be considered equal to the velocity of the centre of mass times a global particle–wall interaction coefficient, c. Besides, it was found that the most probable value of the local coefficient c in the DEM simulations is similar to the global value experimentally obtained. As expected, the DEM results showed that this particle–wall interaction coefficient, c, increases with the particle–wall friction coefficient. Coincidence between simulations and experiments is maximum if an angle of internal friction very close to 30° is considered in the DEM particle–wall interaction.
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•A numerical study of a pseudo-2D bed is carried out using the MFIX-DEM code.•The frictional force of the walls on the particles is studied.•The DEM results are consistent with previous experimental findings.•The frictional force is proportional to a particle–wall interaction coefficient.•The particle–wall interaction coefficient increases with the particle–wall friction.
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 0.206m span and 0.01m 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 1.15mm 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.
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•Experiments of a triangular fluidized bed subject to vibration were performed.•PIV in a relative reference system were used to measure the particle velocity.•Different granular patterns depending on the experimental conditions were observed.•Gas injection through the bed inclined walls can reverse the circulation pattern.•The particle velocity field follows several subcycles of oscillation.
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•A new wall-friction model is developed for 2D simulations of pseudo-2D beds.•The wall-friction accounts for the front and back walls drag on the solids motion.•The model combines ...experimental evidence and simplifications of KTGF.•Simulation results are validated with two different experimental pseudo-2D beds.•The use of the proposed wall-friction model improves standard 2D simulations.
Pseudo-2D fluidized beds have been crucial for the understanding of the dynamics of gas-particle systems. In these systems the distance between the front and back walls is narrow, which restricts and creates a resistance to the solids motion, leading to a different flow behaviour compared to fully 3D systems. This interaction of the particle motion with the walls can be significant and should not be neglected in numerical simulations. The present work develops a new model to easily account for the friction effect between the walls and the particles in a pseudo-2D bed. The model is based on experimental results combined with simplifications of the shear force on a wall provided by the kinetic theory of granular flows. The dependence on the particle diameter and bed thickness is directly introduced in the model through the use of a straightforward expression that is easy to code and does not lead to numerical divergence. To test the model two beds of different thickness were simulated, and the resulting time-averaged solids concentration and velocity as well as bubble properties were compared with experiments. It is shown that the numerical results with the new wall-friction model improve the prediction of the standard 2D-simulations.
•A novel design of a confined packed bed thermal energy storage system is presented.•Particles are mechanically confined to prevent their fluidization.•The system is optimized considering ...thermodynamic and economic aspects.•The thermocline evolution is analytically considered in the optimization model.•Optimal dimensions and operating velocities are obtained for a case study.
Concentrated solar power is a suitable technology for production of green electricity. However, to attain a uniform electricity production, concentrated solar power should be coupled with large Thermal Energy Storage (TES) systems. Among the different technologies of TES systems, storage of sensible heat in granular material is widely used due to its simple operation. These TES systems store energy as an increase of temperature of a large mass of small solid particles, through which a fluid circulates exchanging heat. TES systems are typically operated in a fixed bed regime, maximizing their exergy output, thus limiting the maximum allowable velocity of the fluid flow. In this work, a novel confined bed is proposed to mechanically prevent the motion of the solid particles conforming the TES system even for high fluid velocities, to guarantee that the exhaust temperature of the fluid is maximum during a discharge process. In this novel confined bed, a thermocline evolves from bottom to top of the system, separating the low and high temperature of the bed during the discharge process. An analytical model was applied to describe the evolution of the thermocline and the effect of the different operating parameters on the thermocline thickness.
The effect of the thermocline thickness was combined with a thermo-economic analysis of a confined bed TES system proposed for a case of study. The new confined bed here proposed was optimized considering thermodynamics aspects, namely the fluid exergy increment in the bed, and economic factors, specifically the total investment cost of the TES system. The optimization resulted in low values of the fluid velocity, between 0.2 and 0.4 m/s, but still higher than the minimum fluidization velocity of sand particles of 750 μm, justifying the requirement of a confined bed, and low bed aspect ratios, between 0.25 and 0.9, to prevent excessively high fluid pressure drops. However, the bed aspect ratio increases significantly for higher granular material particle sizes, up to a ratio of bed height to diameter of 3 for a particle size of 10 mm and a TES demand time of 6 h.
The beam dump facility (BDF) is a project for a new facility at CERN dedicated to high intensity beam dump and fixed target experiments. Currently in its design phase, the first aim of the facility ...is to search for light dark matter and hidden sector models with the Search for Hidden Particles (SHiP) experiment. At the core of the facility sits a dense target/dump, whose function is to absorb safely the400GeV/cSuper Proton Synchrotron (SPS) beam and to maximize the production of charm and beauty mesons. An average power of 300 kW will be deposited on the target, which will be subjected to unprecedented conditions in terms of temperature, structural loads and irradiation. In order to provide a representative validation of the target design, a prototype target has been designed, manufactured, and tested under the SPS fixed-target proton beam during 2018, up to an average beam power of 50 kW, corresponding to 350 kJ per pulse. The present contribution details the target prototype design and experimental setup, as well as a first evaluation of the measurements performed during beam irradiation. The analysis of the collected data suggests that a representative reproduction of the operational conditions of the beam dump facility target was achieved during the prototype tests, which will be complemented by a postirradiation examination campaign during 2020.
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