Week 1- Mixing Tee

Aim:

• To set up steady-state simulations and to compare mixing effectiveness then the hot inlet temperature is 36 C and the cold inlet temperature is 19 C.
• To compare the K-epsilon and k-omega models in the small-T model and get the proper result in the two models.
• Simulate the given velocity and momentum ratio in each case.
• Compare results in each model in a future case K-epsilon is best suited for the free shear flow through a wall then this model will be taken and plotted the results in two cases.

Objectives:

• Open an ANSYS workbench and set up a fluent simulation setup, and rename it.
• And then set a fluid setup and loading geometry.
• Identify the working fluid in the loaded geometry and run simulation there, close the space claim after opening.
• Then specify the boundary names for each setup in mesh setup before meshing for the fluid flow volume. And then update it.
• An open simulation setup has then given all the parameters for the simulation in the set-up section, later on, then including the simulation type, then given the all input parameters like velocity and temperature, materials (working fluid), and other metric parameters.
• Learn about the k-epsilon and K-omega SST concepts and then choose the better parameter in the future state model.
• After that simulate our cases to set how many iterations in the models are necessary for the given output result.
• Close the setup and then update it, to open the result section.
• The result of each case's cell count, average outlet temperature, and convergence iteration count was generated.
• Then study the effect of length and momentum ratio in each case.

Introduction:

• Computational fluid dynamics is a branch of fluid mechanics, then uses mathematics, physics, and computational software to visualize how the fluid flow affects its medium and its neighbors. In CFD software is mostly used to predict the fluid flow in the surfaces based on the governing equation like a conversation of momentum, conversation of energy, etc. This study uses ANSYS to give the result of fluid flow in the parameters. ANSYS Fluent is a very important fluid in the CAE domain. They are three steps of the processer namely pre-processor, solving, and post-processor. ANSYS fluent facilitates the modeling flow, turbulence, heat transfer, and chemical reaction.
• Then we will come in a mixing tee that will be used in specifically engineered internal geometry to mix two fluid streams in one stream, efficiently. In this study, we going to find the effectiveness of mixing flow in two different fluids are hot air and cold air are mixed in mixing tee. We will use two different short and long tees in the length of mixing.

Primary condition:

• Insert the K-epsilon and K-omega model in case1

Case1

a- Simulation 1:

inlet velocity - 3m/s

Momentum ratio - 4

b- Simulation 2:

inlet velocity - 3m/s

Momentum ratio - 2

Case2

a- Simulation 3:

inlet velocity - 3m/s

Momentum ratio - 2

procedure:

• To first study the two setup K-epsilon and k-omega SST. Here we are going to use the same geometry and mesh for both simulations. We are going to use the small tee to use the first setup.
• Now click on geometry to import the mixing short tee model. It will redirect to Spaceclaim.
• Then load geometry in setup and CAD cleanup software to prepare CAD geometry for simulation setup.

• We are to analyze the mixing of two fluids, to need to extract the volume of the tee. Go to prepare the menu and click on volume extract. Click the edge selection and click the three outer edges and click on the green check button.

• Hide solid by unchecking from the structure menu to see the volume, which we have extracted.
• Suppress solid now clicking on the suppress physics
• Exit and click on the mesh to open the mesh menu.
• Select an outer face and press the right click of a mouse, then will open a menu. Click on the option to create a named selection and write a suitable name on the face.
• Then going to meshing we will start ANSYS mesh then we can provide the name on all the surfaces on geometry.
• And create two fluent setups for K- epsilon and K- omega SST. Drag the existing mesh into the fluent analysis setup cell. Then update the mesh (Right-click on mesh and use the update button) to convert and write the mesh for the FLUENT analysis.

• And we can also check the quality of the mesh by clicking the option called element quality in the quality section of the mesh panel.
• Then, double-click on the setup option in the fluent cell for trying the k-epsilon method. it will open the fluent interface.
• In the Fluent window, first, see the console window which displays the text, and also accepts text user interface commands. They are a lot of text about loading meshing cells, and text about achromatic setting boundary conditions in fluent as per the name provided to surfaces.
• After clicking the mesh quality button, it prints the following confirmation in the console window. The mesh cell ensures each cell is in the correct format and connected to other cells as expected. check every mesh immediately after reading it. In case any check indicates failure they will be bah mesh that will need repairs prior to simulation.

• Next, in the domain panel by using the check button then check whether our mesh is preferable or not. And we can improve the quality using modify option.

• Then click the units bar to change the units of temperature. fluent store values in SI units. Most post-processing can be converted into other units.
• And we will set the physics of the domain. Keep the general setting parameter as it is. Solver type -> Pressure-based; Velocity formulation -> Absolute; Solver time -> steady.

• Go to the physics menu and select a viscous model. It will open a menu, choose a k-epsilon model and close it.
• In material Click FLUENT DATABASE to add new material. Working fluid is air click the air.
• In the viscus model, select the k-epsilon and realizable for our first type in the simulation. The turbulence model is a complex area. The choice of the model depends on the application. Here the k-epsilon model is used.

• Now go to the boundary option to set the boundary condition. Select inlet X and click on the option edit. It will open a menu, all the parameters will be set it is thermal, radiant, momentum, etc. Put the velocity value in 3m/s and set the temperature to 36 c, for hot fluid.

• And, a similar way create a plot average velocity plot at the outlet boundary condition
• The final step is to initialize the problem setup.
• Initialization creates the initial solution then the solver will iteratively improve, the same converged solution is reached whether the initialization, through convergence is easier if they are similar. Initialization imposes the same value in all cells.
• The hybrid initialization method is an efficient method of initializing the solution based purely on the setup of the simulation with no extra information required. These methods produce a velocity field that conforms to complex domain geometric and a pressure field smoothly connects high and low.
• Now we can run the simulation then enter the number of iterations for which we want to run the simulation and press calculate.
• Repeat the same procedure for cold fluid coming, velocity - 6m/s, temperature - 19c
• We want to generate a temperature report of the outlet and we want a standard deviation of temperature also.
• Go to the solution tab and select definition and select area weight average. In case the opened menu selects temperature is the field variable. Similarly, select the standard deviation in the definitions menu.
• Now click on initialization to calculate approximate values of velocity and pressure. Change the number of iterations to 250 and another click on calculate. If a solution is not converged, then the iterations can be increased.
• Exit setup then double-clicks on the result to open the CFD post. Check the box to see the walls and other parts.
• CFD-post initially displays the outline (wireframe) of the model which can be turned on by checking the surfaces shown in the outline panel in the fluid section.
• Viewer toolbar buttons allow you to manipulate the view.
• Create a panel in the location menu, select plane -> accept the default name plane 1 -> set method to be XY plane, accept z as 0.0, and press apply
• K-Epsilon: As we saw in our first simulation, the k-epsilon turbulence model is the most common model used in CFD to simulate mean flow characteristics for turbulent flow conditions. It is a two-equation model that gives a description of turbulence by means of two transport equations (PDEs). The first is turbulent kinetic energy and the second is rates of dissipation of turbulent kinetic energy. It focuses on the mechanism that affects the turbulent kinetic energy.
• K-omega SST: It is a two-equation eddy viscosity model that is used for aerodynamic applications. It is the combination of that two models. This ensures that the appropriate model is utilized through the flow fluid.

Results:

• The viscous sublayer flow can be simulated by using the k-omega model. Then the k-epsilon model predicts flow from an area far from the wall. Compare to many other models, the SST is less sensitive to free stream conditions.
• The k-omega model is assisted by the shear stress limit in preventing an accumulation of excessive turbulent kinetic energy close to stagnation points. The difference between the two results k-epsilon is the proper mixing of the two air streams compared to the k-omega model we will look to see in the figure. Hence the k-epsilon is used in the future simulation.

Now, let us start our simulation with case 1-a

1. Case-1-a(Short mixing tee):

• a- Simulation-1,
• Inlet velocity - 3m/s
• Momentum ratio - 4
• Inlet velocity of cold air = 3*4 = 12m/s

• Residual plot:

The solution is converging at 310 iterations.

• Average Temperature VS Iterations:

Area weighted average outlet temperature:

The average temperature at the outlet is 18.19 C

• Standard Deviation of Temperature vs Iteration:

• Temperature contour on the plane along the length of the pipe:

• Velocity contour on the plane along the length of pipe:

2. Case-1-b(Short mixing tee):

• b- Simulation-1,
• Inlet velocity - 3m/s
• Momentum ratio - 2
• Inlet velocity of cold air = 3*2 = 6m/s

• Residual plot:

The solution is converging at 250 iterations.

• Average Temperature VS Iterations:

Area weighted average outlet temperature:

The average temperature at the outlet is 28.14 C

• Standard Deviation of Temperature vs Iteration:

• Temperature contour on the plane along the length of the pipe:

• Velocity contour on the plane along the length of pipe:

3. Case-2-a(Long mixing tee):

• a- Simulation-1,
• Inlet velocity - 3m/s
• Momentum ratio - 4
• Inlet velocity of cold air = 3*4 = 12m/s

• Residual plot:

The solution is converging at 300 iterations.

• Average Temperature VS Iterations:

Area weighted average outlet temperature:

The average temperature at the outlet is 32.6 C

• Standard Deviation of Temperature vs Iteration:

• Temperature contour on the plane along the length of the pipe:

• Velocity contour on the plane along the length of pipe:

Conclusion:

• Solving the first simulation on the fluid domain using coarse mesh gives an initial overview of the analysis which convergence results in fact. As the element number increases the convergence time also increases. After obtaining the rough result using the course mesh simulation successfully we can improve mesh quality and get to the final accurate results doing a lot of time which is also extremely helpful for the mesh independence test.
• To get better mixing at outlet high velocity of flow is very effective. Therefore, a higher moment ratio gives a better mixing ratio at the outlet.
• The length of the mixing tee increases because diffusion flow always gives a slightly better mixing of two fluids.
• In this study, we performed a simulation of mixing tee and got insights into various features of fluent. We can plot the graph the momentum ratio is increased, outlet temperature will decrease and there is no effect of velocity even though the length is increased.
• The difference between a short and long mixing tee and the long mixing tee with momentum would be preferable.