Week 4.1: Project - Steady state simulation of flow over a throttle body
Steady-state simulation of flow over a throttle body:
Abstract:
A throttle body controls airflow and plays a major role in the performance and efficiency of a spark-ignition engine. Controlling airflow into the engine is a crucial factor in controlling the engine’s air/fuel ratio. 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. The throttle plate also acts as a restriction to the intake air stream, causing loss of flow energy in the intake air. In this project, steady-state simulation for airflow in an elbow containing a throttle has been carried out using Converge CFD to understand the behavior of airflow inside and a comparative study of pressure and velocity variations across the valve has been done.
Objective:
1. The objective of the project is to setup and simulate steady-state airflow in an elbow with a throttle using Converge CFD and CYGWIN Terminal.
2. Show pressure and velocity contours inside the elbow with throttle present using ParaView software and plot mass flow rate, average velocity, total pressure, and total cell count graphs using Converge software line plotting.
Work Flow involved:
1. Pre-processing (preparing surface geometry, meshing, and configuring the input and data files).
2. Running the simulation.
3. Post-processing (showing of different contours and plots involved).
1. Pre-processing:
1. Upload the given .stl file to Converge studio and change the scale to meters if the given geometry is not in meters by going to the scale option under transform.
2. Transform the normal of each triangle to the inside of the wall since internal flow is happening.
3. Boundary condition – Elbow wall, Throttle, Inlet and Outlet are given.
Geometry:
Case-setup:
1. Application: Time-based.
2. Materials:
i) Gas simulation: Air.
ii) Global Transport Properties: Default.
iii) Species: Add O2 and N2.
3. Simulation parameters:
i) Run parameters:
a. Solver: Steady-state solver.
b. Simulation mode: Full hydrodynamic.
c. Gas flow solver: Compressible.
d. Steady-state monitor: on.
ii) Simulation time parameters:
a. Start time: 0.
b. End time: 15000.
c. Initial time-step: 1e-6.
d. Minimum time-step: 1e-6.
e. Maximum time-step: 1.
f. Maximum convection CFL limit: 1.
iii)Solver parameters:
a. Navier-Stokes solver type: Density-based.
b. Equations: Change preconditioner in Pressure to none.
c. Steady-state solver control: Maximum convection CFL limit: 0.8.
iv) Boundary conditions:
a. Elbow wall: Wall, Surface movement: Law of wall, Temperature boundary condition: Zero normal gradient and Turbulent kinetic energy boundary condition: Zero normal gradient.
b. Throttle: Wall, Surface movement: Law of wall, Temperature boundary condition: Zero normal gradient and Turbulent kinetic energy boundary condition: Zero normal gradient.
c. Inlet: Inflow, Total pressure: 150000.0 Pa, Initial temperature: 300K and add Species: Air.
d. Outlet: Pressure: 100000.0, Temperature backflow: 300K and add Species: Air.
v)Initial conditions and events:
a. Species: Add Air and change the mass fraction of N2 to 0.23 and O2 to 0.77 so that we can see the changes in post-processing and even if the composition is different from the actual mole fractions of air, converge still accepts inputs and gives results according to input.
b. Add region and name it: volumetric region user-defined.
vi) Physical models:
a. Turbulence modeling [RANS_K_EPS_RNG].
vii) Grid control:
a. Base grid:
Converge CFD uses a cut-cell cartesian method. 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.
dx: 2e-3, dy: 2e-3 and dz: 2e-3.
b. Add Fixed embedding. Entity type: Boundary, Boundary ID: Throttle, Mode: Permanent, Scale: 2, and Embed Layers: 2. Embedding is useful if you are looking to refine the near-wall mesh to capture the boundary layer accurately. The scaling factor of 2 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 4e-3. The refined mesh near the throttle valve and the base mesh can be seen in the figures down below.
viii) Output/Post-processing:
a. Select parameters needed and select N2 and O2 in mole fraction under species.
b. Output files: Writing time intervals: Time interval for writing 3D output data files: 150 and Time interval for writing restarting output: 1000.
2. Running the simulation:
After this, we need to export the data. To do that, 'File' > 'Export' > 'Export input files'.
Open Cygwin and navigate to the folder in which the input files are exported and run the command as 'mpiexec.exe -n 4 converge.exe restricted' and then press enter. The simulation will run and in the end, if it shows 'normal termination', the simulation has ended successfully.
3. Post-processing and Results:
All the post-processing of results is done in ParaView software after extracting the output file from converge.
Mass flow rate:
For a steady-state simulation, it is important to check if convergence has been attained. To do so, it is a good idea to analyze mass flow rate plots for inlet and outlet sections. The mass flow rate plot can be seen in the below figure.
Mass_Flow_Rate [bound_id_3]: Inlet.
Mass_Flow_Rate [bound_id_4]: Outlet.
From the above figure, convergence for both the inlet and the outlet section occurs at approximately 0.065 kg/s mass flow rate after approximately 4000 cycles.
Total cell count:
The total cell count is 13142 and is distributed between 4 processors.
Velocity contours:
The link for velocity contour: https://youtu.be/JyxklW_dGqw
From the video, the results can be seen of the varying velocity across a throttle body. When the throttle valve is in a wide-open position, the maximum amount of fluid can pass through that region from inlet to outlet. But the throttle body still creates some disturbance as the minimum area of it is still in between the fluid path. Reduction in fluid velocity is seen around the throttle edges where the fluid strikes from the inlet side. As it passes over the throttle body, it creates a narrow path with respect to the elbow wall due to which there is an increase in velocity and then as the fluid completely passes over the throttle region it regains its original velocity.
Flow visualization with the helo of velocity vectors can be seen in the figure below.
This is the velocity contour at the end of the simulation inside the elbow with the throttle.
Pressure contours:
The link to pressure contour: https://youtu.be/RzeU8KZBYSw
The below figure represents the pressure contour at the start of the simulation.
The below figure represents the pressure contour during the middle of the flow. When the fluid strikes down at the throttle body edges from the inlet side, its velocity decreases. The pressure is inversely proportional to the velocity and from the above figure, we can see the maximum pressure at the region where the velocity becomes minimum.
The below figure represents the pressure contour at the end of the flow
This is the pressure contour at the end of the simulation inside the elbow with the throttle.
Average velocity plot:
Total pressure plot:
Total_Pres [bound_id_3]: Pressure at the inlet: 150000 pa.
Total_Pres [bound_id_4]: Pressure at the outlet which can be seen varying and then settling down below 140000 pa.
Conclusion:
Steady-state simulation of airflow in an elbow with a throttle using Converge CFD is done to understand the behavior of airflow inside and velocity, and pressure contours and plots of mass flow rate, average velocity, total pressure, and total cell count are shown. The maximum amount of fluid can pass through the region when the throttle valve is in a wide-open position, from inlet to outlet. But the throttle body still creates some disturbance as the minimum area of it is still in between the fluid path. Reduction in fluid velocity is seen around the throttle edges where the fluid strikes from the inlet side. Lesser is the restriction across the flow, lower is the loss of energy. Also when the fluid strikes down at the throttle body edges from the inlet side, and we can see the maximum pressure at the throttle body region where the velocity becomes minimum. As it passes over the throttle body, it creates a narrow path with respect to the elbow wall due to which there is an increase in velocity and then as the fluid completely passes over the throttle region it regains its original velocity.