Week 4.2: Project - Transient simulation of flow over a throttle body

Objective: To simulate (Transient) the flow of fluid over the throttle body in a converge studio and post process the results.

1. Show the pressure and velocity contours.
2. Show the mesh (surface with edges).
3. Show the plots for pressure, velocity, mass flow rate and total cell count.
4. show the calculations on how you calculated an end time for the simulation.
5. Create an animation in which it throttle movement should be visible.

Basic of throttle valve:

Defnition of throttle valve

A valve designed to regulate the supply of a fluid (as steam or gas and air) to an engine and operated by a handwheel, a lever, or automatically by a governor. Especially : the valve in an internal-combustion engine incorporated in or just outside the carburetor and controlling the volume of vaporized fuel charge delivered to the cylinders.

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. What is often termed a throttle (in an aviation context) is also called a thrust lever, particularly for jet engine powered aircraft. For a steam locomotive, the valve which controls the steam is known as the regulator.

In an internal combustion engine, the throttle is a means of controlling an engine's power by regulating the amount of fuel or air entering the engine. In a motor vehicle the control used by the driver to regulate power is sometimes called the throttle, accelerator, or gas pedal. For a gasoline engine, the throttle most commonly regulates the amount of air and fuel allowed to enter the engine. Recently, for a GDI engine, the throttle regulates the amount of air allowed to enter the engine. The throttle of a diesel, when present, regulates the air flow into the engine.

To regulate the power output from an engine there have to be some restriction of how much fuel - air mixture that reaches the cylinders. One way to do this is to use some kind of valve that restricts the air, fuel is then regulated to get the correct fuel - air mixture. The most common valve is of butterfly type and is used in almost every car engine manufactured today, the valve is sketched in the below figure

Most of the spark-ignition internal combustion engines operate with a stoichiometric air-gasoline ratio since it enables the catalyst to control emissions level. To control engine power, a throttle valve is used. The objective is to control air and consequently gasoline mass flow.

Valves that are utilized as fluid control devices are typically “throttling valves".

Such valves experience internal velocity and internal pressure gradients (both positive and negative) that conclude with a permanent pressure loss (∆P) from the inlet pipe-to-outlet pipe connections.

Geometry preparation:

• Open the new project and load the stl file of geometry.

• Add the boundaries by clicking create multiple boubdaries to create all boundaries at a once and name them like Elbow_wall, Throttle_valve, top, inlet and outlet by selecting the respective triangles.

• Then click Normal Toggle option and click any one of the triangle which shows the normal that is opposite to the direction of fluid flow and click Apply.
• To ensure that the geometry is clean, click Diagnosis tab and click on Find.
• All the parameters that got checked shows the green colour tick mark.
• Case-setup
• In order to begin the case-setup, click on the case-setup and click begin case-setup and click Time based, since it is not a crank angle based like IC engines or Pumps but a steady-state channel flow setup.
• Then click on Materials and select Air to be predefined mixtures.
• Then open the Gas simulation and Global transport parameters and retain the default setting. And also retain the same setting for the species and Reaction mechanism.
• Then click on the Run Parameter under the Simulation Parameter and choose the transient solver under the solver tab and then choose the Full Hydrodynmic under the simulation mode. And then choose the Compressible under the Gas flow solver.
• we need to calculate the end time of simulation based on the following parameters.

1. Velocity at inlet
2. Characteristics length of domain through which fluid flow going to happen.
3. Number of passes of fluid through the domain in order to reach convergence

To know the dimensions of a domain, mark tick on the geometry bounding box from the options menu of geometry Dock. ideally we should know the complete length of the elbow.

Characteristics length = 0.2m (Approx)

Avg.velocity at inlet = 191 m/s (Approx)

Time to pass a fluid through the elbow = (characteristics length/Avg.velocity)

T = (0.2/191)= 0.00104 sec (for one time pass)

To make a pass of fluid 10 times through elbow

T = (0.00104)*(10) = 0.01 ms (Approx)

• In the simulation time parameter do the following changes.

• In the solver time parameter, we only choose the pressure based solver and retain the remaining setting.

• At the inlet boundary we specify the inlet pressure 150000 Pa and select the predefined mixture as a Air which contains N2 as a 77% and O2 as 23%. And select the temperature to be specified of 300K.

• At the outlet boundary, we specify the 100000 Pa pressure.

• At the Elbow and Throttle valve , we are taken the wall boundary condition.

As we are interested in rorating the throttle since it is a transient problem. This will be done in the following way

Boundary type: Wall

Wall motion type: Rotating

Surface Movement: fixed/Moving

Rotation center and Rotation about(axis) will be evaluated by:

Goto Measure `rightarrow` Direction `rightarrow` Arc Normal `rightarrow` select any 3 vertices along arc `rightarrow` Apply

OR

Goto Measure `rightarrow` Direction `rightarrow` Triangle Normal `rightarrow` Select any Triangle `rightarrow` Apply

In the message log results we will get as

Arc center as Rotating center

Arc Normal(Cell Normal) as Rotating about (axis)

Arc Diameter as Cylinder Diameter

Now we have to provide the rotation rate to the throttle.

Throttle profile info:

• From 0 to 2 ms the valve should rotate by 25 degrees.
• From 2 to 4 ms it should stay at 25 degrees.
• After which 8ms the valve should get back to its original position.

• In Regions and Initializations, click Add as Region 0.

• Goto Physical Models and check the Turbulence Modelling and choose the RNG/k-epsilon model.
• Then in the grid control, the mesh size that want to be created is in meters.
• Output/Post processing:

Time inteval to write 3D output files = (running simulation time)/(Required no of output files)

t = 0.01/10 = 0.001 sec.

• Now when we click on validate all options for validating, we got the errors.
• To Resolve these errors, we go to Regions and initializations and rename the region 0 to Voulmetric region user defined and then go Boundary to assign this region to all the boundaries we have ceated since the error is not assigning the region for those.

• And then click on validate all again and there will be no errors and the validation has been successfully complete.
• At the end all the inputs files have to be exported:

File -->Export-->Export input files

Choice of grid scale in converge

Use fixed embedding to refine the grid at specific locations in the domain where a finer resolution is critical to the accuracy of the solution. For example, when simulating sprays, you can add an area of fixed embedding by the nozzle to resolve the complex flow behavior. Fixed embedding allows the rest of the grid to remain coarse to minimize simulation time. For each fixed embedding, you must specify an embedding scale that indicates how CONVERGE will refine the grid in that location. The embedding > embed_scale parameter in embedded.in, which must be a positive integer, scales the base grid sizes according to

`embed = frac{base grid}{2^(embed scal}`

Mesh

The mesh was refined locally near the throttle(empty circle) using fixed embedding to obtain more accurate results.

Mass flow Rate

• Mass flow rate is defined as the mass of substance which passes per unit time.
• In the initial valve is in opening position which we can easily observe in the above figure that in the start flow is going smooth after 0.001S mass flow rate is going to increase which is 0.06 and after that it is going to decrease. At a time 0.002S valve is in 25 degree position means we can say that valve is in closing position that's why mass flow rate is constant, there is no increase in mass flow rate. At a 0.004S throttle valve is again starting to open,we observe there is a little increase occuring in mass flow rate. And at a 0.008S mass flow rate is again going to smooth because at that position throttle vlave is again open.
• Then Finally at the inlet the value of the mass flow rate is -0.065 (kg/s) and at the outlet the value of mass flow rate is 0.065 (kg/s).
• At the inlet negative sign shows that the mass is going to inside and the outlet positive sign shows that the mass is going to outside.

Velocity

• In the initial valve is in opening position which we can easily observe in the above figure that in the start velocity goes smooth after 0.001S velocity is going to decrease. At a time 0.002S valve is in 25 degree position means we can say that valve is in closing position that's why velocity is constant, there is no increase in velocity. At a 0.004S throttle valve is again starting to open,we observe there is a little increase occuring in velocity. And at a 0.008S velocity going to smooth because at that position throttle vlave is again open.
• At inlet the velocity is decreased and reaches to -199 m/s and at the outlet the velocity is increased and reaches to 220 m/s.
• It occurs since the pressure is inversely proportional to the velocity.

Pressure

• At the inlet plot of pressure, we observe that initially total and static pressure is same which is 150,000 Pa. And we also know that total pressure = static pressure+dynamic pressure and initially dynamic pressure is zero.
• As the time passes the static pressure is going to decrease due to increase in dynamic pressure.
• After 0.001S the static pressure is going to decrease rapidly due to fastly movement of fluid.

• Finally at the outlet the static pressure reaches to 136,000 Pa and the value of the total pressure is increases due to increase in dynamic pressure.

Total pressure

Static pressure

Total cells

• Above figure represent the total cell count.
• And the major reason behind getting this irregular structure in the plots is the rotation of throttle. Since we rotate the valve to 25 degrees fom 2 to 4 ms total cell count has a specified value when throttle valve starts rotating fixed embedding helps us to provide the required no of elements to capture data throughout rotation process then the total cells keeps going on changing
• In the initial valve is in opening position which we can easily observe in the above figure that in the start total cell count is 71,000 after 0.0004S cell count is going to increase. At a time 0.002S valve is in 25 degree position means we can say that valve is in closing position that's why cell count is constant. At a 0.004S throttle valve is again starting to open,we observe that there is a little increase occuring in cell count and at after 0.005 there is a decrease occuring in cell count. And after 0.008S to onwad total cell count is again going to smooth because at that position throttle vlave is again open.

Pressure Contours

Observation from plot

In the above plot we can easily observe the two regions which we can see

• Red region shows the high pressure which is between 1.4 to 1.5 bar.
• And blue region shows the low pressure. The pressure drop because the flow is constricted.

• On the throttle, we also observe the two region which is red and green. Red colour shows the high pressure region and green colour shows the low pressure region. In high pressure region fluid impose the pressure on the throttle as it flows over the throttle body.
• And the low pressure region(green region) is due to the flow separation happening near the axis of rotation of the throttle.

Velocity Contours

Observation from plot

In the above plot we can easily observe the three regions which we can see

• Top left blue colour region shows that the velocity decreases as it is near about about the throttle.
• Red region shows the high velocity which is approximately 310 m/s. This is the narrow path where the fluid passses and that's why velocity is increases.
• And bottom right blue region shows the recirculation region. Flow separation is also occuring here, eddies are formed here and velocity drops as low as 20 m/s.

On the throttle, we also observe the two region which is red and blue. Red colour shows the high velocity region and blue colour shows the low velocity region.

Velocity animation

Pressure animation

Conclusion

The following different characteristics and observations are concluded from the above results.

• The throttle valve provide the 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 the throttle body.
• The region of recirculating flow, formed downstream of the throttle shaft is known as wake region. The recirculation of air is caused by the viscous airflow around the throttle plate surface. The medium is displaced when it leaves the surface of throttle shaft, resulting in turbulent eddies being formed. The presence of eddies make the flow through the wake region turbulent and hence, it should be minimum.
• There is a region of adverse pressure gradient formed due to excessive loss of momentum near the flow contracting surface. Due to the adverse pressure gradient, the flow near the surface of a body is decelerated. Thus, this pressure gradient precludes the flow from progressing downstream past a certain point, called the point of separation. At this point, the flow becomes separated from the contacting surface of the throttle plate. The position of the point of separation is also an important factor to be considered while analyzing the pressure contours.
• The stagnation point is a region formed in the airflow field where the local velocity of air approaches zero. In the throttle flow, the stagnation point is formed at the tip of the leading edge of the throttle plate where the air is brought to rest due to sudden obstruction inflow. In this region, the static pressure increases as the velocity drops.