Mixing Tee
Title :- Mixing Tee Effectiveness
Introduction :-
In the turbulent mixing tee model, we use two fluid which has two properties in the different inlet. The fluids are in different temperatures and different velocities which are playing an important role in the T-junction of the model. Temperature fluctuation is happening in the mixing tee model with respect to the distance covered by the mixing air. We use here two types of models where one has a shorter length and another one has a longer length. We are using Ansys fluent for the CFD simulation of the turbulence mixing model. We are considering some models to define the appropriate values of the mixing air.
In our practical life, we use air conditioning systems for the human body’s comfort, but this comfort temperature will be different when the season will change. In this condition air conditioner gives a lower temperature air where the human body needs more temperature in the atmosphere for this condition, we use the atmospheric temperature with some velocity to increase the temperature of the air, for this condition we use a mixing tee. This mixing of air creates turbulence conditions which fluctuate the temperature in the wall and outlet. In the Ansys fluent mesh parameter and solving the problem by CFD simulation and giving a detailed description of the result, we can use these data to compare with the experimental data.
Procedure :-
Here we consider two models as compared to the length of the pipe one is a shorter length the other is a longer length where other dimensions are the same. In this CFD simulation of turbulence mixing tee, we are considering two models for the solution to the problem, one is k-epsilon with RANS (Reynolds-Average Navier-stokes equation) and another one is the k-omega SST model. We are using these two models of the equation for solving the problem. The temperature near the wall is fluctuating after the mixing of hot air and cold air with some velocity. We can use the temperature and velocity counter-plot for the detailed analysis of the problem. When seen in the counter-plot diagram of temperature, after mixing the two-temperature air temperature will start fluctuating, this temperature diagram also depends upon the mesh element size.
In the temperature counter plot diagram also can see the velocity profile in the different positions, in the near the wall the velocity will be zero because of the no-slip condition, here the velocity profile will fluctuate as compared to the element mesh size. In the result after the simulation, some iterations will take to converge the solution. For the converge solution we use basically the discretization methods. These discretization methods are basically the computer language which use to solve the partial differential equations.
Discretization methods are 3 types
- Finite element method
- Finite difference method
- Finite volume method
We use these methods for structural analysis, conservation and for hyperbolic functions. For computational fluid dynamics, we use the finite volume method which gives a more accurate result. We can use any of the methods for any type of analysis problem but in CFD problems we use NS equations which conserve mass, momentum and energy equations. After meshing the fluid domain which gives a much more amount of small volume in the iteration solves each of the volumes to conserve the solution.
Pre-Processing & Solver Setting :-
We are using Ansys fluent for solving this problem, in Space Claim, we create two models for comparing the results. in the first case, we will take the length of the pipe 200mm with 40mm diameter and the T-section with a length of 40mm and a diameter is 20 mm. After creating the pipe wall, we can define the inside volume where fluid will flow in the simulation, we use this fluid volume as given below image as like this another model will create with the same dimensions except the length of the pipe which will take as 300mm. after the creating model we can open the mesh module.
Steps :-
1. Open new project, by clicking 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 :-
Case 1 :- 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.
In this case k-epsilon is giving better results because solution was converged after few iterations, whereas after using k-omega model solution was not converged. K-omega is preferred in studying fluid behaviour near wall, but here it is not the case. So, k-epsilon model is recommended to study of fluid far from wall i.e. flow region, in further cases.
Plot and contours from k-epsilon model
Fig. Residuals
Fig. Average Temperature vs Iterations
Fig. Standard Deviation of Temperature vs Iteration
Fig. Temeprature Contour
Fig. Velocity Contour
Fig. Temperature Streamline
Fig. Velocity Streamline
Plot and contours from k-omega model
Fig. Residuals
Fig. Average Temperature vs Iterations
Fig. Standard Deviation of Temperature vs Iteration
Fig. Temeprature Contour
Fig. Velocity Contour
Fig. Temperature Streamline
Fig. Velocity Streamline
Case 2 :- 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.
We used realized k-epsilon model and solution was converged after 156 iterations.
Fig. Residuals
Fig. Average Temperature vs Iterations
Fig. Standard Deviation of Temperature vs Iteration
Fig. Temeprature Contour
Fig. Velocity Contour
Fig. Temperature Streamline
Fig. Velocity Streamline
Comparison table :-
|
Parameters |
Cell Count |
Average outlet temperature |
Number of iterations to converge |
|
Case 1 |
12812 |
30.20C |
199 |
|
Case 2 |
15588 |
29C |
157 |
Mesh Independent Study :-
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.
Fig. Residuals
Fig. Average Temperature vs Iterations
Fig. Standard Deviation of Temperature vs Iteration
Conclusion :-
Ansys fluent is high performance CFD software recognized for accuracy, robustness etc. it is not confined to only aerospace and mechanical domain, it has various application like to simulation of calculation of air flow on air craft wing, combustion in furnace, bubble columns in oil refineries, combustion in IC engine etc. In this study we performed simulation on simple geometry of mixing tee and got insights of various feature of fluent. we can conclude by analysing graph that If momentum ratio is increased, outlet temperature will decrease and there is no effect of velocity even though length is increased.