Steady state simulation of flow over a throttle body   (Converge CFD, Skill Lync, Easy)

Aim:

To simulate a case of steady flow over the throttle valve using Converge CFD.

Introduction:

The throttle valve, used in IC engines, is a circular disc placed near to intake manifold which controls the mass flow rate of air fuel mixture entering the combustion chamber. Depending on the vaccum inside intake manifold and throttle position, the flow rate keeps changing. When the valve is open, the fuel air mixture is sucked inside cylinders. The accelerator pedal of car is linked to this valve. In this report, a steady state simulation of fully open throttle valve is done using Converge CFD. The model used for simulation is:

The original throttle valve has a seal and a shaft. The seal prevents any leakage when the valve is closed and shaft is used for rotation of valve. These two parts are not considered in this analysis. Also, the original flow for this case happens due to suction at outlet, i.e the flow happens due to the pressure difference. Similarly, in this simulation the pressure at outlet is lower than inlet, but the pressure difference values are different than in actual scenario.

Following are the details of simulation:

Mesh Sizes: (Element size along X, Y, Z axes) (in metres): (0.002, 0.002, 0.002)

The feature of 'Fixed Embedding' is used on 'throttle body' with scale as 2 and layers as 3.

Type of simulation: Steady state with only gas flow simulation.

Steady State monitor properties: (Used to stop simulation when difference between two monitor variable values in sample is lower than tolerance)

Monitor variable: Mass flow rate at outlet calculated from mass average flow otput file.

Monitor start delay: 200 cycles

Sample size: 200 cycles

Relative Tolerance: 0.001

Boundaries:

Inlet: Inflow boundary condition with pressure inlet at 150000 Pa.

Outflow: Outflow boundary condition with pressure outlet at 100000 Pa (difference of 50000 Pa)

Pipe wall and throttle body surfaces : No Slip walls

Initial region pressure = 100000 pa

Turbulence model: RNG k-e model with standard wall functions.

Solver Properties:

Solver: Pressure based 'PISO' solver with 'step' as flux limiter.

Details: 2 minimum iterations and 20 max iterations per cycle. Tolerance = 0.001

Minimum Cycles = 0

Maximum Cycles = 15000

Minimum time step = 1e-9 sec

Maximum time step = 0.003 sec

Max Convection and diffusion CFL: 1

Max Mach CFL: 50

Results:

1) The simulation ran upto 6268 cycles.

2) Mesh:

The 'fixed embedding' created a fine mesh near the 'Throttle body'. The scale parameter (n) determines final size of elements as `dx/(2^n)` and layers determines number of layers of fine mesh along the boundary. The software creates the gradual fine mesh by dividing neighbouring cells of boundary into half the size of previous cell, till the finest size becomes as decieded by scale parameter.

3) Total cells:

Since the mesh embedding was permanent, the cells didn't change in number.

4) Velocity Contour:

The velocity inside the pipe remained nearly equal between 200 and 250 m/s. The velocity behind the valve plate is less than 50 m/s. The shows the formation of wake. The velocity of air in the gaps of thottle valve is higher. This is because the pressure loss around that points is higher than surroundings.

5) Velocity graph:

The velocity at inlet and outlet nearly match with each other.

6) Pressure Contours:

The low pressure in wake region can be seen. Also, high pressure zone at stagnation point can be seen.

7) Pressure graph:

The inlet pressure stayed 150000 Pa because it was boundary condition value. The outlet pressure value reached at 137750 Pa.

8) Mass flow rate

The mass flow rate at inlet and outlet are same, indicating mass conservation.

9) Streamlines:

The streamlines from inlet shows that throttle plate doesn't create a lot of turbulence in this position. Its obvious because the opposing area is very low compared to cross section of pipe.

10) Animation:

Conclusion:

The steady state simulation of flow over throttle body is done using Converge CFD and appropriate post processing was also done.