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Comparison between DNS and RANS approaches for liquid metal flows around a square rod bundle
Full text linkThe thermal-hydraulic characteristics of liquid metal flows around rod bundles are of great interest for the research and design of fourth generation nuclear reactors. Currently, a large research effort is aimed at the development of accurate numerical models for low Prandtl number fluid flows, since the data available in the literature are quite scarce. Direct Numerical Simulation (DNS) is undoubtedly the most accurate approach, but its large requirements of computational resources and time make it less practical than other simplified methods such as the Reynolds-Average Navier Stokes (RANS) approach. The present paper provides a comparison between numerical results of a flow of liquid Lead-Bismuth Eutectic (LBE) at Pr=0.031 around four vertical cylindrical rods arranged in a square lattice, obtained by DNS and RANS. Several turbulence models are considered, including the standard k-ε, k-ω SST, and two Reynolds stress models, namely the ones by Launder, Reece and Rodi (LRR), and Speziale, Sarkar and Gatski (SSG). The accuracy of these models is assessed by comparing the mean Nusselt number, the pressure drop, and local field distributions with those obtained by DNS
Direct Numerical Simulation of liquid metal forced and mixed convection in a square rod bundle
Full text linkThis paper reports results of Direct Numerical Simulations of fully developed, forced and mixed convection of Liquid Lead Bismuth Eutectic (LBE) around four vertical cylindrical rods arranged in a square lattice at Pr = 0.031. The solutions provided here are placed in a context where very few data are available for low-Prandtl flows, especially in mixed convection conditions, and can hence be useful for the development and validation of advanced turbulent heat transfer models. The equations are discretized using a Finite Volume implementation of a second order projection method. The irregular cylindrical boundaries are handled with an original Immersed Boundary technique. A single friction Reynolds number value is set (Reτ = 180) and buoyancy effects are accounted for by imposing the Rayleigh number, under the Boussinesq approximation. A Ra-value Ra = 2.5 ×104 is selected in order to investigate the main differences between forced and mixed convection at low Prandtl number values. Time-averaged velocity and temperature fields are shown, in order to discuss the main features of the flow and thermal fields. First order statistics are also presented, highlighting the effect of aiding buoyancy on turbulent heat and momentum transport
Thermodynamics of complexation of cadmium(II) by open-chain N-donor ligands in dimethyl sulfoxide solution
No full textThermodynamic parameters for the complexation of Cd-II by the amines n-butylamine (n-but), ethylenediamine (en), N,N'-dimethylethylenediamine (dmen), N,N,N'-trimethylethylenediamine (trmen), N,N,N',N'-tetramethylethylenediamine (tmen), diethylenetriamine (dien), N,N"-dimethyldiethylenetriamine (dmdien), and N,N,N',N",N"-pentamethyldiethylenetriamine (pmdien) have been determined in dimethyl sulfoxide (DMSO) solution by potentiometric and calorimetric techniques at 298 K and 0.1 mol dm(-3) ionic strength (NEt4ClO4). Only mononuclear complexes are formed, in which the polyamines act predominantly as chelating agents. All the complexes are enthalpy-stabilized, whereas the entropy changes counteract the complexation. The steric requirements of the donors have a strong influence on the stoichiometry and nature of the Cd-II complexes formed. Comparisons are made with data reported for some of these systems in water and with available data on silver(I) complexation by the same amines in DMSO, the latter showing that the selectivity of the ligands towards metal ions is also influenced by N-alkylation
A comprehensive numerical study on heat transfer and friction characteristics of offset-strip fins
Full text linkOffset-strip fins are among the most used geometries in compact heat exchangers. The geometric and flow parameters of the fins affect their heat transfer effectiveness and head losses. Hence, accurate predictive models are needed to guide the design process. However, most of the correlations available in the literature are valid only for a limited set of geometric configurations and flow regimes. This work discusses the derivation of multivariate response surfaces for the equivalent Darcy and Colburn factors in offset-strip fins. These surfaces feature clear applicability ranges and extend over wide Reynolds and Prandtl number ranges (50≤Re≤12000, 0.71≤Pr≤190). In addition, a novel empirical model for the Prandtl number scaling exponent is proposed. The analysis is carried out through a Design of Experiment approach, performed with computational techniques. Each numerical experiment is carried out by CFD analysis of a periodic fin geometry. The obtained response surfaces approximate CFD results with a mean deviation of ±8.4%. Moreover, application of the correlations to the analysis of complete heat exchangers issued mean and maximum deviations of ±7.8% and ±20%, respectively, thus highlighting the usefulness of the proposed models for the accurate modelling of offset-strip fins in heat transfer applications
Thermodynamic studies on the complexation of cobalt(II) with nitrogen donor ligands in dimethyl sulfoxide
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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
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