Transient flow simulation through a throttle body
ABSTRACT
Computational Fluid Dynamics (CFD) is a branch of fluid mechanics that depends on various numerical analysis and data structures that are used for finding solutions and analyzing the complex problems that concern fluid flow. The objective of the current project is to determine the variation of flow parameters like pressure, temperature, velocity and density across the throttle body using CFD. In the current work, the study of flow through the throttle body is carried out by using the finite volume method. The modeling, discretization and analysis are carried out in CONVERGE Studios.
INTRODUCTION
A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term throttle has come to refer, informally, to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal.
Performance of a spark engine mainly depends on three variables i.e. air intake, fuel injection and spark ignition. The air intake system delivers oxygenated air into the combustion chamber of an engine, where fuel injection and timely spark occurs to generate power for the vehicle. A proper air intake system delivers the required amount of clean air into the engine as per the engine load, to deliver power and better gas mileage during operation. The design of air intake system requires optimization of the airflow, reduction of inertia of moving parts to improve response and reduction of losses. For ideal airflow, the intake system should increase the velocity of the air until it travels into the combustion chamber while minimizing turbulence and restriction to flow.
CFD
There are 3 stages involved in CFD modeling. The first is the preprocessing stage, which involves defining the problem through creating the geometry and setting up the grid or mesh and defining the physics of the problem (including the boundary conditions and the turbulence model). The second stage is the application of the solver whereby the software uses the information provided in the pre-processing stage to generate the numerical results. Finally, post-processing of results takes place where the numerical results are interpreted, visualized and analyzed.
This report is primarily concerned with observing the flow through the closed conduit and performing the grid dependence test and comparing the results in each case.
The work detailed in this report was conducted using CONVERGE Studios, which generated the input files for the given problem statement. The post-processing is done using the Paraview software.
COMPUTATIONAL GEOMETRY AND MESH:
The following figure shows the computational geometry utilized for this simulation and the different boundaries flagged for applying appropriate boundary conditions.
The throttle valve is set initially at 0 degrees representing the completely open configuration. The throttle valve is assigned a rotation to go from 0 to 25 degrees and is kept stationary at 25 degrees for the rest of the simulation.
A computational mesh with a base size of 2mm is generated over the domain. In order to refine the mesh around the throttle valve, a permanent embedding of 2 layers with a scale of 3 is defined around that region, the following figures show the computational mesh along the section plane sliced in between the domain and zoomed in near the throttle valve.
SETUP OF THE FLOW PHYSICS:
The solver is a pressure-based steady-state solver for a general flow problem. The different surfaces and the boundary conditions applied are listed below:
Inlet:
Boundary Type: INFLOW
Specified Total Pressure: 1.5 bar
Material: Air
Temperature: 300K
Outlet:
Boundary Type: OUTFLOW
Specified Total Pressure: 1 bar
Material: Air
Backflow Temperature: 300K
Pipe Wall:
Boundary Type: WALL
Wall Motion: Stationary
Velocity Boundary Conditions: Law of wall
Temperature Boundary Condition: Law of Wall
Throttle:
Boundary type: wall
Wall motion: Rotating (About the axis passing through the center if the shaft of the throttle valve)
Time (sec) Angle
0.0 0
0.002 25
0.01 25
Velocity Boundary Conditions: Law of Wall
Temperature Boundary Conditions: Law of Wall
A volumetric region is generated to simulate the fluid domain and is initialized with a pressure of 101325 Pa (1 atm) and a temperature of 300K. the RNG k- epsilon model is used to model the turbulence in the flow. The simulation is run in parallel using 4 processors for a total of 15000 cycles with an initial time-step of 1 x 10^-9 sec and a maximum time step of 1 sec to ensure convergence to the steady-state simulation.
RESULT AND DISCUSSION:
The plots shown below represents the convergence of the solution to a steady-state value. The “ bound_id_2” refers to the ‘Inlet” boundary and the “Bound_id_3” refers to the “Outlet” boundary.
Cell Count
The total number of cells generated during the simulation starts 41454 and increases over the successive time-steps to 45201 cells because if the moving throttle valve.
Mass flow rate
The inlet and the outlet mass flow rate are both equal at a magnitude of 4.47 x 10^-2 kg/s with an imbalance of only 6.7 x 10^-5 kg/s which indicates a good degree of mass conservation by the solver.
Pressure:
The inlet pressure id higher than the outlet pressure which is expected because fluid flows from a high-pressure region to the low-pressure region. The pressure upstream of the throttle valve keeps on increasing as the throttle valve blocks more and more volume of the flow passing through.
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Velocity
The velocity contour and the vectors clearly show the separated flow behind the throttle valve and the recirculation region just downstream of the valve due to its rotating motion.
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Streamlines and Surface Pressure:
The recirculation of the flow is more clearly visible through the plotted stream lines.
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