259 research outputs found
Compressible flow through a porous medium: choking at pore scale and its implications
abstract: Production from a high pressure gas well at a high production-rate encounters the risk of operating near the choking condition for a compressible flow in porous media. The unbounded gas pressure gradient near the point of choking, which is located near the wellbore, generates an effective tensile stress on the porous rock frame. This tensile stress almost always exceeds the tensile strength of the rock and it causes a tensile failure of the rock, leading to wellbore instability. In a porous rock, not all pores are choked at the same flow rate, and when just one pore is choked, the flow through the entire porous medium should be considered choked as the gas pressure gradient at the point of choking becomes singular. This thesis investigates the choking condition for compressible gas flow in a single microscopic pore. Quasi-one-dimensional analysis and axisymmetric numerical simulations of compressible gas flow in a pore scale varicose tube with a number of bumps are carried out, and the local Mach number and pressure along the tube are computed for the flow near choking condition. The effects of tube length, inlet-to-outlet pressure ratio, the number of bumps and the amplitude of the bumps on the choking condition are obtained. These critical values provide guidance for avoiding the choking condition in practice.Dissertation/ThesisM.S. Mechanical Engineering 201
Computational modeling of coupled free and porous media flow for membrane-based filtration systems: a review
We review different mathematical models proposed in literature to describe
fluid-dynamic aspects in membrane-based water filtration systems.
Firstly, we discuss the societal impact of water filtration, especially in the
context of developing countries under emergency situations, and then review
the basic concepts of membrane science that are necessary for a mathematical
description of a filtration system.
Secondly, we categorize the mathematical models available in the literature
as (a) microscopic, if the pore-scale geometry of the membrane is accounted for;
(b) reduced, if the membrane is treated as a geometrically lower-dimensional
entity due to its small thickness compared to the free flow domain; (c) mesoscopic, if the characteristic geometrical dimension of the free flow domain and
the porous domain is the same, and a multi-physics problem involving both
incompressible fluid flow and porous media flow is considered. Implementation
aspects of mesoscopic models in CFD software are also discussed with the help
of relevant examples
Volumen-gemittelte RANS-Simulation turbulenter Strömung über porösen Medien
Recent advances in acoustic research have revealed that making the trailing edges of aircraft wings porous results in significant noise reductions. Such usage of porous media on aerodynamic bodies amongst others requires the development of accurate prediction tools of how the aerodynamics are affected by the presence of porous parts. The present work is a step towards understanding this and demonstrates a complete development process, from the derivation of the theoretical methods over to the integration of the theory into a finite-volume flow solver up to the validation of the methods with DNS-data and experiments.
The derivations of this work are based on the aerodynamic condition of high Reynolds numbers, very fine porous structures and flow velocities up to the range of transonic Mach numbers. These requirements mirror the premise that the porous media are used in civil aviation for noise reduction purposes. The overall strategy to derive the theoretical framework for the simulation of flow over porous media under the given conditions is based on averaging the Navier-Stokes equations in space and time, while always keeping the equations in their compressible form. The unknown terms which occur from the averaging process are modelled with the Darcy and Forchheimer terms which describe the effect of the porous medium on the air. A Reynolds-stress model is used for modelling the turbulent effects.
Special conditions are derived at the surfaces of the porous media such that the flow that penetrates across the so-called nonporous-porous interface continues through the porous regions in a physically correct way. The relationships include conservation of fluxes and jump conditions for several gradients.
The implementation of the theoretical models into a finite-volume flow solver is briefly presented. After verification with simple test cases, the models are extensively calibrated and validated. The calibration process adjusts the unknown parameters of the models with data from direct numerical simulations in a partly porous channel resulting in good agreement for both velocity and Reynolds-stress profiles. For the final validations, aerodynamic wind-tunnel experiments of a wing with porous trailing edge are performed. Measurements of the lift coefficient and of the flow field over the porous trailing edge compare well with the the numerical results.Kürzlich durchgeführte akustische Untersuchungen zeigen, dass poröse Tragflügelhinterkanten den Flugzeuglärm deutlich verringern können. Die Simulationsmöglichkeiten für solche Verwendungen von porösen Materialien an Flugzeugen und auch an anderen aerodynamischen Gegenständen sind bisher sehr begrenzt. Die vorliegende Arbeit nimmt sich diesem Problem an und beschreibt den gesamten Entwicklungsprozess einer möglichen Erweiterung für numerische Strömungslöser zur Berechnung turbulenter Strömungen über porösen Materialien, beginnend mit der Herleitung der theoretischen Modelle, über deren Integration in einen Strömungslöser, bis hin zur Validierung der Modelle anhand von DNS-Daten und Experimenten.
Die Randbedingungen der zu simulierenden Strömungen kommen aus der zivilen Luftfahrt und sind unter anderem hohe Reynoldszahlen, sehr feine poröse Strukturen und Geschwindigkeiten im transonischen Machzahlbereich. Um solche Strömungen effektive lösen zu können werden die Navier-Stokes-Gleichungen in ihrer kompressiblen Form räumlich und zeitlich gemittelt. Dadurch entstehen zu modellierende Terme, welche zum einen den Effekt von porösen Materialien auf die Strömung und zum anderen den Effekt der Turbulenz beschreiben. Das verwendete Modell der für die porösen Materialien zuständigen Terme basiert auf den Gesetzen von Darcy- und Forchheimer, und Turbulenzterme werden mit Hilfe eines Reynoldsspannungsmodells modelliert.
An der Übergangsfläche zwischen porösem Medium und freier Strömung werden zusätzliche Bedingungen notwendig. Denn beim Auftreffen der Strömung auf ein poröses Medium muss ihr Zustand so transformiert werden, dass sie ihren Weg im porösen Medium physikalisch sinnvoll fortsetzt. Die Transformationsregeln beruhen auf Erhaltungsgleichungen sowie Sprungbedingungen für Gradienten einzelner Strömungsvariablen.
Nach der Herleitung der benötigten Theorie wird kurz auf deren Implementierung in einen finite-Volumen Strömungslöser eingegangen. Die Funktionsfähigkeit wird anhand von einfachen Testfällen gezeigt, um sich dann der Kalibrierung und Validierung der theoretischen Modelle zu widmen. Die Lösungen mit den final festgelegten Parameterwerten zeigen gute Übereinstimmung mit DNS-Daten. Für die Validierung werden aerodynamische Windkanaluntersuchungen an dem anfangs beschriebenen Flügel mit poröser Hinterkante durchgeführt. Der Effekt der porösen Hinterkante auf den Auftriebsbeiwert wird durch die numerischen Simulationen gut wiedergegeben
NUMERICAL COUPLING OF 2.5D RESERVOIR AND 1.5D WELLBORE MODELS IN ORDER TO INTERPRET THERMOMETRICS
International audienceThe paper deals with the numerical coupling of an axisymmetric reser- voir model, governed by Darcy-Forchheimer's equation together with a nonstandard energy balance, and a quasi 1D wellbore model described by the compressible Navier- Stokes equation
HPTAM, a two-dimensional Heat Pipe Transient Analysis Model, including the startup from a frozen state
A two-dimensional Heat Pipe Transient Analysis Model, 'HPTAM,' was developed to simulate the transient operation of fully-thawed heat pipes and the startup of heat pipes from a frozen state. The model incorporates: (a) sublimation and resolidification of working fluid; (b) melting and freezing of the working fluid in the porous wick; (c) evaporation of thawed working fluid and condensation as a thin liquid film on a frozen substrate; (d) free-molecule, transition, and continuum vapor flow regimes, using the Dusty Gas Model; (e) liquid flow and heat transfer in the porous wick; and (f) thermal and hydrodynamic couplings of phases at their respective interfaces. HPTAM predicts the radius of curvature of the liquid meniscus at the liquid-vapor interface and the radial location of the working fluid level (liquid or solid) in the wick. It also includes the transverse momentum jump condition (capillary relationship of Pascal) at the liquid-vapor interface and geometrically relates the radius of curvature of the liquid meniscus to the volume fraction of vapor in the wick. The present model predicts the capillary limit and partial liquid recess (dryout) in the evaporator wick, and incorporates a liquid pooling submodel, which simulates accumulation of the excess liquid in the vapor core at the condenser end
Application of Computational Fluid Dynamics to Near-Wellbore Modeling of a Gas Well
Well completion plays a key role in the economically viable production of hydrocarbons from a reservoir. Therefore, it is of high importance for the production engineer to have as many tools available that aid in the successful design of a proper completion scheme, depending on the type of formation rock, reservoir fluid properties and forecasting of production rates. Because well completion jobs are expensive, most of the completed wells are usually expected to produce as much hydrocarbon and as fast as possible, in order to shorten the time of return of the investment. This research study focused on the evaluation of well performance at two common completion schemes: gravel pack and frac pack. Also, the effects of sand production on well productivity and its associated erosive effects on the wellbore, downhole and tubular equipment were also a motivation in considering the inclusion of a decoupled geomechanics models into the study. The geomechanics-hydrodynamics modeling was done using a computational fluid dynamics (CFD) approach to simulate a near-wellbore model, on which diverse physical processes interact simultaneously, such as nonlinear porous media flow (Forchheimer formulation), turbulence kinetic energy dissipation, heterogeneous reservoir rock properties and particles transportation. In addition, this study considered a gas reservoir whose thermodynamic properties were modeled using the Soave-Redlich-Kwong equation of state. In general, this study is divided into: 1. Verification of a CFD simulation results against its corresponding analytical solution. 2. Analysis of well completion performance of each of the proposed completion schemes. 3. Effect of using Darcy’s law on the prediction of well completion performance. 4. Sand production and erosive damage analysis. The CFD approach used on this research delivered promising results, including pressure and velocity distribution in the near-wellbore model as well as three-dimensional flow patterns and effects of sanding on the wellbore integrity
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