Acoustic streaming in pulsating flows through porous media

When a body immersed in a viscous fluid is subjected to a sound wave (or, equivalently, the body oscillates in the fluid otherwise at rest) a rota tional fluid stream develops across a boundary layer nearby the fluid-body interphase. This so-called acoustic streaming phenomenon is responsible for a notable enhancement of heat, mass and momentum transfer and takes place in any process involving two phases subjected to relative oscillations. Understanding the fundamental mechanisms governing acoustic streaming in two-phase flows is of great interest for a wide range of applications such as sonoprocessed fluidized bed reactors, thermoacoustic refrigerators/engines, pulsatile flows through veins/arteries, hemodialysis devices, pipes in off-shore platforms, offshore piers, vibrating structures in the power-generating industry, lab-on-a-chip microfluidics and microgravity acoustic levitation, and solar thermal collectors to name a few. The aim of engineering studies on this vast diversity of systems is oriented towards maximizing the efficiency of each particular process. Even though practical problems are usually approached from disparate disciplines without any apparent linkage, the behavior of these systems is influenced by the same underlying physics. In general, acoustic streaming occurs within the interstices of porous media and usually in the presence of externally imposed steady fluid flows, which gives rise to important effects arising from the interference between viscous boundary layers developed around nearby solid surfaces and the nonlinear coupling between the oscillating and steady flows. This paper is mainly devoted to highlighting the fundamental physics behind acoustic streaming in porous media in order to provide a simple instrument to assess the relevance of this phenomenon in each particular application. The exact microscopic Navier-Stokes equations will be numerically solved for a simplified 2D system consisting of a regular array of oscillating cylinders subjected to an externally imposed steady flow. Results on the pressure drop associated with viscous losses will be compared with predictions from a simple analytical model in which the interaction between the streaming flows developed around the particles and between the oscillating and steady flows are neglected

Role of Particle Size on the Cohesive Behavior of Limestone Powders at High Temperature

Thermal Energy Storage (TES) using granular solids is gaining momentum in the last years. With no degradation up to very high temperatures and very low price, the use of some granular materials such as sand or SiC would be feasible for storing sensible heat at large scale. A further step beyond TES is thermochemical energy storage (TCES) wherein the granular solids undergo a highly endothermic reaction at high temperature. Energy can be in this way more efficiently stored in the long term and released on demand by means of the reverse exothermic reaction. The Calcium Looping process, based on the calcination/carbonation of CaCO3, is being actively investigated for this purpose. However, a caveat of using granular solids for energy storage is the possible increase of interparticle adhesive forces with temperature which would severely hamper the flowability of the solids in the process. The cohesiveness of granular materials is essentially determined by particle size. In this paper we investigate the dependence of the tensile yield strength and compressibility of CaCO3 powders on temperature and consoli- dation stress using samples of narrow particle size distribution in the relevant range between ∼30 and ∼80 μm particle size and temperatures up to 500◦C. Our experimental results show that powder cohesiveness is greatly increased with temperature especially in the case of the finest powders whose tensile yield strength can be increased by up 2 orders of magnitude. The increase of cohesiveness with temperature is further enhanced with a previously applied consolidation stress, which is particularly relevant for applications wherein large amounts of solids are to be stored at high temperature. Experimental data are consistent with the predictions by a contact mechanics model assuming that the solids deform plastically at interparticle contacts. A main conclusion from our work is that some mechanical properties of the solids, specially the mechanical hardness, and how they change with temperature, play a critical role on the flowability of the solids as affected by an increase of temperature.Versión aceptada del artículo. La versión final puede consultarse en: https://doi.org/10.1016/j.cej.2019.123520Ministerio de Economía y Empresa (contract No. CTQ2017-83602-C2-2-R, Feder Funds

Ordering of hard spheres inside hard cylindrical pores

Isothermal-isobaric simulations on the ordering behavior of hard spheres upon confinement are presented. The radii of the confining cylinders go from 1.1 to 2 in units of the diameters of the hard spheres adsorbed. In all the range of pressures considered the spheres were located in concentric layers, as many as the radius of the hard cylinder would permit. When the pressure increases, the hard spheres go from being loosely arranged to the formation of ordered structures. This change is marked in all cases by a distinct break in the density of spheres in a narrow pressure range. When the tube radius is smaller than 1.5, the high-pressure ordering is determined by the number of coplanar spheres you can have within a circle of radius equal to that of the confining tube. For wider tubes, the change upon compression is determined by the formation of defected two-dimensional triangular lattices wrapped to fit inside the particular cylinder we are considering.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Multi-species simulation of Trichel pulses in oxygen

A multi-species model consisting of seven species has been implemented to simulate the generation and development of Trichel pulses in oxygen between a sphere (the cathode) and a plane (the anode). The spatial and temporal evolution of species is obtained by solving the continuity equations of species using a classical one-dimensional model of negative corona discharge. The chemical kinetics of corona discharge includes electron impact reactions (ionization, dissociative and non-dissociative electron attachment, molecular dissociation, etc.), charge transfer reactions and reactions between neutral species

Titania Coatings: A Mechanical Shield for Cohesive Granular Media at High Temperatures

Fine granular media are pivotal in thermochemical energy storage technology. Reactors based on granular materials store the heat using reversible reactions at high temperatures. Yet, powders become increasingly cohesive in those conditions. The rise of powder cohesion at high temperatures is one of the most irksome phenomena still limiting the scalability of this technology. We found titania coatings comprise an excellent solution to control cohesion in fine limestone powders at high temperatures. Limestone is the main component in granular flows running solid-based storage circuits based on the calcium looping process. It is also involved in many other industrial applications. Titania layers were used to shape stiffer carbonate surfaces at high temperatures (close to the Tamman point). The experiments conducted in this work investigated the benefits of these layers, examining the powder cohesion as the contact between particles evolved from rigid to plastic surfaces. In doing so, samples were subjected to different temperatures varying from 25 oC to 500 oC and preconsolidations up to 2 kPa. The results revealed that titania coatings shield (mechanically) carbonate particles, making surfaces more resilient to deformation while particles interact. The efficiency of titania layers was compared with samples coated with nanosilica, which is a solution broadly accepted nowadays for limestone powders. The experiments tackled one of the weaknesses of nanosilica coatings, namely their efficiency when particles are barely coated. Interestingly, at high temperatures, samples treated with titania outperformed those layered with nanosilica for surface coverages around 9 %. Moreover, despite such a moderate amount of coverage, samples coated with titania reached an easy-flow regime even at high temperatures. However, samples treated with nanosilica fluidized less uniformly, and their flowability fell into a cohesive-flow regime in similar conditions. In conclusion, titania coatings represent an excellent alternative to deal with those flowability issues that still limit the scalability of solid-based storage technology.Versión aceptada del artículo. La versión final puede consultarse en: https://doi.org/10.1016/j.cej.2022.138123Ministerio de Economía y Competitividad (contract No. CTQ2017-83602-C2-2-R