1,721,024 research outputs found
Natural convection of water-based nanofluids in three-dimensional enclosures
No full textThe single-phase approach, in which nanofluids are treated as pure fluids assuming that the solid and liquid phases are in local thermal and hydrodynamic equilibrium, has been used with good results to simulate forced convection flows. Conversely, its extension to natural convection has revealed to be more problematic, as the obtained numerical results are substantially different from the experimental data, mainly due to the effects of the slip motion occurring between the suspended nanoparticles and the base liquid, whose consequent nonuniform distribution of the solid phase concentration can significantly affect both heat and momentum transfer. In the present article, a two-phase model based on a double-diffusive approach is proposed to study the natural convection flows of water-based metal oxide nanofluids inside cylindrical and rectangular enclosures heated and cooled at their opposite sides with the scope to reproduce a number of experimental data-sets available in the literature. Given the assumption of local thermal equilibrium between nanoparticles and host liquid, and considering the Brownian and thermophoretic diffusion as responsible for causing significant relative velocity between the solid and liquid phases, the governing equations are solved by a control-volume numerical method implemented using the open-source platform OpenFOAM (Open Field Operation and Manipulation). It has been found that the nanoparticle diffusion gives the major contribution to the decrease of the heat transfer rate red usually observed experimentally in natural convection flows of nanofluids inside differentially-heated enclosures. This means that a two-phase model, in conjunction with proper correlations for the evaluation of the nanofluid effective physical properties, must be necessarily enforced to obtain reliable results for many engineering applications
First analysis of a novel design of a solar chimney with absorber elements distributed in the air channel
No full textThe study of solar chimneys arouses great interest because of the wide variety of its applications, above all passive heating, cooling and ventilating of buildings. Mathematical models to predict the obtainable air flow rates and the conversion efficiency have been developed. Some studies focused on the possibility of increasing the amount of heat transferred to the air, and thus the efficiency, through an increase of the absorbing surface area inside the channel. The present work arises in this research line, and in this study a never investigated novel solar chimney configuration is proposed, with four arrays of cylindrical absorber elements in the air channel. Aim of this system is not only to increase the absorbing surface, but also the convection exchange coefficient between absorber surface and air, thus increasing thermal efficiency compared to a traditional system. A numerical model has been developed to investigate the proposed system, with an accurate approach to the radiative heat transfer exchanges, that have usually been rather neglected in previous studies. Convection heat transfer coefficient have been calculated and simulations have been conducted both in the hypothesis of natural and mixed convection. Head losses have also been taken into account. Thermal efficiency and air flow rate have been calculated for different values of the solar height. Results show that in the investigated configuration (a direct power equal to 669.1 W has been considered) the efficiency is always higher than in the traditional system, reaching values up to 61%. Moreover, efficiency value does not decrease as solar height increases, as it happens in the traditional system, where efficiency maximum value is 38% for a 10 degrees solar altitude. Compared to the traditional system efficiency values, the proposed system values result increased, between a minimum of 47% at a solar altitude equal to 3 3 degrees and a maximum of 128% at a solar altitude equal to 71 degrees. Also airflow rates are higher, from a minimum of 7% to a maximum of 21% respectively for the solar altitude values of 33 degrees and 71 degrees. With reference to the average of the efficiency values (in the ten solar altitude values examined in the range 10 degrees -71 degrees), it has been verified that is 68% higher for the configuration with cylinders than for the traditional one. The average air flow rate is 11% higher.Also the power lost by the glass surface do not depend on solar height, and results much lower in the proposed system than in the traditional; the power loss due to the direct power on the inlet opening in the proposed system is lower than in the traditional system, especially for high solar altitudes, having a value up to 135 W, while in the traditional system it reaches 280 W. Moreover, the proposed system efficiency values in several cases results also higher than those found with solutions presented in other studies to increase the absorbing surface.Results therefore show considerable advantages in adopting a novel solar chimney system such as the one proposed in the present work, and its performance appears to be worthy of further investigation, focusing on convection mode and on the optimization of the arrangement of the cylinders (position, number of arrays,..) with respect to the solar height
Measurements of the physical properties of flame-retardant intumescent paints
No full textFire protection of exposed structures may effectively be achieved by the use of intumescent paints,
which create a cellular charred foam cushion upon the substrate when temperature gets high. Such protective
foam, which develops owing to the chemical reactions occurring among the several active ingredients, has the
main role of keeping the structural damage to a minimum, or at least buying time until help can arrive.
Unfortunately, physical properties of intumescent paints after the chemical reaction has occurred are not widely
known, which makes impossible to carry out reliable fire-resistance evaluations for the coated structure. In this
context, first-approach measurements of the thermal conductivity of the cellular foam are executed, and the
main results obtained are presented. Mass density and weight-loss data are also reported
Experimental verification of the acoustic performance of diffusive roadside noise barriers
No full textThe acoustic performance of pairs of diffusive roadside barriers is tested experimentally on a 1:10
scale model, and compared to that of more traditional specularly reflecting barriers. Significant
attenuation benefits are detected not only in the shadow zone behind the barriers, but also in the
unprotected zone immediately above the barriers, thus proving that diffusive traffic faces of the barriers
may effectively help in counteracting multiple reflection effects. In addition, a radiosity-based
theoretical model developed for the evaluation of the sound field behind pairs of diffusive noise barriers
is described, and its ability to predict the extra SPL attenuation deriving from the replacement
of geometrically reflecting barriers with diffusely reflecting barriers is verified
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Buoyancy-induced convection of water-based nanofluids from an enclosed heated cylinder
No full textLaminar natural convection of nanofluids in a square cooled cavity enclosing a heated horizontal cylinder is studied numerically. This paper aims to investigate in what measure the nanoparticle size and average volume fraction, the cavity width, the cylinder diameter and position, the average temperature of the nanofluid and the temperature difference imposed between the cylinder and the cavity walls, affects the basic heat and fluid flow features, as well as the thermal performance of the nanofluid relative to that of the base liquid
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