Week 1- Mixing Tee

AIM: To create two versions of the longer and short 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.
• Use k-epsilon and k-omega SST model.
• Case 1
  • Short mixing tee with a hot inlet velocity of 3m/s.
  • Momentum ratio of 2, 4.
• Case 2
  • Long mixing tee with a hot inlet velocity of 3m/s.
  • Momentum ratio of 2, 4.

THEORY AND EXPLANATION:

Computational fluid dynamics is a branch of fluid mechanics, which uses mathematics, physics and computational software to visualize how fluid flows and affects its medium and its neighbours. In CFD mostly computers are used to predict the fluid flow phenomena based on some governing equations like conservation of momentum – Navier stokes equation, conservation of energy etc. In this study we are going to simulating effect of fluid using Ansys fluent software. Ansys fluent is a very important tool in CAE domain, which is allows you to perform all the three major steps of any simulation pre-processing, solving and postprocessing. Ansys fluent facilitates for modelling flow, turbulence, heat transfer and chemical reactions. With Ansys we can accomplice more in less time and with less training.

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

PROCEDURE:

1. Click on Fluid flow(Fluent) in Ansys Workbench.

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

3. We are to analyse the mixing of two fluid, so there is role of parts other than two pipes. To supress that we need to extract volume of tee. Go to prepare menu and click on volume extract. Switch to edge selection and select three outer edges and click on green check button.

4. Hide solid by unchecking it from structure menu to see volume, which we have extracted. Supress solid by clicking on option supress for physics.

5. Exit from Spaceclaim and click on mesh to open mesh menu.

6. 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.

7. Now click on mesh option and choose generate mesh. It will generate mesh according to some predefined criteria. Element size can be selected but it is going to affect simulation time. Section plane can be created to see inner portion of geometry

8. 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 to 1 so we can say it is good quality.

9. 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.

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

11. Click on material and choose air as working fluid. Check working fluid by clicking on cell zone conditions option, if it is air then proceed.

12. 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.

13. Repeat the same procedure for cold fluid coming, velocity –6 m/s, temperature – 19C.

14. We want to generate temperature report of outlet and we want standard deviation of temperature also. 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.

15. Now click on initialization to calculate approximate values of velocity and pressure.

16. Change the number of iterations to 250 and click on calculate. If solution is not converged, then iterations can be increased.

17. 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 of 0.4 to inside of wall. Colour can also be changed to white for clear visibility.

18. Now insert a XY plane in wall by clicking on plane in location option of insert.

19. Go to colour of plane and change it by variable choose option temperature. Similarly, velocity contours can be created by just changing variable as velocity.

20. Right click on plane which we created and select streamline. Go to appearance and select temperature to see plot line on plane. Similar procedure can be repeated for velocity plot line.

RESULTS:

CASE1: In this short mixing tee is used, which was mixing two fluid with temperature 6C and 19C. Velocity of hot fluid was 3m/s and cold fluid was 6m/s. we used two different turbulent solvers for it first was realized k-epsilon and second was k-omega SST. The k-epsilon is giving better results because solution was converged after few iterations

Plot and contours from k-epsilon model:

Residuals:

Average Temperature vs Iterations:

Standard Deviation of Temperature vs Iteration:

Temperature contour:

Velocity contour:

Plot and contours from k-omega model:

Residuals:

Average Temperature vs Iterations:

Standard Deviation of Temperature vs Iteration:

Temperature contour:

Velocity contour:

CASE2: In this case long mixing tee is used, which was mixing two fluid with temperature 6C and 19C. Velocity of hot fluid was 3m/s and cold fluid was 12m/s.

Residuals:

Average Temperature vs Iterations:

Standard Deviation of Temperature vs Iteration:

Temeprature Contour:

Velocity Contour:

Mesh independent test allows mesh refinement without affecting results. After some tests we can be ensured by solution that mesh refinement won’t affect the result. The following graphs are showing the results after mesh refinement on short mixing tee, earlier element size is used as 0.0195 and now it got 0.001.

CONCLUSION:

• k- epsilon model predicts well far from the boundaries (wall) and k- omega model predicts well near the wall. Even though it depends on Y+. Therefore, K-epsilon model is majorly used for free flow away from the wall and as in this project, there is no dealing with the wall of the mixing tee for change in temperature along the length of it, so K-epsilon model was chosen for further simulations.
• As seen in the Case-1-a and Case-1-b, the temperature at the outlet drops down significantly due to high velocity of cold air from the inlet. This is possible due to the turbulent mixing at the tee joint between hot air and cold air because of the high velocity of cold air., which is not possible in case of low-velocity cold air.