Transient simulation of flow over a throttle body
Aim- To study the flow characteristics of fluid passing over a throttle body in an elbow-shaped tube.
Governing equation- The equation used to solve this case is Navier-Stokes Unsteady equations. It comprises of 5 equations continuity, x-momentum, y-momentum, z-momentum, and energy equation. But for this case, the energy equation is not used since there is no gain or loss of energy and the temperature is constant throughout the simulation.
Objective- The objective of this problem is to simulate fluid flow over a throttle body at different angles in an elbow-tube. A base mesh is created with refinement near the throttle body and the pressure, velocity contours plot are made.
Description- First the geometry to be simulated is imported in Converge Studio and then the geometry is checked for any errors. The geometry is grouped into faces so that respective boundary conditions can be assigned and then the case setup is done. For this project, we will be running a transient simulation with start time 0ms and end time 10ms. It is a turbulent flow simulation hence RNG k-epsilon turbulence model is selected. The mesh size used is 1e-3m in all directions. In this project, the throttle body surface is refined because it helps in properly capturing the flow separation taking place and so a fixed embedding of scale 1 is given which means the cell size at the surface will be of 1mm and no. of layers 2.
Geometry-
From the above image you can see the position of the throttle body inside the elbow-tube. The throttle is given rotating boundary conditions such as it rotates from 0-25deg in 0-2ms and stays at 25deg until 4ms and after that moves back to its original position of 0deg.
Material-Air
Boundary Condition-Inlet: Pressure inflow of 1.5e+5 Pa
Outlet: Pressure outflow of 1e+5 Pa
Elbow wall: Fixed wall
Throttle body: Rotating wall
Initial Condition- Pressure:101325 Pa and temperature:300K
Mesh- Mesh size:1e-3m with fixed embedding scale=1 and no. of layers=2.
A fixed embedding of scale 1 and no. of layers 2 is used on the throttle surface because we want to capture accurately the flow around the throttle and also in the gaps between the throttle and the elbow wall.
Results-
Cell count
The above plot shows how the cell count varies as the throttle position changes. The max cell count is between 2-4ms when the throttle is at max angle of 25deg and then it gradually decreases as the throttle comes back to its original position of 0deg.
Mass flow rate
In the above plot, bound_id_1 represents inlet, and bound_id_2 represents the outlet. The mass flow rate is almost constant between 2-4ms because the throttle is at 25deg between that time. After 4ms as the throttle starts moving towards 0deg, the mass flow rate increases. So from the plot it is evident that the simulation as converged as the mass flow rate at the inlet and outlet is almost the same.
Static Pressure line plot
As the air flows towards the outlet the static pressure reduces at the inlet.
Velocity line plot
The velocity is almost constant between 2-4ms because the throttle is at 25deg between that time. After 4ms as the throttle starts moving towards 0deg, the mass flow rate increases. The velocity at the outlet is higher than the inlet because of the throttle body which reduces the area of the elbow at the center and hence accelerates the air.
Pressure contour
The pressure in the front part of the throttle is high because it is resisting the flowing fluid and hence the fluid gets stagnated at that point. The pressure is about 1.5e+5 Pa.
Velocity contour
The maximum velocity of 340m/s is observed when the throttle is at 25deg where the max flow of air is obstructed. The velocity is high at the small gaps at the upper and lower parts of the throttle. The velocity at the outlet region is also high compared to inlet because the air accelerates due to a reduction in the elbow area by the throttle body.
Velocity streamlines
From this streamline of velocity we can see how the flow occurs over the throttle as it rotates and also we can see the recirculation region behind the throttle. This plot also verifies whether the case is running properly or not.
Conclusion
From the above plots, we can understand how the flow behaves when the throttle rotates and accordingly design the elbow and throttle as per the output requirements.