Simulation on Mixing Effectiveness over a T channel.

Objective:

We have created two versions of the mixing tee. One of them is longer than the other. Our objective is to set up steady-state simulations to compare the mixing effectiveness when the hot inlet temperature is 36⁰C & the Cold inlet is at 19⁰C. 

Using the k-epsilon and k-omega SST model for the first case and based on your judgment use the more suitable model for further cases. Giving the reason for choosing a suitable model is compulsory. 

1. Case 1 
   • Short mixing tee with a hot inlet velocity of 3m/s.
   • Momentum ratio of 2, 4. 
2. Case 2 
   • Long mixing tee with a hot inlet velocity of 3m/s.
   • Momentum ratio of 2, 4.

Momentum ratio = velocity at cold inlet / velocity at hot inlet

In a table, compare the following values from the 2 cases. 

1. Cell count
2. Average outlet temperature
3. Number of iterations for convergence

Expected results:

1. Velocity and temperature contour plots on the cut planes along and across the pipe.
2. Velocity and temperature line plots along and across the length of the pipe.
3. Perform Mesh independent study for any one case.
4. Discuss the effect of length and momentum ratio on results. 

For each simulation, you need to show the convergence plot 

Reference Research paper:- https://www.sciencedirect.com/science/article/pii/S0029549309005676

[Note - It is just a reference research paper, those who do not have access to this paper can use any other research paper. Try to imitate the results from this paper, even if answers don't match]

Reference Formulas:-

Solution:

 CFD modeling of a mixing tee that is often found in the industry. Traditional simulation is validated against experiment, as well as a new commercially available method that offers the possibility of substantial solution time reduction.  In fact, the new method is shown to give accurate results in a much shorter computer time than the traditional analysis, allowing a much more rapid turnaround of difficult problems such as the turbulent mixing behavior of industrial mixing tees.

When there is a large temperature difference between two fluid streams, large temperature fluctuations can occur, which can lead to thermal fatigue of the piping system, even at “steady-state” bulk flow conditions. Advanced CFD modeling is capable of predicting these fluid temperature fluctuations at the mix point, as well as characterizing the corresponding temperature variations in the pipe wall itself. Specifically, large eddy simulation (LES) is required to capture the time-varying turbulent behavior at the mix point, which is significantly more time and resource-intensive than traditional two-parameter (e.g. k-e and k-w) Reynolds Averaged Navier-Stokes (RANS) simulation which is the workhorse of most industrial CFD simulation. In the first part of this twin blog, both the traditional LES approach and a new hybrid RANS-LES method are used to predict the mixing of the two fluid streams and the results are compared with test data reported in the literature for validation purposes. The new hybrid model is termed stress-blended eddy simulation (SBES) and has recently been included in the commercial software ANSYS/Fluent. SBES retains most of the fidelity of the traditional LES approach in a fraction of the time for typical problems. The SBES approach is then used in Part 2 as part of an actual industrial application to predict temperature fluctuations in a mixing tee. The simulation is used to determine the length of the thermal sleeve required to protect the pipe from thermal fatigue.

Turbulence is characterized by eddies with different space and time scales. In LES, large eddies are resolved directly, while small eddies are modeled using a subgrid model. LES requires significantly finer meshes and a smaller time step than those for a RANS model. SBES is a hybrid RANS-LES turbulence model, in which the boundary layer is modeled using an unsteady RANS model, while the LES model is applied to the core turbulent region where large turbulence scales play a dominant role. As such, SBES allows a coarser mesh and a larger time step than LES does.

The predicted time-averaged axial velocity component (u) and vertical velocity component (v) compare satisfactorily with the test data. The predicted fluctuations of the velocity components are also in good agreement with the measurements for both the LES and SBES simulations. It is whorth noting that the SBES simulation takes a much shorter time to solve than the LES simulation for the same basic results.

The model predictions show that the results from both the LES and SBES models are in excellent agreement with the measurements of the time-averaged and RMSE temperature and velocity components of the flow in a mixing tee. The SBES model can predict turbulent flow structures in the thermal mixing process. Since the SBES model requires a much lower computational cost and provides a faster turn-around of difficult problems than the LES model does, this approach will be used in Part 2 to predict temperature fluctuations in a mixing tee, which blends light gas oil with a recycled gas, to determine the length of a thermal sleeve which is used to protect the pressure boundary piping.

 

So first we are going to take a short mixing tee for simulation and here we extract and import the STEP file to Ansys-fluent. And the imported file is run in space claim, which is again an in-build CAD clean-up software in Ansys -Fluent. This is how we it looks like in the space claim. Although the steps to reach the solutions are the same only varies with grid dependencies test and variable momentum ratio test for both short and long mixing tee.