Orifices are essential devices for measurement and control of flow. It is important to define the flow field and understand the flow characteristics behind an orifice for the sake of reliability measures in many hydraulic engineering applications. Since analytical and experimental solutions are restricted, a numerical solution is obtained using volume of fluid (VOF) method with the CFD solver, FLUENT, for sharp crested orifices, orifice tubes and slots. The results are compared to the available data in the literature / also a large spectrum of data collection has been achieved.

Afin de répondre à des problématiques bâtimentaires et des enjeux de sécurité incendie, cette thèse aborde l’écoulement de convection mixte à travers un orifice horizontal reliant deux compartiments. L’objectif est d’améliorer la connaissance et la modélisation de l’échange de gaz de masse volumique variable à travers l’orifice. Une étude expérimentale à échelle réduite couplée à une approche théorique est proposée. L'étude est d'abord focalisée sur l’influence du rapport géométrique L/D de l’orifice sur la variation de débit échangé pour un régime de convection naturelle. Les mesures non intrusives de ces débits, par suivit de l'interface entre deux liquides non miscibles lors d'une première approche densimétrique, ainsi que par Stéréo PIV en sortie d'orifice dans une approche thermique, permettent de décrire le processus d'échange bidirectionnel et de conforter les corrélations existantes.Des expériences en régime de convection mixte visent ensuite à caractériser l’influence d’une ventilation mécanique (en soufflage et en extraction) sur les débits échangés. La confrontation des corrélations existantes avec les points expérimentaux montre des écarts importants. Une modification de la corrélation de Cooper 89 est proposée et permet d'en accroître la précision. En parallèle, une approche théorique issue des équations de Navier Stokes simplifiées et sous l’approximation de Boussinesq permet de discuter la construction des corrélations existantes. L'ajustement de coefficients de pertes de charge à partir des points expérimentaux permet de proposer un modèle plus performant que ceux disponibles dans la littérature. / To answer to building issues and fire safety challenges, this thesis deals with the mixed convection flow through a horizontal orifice linking two compartments. The aim is to improve the understanding and modeling of the exchange of variable density gas through the opening. A small scale experimental study and a theoretical approach are proposed.The study is first focussed on the impact of the geometrical ratio L/D of the opening on the exchanged flow rate variation for free convection regime. Non-intrusive measurements of these flow rates, via the tracking of the interface between two non miscible liquids in an isothermal approach, and thanks to the SPIV in a thermal approach, permit to describe the bidirectional exchange process and to consolidate existing correlations.Experiments in mixed convection regime aim to study the impact of mechanical ventilation (in blowing and extracting mode) on the exchanged flow rates. The comparison between existing correlations and experimental data shows large differences. A change making the coorelation of Cooper 89 more accurate is proposed. A theoretical approach from the simplified Navier Stokes equations and with the Boussinesq approximation permits to discuss the construction of existing correlations. From this theory, a model more accurate than those available in the literature is proposed thanks to an adjustment of discharge coefficients from experimental data.

The impinging solar receiver, bearing the merits of high heat transfer coefficient and compact structure, has a great potential in the field of solar dish Brayton system. Despite the wide application of cylindrical structure in the impinging solar receiver, the research on orifice array optimization against curvature surfaces is rare.In this paper, the main objective is to study the heat transfer and pressure drop characteristics of an orifice impinging array under a constant mass flow rate and a constant surface temperature boundary condition for the future impinging receiver design. Various orifice shapes were studied via numerical tools (Ansys Fluent 14.0) and their performances in both pressure drop and heat transfer coefficient were compared. The upstream fillet orifice was found to have the lowest pressure drop with moderate compromise in heat transfer coefficient. Moreover, a mathematical optimization model, based on empirical correlations, was developed for the orifice impinging array on the convex cylindrical surfaces. This model can provide an appropriate range of orifice number and orifice diameter, from which the key factors of the array including the ratio of height and orifice diameter H/D, orifice interval, number of orifices in each tier circumferential and tier numbers can be calculated. Several validation cases were also conducted by Ansys Fluent.

An experimental investigation was conducted using particle image velocimetry to study the effect of nozzle geometry on turbulent free orifice jets. The nozzle geometries studied include the round, cross, flower, star, rectangular and elliptical nozzles (aspect ratio 2). The spread rate of the rectangular nozzle was 61% greater than the square nozzle while the elliptical nozzle was 45% greater than the round nozzle using the conventional half velocity width. The superior mixing capacity of the rectangular and elliptical nozzles is attributed to the axis-switching mechanism. Evaluation of the energy budget showed a higher level of production of turbulence and convection of the mean flow for the rectangular nozzle compared with the round nozzle. Two-point auto-correlation function revealed larger structures in the non-circular nozzles and in particular the rectangular nozzle. The Kolmogorov and Taylor microscales however, did not show any significant dependency on nozzle geometry. / October 2017

This thesis reports the methods to simulate flows withcomplex boundary such as orifice flow. The method is forgeneral purposes so that it has been tested on different flowsincluding orifice flow. Also it contains a chapter about theexperiment of orifice flow. Higher-order precision interpolation schemes are used inumerical simulation to improve prediction at acceptable gridrefinement. Because higher-order schemes cause instability inconvection-diffusion problems or involve a large computationalkernel, they are implemented with deferred correction method. Alower-order scheme such as upwind numerical scheme is used tomake preliminary guess. A deferred (defect) correction term isadded to maintain precision. This avoids the conflict betweenprecision order and implementation difficulty. The authorproposes a shifting between upwind scheme and centraldifference scheme for the preliminary guess. This has beenproven to improve convergence while higher order schemes havewider range of stability. Non-orthogonal grid is a necessity for complex flow. Usuallyone can map coordinate of such a grid to a transformed domainwhere the grid is regular. The cost is that differentialequations get much more complex form. If calculated directly innon-orthogonal grid, the equations keep simple forms. However,it is difficult to make interpolation in a non-orthogonal grid.Three methods can be used: local correction, shape function andcurvilinear interpolation. The local correction method cannotinsure second-order precision. The shape function method uses alarge computational molecule. The curvilinear interpolationthis author proposes imports the advantage of coordinatetransformation method: easy to do interpolation. A coordinatesystem staggered half control volume used in the coordinatetransformation method is used as accessory to deriveinterpolation schemes. The calculation in physical domain withnon-orthogonal grid becomes as easy as that in a Cartesianorthogonal grid. The author applies this method to calculate turbulentorifice flow. The usual under-prediction of eddy length isimproved with the ULTRA-QUICK scheme to reflect the highgradients in orifice flow. In the last chapter, the author quantifies hydraulicabruptness to describe orifice geometry. The abruptness canhelp engineers to interpolate existing data to a new orifice,which saves detailed experiments

A study of the pressure drop characteristics of the flow of highly viscous fluids through small diameter orifices was conducted to obtain a better understanding of hydraulic fluid flow loops in vehicles. Pressure drops were measured for each of nine orifices, including orifices of nominal diameter 0.5, 1 and 3 mm, and three thicknesses (nominally 1, 2 and 3 mm), and over a wide range of flow rates (2.86x10sup-7/sup Q 3.33x10sup-4/sup msup3/sup/s). The fluid under consideration exhibits steep dependence of the properties (changes of several orders of magnitude) as a function of temperature and pressure, and is also non-Newtonian at the lower temperatures. The data were non-dimensionalized to obtain Euler numbers and Reynolds numbers using non-Newtonian treatment. It was found that at small values of Reynolds numbers, an increase in aspect ratio (length/diameter ratio of the orifice) causes an increase in Euler number. It was also found that at extremely low Reynolds numbers, the Euler number was very strongly influenced by the Reynolds number, while the dependence becomes weaker as the Reynolds number increases toward the turbulent regime, and the Euler number tends to assume a constant value determined by the aspect ratio and the diameter ratio. A two-region (based on Reynolds number) model was developed to predict Euler number as a function of diameter ratio, aspect ratio, viscosity ratio and generalized Reynolds number. This model also includes data at higher temperatures (20 and le; T and le; 50supo/supC) obtained by Mincks (2002). It was shown that for such highly viscous fluids with non-Newtonian behavior at some conditions, accounting for the shear rate through the generalized Reynolds number resulted in a considerable improvement in the predictive capabilities of the model. Over the laminar, transition and turbulent regions, the model predicts 86% of the data within and plusmn25% for 0.32 l/d (orifice thickness/diameter ratio) 5.72, 0.023 and beta; (orifice/pipe diameter ratio) 0.137, 0.09 Resubge/sub 9677, and 0.0194 and mu;subge/sub 9.589 (kg/m-s)

<p>This thesis reports the methods to simulate flows withcomplex boundary such as orifice flow. The method is forgeneral purposes so that it has been tested on different flowsincluding orifice flow. Also it contains a chapter about theexperiment of orifice flow.</p><p>Higher-order precision interpolation schemes are used inumerical simulation to improve prediction at acceptable gridrefinement. Because higher-order schemes cause instability inconvection-diffusion problems or involve a large computationalkernel, they are implemented with deferred correction method. Alower-order scheme such as upwind numerical scheme is used tomake preliminary guess. A deferred (defect) correction term isadded to maintain precision. This avoids the conflict betweenprecision order and implementation difficulty. The authorproposes a shifting between upwind scheme and centraldifference scheme for the preliminary guess. This has beenproven to improve convergence while higher order schemes havewider range of stability.</p><p>Non-orthogonal grid is a necessity for complex flow. Usuallyone can map coordinate of such a grid to a transformed domainwhere the grid is regular. The cost is that differentialequations get much more complex form. If calculated directly innon-orthogonal grid, the equations keep simple forms. However,it is difficult to make interpolation in a non-orthogonal grid.Three methods can be used: local correction, shape function andcurvilinear interpolation. The local correction method cannotinsure second-order precision. The shape function method uses alarge computational molecule. The curvilinear interpolationthis author proposes imports the advantage of coordinatetransformation method: easy to do interpolation. A coordinatesystem staggered half control volume used in the coordinatetransformation method is used as accessory to deriveinterpolation schemes. The calculation in physical domain withnon-orthogonal grid becomes as easy as that in a Cartesianorthogonal grid.</p><p>The author applies this method to calculate turbulentorifice flow. The usual under-prediction of eddy length isimproved with the ULTRA-QUICK scheme to reflect the highgradients in orifice flow.</p><p>In the last chapter, the author quantifies hydraulicabruptness to describe orifice geometry. The abruptness canhelp engineers to interpolate existing data to a new orifice,which saves detailed experiments</p>

The master’s thesis deals with computational fluid dynamics (CFD) of two orifices, that have different shapes of holes but similar cross-sectional flow areas. The first of them is orifice with circular-shaped hole, which is used for maintenance free measurement of flow. The second one is orifice with fractal-shaped hole, inspired by von Koch snow-flake. This thesis follows bachelor thesis, in which was experimentally examined, that fractal-shaped orifices have better hydraulic properties (hydraulic losses and lower pressure pulsations) than circle-shaped one. The main target is to confirm this conclusion based on experiment, this time using CFD with various types of turbulence modelling ap-proaches. Both single phase (cavitation free) and multiphase numerical simulations were realized. Each model was compared from perspective of hydraulic and dynamic charac-teristics.

There are some basic information about pressure sensors and flow in the first part of my diploma thesis. For example turbulent and laminar flow, construction of pressure sensors and basic information abaut Ansys and –Fluent. Main part of this thesis is focused on CFD simulation of pressure and velocity in the vicinity of pressure sensors. I confront this simulated values with measured values in final part of this thesis.

In this thesis computational models for cavitating flows around orifice plates has been studied and compared. The goal was to fit a model with experimental data and this was done with some success, although problems with numerical stability, long calculation times and geometry overfitting remain. Cavitation is a complex fluid phenomenon that can occur in pressurized liquid flows. It starts when the liquid pressure is lowered below the boiling pressure and water that undergoes cavitation forms vapor which later implodes violently. This process can cause problems such as noise, vibrations and corrosion in piping systems. Loud noise is a nuisance, however powerful vibrations and corrosion can have serious consequences for the structural integrity of pipes. The for example lessened performance, leakages or even failure. Therefore the minimization of cavitation is often a goal in orifice and piping design. Vattenfall AB, together with Forsmark and Ringhals nuclear plants have studied cavitating flows around orifice plates used for flow limitation. A set of data from laboratory tests made by Vattenfall was used as the basis of analysis. Existing computational models in OpenFOAM were tested and evaluated based on their ability to model the experimental data accurately, as well as their computational performance and stability. The cavitation phenomenon was difficult to simulate using established methods so a new method was created and verified. It is based on the Kunz cavitation model together with Large Eddy Simulations, but with turbulence as a predictor of cavitation. The new computational model will serve as a tool for knowing how to design orifices in the future, so that laboratory experiments will not have to be conducted for each new piping design.