Transient simulation of flow over a throttle body
CONVERGE CFD- Simulation of Flow Over a Throttle Body
Objective:
- To perform Transient-state simulation of fluid flow over a throttle body.
- To post-process results and show pressure and velocity contours for both the cases.
- To show the plots for pressure, velocity, mass flow rate and total cell count.
Throttle Body:
A throttle body is essentially a large air valve that sits between the intake manifold and the air intake pipe. An engine has one throttle body, which bolts to the intake manifold\'s plenum (the large air cavity in the center). A throttle body controls airflow into the engine, a crucial factor for controlling the engine\'s air/fuel ratio. Throttle bodies come in several shapes, sizes, and configurations and can perform several tasks outside of their normal air-metering duties.
Throttle bodies usually use butterfly valves. These valves consist of a casing with a large hole in the center, which is filled with a steel plate called a butterfly, or throttle plate. The throttle shaft is a rod that passes through the housing and is connected to the throttle cable on the outside and the throttle plate on the inside. When the throttle shaft turns, it rotates the throttle plate, allowing air to go around it and into the engine.
Meshing Technique:
Converge CFD uses a cut-cell cartesian method. One of the most obvious benefits of the cut-cell Cartesian approach is the fact that you have a body-fitted mesh. This ultimately means that your mesh fits perfectly with your geometry, reducing the number of inaccuracies that propagate through the rest of the domain. Because of this, the cut-cell Cartesian approach makes handling complex and moving geometries much easier and accurate by cutting the cells rather than stretching and compressing the cells to fit. The other benefit of the cut-cell Cartesian method is added cell refinement at the wall, as this will have a positive impact on accuracy. Mesh refinement can be performed at locations where the flow properties need to be captured accurately. A method called \"Fixed Embedding\" is deployed in this case with the region near the throttle body being the region of interest.
The scaling factor of 3 has been used with 2 layers of fixed embedding. The base grid used in this simulation is 2e-3 in x, y, and z-direction. This means that after scaling has been applied for the mesh near the throttle valve region, the mesh will be of value 6e-3. The refined mesh near the throttle valve and the base mesh can be seen in figure 4 and figure 5 respectively.
Transient Simulation:
Transient means, something that fades with time. Transient analysis obtains results as a function of time. Transient simulations are used to obtain accurate results as a function of time including fluctuations, time progression or reactions to a transient boundary. Transient simulation is however typically expensive in terms of computational power and or time, for applications where long period transient effects are to be studied.
The dynamic analysis in the time domain is called Transient Dynamic Analysis. The input can be the time history of loading and then the output will be the time history of response. That means, load vs. time will be the input, Response (displacement, stresses, velocities, acceleration) vs. time will be the result. In dynamic analysis, there is frequency response analysis and transient response analysis. One is where you give loading as a function of frequency and the other as a function of time.
In this model, a rotation is given to throttle body w.r.t time for which a transient simulation is required to done to analyze velocity and pressure for a flow flowing through a throttle body for every second of its rotation.
Pre-processing:
Case Set-up: General Flow
Materials:
Predefined Mixtures: Air
(Gas Simulation)
Simulation Parameters
Solver: Transient
Mode: Full Hydronic
Start time: 0 sec
End time: 0.01 sec
Initial time-step: 1e-9 s
Minimum time step: 1e-9 s
Maximum time step: 1.0 s
Regions and Initialization
Added Region (Air)
Pressure: 101325 Pa
Species: Air - O2 = 0.23, N2 = 0.77
Boundary Conditions
Inlet: Inflow 150000 Pa Air
Outlet: Outflow 101325 Pa Air
Throttle: Wall (Motion type: Rotating)
|
Second |
Angle |
|
0.0 |
0.0 |
|
0.002 |
25.0 |
|
0.004 |
25.0 |
|
0.008 |
0.0 |
Elbow wall: Wall
Velocity boundary conditions: Law of wall
Wall motion type: Stationary
Surface Movement: Fixed
Temperature boundary conditions: Law of wall (300K)
Base Grid: Mesh in dx, dy and dz directions = 0.002
Fixed Embedding
Fixed embedding is used to refine the mesh around the desired regions. In this case, the regions of interest are the inner walls of the channel. Therefore, the fixed embedding is applied with a scale of 3 and 2 embedded layers.
Output files:
Time interval to write 3D output data files = 0.001 sec
This means that after every 0.001 sec, 3D output data files will be written and saved. Since we are running the simulation for 0.01 sec, we shall have 10 3D output data files.
Time interval for writing text output file = 1e-06 sec
Calculation of an ‘End time’ for the simulation:
Bounding box characteristic dimension (from CONVERGE Studio):
Lx = Ly = Lz = ~0.2 (max.)
From Steady-state simulation analysis:
Avg. velocity @ inlet = 200 m/s
Flow through time = (Lx/Ly/Lz) ÷ (Avg. Velocity)
∴Flow through time = 0.2/200 = 0.001sec.
This is the time taken by fluid flow through the pipe for once
To be on a safe side for complete simulation, we can take eight to ten times the value we calculated (flow-through time) for ‘End time’
∴End time for simulation is taken as equal to 0.01 sec.
Results:
From the above figure, we can see that the convergence for both the inlet and the outlet section occurs at approx. 0.065 kg/s mass flow rate after approx. 0.008 sec. The negative sign at the inlet indicates that the flow is into the system and the positive side at the outlet flows out of the system.
bound_id_2 = Inlet
bound_id_3 = Outlet
The simulation is run in parallel. The converge uses MPI programming to parallelize the solver, which means that the solver is equally distributed to given no of processors in the working computer. In the present problem, the solver is run with 4 processors which equally distributing the cell structure to each of the processors.
Link: https://www.youtube.com/watch?v=aWgY7NEOXsg&feature=youtu.be
The discharge must remain constant for fluid flow through a throttle body at 0 deg and 25 deg position. This means that to satisfy the continuity equation (Area*Velocity=constant), velocity increases in sections where the flow area is reduced (at 25 deg rotation). A maximum flow velocity of around 300m/s is obtained when Valve is rotated through an angle of 25 degrees.
- View from inlet side @ 25 deg. throttle rotation
- View from outlet side @ 25 deg. throttle rotation
Link: https://www.youtube.com/watch?v=CLVhRN3Aih0&feature=youtu.be
The Pressure Profile for Throttle Valve is the most important as it can be used to determine the thickness, material and other physical properties the Valve Body should have to bear the pressure load. Initially, the pressure gradient across the throttle valve was not much because the valve was in an open position. However, as the valve rotates, the fluid flow is obstructed which results in the conversion of Kinetic Energy into Pressure Energy resulting in higher Pressure Gradient across the throttle body.
When the throttle valve rotates at 25 deg. Position, the maximum area of it creates an obstruction to fluid flow due to which there will be a high-pressure rise at its surface facing toward the inlet side (figure 16) and low pressure on its opposite face facing towards the outlet side (figure 17). As velocity is inversely proportional to pressure, from the pressure profile and velocity profile, it is observed that both having the opposite effect on fluid flowing over the throttle region.