Comparison of the heat transfer efficiency of nanofluids

The continuously increasing power involved in many applications, coupled with the very small size of a number of component devices, is pushing the technical community to look for more efficient heat transfer systems, to remove the heat generated and keep the system under controlled operating conditions. In particular, significant interest has been devoted to the use of the so-called nanofluids, obtained by suspending nano-sized particles in conventional heat transfer liquids. According to some literature, these suspensions present enhanced heat transfer capabilities, without the inconveniencies of particles settlement and clogging of the channels encountered using larger particles. However, other results show that the actual improvement in the heat transfer efficiency may depend on the adopted working conditions and on the reference parameters (fluid velocity, Reynolds number, pressure drop, etc.) assumed to compare the performances of the nanoparticles suspensions with those of the clear thermal fluid. In the present work heat transfer experiments were carried out on a number of nanofluids systems, varying the type and the concentration of the nanoparticles, and the fluid dynamic regime. The investigated suspensions gave rise to heat transfer coefficients different from those of their respective clear thermal fluid, the thermal efficiency being higher or lower, depending on the fluid dynamic parameter used as a base for comparing the systems. Generally speaking, in most cases nanofluids may give an advantage from the heat transfer point of view only when the conditions are unfavorable for the traditional thermal fluid

Flexural-Torsional Flutter and Buckling of Braced Foil Beams under a Follower Force

The flutter and buckling behavior of a cantilever foil beam, loaded at the tip by a follower force, are addressed in this paper. The beam is internally and externally damped and braced at the tip by a linear spring-damper device, which is located in an eccentric position with respect to beam axis, thus coupling the flexural and torsional behaviors. An exact linear stability analysis is carried out, and the linear stability diagram of the trivial rectilinear configuration is built up in the space of the follower load and spring’s stiffness parameters. The effects of the flexural-torsional coupling, as well as of the damping, on the flutter and buckling critical loads are discussed

The macroscopic behavior of pantographic sheets depends mainly on their microstructure: experimental evidence and qualitative analysis of damage in metallic specimens

Recently the exotic properties of pantographic metamaterials have been investigated, and various mathematical models (both discrete and continuous) have been introduced. However, the experimental evidence available up to now concerns only polyamide specimens. In this paper, we use specimens printed using metallic powder. We prove experimentally that the main qualitative and quantitative features of pantographic sheets in planar deformation are independent of the constituting materials, at least when they can be regarded as homogeneous and isotropic at micro-level. Of course, the absolute value of Young’s modulus of constituent material affects the overall reaction force needed to the hard device to impose a given displacement: A first investigation on this effect is also attempted

Buckling of Planar Micro-Structured Beams

In this paper, a Timoshenko beam model is formulated for buckling analysis of periodic micro-structured beams, uniformly compressed. These are planar grid beams, whose micro-structure consists of a square lattice of equal fibers, modeled as Timoshenko micro-beams. The equivalent beam model is derived in the framework of a direct one-dimensional approach and its constitutive law, including the effect of prestress of the longitudinal fibers, is deduced through a homogenization approach. Accordingly, micro–macro constitutive relations are obtained through an energy equivalence between a cell of the periodic model and a segment of the equivalent beam. The model also accounts for warping of the micro-structure, via the introduction of elastic and geometric corrective factors of the constitutive coefficients. A survey of the buckling behavior of sample grid beams is presented to validate the effectiveness and limits of the equivalent model. To this purpose, results supplied by the exact analyses of the equivalent beam are compared with those given by finite element models of bi-dimensional frames

Multi-scale design of an architected composite structure with optimized graded properties

A design framework is here presented for the development of an architected solid with targeted mechanical properties thanks to an optimized porosity distribution. A 2D lattice of regular hexagons is considered as core element of a sandwich panel and a Bloch-Floquet-based approach is adopted to derive homogenized equivalent properties. The density distribution of the equivalent continuum is taken as objective function to be minimized in the optimization process. To this end, suitable constraints are designed to avoid empty regions and ensure a minimized density where required by the mechanical actions. A de-homogenization process is carried out on the optimized equivalent continuum to derive the configuration of regular hexagons with optimally varying wall thickness. Static and buckling responses of the optimized architected solid are compared with that of a 2D continuum whose material density distribution is determined through a classical topology optimization. It is shown that the architected 2D solid can absorb higher strain energy, with respect to classically optimized structures, which suffer a buckling-driven collapse below the elasticity threshold. The architected solid is also shown to have an improved energy absorption capability, that may increase considerably its performance, depending on the ductility of the adopted material

Air cooling of Li-ion batteries. An experimental analysis

Electric vehicle industry has been rapidly developing internationally due to the renewed interest in low- or zero-emission vehicles. So far, Lithium-Ion batteries, is the technology that best fits the needs of electric vehicles, due to their large specific energy density and specific power, making these cells ideally suited for high rate-of-discharge applications such as during acceleration. In spite of this, there are safety concerns using Lithium-ion cells because of the use of high energy materials. Heat is a major battery killer and Lithium secondary cells need careful temperature control. Operating at high temperatures brings on a set of different problems which may result in the destruction of the cell: unless heat is removed faster than it is generated, a thermal runaway may occur. Tests with an air cooling system were carried out using an experimental loop specially designed: the tests were performed on a module consisting of four pouch cells, manufactured by EiG, connected in series. The module was discharged with a current of 80 A (4C discharge) and cooled with air at velocity ranging from 0 to 7 m/s. It was found that it is possible to completely discharge the batteries using air velocities higher than (or equal to) 4 m/s. Increasing the air velocity the temperature difference between the centre of the cell and the electrodes decreases. In spite of this, it was not possible to reach a uniform temperature distribution within a cell, and within the pack, using air as cooling flui

Thermal analysis of Lithium-ion batteries: an experimental investigation

The performance and stability of secondary batteries depend on the working temperature of the cells. This paper describes a set of experimental tests carried out to better understand the thermal behavior of Lithium-ion batteries under load. Different types of batteries have been analyzed to check the influence of a number of parameters that characterize the cells. The generation of hot spots has been registered, their presence being independent of the cell geometry and size; instead, the battery’s history and age, appear the main factors in determining the onset of hot spots on the surface of the cell, with increases of more than 20-30 degrees with respect to the average surface temperature. Nickel Manganese Cobalt-oxide cells presented major problems from the thermal point of view, while new LiFePO4 cells could withstand charge/discharge rates well beyond the maximum allowed values with no signs of excessive heating

Measurements of rising velocity of a small bubble in a stagnant fluid in one- and two-component systems

Despite its apparent simplicity, the problem of accurate determination of the shape and velocity of a bubble in a still fluid is far from being solved. Most of the models available in the literature make use of three dimensionless parameters to correlate the data, but, according to experimental evidence, the number of independent variables ruling the phenomenon is continuously increasing. The purpose of the present paper is to assess the degree of accuracy of relatively simple and general correlations available in literature for the terminal rising velocity and aspect ratio of a bubble over a wide experimental data set, obtained by injecting bubbles in two different fluids (water and FC-72, a fluoroinert trade mark of 3M) with different injection devices (nozzles and orifices). Both two-components (gas in a different liquid) and one-component (vapor in its liquid) systems were considered. To this aim, a simple experimental apparatus was built and the bubble velocity and shape were determined by analyzing the images taken with a high-speed camera. Measured shapes (aspect ratio) and rising velocities of single bubbles were compared with available correlations. The comparison showed that the aspect ratio was well correlated by the Tadaki number or, to a little lesser extent, by the Weber number. Among the correlations tested, the ones proposed by Taylor & Acrivos and Vakhrushev & Efremov gave the better results in the observed range. Concerning the terminal rising velocity, several correlations were selected and their predictions exhibited generally an error up to ±50% throughout the observed range, the best accuracy being given by the recent model of Tomiyama et al

Antioxidant responses to arsenic stress in grafted melon plants

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Risposta antiossidante allo stress da arsenico in piante di melone innestate

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