Mixing Tee Efficiency

Amitabh Das

Advanced CFD using ANSYS Fluent

SKILL LYNC

Date of Submission: 10/02/2019

CASE I

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

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

For momentum ratio of 2 and velocity at hot inlet being 3 m/s,

Velocity at cold inlet = 3x2 = 6m/s

Assumption: Cold inlet temperature as 10 °C and hot inlet temperature as 25°C.

By default, mesh size of 10mm is chosen as default by ANSYS. Running the simulation for 100 iterations gives the results with slightly non-convergent standard deviation and weighted average of temperature values. Hence the iterations are increased to 150. The results are fairly convergent.

Trial cases

Cell size	10mm
No.of cells	13014
Average outlet temperature	19.9 or 20°C approx..
No.of iterations	150 for avg. temp. and stand. dev. ( 400 for convergence of residues)
Standard deviation	1°C approx.

From the results, it was observed that the value of average outlet temperature and standard deviation of temperature were fairly constant after 150 iterations. However, the residuals had decreasing values until 400 iterations. The simulations were performed for cell size 10.

Fig. 1.1 Average Temperature

Fig. 1.2 Standard Deviation of Temperature

Fig. 1.3 Residuals plot

Fig. 1.4 Temperature contour

Fig. 1.5 Velocity Contour

Now solving using lower cell size was done. The cell size was reduced to check better quality meshing and more precise results. Cell size was chosen as 5mm.

Cell size	5mm
No.of cells	14453
Average outlet temperature	20°C approx..
No.of iterations	200 for std dev and avg temp.
Standard deviation	1.5 °C approx..

Fig. 1.6 Avg.temperature

Fig. 1.7 Standard Deviation

Fig. 1.8 Residuals

Fig. 1.9 Velocity Contour

Fig. 1.10 Temperature Contour

CASE II

• Long mixing tee with hot inlet velocity of 3m/s
• Momentum ratio of 2
• Velocity at cold inlet = 3x2 = 6 m/s
• Assumption: Cold inlet temperature as 10 °C and hot inlet temperature as 25°C.

Cell size	5mm
No.of cells	18180
Average outlet temperature	20°C approx..
No.of iterations	200 for avg. temp. and stand. dev. ( 400 for convergence of residues)
Standard deviation	1°C approx.

Fig. 2.1 Standard Deviation of Temperature

Fig. 2.2 Average Temperature

Fig. 2.3 Residuals plot

Fig. 2.4 Velocity Contour

Fig. 2.5 Temperature Contour

CASE III

• Short mixing tee with hot inlet velocity of 3m/s
• Momentum ratio of 4
• Velocity at cold inlet = 3x4 = 12 m/s
• Assumption: Cold inlet temperature as 10 °C and hot inlet temperature as 25°C.

Cell size has been chosen as 5mm.

Cell size	5mm
No.of cells	14453
Average outlet temperature	17.3°C approx..
No.of iterations	200 for avg. temp. and stand. dev. ( 400 for convergence of residues)
Standard deviation	1°C approx.

Fig. 3.1 Average Temperature

Fig. 3.2 Standard Deviation of Temperature

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Fig. 3.3 Residuals plot

Fig. 3.4 Velocity Contour

Fig. 3.5 Temperature Contour

	Cell count	Average outlet temperature (°C)	No.of iterations for convergence	Standard deviation
Case1	14453	20	200	1.5
Case2	19017	20	200	1
Case3	14453	17.3	200	1

Conclusion: The model chosen for case 1 and case 3 are short mixing tee. By increasing the momentum ratio in case of Case3, the final outlet temperature decreased. The long tee in case 2 gives better mixing characteristcis with momentum ratio 2. Thus, with same momentum ratio and same cell size, the long tee gives better efficiency.