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Is it a turbulent flow or laminar flow?

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Old   January 7, 2015, 21:43
Default Is it a turbulent flow or laminar flow?
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Hi guys,

I am trying to simulate the flow through a heat sink with several fins. the geometry model is shown as the below picture.

1.jpg

According to the length of the fin, the Reynolds number is 9e+4, which is less than the number of transition. so, the flow should be laminar? I'm not sure if I should treat it as a laminar flow, and then use laminar model to simulate the flow?

What's more, the flow starts from a fan. Is it resonable that the flow has become turbulent when it leaves the fan blade? so I sould simulate it by using a turbulent model.

Thanks in advance.
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Old   January 19, 2015, 20:42
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Could anyone give me any suggestion?
Or.. is there anything I didn't descrbe clearly?
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Old   January 19, 2015, 21:00
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Quote:
Originally Posted by ringtail View Post
Hi guys,

I am trying to simulate the flow through a heat sink with several fins. the geometry model is shown as the below picture.

Attachment 36392

According to the length of the fin, the Reynolds number is 9e+4, which is less than the number of transition. so, the flow should be laminar? I'm not sure if I should treat it as a laminar flow, and then use laminar model to simulate the flow?

What's more, the flow starts from a fan. Is it resonable that the flow has become turbulent when it leaves the fan blade? so I sould simulate it by using a turbulent model.

Thanks in advance.
In which direction (x,y,z) is the freestream flow directed? Is the freestream flow
inline with the fins or perpendicular.

If there is any question about laminar or turbulent, you may want to run laminar
flow as an initial limiting case. The laminar case should produce the lowest heat
flux values without the additional energy transport due to the motion of turbulent
eddies.
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Old   January 20, 2015, 02:55
Default Is it a turbulent flow or laminar flow?
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Laminar flow will be suitable for this case without caring of near fan treatment ....
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Old   January 21, 2015, 00:34
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Quote:
Originally Posted by Firas abd ali View Post
Laminar flow will be suitable for this case without caring of near fan treatment ....
Yes, thats true but as the flow leaves from the fan, it has already been disturbed and it is quite reasonable to treat it as a turbulent flow in this case.
Note: Freestream turbulence should be taken into account before deciding whether it a laminar or turbulent flow. Reynolds number doesn't alone play the role.
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Old   January 22, 2015, 19:24
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Quote:
Originally Posted by ringtail View Post
Hi guys,

I am trying to simulate the flow through a heat sink with several fins. the geometry model is shown as the below picture.

Attachment 36392

According to the length of the fin, the Reynolds number is 9e+4, which is less than the number of transition. so, the flow should be laminar? I'm not sure if I should treat it as a laminar flow, and then use laminar model to simulate the flow?

What's more, the flow starts from a fan. Is it resonable that the flow has become turbulent when it leaves the fan blade? so I sould simulate it by using a turbulent model.

Thanks in advance.
turbulent is prevalent in nature. Your criterion of the transition from laminr to turbulent only make sense for the simple plate flow or pipe flow. The flow is turbulent unless your heat sink if micro fin and micro channel.
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Old   January 22, 2015, 20:32
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Hi, Troy
Thank you~~The flow the inline with fin.
Yes, I agree with you. The heat transfer in laminar case is smaller than turbulent one. But I wanna know the real boundary layer thickness on the fins, so it's essential to know the flow model in advance. And I think the best suitable gap between fins, shoud be the double of the boundary layer thickness.
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Originally Posted by tas38 View Post
In which direction (x,y,z) is the freestream flow directed? Is the freestream flow
inline with the fins or perpendicular.

If there is any question about laminar or turbulent, you may want to run laminar
flow as an initial limiting case. The laminar case should produce the lowest heat
flux values without the additional energy transport due to the motion of turbulent
eddies.
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Old   January 22, 2015, 20:37
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Hi Firas, I am still a bit confused about this.
If the free stream is disturbed by the fan, hasn't it become turbulent flow in front of the fin? so, the flow around the fins should be turbulent since the turbulent flow won't transit to laminar flow even I don't care about the flow near the fan. But I'm not sure about my opinion.

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Laminar flow will be suitable for this case without caring of near fan treatment ....
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Old   January 22, 2015, 20:47
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yes, Venkatesh , I totally agree with you. And I want have a deeper discussion about this topic.
How should we make a appraisal of the disturb if it's big enough to triggle the transition? and how should we assess the turbulent intensity of the free stream after it leaves the fan? (you know, it may be the key parameters to set the bc in FLUENT)
Quote:
Originally Posted by Venkatesh Devaraj View Post
Yes, thats true but as the flow leaves from the fan, it has already been disturbed and it is quite reasonable to treat it as a turbulent flow in this case.
Note: Freestream turbulence should be taken into account before deciding whether it a laminar or turbulent flow. Reynolds number doesn't alone play the role.
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Old   January 22, 2015, 20:52
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the heat sink is about 400 mm long , 300 mm width and 200 mm high.
the gap between the fins is about 5 mm.
Quote:
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turbulent is prevalent in nature. Your criterion of the transition from laminr to turbulent only make sense for the simple plate flow or pipe flow. The flow is turbulent unless your heat sink if micro fin and micro channel.
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