Array Of Cylindrical Rods Research Articles

OPTIMIZATION AND THERMAL PERFORMANCE INVESTIGATION OF CYLINDRICAL RODS FILLED WITH PCM MATERIAL USING CFD SIMULATION METHOD

A phase-change material (PCM) thermal energy storage (TES) has been studied as part of a project to develop a solar-based room-heating system for extreme cold areas using ANSYS Fluent. The purpose of using a PCM thermal storage is to overcome the intermittency of solar and provide room heating during nighttime as a sustainable and clean solution. A computational fluid dynamic (CFD) model is developed for a cubical TES unit having an array of cylindrical rods of stainless steel filled with inorganic PCM as thermal storage media. Heat-transfer fluid (HTF) selected for this study is water. In the present study, the diameter of cylindrical rods is optimized by comparing the melting characteristics of PCM with three different diameters, and thermal performance of the selected option is investigated and discussed. ANSYS Fluent software is used for numerical modeling of a commercially available inorganic PCM material Save-HS48 (Pluss Advanced Technologies, India) and the results were validated using experimental data provided by the PCM manufacturer. 3D transient option with solidification and melting in Ansys Fluent software is selected for the investigation. A four-hour charging time is assumed for the numerical modeling and discharging for a particular case was studied with suitable assumptions for full-cycle analysis. Thermo-physical properties of Save-HS48 PCM are found to be suitable for solar room-heating application and effectiveness of all the cases simulated in the present study provides suitable results for further experimental study of a PCM-TES unit.

Shear flow over cylindrical rods attached to a substrate

Shear flow over a periodic array of cylindrical rods attached to a substrate is studied as a model of flow over a nanomat consisting of aligned carbon nanotubes. The objectives are to evaluate the macroscopic slip velocity, compute the hydrodynamic load exerted at the rod side surface and tip, and estimate the flow-induced deflection. The hydrodynamic traction and macroscopic slip velocity are computed by solving the equations of Stokes flow for a doubly periodic square or hexagonal arrangement using a boundary-element method. The results illustrate the dependence of the slip velocity on the surface coverage expressed by the ratio of the rod radius to separation, and confirm the occurrence of hydrodynamic screening due to surrounding rods confining the traction near the exposed tip of each rod. An estimate for the flexural stiffness of nanotubes is made using available information on the flow-induced deflection. Computations for shear flow past an isolated attached rod are carried out using a highly accurate boundary-element method coupled with a finite-element method for solving the Euler–Bernoulli beam equation, and an iterative procedure involving a boundary-element implementation coupled with a boundary-value formulation involving ordinary differential equations for describing large beam deformation. The results illustrate the precise shape of deflected rods.

Acoustic analog of electronic Bloch oscillations and Zener tunneling

The observation of Bloch oscillations in sound propagation through a multilayer of two different fluidlike components is predicted. In order to obtain the equivalent to the acoustic analog of a Wannier-Stark ladder [E. E. Mendez et al., Phys. Rev. Lett. 60, 2426–2429 (1988)], a set of cavities with increasing thickness is employed. Bloch oscillations were theoretically predicted as time-resolved oscillations in transmission in direct analogy to electronic Bloch oscillations in semiconductor superlattices [J. Feldmann et al., Phys. Rev. B 46, R7252–R7255 (1992)]. Finally, an experimental setup is proposed to observe the phenomenon by using arrays of cylindrical rods in air, which acoustically behaves as a fluidlike system with effective sound velocity and density [D. Torrent et al., Phys. Rev. Lett. 96, 204302 (2006)]. For the proposed system, Bloch oscillations and Zener tunneling are confirmed by using multiple scattering simulations. [Work supported by MEC of Spain.]

A mechanism of pore pressure accumulation in rapidly sliding submerged porous blocks

The generation and dissipation of pore pressure excess due to the rapid, mutual sliding of two water-saturated blocks which are composed of glued arrays of cylindrical rods is herein theoretically examined. The mathematical model consists of three ordinary differential equations describing the motion of a solid mass coupled with the continuity of a fluid mass. These equations are numerically solved for the following conditions: (a) motion induced by the constant horizontal velocity of the sliding mass; (b) constant drag force acting in a horizontal direction; (c) gliding on the irregular, inclined surface that envelops the rods. Theoretical predictions for case (a) favorably compare with experimental results reported in the literature from an investigation carried out on a physical model. Primary results show that the pore pressure excess depends upon physical and geometrical parameters, and furthermore, that displacement of the rods and pore pressure excess are strongly coupled. Maximum pore water pressure occurs when “fluidization”, i.e. the complete balancing of submerged block weight by pore pressure excess, is achieved. The latter does not coincide with the rising of the upper block (hydroplaning). A cyclic motion is reached under condition (a), when dynamic pore water pressure increases from a negative to a maximum positive value and then falls again to a negative value. On the other hand, non-cyclical motion and monotonically increasing pore water pressure are obtained under sliding conditions (b) and (c). The response of the model under condition (c), which is of particular interest for run-out analyses of flow slides, is compared to the sliding response of ordinary submerged blocks on a slope, showing significant differences and pointing out the roles of particle diameter, layer extent and medium permeability. The occurrence of different pore pressure states for fluidization and hydroplaning is highlighted. This, hopefully, can open the way to a reliable treatment of the mechanics of sub-marine flow slide propagation.

On the Mathematical Description and Simulation of Turbulent Flow in a Porous Medium Formed by an Array of Elliptic Rods

Many engineering and environmental system analyses can benefit from appropriate modeling of turbulent flow in porous media. Through the volumetric averaging of the microscopic transport equations for the turbulent kinetic energy, k, and its dissipation rate, ε, a macroscopic model was proposed for such media (IJHMT, 44(6), 1081-1093, 2001). In that initial work, the medium was simulated as an infinite array of cylindrical rods. As an outcome of the volume averaging process, additional terms appeared in the equations for k and ε. These terms were here adjusted assuming now the porous structure to be modeled as an array of elliptic rods instead. Such an adjustment was obtained by numerically solving the microscopic flow governing equations, using a low Reynolds formulation, in the periodic cell composing the medium. Different porosity and Reynolds numbers were investigated. The fine turbulence structure of the flow was computed and integral parameters were presented. The adjusted model constant was compared to similar results for square and cylindrical rods. It is expected that the contribution herein provide some insight to modelers devoted to the analysis of engineering and a environmental systems characterized by a porous structure saturated by a fluid flowing in turbulent regime.

Finite element analysis of an array of cylindrical rods or rectangular bars between ground planes

The structure of many microwave filters consists of arrays of cylindrical rods or rectangular bars located between parallel ground planes. In this paper, a design method and design data to synthesize filters such as interdigital and comb-line filters are presented. An analysis of these filters based on finite and infinite element methods is developed. Quasi-TEM mode, the lowfrequency propagation, along such a structure is described in terms of even and odd modes. A variational principle is used to compute the Normalized Maxwellian Capacitance Matrix Per Unit Length (NMCPUL). The NMCPUL may be used to synthesize filters. The results obtained by the present method agree with the reported results in the literature.

Green's-function theory for row and periodic defect arrays in photonic band structures.

The defect modes associated with a row or a periodic array of defects in a two-dimensional photonic band structure are studied using exact Green's-function methods. Specifically, we consider the above-described defect problems created in a pure photonic band-structure system that models an experimental system recently investigated by McCall et al. This is a square lattice array of cylindrical rods formed of linear dielectric material with \ensuremath{\epsilon}=9 surrounded by vacuum and of filling fraction f=0.4488. In one study a row of rods is replaced in the photonic band structure by a row of impurity rods that may be of either linear or nonlinear dielectric material, and conditions are determined for the existence of impurity modes, associated with the impurity array, in the photonic band gap. The electric fields of the impurity modes are computed for impurity modes in the gaps. In a second study, a two-dimensionally periodic array of defects is introduced into the photonic band structure such that the periodicity of the defect array may or may not coincide with the periodicity of the original photonic band structure. The defect rods are treated in the cases for which the impurities are formed from both linear and nonlinear dielectric materials, and the conditions necessary for defect modes to exist in the photonic band gaps are determined. \textcopyright{} 1996 The American Physical Society.

Influence of the porosity of a porous medium on the effective viscosity in Brinkman's filtration equation

Brinkman's filtration equation is usually used to describe the low-Reynolds-number flow in porous media in situations where velocity gradients are non-negligible. In this paper a model problem is analysed to determine the influence of the porosity of the porous medium on the effective viscosity in Brinkman's filtration equation. In the idealized problem we consider axial flow through infinite and streaked arrays of cylindrical rods. Suppose that in such porous medium the flow is described by Brinkman's filtration equation then the effective viscosity can be calculated as a function of porosity. Contrary to the Einstein correction for dilute suspensions it is shown in this paper that the effective viscosity may be less than the viscosity of pure fluid.

An Analysis of Heat Transfer to Axial Dispersed Flow between Rod Bundles under Reactor Emergency Cooling Conditions

Analysis is carried out for dispersed flow heat transfer under reactor emergency cooling conditions. The present formulation explicitly reveals an extra dependence of the heat transfer coefficient and Nusselt number on the mean vapor temperature for droplet dispersed flow which is not found in single phase flow heat transfer. The heat transfer results obtained from three different geometries—an infinite square array of cylindrical rods, an annulus and a circular pipe—are compared; all have the same hydraulic diameter. It is found that, within the framework of the present analysis, results for the annulus and the rod bundles agree well when the pitch-to-diameter ratio is 1.5 or greater. The circular pipe is in general a poor approximation for rod bundle geometries except at a pitch-to-diameter ratio of about 1.3 which is typical of present day light water reactor fuel assemblies.

The effects of thermal radiation on the temperature distribution in fuel rod arrays

Radiative transfer through an array of cylindrical rods in a non-conductive and non-convective medium is analyzed on the basis of a new conceptual model. The rod array geometry is modeled by defining an appropriate unit cell, for which the characteristic thermal radiation parameters of forward and backward reflectance and cell transmittance are determined. A two-flux radiation model is then utilized to determine the net thermal radiation to any surface within the subassembly. This radiant flux acts as the time-dependent boundary conditions for the transient temperature variation of individual rods and the subassembly can wall. By approximating the radiant flux with a Fourier series and applying Duhamel's principle, the temperature profile can be expressed in terms of a convergent series. This new methodology is applied to different reactor accident analyses, and it is found that after clad melting, the azimuthally-dependent net radiant flux results in the development of significant temperature differences across the fuel elements close to the subassembly can wall.

Performance of Ag-In-Cd in Pressurized Water Reactors

An alloy composed of 80% Ag, 15% In, and 5% Cd was selected for the first commercial pressurized water reactors on the basis of its attractive nuclear properties, ease of fabrication, and low relative cost. Representative samples of a nickel-plated Ag-In-Cd control rod from Yankee Core I showed that the plating had not protected the base metal from the reactor coolant. However, the alloy displayed good irradiation stability, as evidenced by only minor changes in dimensions, microstructure, and mechanical properties. Stainless steel clad Ag-In-Cd absorbers have performed well in several reactors, both as cruciform arrays of cylindrical rods and as clusters of rods arranged in concentric rings.