Compact device for mass transfer between liquid films and vapour or gas

University Carlos III of Madrid (Spain) offers a method and device for mass transfer between liquid and vapour or gas. Applications to absorbers and desorbers in absorption chiller technology, evaporators, condensers and chemical reactors. The research group is trying to find companies for further development, commercial viability assessment and commercial exploitation

Dispositivo compacto de aplicación en tecnología frigorífica de absorción, evaporadores, condensadores y reactores químicos

La Universidad Carlos III de Madrid ofrece un dispositivo de transferencia de masa entre fases gaseosa y líquida, para tecnología frigorífica de absorción. La tecnología mejora la eficiencia y reduce significativamente el volumen de elementos como los absorbedores y desorbedores de máquinas frigoríficas de absorción, y otros dispositivos de transferencia de masa. Se solicita colaboración para profundizar en el desarrollo, realizar un estudio de viabilidad técnico-económica para su explotación comercial

Approximate analytic temperature distribution and efficiency for annular fins of uniform thickness

The salient feature in the quasi one-dimensional differential equation for annular fins of uniform thickness is without question the presence of the variable coefficient 1/r multiplying the first order derivative, dT /dr. A good-natured manipulation of the variable coefficient 1/r is the principal objective of the present work. Specifically, the manipulation applies the mean value theorem for integration to 1/r in the proper fin domain extending from the inner radius r¹ to the outer radius r². It is demonstrated that approximate analytic temperature profiles and heat transfer rates of good quality are easily obtainable without resorting to the exact analytic temperature distribution and heat transfer rate embodying modified Bessel functions. For enhanced visualization, the computed temperature profiles, tip temperatures and fin efficiencies of approximate nature are graphed and tabulated for realistic combinations of the normalized radii ratio c and the thermo-geometric fin parameter ξ of interest in thermal engineering applicationsPublicad

Compressible-gas two-fluid modeling of isolated bubbles in a vertically vibrated fluidized bed and comparison with experiments

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.This work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660).Publicad

Bulk oscillation and velocity wave propagation in a vibrated fluidized bed at minimum fluidization conditions

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

Experimental quantification of the particle-wall frictional forces in pseudo-2D gas fluidised beds

In this work a novel measurement technique for pseudo-2D fluidised beds is developed. The objective is to give an estimation of the overall frictional force between the solids and the front and rear walls of the bed. For doing this, the measured pressure signal in the bed is processed in combination with the solids distribution (i.e. centre of mass position, velocity and acceleration) obtained from digital image analysis of the optically accessible front view of the bed. This is performed by acquiring the pressure signal in the bed simultaneously to the digital images. Both the pressure and the digital images are connected through a simple force balance in the bed, and a particle-wall interaction coefficient is obtained assuming that the overall frictional force is proportional to the centre of mass velocity. The particle-wall interaction coefficient found using this technique is of the order of 40-120 kg/m(2) s in the bed tested, and the standard deviation of the frictional forces reaches more than 70% of the weight of the bed. Therefore, the results indicate that the contribution of the particle-to-wall friction on the fluctuation of the pressure drop in a pseudo-2D bed is not negligible.This work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660).Publicad

Thermal and mechanical stresses in bayonet tubes of solar central receivers working with molten salt and liquid sodium

One of the most promising technologies for solar thermal power are solar power towers (SPTs), in which direct solar radiation is redirected by heliostats to a receiver located on top of a tower. The technology used by SPT allows obtaining high thermal efficiencies as well as a high number of hours of operation thanks to thermal storage. However, the high thermal gradients to which the receiver is subjected, in addition to the corrosion of the molten solar salt, can cause the rupture of the receiver and this limits the maximum irradiation the receiver can withstand. To overcome this problem there are different strategies, such as the use of working fluids that are less corrosive than molten salts or the development of new designs of the receiver to avoid overheating of the pipes. In this work we analyze the thermal and structural behavior of a new design of SPT receiver in which bayonet tubes are used instead of simple tubes. A bayonet tube consists of a tube inside another one. In the bayonet tube the working fluid first circulates through the inner tube and then through the annular section between the tubes. An eccentric bayonet tube, created by displacing the inner tube with regards the outer tube, reduces the overheating of the fluid and the outer tube wall as will be shown later. Besides, this work also assesses the effect of using either molten salt or liquid sodium as a working fluid on the thermal and structural behavior of the absorber tube. Since the extreme thermal conditions of central receivers preclude a detailed experimental analysis, the analyses of the present work are performed through multi-physics (CFD – FEM) simulations of the working fluid flow in the annular section and the stresses in the outer wall of the bayonet tube, which are the most critical elements of the receiver. In particular, to perform the hydrodynamic and thermal analysis of the fluid section and the outer wall of the tube, the RANS equations of the fluid together with the turbulent RSM model and the head diffusion equation of the wall were solved using ANSYS Fluent v18 CFD code. Boundary conditions of temperature and non-uniform irradiation were selected to represent typical operative conditions of receivers. Subsequently, using the temperature profiles obtained from the CFD simulations for each working fluid, ANSYS Workbench v18 was employed to obtain the thermal and mechanical stresses in the outer tube as a function of its different constraints, including the attachment of the tube. The results obtained with the CFD – FEM simulations show that, regardless of the working fluid, the eccentricity of the bayonet tube decreases local peaks of temperature in the flow and temperature gradients in the outer tube wall, which leads to a reduction of the wall stresses of the SPT receiver. Furthermore, thanks to its high conductivity, liquid sodium is able to yield lower temperature gradients and stresses in the wall, independently of the kind of tube, compared to molten salt

Modelling non-isothermal absorption of vapour into expanding liquid sheets

The problem of non isothermal absorption of vapour into freely expanding liquid sheets is addressed in this study. This is done in the context of four models that characterise the coupled heat and mass transfer in the liquid phase: a nonlinear model retaining the effect of sheet growth, an approximate model for slowly increasing mass flow rate in the sheet, a large Lewis number model and finally, a boundary layer model. These models have been numerically or analytically solved and applied to the comparative analysis of two different working pairs, LiBr H₂O and LiNO₃NH₃, under conditions representative of adiabatic absorption in refrigeration systems. The limits of applicability of each model have been assessed and the sensitivity of the results to the sheet aperture angle, heat of absorption and initial subcooling has also been tested. For equal initial mass fraction and subcooling, the models indicate that Sherwood number and the rate of absorption in laminar expanding sheets for the LiNO₃NH₃solution are always superior to those for the LiBr H₂O solutionThe authors wish to express their gratitude to Dr. M. Venegas for her useful comments. This work has been partially funded by the Spanish Government Research Grants DPI 2002 02439 and ENE 2005 08255 CO2 02, as well as by the Autonomous Community of Madrid & UC3M through CCG07 UC3M/AMB 3412 project. Their contribution is greatly appreciatedPublicad

Effect of vertical vibration and particle size on the solids hold-up and mean bubble behavior in a pseudo-2D fluidized bed

The solids hold-up and mean bubble behavior in a vertically-vibrated fluidized bed are experimentally studied in the present work by Means of Digital Image Analysis (DIA) for four different powders with Geldart classifications A, B and A/B. The bed has a small thickness (i.e. pseudo-2D bed) and operates in bubbling regime subject to a wide range of gas superficial velocities, vibration frequencies and vibration amplitudes. Mean parameters of the bed and the bubbles, such as solids hold-up, bubble fraction, bubble number density and bubble diameter and velocity, are characterized here by averaging the results over time and space. The results reveal that vibration of the bed promotes a confinement of the bubble path to the central section of the bed. This bubble confinement is more intense for the smallest particles tested and for high vibration strengths and creates two different bubble regimes in the bed. In particular, close to the distributor, the bubble velocity decreases when increasing the vibration amplitude of the bed vessel because bubbles are smaller and less confined, and they behave like isolated bubbles. The behavior of bubbles changes when they are far from the distributor, where the interaction between bubbles becomes greater due to their bigger size and the confinement of bubbles induced by vibration. This confinement promotes coalescence of bubbles. It is shown that consideration of these two different regimes of bubble dynamics allows to shed light on understanding the apparently contradictory results encountered in the literature regarding bubble behavior in bubbling vibrated fluidized beds.This work has been partially funded by the Universidad Carlos III de Madrid, Spain, Ayudas a la Movilidad 2014.Publicad

Numerical simulation of a 3-D gas-solid fluidized bed: Comparison of TFM and CPFD numerical approaches and experimental validation

This paper presents the results of a 3-D numerical simulation of a freely bubbling fluidized bed, based on the Eulerian¿Lagrangian approach, using the software Barracuda (CPFD-Barracuda). The main results obtained were assessed in terms of frequency analysis, bubble pierced length, bubble size, bubble passage frequency and bubble velocity. The results obtained were also compared with experimental data obtained in a 3-D fluidized bed using pressure and optical probes, and with the numerical results using the more common Eulerian-Eulerian approach, implemented in the commercial software Fluent (TFM-Fluent). The results show that CPFD-Barracuda satisfactorily predicts the global behaviour of bubbling beds with a low computational cost, although it computes smaller bubble sizes and lower bubble velocities than TFM-Fluent and experiments. Additionally, the spectra of pressure and particle volume fraction obtained with CPFD-Barracuda resemble those from the experiments and the TFM-Fluent simulations, but with a larger contribution of lower frequencies. The peaks of the pressure spectra from CPFD-Barracuda are close to those from the experiments and the TFM-Fluent simulations, whereas those in the solid volume spectra seem to be underestimated by CPFD-Barracuda. The results also indicate that the particle fraction threshold value chosen to distinguish bubbles contours notably influences the results of the bubble characteristics, especially for TFM-Fluent, whereas CPFD-Barracuda is less sensitive to this threshold value.This work was partially funded by the Ministerio de Economía y Competitividad (Projects ENE2016-78908-R and RTI2018-096664- B-C21 (MICINN, FEDER/UE)) of the Spanish Government, the Regional Government of Castilla-La Mancha (project SBPLY/17/180501/000412) and the Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (RED2018-102431-T)