Mixing efficiency calculation of mixing tee for various lengths

Mixing efficiency calculation of mixing tee for various lengths

Aim:

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

Use the k-epsilon and k-omega SST model for the first case and based on your judgment use the more suitable model for further cases. Giving the reason for choosing a suitable model is compulsory.

Procedure:

1.The given two models are inputed into ansys workbench

2.Meshing is done with element size = 3mm

3. The boundaries are defined and desiredd velocities are defined at two inlets and 0 gauge pressure given at outlet

4. The solution is run for 200 iterations

5. The results obtained are plotted as per requirment

Results:

Case 1:

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2.
• nodes = 8350
• elements = 39817
• k-epsilon model is used

Since cold inlet velocity is only 6 m/s the proper mixing is not happening within the tee as seen above.

The outlet temperature is 30.3 C.

Case 2:

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 4.
• nodes = 8350
• elements = 39817

Since cold inlet velocity is 12 ms the mixing happens properly and the outlet temperature further drops to 27.5 C.

Case 3:

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2.
• nodes = 1171
• elements = 56556
• k-omega sst model is used.

There is not a big change in outlet temperature in increasing the tee length. The temperature still hovers around 30.5 C.

Case 4:

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 4.
• nodes = 1171
• elements = 56556

Increasing cold inlet velocity has similar result as that of short tee.

Conclusion:

Cell dependency does not have much effect for element size of 3mm and 1mm as the outlet temperature and velocity changes by only a small decimal.

The length of the tee has no effect on the average outlet temperature proving ineffectiveness in increasing the tee length.

The increase in cold inlet velocity has a drastic change in the outlet temperature as the temperature drops by 4 C when momentum ratio is increased from 2 to 4.

k-epsilon and k-omega models did not give much change in the output values but computation time for k-omega sst model seems to be taking more time to converge compared to k-omega model.