Project 1 - Surface preparation and Boundary Flagging (PFI) - Part 1

OBJECTIVE:

• To perform surface preparation and boundary flagging tasks on our IC Engine case and setup a No Hydro simulation.

PROCEDURE:

When importing our geometry and running the diagnosis in Converge CFD we are prone to get a lot of errors in the design geometry. First we flag recognisable boundaries and hide certain parts of the geometry to start working on our problem triangles.

• Make use of Boundary - Flag option to select the surface to assign them to an individual component say Piston and Liner etc.
• The inflow and outflow boundaries are described as circles and oval shapes respectively.
• Sometimes the triangles exceed their respective boundaries and spread to other parts in this case make use of Boundary - Fence option to assign fences to individual boundaries and flag them accordingly.
• The spark plug and spark plug terminal can also be flagged in the same way seen in the image given below:

• When flagging the cylinder head(indicated in orange) we find that the valves from intake port(indicated in green) are superimposing with that of the cylinder head, We can infer that the intake valves are pushed so high that it is part of the cylinder head as seen in the image shown below:

• Before resolving this issue we first have to flag more boundaries so that we are left with minimum components to handle with. The intake and exhaust valves are flagged into top, angle and bottom boundaries respectively as shown in the figure below:

• There will be certain profiles which are left in the exhaust section and all these should be a part of the exhaust port section as shown in the figure below:

• To resolve issues with super imposing triangles first we need to create two fences and extend our port boundaries close to the valve boundaries. This is given in the image below:

• Afterwards we make a three point arc on the circumference of the circle and get the arc normal. The coordinates of the vectors are taken and we translate by 1mm. This will push the intake port and the issue with the super imposing triangles are resolved as shown in the figure below:

• We still have our problem triangles indicated by the pink areas to be resolved as well. Using the Repair-Patch option the open edges shown in figure below can be resolved:

• We are then left with normal orientation errors in areas of piston and liner which can be solved by making sure that normals point inside these regions so that the movement of the fluid is inside the engine as shown in the image given below:

• Before we move on to setup initial and boundary conditions we have to divide our intake port into two parts because fuel is injected through one half of the intake port and other part has no fuel spray. To initialize the fuel that comes into the combustion chamber we split the valves into two boundaries namely Intake Port A(with fuel) and Intake Port B(without fuel) respectively as shown in figure below:

After we are done flagging boundaries to the individual components we assign regions. A region is a closed volume. One region can interact with the other one but the interface should be clean as seen in the image below:

The valves should interact with the intake-combustion chamber and combustion chamber-exhaust regions. One region can interact with other region but interface should be clean.

DISCONNECT TRIANGLES CONCEPT:

From the picture shown above we can see two circles. These are the areas where disconnect triangles are created. They restrict the flow which enters the cylinder region. When the triangles disappear flow is allowed. If the disconnect triangles are not present our valve is quite open and we would be underpredicting the mass flow which enters our computational domain.

INITIAL DATA:

• The picture given below describes all the data required for our engine geometry,

• The simulation data is given in the picture given below:

• For our simulation we are provided with mechanism files which take care of our gas phase chemical reactions and for each gas phase species we are provided with thermodynamic properties. We are only interested in mesh generation and not running the entire CFD simulation. We use Isoctane (IC8H18) which is similiar to gasoline as our primary fuel.
• The boundary conditions are provided as shown in the image given below:

1. The inflow boundary has a mass fraction of 1 which contains the constituents of air i.e. Nitrogen(0.77) and Oxygen(0.23).
2. Combustion of our fuel is given by the equation: C₈H₁₈ + 12.5*(O₂ + 3.76N₂) = 8CO₂+9H₂O+(3.76*12.5) N₂
3. We assume that combustion is stoichiometric and the concentration of species in our outflow boundary when combustion products are released is given by the following table:

STOICHIOMETRIC CALCULATOR
SPECIES	MOLES	MOLECULAR WEIGHT (g/mol)	MASS	MASS FRACTION
CO2	8	44.01	352.08	0.192304
H2O	9	18.01528	162.1375	0.088559
N2	47	28.0134	1316.63	0.719137
TOTAL			1830.847	1

The different regions are provided with boundary conditions shown in the images below:

• Based on the valve timing for the intake and the exhaust profiles we must connect and disconnect from the combustion chamber using cyclic option in Events box.

• Since we are having regions and the two intake ports A and B are seperated we provide additional permanent event to it as seen in the image below:

• • A standard RNG k-ε model is used as our turbulence model as lot of experiments where conducted to determine constants suitable for our IC Engine case followed by a base grid size of 4mm.
    
  • Embedding is provided to refine the grid at specified locations to improve accuracy of our solutions. An embedding for intake and exhaust valve angles provides 0.5mm cells as mesh refinement. An additional embedding is given for our spark plugs so when energy is sourced into spark plug the combustion propagates. The embedding can be visualized in the picture given below as small and large embeddings with 2mm and 1mm cells respectively:

  CONCLUSION:

  Following all the steps described in our procedure and running the No Hydro Simulation we will end up with the following animation shown below describing our valve movements defined by their valve timing profiles: