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 pipe i.e. short pipe and long pipe.

About:

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. 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.

Resference :https://en.wikipedia.org/wiki/Mixing_(process_engineering)

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.

For this, we have created two versions of the mixing tee. One of them is longer than the other.

It is set up as steady-state simulations to compare the mixing effectiveness when hot inlet temperature is 36⁰C & the Cold inlet is at 19⁰C.

1. Case 1
   • Short mixing tee with a hot inlet velocity of 3m/s.
   • Momentum ratio of 2.
   • K-epsilon model
2. Case 1.1
   • Short mixing tee with a hot inlet velocity of 3m/s.
   • Momentum ratio of 2.
   • K-Omega SST model.
3. Case 2

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio 4.
• K-epsilon model.

4. Case 3

• Long mixingtee with a hot inlet velocity of 3m/s.
• Momentum ratio 2,4.
• K-epsilon model.

Momentum ratio is defined as the ratio of velocity at the cold inlet to the velocity at the hot inlet.

CASE setup-

CAD cleanup/Pre-processing- SpaceClaim.

The CAD model is imported into the SpaceClaim to extract the flow volume field from the model. The extracted volume is shown below:

Mesh

The boundary names are applied in this section and a 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 setup to visualize the properties (such as Temperature, velocity etc.) for which simulation is run.

Case 1:

Element quality:

Elements : 104931

Residual Plot

Case 1.1:

Case 2:

Case 3:

Elements:140811

Momentum ratio: 2

Momentum ratio :4

Line plot:

CONCLUSION:

• And as the length of the pipe increases the mixing has a lot of time and increases the temperature losing its main objective.
• In both cases of K-epsilon snd k-omega, k-epsilon has better results as we see in the residual plot these converge faster compared to k-omega
• By doubling the chilled air velocity at the inlet results in better outlet mixing temperature which pretty much an acceptable result for us.