Flow over a Throttle body - Using CONVERGE CFD

In a traditional spark ignition gasoline engine, the throttle body is the part of the air intake system that controls the amount of air that flows into an engine's combustion chamber. It consists of a housing unit that contains a throttle plate (butterfly valve) that rotates on a shaft.

When the accelerator (gas pedal) is pushed down, the throttle plate opens and allows air into the engine. When the gas pedal is released, the butterfly closes and effectively chokes off (throttles) air flow into the combustion chamber. This process effectively controls the speed of the engine and ultimately, the speed of the vehicle.

 

III. Geometry Cleanup in Converge Studio : 

Geometry of Elbow: 

                        

Half Cross-section : In the middle the Throttle is visible. 

                        

The Outer Edge of the Pipe is deleted as it is not needed and only the internal surface of the pipe is required, The inlet and outlet area is patched with triangles to complete the Fluid Volume. 

                       

Cross-section Geometry after naming the Parts as per Geometry boundary. 

                        

 

 

                                                STEADY STATE - SIMULATION 

The geometry shown above is used for both Steady and Unsteady state hence shown above. From here we will Discuss only Steady state parameters of the Project. 

IV. Meshing : 

The mesh is created with 0.002 m grid and Fixed embedding is done at the Throttle valve with 3 Scale and 2 layers as shown below. It creates layered mesh and refines that zone to capture the physics in that area accurately. With this refinement we can keep a slight coarse mesh on other not so essential areas of the body. 

                            

                           

 

Total Cell Count : 35510 Cell elements created .

      

 

V. Case Setup : 

1. Application type : Time Based

2. Material : Air mixture ( Gas simulation) 

3. Gas Species : 02 , N2 .

4. Solver : Steady state solver , Fully Hydrodynamic. 

5. Gas flow solver : Compressible.

6. Misc : Momentum , Energy on 

7. Steady state monitor :  On. - min cycles : 500

8. Monitor Variables : Mass flow rate at Outlet

 

9. Simulation Time parameters : 

i. Start 0 cyc

ii. End : 15000 cyc. 

iii. Initial Time-step : 1e-9s

iv. minimum Time-step  1e-9s

v. Max time-step 1s.

 

10. Solver scheme : Piso algorithm : Pressure Based

11 Pressure : preconditioner SOR.

 

12. Boundary Conditions : 

i. Inlet : Inflow ; Pressure : 1.5e5 Pa ; Temp : 300k ; Species : O2 , N2.

ii. Outlet : Outflow ; Pressure : 1e5 Pa ; Temp : 300k; Species : O2, N2. 

iii. Wall - Elbow:  Law of wall

iv. Throttle : Law of wall

vi. TKE : Zero normal gradient

 

13. Regions & Initialization : 

14 Volumetric Region created : Species : O2,N2 .

15. Output Files : Time interval for writing 3D output data files : 100 cyc.

16. Max restart files saved : 3 

17. Turbulence Model :RNG k-epsilon

 

VI. Solution & Post Processing : 

The Simulation run took 1155.13 seconds to finish. It reached convergence before the Set number of cycles. 

Velocity : 

The top part is the inlet and the bottom is the outlet side, We can see the velocity at steady state in final iteration is more towards the outlet, as this valve is open. The valve geometry is not exact to scale and hence there is flow from the top and bottom. The maximum velocity reached is 270 m/s.

                         

This is with the Throttle valve enabled, we can see the flow at the side of throttle valve is less as it is a solid surface the flow attaches on it. 

                          

 

Velocity Plot: 

The positive velocity represents the inflow while negative velocity represnts the outflow. Here we are observing the average value the elbow with throttle valve fully open achieves. The inlet is nearly 200 m/s while outlet is 150 m/s.

         

 

 

 Pressure :

The prressure contour shows the pressure on the inlet pipe is higher than the outlet side. The max pressure set for the simulation is 1.5e5 which doesnt exceeds as seen on the legend. The pressure is more on the throttle valve surface edge as the fluid hits there and gets stagnant which leads to velocity drop as seen in above contour and increase in pressure as seen below. 

                          

Pressure Plot : 

The pressure Plot shows all Total and static pressures. The pressure at inlet is 1.5e5 and outlet is 1e5 which is the Boundary in the plot, The pressure variation along the inlet and outlet is shown by the lines in between the stationary lines. These converge uniformly in the end from inlet to outlet creating a steady state. 

        

 

Mass-flow- Rate : 

The mass flow rate appears to be conserved as we're running Mass conservation equation. The positive flow is at inlet and the negative one represents outlet. The value is ~ 0.006 kg/s 

       

 

 

 

                                                TRANSIENT STATE - SIMULATION 

 

 

From here, we shall discuss the Transient state parameters. In the transient case the throttle valve is rotated upto 25 degrees and then repositioned into its opened start position. The simulation is Ran for 10 milliseconds of Flow time. 

                      

 

VII. Geometry Preparation in Converge Studio :

As mentioned above the geometry cleanup is same as steady state, The only preparation required for transient case is the rotation of throttle valve in the Joint. 

This is the Throttle valve, After hiding the geometry we enable only the valve. To rotate the valve we need to find its center of axis and normal of the valve about which we desire rotation. 

                            

 

We see the cylindrical shape on the throttle valve above, we hide the rest of the surfaces and keep these enabled, as theyre at the center of the Throttle at its center lies the center of the valve, Hence from converge studio we use measure tool and select Arc normal, Using this then we select 3 vertex on the arc of the below shown cylinder, it doesnt matter which vertex. 

This gives the value of Arc center and Arc normal values which we paste on the Boundary condition of the Throttle instead of Law wall as we selected before. Once these co-ordinates as shown in the below picture are placed in the Boundary conditions are set.  More details are provided in Case setup. 

                            

 

VIII. Meshing : 

The mesh is kept slightly Coarser than steady state here to save computational time. The element size is 0.0035 m with Fixed embedding of 3 scale and 2 layers as before. The mesh is shown below: 

                            

Total Cell Count :

The total cell count is not constant because Our mesh is being continiously refined as the Throttle valve moves. The cell count is varying exactly on the time of flow we have rotated the valve and becomes constant when its not rotating. The maximum flow time is 0.01. More detail regarding the flow time is provided later in the report. 

 

       

 

IX. Flow Time Calculation :

By this calculation we determine how much end time is required for the simulation. This varies majorly on the geometry, fluid velocity etc. 

                                  

From the above bounding box, we can see the length of the pipe in x and Y direction in global co-ordinates from the origin.  The Xdir is ~0.1m and y is ~0.12m  

So we assume the total length of pipe as 0.2 m from xy direction.  

Now from steady state analysis above, we know that the average velocity is 200 m/s on the inlet side. So the fluid flows at average velocity of 200 m/s. 

Since Speed = Distance/Time . we have both distance and speed from which we can calculate flow time. 

Time = Distance / speed = 0.2/200= 0.001 s 

It takes 0.001 seconds for 1 flow. Hence to capture the physics for a few more flows, say 10 flows for example. 

0.001 * 10 = 0.01 seconds. 

Hence end time or Flow time is set to be 0.01 seconds. 

 

X. Case Setup : 

1. Application type : Time Based

2. Material : Air mixture ( Gas simulation) 

3. Gas Species : 02 , N2 .

4. Solver : Transient solver ; Time -based simulation ; Fully Hydrodynamic. 

5. Gas flow solver : Compressible.

6. Misc : Momentum , Energy on 

 

7. Simulation Time parameters : 

i. Start 0 s

ii. End : 0.01 s. 

iii. Initial Time-step : 1e-9s

iv. minimum Time-step  1e-9s

v. Max time-step 1s.

 

8. Solver scheme : Piso algorithm : Pressure Based

9 Pressure : preconditioner SOR.

 

10. Boundary Conditions : 

i. Inlet : Inflow ; Pressure : 1.5e5 Pa ; Temp : 300k ; Species : O2 , N2.

ii. Outlet : Outflow ; Pressure : 1e5 Pa ; Temp : 300k; Species : O2, N2. 

iii. Wall - Elbow:  Law of wall

iv. Throttle : wall - motion - Rotate ; Surface movement : Moving 

               

v. Rotation rate : As shown in pic above, right hand side.  It means the valve will turn to 25 deg from 0 - 0.002 seconds. Stay at 25 deg upto 0.004 seconds, Then rotate back to starting position till it reaches 0.008 seconds and stay like that upto 0.01 seconds.

 

11. Regions & Initialization : 

12 Volumetric Region created : Species : O2,N2 .

13. Output Files : Time interval for writing 3D output data files : 0.00065 seconds (nearly 16 output files)

Time interval for text output : 1e-6 seconds.

16. Max restart files saved : 3  

17. Turbulence Model :RNG k-epsilon

 

XI. Solution and Post Processing : 

The Simulation run took 8492.08 seconds / 2.35 hrs to finish. 

Velocity : 

The maximum velocity attained is increased from the steady state, its around 320 m/s for transient case whereas it was 270 m/s for steady state.  The physics of the flow and velocity distribution is same as explained in the steady state except for the velocity value itself. 

The contour shown is at the last flow time 0.01 second.

                           

Velocity Plot : 

The average velocity obtained here is also ~ 200 m/s as reached in steady state . This value represents inlet and outlet side of the elbow pipe. The flow is constant velocity for 0.002 - 0.004 and 0.008 - 0.01 as we had the valve constant and not moving and on the other flow time we have variations. 

        

 

Pressure :

The pressure distribution is same as explained in the steady state. The max pressure set 1.5e5. The lowest pressure observed decreased from the steady state in here. The low pressure obtained near the cylinder region of the throttle is due to its out bulge shape where velocity increases as seen from above, hence pressure reduces in that linear zone. The contour shown is at 0.01 second. 

                           

Pressure Plot :

The pressure distribution at inlet and outlet is shown, The central variation lies between the maximum and minimum given pressures and displays how it varies every flow time. There is more variation initially when the throttle closes whereas during when it opens 0.004 - 0.008 there is linear drop and increase. 

       

 

Mass Flow rate : 

Mass conservation is observed in the transient flow as well. Plot represents the inlet and outlet of the elblow. The mass flow rate is same as steady state. 

        

 

 XII. Throttle Movement Animation : 

The animation is created in paraview. Steamlines are colored with velocity as well as the throttle body. The animation shows the throttle movement as set in the boundary condition. 

                 

 

                 

 

 

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keywords - VALVE, CFD, CAE, CONVERGE-CFD, PARAVIEW