Mixing efficiency simulation in T-shape tube.

Aim:- To perform mixing efficiency simulation in T-shape tube.

Introduction:-

The mixing process of hot and cold fluids in a tee junction is chaotic (turbulent) in nature and can result in high cycle thermal fatigue of the junction. This random quasi steady state phenomenon of hot and cold shocks can lead to fatigue cracks and possible leakage, causing a structural integrity and process safety concern. Standard Computational Fluid Dynamics (CFD) two parameter turbulence models (Reynolds Averaged Navier Stokes Models) cannot predict this phenomenon, and advanced analysis, specifically Large Eddy Simulation (LES) is required to obtain meaningful results. This paper presents a comprehensive analysis of a petrochemical industry mixing point, including LES, a standard two parameter CFD turbulence model, finite element stress analysis, fatigue and fatigue crack growth approaches. This investigation is driven by extensive analysis of operational data, and is rigorously benchmarked against the observed in-service failure of the mixing tee.

Theory :-

In this project, We have worked upon how two different fluid having different temperature gets mixes with each other and produces a diffused flow into the domain of tube having T-shape or in simpler words, how diffusion takes place in T-joint tube. Here, Both fluid have different inlet velocity, fluid which comes from y axis inlet and velocity which comes from x axis inlet. The main aim behind this mixing of two different fluid in T-shape tube is to anlayse that how diffusion and turbulence take place between the two fluid.

To fulfill such desire, We have made up a model whose geometry is in T-shape. The model have been created and developed in CAD package 'Solidworks'. In CAD package, We basically have created two types of model i.e short mixing Tee and long mixing Tee. Then this models have been imported in Ansys simulation package, Where geometry have been cleaned up, meshed, setup and finally post-processed the results.

Here, We have imported two types of geometry in Space-claim :-

One is short mixing tee whose dimension is approx 0.26 m

and, another one is long mixing tee 0.19 m

Since, We are interested in knowing that what happens when diffusion takes in T-joint tube rather how it happens within the T-joint tube. Hence, We have performed steady state solution which gives final state solution.

We have used following setup for simulation which is given below :-

a. Compared k-epsilon and k-omega SST model for the first case with reason.

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.

Note:- Momentum ratio = velocity at cold inlet / velocity at hot inlet

b. In a table, We have compared the following values from the 2 cases.

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

c. 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.

Applications :-

(i) In the nuclear reactors and general plants, an elbow is often used near the mixing tee and it brings biased velocity distribution and also the secondary flow. In this study, influences of upstream elbow in the main pipe are studied in a water experiment of mixing tee with the elbow.

(ii) In analysis of ventilation inside chamber, where comporessed cold air is mixed with hot air present inside the room and the relative temperature is adjusted according to temperature sensors used by the devices present inside chambers which governs the compressed air motion.

Preprocessing and solver setting approach :-

1. Imported Geometry in Space-claim

In this, We imported the geometry(Tee-shape tube) in Space-claim, where the geometry was in raw state as created in CAD package software. Here, We have represented the geometry in trimetric view given below:-

2. Cleaned up geometry in Space-claim

Since Geometry was in solid state where the body was having thickness which need to be remove so as to make geometry simple and can be converted into volumetric form. The newly updated geometry in volumetric form was as given below,

3. Meshed geometry in Design-modeler

Here, We imported the volumetric geometry created in Space-claim after cleaning up the geometry. Here, we applied meshing on the volumetric region with base mesh size of 2.5e-3 m to 1.5 2.5e-3 m. We also make sure that mesh quality should be of good quality.

Here, We have set the element size as parametric as We have to perform grid dependence test for convergence. Hence, We need base grid size of diiferent length so we did this.

The Nomenclature was also defined in this module where notation was given to each side of pipe.

For example:- inlet for x and y axis orifice, outlet and walls.

4. Created Intial Setup for simulation

Here, We have created the setup for simulation where we have provided double precision and displayed mesh after reading so that We can visualize the mesh obtained in design moduler.

5. Detailed Soultion setup in Ansys fluent

A. Boundary condition :-

(i) Inlet velocity for x -axis :-

Here, We have given value of velocity for x-axis 3 m/s and gauge pressure 0 Pa and value for all other parameters have been taken default.

(ii) Inlet velocity for y -axis :-

Here, We have assumed value of velocity for y axis 6 m/s and gauge pressure 0 Pa whereas value for all other parameters have been taken default.

(iii) Wall condition for pipe :-

Wall conditions have been taken in the following way :-

• Stationary wall motion
• No Slip condition for shear stress
• Standard Roughness Model have been assumed at the surface
• Other parameters have been default value

(iv) Outlet condition for pipe :-

Here, values have been assumed by default for all the conditions.

B. Turbulence Model :-

This condition is generally provided for analysing the turbulent behaviour in the flow of the fluid. For analysing the turbulent behaviour, We have assumed condition for this model 'K-epsilon' as well as 'K-omega' for different case. While values for all other conditions are taken defaults values.

6. Post-processed results from the Ansys fluent :-

A. Comparison between K-epsilon and K-omega model for short mixing Tee on the basis of Convergence.

(i) When Momentum ratio is 2 :-

Using K-epsilon model

Using K-Omega model

(ii) When Momentum ratio is 4 :-

Using K-epsilon model

Using K-epsilon model

Here, We compared cases for both turbulence models on the basis of standard deviation of temperature and We get information that Standard deviation is higher in case of k-omega turbulence model than k-epsilon turbulence model.

Since, a highly dispersed temperature value was not acceptable for solution on simulation for mixing of different temperature based inflow velocity. So, We had chosen K-epsilon Standard turbulence model which was having less dispersive range of value. Hence, We rely over this model.

B. Comparison between short and long mixing Tee for momentum ratio of 4 :-

1. Cell count :-

Short mixing Tee

Long mixing Tee

Here, We have created table above for both short and long pipe where We have mention down elements and nodes down for all the grid sizes used for simulation.

From table,We got information that as mesh size would decrease, number of nodes and elements would simultaneously increase. Also, We also got information that increase in model size would lead to an increase in number of elements and nodes too.

2. Average outlet temperature :-

Here, We had created Average outlet temperature table for Short and long pipe on the basis of different momentum ratio as a solution from simulation. From Average outlet temperature based table, We got the following informative results:-

(i) When We increase the momentum ratio by increasing the cold inlet velocity of y-axis, We observed a sudden drop of temperature by more than 3 degree celcius. So, We concluded that increasing in momentum ratio would lead to a significant decrease in temperature(Temperature drop).

(ii) We also get information from table that When We increased the size of mixing Tee model especially in terms of length of model, there was no effect of it on change of temperature(temperature drop). Hence, We concluded that increase in size would only lead to increase in mesh size which was only a shear waste of time for simulation.

3. Number of iterations for Convergence :-

For Mesh size 2mm

Here, from above table We get information that increase in momentum ratio leads to an increase in number of iterations for getting a convergence solution for both type of cases.

One more informative thing We get that increase in model size or length of pipe also lead to an increase in number of iterations for getting a convergence solution.

So, Here conclusion We made that increase in mesh size causes increase in number of iterations for convergence for solution.

Temperature contour plot on the cut planes along and across the pipe for Short mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Velocity contour plot on the cut planes along and across the pipe for Short mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Temperature contour plot on the cut planes along and across the pipe for Long mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Velocity contour plot on the cut planes along and across the pipe for Long mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Temperature line plot on the cut planes along and across the pipe for Short mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Velocity line plot on the cut planes along and across the pipe for Short mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Temperature line plot on the cut planes along and across the pipe for Long mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Velocity line plot on the cut planes along and across the pipe for Long mixing Tee :-

For momentum ratio of 2

For momentum ratio of 4

Mesh independent study case of short mixing Tee :-

Grid size of 25 mm

Grid size of 20 mm

Grid size of 15 mm

Grid size=12.5 mm

Here, We had plotted Convergence plots of three base mesh size and by observing all three mentioned plots We get information that Convergence was relatively increasing as We were decreasing the base mesh size.

But, this convergence in relation with base mesh size is limited to only some grid size i.e when We were decreasing the mesh size, We found that after a particular mesh size, We didn't able to find any significant change in convergence terms where everthing else in plots were repeating pattern. The mesh size at which this happen was 20 mm, So We could say that the grids size become independent of convergence at mesh size 20 mm.