Flow through a throttle body (Steady State)
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 a 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.
The 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 a 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 the 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 at 0 degrees representing the completely open configuration. For the steady-state analysis, the valve is not moved at all. 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: stationary
Velocity boundary condition: Law of Wall
Temperature Boundary Condition: 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 simulations is 41,572 out of which 10,345 cells are contributed by the 1st Processor; 10,843 cells are contributed by the 2nd Processor; 10,129 cells are contributed by the 3rd Processor and around 10,255 cells are contributed by 4th Processor.
Mass Flow Rate:
The inlet and the outlet mass flow rate are both equal at a magnitude of 0.06572 kg/s with an imbalance of only 7 x 10^-8 Which indicates a good degree of mass conservation by the solver.
Pressure:
The inlet pressure is higher than the outlet pressure which is expected because fluid flows from a high-pressure region to the low-pressure region. There is a high-pressure region along the tip of the throttle where the flow first hits it indicating a stagnation point for the flow. There is also a very high-pressure drop near the tip of the throttle because of the tiny gap between the valve and the pipe wall.
Velocity:
The velocity contour and the vectors clearly show the separated flow behind the throttle valve and the high-velocity flow in the tiny gap between the valve and the pipe wall.