SI-PFI Engine - Surface preparation, Boundary Flagging & No-Hydro Case Setup using CONVERGE CFD (Part 1/2)
PART 2 - Multidimensional Simulation of SI-PFI Engine using Converge CFD (FULL-HYDRO CASE)
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I. INTRODUCTION
Port-fuel-injection engine as its name suggests is an engine where fuel is injected in the port i.e in its intake port from where it goes inside the combustion chamber passing through the valve opening. In this project a single cylinder of a PFI engine is simulated for combustion using Converge software. The part 1 of this project comprises of Surface preparation, Boundary flagging & Setting up the case for No-hydro simulation. No-Hydro simply means that Converge will not solve any equations for solving the simulation. This is done to check the mesh generation and any translatory movements in the geometry before we proceed with Full hydrodynamic simulation.
II. OBJECTIVE
1. Boundary Flagging & Regions - Extract Boundaries from Geometry, Name them and Set their appropriate regions.
2. Surface preparation - Remove all Surface errors and perform Surface cleanup.
3. No-Hydro Case setup - Run the Case for Checking Mesh motions & Movements in the Geometry for the case.
III. GEOMETRY IN CONVERGE
This is the PFI geometry imported as an stl file in converge studio. This is the base geometry which has alot of surface errors and boundary needs to be flagged appropriately along with other case setup. We begin by running a diagnosis to identify the types of errors the geometry consists of, then we fix them one by one while flagging boundary simultaneously as it becomes easy to select that particular boundary to either hide or translate to further work on it.
III.1 Diagnosis
As we can see there are alot of different errors in the geometry including intersections, open edges, overlapping triangles, normal orientation. The pink highlight in the picture gives the location of the surface that needs to be fixed.
III.2 Boundary Flagging
There are in total 16 boundaries flagged in this geometry. And this can be started from any part of the geometry as long as you can select that portion right and view it / hide it as necessary and sometimes to capture the portion of the boundary, fence around the edges or vertices are created.
1. Liner & Piston
Fence around the edges of top & bottom of liner Selection of Liner for boundary flagging
Once the fence is created selecting that portion of the geometry becomes easier and they're then flagged according to its part name. From below picture Liner (cylinderical portion) & Piston (bottom) are flagged.
Boundary flagged for Liner & Piston - (Color coding mentioned on left side)
2. Cylinder Head
To flag cylinder head we create fence on top of the connected surface (wall) of cylinder head & intake / exhaust ports. The intersection error was present near the area fence are created but they are fixed are the boundary for that part has been flagged. For the flow of this report flagging is shown first then the surface clean up.
Fence created on beginning of ports as top of cylinder head
The boundary for cylinder head flagged & named.
Boundary flagging for Cylinder head
3. Intake Port / Inflow / Exhaust Port / Outflow
The intake port is split into two boundaries & later into two regions to provide separate different boundary conditions. The intake port 1 is where the Fuel spray is injected from. The inflow is also flagged. The intake is split into two regions by using same fence method created by two vertices and then fence are created for flagged boundaries.
Intake Port + Inflow
Then the outflow is flagged and using boundary fence the rest of exhaust port region is selected and flagged.
Exhaust Port + Outflow flagged
4. Valves
After hiding the Intake & Exhaust port, Liner and cylinder head there are valves inside the ports which goes translates into the chamber. The valve geometry is flagged in 3 categories as mentioned. This is done to make creating events easy between the cylinder and port when converge creates disconnecting triangles to stop the flow & then later resume it. The bottom is later joined in regions of cylinder as it forms the internal top part during opening and closing. Also these parts are given translatory motion from calculating their normal.
Valves
5. Spark plug & Spark Plug terminal
The spark plug & its terminal is flagged using fence. The separate b.c for spark plug and terminal is necessary as they are at a higher temperature than rest of components as this is where the spark occurs.
Spark Plug Spark Plug Terminal (where spark generates) - BLUE COLORED
6. Small Cylinder flagged to Intake and Exhaust port boundaries on both sides.
This is done to prevent the shape of the cylinder on top of the valve where its connected to distort as the valve moves up and down. Flagging this small cylinder to it retains the shape & hence simulation correctness.
7. All Boundaries flagged - Displaying boundary + Geometry.
IV SURFACE PREPARATION :
1. Intersections :
We have 1574 intersection errors at the inlet valve side where valve angle protrudes out of the cylinder head wall surface. The red selection shows how the triangles are intersecting each other which causes this error. As geometry is made up of triangles in converge we can see the triangles of valve angle & cylinder are crossing each other, The mixing zone is highlighted while the rest is colored without edges.
To solve this error the valve must be pushed down which is done by obtaining the valve arc normal at the top valve circle and using translate option and choosing selected boundary i.e valve top, angle & bottom it is translated down in increments of 0.0001 m until the surface appears clean of the intersection & diagnosis is ran again to verify the error has been solved.
Intersection error
The settings of translate, direction co-ordinates & intersection error free geometry.
Intersection error solved
2. Open Edges :
Sometimes such open edges are formed in the geometry as shown below, type 1 is straight forward open - has a hole like structure on the surface while type 2 is more like open at small points and intersected type triangles are formed due to open edges.
Open edges type 1 Open edges type 2
Simply patching the open edges by selecting its open edge from repair - patch & converge automatically creates a triangulated surface over it. And for the type 2 simply delete the triangles and when the open edge is found behind just patch it like type 1 problem.
Open edge fixed
3. Normal orientation / Overlapping Tris
Overlapping tris are triangles that overlapp each other, usually they need to be fixed by deleting some over lapping triangles & patching them, however in this case they were mixed with Normal orientation problem & fixing its normals also fixed the overlapping tris. As from the image below the normal toggles are pointing out we simply need to flip them inside by going into Transform tab, Normal & selecting any triangle in that area and clicking accept. Converge automatically corrects the orientation.
Overlapping Tris
Normal Orientation Normals / Overlapping tris fixed
4. All Surfaces cleaned & prepared & boundary flagged.
V REGIONS :
Regions is the next step done afterwards, They have their own boundary conditions & can be coupled with events as in this case to execute a cyclic & permanent events explained later in the report. Also other boundary like spray modeling require these to be created. Different regions can have different physics of mesh motion & temperature just like boundaries.
Regions
Which boundary belongs to what region is shown below from the Case setup.
VI. NO-HYDRO CASE SETUP
The entire case setup is explained in tabulated format. Boundary conditions in detail are mentioned & regions & events created. In nohydro setup no CFD equations are solved, only mesh and motion of translatory geometries is observed before running a full hydrodynamic simulation. This is done to save time & find errors aswell as costs. The temperature boundary conditions are not exactly to scale and have been guessed as per the geometry & relevant assumptions are made.
Events shown below in the table are created through regions created. They're between cylinder and intake + Exhaust ports by means of valve event. which basically means converge will read the valve lift profile and when the valve is supposed to be closed it will create disconnect triangles between cylinder and ports which stops the flow & separates them and then reopen as per the lift profile provided. There is permanent open event between intake port 1 & 2.
Uploaded data.
3. mech.dat
4. therm.dat
Note - In Boundary Conditions TKE B.C - Zero Normal Gradient & Turbulent dissipation - Wall Model for all unless specified otherwise.
VI.1 Stoichiometric Combustion equation :
The combustion is taken to be stoichiometric & the product formed is shown in the equation according to which we calculate mass fraction and feed in the case setup.
`C_8H_18 + 12.5(O_2+3.76N_2) -> 8CO_2 + 9H_2O + (3.76*12.5)N_2`
Mass fraction calculation
VI.2 Meshing - Grid Control :
Default standard grid size is taken to be :
Base Grid size : 0.004 m
Fixed embedding :
This embedding is provided near angles of valve as the space there is very small and 0.004 m mesh size will give inaccurate results and as we know the distance from approximations and when we lowered the valve during preparaing surface, and converge laws of adding half cell we need it to be 0.5 mm and hence fixed embedding is provided to refine that area to capture the combustion details and calculate them properly. Similarly the cylinder is also very coarse and cylinderical shape type embedding is provided onto it.
- The simulation was further coarsened by setting grid scaling by -2 later which coarsened the mesh by factor of 4. Animation results are from this mesh.
1. Intake valve angle & Exhaust valve angle :
Entity type - Boundary
Mode - Permanent
Scale - 3 , Layers - 1.
2. Big cylinder Embedding & Small Cylinder Embedding
Pink - Big cyl ; White - Small cyl
VII. POST PROCESSING - PARAVIEW
The mesh embedding created is observed in the clip plane which was coarsened later by factor of 4 as the original mesh was computationally heavy. However the objective is achieved with this mesh and at a faster time period of 540 seconds running on 4 processors. The next animation is faded and gives a more 3D outlook of the whole process.
Meshing Animation Clip -
Animation -
keywords - CFD, COMBUSTION, IC-ENGINE-CFD, CONVERGE-CFD, PARAVIEW, CAE.
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