Week 1- Mixing Tee

AIM:

To set up steady-state simulations to compare the mixing effectiveness when the hot inlet temperature is 36⁰C & the Cold inlet is at 19⁰C creating two versions of the mixing tee.

OBJECTIVE:

• Is to set up steady-state simulations to compare the mixing effectiveness when the 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.

1. Case 1
   • Short mixing tee with a hot inlet velocity of 3m/s.
   • Momentum ratio of 2, 4.
2. 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.

1. Cell count
2. Average outlet temperature
3. Number of iterations for convergence

• Expected results:

1. Velocity and temperature contour plots on the cut planes along and across the pipe.
2. Velocity and temperature line plots along and across the length of the pipe.
3. Perform Mesh independent study for any one case.
4. Discuss the effect of length and momentum ratio on results.

PROCEDURE AND RESULTS

Case 1

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4

Geometry:

The Geometry has been provided in the challenge interface. The first case is simulated with the short T-joint with the momentum ratio 2.

Meshing:

creating the Mesh file, we name the geometry for the further use

1. Inlet x (inlet where the hot air come from)

2. Inlet y (inlet where the cold air come from)

3. The walls

4. Outlet

We have done mesh refinement by an element size = 0.002m and keeping the rest of it default

with an element quality of:

After creating the Mesh file, we name the geometry for the further use

1. Inlet x (inlet where the hot air come from)

2. Inlet y (inlet where the cold air come from)

3. The walls

4. Outlet

After updating the mesh file we go to the Setup part.

Setup

Once the mesh was done, we initiated Setup which uses Ansys fluent, in the launcher we selected 4 processors coz that was max allowed by our license. After that In fluent the first thing we performed was mesh check, after we got ok from there we went into physics where we setup the general settings like units and things and in the zones we gave all our simulations Initial conditions after which we selected Energy equation and in viscous we selected the model type of our choice. After that we used the definition tool to specify the plot outputs we wanted from the simulation and then we initialised the solution it ran for 10 iterations and once it was done we calculated values for 200 iterations and then closed the Ansys fluent.

Solution

SETUP	Short TEE (MR=4 & Inlet Hot Velocity=3m/s)
	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
Boundaries	Inlet x (velocity inlet) Velocity Magnitude 3m/s Temperature – 36 degrees c. Inlet y (velocity inlet) Velocity Magnitude 6m/s Temperature – 19 degrees c. Outlet x (Pressure outlet) Gauge Pressure – 0 pas. Walls – Stationary walls	Inlet x (velocity inlet) Velocity Magnitude 3m/s Temperature – 36 degrees c. Inlet y (velocity inlet) Velocity Magnitude 6m/s Temperature – 19 degrees c. Outlet x (Pressure outlet) Gauge Pressure – 0 pas. Walls – Stationary walls

The plotted output graphs are shown for K-epsilon model type.

After that we initiated results which is basically CFD post processing tool here we made our boundaries a bit transparent and plotted planes across and along the pipe using the insert options and the panes were colored with variables to show us the flow of fluid int the pipe along the planes.

Post processed results of K-epsilon short Tee with momentum ratio of 2.

K-epsilon vs K-omega SST

K-epsilon turbulence model is a two-equation model that gives description of turbulence by using two transport equations (PDEs) It has 2 variables first is turbulent kinetic energy and the other is rate of dissipation of turbulent KE. It majorly is about the mechanism affecting tubulent KE.

K-omega SST is also a two equation model though it is a eddy-viscosity model mainly used for aerodynamic purposes. It is a hybrid model, wilcox model is applied near the wall and k-epsilon in the fluid stream.

In our model we in particularly are not dealing with the walls hence we will be using the k-epsilon model type.

Calculation plot of short tee with momentum ratio 4.

Postprocessed results of short tee with momentum ratio 4.

Case 2

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4

Geometry:

The Geometry has been provided in the challenge interface. The first case is simulated with the long T-joint with the momentum ratio 2.

Meshing:

creating the Mesh file, we name the geometry for the further use

1. Inlet x (inlet where the hot air come from)

2. Inlet y (inlet where the cold air come from)

3. The walls

4. Outlet

with an element quality of:

After creating the Mesh file, we name the geometry for the further use

1. Inlet x (inlet where the hot air come from)

2. Inlet y (inlet where the cold air come from)

3. The walls

4. Outlet

After updating the mesh file we go to the Setup part.

Setup

Once the mesh was done, we initiated Setup which uses Ansys fluent, in the launcher we selected 4 processors coz that was max allowed by our license. After that In fluent the first thing we performed was mesh check, after we got ok from there we went into physics where we setup the general settings like units and things and in the zones we gave all our simulations Initial conditions after which we selected Energy equation and in viscous we selected the model type of our choice. After that we used the definition tool to specify the plot outputs we wanted from the simulation and then we initialised the solution it ran for 10 iterations and once it was done we calculated values for 200 iterations and then closed the Ansys fluent.

Solution

SETUP	K-epsilon
Solver Type	Pressure Based
Velocity Formulation	Absolute
Time	Steady
Steady
Viscous Model	Realizable k-epsilon with standard wall function
Material	Fluid type: Air
Cell zones Volume_volume	Fluid type: Air
Boundaries	Inlet x (velocity inlet) Velocity Magnitude 3m/s Temperature – 36 degrees c. Inlet y (velocity inlet) Velocity Magnitude 6m/s Temperature – 19 degrees c. Outlet x (Pressure outlet) Gauge Pressure – 0 pas. Walls – Stationary walls

Plotted calculations of long tee with momentum ratio 2.

Postprocessed results of long tee with momentum ratio 2.

Plotted calculations of long tee with momentum ratio 4

Postprocessed results of long tee with momentum ratio 4.

We performed mesh independent study for long tee with momentum ratio 2.

Here we went into mesh and in mesh properties we went to default where we changed element size to 1 and regenerated the mesh after which all other steps were updated and the postprocessed results for the mesh are attached below.

CONCLUSION:

K-epsilon model predicts flow in the fluid stream while K-omega does it for that as well as near the boundaries.

As no.of elements increases the simulation time increased.

For better mixing of the flow higher inlet velocity is very much effective and as the length increases we dont see that huge of a effect but with high velocity and increased length we observe better mixing due to diffusion.

SHORT TEE, 2
0.01 ( Element size)	12692 (number of cells)	30.25 C (outlet temperature)	200(Number of iterations)
SHORT TEE, 4
0.01	12692	27.5 C	180
LONG TEE, 2
1	2056	30.25 C	200
0.01	15530	30.25 C	185
LONG TEE, 4
0.01	15530	27.5 C	170