Mixing efficiency

• OBJECTIVE To perform simulation in flow through T- joint pipe and check effectiveness of mixing y using ansys fluent software for different momentum ratio and varying length of pipe :

• PROCEDURE :

• After opening ansys workbench click on ansys fluent in toolbox and drag it into project schematic page as shown below:

Mainly following four parts included.

1). Pre-procesing

2). Meshing.

3). Solving .

4). Post- processing.

• Pre- processing

It's include preparation of geometry which is being use in our simulation part .

Here we have short Tee and long Tee geometry already prepared in file. And We can use space claim to prepare geometry to prepare geometry click on geometry.

Normally what we see is when flow through pipe most of time connected with part like flanges .In fact we don't require those part of pipe while simulating flow.

As here we can show by pressing button on prepare we can prepare geometry.

Select option on volume extraction and select the edge which is not require and just by click right button we are able to create geometry.

After that former geometry which will not use in suspress to prevent those geometry being use in fluent.

• Meshing

Once geometry created we need to create divided geometry in the no. Of small element (Mesh) for that click on mesh and then click on edit.

Choose option mesh From where we can choose our element size Ansys creates default element size which is fits in geometry Check the mesh quality.

Min. Mesh quality should be maintain around 30 %.

As we have pipe here I choose with inflation layer which creates layer selected size (thickness) and no. Of layer on the periphery part of Tee- joint

Click on generate mesh which will create mesh of selected element size :While using inflation layer select body in geomentry and 2 faces in geometry

By selecting plane we can see mesh aross plane as shown below:

• Give boundary selection by click (control+f) to select face then give create name selection on inlet , outlet and wall

• Solving :

1. Select pressure base solver type since we are dealing with low mach number.
2. Select absolute in velocity formulation
3. Selct steady state condition
4. Selct mesh quality which will check mesh.

• Select on physics

1. Select on energy
2. Following two turbulence model we are using for simulations.

Unfortunately, there is no particular turbulent model which is universally accepted as remarkable for all types of flow problems. There are many considerations like accuracy level required, resources available for computation and time involved in choosing a correct turbulent model. There are many advantages as well as limitations in every model and it has to be surely taken into account for the variation of results to the problem statement. It is impossible to categorize that a certain model is best suited for specific applications.

• K-epsilon (relaizable) model : The baseline two-transport-equation model solving for kinetic energy k and turbulent dissipation ε. Turbulent dissipation is the rate at which velocity fluctuations dissipate. This is the default k–ε model. Coefficients are empirically derived; valid for fully turbulent flows only
• K-W SST model : two-transport-equation model solving for kinetic energy k and turbulent frequency ω. This is the default k–ω model. This model allows for a more accurate near wall.

• Give boundary condition which include temperature , velocity accordingly different momentum ratio at selected face.

To visulize and and to make better prediction of result we will select area weighted avg. and standard deviation at outlet face from defination.

Initialize and click on run calculation and observe residual and other plot.

• DIFFERENT CASES

1. Short-tee with momentum ratio =2 with following turbulence model.

1). K-epsilon model

2). K-w model

2. Short-tee with k-e model with following momentum ratio

1). Momentum ratio = 2

2). Momentum ratio = 4

3. Long–tee with k-e model with following momentum ratio

1). Momentum ratio = 2

2). Momentum ratio = 4

4. Mesh indepence test with following element size.

1). Element size = 5e-3

2). Element size = 3e-3

3). Element size = 2e-3

Case-1 :

1. Short-tee with momentum ratio =2 with following turbulence model.

1). K-epsilon model

2). K-w model

• Geometry:
• Short tee with element size = 0.002
• inflation layer = 5 inflation layer with growth rate = 1.2
• No. of nodes = 49337
• No. of elements = 141210
• Boundary condition :

• Hot Inlet temp : 36 celcius & Hot inlet Velocity = 3m/s
• cold Inlet temp : 19 celcius & cold inlet Velocity = 6m/s
• Pressure at outlet = 0 guage pressure

.

K-epsilon MODEL

• Residuals plot

• contour plot of velocity and temp (along length of pipe).

• contour plot of velocity and temp (across length of pipe).

• velocity and temp plot (along length of pipe)

• velocity and temp plot (across length of pipe)

K-w MODEL

Residuals plot

• contour plot of velocity and temp (along length of pipe).

• contour plot of velocity and temp (across length of pipe).

• velocity and temp plot (along length of pipe).

• velocity and temp plot (across length of pipe).

Here manually calcalated value at outlet temp shown for tee joint as shown here which shows that k-e model has value very nearer to that which ensures the k-e model is good for our solution which also takes less iteration to solve problem.

• In addition that standart deviation for k-e nodel shows less deviation which ultimately shows accuracy as shown in calculated value.
• But when its comes to capturing wall effect from contour it can be shown that k-w model is best.

Case-2 :

• Short-tee with k-epsilon with following momentum ratio

1). momentum ratio = 2 (iteration Run =350)

2). momentum ratio= 4 (iteration run = 450)

• Geometry:
• Short tee with element size = 0.002
• No. of nodes = 49337
• No. of elements = 141210
• Boundary condition :

• Hot Inlet temp : 38 celcius & Hot inlet Velocity = 3m/s
• cold Inlet temp : 1) 19 celcius & cold inlet Velocity = 6m/s 2) 19 celcius & cold inlet Velocity = 12 m/s

• Pressure at outlet = 0 guage pressure

• Residul plot (momentum ratio =2)

• Residul plot (momentum ratio =4 )

• contour plot of velocity and temp along length of pipe (momentum ratio =2)

• contour plot of velocity and temp along length of pipe (momentum ratio =4)

• contour plot of velocity and temp across of pipe (momentum ratio =2)

• countour plot of velocity and temp across length of pipe(momentum ratio =4)

• velocity and temp plot along length of pipe(momentum ratio =2)

• velocity and temp plot along length of pipe(momentum ratio =4)

• velocity and temp plot across length of pipe(momentum ratio=2)

• velocity and temp plot across length of pipe(momentum ratio=2)

• As increase with the velocity of cold inlet fluid there is change in temp is about 3 degree and velocity increase which ensures better mixing also plot shows value for momentum ratio =4 which rises smoothely compared to momentum ratio=2 after mixing at T- joint whithout having abrupt change .

Case-3 :

• Long -tee with k-epsilon with following momentum ratio

1). momentum ratio = 2 (iteration run =500)

2). momentum ratio= 4 (iteration run =500)

• Geometry:
• Short tee with element size = 0.002
• inflation layer = 5 layer with growth RATE=1.2
• no. of nodes =66248
• no, of element =186261
• Boundary condition
• Hot Inlet temp : 36 celcius & Hot inlet Velocity = 3m/s
• cold Inlet temp : 1) 19 celcius & cold inlet Velocity = 6m/s

2) 19 celcius & cold inlet Velocity = 12 m/s

• Pressure at outlet = 0 guage pressure

• Residul plot (momentum ratio =2)

• Residul plot (momentum ratio =4)

• contour plot of velocity and temp along length of pipe (momentum ratio =2)

• contour plot of velocity and temp along length of pipe (momentum ratio =4)

• contour plot of velocity and temp across length of pipe(momentum ratio =2)

• contour plot of velocity and temp across length of pipe(momentum ratio =4)

• velocity and temp plot along length of pipe(momentum ratio =2)
• velocity and temp plot along length of pipe(momentum ratio =4)

• velocity and temp plot across length of pipe(momentum ratio =2)

• velocity and temp plot across length of pipe(momentum ratio=4)

• As increase with the velocity of cold inlet fluid there is change in temp is about 3 degree and velocity increase which ensures better mixing also plot shows value for momentum ratio =4 which rises smoothely compared to momentum ratio=2 after mixing at T- joint whithout having abrupt change
• Here as incresing length pf pipe velocity gets constant as proceding with length because enough space is avalaible to get better mixing..

• Case-4 :

short -tee with k-epsilon with following element size :

1). Element size = 5e-2 (iteration run =350)

2). Element size = 5e-3 (iteration run =350)

3). Element size = 2e-3 (iteration run =350)

• Element size = 5e-2
• no. of nodes =2837
• no, of element =12760
• Residual plot

• contour plot (momentum ratio = 2)

• Temp & velocity plot

• Element size = 5e-3
• no. of nodes =3154
• no, of element =14330
• Residual plot

• contour plot (momentum ratio = 2)

• Temp & velocity plot

• Element size = 2e-3
• no. of nodes =21345
• no, of element =105865
• Residual plot (momentum ratio = 2)

• contour plot (momentum ratio = 2)

• Temp & velocity plot

From above all plot of velocity and temp. and velocity and temp. contour it can Shawn that greed indepedence test achieved as we increses element size (small mesh size)

Here from plot of velocity and temp values get constant and from contour plot we can see effective mixing but as we increases mesh size ultimately it increases computational cost thus one have to ensure that creteria for choose mesh size .

Conclusion :

		short-tee
			Temp.(K)		Velocity(m/s)		Standard Deviation
Ratio=2		k-e	303.544		4.5		1.497
		k-w	303.58		4.502		1.952
Ratio=4		k-e	300.68		6.0168		1.1152
		k-w	301.06		6.028		2.12

		Long-tee
			Temp.(K)		Velocity(m/s)		Standard Deviation
Ratio=2		k-e	303.5		4.49		0.84
		k-w	303.67		4.498		1.394
Ratio=4		k-e	300.62		5.998		0.5649
		k-w	300.86		6.002		1.577

• Effect of momentum ratio : As increases the velocity of inlet outlet temp . decreses (approx. 3 degree celcius ) because of less diffusion and increases in the outlet velocity (approx. 30% more) it can be shawn from velocity and temp contour and plot and nuetralize process of mixing of both hot and cold fluid very effectively specially in long pipe because there is enough space available to do so.
• at outlet frpm plot of across pipe for momentum ratio =4 velocity remains almost constant & temp.max higher which is equal to 28 fegee on top surface while in momentum ratio=2 max. temp on bottom side which is equal to 32 degree.

• Effect of increse pipe length : for same momentum ratio of short and long pipe there is minor difference in value of outlet avg. temp and velocity (plot and cotour favours values )but when we consider standard deviation is very low compared to short-tee which favours toward use of long –tee. One thing must consider is that while using long tee as no. of node increses computation time also increses which ultimately leads to computation lost