Steady-state simulations on the mixing tee geometry with the help of Ansys Fluent

Title:Steady-state simulations on the mixing tee geometry with the help of Ansys Fluent

Objective:

1.To set up steady-state simulations for compare the mixing effectiveness.

2.Use the k-epsilon and k-omega SST model.

3. Vary the momentum ratio for each case.

Expected Outcomes:

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.

Given Data

1.hot inlet temperature is 36⁰C 

2.Cold inlet is at 19⁰C.

3.hot inlet velocity of 3m/s.

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

Theor & Application

The extreme uses of Jet mixing of two fluid streams in a pipeline are in the chemical industry in pipeline transport process and in tubular apparatus for the purpose of dilution, heat transfer and reaction (combustion). Rapid mixing in the shortest distance downstream from the injection point is usually required. Effective use of turbulence increase reactant contacts and decreases reaction times, which can significantly reduce the cost of producing many chemicals.Efficient mixing is necessary to obtain profitable yields or to eliminate excessive corrosion in reactor or combustion chamber. An effective, simple method to mix two fluids with in a pipeline is to introduce feed jets such that jet contact with pipeline walls in minimized and mixing occurs rapidly with in the turbulent core of the pipeline.

                                         Fig. Layout of Mixing Tee

Procedure to  setup case

1. Open the workbench & drag & drop out Flow Fluent in to project schematic

2. Open the spaceclaim by right click on geometry

•  open the geometry from saved location 
• volume extracted from prepare option
• hide the solid materials
• supress the solid materials   

3. Open Mesh  from workbench

• name given to evey parts by face selction
• mesh size changes from mesh details it's taken 0.0012
• check the mesh metric matrix by select element quality
• take the secton plane from centre of geometry   

4. Open the setup & Solution

• select the basic condtion for steady state solution like pressure based solvers, energy for measure temprature
• select the k-epsilon & k-omega model for turbulent flow.
• setup the boundry condition as per given data.
• in solution select the report for temprature like standard deviation temp. & average temprature.
• initialize the domain
• run for calculation by enter the no.of iteration

5. Open the Result(CFD- post)

• Open the gemoetry by uncheck the outlet, inlet
• select the xy plane for plot the contour of temprature & velocity
• plot the line plot for temprature & velocity by drawn the line along & across the pipe

Geometry 

               Fig.Short tee after volume extraction & hide solid part in spaceclaim

Mesh Model

CFD post model

CASE I: Short mixing tee with hot inlet velocity 3m/s & use of momentum ratio 2

• In first case there is use of hot inlet velocity 3m/s & cold velocity 6m/s, while temprature 36 & 19 degree celcius for respective opening. 
• Apply the k-epsilon & k-omega model for simulate the turbulent flow based upon this will choose best model & will apply for further case.

K-epsilon Model Results

Residual graph for convergence of solution

Average temprature

Standard deviation temprature

Temprature contour

Velocity Contour

K-Omega model Result

Residual graph for convergence of solution

Average temprature

Standard deviation temprature

Temprature contour

Velocity Contour

• From  the above table we can see that  k-omega model have average temprature is lower than the k-epsilon model
• standard deivation temprature is lower for k-omega model
• from the tempraure & velocity contour we can see that mixing efficiency for k-omega model is better than the k-epsilon model.
• Also from research paper its found the k-ω model is useful in many cases where the k-ε model is not accurate, such as internal flows.
• k-ω model is a low Reynolds number model, but it can also be used in conjunction with wall function
• Its correct that k-omega model take more iteration for convergence but here , I focous on good result of temprature & mixing efficiency. so finally i choose the k-omega model for rest of cases.  

CaseI : Short mixing tee with momentum ratio 4

Residual plot

Average temprature

Got lower temprature 27.5 degree celcius by use of k-omega model

Standard deviation temprature

got lower standard deviation temprature 2 degree celcius

Temprature contour

Velocity contour

Line plot for temprture along the length of pipe

Line plot for Velocity at outlet

• the above line plot for temprature & velocity is got in CFD -post. by drawn the line along the length & across the pipe.
• from temprature plot we can see that from hot lnlet there is higher temprature as the length is proceed at mixing point there is temprature is suddenly drop out due to mixing of cold air, as proceed to the last point of length there is temprature is try to maintain the constant.
• from velocity plot is ploted along the cross-sectional length, so that there is velocity is vary , at the wall area there is higher velocity compare with the centre point  velocity.

CASE II: Long mixing tee with momentum ratio 2

Residual plot

Average temprature

Average temprature 30.6degree celcius

Standard deviation temprature

Standard deviation temprature 2.25 degree celcius

Temprature contour

Velocity contour

Line plot for temprture along the length of pipe

Line plot for Velocity at outlet

CASE II: Long mixing tee with momentum ratio 4

Residual plot

Average Temprature

Average temprature 27.5 degree celcius

Standard deviation temprature

standard deviation tempratre 1.4 degree celcius

Line plot for temprture along the length of pipe

Line plot for Velocity at outlet

Mesh Independant study

Mesh indpendant study is done on short tee with momentum ratio 4, so here we can see that cell count is increases in fine mesh but it is observed that result average temprature is same for all three cases. so, it is better to compare the mesh independant study by temprature & contour plot

Temprature contour coarse mesh

Temprature contour Medium mesh

Temrature contour Fine mesh

Velocity contour at coarse mesh

Velocity contour at Medium mesh

Velocity contour at Fine mesh

• from the above contour plot we can easily see that for coarse & meduim mesh  there is temprature & velocity flow is not smooth & also mixing of two fluids is not smooth.
• But in case of fine mesh we can see that there is temprature & velocity is so smooth & mixing of two fluids is also smooth.
•  from above we can see that if we want good result & smooth flow of two mixing fluids , there mesh factor definately depends.
• so, fine mesh always gives good result than the coarse & meduim mesh 

Effect of length & momentum ratio on result

1. momentum ratio affects on average temprature. as from above table we can see that for for momentum ratio 4 i.e inlet hot velocity 3m/s & inlet cold velocity 12m/s, we can see that we got less average temprature i.e 27.5 for both case . also there is standard deviation temprature is lower comapare with momentum ratio 2.

2. length is not that much affected on average temprature, if we increase the cold velocity 12m/s , we got same result as short tee. But yes it deffinatley affected on convergence iteration of solution, long tee take more cell count so that it take more time & iteration for convergence.

Comparison of all cases

conclusion 

• from the above comaprison we can see that the lowest temprature we got 27.5  for both momentum ratio 4 but short tee we got less no.of iteration for convergence.
• short tee with momentum ratio 2 is take less no.of iteration 105 but there is higer average temprature.
• so, there is best combination is short tee with momentum ratio 4 is give better result.
• k-moega model is better work in internal fluids & give better result compare with k-epsilon model.