Mixing efficiency

Given data:

cold inlet temprature X=19⁰C.

hot inlet temperature Y= 36⁰C

There are two cases short and long pipes cases respectively.

momentum ratio = 2,4.

hot inlet velocity of 3m/s. Thus with two Momentum ratio the two sub cases of cold inlet velocity is 6m/s and 12m/s.

With these data the case(1) with short pipe is simulated with k-epsilon and k-omega solvers. First the geometry as to be prepared. once the workbench is opened, fluent dialogue box is opened. and geometry is selected. then the design file is imported to ansys.

Once the geometry file is imported select prepare==> volume extract==> select edges. Select the necessary edges so that unwanted geometry is removed. after slecting edges click ok box. new volume is created. thus uncheck the solid. then right the solid and select supress for physics becase the new is necessary for simulation for the old one. then close the geometry window.

worked geometry:

Then open the mesh in workbench to create mesh over the geometry. now selected thw mesh button. the mesh with random element size is created. but we need a fine mesh thus select the element size and enter 0.001m as element size. click generate button to generate a fine mesh.

once the mesh is created, click ctrl+f. now select a face and click n and enter the name of the face like inlet x, inlet y, outlet and wall. this step will be will be helpful for setting boundary condition using this names. thus close the mesh window.

now open the setup window to solve. under domain, check the mesh. once the mesh checking is done select physics in tool bar.

Select the the following:

then select enegery==> viscous. now we have two solving methods. they are k-epsilon and k-omega viscous. In order to compare one is selected and solved and answer is table. then other solver is selected following the same step and solved tabling those answers to compare. After selecting viscous click either k-epsilon or k-omega.

select the following:

now select zones==>cell zones==>volume_volume and check the type of fluid and click ok. The select boundary to set the boundary conditions. now use the name enetered in mesh window to enter the values. double click the name and enter the boundary conditions

for inlet X:

thus do the same for inlet y with velocity 6 and temperature 19. for output the default data is perfect to solve this. then select solution tab and click definition =>new=>surface report=>areea weighted average. and do the follow:

this will u find the average temperated at outlet. follow the same method to find the standard deviation of temperature. In the initialization tab select hybrid and intialise. Then enter the number of iteration like 200 to run and click calculate button. Run the iteration till the solution converge. Once converged table the outlet temperature and number of iteration for comparison. now close the setup and update the result and open result then click location==>plane and enter the place in pipe and apply for the contour to appear. thus the result is saved.

The same process is done for sub-case with momentum value 4 whereas the in boundary==> inlet y==> edit. the velocity is 12m/s is entered and solved. for both the k-epsilon and k-omega solver methods.

Result for short pipe T-mixing simulation:

temprature:

momentum ratio: 2,4 respectively

velocity:

k-epsilon and k-omega comparison:

Solver	No. of iteration for momentum ratio 2	avg. outlet temperature momentum ratio 2	No. of iteration for momentum ratio 4	avg. outlet temperature momentum ratio 4
k-omega	290	30.50C	383	280C
k-epsilon	122	30.50C	194	280C

from the results its ceratian that k-epsilon is effiecient that k-omega due the time consumption of the number of iteration to solve the problem. Thus k-epsilon is selected to solve the long T-mixing problem which is the case 2.

The process is same with different geometry. so, follow the same steps with k-epsilon solver method.

geometry:

worked geomtry:

mesh:

Since the process is same, every single step is followed with long pipe geometry for both momentum ratios.

Results for long pipe mesh:

momentum ratio: 2,4 respectively

temperature:

velocity:

thus,the stimuation of t-mixing is done for long and short pipe.

Line plot for short and long pipe:

Velocity at mementum ratio 2:

temperature at mementum ratio 2:

velocity at mementum ratio 4:

temperature at mementum ratio 4:

Comparison table of both cases:

case	nodes	elements	no of iteration at M.R=2	average outlet temparature M.R=2	no of iteration at M.R=4	average outlet temparature M.R=4
short	110098	573264	122	30.50C	194	280C
long	146509	761990	127	30.50C	182	27.50C

element size=0.001m

Effect of length on results:

The lenght of the pipe doesn't effect the mixing characteristics in big time. The effect of the pipe can be neglected considering the output. The average outlet velocity of both short and long pipe are almost same and number of iteration have very small negelible difference. Thus it is concluded that the lenght of pipe doesnot effect the flow.

Effect of momentum ratio on results:

The momentum ratio as big effect on the output we get. For momentum ratio 2 the average outlet temperature is 30.5⁰C whereas for momentum ratio 4 the average outlet temperature is 28⁰C. The momentum ratio is ratio of velocity between the inlets. Thus it is concluded that higher momentum ratio gives the average outlet nearer to the coolest inlet.