Papers by Domingo Santana
Novel Approach to Characterize Fluidized Bed Dynamics Combining Particle Image Velocimetry and Finite Element Method
Industrial Engineering Chemistry Research, 2009
ABSTRACT
Unfolding the Phase Space Structure of Noisy Time Series by means of Angular First-Return Maps
Discrete Dynamics in Nature and Society, 2015
Multiresolution Analysis of a Drying Process in a Rotating-Distributor Fluidized Bed
Drying Technology, 2015
ABSTRACT Humidity and temperature measurements together with pressure fluctuation signals were em... more ABSTRACT Humidity and temperature measurements together with pressure fluctuation signals were employed to analyze a drying process in a lab-scale bubbling fluidized bed. The experimental facility was equipped with a rotating distributor. The operational conditions reported in sludge, paste and granular drying processes were reproduced using silica sand as wet material and as inert medium. Experiments were performed changing the bed aspect ratio and the water-sand content in the bed operated at ambient conditions. Four drying periods were found analyzing humidity and temperature signals. The multi-resolution approach of the pressure fluctuation signals showed the effect of the sample drop over the bed surface on the fluidization conditions, relating the drying periods with the bed dynamics. The drying process can affect the low and high frequency details of the pressure signal when the bed state changes towards defluidization, or just the high frequency detail if the bed was at the maldistributed regime. The drying conditions needed to use the pressure signals to control the drying process are defined by means of a statistical monitoring approach. A comparison between the static and the rotating distributor tests showed clear benefits for the operation with the rotating distributor for shallow and deep beds. When the distributor rotates the average improvement of the drying rate was 11% whereas the drying time is reduced 40% for the tests of the maldistributed and defluidized regimes.
Comparison of simplified heat transfer models and CFD simulations for molten salt external receiver
Applied Thermal Engineering, 2014
ABSTRACT In the absence of experimental correlations of the solar external receiver performance, ... more ABSTRACT In the absence of experimental correlations of the solar external receiver performance, it is particularly necessary to develop thermal models to optimize the receiver operating modes and to properly design such equipments. Since CFD models require an enormous computational cost to simulate a receiver, two simplified bi-dimensional implicit-scheme models have been developed. Both models consider axial and circumferential variations on the heat flux absorbed by the tubes and on the wall temperature. One assumes homogeneous surface boundary condition and the other assumes homogeneous surface heat flux at the discretized tube wall. The effects of mass flow rate, and wind velocity on the receiver performance have been analysed considering steady state operation. The results have been compared with a simulation carried out with ANSYS Fluent. Both simplified models are able to predict the heat fluxes, the salt and the tube wall temperature with a deviation lower than 6% compared to CFD simulations. The analysis of the developed models has been also compared with a model usually employed that does not consider variable circumferential temperature. The results show that keeping constant the circumferential tube wall temperature leads to lower wall temperature, underestimating the film temperature, the thermal stress and the salt decomposition.
International Journal of Multiphase Flow, 2006
The bubble size, surface and volume distributions in two and three phase flows are essential to d... more The bubble size, surface and volume distributions in two and three phase flows are essential to determine energy and mass transfer processes. The traditional approaches commonly use a conditional probability density function of chordlengths to calculate the bubble size distribution, when the bubble size, shape and velocity are known. However, the approach used in this paper obtains the above distributions from statistical relations, requiring only the moments inferred from the measurements given by a sampling probe. Using image analysis of bubbles injected in a water tank, and placing an ideal probe on the image, a sample of bubble diameter, shape factor and velocity angle are obtained. The samples of the bubble chord-length are synthetically generated from these variables. Thus, we propose a semi-parametric approach based on the maximum entropy (MaxEnt) distribution estimation subjected to a number of moment constraints avoiding the use of the complex backward transformation. Therefore, the method allows us to obtain the distributions in close form. The probability density functions of the most important length scales (D, D 20 , D 30 , D 32 ), obtained applying the semi-parametric approach proposed here in the ellipsoidal bubble regime, are compared with experimental measurements.
District cooling network connected to a solar power tower
Applied Thermal Engineering, 2015
ABSTRACT During the operation of solar power towers there are occasions, commonly in the summer s... more ABSTRACT During the operation of solar power towers there are occasions, commonly in the summer season, where some of the heliostats have to stop focussing on the central receiver, located at the top of the tower, because the maximum thermal power that the receiver can withstand has been reached. The highest cooling demand takes place at these same occasions. In the present paper, the exceeding heliostats are focused to a second receiver, using the extra heat as a source of absorption chillers. The chilled water is used to cool buildings or offices, using a district cooling network, reducing greatly the electricity usage.
Renewable Energy, 2015
This paper presents a methodology to project the flux distribution from the image plane into the ... more This paper presents a methodology to project the flux distribution from the image plane into the panels of any central receiver in Solar Power Tower plants. Since analytic functions derived from the convolution approach are conveniently defined on the image plane, its oblique projection solves the distorted spot found in actual receivers. Because of its accuracy describing the flux distribution due to rectangular focusing heliostats, we make use of the analytic function on the image plane by . Based on the projection method, we have developed a computer code successfully confronted against PSA measurements and SolTrace software, either for flat plate or multi-panel cylindrical receivers. The validated model overcomes the computation time limitation associated to Monte Carlo technique, with a similar accuracy and even higher level of resolution. For each heliostat in a field, the spillage is computed besides the rest of optical losses; parallel projection is used for shading and blocking. The resulting optical performance tool generates the flux map caused by a whole field of heliostats. A multi-aiming strategy is investigated on the basis of the radius of the reflected beams, estimated from error cone angles.
Powder Technology, 2005
Some results on particle image velocimetry (PIV) in 2-D freely bubbling fluidized beds are presen... more Some results on particle image velocimetry (PIV) in 2-D freely bubbling fluidized beds are presented. The PIV applications were used in order to determine the initial particle velocity of bubble eruptions. A two-dimensional non-reacting fluidized bed was constructed to measure the origin of the ejected particles and the initial particle velocity distribution, using coarse sand particles. The bubble ejection mechanism was observed taking into account the origin of particles ejected, the initial particle velocity distributions as well as the effect of other neighbor exploding bubbles. Our results show that the assumption of linear dependence of initial velocity with the angle predicts the velocity faithfully only for purely vertical-ascent bubbles. Measurements of ejection velocities show that initial velocities in the combined layer are higher than those of the particles in the nose of the leading bubble. Avoiding coalescence of bubbles at the bed surface can lead to less particle entrainment out of the bed and consequently to shorter fluidized beds. D
Characterization of flow-induced vibrations in gas–solid fluidized beds: Elements of the theory
Chemical Engineering Science, 2013
ABSTRACT This paper revisits the basic hypothesis underlying the measurement of flow-induced vibr... more ABSTRACT This paper revisits the basic hypothesis underlying the measurement of flow-induced vibration in fluidized beds. A novel theoretical approach based on the standing pressure field characterizing the bed dynamics is proposed to link the pressure fluctuations to the measured accelerometer signals. The model provides a reliable prediction of the carrying frequency band and helps in designing the accelerometer measurement process. The model was tested with previous results reported in the literature as well as with piezoelectric accelerometer measurements collected from a lab-scale experimental facility. A study on accelerometer measurements was conducted to identify the main limitations expected for measuring flow-induced vibrations in a gas–solid fluidized bed. The structural response of the vessel to flow-induced vibration was mostly determined by the “bed acoustics” that can be dominated by either elastic or compression waves. Finally, the survival of an envelope process on the measured accelerometer signal guaranteed the quality of the flow dynamical information collected during the measurement process.
Moving bed syngas conditioning: Modelling
Applied Thermal Engineering, 2014
ABSTRACT This paper presents a modelling approach for simulating tars and particulate (dust) remo... more ABSTRACT This paper presents a modelling approach for simulating tars and particulate (dust) removal in a moving bed heat exchange filter (MBHEF) in order to satisfy gas requirements of end-use syngas applications: engines and turbines. The two-dimension, adiabatic, steady-state proposed model accounts for two-phase (gas and solid) and neglects conduction and mass diffusion. Tars condensation is modelled through representative tar class lumps: phenol (class 2), naphthalene (class 4), pyrene (class 5). The model also considers tar concentration influence on the tar dew point. The filtration model is taken from literature. A sensitivity analysis is performed varying the particle size and the superficial gas velocity. Maps of temperature and tars abatement efficiency are presented. The simulation results indicate the feasibility of the use a MBHEF as tars removal equipment benefiting its advantages against others gas-cleaning methods with acceptable pollutant removal efficiencies, ranging 88-94% for ranges studied. Results also point out low gas velocities (0.5-1 m/s) and high particle size (400-700 mu m) for reducing operational costs in MBHEFs with compact size.
ABSTRACT One of the main problems of solar power tower plants with molten salt as heat transfer f... more ABSTRACT One of the main problems of solar power tower plants with molten salt as heat transfer fluid is the reliability of central receivers. The receiver must withstand high working temperatures, molten salt corrosion and important solar flux transients that lead to thermal stresses and fatigue. Despite these difficulties, it is necessary an estimation of the receiver thermal efficiency in order to have an accurate estimation of the investment cost of the solar plant and to assure the lifetime estimation of the receiver.A thermal, mechanical and hydrodynamic analysis of these receivers has been developing in this work, assuming constant heat flux in each axial discretized section of the tube wall but considering circumferential temperature variations in the perimeter of the tubes caused by the difference between the heat flux received by the front part of the tubes and by the rear part.The thermal analysis shows that the radiation losses are higher than in literature, and consequently the thermal efficiency is lower too. This is due to the fact that the effective tube wall temperature for radiation is higher than the mean tube wall temperature, especially if the rear temperature of the tubes is considered. Besides, it has been found that the highest temperatures and thermal stresses are sited on the eastern and western panels of the receivers.Film temperature is the most limiting parameter for the receiver design due to it is responsible for salt decomposition and tube corrosion. Therefore, once the tube material is chosen, the film temperature cannot exceed a critical value over which the corrosion ratio raises rapidly. Small tube diameters and low number of panels results in low film temperatures, although this kind of design increases the pressure drop. Therefore, a compromise between film temperature and pressure drop can lead to a receiver design that ensures its lifetime, and at the same time, optimizes the investment and operational cost of the receiver.
Wide band energy analysis of fluidized bed pressure fluctuation signals using a frequency division method
ABSTRACT A statistical method based on approximation of the cumulative energy distribution by Stu... more ABSTRACT A statistical method based on approximation of the cumulative energy distribution by Student's t-distribution is proposed for the unbiased frequency domain division. The proposed method fixes the number of samples needed to estimate the power spectrum and its corresponding cumulative energy distribution using the Kolmogorov–Smirnov test. The reliability of the method to divide the frequency domain was shown for different fluidization velocities by changing the bed aspect ratio and using different pressure probes. Water-induced defluidization tests were conducted to illustrate the use of wide band energy as a monitoring tool. The Student's t-distribution results are compared with an analysis performed using the traditional visual inspection method. The energy of the power spectrum contained within the frequency regions obtained by the visual method is not able to detect changes in the bed aspect ratio or the start of the rotating distributor. No meaningful differences could be observed in the frequency regions using different quality pressure sensors because the approach using Student's t-distribution focuses on the sharp energy increase produced by the primary frequencies of the power spectrum. The sensitivity exhibited by the proposed frequency division approach for the range of fluidization conditions tested improves the use of the energy contained in these regions as a diagnostic tool in fluidized bed processes.
Chemical Engineering Journal, 2008
A simplified model for gas-solid reactions in fluidised bed (FB) is proposed. Such models already... more A simplified model for gas-solid reactions in fluidised bed (FB) is proposed. Such models already exist for catalytic gas-solid reactions (CGSRs), providing general description of the system in terms of main governing parameters. Expansion of this approach to non-catalytic gas-solid reactions (NCGSRs) is difficult, because the solid reactant takes part in the reaction. Therefore, FB reactor models for NCGSR are usually devised only for specific cases, and a general analysis has not been presented up to date. The present model allows analysis of different types of NCGSR in a generalised way, handling catalytic reactions as a particular, simpler, case. It is shown that the reactor behaviour can be described by three governing dimensionless parameters. Two additional parameters, quantifying the importance of diffusion effects in single particles are also identified, and their impact on reactor behaviour is analysed. Possible simplifications are explored. Model limitations, that is, assumption of isothermal bed and particle and the occurrence of only one reaction, are discussed. Examples are outlined to show the applicability of the method.
Director/es: Macías Machín, Agustín Título: Simulación y experimentación de un generador de partículas usando un lecho fluidizado
Fluidized bed with a rotating distributor operated under defluidization conditions
Chemical Engineering …, Jan 1, 2012
The fluidization conditions of a rotating distributor applied to a 3-D bubbling fluidized bed was... more The fluidization conditions of a rotating distributor applied to a 3-D bubbling fluidized bed was studied to assess its potential use as a counteracting measure of defluidization phenomena. The performance of the fluidized bed operating under nominal conditions ...
Aiche Journal, Jan 1, 2010
A phenomenological discrete bubble model is proposed to help in the design and dynamic diagnosis ... more A phenomenological discrete bubble model is proposed to help in the design and dynamic diagnosis of bubbling fluidized beds. An activation region mechanism is presented for bubble formation, making it possible to model large beds in a timely manner. The bubbles are modeled as spherical-cap discrete elements that rise through the emulsion phase that is considered as a continuum. The model accounts for the simultaneous interaction of neighboring bubbles by including the trailing effects due to the wake acceleration force. The coalescence process is not irreversible and therefore, the coalescing bubble pair is free to interact with other rising bubbles originating the splitting phenomena. To validate the model, the simulated dynamics are compared with both experimental and literature data. Time, frequency, and state space analysis are complementarily used with a multiresolution approach based on the empirical method of decomposition to explore the different dynamic scales appearing in both the simulated time series and those obtained from experimental runs. It is concluded that the bubble dynamics interactions play the main role as the driver of the resulting bed dynamics, matching the main features of measured bubble dynamics. Exploding bubble phenomena have been identified by establishing a direct relation between the bubble generation, interaction and eruption, and the measured signals. © 2010 American Institute of Chemical Engineers AIChE J, 2011
The performance of a novel rotating distributor fluidized bed is presented. The pressure drop and... more The performance of a novel rotating distributor fluidized bed is presented. The pressure drop and the standard deviation of pressure fluctuations, σp, 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.
… on Fluidization-New …, Jan 1, 2007
A new non-intrusive method for measuring the throughflow velocity crossing the dome of erupting b... more 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 (40 deg < < 140 deg) and the total throughflow increases with the superficial gas velocity.
Annals of the New …, Jan 1, 2009
This work provides an exergy analysis of a moving bed heat exchanger to obtain for a range of inc... more 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.
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Papers by Domingo Santana