Week 1- Mixing Tee

Aim:

To understand the effects of changing pipe length and coolant flow velocity on mixing effectiveness of two fluids in a T joint pipe system.

Theory and objective:

The hot fluid from left end along x axis mixes with cold fluid that flows through Tee pipe along y axis. The operation is carried out in HVAC industry to circulate cold air around the system. Problem is modelled in CFD. Effectiveness of mixing is best understood by evaluating std deviation of temperature at outlet. Effect of changing pipe length and coolant flow velocity on mixing effectiveness is studied. the analysis results can also be used to understand the thermal stress at Tee junction.

To understand this, two versions of the mixing tee are created. One of them is longer than the other.

A steady-state simulations is set up to compare the mixing effectiveness when hot inlet temperature is 36⁰C & the Cold inlet is at 19⁰C.

1. Case 1
• Short mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4.
2. Case 2
• Long mixing tee with a hot inlet velocity of 3m/s.
• Momentum ratio of 2, 4

Working steps:

1. Prepare geometry (volume extraction)
2. Mesh
3. Set up physics
4. Pick viscous model
5. Set Material
6. Apply BC
7. Output def
8. Solve
9. Result validation
10. Post process

mesh size throughout the analysis = 0.004m.

Case 1a : Vh=3m/s ; Vc=6m/s. Th=36°C and Tc=19°C. K-epsilon vs K-omega .

Graph comparison:

. K-epsilon

K-omega

Case 1b : Vh=3m/s ; Vc=12m/s. Th=36°C and Tc=19°C. K-epsilon vs K-omega .

K-epsilon

K-omega

Long tee

Node: 6211 ; Element: 28888

Case 2a : Vh=3m/s ; Vc=6m/s. Th=36°C and Tc=19°C. K-omega .

Case 2b : Vh=3m/s ; Vc=12m/s. Th=36°C and Tc=19°C. K-omega .

S.No	Model	Cell count	Avg output temp (°C)	No. of iter	theoretical value at outlet (°C)
1	Case1a	21977	30.25	212	30.322
2	Case 1b	21977	27.5	158	27.48
3	Case2a	28888	30.3	124	30.322
4	Case2b	28888	27.5	100-150	27.48

mesh independence

S.No	Model	mesh size	cell count	avg output temp
1	Case1a	0.010219	12760	30.25
2	Case 1a	0.004	21977	30.25

Conclusion:

1.Mixing effectiveness: Most effective mixing has been achieved in long pipe with momentum ration 4 as it shows least std. deviation. Increasing length increases mixing effectiveness. Increasing mass flow of cold fluid, increases the effectiveness and decreases outlet temp. it is recommended to increase pipe length to improve effectiveness, as increasing fluid mass flow rate requires more energy.

2.Turbulence model: both models show close results. Still K omega model is used for case2 as it converges faster.