Mixing efficiency

MIXING EFFECTIVENESS OF MIXING T-JUNCTION

AIM- To perform a steady state simulation on Ansys fluent and compare mixing effectiveness of two different lengths of mixing T-junction

INTRODUCTION

Mixing T-Juction are used in various industries to obtain any fluid (like air or water) at an certain temperature. They are used in various power plants for mixing the steams at different temperatures. They are also used in HVAC system to cool a particular area at a particular temperature and many other applications. So it is very important that the mixing T-junction that we are using his the right mixing effectiveness to obtain the desired temperature at the outlet.

PROCEDURE

• Making of T-junction

For this simulation two mixing T-Junction were made on solidworks. The diameter of the inlets and the outlet were same. The only change was in the length of the two T- junction. The length of the short and long T-junction is 192.37 mm and 268.32mm respectively. The diameter of inlet of cold air,hot air and the outlet is same for both the short and long T-junction is same. The image of both the T-junction are give below

Figure 1- Short T-junction

Figure 2-Long T-junction

• Meshing

After making the CAD part for both the T-junctions,the file was uploaded of space claim in Ansys workbench and the only the volume in contact with the air was extracted for both the part by using "extract volume" option in the prepare section of space claim.Now the part is ready for meshing. While meshing the element size was set to 1.1e-3 for the short T-junction and 1.2e-3 for the long T- junction.This was the smallest element size possible as the Ansys 2020 R1 Academic licence don't allow element size smaller than that.The image of the Mesh,Meshing Details and mesh quality charts for both the T-junction are given below.

For the short T-junction-

Figure 3- Meshing of short T-section

Figure 4-Meshing detail & Element Quality chart

For long T-junction-

Figure 5-Meshing for long T-junction

Figure 6- Details of Mesh and Element Quality Chart.

SETUP FOR SIMULATION

The mesh was update and then the condition for the simlution were ste. There are four cases the simulation as the momentum ratio for both the T-junctions is given as 2&4.All four cases are simulated on double presion setting.Pressure based solver at steady state is used for the four cases.For the turbulance both k-epsilon and k-omega SST model can be used but I used k-epsilon model as in all the four cases the reynold number is high and k-omega is not that effective at high reynold number.The velocity of the hot air is given as 3 m/s at 36 degree celcius and the temperature of the cold is 19 degree celcius for all the four cases.

FIGURE 7- Setting for solving the Simulation

Case 1-Short T-Junction with Momentum Ratio=2(Velocity of cold air inlet is 6m/s)

The number of iteration run are 350.Plots of Area wieghted average and Standard devation of temperature at the outlet were plotted as shown in the image below.

Case 2-Short T-Junction with Momentum Ratio=4(Velocity of cold air inlet is 12m/s)

The number of iteration run are 350.Plots of Area wieghted average and Standard devation of temperature at the outlet were plotted as shown in the image below.

Case 3-Long T-Junction with Momentum Ratio=2(Velocity of cold air inlet is 6m/s)

The number of iteration run are 350.Plots of Area wieghted average and Standard devation of temperature at the outlet were plotted as shown in the image below.

Case 4-Long T-Junction with Momentum Ratio=4(Velocity of cold air inlet is 12m/s)

The number of iteration run are 350.Plots of Area wieghted average and Standard devation of temperature at the outlet were plotted as shown in the image below.

OBSERVATION TABLE

CASE NO.	CELL COUNT	AVERAGE OUTLET TEMP(CELCIUS)	NUMBER OF ITERATION FOR CONVERGENCE
Case 1(Short/Momentum ratio=2)	456020	30.388	146
Case 2(Short/Momentum ratio=4)	456020	27.562	210
Case 3(Long/Momentum ratio=2)	500983	30.457	131
Case 4(Long/Momentum ratio=4)	500983	27.474	175

CONCLUSION

From the above simulation ran it can be concluded that the inlet velocity of the cold air is as dominant factor to mixing effectiveness as it can be seen in cases 1&2 and also in case 3&4. The length of the mixing T-junction does not effect the mixing effectiveness that much as inlet velocity of cold air.This can be seen in case 1&3 and also in case 2&4 as there is not any significant change in outlet temperature.