Mixing efficiency
Aim: To plot residual , area-weighted average of temperature , standard deviation of temperature , temperature and velocity while analysing the Tee sections : short and long .
Software used: ANSYS FLUENT
Given:
Two versions of the mixing tee are given. One of them is longer than the other.
Steady-state simulations are set up to compare the mixing effectiveness when hot inlet temperature is 360C & the Cold inlet is at 190C.
We have used the k-omega SST model for the cases as the results are good and it is the most suitable model so far.
There are two cases:
- 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
By that formula , cold inlet velocity for both short mixing tee and long mixing tee will be 6 m/s and 12 m/s each.
Case 1:
SHORT MIXING TEE
Fluid Flow ( Fluent) is been used as an analysis system.
Ansys SpaceClaim utility was used for geometry model.
Mesh size used is 0.0015m
The hot inlet temperature is 36°C(309K) and cold inlet temperature is 19°(292K)
Here X-direction inlet is the hot inlet and Y-direction inlet is the cold inlet.
(a) Short mixing tee with hot inlet velocity of 3m/s and Momentum ratio of 2
Residual Plot:
Area-weighted average of temperature(C):
The Average Outlet temperature is 30.50°C (303.5K)
Standard Deviation of temperature:
Standard Deviation is 2.5
Temperature plot:
As the inlet temperature is 292.15 K and the velocity is 6m/s, the cold air was not able to manipulate and mix with the hot air and we get a balanced outlet temperature which covers the whole range. Here at the outlet cross-section, the temperature ranges between 300K to 309K.
Velocity Profile Plot:
Here at the outlet cross-section, the velocity ranges between 1m/s to 4.11 m/s.
The highest velocity reaches up to 8.22 m/s at tee intersection.
(b) Short mixing tee with hot inlet velocity of 3m/s and Momentum ratio of 4:
Residual Plot:
Area-weighted average of temperature(C):
The average outlet temperature is 27.60°C (300.6K).
Standard Deviation of temperature:
Standard Deviation is 1.60
Temperature Plot:
Here at the outlet cross-section, the temperature ranges between 299 to 305K.
Velocity Profile Plot:
Here at the outlet cross-section, the velocity ranges between 3.76 to 7.53 m/s.
The highest velocity reaches up to 15.06 m/s at tee intersection.
Case 2:
LONG TEE MIXING
Fluid Flow ( Fluent) is been used as an analysis system.
Ansys SpaceClaim utility was used for geometry model.
Mesh size used is 0.0015m
The hot inlet temperature is 36°C(309K) and cold inlet temperature is 19°(292K)
Here X-direction inlet is the hot inlet and Y-direction inlet is the cold inlet.
(a) Long mixing tee with hot inlet velocity of 3m/s and Momentum ratio of 2:
Residual Plot:
Area-weighted average of temperature(C):
The average outlet temperature is 30.30°C(303.3K).
Standard Deviation of Temperature:
Standard Deviation is 1.12
Temperature Plot:
Here at the outlet cross-section, the temperature ranges between 300.66K to 304.91K.
Velocity Profile Plot:
Here at the outlet cross-section, the velocity ranges between 1.91 to 4 m/s.
The highest velocity reaches up to 7.63 m/s at tee intersection.
(b) Long mixing tee with hot inlet velocity of 3m/s and Momentum ratio of 4:
Residual Plot:
Area-weighted average of temperature(C):
The average outlet temperature is 27.60°C(300.6K)
Standard Deviation of temperature:
Standard Deviation is 1.22
Temperature Plot:
Here at the outlet cross-section, the temperature ranges between 298K to 304.91K.
Velocity Profile Plot:
Here at the outlet cross-section, the velocity ranges between 3.63 to 7.26 m/s.
The highest velocity reaches up to 14.53 m/s at tee intersection.
Comparison of Cell Count, Average Outlet Temperature and No. of Iterations for Convergence for all three cases:
| CASES | CELL COUNT | AVG. OUTLET TEMPERATURE | NO. OF ITERATIONS FOR CONVERGENCE |
| CASE 1 (a) | 211671 | 30.50°C (303.5K) | 234 |
| (b) | 211671 | 27.60°C (300.6K) | 220 |
| CASE 2 (a) | 288343 | 30.30°C (303.3K) | 199 |
| (b) | 288343 | 27.60°C (300.6K) | 180 |
CONCLUSION:
By these results we concluded that average of two extreme temperature is observed as the hot inlet temperature was 36°C(309K) and cold inlet temperature was 19°C(292K).
As the cold air velocity increases from the inlet y, the outlet air temperature obtained is cooler.
For the velocity profile plot as the hot and cold inlet velocity increases, the mixing efficiency increases and we get more uniform air at the outlet.
The velocity parameter plays an important role in determining the mixing efficiency.
The average outlet temperature depends upon the inlet hot and cold temperature.
The less the temperature of the cold air, the less outlet air obtained.