Aim :-
For this challenge, we have created two versions of the mixing tee. One of them is longer than the other.
Your job is 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.
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.
Momentum ratio = velocity at cold inlet / velocity at hot inlet
In a table, compare the following values from the 2 cases.
Cell count
Average outlet temperature
Number of iterations for convergence
Expected results:
Velocity and temperature contour plots on the cut planes along and across the pipe.
Velocity and temperature line plots along and across the length of the pipe.
Perform Mesh independent study for any one case.
Discuss the effect of length and momentum ratio on results.
For each simulation, you need to show the convergence plot
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.
BASELINE SIMULATION-
For inlet velocity of 3m/s and momentum ratio of 2
MESH:
Residual plot:
Standard Deviation:
Temperature:
Velocity:
Contours-
1) Temperature:
Velocity:
As seen from the above contour pictures, the flow field of air in velocity and temperature is diffused and not adequately picturised due to coarse mesh. The number of elements is 12,760 which is less to capture the physics properly. Therefore, a finer mesh is set up for further simulation.
CASE 1- SHORT TEE
a) For inlet velocity of 3m/s and momentum ratio of 2
Mesh size- 0.001m
A mesh element metric that identifies the quality of mesh was employed and it can be seen that the elements of the lowest quality are minimal. Most of the elements have quality in the range of 0.7-1. Hence, the mesh is of acceptable quality.
Residual:
Standard-deviation:
Temperature plot:
Contour-
1) Temperature:
2) Velocity:
b) For inlet velocity of 3m/s and momentum ratio of 4
Mesh size- 0.001 m (mesh remains the same as the above case)
Residual plot:
Standard Deviation:
Temperature plot:
Velocity plot:
Contour-
1) Temperature:
2) Velocity:
CASE 2- LONG TEE
a) For inlet velocity of 3m/s and momentum ratio of 2
Mesh size- 0.001 m
A mesh element metric that identifies the quality of mesh was employed and it can be seen that the elements of the lowest quality are minimal. Most of the elements have quality in the range of 0.7-1. Hence, the mesh is of acceptable quality.
Mesh: The number of elements is around 1,40,000.
Residual:
Standard deviation:
Temperature plot:
Velocity plot:
Contour-
1) Temperature:
2) Velocity:
b) For inlet velocity of 3m/s and momentum ratio of 4
Mesh size- 0.002 m
Mesh- It is same as the above case.
Residual:
Standard Deviation:
Temperature plot:
Velocity:
Contour-
1) Temperature:
2) Velocity:
Effect of Turbulence model:
K-
ω
SST model used for carrying out this experiment is Short TEE. Mesh remains the same as the Short tee mesh picture is visualized. The inlet velocity is 3 m/s and momentum ration is 2.
Residual:
Standard Deviation:
Temperature Plot:
Velocity plot:
Contour-
1) Temperature:
2) Velocity:
CONCLUSION:
As seen in the Case-1-a and Case-1-b, the temperature at the outlet drops down significantly due to high velocity of cold air from the inlet. This is possible due to the turbulent mixing at the tee joint between hot air and cold air because of the high velocity of cold air., which is not possible in case of low-velocity cold air.
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.
k- epsilon model predicts well far from the boundaries (wall) and k- omega model predicts well near the wall. Even though it depends on Y+. Therefore, K-epsilon model is majorly used for free flow away from the wall and as in this project, there is no dealing with the wall of the mixing tee for change in temperature along the length of it, so K-epsilon model was chosen for further simulations.
