Week 1- Mixing Tee
Aim:-
To simulate the fow of air through mixing tee pipe and understand the effect of lenght, momentum ratio of velocities for mixing air by using two types of pipes(i.e Short pipe and Long pipe).
Introduction:-
In industrial process engineering, mixing is a unit operation that involves the manipulation of
a heterogeneous physical system with the intent to make it more homogeneous. Mixing is performed to allow heat
and/or mass transfer to occur between one or more streams, components or phases. Modern industrial processing
almost always involves some form of mixing. Mixing tee is a type of mixing mechanism. Mixing Tees utilizes a
specifically engineered internal geometry to efficiently mix two fluid streams into one combined stream. Mixing tees
are widely used in the petrochemical, HVAC industry etc. in which two fluid streams with different physical and/or
chemical properties mix together.
Hot air flowing in the main pipe is mixed with cold air flowing through a tee. The standard deviation of temperature is
computed to quantify the degree of mixing. The velocity and temperature fields are also computed. The effects of the
mesh size, turbulence model on results were examined.
For this, we have created two versions of the mixing tee. One of them is longer than the other.
It is set up as steady-state simulations to compare the mixing effectiveness when hot inlet temperature is 36°c & the
Cold inlet is at 19°c.
CASE Setup :-
CAD clean up/Pre-process-SpaceClaim - The CAD model is imported into the SpaceCliam to extract the flow volume field from the model.
Meshing :- Apply boundary names at different faces and generate the mesh by giving the value element size. Apply the element metrics to check the quality of the mesh elements.
Setup :- In this section, we set up the simulation. Turbulance models are applied like kε and kω SST for solving turbulant mixing air. And then we apply cell zone condition for simulation. we apply different boundary conditions like inlet , outlet boundary conditions. Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution :- We get the plot of residual, Average outlet temperature, Average outlet velocity and temperature deviations. From report definition we can compute the values of Average outlet temperature, Average outlet velocity and temperature deviations.
Results :- Post processing is carried in this section in which different contours are obtained to visualize the properties (such as temperature and velocity) for which simulation is run.
CASE 1:- SHORT TEE PIPE
Set up of Geomery
CAD clean up - The CAD model is imported into the SpaceCliam to extract the flow volume field from the model. The extracted volume is shown below in fig.
Meshing :-
The element size = 1mm (0.001m)
The number of elements = 569312
Element Metrics:-
A mesh element metrics that identify the quality of mesh is employed and it can be seen that the element of lowest quality are minimal. Most of the elements have better quality in the range of 0.7-1.
a) Momentum ratio = 2
Inlet velocity at x direction (velocity at hot inlet)=3 m/s at 36 degree celsius.
Momentum ratio= velocity at cold inlet/ velocity at hot inlet
Inlet velocity at y direction (velocity at cold inlet)=6 m/s at 19 degree celsius.
i) kε - realizable model
Setup :- Turbulance models kε - realizable is applied for solving turbulant mixing air. Select the material as air. And then we apply cell zone condition for simulation. we apply different boundary conditions like velocity-inlet , pressure-outlet at respective temperatures . Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution:-
Residuals
Average outlet temperature: 30.41 degree Celsius
Deviation of temperature: 1.9 degree Celsius
Average output velocity: 4.51 m/s
Results :-
Contours
Temperature contour
velocity contour
ii) kω - SST model
Setup :- Turbulance models kω - SST is applied for solving turbulant mixing air. Select the material as air. And then we apply cell zone condition for simulation. we apply different boundary conditions like velocity-inlet , pressure-outlet at respective temperatures . Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution:-
Residuals
Average outlet temperature: 32.47 degree Celsius
Deviation of temperature : 3.24 degree Celsius
Average output velocity: 3.75 m/s
Results:-
Temperature contour
Velocity contour
b) Momentum ratio = 4
Inlet velocity at x direction (velocity at hot inlet)=3 m/s at 36 degree celsius.
Momentum ratio= velocity at cold inlet/ velocity at hot inlet
Inlet velocity at y direction (velocity at cold inlet)=12 m/s at 19 degree celsius.
i) kε - realizable model
Setup :- Turbulance models kε - realizable is applied for solving turbulant mixing air. Select the material as air. And then we apply cell zone condition for simulation. we apply different boundary conditions like velocity-inlet , pressure-outlet at respective temperatures . Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution:-
Residuals
Average outlet temperature: 27.56 degree Celsius
Deviation of temperature : 1.37 degree Celsius
Average output velocity: 6.06 m/s
Results:-
Temperature contour
Velocity contour
ii) kω - SST model
Setup :- Turbulance models kω - SST is applied for solving turbulant mixing air. Select the material as air. And then we apply cell zone condition for simulation. we apply different boundary conditions like velocity-inlet , pressure-outlet at respective temperatures . Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution:-
Residuals
Average outlet temperature: 27.57 degree Celsius
Deviation of temperature: 1.58 degree Celsius
Average output velocity: 6.08 m/s
Results:-
Temperature contour
Velocity contour
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. K-epsilon model gives lesser deviation compare to k-omega model. so K-epsilon model is chosen for further simulations.
CASE 2:- LONG TEE PIPE
Set up of Geomery
CAD clean up - The CAD model is imported into the SpaceCliam to extract the flow volume field from the model. The extracted volume is shown below in fig.
Meshing :-
The element size = 1mm (0.001m)
The number of elements = 757117
Element Metrics:-
A mesh element metrics that identify the quality of mesh is employed and it can be seen that the element of lowest quality are minimal. Most of the elements have better quality in the range of 0.75-1.
a) Momentum ratio = 2
Inlet velocity at x direction (velocity at hot inlet)=3 m/s at 36 degree celsius.
Momentum ratio= velocity at cold inlet/ velocity at hot inlet
Inlet velocity at y direction (velocity at cold inlet)=6 m/s at 19 degree celsius.
Setup :- Turbulance models kε - realizable is applied for solving turbulant mixing air. Select the material as air. And then we apply cell zone condition for simulation. we apply different boundary conditions like velocity-inlet , pressure-outlet at respective temperatures . Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution:-
Residuals
Average outlet temperature: 30.48 degree Celsius
Deviation of temperature: 1.2 degree Celsius
Average output velocity: 4.51 m/s
Results :-
Contours
Temperature contour
velocity contour
b) Momentum ratio = 4
Inlet velocity at x direction (velocity at hot inlet)=3 m/s at 36 degree celsius.
Momentum ratio= velocity at cold inlet/ velocity at hot inlet
Inlet velocity at y direction (velocity at cold inlet)=12 m/s at 19 degree celsius.
Setup :- Turbulance models kε - realizable is applied for solving turbulant mixing air. Select the material as air. And then we apply cell zone condition for simulation. we apply different boundary conditions like velocity-inlet , pressure-outlet at respective temperatures . Then we prepare the report definition for Average outlet temperature, Average outlet velocity and temperature deviations. After this initialize the process by clicking on hybrid initialization. Certain No of iterations are set for simulation.
Solution:-
Residuals
Average outlet temperature: 27.48 degree Celsius
Deviation of temperature: 0.92 degree Celsius
Average output velocity: 6.04 m/s
Results :-
Contours
Temperature contour
velocity contour
Comparision between two cases:-
Mesh independent study:-
Here I take the short tee joint pipe for momentum ratio=2
From the above table , we can say that mesh is independent that means there is no large change in outlet by increasing the number of elements.
Effect of length and momentum ratio on results:
From above results we can say that outlet temperature is independent of length of the mixing tee.
Momentum ratio effects the outlet temperature. By increasing the momentum ratio outlet temperature is
dropping.
Conclusion:-
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.
The temperature contour at the outlet for short tee cases shows that some of the high temperatures flow sips
through without properly mixing with the flow. Whereas, in the temperature contour of long tee cases, the high
temperatures flow thoroughly mixes with the rest of the flow.
Therefore, we can recommend using the long tee and high-velocity inlet for application. This is because we
have a low standard deviation in case of long tee compared to the short tee for both momentum ratios