7,350 research outputs found
Investigação experimental de escoamentos bifásicos com mudança de fase de uma mistura binária em um tubo de venturi
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2014Escoamentos bifásicos em singularidades (válvulas, orifícios ou tubos de Venturi) são encontrados em diversas aplicações de engenharia. Exemplos industriais de escoamentos bifásicos com a presença da cavitação (flashing) em dispositivos de expansão podem ser encontrados em usinas geotérmicas (bocais), sistemas emergenciais (válvulas de alívio), sistemas de refrigeração (válvulas, tubos curtos e tubos capilares) e dispositivos de medição de vazão (tubos de Venturi). Um aparato experimental foi projetado e construído a fim de estudar os escoamentos bifásicos de misturas binárias com mudança de fase por cavitação em um bocal convergente-divergente (tubo de Venturi). Discute-se a influência do fluxo mássico da mistura, da concentração mássica do componente volátil na fase líquida e do grau de sobrepressão (subsaturação) na entrada da seção de teste sobre as distribuições de pressão estática e temperatura ao longo da seção de teste. A utilização de uma seção de teste transparente permitiu a observação visual, tanto da mudança de fase líquido-vapor como do escoamento bifásico, por meio de imagens capturadas a alta velocidade. O método de Velocimetria por Imagem de Partícula (PIV) possibilitou a avaliação quantitativa dos campos de velocidades e das taxas de dissipação viscosas dos escoamentos. Resultados experimentais obtidos para as misturas de R-134a (componente volátil) e óleo lubrificante POE ISO 10 (componente não-volátil) mostraram que a viscosidade da fase líquida tem um efeito significativo sobre a queda de pressão na garganta e sobre a recuperação de pressão na seção divergente, sendo esta última inversamente proporcional à concentração de refrigerante na fase líquida. A análise visual da cavitação de bolhas no bocal convergente-divergente revelou que a ocorrência do escoamento bifásico na garganta e a jusante dela é bastante sensível a mudanças nas condições do escoamento. As bolhas de vapor presentes no escoamento atuaram, juntamente com a rodamina B, como partículas rastreadoras para o PIV. Verificou-se que à medida que a concentração mássica aumenta, os níveis de velocidade na região da garganta diminuem. Os resultados experimentais obtidos pelo método PIV permitiram concluir que a taxa de dissipação viscosa é alterada majoritariamente pela viscosidade da mistura.Abstract: Flows through singularities (e.g., valves, orifices or Venturi tubes) are encountered in several engineering applications. Industrial examples of two-phase flashing flows in expansion devices can be found in geothermal power plants (nozzles), emergency systems (safety relief valves), refrigeration systems (valves, short tubes and capillary tubes) and flow metering devices (Venturi tubes). An experimental facility was designed and constructed in order to study two-phase flashing flows of binary mixtures in a converging-diverging nozzle (Venturi tube). The effects of mass flux, concentration of the volatile component in the liquid phase and inlet sub-saturation (over pressure) on the axial distributions of static pressure and temperature is discussed. A transparent test section enabled visual observation of the two-phase flow in the Venturi by means of high-speed visualization (HSV) and quantitative assessments of the velocity field and the viscosity dissipation rates by means of a particle image velocimetry (PIV) method. Experimental results generated for mixtures of R-134a (volatile component) and POE ISO 10 lubricating oil (non-volatile component) showed that the liquid phase viscosity has a significant effect on both the throat pressure drop and pressure recovery in the diverging section, with the latter being directly proportional to the refrigerant concentration in the liquid phase. Visual analysis of bubble cavitation in the converging-diverging nozzle revealed that the occurrence of two-phase flow in the throat and downstream of it is quite sensitive to changes in the conditions of the flow. R-134a vapor bubbles and rhodamine B acted as PIV tracking particles. PIV analysis showed that as refrigerant concentration increases, Venturi throat velocity levels decreases. Viscosity dissipation rates are mainly altered by the mixture viscosity
Wet Gas Flow Metering Technique Using a Venturi With Conductance Sensors
No full textWet gas metering is becoming an increasingly important problem to the oil and gas industry. Much research has been done to measure the water and gas flow rates of two-phase flows using a Venturi meter. The Venturi meter is a favoured device for metering of unprocessed wet natural gas production flows. In this thesis, various combinations of techniques have been employed in annular gas-liquid two phase flows to measure the flow parameters (e.g. liquid film thickness, gas volume fraction and gas and water flow rates). One of the most useful techniques which was used and which has proven attractive for many previous multiphase flow applications is the electrical conductance technique used in conjunction with the Venturi.
In this thesis, research has been done on designing a novel wet gas flow metering technique, which combines a Venturi with conductance sensors at the inlet and throat to measure the gas and the water flow rates and gas mass flow rate in vertical annular (wet gas) flows. Two ring conductance sensors at the inlet of the Venturi were used to measure the film velocity by cross correlation, one of the inlet ring conductance sensors was used to measure the film thickness and the inlet gas volume fraction. A ring conductance sensor at the throat was used to measure the gas volume fraction at the Venturi throat. A digital level sensor was also used to measure the film thickness (and the gas volume fraction) at the inlet of the Venturi. The reason for measuring the film thickness, the film velocity, the gas volume fraction at the inlet and the throat of the Venturi was to determine the gas and water flow rate and the gas mass flow rate in annular wet gas two phase flow using a variety of mathematical models of Venturis.
This work included both static ‘bench’ and flow loop experiments. In the flow loop experiments, which were limited to air-water two-phase flow, the test section included a Venturi meter with a 50mm inlet diameter with conductance sensors at inlet and the throat. Reference measurements of the water and air were made with a turbine flow meter and a variable area flow meter respectively. A Honeywell DP cell sensor was used to measure the differential pressure between the inlet and the throat of the Venturi.
A NI USB 6009 data acquisition device was used to integrate the system measurements and to control the operation of the overall Venturi system. A program was created using Labview software to read the input signals from the throat conductance ring sensor; the inlet conductance ring sensors; the digital level sensor; the DP cell sensor; the variable area flow meter and the turbine meter. From these measurements the gas volume fraction at the Venturi inlet and the throat, the differential pressure between the inlet and the throat of the Venturi, the film thickness and the film velocity at the Venturi inlet were calculated and were used with appropriate mathematical models to calculate the gas volumetric rate; gas mass flow rate; and the water mass and volumetric flow rates. Reference values of the gas a water flow rates were also calculated from the turbine meter and variable area flow meter outputs
Modeling of a homogenous gas-water two phase flow through a Venturi and vertical pipe (A prediction of pressure drop sign change in two phase flow)
No full textIn two phase flow, differential pressures technique can be used to measure the volume fraction of the gas phase. In the
case where no restriction is available in the pipeline, the differential pressure technique can be used only in vertical or
inclined pipelines. Two phase air-water pressure drop across a Venturi meter may change its sign from positive to negative
due to change in the compressibility of the gas phase. In other words, the inlet of the venturi (upstream section) is not
always positive as in a single phase flow. A new model to predict the sign change of the two phase pressure drop across a
Venturi was developed and checked against data recently obtained from an air-water flow rig at the University of
Huddersfield. The predication of a two phase pressure drop through a vertical pipe was also investigated and compared with
experimental data. Four sets of data were investigated. In each set the water volumetric flow rate was fixed while the gas
volumetric flow rate was varied (see table-1). It was inferred from the model proposed in this paper and the experimental
data that the sign of the differential pressure drop across Venturi meter and parallel pipe for homogenous air-water flow
depends mainly on the parameters C1, C2, Uh and K. Therefore, if C1 > C2 then, the differential pressure drop across Venturi
tends to be negative and if Uh
2 > K then the differential pressure drop across a 1m long Perspex pipe will be negative
Reti neurali: applicazioni a modelli economico-finanziari non lineari
No full textL’analisi matematica di sistemi economico-finanziari complessi discreti e continui e la loro eventuale dipendenza da un numero elevato di parametri, comporta l’utilizzo di strumenti analitici ed informatici alquanto sofisticati. Questi modelli sono generalmente formalizzati da sistemi dinamici che si esprimono in equazioni differenziali non lineari, discrete o continue, le cui soluzioni: possono evolvere verso stati finali stabili rappresentati da punti fissi, possono essere oscillatorie o pseudo-oscillatorie, cioè tali da determinare un ciclo limite o possono presentare andamenti irregolari, cioè caotici, che dipendono in modo cruciale dalle condizioni iniziali (Lorenz H.W. (1989), Johnson R. A., (1991), Jarsulic M.(1993), Amendola M.-Gaffard J. (1998), Neri U. Venturi B. (1999), Mattana Venturi (1999), Venturi B., (2000))
Dendrimers as Multielectron Storage Devices
No full textDendrimers are complex, repeatedly branched tree-like compounds that can be synthesized with well-defined composition and a high degree of order.
Incorporation of redox-active units in a dendritic architecture enable us to gain information on: (i) dendrimer structure and superstructure; (ii) self-assembly processes; (iii) degree of electronic interaction and communication between redox units located in different sites; (iv) changes in conformation brought about by electron transfer processes. Electroactive dendrimers are attracting increasing interest in view of their possible application as sensors, catalysts, enzyme mimics, in which a redox centre is buried inside the dendritic nanoenvironment, and, last but not least, multielectron storage devices
Entriamo in un’epoca di necessario Illuminismo
No full textUn commento a due documenti inediti di Franco Venturi rinvenuti nell'archivio
NON LINEAR ECONOMIC-FINANCIAL MODELS WITH HOMOCLINIC ORBITS
No full textIn this paper we use global bifurcation theory as understand complicated stability phenomena of general
three-dimensional, economic financial models. ( see also Benhabib J., and Nishimura K., 1979; Benhabib J.,
1992; .Mattana P. and Venturi B. 1999; Fiaschi and Sordi, 2002; De Cesare L. and Sportelli M., 2004; Cai
J., 2005, Mattana 2004, Nishimura K., Shigoga T., Yano M, 2006, Neri and Venturi 2007).
We show that many theoretical results of global indeterminacy of equilibrium can be relating to systems
having a homoclinic orbit biasintotic to a stationary point at some value of the parameters. These outcome
depend upon the eigenvalues of the Jacobian matrix of the flow evaluated at the stationary point.
We apply these results to a reduced form of the endogenous growth models due to Lucas and Romer.
We use graphical and rigorous arguments to prove the existence of homoclinic orbits in these models for
some parameters values.
In order to understand the structure of the solutions of the systems presented, we have elaborated a
numerical simulation
Electroactive Rotaxanes and Catenanes
No full textThe described systems show the potentialities of electrochemical techniques (the various kinds of voltammetry, chronoamperometry, coulometry, impedance spectroscopy, spectro- and photo-electrochemistry) in characterizing complex systems such as redox-active rotaxanes and catenanes. They provide, indeed, a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. Electrochemistry is, therefore, a powerful tool to ‘read’ the state of the system.
In suitable designed rotaxanes and catenanes, however, electrochemistry can play a more important role. By causing the occurrence of endoergonic heterogeneous electron-transfer processes electrochemistry can, indeed, provide the energy needed to modify the noncovalent interactions that stabilize a certain rotaxane and catenane structure promoting mechanical movements. In such cases electrochemistry plays the dual role of ‘writing’ and ‘reading’ the system: by means of electrons and/or holes it supplies the energy to make theses systems work as molecular machines, and by means of the various electrochemical techniques it is used for controlling and monitoring the operation performed by the system.
Although the examples described here evidence that electrochemists have learned how to deal with increasingly complex molecular and supramolecular structures, it is important to notice that electrochemistry is only a part of the game: as the complexity of the systems studied increases, the contribution from many disciplines in a joint and collaborative effort is needed. For the intriguing structures here described this statement is particularly true. The goal of transforming molecular devices and machines into practically useful products requires, indeed, that people belonging to different fields, like chemistry, solid-state physics, biology, computer science, mathematics, materials sciences, etc., work together and learn a common language
Il paesaggio che muta: etiche ed estetiche ai confini dell’esperienza. Una conversazione con Massimo Venturi Ferriolo
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