Mixing efficiency

Steady-state simulations to compare the mixing effectiveness of two different mixing tees

Mixing Tees are a simple, cost effective method of providing chemical diffusion to a system.

Momentum ratio is the ratio of velocity of cold air(inlet) to the velocity of hot air(inlet)

AIM

To set up steady-state simulations to compare the mixing effectiveness when hot inlet temperature is 36°C & the Cold inlet is at 19°C.

Case 1: Short Mixing Tee with Momentum ratio=2

i)Dimensions

	Diameter(mm)	Length(mm)
Pipe along X-axis	33.876	192.376
Pipe along Y-axis	16.962

ii)Mesh

Element Type	Tetrahedral
Element Size	2.e-003
Cell Count	105865

The Mesh quality is shown in the above fig. It is between 30% and 100% and few elements have quality below 70%, so this mesh is good and further procedures can be done

iii)Setup

The fluid used for simulations is air. The properties are as shown below

The equations solved are RANS and Energy equation. The Turbulence model selected is k-ε turbulence model because all the effects of turbulence are modelled, and it is the most used in Industrial applications

iv)Boundary Conditions

Boundary	Conditions
Hot Inlet	Velocity= 3m/s Temperature=19°C
Cold Inlet	Velocity=6m/s Temperature=36°C
Wall	Gauge Pressure=0 bar
Outlet	Gauge Pressure=0 bar

v)Results

Temperature Line and Plane Contours

Velocity Line and Plane Contours

Case 2(Short Mixing Tee with Momentum ratio=4)

i)Dimensions

As shown in the above case

ii)Mesh

As shown in the above case

iii)Setup

The fluid used is air and details are shown above

The equations solved are RANS and energy equation.The Turbulence model selected is k-ε turbulence model because all the effects of turbulence are modelled, and it is the most used in Industrial applications.

iv)Boundary Conditions

Boundary	Conditions
Hot Inlet	Velocity= 3m/s Temperature=19°C
Cold Inlet	Velocity=12m/s Temperature=36°C
Wall	Gauge Pressure=0 bar
Outlet	Gauge Pressure=0 bar

v)Results

Temperature Line and Plane Contours

Velocity Line and Plane Contours

Case 3: Long Mixing Tee with MR=2

i)Dimensions

	Diameter(mm)	Length(mm)
Pipe along X-axis	33.876	268.328mm
Pipe along Y-axis	16.962

ii)Mesh

Element Type	Tetrahedral
Element Size	2.e-003
Cell Count	140525

The Mesh quality is shown in the above fig. It is between 30% and 100% and few elements have quality below 70%, so this mesh is good and further procedures can be done.

iii)Setup

The fluid used in the above simulation is air. The details are shown in Case 1

The equations solved are RANS and energy equation. The Turbulence model used K-epsilon as it is observed by trial and error it takes lesser no of iterations.

iv)Boundary Conditions

Boundary	Conditions
Hot Inlet	Velocity= 3m/s Temperature=19°C
Cold Inlet	Velocity=6m/s Temperature=36°C
Wall	Gauge Pressure=0 bar
Outlet	Gauge Pressure=0 bar

v)Results

Temperature Line and Plane Contours

Velocity Line and Plane Contours

Case 4(Long Mixing Tee with Momentum Ratio=4)

i)Dimensions

As shown in the above case

ii)Mesh

As shown in the above case

iii)Setup

The fluid used is air and details are shown above

The equations solved are RANS and energy equation. The Turbulence model used K-epsilon as it is observed by trial and error it takes lesser no of iterations.

iv)Boundary Conditions

Boundary	Conditions
Hot Inlet	Velocity= 3m/s Temperature=19°C
Cold Inlet	Velocity=12m/s Temperature=36°C
Wall	Gauge Pressure=0 bar
Outlet	Gauge Pressure=0 bar

v)Results

Temperature Line and Plane Contour

Velocity Line and Plane Contour

Results and Conclusions

	Short Mixing Tee with MR=2	Short Mixing Tee with MR=4	Long Mixing Tee with MR=2	Long Mixing Tee with MR=4
Cell Count	105865	105865	140525	140525
No of iterations for convergence	65	70	64	72
Average outlet Temperature	30.30367°C	27.55295°C	30.41893°C	27.48428°C

Conclusion

It is observed that changing the length keeping the momentum ratio does not have much effect on the outlet temperature. However, changing the momentum ratio keeping the length constant has significant effects.