TRANSIENT SIMULATION OF FLOW OVER A THROTTLE BODY using Converge CFD

Abstract:

         Throttle valve plays a major role in performance and effeciency of a spark ignition engine.the design of air intake system is of utmost importance in order to improve its power and fuel efficiency. The amount of air entering the engine is controlled by the throttle valve. However, it also acts as a restriction to the intake air stream, causing loss of flow energy in the intake air. For this reason, the fluid flow analysis for the flow through the throttle valve for different cross-sections of the throttle shaft has been carried out using Converge CFD and a comparative study of pressure and velocity variations across the valve has been made.

Objective:

The objective of this project is to simulate steady state airflow over a simple elbow with a throttle using Converge CFD.

WORKFLOW FOR A CONVERGE CFD SIMULATION:

The workflow in CONVERGE consists of three steps:

1. Pre-processing (preparing the surface geometry and configuring the input and data files)
2. Running the simulation
3. Post-processing (analyzing the *.out ASCII files in the Case Directory and using a visualization program to view the information in the post*.out binary files saved in the output subdirectory).

1. Pre-processing:

Pre-processing involves three sub-steps

a. Preparing the surface geometry

The geometry dock provides us the option to create our desired geometry. In this case, it is the throttle valve. Converge studio though allows us to create a geometry it also allows user to import a geometry made in a cad program.

Steps for importing geometry: File>import>import STL

the window after importing geometry

Setting up the case:

Converge provides us with a handy wizard like feauture for setting up the case as sbown below:

1. 1.  It is a general flow case.
   2. Oxygen (O2) and Nitrogen (N2) are the two main species.
   3. Solver: Transient solver

2.  

             4.Simulation time parameters: 

1. Since this is a Transient solver we have to set start time and End time .

   For the Start Time we shall select 0 seconds , the End time can be derived by dividing the Characteristics Length by Average Velocity can multiply by the number of time we want to run the cycle for 4 times , we multiply by 4 and we get the End time .

   Characteristics Length is the important length along which the fluid occurs for most time period , ie 0.11 m along y direction approximated to 0.2 , and the average velocity from the velocity graph is assumed 80 m/s.

    `L/(Vavg)=0.2/80=0.0025`

   `0.0025 * 4 cyc = 0.01s`

1. 1. 1. Start time: 0 second
      2. End time  : 0.01 seconds
      3. Initial and minimum time step: 1e-9
      4. Maximum time step: 1

            5.Boundary Conditions:

                   a.At inlet: Pressure=1.5 bar; Temperature=300 K.

                   b.At outlet: Pressure=1 bar; Temperature=300 K.

                   c.The elbow wall are defined as walls with default parameters.

                   d. The throttle is defined as a rotating wall  

To locate the centre and axis of the geometry we shall measure it by arc method selecting 3 points on the arc and we get the centre and axis as follows , we shall paste the coordinates in the centre and axis columns respectively

the rotation centre and rotation angle are written as a seperate input file as shown below.

            6.Regions and initialization: Species O₂ =23%; N₂=77%; Temperature= 300 K; Pressure= 101325 Pa.

               Regions and Initializations: Rename the fluid flow volume as volumetric flow region.

            7.Base Grid: base grid mesh sizes are as follows

• dx= 0.004 m; dy= 0.004 m; dz=0.004 m.

            8.Select the required output variables like pressure, density, temperature etc. whose results need to be analyzed.

After setting up the case, the input files are exported in a directory.

The final geometry after applying the boundary conditions is shown below:

2. Running the simulation:

        After exporting all the required input files the simulation is ran by using cygwin.

        Cygwin is a POSIX-compatible environment that runs natively on Microsoft Windows. Its goal is to allow programs of Unix-like systems to be recompiled and run natively on Windows with minimal source code modifications by providing them with the same underlying POSIX API they would expect in those systems.

        The executable file provided by the convergent science is executed using cygwin with the help of Microsoft Message Passing Interface (MSMPI).

        Microsoft Message Passing Interface is an implementation of the MPI-2 specification by Microsoft for use in Windows to interconnect and communicate between High performance computing nodes.

        The output files are generated in the same folder where the input files are executed using the executable.

Time required for the simulation and the summary of usage is as folllws:

Program used 9010.568743 seconds.

Summary of total time for:
load balance = 10.74 seconds ( 0.12%)
solving transport equations = 2850.34 seconds (31.63%)
move surface and update grid = 2333.41 seconds (25.90%)
update boundary conditions = 2923.89 seconds (32.45%)
combustion = 0.00 seconds ( 0.00%)
spray = 0.02 seconds ( 0.00%)
writing output files = 810.37 seconds ( 8.99%)

3) Post-processing:

• Meshing: Mesh is generated for the given file at different time steps and throttle angle as shown below.

Mesh at 0 ms and 0 degree throttle angle:

Mesh at 2 ms and 25 degree throttle angle:

In order to make the simulation flow around the throttle to be smooth fixed embedding technique is used

Focussed image of the mesh around the throttle 0 ms and 0 degree throttle angle is shown below:

Focussed image of the mesh around the throttle 2 ms and 25 degree throttle angle is shown below:

Cell Counts:

Total cell count at each time step is shown below:

Mass flolw rate:

It can be seen from the above plot that the mass flow rate initially take a little turbulence and finally converges as expected . It can be clearly seen from the graph that the law of conservation of the mass is followed as the inflow mass flow rate is balanced by the outflow mass flow rate. It can be seen that initially due to the movement of throttle mass flow rate fluctuations at that time period are high. From 0.002 to 0.004 the fluctuations decreases and also approximately after 8 milliseconds the mass flow rate converges.

Velocity profile and contour:

velocity contour over the entire region at 0.002 second is shown below:

velocity contour over the throttle body at 0.002 second is shown below:

velocity contour over the entire region at 0.004 second is shown below:

velocity contour over the throttle body at 0.004 second is shown below:

velocity contour over the entire region at final time step is shown below:

velocity contour over the throttle body at final time step is shown below:

Conclusions:

1. The throttle plate and the throttle shaft provide restriction to the flow of intake air.
2. Lesser is the restriction across the flow, lower is the loss of energy. This increases the velocity at the outlet of throttle body. Thus it can be seen from the above contours at different time step showing velocity changes across the flow.
3. At 0.002 seconds the throttle position is at 25 degrees. thus due to the sudden restriction in flow the velocity decreases.
4. Since the flow area is minimized the flow velocity decreases but after the flow passes over the throttle the velocity reaches its highest point due to the sudden expansion of area.
5. The plot is similar to that of the mass flow rate. The velocity fluctuates at the beginning due to the obstruction in the flow after which it increases constantly.

Stream tracer view:

 link for youtube video : https://youtu.be/fFSeAYO5BcI

Pressure plots and contours:

Pressure contour over the entire region at 0.002 second is shown below:

Pressure contour over the throttle body at 0.002 second is shown below:

Pressure contour over the entire region at 0.004 second is shown below:

Pressure contour over the throttle body at 0.004 second is shown below:

Pressure contour over the entire region at final time step is shown below:

Pressure contour over the throttle body at final time step is shown below:

1. Initially when the throttle is at 25 degrees there is maximum obstruction to the fluid flow so the pressure increases to its highest point.

2.  The Total inlet pressure is as observed is as set in our case ie 150000 Pa . The static pressure drops at the inlet with a significant dip at around 0.002 where the throttle rotates  and gets into normal curve after that as the throttle gets back to its place .

3. At the Outlet the total pressure increases and the static pressure remains same as set in the case set up ie 100000 Pa.

Stream tracer view:

Link for youtube video : https://youtu.be/MUdmmKzTw-M

Conclusions:

1. The mass flow rate initially take a little turbulence and finally converges as expected . It can be clearly seen from the graph that the law of conservation of the mass is followed as the inflow mass flow rate is balanced by the outflow mass flow rate. It can be seen that initially due to the movement of throttle mass flow rate fluctuations at that time period are high. From 0.002 to 0.004 the fluctuations decreases and also approximately after 8 milliseconds the mass flow rate converges.

2. The throttle plate and the throttle shaft provide restriction to the flow of intake air. Lesser is the restriction across the flow, lower is the loss of energy. This increases the velocity at the outlet of throttle body.Since the flow area is minimized the flow velocity decreases but after the flow passes over the throttle the velocity reaches its highest point due to the sudden expansion of area.

3. The Total inlet pressure is as observed is as set in our case ie 150000 Pa . The static pressure drops at the inlet with a significant dip at around 0.002 where the throttle rotates  and gets into normal curve after that as the throttle gets back to its place .

4. At the Outlet the total pressure increases and the static pressure remains same as set in the case set up ie 100000 Pa.

5. Simulation time is much more as compared to the steady State Simulation .