Week 1- Mixing Tee

Challenge No-1 Mixing tee Long.

Aim :

01. Set up a steady state simulation to compare the mixing effectiveness when hot inlet temperature in 36’C & the cold inlet is at 19’C.

02. Try both the K-epsilon & K-omega SST models, which one is better for the simulation.

03. Momentum ratio is same for the both case.

Solution:

01. First step is to load the geometry in the ansys work bench. & start editing the ansys space claim.

02. After opening the geometry in the space claim, geometry will look like in this manner.

03. Go to the PREPARE TAB & select VOLUME EXTRACTION > EDGE SELECTION. Select all the edges of the object and click the complete option to get the inside component, which will be in contact with the fluid volume.

04. Now, suppress the outer body, option called SUPRESS FOR PHYSICS, and it is looking like in this manner.

05. Now, open the ansys mesh, & provide the names to the different entities of the object.

06. In the SMART SELECTION COLUMN, There are face selection. Click on that option, and provide the names to the entities.

07. Now, cut the object from the middle, with the help of the NEW SECTION PLANE OPTION.

Bounding Box: Bounding box is a imaginary box, which covers the whole body. On the basis of that box, ansys generate the mesh.

Ansys will understand automatically the term, inlet, outlet & walls.

Now check the mesh quality, by clicking the check command. Ansys will check the mesh.

After checking the mesh, select the solver, solver will be pressure based, absolute & steady for the physical conditions with the air fluid CFD analysis.

Setting up physics: Set the solver based model for the mixing tee, for this model we have used both the solver. 1. K-Epsilon 2. K-Omega. For the different momentum ratio 2 & 6.

Create the material property: For CFD analysis, we need to create or edit the material property. We have to select the material only here. Other then there is no way to select the material.

Setting the cell zone: Select the material for the cell zones.

Select the boundary conditions: Select the boundary conditions for the cell body, one by one. Define the inlet velocity of the hot air inside the mixing tee from the ‘inlet-x’. for the first condition we have to have the momentum ratio should be 2. And for the second term, the momentum ratio should be 6. For fulfilling this condition. So, momentum ratio for the first experiment is 3:6 & for second experiment the momentum ratio is 3:12. And inlet air temperature from ‘x’ is 25’C and 10’C for the air temperature inlet from axis ‘y’.

Define the reports of experiment: Select the report files for the experiment.

We have selected four reports, for the outlet function in terms of the temperature only.

1. Area weighted average of temperature.

2. Scaled residuals.

3. Standard deviation of the temperature.

4. Total mesh element counts.

5. Area weighted average of velocity at outlet.

6. Standard deviation for the velocity magnitude.

Total mesh elements are 106544 by defining the mesh length to 0.002mm in the mesh platform.

K-Epsilon Report : These are the total reports for the ‘K-Epsilon’ experiment model of short tee.

These are the two CFD analysis results on the basis for Velocity & temperature of short mixing tee.

K-Omega report: These are the total report of the K-Omega report for short tee.

K-Epsilon Report for long tee :

K-Omega report for long tee :

K-Epsilon - As we saw in our first simulation, the K-epsilon turbulence model is the most common model used in CFD to simulate mean flow characteristics for turbulent flow conditions. It is a two-equation model that gives a description of turbulence by means of two transport equations (PDEs). the first transported variables are the turbulent kinetic energy (k) and the second transported variable is the rate of dissipation of turbulent kinetic energy.

k-epsilon focuses on the mechanism that affects the turbulent kinetic energy.

K-Omega SST - The SST k-omega turbulence model is a two-equation eddy-viscosity model that is used for many aerodynamic applications. It is a hybrid model combining the Wilcox k-omega and the k-epsilon models. A blending function, F1, activates the Wilcox model near the wall and the k-epsilon model in the free stream. This ensures that the appropriate model is utilized through the flow field.

Main Difference - The k- omega model is well suited for simulating flow in the viscous sub-layer. On the contrary, The k-epsilon model is ideal for predicting flow in the regions away from the wall

The SST model exhibit less sensitivity to free stream conditions (Flow outside the boundary layer) than many other turbulence models.

The shear stress limiter helps the k-omega model avoid a build-up of excessive turbulent kinetic energy near stagnation points.

If we observe our simulated figure, we can notice, in the k-omega model flow is more like laminar and not giving proper turbulent results, in contrast, simulation by k-epsilon, is giving proper mixing of the two air stream.

Short Mixing Tee
For momentum ratio - 2
Element size in m	No. Of element	Average velocity at outlet in m/s.	Average outlet temperature in C.	No. Of iteration
0.002	106544	4.5038	20°	185
For momentum ratio - 4
Element size in mm	No. Of element	Average velocity at outlet in m/s.	Average outlet temperature in C.	No. Of iteration
0.002	106544	6.0632	18°	111
Long Mixing Tee
For momentum ratio - 2
Element size in mm	No. Of element	Average velocity at outlet in m/s.	Average outlet temperature in C.	No. Of iteration
0.002	106544	4.5044	20°	250
For momentum ratio - 4
Element size in mm	No. Of element	Average velocity at outlet in m/s.	Average outlet temperature in C.	No. Of iteration
0.002	106544	6.0605	18°	193