Mixing efficiency

Objective: To simulate the flow of air through mixing tee for the study of the effectiveness of mixing with the length of mixing tee i.e short and long mixing tee. while using 2 and 4 value of momentum ratio.

For short mixing tee,both kepsilon and k-omega sst model are used with momentum ratio equal to 2 and 4.

Importance of Mixing:

Mixing effectiveness is required in HVAC system for controlling the outlet temperature of the plant.For that we are simulating hot air at 36 degree Celcius and cold air at 19 degree Celcius through short and long pipe for understanding the effect of velocity of air and length of pipe on mixing process.Change in velocity will create Turbulence effect which will affect mixing process.

Case 1

• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4.

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

Case 1-For momentum ratio =2:

Case setup:

1.Geometry:

Extracted volume of fluid (through Spaceclaim):

Mesh Generated(Section view):

Simulation Results:

For Temperature:

Temperature Contour Plot:(along length of pipe)

Velocity Contour Plot:(along length of pipe)

Temeratue and Velocity plot across the pipe (at outlet):

Temperature and Velocity Line Plot along the length of the pipe:

Temperature and Velocity Plot across the pipe (at Outlet):

Residual Plot of simulation:

Kepsilon model gives better results than K-omega sst model for same mesh size without inclusion of any inflation for simulation of air flow through short pipe with momentum ratio equal to 2.

Better in the sense,the Average temperature obtained by convergence of solution after simulation is closer to value obtained through Analytical solution (by feeding values in Formula given above).

So Kepsilon model is preferred for further cases.

2.For momentum ratio=4:

Simulation Results:

For Temperature:

For Velocity

Temperature and Velocity Contour Plots

Temperature and Velocity Line Plot along the length of the pipe:

Temperature and Velocity Plot across the pipe (at Outlet):

Residual Plot of simulation:

Case 2

• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4.

1. For momentum ratio=2.

Simulation Results:

For Temperature:

Temperature and Velocity Contour plots:

Residual Plot:

Mesh Independent Study:

For long mixing tee with momentum ratio equal 4.

a.For Mesh grid size =0.01m and adding Inflation at walls and outlet:

b.For Mesh grid size =0.003m and adding Inflation at walls and outlet:

From both cases it is found that,

Iteration for convergence of solution are alomost same.

Average temperature at outlet is 300.48 for mesh size 0.01m and 300.51 for mesh size 0.003m.

0.003m grid gives better contour plots then 0.01m.

Otherwise not much difference in the results also.

So this conclude Mesh Independent Study.

For Momentum Ratio equals 2.(using Kepsilon Model)

	Cell Count	No. of Iteration for convergence	Average Outlet Temperature	Average Outlet Temperature(By formula)
Short Mixing Tee	39803	130	303.201	303.189
Long Mixing Tee	56504	180	303.386	303.189

For Momentum Ratio equals 4.(using Kepsilon Model):

	Cell Count	No. of Iteration for convergence	Average Outlet Temperature	Average Outlet Temperature(By formula)
Short Mixing Tee	39803	149	300.586	300.339
Long Mixing Tee	78208	85	300.511	300.339

Effect of Length and Momentum Ratio on Mixing Efficiency of air in Tee:

Now we will discuss what outcomes we have collected from simulation considering effect of length and Momentum ration on Mixing of air in a tee.

• As we can see from above results shown,even with increase in length of mixing tee.There is almost no change in outlet mixing temperature.

For short tee of 192 mm length and for long tee of 268 mm length,there is not much change in average outlet temperature.So there is no need to increase area of HVAC system and using long mixing tee ,as results obtained are almost the same while using short tee.

• With the increase in momemtum ratio ,the average outlet temperature decreases .This represent that with the increase in velocity of cold inlet,the mixing efficiency increases.