Finite element formulation to study thermal stresses in nanoencapsulated phase change materials for energy storage

Nanoencapsulated phase change materials (nePCMs) – which are composed of a core with a phase change material and of a shell that envelopes the core – are currently under research for heat storage applications. Mechanically, one problem encountered in the synthesis of nePCMs is the failure of the shell due to thermal stresses during heating/cooling cycles. Thus, a compromise between shell and core volumes must be found to guarantee both mechanical reliability and heat storage capacity. At present, this compromise is commonly achieved by trial and error experiments or by using simple analytical solutions. On this ground, the current work presents a thermodynamically consistent and three-dimensional finite element (FE) formulation considering both solid and liquid phases to study thermal stresses in nePCMs. Despite the fact that there are several phase change FE formulations in the literature, the main novelty of the present work is its monolithic coupling – no staggered approaches are required – between thermal and mechanical fields. Then, the FE formulation is implemented in a computational code and it is validated against one-dimensional analytical solutions. Finally, the FE model is used to perform a thermal stress analysis for different nePCM geometries and materials to predict their mechanical failure by using Rankine’s criterion

Comparación de la posición del incisivo inferior en radiografías de sujetos de 10 a 35 años de edad con diferentes clases esqueléticas y biotipo facial

Comparar la posición del incisivo inferior con diferente clase esquelética y biotipo facial, en sujetos de 10 a 35 años de edad MATERIAL Y MÉTODO: Se realizó un estudio observacional, descriptivo transversal en 120 pacientes que asistieron al Centro Radiológico Digital Dent del distrito de Trujillo en el 2016. RESULTADOS: La posición del incisivo inferior (PII) en la clase esquelética tuvo como promedio 5.55°, la clase II alcanzó en promedio 7.37°.y los de clase esquelética III tuvieron en promedio 2.30°. Hubo relación entre el ángulo interincisal con la clase esquelética con un r = - 0.221,(p=0.015). El promedio de la PII en el biotipo dolicofacial fue 7.51°, en el biotipo mesofacial 6.2° y en el biotipo braquifacial el promedio fue de 4.19 °. Hubo relación entre el ángulo interincisal con el biotipo facial con una r = 0.195 (p= 0.032). La PII se relacionó moderadamente de manera negativa con la clase esquelética, con una r= -0.476 p<0.001. Así mismo la PII se correlacionó, con la biotipo facial con un r = 0.457 (p<0.001). CONCLUSIONES: La posición del incisivo inferior se relacionó con la clase esquelética, así como el ángulo interincisal con el biotipo facial. Por otro lado se halló relación entre la posición del incisivo inferior con las clases esqueléticas y el biotipo facial.Determine the position of the incisive lower in subject of 10 to 35 years of age with different class skeletal and biotype facial.. MATERIAL and METHOD: An observational, descriptive and cross- sectional study. was in 120 patients who attended the Digital Dent Radiological Center of the District of Trujillo in 2016. RESULTS: the position of the incisive lower (PII) in the class skeletal had as average 5.55 °, the class II reached in average 7.37 ° .and those of class skeletal III had in average 2.30 °. There was a relationship between the interincisal angle with the skeletal class with a r = - 0.221,(p=0.015). The average of the IIP in the biotype dolichoface was 7.51 °, in the biotype mesoface 6.2 ° and in the biotype braquiface the average was 4.19 °. There were relationship between the angle interincisal with the face biotype with r = 0.195 (p = 0.032). The PII is related moderately of way negative with the class skeleton, with an r =-0.476 P = 0.000. Likewise the PII is correlated, with the face biotype with an r = 0.457 (p < 0.001). CONCLUSIONS: The lower incisor position related to the skeletal class as well as the interincisal angle with face biotype. On the other hand we found relationship between the position of the lower incisor with skeleton classes and facial biotype

Characterization study of a thermal oil-based carbon black solar nanofluid

Carbon nanoparticles are very useful in solar thermal applications, since they absorb much of the solar spectrum, are cheap and have excellent optical properties. Carbon nanoparticlesthermal oil-based nanofluid was prepared using two-step method with diphenyl sulfone as surfactant to achieve that nanoparticles remain suspended even at high temperatures. The size particle distribution was studied using two Dynamic Light Scattering systems at room and high temperature and also evaluated before and after exposing the nanofluid to a thermal treatment so that conditions closer to those in real applications were replicated. Moreover, the morphological changes due to the thermal treatment were observed with Transmission Electron Microscopy. Finally, the optical properties as the ballistic transmittance, absorption coefficient and scattering albedo of the base fluid as well as of the nanofluid were measured using a spectrophotometer with and without integrating sphere. The results of this study contribute to the knowledge about these solar nanofluids that are promising alternatives to the conventional solar collectors

On the relationship between the specific heat enhancement of salt-based nanofluids and the ionic exchange capacity of nanoparticles

Nanoparticles have been used in thermal applications to increase the specific heat of the molten salts used in Concentrated Solar Power plants for thermal energy storage. Although several mechanisms for abnormal enhancement have been proposed, they are still being investigated and more research is necessary. However, this nanoparticle-salt interaction can also be found in chemical applications in which nanoparticles have proved suitable to be used as an adsorbent for nitrate removal given their high specific surface, reactivity and ionic exchange capacity. In this work, the ionic exchange capacity mechanism for the nanoparticles functionalization phenomenon was evaluated. The ionic exchange capacity of silica and alumina nanoparticles dispersed in lithium, sodium and potassium nitrates was measured. Fourier-transform infrared spectroscopy tests confirmed the adsorption of nitrate ions on the nanoparticle surface. A relationship between the ionic exchange capacity of nanoparticles and the specific heat enhancement of doped molten salts was proposed for the first time

Influence of the production method on the thermophysical properties of high temperature molten salt-based nanofluids

The potential use of molten salt-based nanofluids as thermal energy storage material in Concentrated Solar Power plants has gained attention over the last years due to their enhanced storage capacity. The possible effects of the salt-based nanofluid production at industrial scale have not been yet investigated, as this could influence the nanoparticles agglomeration and therefore their thermal and flow properties. Four methods were evaluated for the production of solar salt-based nanofluids containing 1 wt% of silica nanoparticles. The particle size distribution, the stability, the rheological behaviour and the specific heat of the samples were measured. Nanofluids prepared by means of a dry mixing method presented the lowest viscosity, trimodal particle size distribution and lack of stability. The commonly used dissolution method coupled with oven drying in a petri dish as well as the ball milling method presented non-Newtonian behaviour and intermediate values of particle size and stability. The new spray drying method proposed provided a monomodal particle size distribution with high stability but the highest viscosity and shear thickening behaviour. Results suggest that the four methods evaluated are appropriate for specific heat enhancement (up to 21.1%) but a commitment between stability and viscosity has to be achieved

Modeling of Drying Curves of Silica Nanofluid Droplets Dried in an Acoustic Levitator Using the Reaction Engineering Approach (REA) Model

The use of nanoparticles has become of great interest in different industrial applications. The spray drying of nanofluids forms nanostructured grains, preserving the nanoparticle properties. In this work, individual droplets of silica nanofluids were dried in an acoustic levitator. Tests were carried out under different experimental conditions to study the influence of the variables on the drying process. The drying curves were experimentally obtained and an REA model was used to obtain the theoretical curves and the correlations for the activation energy. The critical moisture content theoretically obtained was used to predict the grain diameter

Influence of Particle Size on the Drying Kinetics of Single Droplets Containing Mixtures of Nanoparticles and Microparticles: Modeling and Pilot-Scale Validation

The particle size of the primary particles is an important parameter influencing the drying behavior of droplets. In this work, the influence of particle size on the drying kinetics and grain properties was analyzed for droplets containing silica nanoparticles, microparticles, and mixtures of the two. The presence of microparticles was found to increase the drying rate and shrinkage of the droplet. The drying curves were modeled using a reaction engineering approach (REA) model. Finally, different suspensions were dried in a pilot-scale spray dryer in order to prove the influence of the particle size obtained in the levitator tests

Microstructure and mechanical properties of grains of silica nanofluids dried in an acoustic levitator

The microstructure and mechanical properties of spray-dried grains are key factors in many applications. In this work single droplets of silica nanofluids were dried in an acoustic levitator under different experimental conditions of solid mass fraction, pH value, salt concentration, drying temperature and initial droplet volume. ANOVA method was used to determine their influence on the final grain diameter, the shell thickness and the mechanical strength. The solid content and the droplet volume are the variables that exert an influence on these three properties. In addition, the mechanical strength is influenced by the pH value. The maximum packing fraction of the particles inside the shell was obtained by modeling the viscosity data with the Quemada equation. The packing fraction was then used to calculate the shell thickness. The internal microstructure of the grains was observed by SEM and the shell thickness was measured. Experimental and calculated results show good agreemen

Characterization of silica–water nanofluids dispersed with an ultrasound probe: A study of their physical properties and stability

The stability and agglomeration state of nanofluids are key parameters for their use in different applications. Silica nanofluids were prepared by dispersing the nanoparticles in distilled water using an ultrasonic probe, which has proved to be the most effective system and gives the best results when compared with previous works. Results were obtained concerning the influence of the solid content, pH and salt concentration on the zeta potential, electrical double layer, viscosity, elastic and viscous moduli, particle size and light backscattering. Measurement of all these properties provides information about the colloidal state of nanofluids. The most important variable is the solid content. Despite the agglomeration due to high concentration, nanofluids with low viscosity and behaving like liquid were prepared at 20% of mass load thanks to the good dispersion achieved with the ultrasonic treatment. The pH of the medium can be used to control the stability, since the nanofluids are more stable under basic conditions far from the isoelectric point (IEP) and settle at pH = 2. Therefore, stable nanofluids for at least 48 h, with high solid content, can be prepared at high pH value (pH > 7) due to the electrostatic repulsion between particles