Mixing efficiency

AIM:- To study mixing efficiency of mixing Tee for different Momentum ratio.

Mixing Tee:- It is a device used in many mixing/diffusion process, and it also provide proper mixing of quantities to get desired result at output.

 As shown in above figure, by adjusting chilled air velocity, we can get desired output.

Work flow of simulation:-

• Open Ansys workbench, double click/drag drop Fluid Flow (Fluent) analysis system to project schematic.

         

Here we see steps to follow to solve a CFD problem.

      

1. Geometry: Spaceclaim, it is used for import the geometry and other CAD related functions like geometry clean-up etc.
2. Mesh: used to generate Ansys Mesh and other mesh control option there, also assign name which helps during BCs steup.
3. Setup: used to define physics of problem, like BCs, material, and type of solving method, iterations etc.
4. Solution: where runtime results visualization done.
5. Results: CFD-Post, where Post Processing of Results will be done.

Case 1:- Short Tee

1. Geometry Cleanup:- Import short tee geometry into spaceclaim, spaceclaim gives various facility like CAD softwares, so we prepare geometry as per our simulation requirement.

                                                                     

 As per our simulation we are only interested in fluid flow analysis throught our short tee geometry, so we extract fluid volume from short tee geometry. Spaceclaim provide various                                           option for extract the volume.

 Goto Prepare -> Volume Extract -> Select edges

 

                                                                             

After extract flow volume, subpress for solid geometry so that it does not take part in mesh generation and futher simulation process.

b. Mesh Generation:-

In this step boundary naming also done. Boundary naming helps in for easy & correct visual of inlet/outlet and other geometry features so we can apply correct condition at correct place.

Mesh generation & element quality check is shown below-

we choose 1E-3 m element size.

                                                                        

Goto Mesh -> Quality -> Mesh Metric -> Element Quality

                                                                       

please check element quality should not be less than 5%.

c. Fluent Setup:-

At Fluent launcher choose Double precision option & solver process.

                                                                               

 

Condition 1:-

Short Tee with a hot inlet velocity of 3 m/s & Momentum ratio of 2

SETUP	K-epsilon	K-omega
Solver Type	Pressure Based	Pressure Based
Velocity Formulation	Absolute	Absolute
Time	Steady	Steady
Viscous Model	Realizable k-epsilon with standard wall function	k-omega SST model
Material	Air	Air
Cell Zones
Volume_Volume	Fluid type- Air	Fluid type- Air
	Inlet x (velocity inlet)	Inlet x (velocity inlet)
	Velocity Magnitude= 3 m/s	Velocity Magnitude= 3 m/s
	Temp.= 36 degree C	Temp.= 36 degree C
Boundaries	Inlet y (velocity inlet)	Inlet y (velocity inlet)
	Velocity Magnitude= 6 m/s	Velocity Magnitude= 6 m/s
	Temp.= 19 degree C	Temp.= 19 degree C
	Outlet (pressure outlet)	Outlet (pressure outlet)
	Gauge Pressure= 0 pas.	Gauge Pressure= 0 pas.
	Walls= Stationary walls	Walls= Stationary walls

After placing all input condition solution is initialize by Hybrid Initialization.

 

d. Results:-

                                                                      For k-epsilon model                                                                                                                  For k-omega model

Residuals plot:-   

                                                  

                                                           Approx. 122 iterations taken to converge solution.                                                             Approx. 250 iterations taken to converge solution.

Average weighted area Plot for Temp.:-

                                                

                                                       From the above snap outlet temp. of mixture is 30.4 degree C.                                              From the above snap outlet temp. of mixture is 30.5 degree C   

 

Standard deviation plot:-

                                              

                                                        From the above snap standard deviation is 1.77                                                                    From the above snap standard deviation is 2.45

Contour plots:-

1) For Temp. distribution

                                                

2) For Velocity. distribution

                                                 

Line plots:-

1) For Temp. distribution

                                             

2) For Velocity. distribution

                                            

Summary:- Short Tee with MR=2

	K-epsilon	K-omega
Cell count	573264	573264
Avg. Outlet Temp.	30.4 degree C	30.5 degree C
No. of Iterations	approx. 122	approx. 250
Standard deviation	1.77	2.45

 

Short Tee with a hot inlet velocity of 3 m/s & Momentum ratio of 4

SETUP	K-epsilon	K-omega
Solver Type	Pressure Based	Pressure Based
Velocity Formulation	Absolute	Absolute
Time	Steady	Steady
Viscous Model	Realizable k-epsilon with standard wall function	k-omega SST model
Material	Air	Air
Cell Zones
Volume_Volume	Fluid type- Air	Fluid type- Air
	Inlet x (velocity inlet)	Inlet x (velocity inlet)
	Velocity Magnitude= 3 m/s	Velocity Magnitude= 3 m/s
	Temp.= 36 degree C	Temp.= 36 degree C
Boundaries	Inlet y (velocity inlet)	Inlet y (velocity inlet)
	Velocity Magnitude= 12 m/s	Velocity Magnitude= 12 m/s
	Temp.= 19 degree C	Temp.= 19 degree C
	Outlet (pressure outlet)	Outlet (pressure outlet)
	Gauge Pressure= 0 pas.	Gauge Pressure= 0 pas.
	Walls= Stationary walls	Walls= Stationary walls

After placing all input condition solution is initialize by Hybrid Initialization.

 

d. Results:-

                                                                                    For k-epsilon model                                                                                                         For k-omega model

Residuals plot:-   

                                              

                                                                     Approx. 194 iterations taken to converge solution.                                                          Approx. 449 iterations taken to converge solution.

Average weighted area Plot for Temp.:-

                                             

                                                      From the above snap outlet temp. of mixture is 27.55 degree C                                                     From the above snap outlet temp. of mixture is 27.79 degree C 

Standard deviation plot:-

                                           

                                                      From the above snap standard deviation is 1.26                                                                         From the above snap standard deviation is 1.99

Contour plots:-

1) For Temp. distribution

                                      

2) For Velocity. distribution

                                     

Line plots:-

 

1) For Velocity. distribution

                                     

Summary:- Short Tee with MR=4

	K-epsilon	K-omega
Cell count	573264	573264
Avg. Outlet Temp.	27.55 degree C	27.79 degree C
No. of Iterations	approx. 194	approx. 449
Standard deviation	1.26	1.99

 Conclusion:-

	K-epsilon(MR=2)	K-omega(MR=2)	K-epsilon(MR=4)	K-omega(MR=4)
Cell count	573264	573264	573264	573264
Avg. Outlet Temp.	30.4 degree C	30.5 degree C	27.55 degree C	27.79 degree C
No. of Iterations	approx. 122	approx. 250	approx. 194	approx. 449
Standard deviation	1.77	2.45	1.26	1.99

 

1) For MR 2 & 4, Standard deviation of k-epsilon model is lower than k-omega model. Hence k-epsilon model has better mixing efficiency than k-omega model.

2) For MR 2 & 4, k-epsilon model takes less number of iterations than k-omega model. Hence k-epsilon model is faster to converge solutions.

Hence we use k-epsilon model for future calcutions because it is more accurate & take less time to converge.

Case 2:- Long Tee

1. Geometry Cleanup:-Import Long tee geometry into spaceclaim, spaceclaim gives various facility like CAD softwares, so we prepare geometry as per our simulation requirement.

                                                                           

As per our simulation we are only interested in fluid flow analysis throught our long tee geometry, so we extract fluid volume from long tee geometry. Spaceclaim provide various option for extract the volume.

Goto Prepare -> Volume Extract -> Select edges

                                                                       

After extract flow volume, subpress for solid geometry so that it does not take part in mesh generation and futher simulation process.

b. Mesh Generation:-

In this step boundary naming also done. Boundary naming helps in for easy & correct visual of inlet/outlet and other geometry features so we can apply correct condition at correct place.

Mesh generation & element quality check is shown below-

we choose 1E-2 m element size.

                                                                      

Goto Mesh -> Quality -> Mesh Metric -> Element Quality

                                                                     

please check element quality should not be less than 5%.

c. Fluent Setup:-

At Fluent launcher choose Double precision option & solver process.

 

Condition 1:-

Long Tee with a hot inlet velocity of 3 m/s & Momentum ratio of 2

SETUP	K-epsilon
Solver Type	Pressure Based
Velocity Formulation	Absolute
Time	Steady
Viscous Model	Realizable k-epsilon with standard wall function
Material	Air
Cell Zones
Volume_Volume	Fluid type- Air
	Inlet x (velocity inlet)
	Velocity Magnitude= 3 m/s
	Temp.= 36 degree C
Boundaries	Inlet y (velocity inlet)
	Velocity Magnitude= 6 m/s
	Temp.= 19 degree C
	Outlet (pressure outlet)
	Gauge Pressure= 0 pas.
	Walls= Stationary walls

After placing all input condition solution is initialize by Hybrid Initialization.

 

d. Results:-

                                                                                                                                 For k-epsilon model                                                                             

Residuals plot:-   

                                                                     

                                                                                                          Approx. 239 iterations taken to converge solution.

Average weighted area Plot for Temp.:-

                                                                        

                                                                                               From the above snap outlet temp. of mixture is 30.28 degree C.

Standard deviation plot:-

                                                                        

                                                                                                     From the above snap standard deviation is 1.17

Contour plots:-

1) For Temp. distribution

                                                                        

2) For Velocity. distribution

                                                                       

Line plots:-

1) For Velocity. distribution

                                                                      

Summary:- Long Tee with MR=2

	K-epsilon
Cell count	15368
Avg. Outlet Temp.	30.28 degree C
No. of Iterations	approx. 239
Standard deviation	1.17

 

Long Tee with a hot inlet velocity of 3 m/s & Momentum ratio of 4

SETUP	K-epsilon
Solver Type	Pressure Based
Velocity Formulation	Absolute
Time	Steady
Viscous Model	Realizable k-epsilon with standard wall function
Material	Air
Cell Zones
Volume_Volume	Fluid type- Air
	Inlet x (velocity inlet)
	Velocity Magnitude= 3 m/s
	Temp.= 36 degree C
Boundaries	Inlet y (velocity inlet)
	Velocity Magnitude= 12 m/s
	Temp.= 19 degree C
	Outlet (pressure outlet)
	Gauge Pressure= 0 pas.
	Walls= Stationary walls

After placing all input condition solution is initialize by Hybrid Initialization.

 

d. Results:-

                                                                                                                                 For k-epsilon model                  

Residuals plot:-   

                                                                                                  

                                                                                                        Approx. 121 iterations taken to converge solution.

Average weighted area Plot for Temp.:-

                                                                      

                                                                                                     From the above snap outlet temp. of mixture is 27.49 degree C.

Standard deviation plot:-

                                                                      

                                                                                                        From the above snap standard deviation is 0.75

Contour plots:-

1) For Temp. distribution

                                                                       

2) For Velocity. distribution

                                                                     

Line plots:   

1) For Velocity. distribution

                                                                    

Summary:- Long Tee with MR=4

	K-epsilon
Cell count	15368
Avg. Outlet Temp.	27.49 degree C
No. of Iterations	approx. 121
Standard deviation	0.75

 

	K-epsilon (MR= 2)	K-epsilon (MR= 4)
Cell count	15368	15368
Avg. Outlet Temp.	30.28 degree C	27.49 degree C
No. of Iterations	approx. 239	approx. 121
Standard deviation	1.17	0.75

Conclusion:-

	Short Tee		Long Tee
	K-epsilon(MR=2)	K-epsilon(MR=4)	K-epsilon(MR=2)	K-epsilon(MR=4)
Cell count	573264	573264	15368	15368
Avg. Outlet Temp.	30.4 degree C	27.55 degree C	30.28 degree C	27.49 degree C
No. of Iterations	approx. 122	approx. 194	approx. 239	approx. 121
Standard deviation	1.77	1.26	1.17	0.75

 

1) As we see in above table long tee outlet temp. is approx similar to short tree in respective MR, hence longer length does not have significant effect on efficiency of mixing tee.

2) The tee mixing effectiveness increases when the velocity of the cold inlet fluid increases. Hence MR 4 has better effectiveness than 2.

3) From the grid independence test, we can conclude that the finer the size mesh, the better the accuracy of the solution. Hence at the finer grid size, we got better mixing efficiency. But at the same time, computional time increases significantly.