EXPERIMENTAL ANALYSIS OF A SINGLE-PHASE PASSIVE COOLING SYSTEM

The Fukushima Daiichi Accident highlighted the need for new sustainable technologies with high reliability for removing thermal load, in the thermal power area, with focus on nuclear power plants. This technology is designed for heat transfer from a hot source to a cold source by natural convection, without the need of active components, such as pumps or ventilators, reducing costs and improving reliability. In order to analyze the system parameters of such passive systems, with focus on its thermo-hydraulic stability, an experimental campaign was performed using a reduced model built at State University of Rio de Janeiro – UERJ – with a Single-phase Passive Cooling System. Thus, the objective of this work is the experimental characterization of such systems for the analysis of the physical phenomena that drives the flow to unstable regimes and also to validate a 1D numerical model developed within this research project to simulate this kind of systems. Keywords: Natural Convection, Thermal Energy, Nuclear systems

Phytoplankton community structure in one sector of Guanabara Bay (RJ, Brazil) during 2011 and 2012

This study analyzed the temporal variability of phytoplankton assemblages in the surface waters of Guanabara Bay (RJ, Brazil), at six stations in front of Icaraí Inlet from April/2011 to April/2012. Our results highlight the great contribution of diatoms, dinoflagellates and cyanobacteria, represented by 111 taxa typical of estuarine and coastal areas. The coexistence of benthic and planktonic species suggests considerable hydrodinamism in these waters. All variables were homogeneous (p >; 0.05) between the stations, but differed between sampling periods. On average, phytoplankton abundance (107 cells.L-1) was higher than that of other estuaries and its temporal behavior was closely correlated (p ; 0,05) entre as estações de coleta para todas as variáveis, mas diferenças entre campanhas. A densidade média (107 cel.L-1) foi superior à de outros sistemas estuarinos e seu comportamento temporal esteve altamente correlacionado (p < 0,01) com diatomáceas e cianobactérias. Por sua vez o padrão de distribuição da riqueza (7 a 27 táxons) apresentou alta correlação positiva (p < 0,01) com dinoflagelados e diatomáceas. Baixos índices de diversidade (< 2,0 bits.cel-1) em 90% das amostras reafirmaram o equilíbrio instável do sistema, típico de ambientes sujeitos à eutrofização. A análise da estrutura das populações estabeleceu que 10% dos táxons são residentes, 12% visitantes e 78% acidentais, reafirmando a influência do aporte continental e/ou águas oceânicas. Entre os táxons "típicos", destacaram-se cianobactérias da Ordem Oscillatoriales, diatomáceas Ceratoneis closterium (=Cylindrotheca closterium) e Leptocylindrus minimus e o dinoflagelado Prorocentrum triestinum

Surface tension implementation for Gensmac 2D

In the present work we describe a method which allows the incorporation of surface tension into the GENSMAC2D code. This is achieved on two scales. First on the scale of a cell, the surface tension effects are incorporated into the free surface boundary conditions through the computation of the capillary pressure. The required curvature is estimated by fitting a least square circle to the free surface using the tracking particles in the cell and in its close neighbors. On a sub-cell scale, short wavelength perturbations are filtered out using a local 4-point stencil which is mass conservative. An efficient implementation is obtained through a dual representation of the cell data, using both a matrix representation, for ease at identifying neighbouring cells, and also a tree data structure, which permits the representation of specific groups of cells with additional information pertaining to that group. The resulting code is shown to be robust, and to produce accurate results when compared with exact solutions of selected fluid dynamic problems involving surface tension

Dynamics of a turbulent jet interacting with a free surface.

The dynamics of free surface turbulence at low Froude numbers has been investigated by direct numerical simulations of a series of temporally growing round turbulent jets issued below and parallel to a clean free surface. The jets had a Reynolds number of 10000 based on initial jet diameter and centerline velocity (Re\sb\lambda\approx 100), a Froude number of 0.1 and were issued at depths of 1.0, 1.5, and 2.0 jet diameters below the surface, respectively. A passive scalar with a Schmidt number of 0.7 was also included in the simulation. The statistics and structure of the jet was found to be in good agreement with experimental measurements in free-surface jets. As the jet approaches the free-surface, the vertical turbulent fluctuations are damped in a 'surface layer' of magnitude on the order of one lateral Taylor microscale, and the horizontal turbulent fluctuations are amplified. Two classes of organized vortical structures could be identified within the surface layer of the jet; (i) vortex tubes with axis parallel to the free surface oriented along the direction of the mean flow strain field and, (ii) vortex tubes with axis normal to the free surface connected to the surface. The interaction of these structures with the free surface leads to the establishment of a secondary flow which pumps the flow upwards towards the free surface at the jet centerplane and outwards towards the edges of the jet on the surface. This phenomenon, known as the surface current, has also been observed in experimental studies. The parallel vortex tubes are subject to three-dimensional dynamics and the usual cascade of energy to the small scales. The reconnected vortex tubes, on the other hand, are not subject to strong vortex stretching near the free surface and form long-lasting coherent structures which grow with time and occasionally merge, leading to a reverse cascade of energy towards the large scales and the establishment of a nearly two-dimensional turbulent state in the immediate vicinity of the free surface. The implications of this two-dimensionality for the modeling of free-surface turbulence is discussed. The ability of various subgrid scale models in capturing these free-surface effects is assessed by a priori tests. The dynamic two-component model developed by Ansari et al. (1994) is shown to be a promising model for LES of free-surface turbulence.Ph.D.Mechanical Engineering and Scientific ComputingUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104346/1/9513424.pdfDescription of 9513424.pdf : Restricted to UM users only

An introduction to compressible flows with applications: quasi-one-dimensional approximation and general formulation for subsonic, transonic and supersonic flows

This book offers a concise and practical survey of the principles governing compressible flows, along with selected applications. It starts with derivation of the time-dependent, three-dimensional equation of compressible potential flows, and a study of weak waves, including evaluation of the sound speed in gases. The following chapter addresses quasi-one-dimensional flows, the study of normal shock waves, and flow in ducts with constant cross section subjected to friction and/or heat transfer. It also investigates the effects of friction and heat transfer in ducts with variable cross section. The chapter ends by pointing to the analogy between one-dimensional compressible flows and open channel hydraulics. Further, the book discusses supersonic flows, including the study of oblique shock waves, and supersonic flows over corners and wedges. It also examines Riemann problems, numerical resolution of the wave equation, and of nonlinear hyperbolic problems, including propagation of strong waves. A subsequent chapter focuses on the small perturbation theory of subsonic, transonic and supersonic flows around slender bodies aligned or almost aligned to the uniform inflow. In particular, it explores subsonic and supersonic flows over a wavy wall. Lastly, an appendix with a short derivation of the Fluid Mechanics basic equations is included. The final chapter addresses the problem of transonic flows where both subsonic and supersonic are present. Lastly, an appendix with a short derivation of the Fluid Mechanics basic equations is included. Illustrated with several practical examples, this book is a valuable tool to understand the most fundamental mathematical principles of compressible flows. Graduate Mathematics, Physics and Engineering students as well as researchers with an interest in the aerospace sciences benefit from this work

An unstructured CVFEM and moving interface algorithm for non-Newtonian Hele-Shaw flows in injection molding

Purpose - The purpose of this paper is to develop a novel unstructured simulation approach for injection molding processes described by the Hele-Shaw model. Design/methodology/approach - The scheme involves dual dynamic meshes with active and inactive cells determined from an initial background pointset. The quasi-static pressure solution in each timestep for this evolving unstructured mesh system is approximated using a control volume finite element method formulation coupled to a corresponding modified volume of fluid method. The flow is considered to be isothermal and non-Newtonian. Findings - Supporting numerical tests and performance studies for polystyrene described by Carreau, Cross, Ellis and Power-law fluid models are conducted. Results for the present method are shown to be comparable to those from other methods for both Newtonian fluid and polystyrene fluid injected in different mold geometries. Research limitations/implications - With respect to the methodology, the background pointset infers a mesh that is dynamically reconstructed here, and there are a number of efficiency issues and improvements that would be relevant to industrial applications. For instance, one can use the pointset to construct special bases and invoke a so-called ""meshless"" scheme using the basis. This would require some interesting strategies to deal with the dynamic point enrichment of the moving front that could benefit from the present front treatment strategy. There are also issues related to mass conservation and fill-time errors that might be addressed by introducing suitable projections. The general question of ""rate of convergence"" of these schemes requires analysis. Numerical results here suggest first-order accuracy and are consistent with the approximations made, but theoretical results are not available yet for these methods. Originality/value - This novel unstructured simulation approach involves dual meshes with active and inactive cells determined from an initial background pointset: local active dual patches are constructed ""on-the-fly"" for each ""active point"" to form a dynamic virtual mesh of active elements that evolves with the moving interface

3D ALE Finite-Element Method for Two-Phase Flows With Phase Change

We seek to study numerically two-phase flow phenomena with phase change through the finite-element method (FEM) and the arbitrary Lagrangian-Eulerian (ALE) framework. This method is based on the so-called one-fluid formulation; thus, only one set of equations is used to describe the flow field at the vapor and liquid phases. The equations are discretized on an unstructured tetrahedron mesh and the interface between the phases is defined by a triangular surface, which is a subset of the three-dimensional mesh. The Navier-Stokes equation is used to model the fluid flow with the inclusion of a source term to compute the interfacial forces that arise from two-phase flows. The continuity and energy equations are slightly modified to take into account the heat and mass transport between the different phases. Such a methodology can be employed to study accurately many problems, such as oil extraction and refinement in the petroleum area, design of refrigeration systems, modeling of biological systems, and efficient cooling of electronics for computational purposes, which is the aim of this research. A comparison of the obtained numerical results to the classical literature is performed and presented in this paper, thus proving the capability of the proposed new methodology as a platform for the study of diabatic two-phase flows

Nano-patterning of surfaces by ion sputtering: Numerical study of the damping effect on the anisotropic Kuramoto-Sivashinsky equation

Series Editors: da Costa Mattos, Heraldo, Martins Costa, Maria Laura, Laredo dos Reis, João.This paper presents a numerical approach to a model describing the pattern formation by ion beam sputtering on a material surface. This process is responsible for the appearance of unexpectedly organized patterns, such as ripples, nanodots and hexagonal arrays of nanoholes. A numerical analysis of preexisting patterns is proposed to investigate surface dynamics, based on a model derived from a anisotropic damped Kuramoto-Sivashinsky equation, in a two dimensional surface with periodic boundary conditions. While deterministic, its highly nonlinear character gives a rich range of results, making it possible to describe accurately different patterns. A finite-difference semi-implicit splitting scheme is employed on the discretization of the governing equation. Simulations were conducted with realistic coefficients related to physical parameters (anisotropies, beam orientation, diffusion). The stability of the numerical scheme is verified with time step and grid spacing tests for the pattern evolution. Hexagonal patterns were obtained from a monomodal initial condition for a higher value of the damping coefficient, while spatiotemporal chaos appeared for lower values. The hexagonal ordered character of the structure was shown to be directly proportional to the damping coefficient.N