1、Workshop 02Using the Discrete Phase Model (DPM)Workshop Description:This workshop shows how to use the Discrete Phase Model (DPM) within Fluent. In the previous workshop we simulated the flow of a single-phase fluid within a pipe T-piece. This workshop will use the same T-piece geometry. The DPM ena

2、bles us to compute the trajectories of a stream of particles / droplets, based on their density and diameter. Learning Aims:This workshop will cover how to set up and run a DPM simulation: defining particle materials- including turbulent (stochastic) effects injecting particles into the domain - pre

3、dicting where erosion will occurUse either constant or a distribution profile for the particle diameterLearning Objectives:To understand how Fluent can be used to solve for the flow of a discrete phase, and the key controls used to produce a reliable result. IIntroductionIntroductionModel SetupBasic

4、 DPMModificationsCFD-PostSummarySimulation to be performedThe pipe simulated in workshop 1 is to be fitted in a petrochemicals site. The working fluid will be propane, and upstream some water droplets are injected into the pipe (this is done to dissolve any salts in the gas stream, though that proce

5、ss is not considered here)This simulation will consider how these water droplets are carried by the gas flow, and to what extent they impact on the pipe wall We will use a range of droplet sizes, and predict where erosion (or in practice, corrosion) may occur on the pipe wallIntroductionModel SetupB

6、asic DPMModificationsCFD-PostSummaryWe suggest you start a fresh workbench project for this, although the starting point will be the model built in workshop 1Start a new workbench sessionDrag a Fluent component system onto the projectRight-click on Setup, and select Import Fluent Case, and BrowseBro

7、wse to and select the file“tpiece_model_from_ws1.cas.gz” Click OK on the Fluent Launcher screenLoading a Mesh and starting FluentIntroductionModel SetupBasic DPMModificationsCFD-PostSummary We know the mesh is OK (we checked it for Workshop 1). However we need to change the working fluid to PropaneF

8、rom the navigation pane (left-hand column), select Materials then, Create/Edit.click Fluent Database. to open a new windowselect propane (c3h8), press Copy, then close both windowsSetup of flow fieldIntroductionModel SetupBasic DPMModificationsCFD-PostSummary Solution Setup Materialsclick on air the

9、n delete This will work, since air is not currently in useclick on water-liquid then delete This will fail, since water is still in use by the cell zoneCell Zone Conditions fluid EditIn the pull-down list next to Material Name, change to water to propane, then OKRevisit the Materials setup, and try

10、again todelete water-liquid This will now be possibleAssigning MaterialsA common mistake is to merely create the new fluid material, and not assign it to the cell zone. Fluent will still use the default material in the cell zones. Since you cannot delete a material that is in use, this step presents

11、 a useful check (especially in complex models). If the unwanted materials are still in use somewhere you cannot delete them.IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryObtaining new flow fieldAll settings (Solution Methods and Solution Controls) are to remain unchanged from the first

12、 workshopSolution Initialization Hybrid Initialization InitializeRun Calculation 150 iterations CalculateThe computation takes about 1 minute, and you should see convergence before 150 iterations are reached.IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryNote that in addition to the res

13、iduals reaching the convergence criteria, the solution monitors no longer change. If the lines in these graphs were not flat, it would be necessary to continue iterating.Basic DPM setup 1From the navigation pane (left-hand column), select Models, Discrete Phase, then EditOn the Discrete Phase Model

14、pane, select InjectionsIn the Injections panel, select Create This will open up the injections panelIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryBasic DPM setup 2Set up a new injection as follows:Set injection type to surfacePick surface inlet-zKeep default material anthraciteKeep def

15、ault diameter uniformX-Velocity 0 (m/s)Y-Velocity 0 (m/s)Z-Velocity -1 (m/s)Diameter 1e-04 (m)Temperature 90(c)Total Flow Rate 1 (kg/s)Tick Scale Flow Rate by Face AreaSelect OK to close this windowClose the Injections WindowOK on the Discrete Phase Model windowOnly when we have set up a DPM injecti

16、on can we get access to define the particle material. We will change this on the next slide.There are many ways to introduce DPM particles (parcels). Although here we use a boundary surface present in the geometry, we could choose to inject at XYZ point co-ordinates anywhere within the model.Introdu

17、ctionModel SetupBasic DPMModificationsCFD-PostSummaryDefining DPM materialsMaterials Inert Particle anthracite EditDefining an injection means we can now set up the material properties for the dropletsChange the name from anthracite to water-dropletsChange the Density to 1000 kg/m3Click change/creat

18、e, then answer YesIf we selected No then we would have both anthracite and water-droplets in the model. Selecting Yes overwrites so we just have water-droplets present. The injection defined on the last slide will automatically take this new material instead of anthracite.IntroductionModel SetupBasi

19、c DPMModificationsCFD-PostSummaryFirst display of particle tracks 1In its simplest form, the DPM can just be used as a post-processing exercise (the coupling is one-way between the continuous (propane) phase and the droplets). We can go straight to a post-processing action. When we display the parti

20、cle tracks, the solver computes how these particles (of this diameter, density etc) are carried by the flow.Results Graphics and Animations Particle Tracks Set Up.Select Draw Mesh, then in the pop-up window:select Edges, Edge Type OutlineSelect all surfaces except interior-fluidDisplay, then close j

21、ust the Mesh display windowIf you do not see the pipe outline on screen, then you need to use the pull-down panel immediately above the graphic window, and change it from the ConvergenceHistory plots to MeshIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryFirst display of particle tracks

22、2On the Particle Tracks window, selectColour by Particle Variables Particle DiameterPick the injection injection-0Click DisplayObserve also the bottom line of the TUI window. You should see:Number tracked 158Number escaped 158This will be discussed next.IntroductionModel SetupBasic DPMModificationsC

23、FD-PostSummaryDiscussion on DPM 1In this example, Fluent has released one droplet from each face on the inlet-z boundary. There are 128 faces in the mesh here, hence 158 trajectories. Each droplet has a diameter of 1x10-4 m, and a density of 1000 kg/m3Therefore each droplet has a mass of 5.2x10-10 k

24、g (4/3 rpr3)It is assumed that any droplet released from the same location with the same conditions will follow the same trajectoryOur mass flow rate is 1 kg/sSo each of the 158 droplet trajectories computed is used to represent 1.2x107 actual droplets/sec 1/(5.2x10-10 x158) The droplet (or particle

25、) progresses through the domain through a large number of small steps. At each step, the solver computes the force balance acting on a single droplet (diameter 1x10-4 m) hence considering the drag with the surrounding fluid, droplet inertia, and if applicable gravity. The mass transported is that of

26、 all the droplets in that stream (1.2x107 droplets/sec).IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryDiscussion on DPM 2The coupling of the droplet (DPM) motion with that of the continuous phase can either be one-way or two-way coupled. The present example is one-way coupledBy this we

27、 mean that the fluid affects the momentum / energy of the DPMBut the surrounding fluid flow (propane) remains unaffected by the momentum / energy exchange with the DPMFor this reason, we can use the DPM as a post-processing exercise, and quickly compute the particle solution If required, two-way cou

28、pled behaviour can be enabled by setting Interaction with Continuous Phase on the DPM set-up panelOne would then need to perform additional iterations of the (propane) flow field to convergenceIt is not usually necessary to solve the DPM at every flow iteration Typically the DPM field needs updating

29、 every 5-10 flow iterationsIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryUsing a range of droplet sizesSo far we have looked at droplets of a uniform size. Next we will define a range of diametersDefine Injections Highlight injection-0 then SetChange Diameter-Distribution to Rosin-Ramm

30、lerSet the following values:Min diameter 1e-04 (m)Max diameter 5e-04 (m)Mean diameter 4e-04 (m)All other values should still bethe same as set previouslyObserve that the default is tohave Number of Diameters = 10Click OKIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryDrag slider bar to b

31、ottom to make diameter inputs visible.Trajectories of distributed sized dropletsIn the Particle Tracks panel, selectColor by Particle Variables. Particle DiameterPick the injection injection-0Click DisplayObserve also the bottom line of the TUI window. You should see:Number tracked 1580Number escape

32、d 1580Note how the larger droplet sizes have not made it round the bend, and have impacted on the pipe wall.Recall we asked for a distribution of 10 diameters, and so we now have 10 x 158 trajectories being computed.IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryViewing the range of siz

33、es used 1To plot the size distribution used: Reports Discrete Phase Sample Set Up. Pick boundary Outlet, and release from injection-0, then ComputeAll droplets currently make it to the outlet. This action will write to disk a file called outlet.dpm that will record the profile of droplets at this ou

34、tlet boundary Reports Discrete Phase Histogram Set Up. Select Read then pick the file just saved (outlet.dpm) Select sample Outlet, Variable diameter and weight mass-flow Click Axes., set precision to 5, Apply Select PlotIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryViewing the range o

35、f sizes used 2The Rosin-Rammler Diameter distribution is shown in the histogram. Recall the minimum size was 1x10-4m and the maximum 5x10-4m. Since we specified a mean diameter of 4x10-4m, the histogram is weighted towards the larger-sized droplets.IntroductionModel SetupBasic DPMModificationsCFD-Po

36、stSummaryTrapping droplets on the wallBy default when DPM droplets/particles hit a wall they are reflected off. In this case we want to say that water droplets that impact on the wall will remain there and not bounce off.Boundary conditions wall-fluid EditOn the DPM tab, set the Type to trapClick OK

37、IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryTrajectories with no reflection on wallsIn the Particle Tracks panel, selectColor by Particle Variables. Particle DiameterPick the injection injection-0Click DisplayObserve also the bottom line of the TUI window. You should see:Number track

38、ed 1580Number escaped 857Number trapped 723Number plete 0Now ca. 46% of the water droplets impact on the wall and are removed from the simulation.It is very important to keep an eye on these numbers. Fluent will simulate a finite number of steps (default 500) for each particle stream. If this is not

39、 enough, there may be a significant number plete, in which case the values in Models Discrete Phase need changing. In some flows the particles may naturally e stuck in a recirculation region, and therefore plete is appropriate.IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryIncluding tur

40、bulent effectsThis flow is turbulent, which will impart a random motion on the water droplets. However, our flow solution does not resolve all the small-scale turbulent eddies in the flow. The way we resolve this is to use stochastic tracking. Put simply, a number (in this case 10) particle streams

41、are released from the same point. Each one is given a random kick in each grid cell based on the turbulent intensity. This will indicate how turbulence will modify the trajectories.Define InjectionsHighlight injection-0 then SetSelect Turbulent Dispersion tabTick Discrete Random Walk ModelSet number

42、 of tries to 10OKIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryTrajectories with turbulent effectsIn the Particle Tracks panel, selectColor by Particle Variables. Particle DiameterPick the injection injection-0Click Display This will take noticeably longer than beforeObserve also the b

43、ottom line of the TUI window. You should see:Number tracked 15800Number escaped 8522Number trapped 7276Number plete 2We have just set 10 tries for the turbulent tracking, so we now track 10 x the number of particle trajectories (previously 1580).In this case, the distribution of the number escaped v

44、s the number trapped is barely changed by including turbulent effects.Note that using 10 tries AND 10 particle diameters has resulted in 100 x the number of trajectories (158) originally computed. You need to use these settings with care to keep the compute cost manageable.IntroductionModel SetupBas

45、ic DPMModificationsCFD-PostSummaryWe can ask Fluent to account for how the particles interact with the wall, and so simulate erosion. This option is only available if two-way coupling has been activated. However we only need to perform 1 iteration to collect this data - we are not going to run the m

46、odel until the two-way coupled case converges. First, we will disable turbulent stochastic tracking (for speed reasons) since it was found to have little effect in this case.Define InjectionsHighlight injection-0 then SetUn-Select Discrete Random Walk OKModels Discrete Phase EditSelect Interaction w

47、ith Continuous PhaseSet 1 continuous iteration per DPM iterationGo to Physical Models tabEnable Erosion/Accretion ModelOKSimulating ErosionIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryRun the solver for 1 iteration, so we can compute the erosion quantitySolution Run Calculation 1 iter

48、ation CalculateClick OK if a panel appears asking whether to use changes for the current calculation onlyGraphics and Animations Contours Set Up.Contours of Discrete Phase Variable / DPM Erosion Rate, filled, on wall-fluid, DisplayRotate the view and look at the Z surface of the pipe, in the region

49、where the droplets hit the pipe wallSimulating Erosion 2The functions used to quantify erosion based on how the DPM parcels impact the wall can be set as part of the wall boundary condition.IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryExporting data to CFD-PostSave the project: File S

50、ave ProjectFluent saves the values stored in each FLUID grid cell (so propane velocity, temperature pressure etc). BUT the motion of particles is separate. Their trajectories are overlaid on the grid cells, not stored as part of the grid cells. In order to view particle trajectories in CFD-Post, the

51、se need to be separately exported from Fluent.File Export Particle History DataClick Exported Particle VariablesPick: Particle Velocity Magnitude, Diameter and Temperature, then Add Variables, OKSelect injection-0 and enter the Particle File Name t-piece-dpm, then click WriteExit Fluent and return t

52、o WorkbenchIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryPreparing for PostFrom Component Systems, drag a Results object and drop on the Fluent solution cell.Double-click Results to launch CFD-PostIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryCFD-PostThe results are loade

53、d automatically for the FLUID cellsTo load the DPM particle tracks, select File Import Import Fluent Particle Track FileThe file you need is:Folder_where_project_savedproject_name_filesdp0FLUFluentt-piece-dpm.xmlSelect Open, then OKIntroductionModel SetupBasic DPMModificationsCFD-PostSummaryCFD-Post

54、 2A new item will appear in the model tree “Fluent PT for Water Droplets”. This gives access to the data that has been saved per DPM parcel (which is different from the normal results data which is saved per grid cell).Double-click on Fluent PT for Water DropletsUnder Geometry, set the Maximum numbe

55、r of Tracks to 500Under Color, set mode to Variable, and color by “Water Droplets.Particle Diameter”Click ApplyThis will give a similar image to that we saw in Fluent. We exported 1580 particle tracks from Fluent. By plotting 500 tracks we are showing approximately every third particle track. For cl

56、arity you may want a number less than 500. IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryCFD-Post 3Double-click on Fluent PT for Water DropletsUnder Geometry, set the Maximum number of Tracks to 50Under Color, set mode to Variable, and colour by “Water Droplets.Particle Time”Under Symb

57、ol, tick Show Symbolsset the Max time to User Specified at 5.0, minimum 0.0 and interval 1.0 sKeep default symbol of BallClick ApplyThe particle tracks are coloured by particle time. The colour legend shows it takes about 6.0 s for the water droplets to pass through the model.The symbols are plotted

58、 every 1.0 s along the trajectories. Initially all the symbols are together in the top pipe, however as they meet the main flow more scatter is evident as some tracks are accelerated more than others.IntroductionModel SetupBasic DPMModificationsCFD-PostSummaryWrap-up 1This workshop has shown how Fluent can be used