Week 1- Mixing Tee

AIM:

To perform simulation of flow of air through mixing tee at different temperatures and velocities and understand the effect of length of pipe and momentum ratio of velocity for mixing of air by using a short and a long pipe. This is widely used in HVAC applications.

OBJECTIVES:

1. Velocity and temperature contour plots on the cut planes along and across the pipe.
2. Velocity and temperature line plots along and across the length of the pipe.
3. Perform Mesh independent study for any one case.
4. Discuss the effect of length and momentum ratio on results.

Procedure:

• Setting up Geometry:

The CAD model is imported into the SpaceClaim to extract the flow volume field from the model. We don’t need to perform the simulation on the whole geometry of the model, so we can extract the specific volume from the model on which the simulation is performed.

The 2^nd image shows the extracted volume.

• Meshing:

In this section we close the geometry section in ansys workbench, open meshing. Now, create a mesh around the volume with a size of 0.002m. Finer the mesh, better is the quality of the solution.

This mesh has most of the element metrics in the range of .7-1, which means the mesh is of acceptable quality.

• Naming the faces

Once the meshing is done name the faces according to their position and function, this will help you when you are setting up the simulation in fluent.

• Set-Up

In this step, we have to configure the variables such as velocities, temperatures, etc according to each case for the simulation.

Regarding the type of t=model we are going to be using for solution, let’s choose the k-epsilon model over k-omega SST. The omega model gives us better convergence but we have to know to the fact that convergence doesn’t signify accuracy. In this model, we have to study the conditions far from the wall because of which it is better to go with the realizable K-epsilon model.

Case 1A:

For inlet velocity of 3m/s and momentum ratio of 2.

Inlet-x conditions; 3m/s and 36C.

Inlet-y conditions; 6m/s and 19C.

Plots and graphs;

Case 1B:

For inlet velocity of 3m/s and momentum ratio of 4.

Inlet-x conditions; 3m/s and 36C.

Inlet-y conditions; 12m/s and 19C.

Plots and graphs;

Case 2A: Long Mixing Tee (same meshing conditions)

For inlet velocity of 3m/s and momentum ratio of 2.

Inlet-x conditions; 3m/s and 36C.

Inlet-y conditions; 6m/s and 19C.

Plots and graphs;

Case 2B: Long Mixing Tee (same meshing conditions)

For inlet velocity of 3m/s and momentum ratio of 4.

Inlet-x conditions; 3m/s and 36C.

Inlet-y conditions; 12m/s and 19C.

Plots and graphs;

Mesh Independent Study:

We conduct mesh independence studies in CFD to make sure that the results we get are due to the boundary conditions and physics used, not the mesh resolution. So if the results do not change with mesh density, we have achieved mesh independence. Let’s change the mesh to 0.0015 in 1B case. (facing license issues when reducing to 0.001)

Results:

Case Avg Temp

1A 30.25C

1B 27.48C

2A 30.45

2B 27.5

Conclusions:

1. When comparing the long and short mixing tee with momentum ratio 2, the temperature distribution is not even in the short tee. The temperature distribution is more consistent in the long tee.
2. When comparing the long and short mixing tee with momentum ratio 4, the temperature distribution isn’t even in the short tee. The temperature distribution is more consistent in the long tee.
3. It’s pretty evident from the results that change the length of the mixing tee doesn’t affect the results much but increasing the momentum ratio shows better mixing results and lowers the outlet temperature.