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1、,Introduction to Rotating Machinery Analysis Using Fluent,Frank Kelecy Fluent Inc.,Agenda,Introduction Single moving reference frame (SRF) model Multiple moving reference frame (MRF) model Mixing plane model Sliding mesh model Questions?,Motivation,Flows involving rotating domains occur frequently i
2、n engineering Examples compressors and turbines fans and pumps rotating cavities, seals, and bearings mixing equipment fluid coupling devices and torque converters air motors marine and aircraft propellers and many more Computational Fluid Dynamics (CFD) today plays a central role in the design and
3、analysis of rotating machinery,Examples of Rotating Machinery,gas turbine engine,automotive water pump,tube axial fan,steam turbine,HVAC blower unit,hydro turbine,Goals of the Training,Provide an introduction to rotating machinery modeling Examine the four major classes of rotating machinery problem
4、s Single rotating reference frame (SRF) Multiple rotating reference frame (MRF) Mixing plane Sliding mesh Present details on modeling rotating machinery problems using Fluent Model setup Solution process (steady-state and unsteady) Answer your questions!,Types of Rotating Machinery,In this course, w
5、e will classify rotating machinery as follows: Turbomachinery - machines which add work to or extract work from a fluid compressors, fans, pumps - add work to achieve a pressure rise in the fluid turbines, windmills - extract work from fluid to drive other machines Mixing equipment - machines which
6、are designed to mix fluid (and possibly solid) materials for use in a chemical processing application industrial mixing tanks Rotating tanks, seals, cavities, and other devices disk cavities and labyrinth seals in gas turbine engines electric motor cooling passages disk drives rotating tires on auto
7、motive vehicles All of these applications involve rotating surfaces and domains (and thus may use a rotating reference frame for modeling),Classification of Turbomachinery,Axial machines Flow through the machine is (in general) aligned with the axis of rotation Examples: propellers, axial fans/compr
8、essors/turbines, swirlers Centrifugal machines Flow through the machine is (in general) perpendicular to the axis of rotation Examples: liquid pumps, centrifugal fans/compressors, radial turbines Mixed Flow Flow through the machine is somewhere between axial and centrifugal Example: mixed flow compr
9、essor,Basic Problem Statement,We wish to solve for the flow through a domain which contains rotating components propeller, compressor/turbine blade, radial impeller, etc. stationary and/or rotating surfaces ducts walls, bores and cavities, seal teeth surfaces, etc. Rotation(s) assumed to be steady a
10、ccelerating reference frames can be modeled with source terms (not considered here) Well-posed boundary conditions flowrates, pressures, temperatures, other scalars at inlet/outlet boundaries wall motion, thermal, other BCs at walls Other considerations laminar/turbulent flow, other physics (e.g. mu
11、ltiphase flow, heat transfer) level of interaction between moving/stationary components,Modeling Approaches,Single Rotating Frame (SRF) Entire computational domain is referred to rotating reference frame Multiple Rotating Frame (MRF) Selected regions of the domain are referred to rotating reference
12、frames Ignore interaction effects steady-state Mixing Plane (MPM) Influence of neighboring regions accounted for through use of a mixing plane model at rotating/stationary domain interfaces Ignore circumferential non-uniformities in the flow steady-state Sliding Mesh (SMM) Motion of specific regions
13、 accounted for by mesh motion algorithm Flow variables interpolated across a sliding interface Unsteady problem - can capture all interaction effects with complete fidelity,Single Reference Frame(SRF) Modeling,Introduction to the SRF Model,Many problems which involve rotating components can be model
14、ed using a single rotating reference frame. Why use a rotating reference frame? Flowfield which is unsteady in the stationary frame becomes steady in the rotating frame Steady-state problems are easier to solve. simpler BCs low computational cost easier to post-process and analyze We will discuss is
15、sues related to SRF modeling in this section, but many concepts (e.g. solver settings, physical models, etc.) will also apply to MRF, mixing plane, and sliding mesh modeling.,Illustration of SRF model,blade,hub,domain,rotating reference frame,axis,shroud/casing,Implications of SRF,Single fluid domai
16、n Domain rotates with a constant rotational speed about a specified rotational axis Entire domain moves with the reference frame Boundaries which move with the fluid domain may assume any shape Boundaries which are stationary (with respect to the laboratory or fixed frame) must be surfaces of revolu
17、tion Can employ rotationally-periodic boundaries for efficiency (reduced domain size),Stationary Walls in SRF Models,stationary wall,rotor,baffle,Correct,Wrong!,Wall with baffles not a surface of revolution!,N-S Equations: Rotating Reference Frame,Two different formulations are used in Fluent Relati
18、ve Velocity Formulation (RVF) Obtained by transforming the stationary frame N-S equations to a rotating reference frame Uses the relative velocity as the dependent variable in the momentum equations Uses the relative total internal energy as the dependent variable in the energy equation Absolute Vel
19、ocity Formulation (AVF) Derived from the relative velocity formulation Uses the absolute velocity as the dependent variable in the momentum equations Uses the absolute total internal energy as the dependent variable in the energy equation,Reference Frames,x,y,z,z,y,x,stationary frame,rotating frame,
20、axis of rotation,CFD domain,Assumptions No translation ( ) Steady rotation (w = constant) about specified axis axis passes through origin of rotating frame Ignore body forces due to gravity and other effects Ignore energy sources Definitions Absolute velocity ( ) - Fluid velocity with respect to the
21、 stationary (absolute) reference frame Relative velocity ( ) - Fluid velocity with respect to the rotating reference frame 3-D compressible, laminar forms of the equations presented in the following slides,Assumptions and Definitions,The Velocity Triangle,The relationship between the absolute and re
22、lative velocities is given by In turbomachinery, this relationship can be illustrated using the laws of vector addition. This is known as the Velocity Triangle,Relative Velocity Formulation,(continuity),(x momentum),(y momentum),(z momentum),(energy),Relative Velocity Formulation (2),(relative veloc
23、ity vector),(relative total internal energy),(Fouriers Law),(viscous terms),Relative Velocity Formulation (3),Acceleration terms due to rotating reference frame,Coriolis acceleration,centripetal acceleration,Absolute Velocity Formulation,(continuity),(x momentum),(y momentum),(z momentum),(energy),A
24、bsolute Velocity Formulation (2),(absolute velocity vector),(total internal energy),(Fouriers Law),(viscous terms),Absolute Velocity Formulation (3),Acceleration term due to rotating reference frame,Acceleration reduces to single term involving rotational speed and absolute velocity,SRF Geometries:
25、2-D,2-D Problems 2-D planar geometries rotate about axis normal to x-y plane with specified origin (periodic boundaries are permitted) 2-D axisymmetric geometries rotate about the x-axis,Planar,Axisymmetric,x,y,x,SRF Geometries: 3-D,3-D Problems User defines both rotational axis origin and direction
26、 for the fluid domain Periodic boundaries permitted,origin,rotational axis,Choice of Solver,Same considerations for general flowfield modeling apply to SRF solver choice Segregated Solver: incompressible, low speed compressible flows Examples: Fans, blowers, pumps Coupled Solvers: high speed compres
27、sible flows, above Mach 0.3 Examples: high pressure axial compressors, turbines, turbochargers Velocity Formulation recommendations Use AVF when inflow comes from a stationary domain Use RVF with closed domains (all surfaces are moving) or if inflow comes froma rotating domain NOTE: RVF only availab
28、le in the segregated solver In many cases, either can be used successfully,Boundary Conditions and Physical Models,Basic BCs used in SRF analysis Fluid BC Inflow BCs Pressure Inlet Velocity Inlet Mass Flow Inlet Outflow BCs Pressure Outlet Walls Periodics Physical models Turbulence models DPM Multip
29、hase, real gas, heat transfer,Fluid BCs,Use fluid BC panel to select rotational axis origin and direction vector for rotating reference frame Note: all direction vectors should be unit vectors but Fluent will normalize them if they arent Select Moving Reference Frame as the Motion Type for SRF Enter
30、 rotational speed Translation velocity set to zero,Velocity Inlets,Used for incompressible, mildly compressible flows when inlet velocity is known Can specify absolute or relative velocity vector Can specify vector components in Cartesian or cylindrical coordinates For 2-D, axisymmetric with swirl a
31、nd 3-D problems you can specify tangential velocity as,Pressure Inlets (1),Pressure inlets can be used with either incompressible or compressible flows. Definition of total pressure depends on velocity formulation and compressibility:,incompressible, AVF,incompressible, RVF,compressible, AVF,Pressur
32、e Inlets (2),Specify appropriate total pressure and total temperature If inlet flow is supersonic, specify static pressure such that desired Mach number corresponds to pt/p Specify flow direction vector Can use Cartesian, cylindrical, or local cylindrical coordinate system Frame of flow direction de
33、pends on velocity formulation! You cannot use a frame of reference for the direction which is different from the velocity formulation,Mass Flow Inlets,Prescribe total mass flow rate or mass flux and total temperature for compressible flows Total pressure “floats” since the mass flow rate is fixed Pe
34、rmits flow direction specification in absolute frame only Fluent 6 permits direction specification in Cartesian and cylindrical coordinates,Pressure Outlets,Specify static pressure at the outlet Can employ a radial equilibrium assumption which computes a radial pressure variation from The specified
35、pressure is then assumed to be the hub static pressure,Backflow,Backflow occurs when the static pressure in a cell adjacent to a pressure boundary falls below the prescribed boundary pressure For SRF problems, the direction of the backflow is normal to the boundary in the absolute frame if AVF is us
36、ed normal to the boundary in the relative frame if RVF is used Recommendation As some backflow may occur during the solution process, prescribe reasonable values for all backflow quantities Try to minimize (or eliminate) backflow by extending your outlet boundary further downstream,Wall BCs,Wall BCs
37、 enforce zero normal velocity at all wall surfaces no slip (zero velocity) for viscous flows For moving reference frames, you can specify the wall motion in either the absolute or relative frames Recommended specification of wall BCs for all moving reference frame problems For stationary surfaces (i
38、n the lab frame) use zero Rotational speed, Absolute For moving surfaces, use zero Rotational speed, Relative to Adjacent Cell Zone,Periodic BCs,Rotational periodic BCs rely on the rotational axis specification to transfer information correctly Rotationally periodic boundaries can be used in SRF pro
39、blems to reduce mesh size provided both the geometry and flow are periodic Notes: If you are using the make-periodic command in the TUI, make sure you set the rotational axis in the Fluid BC panel first before creating the periodics Once the periodic BCs have been set, perform a grid check to see if
40、 the reported periodic angles are correct,Turbulence Models for Rotating Machinery,DPM Modeling,You can use DPM and pathline models for SRF problems Particle paths are computed in the relative frame If you want to see particle paths in the absolute frame, you can access the following switch in the T
41、UI: define/models/dpm/tracking/ track-in-absolute-frame Note that particles moving in absolute frame may hit wall surfaces, since the rotation of the frame is not accounted for,particle injection at blade tips,Other Models,Multiphase Models VOF, ASMM, Eulerian (Fluent 6) multiphase models are all co
42、mpatible with SRF modeling in Fluent Examples: mixing tanks, multiphase pumps flows Real Gas Model Can model specific fluids using non-ideal gas equation of state Employs the REFPROP library from NIST For use with the coupled-solvers only! Available fluids include: carbon dioxide, ammonia, butane, e
43、thane, propane, propylene, wide range of refrigerants (e.g. R11, R12, R134a, etc.) Heat Transfer Conduction and radiation models can be enabled with SRF models Note: For conducting solids which are contained in a moving reference frame, you dont need to activate the Moving Reference Frame option!,So
44、lver Settings (1),Segregated solver Pressure-Velocity Coupling Method SIMPLE is sufficient for most problems Use PISO for unsteady problems (e.g.sliding mesh) Pressure Interpolation Standard scheme is acceptable for low speed flows, but for highly swirling flows. use PRESTO! if you have a quad or he
45、x mesh use Body Force Weighted scheme for mixed meshes Other equations - use second order discretizations Can start with first order for stability, especially for problems with high rotational speeds,Solver Settings (2),Coupled solvers Use first order discretizations to begin your calculation - then
46、 switch to second order when the solution is close to convergence Use default Courant numbers as a start (1 for explicit solver, 5 for implicit solver) For coupled-explicit solver Use 4 levels of FAS multigrid for most problems helps propagate solution more rapidly through the domain Use more levels
47、 of you have a very large mesh,Example SRF Calculations,Two examples will now be presented to illustrate typical SRF modeling procedures: 2-D swirling flow through a disk cavity 3-D flow through a propeller fan,Disk Cavity,Disk cavity air flow study based on the experiments of Pincombe, 1981 Disk ge
48、ometry: radius (b) = 443 mm, width = 59 mm, bore = 44.3 mm Solutions obtained for following conditions : Cw = Q/nb=1092, Ref=wb2/n=105 Three different numerical configurations were examined: Case 1 - Stationary frame, moving walls Case 2 - SRF, RVF Case 3 - SRF, AVF All cases used the same mesh (205
49、76 quad cells) , 2D segregated solver (axisymmetric with swirl), incompressible flow, RKE turbulence model, second order discretizations,Disk Cavity - Mesh,both walls rotate,inlet,outlet,axis,inlet tube,Disk Cavity - Stream Function,Case 1,Case 2,Case 3,separated flow,Nearly identical flow patterns observed for all three cases.,Radial Velocity Profile (r/b = 0.633),Radial Velocity Profile (r/b = 0.833),Disk Cavity - Results,Force results Conclusions All three numerical approaches yield ess