Week 1- Mixing Tee

OBJECTIVE –

To simulate the flow of air through mixing tee and understand the effect of length of pipe and momentum ratio of velocity for mixing of air by using two different types of pipes i.e., short pipe and long pipe.

THEORY –

In industrial process engineering, mixing is a unit operation that involves the manipulation of a heterogeneous physical system with the intent to make it more homogeneous. Mixing is performed to allow heat and/or mass transfer to occur between one or more streams, components or phases. Modern industrial processing almost always involves some form of mixing. A mixing tee is a type of mixing mechanism. Mixing Tees utilizes a specifically engineered internal geometry to efficiently mix two fluid streams into one combined stream. Mixing tees are widely used in the petrochemical, HVAC industry etc. in which two fluid streams with different physical and/or chemical properties mix together. Hot air flowing in the main pipe is mixed with cold air flowing through a tee. The standard deviation of temperature is computed to quantify the degree of mixing. The velocity and temperature fields are also computed. The effects of the mesh size, turbulence model on results were examined.

So, to simulate the flow in a mixing tee, we have set up two mixing tees of different lengths.

Case 1

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4.

Case 2

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4.

Where Momentum ratio = velocity at cold inlet/velocity at hot inlet.

To simulate the above cases we're going to use ANSYS FLUENT.

FLUENT SETUP -

Open Ansys Workbench. Drag and drop Fluid Flow (Fluent) onto Project Schematic.

Here, we will see Steps we need to follow to solve a particular engineering system from a CFD standpoint.

1. Geometry: SpaceClaim, where we import the geometry, volume extraction and suppression for physics will be done.
   
   
2. Mesh: Ansys Mesh, where Boundary naming, element sizing and mesh generation with required grid size will be done.
   
   
3. Setup: Ansys Fluent, where actual case setup will be done.
   
   
4. Solution: Ansys Fluent, where runtime results visualization is done.
   
   
5. Results: CFD-Post, where Post Processing of Results will be done.

CAD Clean up -

The CAD model is imported to the SpaceClaim and since the model is a 3D model with thickness, we'll first extract only flow volume area by suppressing everything except the required volume region as shown below.

3D Model -

Flow Volume Model -

Mesh -

In this section, we name the inlet and outlet section using the name selection option and then the mesh is generated.

Setup -

In this section, we apply different boundary conditions like inlet, outlet boundary conditions. Turbulence models are set up like K-epsilon or K-omega for solving the turbulent mixing of air. The number of iterations is set in this section.

CFD-Post -

Post-processing is carried in this section in which different contours are set up to visualize the properties (such as Temperature, Velocity, etc.) for which simulation is run.

CASE 1 - SHORT TEE

a.) For inlet velocity 3m/s, momentum ratio - 2

A mesh element metric that identifies the quality of the mesh is employed and it can be seen that the elements of the lowest quality employed are minimal. Most of the elements have quality in the range of 0.7 to 1. Hence the mesh is of acceptable quality.

Mesh Size = 0.002m

Mesh Count = 1,05,876

Residual Plot -

Standard-Deviation -

Temperature Plot -

CONTOURS -

1.) Temperature -

2.) Velocity -

b.) For inlet velocity 3m/s, momentum ratio - 4

Mesh is the same as in the case above

Residual Plot -

Standard-Deviation -

Temperature Plot -

CONTOURS -

1.) Temperature -

2.) Velocity -

The model we have used above for flow simulation is k-epsilon model and as the part of this challenge we have to solve any one of the above cases using the k-omega(SST) model and then compare the result to k-epsilon model.

Short tee probmen using k-omega(SST) model -

For inlet velocity 3m/s, momentum ratio - 2

Since we only need to change the solver in setup part we don't have to make any changes to the geometry and the mesh part, they are same as explained above.

Residual Plot -

Standard-Deviation -

Temperature Plot -

CONTOURS -

1.) Temperature -

2.) Velocity -

Comparrison between k-epsilon and k-omega(SST) model -

Since for this challenge we're only concerned by the output temperature and not the temperature near wall or anywhere else, its better to use k-epsilon model as compared to k-omegs(SST) model.

As in the case of k-omega(SST) model the model is useful when we want accurate near wall temperature and are not concerned with the final output. k-omega(SST) model uses the concept of y+ to accurately calculate the near wall temperature but as a result takes more time/iterations than k-epsilon model and it also compromise on the far away temperature (output temperature) which is of great concern for this challenge and hence we'll use the k-epsilon model for the remaining cases.

CASE 2 - LONG TEE

a.) For inlet velocity 3m/s, momentum ratio - 2

Mesh quality graph-

Mesh Size = 0.002m

Mesh Count = 1,40,025

Residual Plot -

Standard-Deviation -

Temperature Plot -

CONTOURS -

1.) Temperature -

2.) Velocity -

b.) For inlet velocity 3m/s, momentum ratio - 4

Mesh is the same as in the case above

Residual Plot -

Standard-Deviation -

Temperature Plot -

CONTOURS -

1.) Temperature -

2.) Velocity -

Mesh Independece Study -

In order to verify that the mesh we have used gives correct result, we'll change the element size of the mesh and will perform the same simulation using the new mesh to see if there are any changes in the result. If there is no change in the result than the solution we got is independent of the meash and hence we can trust our result.

CASE - Short tee with momentum ratio = 2

New Mesh -

Mesh Size - 0.005 m

Mesh Count - 14247

Temperature Contour -

As we can see from the above temperature contour the outlet temperature for this case is same as the above case and hence we can say that our result is grid/mesh independent.

Conclusion -

• For the temperature plot, the inlet-x temp. is kept 36⁰C and the cold temp. is kept being 19⁰C and its intersection we see 2 different layers of temperature which makes an average of these two extreme temperatures.
  
  
• As the cold air velocity increases from the inlet y, the outlet air temperature obtained is cooler.
  
  
• For the velocity profile plot as the hot and cold inlet velocity increases, the mixing efficiency increases and we get more uniform air at the outlet.

• The temperature contour at the outlet for short tee cases shows that some of the high temperatures flow sips through without properly mixing with the flow. Whereas, in the temperature contour of long tee cases, the high temperatures flow thoroughly mixes with the rest of the flow.

• Therefore, we can recommend using the long tee and high-velocity inlet for application. This is because we have a low standard deviation in case of long tee compared to the short tee for both momentum ratios.