Steady state simulation of air 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: Pressure based steady solver.
   4. Simulation time parameters: 
      1. Number of cycles: 15000
      2. Initial and minimum time step: 1e-9
      3. 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.

            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: Three cases were simulated in total that had different base mesh grid(all dimensions in m):

• 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 1147.473564 seconds.

Summary of total time for:
load balance = 0.17 seconds ( 0.02%)
solving transport equations = 877.85 seconds (76.50%)
move surface and update grid = 22.67 seconds ( 1.98%)
update boundary conditions = 83.16 seconds ( 7.25%)
combustion = 0.01 seconds ( 0.00%)
spray = 0.01 seconds ( 0.00%)
writing output files = 122.25 seconds (10.65%)

3) Post-processing:

• Meshing: Mesh is generated for the given file as shown below.

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 is shown below:

Cell Counts:

The flow is simulated on four processors in parallel and the cell count used by all four processors is shown below:

cell count in rank 1 is: 3231
cell count in rank 3 is: 2807
cell count in rank 2 is: 2573
cell count in rank 0 is: 2277

Plots and conclusions:

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 takes around 3000 cycles approximately for the mass flow rate to get converged.

Velocity profile and contour:

velocity contour over the entire region is shown below:

velocity contour over the throttle body is shown below:

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.

Pressure plots and contours:

pressure contour over entire region:

velocity contour over the throttle body is shown below:

Throat region has the minimum area available for the air to flow. In order to reduce energy loss, the flow gets accelerated at the throat. Hence, the pressure drop at
throat is minimum due to the maximum velocity.

The static pressure at the start drops due to the increase in velocity, as per conservation law and then gets stable as the simulation proceeds and converges around 4500 cycles. And total pressure increases at the outlet and converges around 4500 cycles.

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 takes around 3000 cycles approximately for the mass flow rate to get converged.

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.

3. Throat region has the minimum area available for the air to flow. In order to reduce energy loss, the flow gets accelerated at the throat. Hence, the pressure drop at
throat is minimum due to the maximum velocity.

4. The static pressure at the start drops due to the increase in velocity, as per conservation law and then gets stable as the simulation proceeds and converges around 4500 cycles. And total pressure increases at the outlet and converges around 4500 cycles.