CFD Computational Fluid Dynamics is a well-established method of patterning fluid flows and can get by with really complex fluid flows. The technique is widely used in the turbomachinery industry and accomplishments in this country are in demand in industry.
OpenFOAM is an unfastened beginning CFD codification, which means that it is available for anyone to utilize at no cost, capable to certain conditions. The advantages of utilizing OpenFOAM as opposed to utilizing conventional commercial codifications are:Customisation. The unfastened beginning codification means that the user can potentially custom-make the codification for their ain specific demands.Cost effectivity. As OpenFOAM is free of charge it could offer a more effectual manner of calculating complex flow, or rapidly patterning many fluctuations of a design. Commercial codifications normally require a licensing fee per processor used, hence to calculate complex jobs on many processors the cost would be high.This undertakings aim is to look into the utility of OpenFOAM for patterning turbomachinery flows at Queens University Belfast.
The undertaking will set about modeling of an bing turbine geometry utilizing OpenFOAM and the consequences will be compared by utilizing the commercially available CFD codification, ANSYS CFX. ANSYS CFX is presently used at Queens University Belfast to pattern turbomachinery flows. The undertaking should besides be able to move as a usher for future pupils who show an involvement in CFD.
PageDrumhead IRecognitions iiContentss iiiTerminology four1 Introduction 12 Technological Review 63 Development of System Model 9Mentions 10Appendix A: Undertaking Management 11
Symbols:Wx Total shaft work ( J/sec )Mass flow rate ( kg/s )C Absolute speed ( m/s )U Blade velocity ( m/s )p force per unit area ( Pa )T Temperature ( K )V Relative Velocity ( m/s )I± Absolute flow angle ( deg )I? Relative flow angle ( deg )I? Turning angle ( deg )H Specific heat content ; tallness ( j/kg K )W Relative speed ; diffuser pharynx breadth ( m/s )Cm Axial Velocity ( m/s )Subscripts:0 stagnancy status0, 1, 2.. Stations in the phaseAbbreviations:CFD Computational Fluid DynamicsCSA Cross-sectional Area
1.1CFD ; an overview
Traditional modeling in technology is to a great extent based on empirical or semi-empirical theoretical accounts. These theoretical accounts frequently work really good for well-known unit operations, but are non dependable for new procedure conditions. Developing new procedures and equipment can be expensive both in footings of clip ingestion and use of adult male power. This is besides the instance when seeking to take the measure from the experimental phase to existent life phase. In order to get the better of this job applied scientists are progressively turning to Computational Fluid Dynamics, CFD.
CFD is a subdivision of fluid mechanics that uses numerical methods and algorithms to work out and analyze jobs that involve fluid flows. Design equations are in topographic point but merely for bing equipment and merely for a limited scope of procedure conditions. Therefore when there are no accurate anticipations available, CFD simulations are run to obtain solutions to the job. A large advantage of this is that the simulation can be repeated with alterations to the parametric quantities implemented so that an optimal solution can be found.
1 History of CFD
CFD is a tool that has merely been developed late, about at the same time with electronic digital computing machines. It has been in the past 40 old ages that it has made dramatic advancement and several powerful computational methods have come up during this period, the most important among them being the finite difference method, the finite component method and the finite volume method.
1.1.2 Mathematicss of CFD
The cardinal footing of about all CFD jobs are the Navier-Stokes equations, ( Andersson, 2001 ) . These are the set of equations that describe the procedures of impulse, heat and mass transportation. These partial derived function equations have no known general analytical solution but can be discretized and solved numerically.
Equations depicting other procedures, such as burning, can be solved in concurrence with the Navier-Stokes equations. Often an approximating theoretical account is used to deduce these extra equations e.g. a turbulency theoretical account.
1.1.3 Applications of CFD
In the last several decennaries the dramatic growing of CFD has led it to going a widely applied technique in the technology industry. Its techniques have been applied in a wide graduated table in the procedure industry to derive insight into assorted flow phenomena, examine different equipment designs or compare public presentation under different operating conditions.
Soon, CFD is being progressively employed by many industries either to cut down fabrication design rhythms or to supply an penetration into bing engineerings so that they may be analysed and improved, ( Leeds 2012 ) . CFD is used across many countries in industry including Aerospace ; flying design, Automotive ; internal burning, Environmental ; fire direction, Mechanical ; pumps, Sports Equipment ; golf balls, Water ; wave burden, Wind ; air current burden and Turbomachinery ; turbines.
Turbomachinery ; an overview
A turbomachine is classed as a machine holding the characteristic to reassign energy between a uninterrupted watercourse of fluid and an component revolving about a fixed axis. Machines that fall in to this class would include pumps, compressors and turbines.
Turbomachines are categorized by their flow way and map. These can include axial, centrifugal and radial. In all turbomachine instances the specifying indispensable equation that must be understood is the Euler turbomachinery equation ( Baines, 1997 ) :
( Eqn. 1 )
This relates the work delivered to the flow, , per unit mass flow rate, to the alteration in of import speeds go forthing at phase 2 ( mercantile establishment ) with regard to those come ining at phase 1 ( recess ) .
The flow gives a alteration in temperature and force per unit area. The work input in each type of turbomachine is affected otherwise. For illustration in an axial machine the work input is affected by alterations in flow angle which in bend affects the force per unit area and temperature alteration. However in a radial machine the work input is besides affected by alterations in radii at the recess and mercantile establishment which in bend affects the force per unit area and temperature alteration. In axial turbines the radii of the recess and mercantile establishment are about changeless therefore this has no consequence on the work input.
A turbocharger is a forced initiation device, driven by the engine ‘s fumes gas turbine, used to let more power to be produced for an engine of a given size. In 1905 Swiss applied scientist Alfred Bchi received a patent for utilizing a compressor driven by exhaust gases to coerce air into a diesel engine to increase power end product. However it was n’t until 1924 that the first turbocharged engine was commercialised and back so the applications were applied to big ships and engines.
The development of turbocharged car applications progressed easy until in 1978 the first turbocharged Diesel rider auto was produced. Today turbocharged engines are really common for car applications in any size scope. Turbochargers have many other applications runing from little bike engines to big ship engines.
The map of an Axial Turbine
There are two basic types of turbine ; radial flow and axial flow turbines.
The huge bulk of gas turbines employ the axial flow turbine and this undertaking is traveling to concentrate entirely on that type. The axial flow turbine is usually the more efficient of the two. The axial flow turbine consists of one or more phases located instantly to the rear of the engine burning chamber. The turbine extracts kinetic energy from the spread outing gases as the gases come from the burner, change overing this kinetic energy into shaft power to drive the compressor and engine accoutrements. The gas, which is restricted by the turbine ‘s flow cross-sectional country, consequences in a force per unit area and temperature bead between the recess and mercantile establishment. This force per unit area bead is converted into the kinetic energy to drive the turbine wheel, ( BorgWarner 2012 ) .
As the force per unit area bead between the recess and mercantile establishment additions so does the turbine public presentation.
Axial Turbine Theory
A turbine converts the energy of a fluid into the energy of a rotating shaft. The connexion is made clear by the first jurisprudence of thermodynamics ( Baines, 1997 ) in which the fluid internal energy is measured by the entire heat content Ho ( where phase 1 refers to the recess, phase 2 refers to the stator outlet/rotor recess and phase 3 refers to the issue ) :
( Eqn. 2 )
The existent mechanism for this transportation is, nevertheless, via an exchange of angular impulse across the rotor, expressed in the Euler turbomachinery equation. For an axial phase there is small or no alteration in radius therefore it is assumed that the blade velocity is changeless so the equation becomes:
( Eqn. 3 )
It can be seen that the work transportation is achieved by set uping the fluid to turn in the blade transition, therefore altering the digressive constituent of the speed suitably, and normally by speed uping the fluid.
This fact can be appreciated if this equation is to the full developed utilizing speed trigons at the recess and rotor phase. The resulting equation is:
( Eqn. 4 )
This equation shows the demand to maximise the rotor recess speed C2 by speed uping the fluid through the stator, minimise the issue speed C3 by a suited pick of speed trigon and set up the fluid to speed up through the blade transition so that W3 & gt ; W2. The turning is achieved by using a suited curvature to the turbine blades, and the enlargement by set uping that the blade transitions have a decreasing CSA in the way of flow in order to speed up fluid, ( Baines 1997 ) .The preliminary analysis of a turbine phase returns from speed trigons. Velocity trigons ( Figure. 1 ) can be used to cipher the basic public presentation of a turbine phase. Gas enters the stationary turbine stator at absolute speed C1 and angle I±1 and accelerates to an absolute speed C2 and angle I±2.
The rotor rotates at speed U. Relative to the rotor, the speed of the gas as it impinges on the rotor entryway is W2. The gas is turned by the rotor and issues, comparative to the rotor, at speed W3. However, in absolute footings the rotor issue speed is C3, ( Saravanamuttoo 2001 ) . The speed trigons are constructed utilizing these assorted speed vectors. Velocity trigons can be constructed at any subdivision through the blading.
Figure 1. Speed trigons for a turbine phase ( courtesy of Saravanamuttoo, 2001 )
Speed trigons at rotor recess and issue spring:
( Eqn. 5 )
( Eqn. 6 )
The Euler turbomachinery equation is:
( ( Eqn.
This can be combined with Eqn. 5 to give:
( ( Eqn. 8 )
From these equations the of import factors which influence the axial turbine design phase can be seen therefore it can be stated that maximising the blade velocity, U, increasing the axial speed, Cm, and increasing the sum of turning, , will increase the specific work end product ( Baines, 1997 and Saravanamuttoo, 2001 ) .
2.1 CFD Methodology
Before get downing a new simulation for any application of CFD it is wise to believe carefully of what it is that should be predicted and what physical phenomena affect the consequences. There are many stairss that have to be defined.
2.1.1 Making the Geometry and Mesh
The organic structure about which flow is to be analysed requires patterning. This by and large involves patterning the geometry with a CAD package bundle. Most commercial CFD programmes include a CAD package bundle for their system but the geometry can normally be imported into the grid-generation programme.
Concurrently, determinations are made as to the extent of the finite flow sphere in which the flow is to be simulated. Grid coevals specifies the physical constellation to be simulated and divides it up into a 3-dimensional grid incorporating a sufficient figure of little parts known as control volume cells so that the Navier-Stokes equations can be solved iteratively. Obtaining an accurate mesh of the computational sphere is every bit of import as specifying the physical theoretical account. So the mesh quality must be evaluated prior to the simulation. Typically the cell size of a mesh should non alter with more than a factor of 1.25 between neighboring cells.
Puting Physical Properties
All the physical belongingss of the fluids must be defined, e.
g. the viscousness and denseness and their temperature, composing and force per unit area dependance.
Turbulent flows are characterized by fluctuating speed Fieldss in which there exist small-scale and high frequence fluctuations. At high Reynolds Numberss, disruptive fluctuations cause a much greater net impulse transportation than syrupy forces throughout most of the flow. This so means that for turbulent flows the Navier-Stokes equations can non be solved straight. Thus, accurate modeling of the Reynolds emphasiss is critical.
A turbulency theoretical account is a agency of come closing the Reynolds emphasiss in order to shut the mean-flow equations.
Boundary Conditionss and Initial Conditions
For a CFD simulation to bring forth a consequence, the environment around the design must be defined. This environment is described by the boundary conditions. These conditions are the inputs for the simulation and so utilizing them decently is necessary for good simulation consequences.
Conditionss at the recess, the mercantile establishment and the walls must all be defined. Boundary conditions due to simplifications of a computational sphere may be introduced i.e. symmetricalness boundary conditions. And advanced conditions can be applied for revolving equipment which are known as periodic boundary conditions.
Solving the CFD Problem
Once the job definition is completed, it is submitted to the convergent thinker for the calculation of a solution. This is the solution measure.
The regulating equations are coupled and nonlinear in nature.Therefore, a guess-and-correct, iterative scheme is adopted to calculate the solution. Although the solution method is automated, user intercession often is required to obtain a stable converged solution.
Post processing is the phase at which CFD consequences are analysed. A CFD solution provides full-field informations ; flux variables at 1000s of locations depending on the mesh. Analysis of consequences will give local information about flow, concentrations, force per unit areas, temperatures etc. However a cardinal value of CFD is its ability to supply accurate anticipations of incorporate measures such as heat transportation rates and mass transportation rates. The station processing phase should besides be used to analyze the quality of the solution.
CFD in turbomachinery
CFD now forms an built-in portion of the design procedure for many unstable machines including compressors, turbines, aero-engines, gas turbines and turbochargers. Simulation of multi-bladed machines brings challenges beyond those encountered in simple wall bounded disruptive flows. And for a successful simulation it is the undermentioned jobs that have to be overcome:The geometrical complexness of, multi-bladed, multi-stage rotating equipment coupled to the physical complexness of unsteady revolving flow.The public presentation anticipation of a rotating machine requires accurate simulation of affiliated or detached boundary beds.
Rotation makes it necessary to account for comparative gesture of multiple rotors and stators.The cost of simulation demands to be sufficiently low to let elaborate surveies of changing input parametric quantities, geometric form optimization etc.
2.2.1 OpenFOAM package
OpenFOAM is an abbreviation which stands for “ Open beginning Field Operation And Manipulation ” , and is an unfastened beginning numerical simulation package with extended capablenesss in work outingfluid flows and other multi-physics jobs. The package is first and foremost a C++ library for the development of numerical convergent thinkers, and public-service corporations for pre and post-processing the solution of continuum mechanics.
The codification is released as free and unfastened beginning package under the GNU General Public License. It was developed by OpenCFD Ltd and distributed by the OpenFOAM Foundation.OpenFOAM is of all time developing as a utile tool for patterning flows in turbomachinery. Its general intent capablenesss are enhanced with turbo-specific characteristics in order to get the better of the aforesaid jobs in patterning turbomachinery flows. These characteristics include traveling mesh computations, individual revolving frame of mention and multiple rotating frame.
There are so interface managing techniques for get the better ofing the job at the interface between stationary and revolving parts. These include the frozen rotor technique, a General Grid Interference ( GGI ) and a cyclic GGI.
ANSYS CFX is a commercial CFD plan used to imitate fluid flow in a assortment of applications. ANSYS CFX combines advanced solver engineering with a modern user interface and an adaptative architecture to do CFD extremely accessible to applied scientists who require in-depth modeling of complex fluid dynamic jobs.ANSYS CFX provides a suite of special-purpose tools for undertakings such as blade row geometry definition, flow way engagement, 1-D public presentation appraisal, accelerated instance apparatus, design of experiment analysis and component-specific post-processing.
More specifically for work outing fluid flows in turbomachinery ANSYS CFX provides general interfaces such as incompressible and compressible flow, revolving machinery with phase interfaces, frozen rotor technique. When these are used in concurrence with the customizable natural philosophies and user interface, and solved, an accurate theoretical account can be produced. A existent in depth analysis can be achieved with the blade-row specific post-processing tool provided by ANSYS CFX.