Mixing efficiency
EFFICIENCY OF MIXING TEE
AIM:-
To set up steady-state simulations to compare the mixing effectiveness of a mixing Tee ,for the following cases:
- Short mixing tee
- Long mixing tee
And to discuss the effect of length and momentum ratio on both cases, and also to perform a mesh independent case study by using CFD in Ansys Fluent Software.
INTRODUCTION:-
Mixing Tees utilizes a specifically engineered internal geometry to efficiently mix two fluid streams into one combined stream to produce a steady flow and used specifically for high pressured applications,such as microbore or analytical gradient HPLC.Hence to analyse the mixing efficiency of a mixing tee we could be able to provide an efficeint deisgn.The conditions can be analysed by using Computerised Fluid Dynamic in Ansys Fluent to showcase the effective mixing of the two fulids at the inlet and can be optimised accordingly.Computational Fluid Dynamics (CFD) is the science of predicting fluid flow,
heat and mass transfer, chemical reactions, and related phenomena by solving numerically the set of governing mathematical equations.It complements testing and experimentation by reducing the total work and cost.
THEORY:
ANSYS CFD solvers are based on the finite volume method,
- Domain is discretised into a finite set of control volume.
- General conservation equations for mass, momentum, energy, species, etc. are solved on this set of control volumes.
- Partial differential equations are discretised into a system of algebraic equations.
- All algebraic equations are then solved numerically to render the solution field.
Initially the process involves three major Categories,
Pre-Processing
- Geometry
- Mesh
- Physics
- Solver Settings
Solve
- Compute solution.
Post Processing
- Examine results.
CASE 1:-
For case 1 we will be using Short mixing tee with Length 200mm and with a hot inlet velocity of 3m/s,with Momentum ratio at 2,4.
Geometry
- Intilally the mixing tee is modeled in Space claim and the boundaries is defined.
- Then the model is prepared by using the option volume extraction to get proper flow area.
- Next step is to hide the unnecessary parts and suppress the physics for that part alone to have a proper simulation.
This is the Base model of a Short Tee. This is the Volume Extracted Model of Short Tee.
Meshing:-
- After Modeling the Short Tee,Next is to Mesh.
- First various parts are named acoordingly via the face selection process.
- After naming the parts, we set the desired element size for meshing.
- For fine meshing I have given element size of 0.002.
Element Quality Meshed Component
Setup:-
- After Meshing it is time to setup the solver.For a proper analysis the following conditions are selected
- Steady state condition.
- Pressure based solver since for lower Mach number it is suitable.
- Energy equation is selected
- And for viscous condition K-Omega SST is selected.
Selection Between K Epsilon and K Omega SST Model:-
K epsilon is suitable for high Reynolds number or in other words proper for simulation of free flow as it relates the rate of kinematic dissipation to the turbulent viscosity while K- omega relates it by frequency meaning that K -omega suits for low Reynolds number regions.Also k- epsilon model predicts well far from the boundaries (wall) and k- omega model predicts well near wall. Even though it depends on Y+.I have done some simulations by using both the types using the same parameters and this is the results I have concluded.
K Epsilon K Omega SST
As we could infer from the above simulation that in the case K-Epsilon model that at the edged (i.e. the boundaries) are not clearly depicted when compared to the K-Omega SST model.Therefore K-Omega SST model is more suitable to be used because of the reason that it clearly depicts the temperatures in the boundaries also.
Solution:-
- After Selecting the Viscousity Model,next step is to assign the boundary conditions to vaarious parts of the mixing TEE.
- Since we did the naming properly in the previous step,Ansys Fluent Software idntifies the specific keywords assigned to it and it atomatically sets the boundary condition accordingly.
- For Inlet X and Inlet Y we should assign as Velocity Inlet.
- Assaign the velocity value and temperature value according to the question.For the velocity of Cold inlet Momentum ratio is given as 2,4.
- Momentum ratio = velocity at cold inlet / velocity at hot inlet.
- By using the above formula we get that inlet velocity for cold inlet as 6m/s & 12m/s.
- Therefore the solution is solved for both the velocities accordingly.
Velocity at 6m/s Velocity at 12m/s
- And for Outlet X it should be assigned with Pressure outlet.
- Also for walls the Boundary conditions should be given as walls for it.
- After setting the boundaries next step is to set definations as to gain additional result data.
- Next step is to Initialize the setup by using Hybrid Initialization.
- After initializing we should give the input number of Iterations so as to calculate or to solve the problem.
POST PROCESSING:-
- After Solving the problem, next step is to display the output in a clear cut manner.
- By deselecting both the inlets and outlets and the wall alone is selected.
- Then the appearance of the wall is altered to desirable to get it almost transparent in nature.
- Then after that a plane is added along and acroos the pipe.
- By doing so we can get the actual simulation result in the plane by changing the color parameter to variable for temperature and velocity.
- Likewise line is also is drawn alonga and across the pipe and plotted the temperature and velocity in a chart.
Case 2:-
For case 2 we will be using Long mixing tee with Length 250mm and with a hot inlet velocity of 3m/s,with Momentum ratio at 2,4.
Geometry
- Intilally the mixing tee is modeled in Space claim and the boundaries is defined.
- Then the model is prepared by using the option volume extraction to get proper flow area.
- Next step is to hide the unnecessary parts and suppress the physics for that part alone to have a proper simulation.
This is the Volume Extracted Model of Long Tee.
Meshing:-
- After Modeling the Long Tee,Next is to Mesh.
- First various parts are named acoordingly via the face selection process.
- After naming the parts, we set the desired element size for meshing.
- For fine meshing I have given element size of 0.002.
Element Quality Meshed Component
Setup:-
- After Meshing it is time to setup the solver.For a proper analysis the following conditions are selected
- Steady state condition.
- Pressure based solver since for lower Mach number it is suitable.
- Energy equation is selected
- And for viscous condition K-Omega SST is selected.
Solution:-
- After Selecting the Viscousity Model,next step is to assign the boundary conditions to vaarious parts of the mixing TEE.
- Since we did the naming properly in the previous step,Ansys Fluent Software idntifies the specific keywords assigned to it and it atomatically sets the boundary condition accordingly.
- For Inlet X and Inlet Y we should assign as Velocity Inlet.
- Assaign the velocity value and temperature value according to the question.For the velocity of Cold inlet Momentum ratio is given as 2,4.
- Momentum ratio = velocity at cold inlet / velocity at hot inlet.
- By using the above formula we get that inlet velocity for cold inlet as 6m/s & 12m/s.
- Therefore the solution is solved for both the velocities accordingly.
Velocity at 6m/s Velocity at 12m/s
- And for Outlet X it should be assigned with Pressure outlet.
- Also for walls the Boundary conditions should be given as walls for
- After setting the boundaries next step is to set definations as to gain additional result data.
- Next step is to Initialize the setup by using Hybrid Initialization.
- After initializing we should give the input number of Iterations so as to calculate or to solve the problem.
POST PROCESSING:-
- After Solving the problem, next step is to display the output in a clear cut manner.
- By deselecting both the inlets and outlets and the wall alone is selected.
- Then the appearance of the wall is altered to desirable to get it almost transparent in nature.
- Then after that a plane is added along and acroos the pipe.
- By doing so we can get the actual simulation result in the plane by changing the color parameter to variable for temperature and velocity.
- Likewise line is also is drawn alonga and across the pipe and plotted the temperature and velocity in a chart.
Mesh Independent Study on Short Mixing TEE:-
- To analyse the mesh impact on results I have considered three diffrent Mesh size for Consideration having the same parameters and same model as reference.
- 1)0.01 2)0.005 3)0.002.
1) Element Size of 0.01(Default):-
This is the default mesh quality given by the software as it is shown it has the least number of elements and nodes when compared with other cases. Element is much larger.
Nodes :2837
Elements :12760
2)Element size of 0.005:-
This is the second case of mesh study which is the average of both the considerations.It has a higher count in nodes and elements when compared with 1st case and lower compared with 3rd case.
Nodes :3152
Elements:14330
3)Element Size of 0.002:-
It has the smallest element size when compared with other cases and it has the highest amount of nodes and elemnts in the sample.Element size is much smaller among all the cases.
Nodes :21347
Elements:105865
| Element Size | 0.01 | 0.005 | 0.002 |
| Nodes | 2837 | 3152 | 21347 |
| Elements | 12760 | 14330 | 105865 |
To get the clear idea of which is the best mesh I have done a simulation on three diffrent mesh quality with same problem,Results are shown below.
Element size of 0.01
Element size of 0.005
Element size of 0.002
According to the simulation results, as the element size keeps on decreasing the temperature flow and quality of the analysis develops drasticaly.In case 1 having a larger element size tends to have a coarse results in the image and tends to decrease slightly in the case 2 where the element quality is much lower and the results in the image 3 is more finer and has great quality than other 2 cases having the smallest element size.But on the downside larger the number of elements higher the rendering time and time taken for solving also increases.Hence element size of 0.002 is optimal for showing much finer results and to take a decent amount of time.
Results:-
By doing almost 4 complete simulations i.e. 2 on short tee and 2 on long tee the following reults are observed.
|
CASE 1(Short Tee) (6m/s) |
CASE 1(Short Tee) (12m/s) |
CASE 2(Long Tee) (6m/s) |
CASE 2(Long Tee) (12m/s) |
|
| Cell Count | 105865 | 105865 | 140025 | 140025 |
| Average Outlet Temperature | 30.80 (C) | 27.50 (C) | 30.85(C) | 27.65 (C) |
| No. of Iterations for Convergence | 110 | 150 | 110 | 150 |
Convergence Plot:-
CASE 1(Short Tee at 6m/s):-
Area Weighted Average of Temperature Standard Deviation of Temperature
CASE 1(Short Tee at 12m/s):-
Area Weighted Average of Temperature Standard Deviation of Temperature
CASE 2(Long Tee at 6m/s):-
Area Weighted Average of Temperature Standard Deviation of Temperature
CASE 2(Long Tee at 12m/s):-
Area Weighted Average of Temperature Standard Deviation of Temperature
Simulation Results:-
Here are the simulation results of both long and short mixing TEE, on Velocity and Temperatures.
CASE 1(Short Tee at 6m/s):-
Temperature along Short TEE at 6 m/s Velocity along Short TEE at 6 m/s
Temperature across Short TEE at 6 m/s Velocity across Short TEE at 6m/s
Temperature Line chart of Short Tee at 6 m/s Velocity Line chart of Short Tee at 6 m/s
CASE 1(Short Tee at 12m/s):-
Temperature along Short TEE at 12 m/s Velocity along Short TEE at 12 m/s
Temperature across Short TEE at 12 m/s Velocity across Short TEE at 12 m/s
Temperature Line chart of Short Tee at 12 m/s Velocity Line chart of Short Tee at 12 m/s
CASE 2(Long Tee at 6m/s):-
Temperature along Long TEE at 6 m/s Velocity along Long TEE at 6 m/s
Temperature across LongTEE at 6 m/s Velocity across Long TEE at 6m/s
Temperature Line chart of Long Tee at 6 m/s Velocity Line chart of Long Tee at 6 m/s
CASE 2(Long Tee at 12m/s):-
Temperature along Long TEE at 12 m/s Velocity along Long TEE at 12 m/s
Temperature across LongTEE at 12 m/s Velocity across Long TEE at 12 m/s
Temperature Line chart of Long Tee at 12 m/s Velocity Line chart of Long Tee at 12 m/s
Conclusion:-
By comaparing the above simulations we could conclude that for maximum efficency of a mixing tee does not depend upon the length of the Mixing TEE but Rather depends upon the Velocity of the cold Inlet.It furthers Explains the Impact of High Inlet Velocity of Cold Inlet Can maximise the efficiency and best suitable for various applications.We could infer the above statement by comparing both the short and lon tee simulations and also from the various moment ratios given.Hence to conclude for maximum efficency of a mixing tee to attain the best case is Short mixing TEE with cold inlet Velocity at 12 m/s.