Navegando por Autor "Vargas, Jose Viriato Coelho"

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Friction and Heat Transfer for Inclined Surfaces in Relative Motion to an Air Stream Under Buoyancy-assisting and Opposing Flow
(2003) Souza, Jeferson Avila; Vargas, Jose Viriato Coelho; Bianchi, Marcus Vinicius Andrade
Heat transfer from a surface in motion relative to either a stationary or moving fluid occurs in many materials processing applications such as hot rolling, extrusion, drawing, and drying. In this study, an analysis has been carried out to predict the convective transport occurring between air and a continuous inclined surface which moves with an assisting or opposing flow with respect to the free stream in the presence of gravity. The steady flow of air is assumed laminar and is modeled by using a two dimensional (2-D) complete set of conservation equations, subject to the appropriate boundary conditions. The equations were solved numerically by employing the finite element method. Predictions for the local dimensionless skin friction and heat transfer are made for different configurations of the relative position of the surface and the free stream. The numerical results of the present study for the buoyancy-assisting and opposing flows on vertical surfaces are validated by direct comparison with the available published data. New results are presented for inclined surfaces with the buoyancy-assisting and opposing flows. The buoyancy-assisting results are then correlated for wide ranges of inclination angles and moving sheet relative velocities.
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Modeling and simulation of industrial FCC risers
(2007) Souza, Jeferson Avila; Vargas, Jose Viriato Coelho; Meien, Oscar Felippe von; Martignoni, Waldir Pedro
Risers are considered vital parts in Fluidized Catalytic Cracking (FCC) conversion units. It is inside the riser reactor that the heavy hydrocarbon molecules are cracked into lighter petroleum fractions such as liquified Petroleum gas (LPG) and gasoline. The FCC process is considered a key process in the world petroleum industry, since it is the main responsible for the porfitable conversion of heavy gasoil into commercial valuable products. This work presents a simplified transient model to predict the response of a FCC riser reactor, i. e., the fluid flow, temperature and concentrations of the mixture components throughout the riser and at the exit. A bi-dimensional fluid flow field combined with a 6 lumps kinetic model and two energy equations are used to model the gasoil mixture flow and cracking process inside the riser reactor. The numerical results are in good agreement with expetimental data, as a result, the model can be utilized for design, and optimization of FCC units. The simulation herein presented shows the applicability of the proposed method for the numerical simulation and control of industrial riser's units.
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Notional all-electric ship systems integration thermal simulation and visualization
(2012) Vargas, Jose Viriato Coelho; Souza, Jeferson Avila; Hovsapian, Rob; Ordonez, Juan Carlos; Chiocchio, Tim; Chalfant, Julie; Chryssostomidis, Chryssostomos; Dilay, Emerson
This work presents a simplified mathematical model for fast visualization and thermal simulation of complex and integrated energy systems that is capable of providing quick responses during system design. The tool allows for the determination of the resulting whole system temperature and relative humidity distribution. For that, the simplified physical model combines principles of classical thermodynamics and heat transfer, resulting in a system of three-dimensional (3D) differential equations that are discretized in space using a 3D cell-centered finite volume scheme. As an example of a complex and integrated system analysis, 3D simulations are performed in order to determine the temperature and relative humidity distributions inside an all-electric ship for a baseline medium voltage direct current power system architecture, under different operating conditions. A relatively coarse mesh was used (9410 volume elements) to obtain converged results for a large computational domain (185m×24m×34m) containing diverse equipment. The largest computational time required for obtaining results was 560 s, that is, less than 10 min. Therefore, after experimental validation for a particular system, it is reasonable to state that the model could be used as an efficient tool for complex and integrated systems thermal design, control and optimization.
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A numerical investigation of the resin flow front tracking applied to the RTM process
(2011) Souza, Jeferson Avila; Rocha, Luiz Alberto Oliveira; Amico, Sandro Campos; Vargas, Jose Viriato Coelho
Resin Transfer Molding (RTM) is largely used for the manufacturing of high-quality composite components and the key stage during processing is the resin infiltration. The complete understanding of this phenomenon is of utmost importance for efficient mold construction and the fast production of high quality components. This paper investigates the resin flow phenomenon within the mold. A computational application was developed to track the resin flow-front position, which uses a finite volume method to determine the pressure field and a FAN (Flow Analysis Network) technique to track the flow front. The mass conservation problem observed with traditional FE-CV (Finite Element-Control Volume) methods is also investigated and the use of a finite volume method to minimize this inconsistency is proposed. Three proposed case studies are used to validate the methodology by direct comparison with analytical and a commercial software solutions. The results show that the proposed methodology is highly efficient to determine the resin flow front, showing an improvement regarding mass conservation across volumes.
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Numerical simulation of FCC risers
(2003) Souza, Jeferson Avila; Vargas, Jose Viriato Coelho; Meien, Oscar Felippe von; Martignoni, Waldir Pedro
The catalytic cracking of hydrocarbons in a FCC riser is a very complex physical and chemical phenomenon, which combines a three-dimensional, three-phase fluid flow with a heterogeneous catalytic cracking kinetics. Several researchers have carried out the modeling of the problem in different ways. Depending on the main objective of the modeling it is possible to find in the literature very simple models while in other cases, when more accurate results are necessary, each equipment is normally treated separately and a set of differential and algebraic equations is written for the problem. The riser reactor is probably the most important equipment in a FCC plant. All cracking reactions and fuel formation occur during the short time (about 4-5s) that the gas oil stays in contact with the catalyst inside the riser. This work presents a simplified model to predict the, temperature and concentrations in a FCC riser reactor. A bi-dimensional fluid flow field combined with a 6 lumps kinetic model and two energy equations (catalyst and gas oil) are used to simulate the gas oil cracking process. Based on the velocity, temperature and concentration fields, it is intended, on a next step, to use the second law of thermodynamic to perform a thermodynamic optimization of the system.
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Temperature and pressure drop model for gaseous helium cooled superconducting DC cables
(2013) Ordonez, Juan Carlos; Souza, Jeferson Avila; Shah, Darshit Rajiv; Vargas, Jose Viriato Coelho; Hovsapian, Rob
The need to transfer large amounts of power in applications where cabling weight and space are a major issue has increased the interest in superconducting cables. Gaseous helium and neon are being considered as possible coolants due to their suitability for the expected operating temperature ranges. Gaseous helium is preferred due to its higher thermal conductivity and relatively lower cost than neon. This paper enhances a previously presented mathematical model of a superconducting cable contained in a flexible cryostat by including flow pressure drops. In this way, the model is capable of properly sizing and minimizing fan power, and allows the prediction of system response to localized heating events (e.g., quenching). A volume element model approach was used to develop a physics model, based on fundamental correlations, and principles of classical thermodynamics, mass and heat transfer, which resulted in a system of ordinary differential equations with time as the independent variable. The spatial dependence of the model is accounted for through the three-dimensional distribution of the volume elements in the computational domain. The model numerically obtains the temperature distribution under different environmental conditions. Pressure drop calculations are based on realistic correlations that account for the wavy nature of the coolant channels. Converged solutions were obtained within the imposed numerical accuracy even with coarse meshes.
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The Temperature Response of Compact Tubular Microalgae Protobioreactors
(2009) Ribeiro, Robert Luis Lara; Mariano, André Bellin; Dilay, Emerson; Souza, Jeferson Avila; Ordonez, Juan Carlos; Vargas, Jose Viriato Coelho
A mathematical and computational modeling of a photobioreactor for the determination of the transient temperature behavior in compact tubular microalgae photobioreactors is presented. The model combines theoretical concepts of thermodynamics with classical theoretical and empirical correlations of Fluid Mechanics and Heat Transfer. The physical domain is discretized with the Volume Element Model (VEM) through which the physical system (reactor pipes) is divided into lumped volumes, such that only one time dependent ordinary differential equation, ODE, results for temperature, based on the first law of thermodynamics. The energetic interactions between the volumes are established through heat transfer empirical correlations for convection, conduction and radiation. Within this context, the main goal of this study is to present a numerical methodology to calculate the mixture (algae + water + nutrients) temperature inside the compact photobioreactor. A pilot plant is under construction, in the Center of Research and Development for Self-Sustainable Energy (NPDEAS), located at UFPR, and the experimental data obtained from this research unit will be used to validate the present numerical solution. Temperature is one of the most important parameters to be controlled in microalgae growth. Microalgae that are cultivated outside their growth temperature range may have a low growth rate or die. For this reason a numerical simulation of the system based on the operating conditions and environmental factors is desirable, in order to predict the transient algae growth temperature distribution along the reactor pipes. The VEM creates an “artificial” spatial dependence in the system or process under analysis by dividing the space (physical domain) into smaller sub domains, namely Volume Elements (VE). Each VE interacts with its neighbors by exchanging energy and/or mass. Thus, each VE is treated as a control volume from classical thermodynamics, i.e., with uniform properties and exchanging mass and energy with its neighbors. The problem is then formulated with the energy equation applied to the fluid VE and to the wall VE. These equations form a system of time dependent ODE’s, which are not dependent on space, therefore eliminating the need for the solution of a system of partial differential equations, PDE’s, depend on time and space, as is the case of traditional numerical methods (e.g., finite element, finite volume and finite differences). The resulting ODE’s were solved using a fourth order Runge- Kutta method with adaptive time step.
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Thermal modeling of helium cooled high-temperature superconducting DC transmission cable
(IEEE, 2011) Souza, Jeferson Avila; Ordonez, Juan Carlos; Hovsapian, Rob; Vargas, Jose Viriato Coelho
The recent increase in distributed power generation is highlighting the demand to investigate and implement better and more efficient power distribution grids. High-temperature superconducting (HTS) DC transmission cables have the potential to address the need for more efficient transmission and their usage is expected to increase in the future. Thermal modeling of HTS DC cables is a critical tool to have in order to better understand and characterize the operation of such transmission lines. This paper introduces a general computational model for a HTS DC cable. A physical model, based on fundamental correlations and principles of classical thermodynamics, mass and heat transfer, was developed and the resulting differential equations were discretized in space. Therefore, the combination of the physical model with the finite volume scheme for the discretization of the differential equations is referenced as Volume Element Model, (VEM). The model accounts for heat transfer by conduction, convection and radiation obtaining numerically the temperature distribution of superconductive cables operating under different environmental, operational and design conditions. As a result, the model is expected to be a useful tool for simulation, design, and optimization of HTS DC transmission cables.
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Thermodynamic optimization of fluidized catalytic cracking (FCC) units
(2011) Souza, Jeferson Avila; Vargas, Jose Viriato Coelho; Ordonez, Juan Carlos; Martignoni, Waldir Pedro; Meien, Oscar Felippe von
In this paper, a thermodynamic optimization procedure for FCC riser units has been developed. The formulation uses a 2D fluid flow and kinetic model to provide the necessary information for the optimization process. The thermodynamic analysis is based on the unit entropy generation minimization, i.e., the minimization of the destroyed exergy in the system. This kind of analysis has been widely used in power generation plants, with large benefits. It was verified that for any given catalyst mass flow rate, there exists an optimum value for the catalyst to oil mass flow rate ratio, COR, for maximum mass flow rate production of gasoline, or any other desired product. Next, the objective function (net exergy production rate) was maximized through the minimization of the destroyed exergy inside the FCC unit. The optimization was conducted with respect to the catalyst to oil ratio (COR). It is important to stress that all optima are sharp, i.e., for example with H/D = 50, the variation ofeE net is greater than 50%, calculated from ðeE net; max eE net; minÞ=eE net; max for 5 < COR < 25. Based on the lack of second law analysis related works for FCC plants in the technical literature and in view of the potential gains suggested by the results, the authors believe that thermodynamic optimization could bring new insight in the quest for better FCC plants. Therefore, a low computational time tool is made available for simulation, control, design and optimization of FCC units.
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Transient Modeling and Simulation of Compact Photobioreactors
(2008) Ribeiro, Robert Luis Lara; Mariano, André Bellin; Souza, Jeferson Avila; Vargas, Jose Viriato Coelho
In this paper, a mathematical model is developed to make possible the simulation of microalgae growth and its dependency on medium temperature and light intensity. The model is utilized to simulate a compact photobioreactor response in time with physicochemical parameters of the microalgae Phaeodactylum tricornutum. The model allows for the prediction of the transient and local evolution of the biomass concentration in the photobioreactor with low computational time. As a result, the model is expected to be a useful tool for simulation, design, and optimization of compact photobioreactors. Numerical solutions of the mathematical model are presented for the visualization of biomass concentration and total production. Several simulations were performed with temperatures ranging from 274K to 300K , and the maximum biomass production was achieved with an operating temperature of 294K .
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A two-phase one-dimensional model for simulation of FCC risers
(2006) Brum, Ary Saad; Souza, Jeferson Avila; Vargas, Jose Viriato Coelho
The FCC (Fluidized Catalytic Cracking) is one of the most important processes in a petroleum refinery plant. The numerical modeling of this process has been performed by several authors who have proposed different mathematical models and reported them in the literature. With the constant increase of computational capabilities, such models have become even more complex and with wider application. The different models address both fluid flow and cracking kinetic, varying from simple one phase and one-dimensional models to three-dimensional and three-phase models. Therefore, there is no common ground regarding the most adequate formulation, and advantages and drawbacks may be identified in each available model. In the present work, a relatively complex model reported in the literature is reproduced. Even though it is a one dimensional model, it includes many physical phenomena, like the dependence of the fluid properties on temperature and the transport equations formulation for both phases (gas and particulate). In a following stage, some simplifications in the mathematical formulation are included to the model and the results obtained with both formulations (with and without simplifications) are compared. The main goal of the present work is to establish a relationship between each included physical phenomenon and its real influence on the capability of the model to predict the products formation inside the riser reactor.
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A volume element model (VEM) for energy systems engineering
(2015) Dilay, Emerson; Vargas, Jose Viriato Coelho; Souza, Jeferson Avila; Ordonez, Juan Carlos; Yang, Sam; Mariano, André Bellin
This work presents a simplified modeling and simulation approach for energy systems engineering that is capable of providing quick and accurate responses during system design. For that, the laws of conservation are combined with available empirical and theoretical correlations to quantify the diverse types of flows that cross the system and produce a simplified tridimensional mathematical model, namely a volume element model (VEM). The physical domain of interest is discretized in space, thus producing a system of algebraic and ODEs with respect to time, whose solution delivers the project variables spatial distribution and dynamic response. In order to illustrate the application of the VEM in energy systems engineering, three example problems are considered: (i) a regenerative heat exchanger; (ii) a power electronic building block (PEBB); and (iii) a notional all-electric ship. The same mathematical model was used to analyze problems (ii) and (iii), that is, the thermal management of heat-generating equipment packaging. In the examples, the converged mesh had a total of 20, 2000, and 7725 volume elements. The third problem led to the largest simulation, which for steady-state cases took between 5 and 10 min of computational time to reach convergence and for the ship dynamic response 50 min (i.e., 80,000 s of real time). The regenerative heat exchanger model demonstrated how VEM allowed for the coexistence of different phases (subsystems) within the same volume element. The thermal management model was adjusted and experimentally validated for the PEBB system, and it was possible to perform a parametric and dynamic analysis of the PEBB and of the notional all-electric ship. Therefore, because of the observed combination of accuracy and low computational time, it is expected that the model could be used as an efficient tool for design, control, and optimization in energy systems engineering.