Week 1- Mixing Tee

1. AIM:- Have created two versions of the mixing tee.

a. Long mixing Tee

b. Short mixing Tee

2. OBJECTIVE:-

a. Velocity and temperature contour plots on the cut planes along and across the pipe.

b. Velocity and temperature line plots along and across the length of the pipe.

c. Perform Mesh independent study for any one case.

d. Discuss the effect of length and momentum ratio on results.

3. INTRODUCTION:- Mixing tee is a device that uses specifically engineered internal geometry to mix two fluid streams into one steam, efficiently. In this study, we are going to find the effectiveness of mixing tee. Hot air and cold air are going to be mixed in a mixing tee. They will have different velocities. We will use two different mixing tees distinguished with length and analyze the effect of length on mixing.

4. SETUP:-

a. Simulations- Steady State

b. Hot inlet temperature- 36°C

c. Cold inlet temperature- 19°C

d. Use the k-epsilon and k-omega SST model

e. Hot inlet velocity of 3m/s

f. Table to compare the values for the 2 cases

• Cell count
• Average outlet temperature
• Number of iterations for convergence

5. GOVERNING EQUATIONS:-

Momentum ratio = velocity at cold inlet / velocity at hot inlet

6. RESULTS AND PROCEDURE-

• Case-1: a)Given parameters- Short mixing Tee
  1. Simulation- Steady State
  2. Hot Inlet temp- 36°C
  3. Cold Inlet temp- 19°C
  4. Hot inlet velocity-3m/s
  5. Colt inlet velocity- 6m/s (from Momentum ratio (2) = velocity at cold inlet / velocity at hot inlet (3))
  6. For K-epsilon
     • Open a new project, by clicking on Fluid flow (Fluent) in the Ansys workbench.

• Now click on geometry to import mixing tee model. It will redirect to Spaceclaim, Ansys’s geometry model manipulator.

• Now Go to prepare menu and click on volume extract. Switch to edge selection and select three outer edges and click on green check button. Exit from Spaceclaim and click on mesh to open mesh menu.

• Select outer face and press right click of mouse, it will open a menu. Click on option create named selection and write suitable name for face. Repeat for other faces.

• Now click on mesh option and choose generate mesh.

• Select the option element quality to see, how many elements are defying quality. Quality is measured on the scale of 0 to 1. In our graph, most of the element are near 1 so we can say it is good quality. Section plane can be created to see inner portion of geometry.

• Exit mesh and open setup. First, Check mesh, it is throwing no error then only we can proceed further. We are going to do pressure based steady state simulation, so choose these options on task page.

• Go to physics menu and select viscous model. It will open a menu, choose realizable k-epsilon model and close it.

• Click on material and choose air as working fluid.

• Now go to boundaries option to set boundary conditions. Select inlet_x and click on option edit. It will open a menu, where all the parameter can be set whether it is thermal, radiation, momentum etc. Put the value of velocity as 3m/s and go to thermal parameter to set temperature as 36C, for hot fluid.

• Repeat the same procedure for cold fluid coming, velocity –6 m/s, temperature – 19C.
• We want to generate temperature report of outlet and standard deviation of temperature. Go to solutions tab and select definition and select area weighted average. In the opened menu select temperature as field variable. Similarly select standard deviation in definitions menu.
• Now click on initialization to calculate approximate values of velocity and pressure for 150 iterations.

• Exit setup and double click on result to open CFD post. Check boxes to see walls and other parts. Click on walls and go to render set transparency to inside of wall. Colour can also be changed to white for clear visibility.
• Now insert a XY plane in wall by clicking on plane in location option of insert.

• Go to colour of plane and change it by variable choose option temperature.

• Right click on plane which we created and select streamline. Go to appearance and select velocity to see plot line on plane.

Case-1: b)Given parameters- Short mixing Tee

1. Simulation- Steady State
2. Hot Inlet temp- 36°C
3. Cold Inlet temp- 19°C
4. Hot inlet velocity-3m/s
5. Colt inlet velocity- 12m/s (from Momentum ratio (4) = velocity at cold inlet / velocity at hot inlet (3))
6. For K-omega SST

Case-2: a)Given parameters- Long mixing Tee

1. Simulation- Steady State
2. Hot Inlet temp- 36°C
3. Cold Inlet temp- 19°C
4. Hot inlet velocity-3m/s
5. Colt inlet velocity- 6m/s (from Momentum ratio (2) = velocity at cold inlet / velocity at hot inlet (3))
6. For K-epsilon

Case-2: b)Given parameters- Long mixing Tee

1. Simulation- Steady State
2. Hot Inlet temp- 36°C
3. Cold Inlet temp- 19°C
4. Hot inlet velocity-3m/s
5. Colt inlet velocity- 12m/s (from Momentum ratio (4) = velocity at cold inlet / velocity at hot inlet (3))
6. For K-omega SST

7. COMPARISON TABLE-

Parameters	Cell Count	Average outlet temperature	Number of iterations to converge
Short Mixing tee Case 1 Epsilon Case 1 Omega	10543 cells 10543 cells	30.1 27.6	150 150
Long Mixing tee Case 2 Epsilon Case 2 Omega	12934 cells 12934 cells	28.7 26.0	160 150

• Analytical formula for T_mix value at outlet is

T_mix = [(m_hot * T_hot) + (m_cold * T_cold)] / (m_hot + m_cold)

Where, m – mass flow rate = ρ * A*V

T_cold - Temperature at cold inlet = 19^oC

T_hot - Temperature at hot inlet = 36 ^oC

T_mix - outlet temperature after mixing

• Mesh independent study: -
  • Here I took a short mixing tee momentum ratio 2 case.

No: of simulations	No: elements	Average Outlet Temperature in Celsius
1	13014	30.256
2	14453	30.211
3	39951	30.272

8. CONCLUSION:

• K-omega model is well suited for simulation flow in the viscous sub-layer. On the other hand, k-epsilon is ideal for predicting flow in the regions away from walls.
• The K-omega exhibit less sensitivity to the free stream conditions than any other turbulence models.
• To get better mixing at outlet high velocity of flow is very effective. Therefore, a higher momentum ratio gives better mixing of hot and cold fluid streams at the outlet.