Artículos de investigación PDI

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A new relationship on transport properties of nanofluids. Evidence with novel magnesium oxide based n-tetradecane nanodispersions
(Powder Technology, 397 (2022) 117082, 2022-02-03) Prado, José I.; Vallejo, Javier P.; Lugo, Luis
The worldwide increasing of thermal energy consumption fosters new technological solutions based on nanomaterials. The use of nanofluids enhances energy efficiency leading to eco-friendlier devices. Thus, researchers are encouraged to understand how modified thermophysical properties improve heat transfer capability. Magnesium oxide based n-tetradecane nanofluids are designed in terms of stability for cold storage application. Thermal conductivity, viscosity, density, and isobaric heat capacity were determined by transient hot wire, rotational rheometry, mechanical oscillation U-tube, and differential scanning calorimetry. Furthermore, a useful relationship on thermal conductivity and viscosity of nanofluids is proposed based on Andrade, Osida and Mohanty theories. Its reliability is checked with the here reported results and literature data of different nanofluids: titanium oxide within water, silver within poly(ethylene glycol), and aluminium oxide within (1-ethyl-3-methylimidazolium methanesulfonate + water). Similar trends have been found for all nanofluids excepting titanium oxide aqueous nanofluids, this differentiated behaviour being expected by the proposed relationship.
A rank-constrained coordinate ascent approach to hybrid precoding for the downlink of wideband massive MIMO systems
(2023-02-01) GONZÁLEZ-COMA, José P.; FRESNEDO, Óscar; CASTEDO, Luis
An innovative approach to hybrid analog-digital precoding for the downlink of wideband massive MIMO systems is developed. The proposed solution, termed Rank-Constrained Coordinate Ascent (RCCA), starts seeking the full-digital pre coder that maximizes the achievable sum-rate over all the frequency subcarriers while constraining the rank of the overall transmit covariance matrix. The frequency-flat constraint on the analog part of the hybrid precoder and the non-convex nature of the rank constraint are circumvented by transforming the original problem into a more suitable one, where a convenient structure for the transmit covariance matrix is imposed. Such structure makes the resulting full-digital precoder particularly adequate for its posterior analog-digital factorization. An addi tional problem formulation to determine an appropriate power allocation policy according to the rank constraint is also pro vided. The numerical results show that the proposed method outperforms baseline solutions even for practical scenarios with high spatial diversity.
Adaptative Integral Sliding Mode Based Course Keeping Control of Unnamded Surface Vehicle
(Journal of Marine Science and Engineering, 2022, 10, 68, 2022) González-Prieto, José Antonio; Pérez-Collazo, Carlos; Sing, Yogangh
This paper investigates the course keeping control problem for an unmanned surface vehicle (USV) in the presence of unknown disturbances and system uncertainties. The simulation study combines two different types of sliding mode surface based control approaches due to its precise tracking and robustness against disturbances and uncertainty. Firstly, an adaptive linear sliding mode surface algorithm is applied, to keep the yaw error within the desired boundaries and then an adaptive integral non-linear sliding mode surface is explored to keep an account of the sliding mode condition. Additionally, a method to reconfigure the input parameters in order to keep settling time, yaw rate restriction and desired precision within boundary conditions is presented. The main strengths of proposed approach is simplicity, robustness with respect to external disturbances and high adaptability to static and dynamics reference courses without the need of parameter reconfiguration.
Addition of ferrocyanide-based compounds to repairing joint lime mortars as a protective method for porous building materials against sodium chloride damage
(Materials and structures, 2021) Feijoo, Jorge; Duygu, Eugenç; Fort, Rafael; Buergo, Mónica Álvarez de
Soluble salts are considered one of the main agents in weathering of porous materials used in building constructions. In this work, a comparison in terms of protection against the damage caused by salts, durability and harmful effects of the application of a joint lime repair mortar, with and without K4Fe(CN)6 in its composition, for bonding sandstone blocks contaminated with NaCl was evaluated. Results show that the introduction of ferrocyanide allowed: 1) to protect the mortar during the curing process, hindering the entry of salts into its pores during drying; 2) to improve the carbonation of the mortar and its adhesion to the porous materials´ surface and 3) to remove chlorides from the surrounding materials, reaching percentages of reduction close to 100%. All of these without causing aesthetic damage to the materials and maintaining a protective capacity against NaCl over time that allows considering this treatment not only as a shock treatment but also as a preventive measure.
Alternating Minimization for Wideband Multiuser IRS-aided MIMO Systems under Imperfect CSI
(IEEE, 2023-11) PÉREZ-ADÁN, Darian; JOHAM, Michael; FRESNEDO, Óscar; GONZÁLEZ-COMA, José P.; CASTEDO, Luis; UTSCHICK, Wolfgang
This work focuses on wideband intelligent reflecting surface (IRS)-aided multiuser MIMO systems. One of the major challenges of this scenario is the joint design of the frequency dependent base station (BS) precoder and user filters, and the IRS phase-shift matrix which is frequency flat and common to all the users. In addition, we consider that the channel state information (CSI) is imperfect at both the transmitter and the receivers. A statistical model for the imperfect CSI is developed and exploited for the system design. A minimum mean square error (MMSE) approach is followed to determine the IRS phase-shift matrix, the transmit precoders, and the receiving filters. The broadcast (BC)-multiple access channel (MAC) duality is used to solve the optimization problem following an alternating minimization approach. Numerical results show that the proposed approach leads to substantial performance gains with respect to baseline strategies that neglect the inter-user interference and do not optimize the IRS phase-shift matrix. Further performance gains are obtained when incorporating into the system design the statistical information of the channel estimation errors.
Analysis of Heat Transfer Characteristics of a GnP Aqueous Nanofluid through a Double-Tube Heat Exchanger
(Nanomaterials 2021, 11(4), 844, 2021) Calviño, Uxía; Vallejo, Javier P.; Buschmann, Matthias H.; Fernández-Seara, José; Lugo, Luis
The thermal properties of graphene have proved to be exceptional and are partly maintained in its multi-layered form, graphene nanoplatelets (GnP). Since these carbon-based nanostructures are hydrophobic, functionalization is needed in order to assess their long-term stability in aqueous suspensions. In this study, the convective heat transfer performance of a polycarboxylate chemically modified GnP dispersion in water at 0.50 wt% is experimentally analyzed. After designing the nanofluid, dynamic viscosity, thermal conductivity, isobaric heat capacity and density are measured using rotational rheometry, the transient hot-wire technique, differential scanning calorimetry and vibrating U-tube methods, respectively, in a wide temperature range. The whole analysis of thermophysical and rheological properties is validated by two laboratories. Afterward, an experimental facility is used to evaluate the heat transfer performance in a turbulent regime. Convective heat transfer coefficients are obtained using the thermal resistances method, reaching enhancements for the nanofluid of up to 13%. The reported improvements are achieved without clear enhancements in the nanofluid thermal conductivity. Finally, dimensionless analyses are carried out by employing the Nusselt and Péclet numbers and Darcy friction factor.
Appraisal of non-destructive in situ techniques to determine moisture- and salt crystallyzation-induced damage in dolostones
(Journal of building engineering, 2022) Fort, R.; Feijoo Conde, Jorge; Varas-Muriel, M.J.; Navacerrada, M.A.; Barbero-Barrera, M.M.; Prida, D. de la
The characterisation of both surface and subsurface pathologies (position, depth, width, …) that affects the porous materials used in building constructions, once in service, is important to establish the most suitable intervention strategy. In this sense, the use of non-destructive techniques allows the analysis of different properties without affecting the material. The present study shows the accuracy of different non-destructive in situ techniques, such as: electrical conductivity and capacitance, infrared thermography, ultrasonic pulse velocity, sound absorption, and electrical resistivity tomography, applied on dolostone ashlar stones outer façade of a sixteenthcentury belltower, affected by moisture and salt induced decay. The joint analysis of the results obtained with different techniques substantially improves the interpretation and characterisation of the detected pathologies, as they complement each other perfectly. Electrical resistivity tomography, which delivers resistivity cross-sections, yields very good results in detecting subsurface pathologies, and sound absorption is particularly useful for stone surfaces. In both cases, the frequency of the electric field and that of the acoustic emission to detect the extent of damage must be established in advance. The joint study of electrical conductivity and capacitance determines the degree of moisture/salts, both at the surface and subsurface, in the materials tested, one of the main causes of scaling and flaking in stony materials. However, the petrological characteristics of the materials used and the identification of the saline phases present must be known in advance to make a correct interpretation of the results.
Biochar Amendments and Phytoremediation: A Combined Approach for Effective Lead Removal in Shooting Range Soils
(Toxics, 2024) Maceiras, Rocío; Pérez-Rial, Leticia; Alfonsín, Víctor; Feijoo. Jorge; López, Ignacio
The increasing contamination of soil with heavy metals poses a problem to environmental sustainability. Among these pollutants, lead is particularly concerning due to its persistence in the environment, with harmful effects on human health and ecosystems. Various strategies that combine phytoremediation techniques with soil amendments have emerged to mitigate lead contamination. In this context, biochar has gained significant attention for its potential to enhance soil quality and remediate metal-contaminated environments. This study aims to investigate the combined effect of biochar amendments on the phytoremediation of lead-contaminated shooting range soils. A series of experiments were conducted to determine the impact of the amount and distribution of biochar on lead removal from soil. Soil samples were incubated with biochar for one week, after which two types of seeds (Brassica rapa and Lolium perenne) were planted. Plant and root lengths, as well as the number of germinated seeds, were measured, and a statistical analysis was conducted to determine the influence of the amendments. After one month, the Pb concentration decreased by more than 70%. Our results demonstrate that seed germination and plant growth were significantly better in soil samples where biochar was mixed rather than applied superficially, with the optimal performance observed at a 10% wt. biochar amendment. Additionally, the combined use of biochar and phytoremediation proved highly effective in immobilizing lead and reducing its bioavailability. These findings suggest that the combination of biochar, particularly when mixed at appropriate concentrations, and Brassica rapa significantly improved lead removal efficiency.
Channel estimation and hybrid precoding for frequency selective multiuser mmWave MIMO systems
(IEEE, 2018-02) GONZÁLEZ-COMA, José P.; GONZÁLEZ-PRELCIC, Nuria; CASTEDO, Luis; HEATH, Robert W. Jr
Configuring the hybrid precoders and combiners in a millimeter wave (mmWave) multiuser (MU) multiple-input multiple-output (MIMO) system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink (DL). In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations (MSs) on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates, the base station designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: 1) a factorization of the purely digital solution, and 2) an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.
CO2 Capture via Adsorption Using Silica Gel
(European Journal of Sustainable Development, 2024) Pérez-Rial, Leticia; Alfonsín, Víctor; Maceiras, Rocio; Feijoo, Rocío, Vallejo, Javier P.
This paper investigates the potential of silica gel as an effective adsorbent for CO2 capture. The study explores the adsorption mechanisms, the efficiency of CO2 uptake, and some factors influencing the adsorption capacity of silica gel. The experimental results demonstrate the significant potential of silica gel for CO2 adsorption under various conditions. The adsorption capacity was found to be highly dependent on parameters such as gas flow rate and the particle size of silica gel. The findings suggest that with optimized conditions, silica gel could be a viable material for reducing atmospheric CO2 levels. This research contributes to the development of sustainable and efficient technologies for mitigating climate change through carbon dioxide capture and storage.
Construction and Building Materials Electroprecipitation of inorganic borates, with different solubility, within monumental
(Construction and building materials, 2023) Feijoo, Jorge; Gómez-Villalba, L.S.; Ríos, A. de los; Fort, Rodrigo
Biodeterioration is a serious threat to the preservation of cultural heritage. Currently, the chemical treatments used to combat it must fulfil a series of restrictions to ensure that they are not harmful to humans or the environment. Borates satisfy these conditions but due to its high degree of leaching its application is limited to indoor areas. The use of electroprecipitation may to increase the range of applicability by allowing to precipitate, along the entire section of the stony materials, of a mixture of boron salts with different solubility (zinc, magnesium, and sodium borates) to cover a wide range of moisture conditions. The results obtained show that electroprecipitation not only increases the penetration depth of boron compounds but also allows the formation of compounds of different solubility, which allows the treatment to last over time. Furthermore, the treatment increased the mechanical properties and reduced the porosity of the stones treated, all without causing significant aesthetic changes.
Design of Linear Precoders for Correlated Sources in MIMO Multiple Access Channels
(IEEE, 2018) SUÁREZ-PASCAL, Pedro; GONZÁLEZ-COMA, José P.; FRESNEDO, Óscar, CASTEDO, Luis
This work focuses on distributed linear precod ing when users transmit correlated information over a fading Multiple-Input and Multiple-Output Multiple Access Channel. Precoders are optimized in order to minimize the sum-Mean Square Error (MSE) between the source and the estimated symbols. When sources are correlated, minimizing the sum-MSE results in a non-convex optimization problem. Precoders for an arbitrary number of users and transmit and receive antennas are thus obtained via a projected steepest-descent algorithm and a low-complexity heuristic approach. For the more restrictive case of two single-antenna users, a closed-form expression for the minimum sum-MSE precoders is derived. Moreover, for the scenario with a single receive antenna and any number of users, a solution is obtained by means of a semidefinite relaxation. Finally, we also consider precoding schemes where the precoders are decomposed into complex scalars and unit norm vectors. Simulation results show a significant improvement when source correlation is exploited at precoding, especially for low SNRs and when the number of receive antennas is lower than the number of transmitting nodes.
Development of paraffinic phase change material nanoemulsions for thermal energy storage and transport in low–temperature applications
(Applied Thermal Engineering, 2019-05-27) David Cabaleiro, Filippo Agresti, Simona Barison, Marco A. Marcos, Jose I. Prado, Stefano Rossi, Sergio Bobbo, Laura Fedele
In this study, new phase change material nanoemulsions (PCMEs) were designed and characterized as possible storage and heat transfer media for low–temperature thermal uses. Water– and (ethylene glycol+water)–based emulsions with fine droplets of n–heptadecane and RT21HC commercial paraffin were produced by a solvent–assisted emulsification method. No phase separation or significant growth in PCM drops were observed for the prepared emulsions through storage, after freeze–thaw cycles and under mechanical shear. Phase change transitions were characterized and a significant sub–cooling was observed, with solidification temperatures up to 13 K below the melting point. One pure alkane and two commercial paraffin waxes with higher melting points were considered as nucleating agents to reduce sub–cooling effect. Although the emulsions exhibited diminutions in thermal conductivity up to 9% with respect to the carrier fluids used as base fluid, enhancements in energy storage capacity (considering an operational temperature interval equal to the sub–cooling) reached 26% in the case of RT21HC nanoemulsion based on the (ethylene glycol+water) mixture that contained 10% in mass of paraffin. In addition, the thermal reliability of the nanoemulsions was verified analyzing the changes in latent heat after storage and throughout 1000 thermal cycles.
Effects of paraffin additives, as phase change materials, on the behavior of a traditional lime mortar
(Construction and building materials, 2022) Feijoo, Jorge; Álvarez-Feijoo, M.A.; Fort, Rafael; Arce, Elena; Duygu, Ergenç
This study refers to the inclusion of phase change materials (PCMs) in porous building materials as an alternative means of improving their thermal behavior, assessing the changes caused in their physical-mechanical and durability properties. Specifically, an organic paraffin wax was selected for direct incorporation into lime mortars using different concentrations by weight. The results show that PCMs improve the thermal properties of the mortar while reducing its accessible porosity. This increases the mortars’ resistance to water and soluble salts. However, excessive PCM content causes stresses within the mortar that can jeopardize its structure.
Electroosmotic permeability in kaolinite and CaCO3 poultice mixtures
(Journal of Cultural Heritage, 2024) Eslami, Naser; Feijoo, Jorge; Paz-García, Juan M.; Franzoni, Elisa; Ottosen, Lisbeth M.
Electrokinetic treatment of masonry for desalination or electroosmotic dewatering depends on a poultice, in which the electrodes are placed, which fulfills several purposes. Poultice composed of kaolinite and CaCO3 have been shown to have good workability and high pH buffering capacity. In this work, the elec- troosmotic (EO) permeability is studied in different kaolinite - CaCO3 mixtures. In addition, the effect on EO of using NaCl as a mixing solution is investigated. A special cell is used to test the EO in the speci- mens. A phenomenological approach, based on the potential gradient and the flux of solution, was used to calculate the EO flow rate and EO permeability coefficient. Results showed that by increasing the con- centration of CaCO3 in the poultice mixture, the EO flow rate decreased and the poultice with 80 % CaCO3 and more did not have any EO flow. Furthermore, the ionic strength in the mixing solution decreases the EO flow rate.
Enhancing the Thermal Performance of a Stearate Phase Change Material with Graphene Nanoplatelets and MgO Nanoparticles
(ACS Applied Materials & Interfaces, 2020-08-17) Jose I. Prado, Luis Lugo
The effectiveness of dispersed nanomaterials to improve the thermal performance of phase change materials (PCMs) is well-proven in the literature. The proposal of new engineered nanoenhanced phase change materials (NePCMs) with customized characteristics may lead to more efficient thermal energy storage (TES) systems. This work is focused on the development of new NePCMs based on dispersions of graphene nanoplatelets (GnPs) or MgO nanoparticles in a stearate PCM. The new proposed materials were developed using the two-step method and acetic acid was selected as surfactant to improve the stability of the dispersions. An extensive characterization of the constitutive materials and the developed dispersions through different spectroscopy techniques is reported. Also, the GnPs nanopowder was explored by using the XPS technique with the aim to characterized the used carbon nanomaterial. The obtained spectra were discussed in terms of the chemical bonds related to the found peaks. The thermophysical profile (density, thermal conductivity, isobaric heat capacity and thermal diffusivity) was experimentally determined once the main components of the NePCMs were characterized and dispersions were designed and developed. The differentiated and distinguished effect of the dispersed GnPs and MgO in the properties of the NePCMs have focused the discussion. A comprehensive analysis of the measurements to elucidate the mechanism that promoted higher improvements using GnPs instead of MgO was performed.
Enzymatic cometabolic biotransformation of organic micropollutants in wastewater treatment plants: A review
(Bioresource Technology Volume 344, Part B, January 2022, 126291, 2021-11-06) Kennes-Veiga, David M.; González-Gil, Lorena; Carballa, Marta; Lema, Juan M.
Biotransformation of trace-level organic micropollutants (OMPs) by complex microbial communities in wastewater treatment facilities is a key process for their detoxification and environmental impact reduction. Therefore, understanding the metabolic activities and mechanisms that contribute to their biotransformation is essential when developing approaches aiming to minimize their discharge. This review addresses the relevance of cometabolic processes and discusses the main enzymatic activities currently known to take part in OMPs removal under different redox environments in the compartments of wastewater treatment plants. Furthermore, the most common methodologies to decipher such enzymes are discussed, including the use of in vitro enzyme assays, enzymatic inhibitors, the analysis of transformation products and the application of several -omic techniques. Finally, perspectives on major challenges and future research requirements to improve OMPs biotransformation are proposed.
Evaluation of the Suitability of Electrokinetic Treatment to Desalinate the Limestone of the Tomb of Cyrus, a UNESCO World Heritage Site in Iran
(Heritage, 2023) Eslami, Nasser; Feijoo, Jorge; Aly, Nevin
The tomb of Cyrus the Great, founder of the Persian Empire, is considered one of the most important monuments of Iran. Its advanced state of deterioration motivated the need to carry out a study focused on analyzing the possible damage caused by the presence of soluble salts, and to assess the suitability of an electrokinetic treatment for their extraction. Preliminary diagnostics carried out on stone samples taken from the tomb confirmed that it is affected by the action of soluble salts, and especially by the presence of nitrates and sulfates. The effectiveness and possible harmful effects caused by electrokinetic treatment were evaluated, under laboratory conditions, using the same limestone that makes up the tomb. The obtained results show that this treatment, in a short period of time, reduces the ionic content, reaching high percentages of anion extraction, without causing any damage, which indicates that it is suitable for this type of stone.
Experimental study of electroosmosis and (Cl-) diffusion in fired-clay bricks
(Construction and building materials, 2024) Eslami, Nasser; Ottosen, Lisbeth M.; Feijoo, Jorge; Franzoni, Elisa, Paz-García, Juan Manuel
Electroosmosis (EO) is considered as a base for methods in drying moist masonry. EO and the other transport mechanisms, namely electromigration and diffusion, that can influence EO in the bricks, were studied in four Danish bricks with different manufacturing years and locations. Brick cubes were cut from each brick type and then saturated in NaCl 0.1 M solution. A cell with four electrodes was used to measure the electroosmotic permeability coefficient using a phenomenological approach based on non-equilibrium thermodynamics. The effective diffusion coefficient for chloride and electromigration coefficient were calculated. Results showed that the EO flux obtained in the bricks did not follow either the magnitude of zeta potential or ionic content present in the bricks. It was observed that there was a correlation between the EO coefficient and the porous structure of the brick, especially with the pore connectivity, since the bricks with higher pore connectivity and, therefore, with higher effective diffusion coefficient had a higher electroosmotic coefficient.
Experimental study on thermophysical properties of alumina nanoparticle enhanced ionic liquids
(Journal of Molecular Liquids, 2019-07-07) Elena Ionela Cherecheş, Jose I. Prado, Marius Cherecheş, Alina Adriana Minea, Luis Lugo
In this experimental study, several alumina Nanoparticle Enhanced Ionic Liquids were prepared and studied in regard to their stability, pH, density and thermal conductivity. These new fluids were manufactured by dispersing aluminium oxide nanoparticles in different mixtures based on water and 1-ethyl-3-methylimidazolium methanesulfonate ionic liquid. Furthermore, thermophysical properties (density, thermal conductivity) of pure and binary mixtures with water and 1-ethyl-3-methylimidazolium methanesulfonate were studied in order to select and propose base fluids to design new advanced heat transfer fluids. The pH of the dispersions was determined as around 8.0 - 8.5. In regard to density, the overall [C2mim][CH3SO3] density is higher by 25% than that of water and the influence of ionic liquid density over the mixtures was found to be much higher than that of water, while for the alumina Nanoparticle Enhanced Ionic Liquids the density respects classical equations. Evaluation of thermal conductivity revealed an increase of up to 13% in thermal conductivity when nanoparticles are added to the base fluids and new correlations based on mass fraction and temperature were proposed.

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