Week 1- Mixing Tee

OBJECTIVE: To set up steady-state simulations to compare the mixing effectiveness when hot inlet temperature is 36⁰C & the Cold inlet is at 19⁰C. Using the k-epsilon and k-omega SST model for the first case and based on our judgment we will use the more suitable model for further cases.

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.

PROCEDURE: We will start with first case of short mixing tee with hot inlet velocity of 3m/s and momentum ratio of 2 i.e., velocity at cold inlet will be 6m/s. We will use both K-epsilon and K-omega SST method to converge the solution and choose the best between the two and same would we used for other cases.

We Initiate the project by dragging and dropping the Fluid Flow(fluent) into the project schematic window.

After that Geometry model was setup using space-claim where we opened our 3D cad file of model and after that extracted the fluid volume with “volume extract” method in which we used the edge selectin tool to select the boundary edges of thee fluid flow volume.

Once this was done we suppressed the outer geometry or shell by using the suppress for physics option in order to keep it away from interfering with the simulation.

After that we closed the Space-claim and Initiated mesh, where we used Face selection tool to select the boundaries of the fluid flow volume and we named the boundaries accordingly.

Next we right clicked on the mesh option from left tool bar and generated mesh. In the available options to see mesh details we plotted the mesh quality graph.

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.

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.

Similarly we dragged another case and connected our previous case mesh with setup of new case to make the whole process connected and time efficient, in setup for the model type this time we selected k-omega SST model type for the calculations rest all other steps were same.

Plotted definitions of K-omega SST for short tee with momentum ratio of 2.

Postprocessed results of K-omega SST with momentum ratio 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.

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