Week 1- Mixing Tee

Mixing Tee

Aim: To perform steady-state simulation by comparing the mixing effectiveness of 2 different models by varying inlet velocity of the cold fluid and provide a suitable model.

Cases :

Case I : Short mixing tee

Case II : Long mixing tee

Boundary conditions :

For both models, the same boundary conditions have been applied.

Boundary condition 1 (Momentum ratio = 2)

Hot inlet Velocity : 3m/s Hot inlet temperature: 39 deg.

Cold inlet velocity : 6m/s Cold inlet teperature : 19 deg.

Boundary condition 2 (Momentum ratio = 4)

Hot inlet Velocity : 3m/s Hot inlet temperature: 39 deg.

Cold inlet velocity : 12m/s Cold inlet teperature : 19 deg.

Geometry :

The geometry for the assignment has been provided as a short mixing tee and a long mixing tee. The Ansys Spaceclaim has been used for geometry cleanup as well as fluid domain extraction.

Meshing :

Meshing has been done using Ansys mesher. Initially, automatic meshing has been generated as per guidance. The refinement and inflation layer has been provided to improve mesh quality. The mesh quality report is shown in the following figure.

Parameter	Case I	Case II	Case III
Element size	2 mm	2 mm	5 mm
Max cell size	25 mm	25 mm	25 mm
Mesh defeaturing size	16 mm	16 mm	16 mm
Node count	17541	50200	16555
Element Count	84468	271225	54221

Setup :

Parameter	Case I	Case II
Turbulent model	K-epsilon	K-Omega
Energy equation	ON	ON
Wall treatment	ON	ON

Solution :

Case I : Baseline simulation

Inlet velocity 3 m/sec and momentum ratio 2

Residual Plot

Standard deviation of temperature and temperature plot

Contour plots

Temperature contour plot

Velocity contour Plot

Case I :

Short mixing tee with inlet velocity 3 m/sec and momentum ratio 2

Residual Plot, Standard deviation of temperature plot, temperature plot, velocity plot

Contour Plots

Temperature and velocity contour

Short tee with inlet velocity 3 m/sec momentum ratio 4

Residual plot, Standard deviation of temperature plot, Temperature plot, Velocity plot

Contour plots

Temperature contour plot and velocity contour plot

Case II :

Long mixing tee

Inlet velocity = 3m/s Momentum ratio = 2

Residual plot, Velocity, Pressure and Temperature plots

Velocity, Pressure, and temperature contour

Long mixing tee

Inlet velocity = 3m/s Momentum ratio = 4

Residual plot, Velocity, Pressure and Temperature plots

Velocity and temperature contour

Long mixing tee

Inlet velocity = 6m/s Momentum ratio = 2

Residual plot, Velocity, Pressure and Temperature plots

Velocity and temperature contour

Case	Cell count	Average outlet temperature (K)	Number of iterations for convergence
Short tee- Momentum ration 2	1,05,751	303.439	192
Short tee- Momentum ration 4	1,05,751	300.699	200-250
Long tee- Momentum ration 2	1,40,510	303.549	250
Long tee- Momentum ration 4	1,40,510	300.639	200-250

CONCLUSION:

1. As seen in Case-1-a and Case-1-b, the temperature at the outlet drops down significantly due to the 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 the case of low-velocity cold air.
2. 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.
3. Therefore, we can recommend using the long tee and high-velocity inlet for application. This is because we have a low standard deviation in the case of a long tee compared to the short tee for both momentum ratios.
4. 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, the 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 the K-epsilon model was chosen for further simulations.