Mixing efficiency of mixing tee

In the following project 3 different variations of the mixing tee are simulated and compared. Ansys spaceclaim utility was used to clean up the geometry. And ansys fluent to process the simulation.

Following are the conditions used for the simulation.

1. Element size is 0.002mm
2. Hot inlet temperature is 300K and cold inlet temperature is 280K
3. Convergence criterion used is stop criterion of 0.001 of standard deviation of temperature.

CASE-1 Short mixing tee with inlet velocity of 3m/s and momentum ratio of 2.

Mesh Profile:

Velocity Plot:

Temperature Plot:

Area weighted average plot:

Standard Deviation:

Residuals Plot

CASE-2 Long mixing tee with inlet velocity of 3m/s and momentum ratio of 2.

Mesh Profile:

Velocity Plot:

Temperature Plot:

Area weighted average plot:

Standard Deviation:

Residuals Plot :

CASE-3 Short mixing tee with inlet velocity of 3m/s and momentum ratio of 4.

Velocity Plot:

Temperature Plot:

Area weighted average plot:

Standard Deviation:

Residuals Plot:

Observations made from above simulations are as follows:

1. Formation of boundary layer is clearly observed near the walls of the mixing tee due to no slip condition for all three simulations.

2. For the CASE-1 Short mixing tee with inlet velocity of 3m/s and momentum ratio of 2, it is clearly observed that temperature distribution at the outlet is not uniform and some hot air presists due to unven mixing.

3.For the CASE-2 Long mixing tee with inlet velocity of 3m/s and momentum ratio of 2, the mixing of air is more uniform than short one and same average temperature at outlet is observed.

4. For the CASE-3 Short mixing tee with inlet velocity of 3m/s and momentum ratio of 4, outlet temperature obtained is reduced due to increase in velocity of cold inlet air. While temperature distribution at the outlet is unven.

5. Three simulations are compared for different values of cell count, average outlet temperature and no. of iterations required for the convergance.

CASE	Cell Count	No. of Iterations required to Converge	Average Outlet Temperature (kelvin)
1 – Short Tee with inlet velocity 3m/s and momentum ratio of 2.	106916 tetrahedral cells	134	293.2
2 - Long Tee with inlet velocity 3m/s and momentum ratio of 2.	141477 tetrahedral cells	270	293.3
3 – Short Tee with inlet velocity 3m/s and momentum ratio of 4	106916 tetrahedral cells	221	290

Conclusion:

For uniform mixing of air long tee is recommended, whereas outlet temperature can be reduced by increasing the velocity of cold inlet air while keeping the length of mixing tee constant.