Week 1- Mixing Tee

AIM: - To set up steady-state simulation to compare the mixing effectiveness when hot temperature is 36⁰C & the Cold inlet is at 19⁰C.

OBJECTIVES: -

<!-- [if !supportLists]-->a)     <!--[endif]-->Set up steady-state simulation for:

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.

<!-- [if !supportLists]-->b)     <!--[endif]-->compare the following values from the 2 cases.

Cell count

       Average outlet temperature

       Number of iterations for convergence.

<!-- [if !supportLists]-->c)      <!--[endif]-->To make the following results.

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

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

       Perform Mesh independent study for any one case.

       Discuss the effect of length and momentum ratio on results.

SOLUTION:

The given challenge was completed using the ANSYS Workbench tool. The steps performed in doing so are as follows:

STEP 1: Prepare the geometry for the simulation.

Start Ansys Workbench and double click on Fluid Flow (Fluent) to begin with the problem.

AIM: - To set up steady-state simulation to compare the mixing effectiveness when hot temperature is 36⁰C & the Cold inlet is at 19⁰C.

OBJECTIVES: -

<!-- [if !supportLists]-->a)     <!--[endif]-->Set up steady-state simulation for:

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.

 

<!-- [if !supportLists]-->b)     <!--[endif]-->compare the following values from the 2 cases.

Cell count

       Average outlet temperature

       Number of iterations for convergence.

<!-- [if !supportLists]-->c)      <!--[endif]-->To make the following results.

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

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

       Perform Mesh independent study for any one case.

       Discuss the effect of length and momentum ratio on results.

SOLUTION:

The given challenge was completed using the ANSYS Workbench tool. The steps performed in doing so are as follows:

STEP 1: Prepare the geometry for the simulation.

Start Ansys Workbench and double click on Fluid Flow (Fluent) to begin with the problem.

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Import the geometry file into the project and perform the clean-up necessary for the simulation, as shown in the figures below:

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Close the SpaceClaim module and in the double click on the Meshing section in the project to move further.

 

 

STEP 2: Meshing.

In this step, first label the important regions of the geometry, such as, the inlets, outlets and wall.

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Next, the above geometry is meshed with the Default element size of approx. 10 mm.

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The mesh will look like this:

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The element size is one of the parameters which will be reduced from the current value of 10 mm upto 2 mm in order to perform the Mesh Independent Study.

Close the Meshing module and update the Mesh in the Workbench Project.

STEP 3: Setup the problem.

 

In this step, double click on Setup to launch the Fluent module and setup the problem that is to be solved.

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The model in the fluent looks like this:

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To begin with the setting up of the problem, first, the mesh is checked whether it is good or not. If the mesh is good to go the fluent will give a completion message in the console panel.

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Next, Go to the Physics panel and:

Check the Energy box.

Set the Solver to Pressure Based & Time to Steady.

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In the Zones section, open up Boundaries and enter the values of the velocity of the hot and cold

Fluids as mentioned in the problem. Change the temperature units to Celsius and update the hot and cold fluid temperatures according to the given problem.

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Hot Fluid:

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Cold Fluid:

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The Outlet face conditions are shown below:

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Next, in the Reports section create a new surface averaged report to gather the average value data and plots of the Temperature & Velocity at the Outlet face.

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Also, check the Report File & Report Plot boxes in the create section to have report and plots ready.

The Initialization is done through Hybrid method.

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Lastly, the number of iterations is submitted equal to 150 and click on Calculate to begin the solving of the problem.

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The above steps are then repeated to perform the analysis for the second case wherein the momentum ratio is equal to 4. The only change that is to be done is to change the velocity magnitude of the cold fluid from 6 m/s to 12 m/s.

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To check the number of iterations elapsed for the convergence, 2 convergence settings are added to the problem setup. This will indicate the convergence iterations.

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Mesh Independent Study:

 

RESULTS:

Following are the different results obtained from the simulations run with different inlet velocity, turbulence model and the geometry.

 

CASE 1: Short Tee Mixing:

 

1. K – Epsilon Turbulence Model, Momentum Ratio = 2:

 

Weighted Residual:

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Average Outlet Temperature:

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From above figures, the average outlet temperature can be seen around 30^oC.

Also, the mixing is of both the fluid streams is not perfect. Since, the hot fluid in the lower regions of the pipe is untouched.

Average Outlet Velocity:

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Above figures show the momentum mixing process during the simulation:

It can be seen that the momentum mixing is occurring throughout the control volume and the average velocity is almost equal to 4.5 m/s.

 

 

 

2. K – Epsilon Turbulence Model, Momentum Ratio = 4:

 

Residual plot:

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Average Outlet Temperature:

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From above figures, the average outlet temperature can be seen around 27.58^oC.

The mixing phenomenon in this case is observed to be better than the previous one, the mixing depth is covering entire diametral depth of the control volume.

 

Average Outlet Velocity:

 

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Above figures show the momentum mixing process during the simulation:

It can be seen that the momentum mixing is occurring throughout the control volume and the average velocity is almost equal to 6.01m/s.

The velocity contour shows a uniform velocity distribution in the near the pipe exit.

3. K – Omega Turbulence Model, Momentum Ratio = 2:

 

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Average Outlet Temperature:

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From above figures, the average outlet temperature can be seen around 30.25^oC.

Also, the mixing is of both the fluid streams is not perfect. Since, the hot fluid in the lower regions of the pipe is untouched.

 

Average Outlet Velocity:

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Above figures show the momentum mixing process during the simulation:

It can be seen that the momentum mixing is occurring throughout the control volume and the average velocity is almost equal to 4.5 m/s.

 

 

4. K – Omega Turbulence Model, Momentum Ratio = 4:

 

Residual Plot:

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Average Outlet Temperature:

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From above figures, the average outlet temperature can be seen around 27.58^oC.

The mixing phenomenon in this case is more uniform, the mixing depth is covering entire diametral depth of the control volume.

 

Average Outlet Velocity: 

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Above figures show the momentum mixing process during the simulation:

It can be seen that the momentum mixing is occurring throughout the control volume and the average velocity is almost equal to 6.022 m/s.

The velocity contour shows a uniform velocity distribution in the near the pipe exit.

 

 

The same results were seen for the Long Tee Mixing simulations, the results for all the analysis done are tabulated below.

 

	COLD FLUID VELOCITY = 6 m/sec		COLD FLUID VELOCITY = 12 m/sec		COLD FLUID VELOCITY = 6 m/sec		COLD FLUID VELOCITY = 12 m/sec
	K-Epsilon Method		K-Epsilon Method		K-Omega Method		K-Omega Method
	Long Tee	Short Tee	Long Tee	Short Tee	Long Tee	Short Tee	Long Tee	Short Tee
Avg. Outlet Temperature	30.3193	30.1824	27.5321	27.5839	30.3980	30.2517	27.5203	27.5839
Avg. Outlet Velocity	4.48	4.49	5.99	6.01	4.48	4.49	5.99	6.02
Convergence Iterations	62	61	65	61	61	63	68	61

Conclusions:

From the above results it can be inferred that:

1. The mixing phenomena is dependent upon the velocity of the fluid undergoing the mixing process, mixing efficiency is more when the velocity of cold fluid is 12 m/s.

2. The exit temperature is also dependent to the fluid velocity; the exit temperature is lower for 12 m/s case.

3. The mesh dependency test shows that the model needs a greater number of iterations for convergence as there are more cells for calculations.

4. The average outlet temperature value for 3 mm mesh and 2 mm mesh is almost equal, therefore, the mesh size of 3 mm will be appropriate for further analysis.