Is it possible to redirect the airflow of a fan in this way?

[amadeok]

Hello,
Can you tell if redirecting the airflow of a fan with a cone would be effecient?
https://ibb.co/gCMv67
That's an image of the idea
thanks

 

[t53186]

It would reduce the fan efficiency by restricting the airflow causing back pressure, slightly. but if you need to redirect the flow give it a try.

 

[lumineZ]

Depends on what you want to do.

You can imagine it like a garden hose. Big diameter in one end going into a small diameter yeld the same volum output but at higher speed.

Thats it.

So airflow with a cone would be nice for a more direct on point / spot cooling.

Also depends on the fan if it can handle the back pressure.

 

[amadeok]

Depends on what you want to do.

You can imagine it like a garden hose. Big diameter in one end going into a small diameter yeld the same volum output but at higher speed.

Thats it.

So airflow with a cone would be nice for a more direct on point / spot cooling.

Also depends on the fan if it can handle the back pressure.

I want to try to make a custom cooling pad by removing the bottom cover, replacing it with a custom made and using noctua fans to cool the processor and gpu. But i'm not sure if using a smaller fan would be better for this, since maybe a fan too big would be wasted airflow if doesnt hit directly the hotspots. What do you think?

 

[Solandri]

It's been 25 years since I last touched this stuff in a fluid dynamics course, so I'm not sure how much I'm remembering. But a contracting duct like you've pictured is used in wind tunnels because it generates a more even flowrate profile at the outlet. With a straight duct, there tends to be lots of eddies and speed variance in the boundary layer (air closest to the duct). When you contract the duct like in your picture, this boundary layer shrinks in sized disproportionately more than the uniform flow layer in the middle. Since wind tunnels are all about uniform airflow speeds, they're all designed with a contracting duct to help reduce the boundary layer and even out the airflow.

https://www.researchgate.net/publication/230271809_Fundamentals_of_Wind-Tunnel_Design

So there actually is some benefit to using a larger fan and contracting its flow with a duct, vs using a fan which is exactly the same size as the opening you want to shoot the air through. You lose fan efficiency, but you gain more uniform airflow speed throughout the diameter of air exiting your contracting duct.

Whether this tradeoff is worth it, I've long since forgotten the math for calculating. Just bear in mind that flow volume (cross-sectional area * airflow rate) remains the same at both ends of the duct. So the reduction in diameter turns into an increase in airspeed. Another reason engineers usually use a contracting duct like you've pictured is noise. Instead of using a small, noisy fan to generate high airspeed, you use a large, quiet fan to generate the equivalent airflow rate, then contract it to generate the airspeed.

 

[lumineZ]

Depends on what you want to do.

You can imagine it like a garden hose. Big diameter in one end going into a small diameter yeld the same volum output but at higher speed.

Thats it.

So airflow with a cone would be nice for a more direct on point / spot cooling.

Also depends on the fan if it can handle the back pressure.

I want to try to make a custom cooling pad by removing the bottom cover, replacing it with a custom made and using noctua fans to cool the processor and gpu. But i'm not sure if using a smaller fan would be better for this, since maybe a fan too big would be wasted airflow if doesnt hit directly the hotspots. What do you think?

Well. Then I dont think the cone is gonna be the main problem to be honest 😀
Sounds like Solandri have a bigger grasp on this than I do to be honest. Ask him when it comes to the cone and airflow.

 

[amadeok]

It's been 25 years since I last touched this stuff in a fluid dynamics course, so I'm not sure how much I'm remembering. But a contracting duct like you've pictured is used in wind tunnels because it generates a more even flowrate profile at the outlet. With a straight duct, there tends to be lots of eddies and speed variance in the boundary layer (air closest to the duct). When you contract the duct like in your picture, this boundary layer shrinks in sized disproportionately more than the uniform flow layer in the middle. Since wind tunnels are all about uniform airflow speeds, they're all designed with a contracting duct to help reduce the boundary layer and even out the airflow.

https://www.researchgate.net/publication/230271809_Fundamentals_of_Wind-Tunnel_Design

So there actually is some benefit to using a larger fan and contracting its flow with a duct, vs using a fan which is exactly the same size as the opening you want to shoot the air through. You lose fan efficiency, but you gain more uniform airflow speed throughout the diameter of air exiting your contracting duct.

Whether this tradeoff is worth it, I've long since forgotten the math for calculating. Just bear in mind that flow volume (cross-sectional area * airflow rate) remains the same at both ends of the duct. So the reduction in diameter turns into an increase in airspeed. Another reason engineers usually use a contracting duct like you've pictured is noise. Instead of using a small, noisy fan to generate high airspeed, you use a large, quiet fan to generate the equivalent airflow rate, then contract it to generate the airspeed.

Thats very interesting however seems a bit out of my league. the idea is for a custom cooling system for a omen 15 2017 laptop. I would replace the bottom cover with a custom made one and use a 200mm noctua fan to cool the cpu and gpu. Howver that fan is already 3cm tall so that would leave me with only 1 or 1.5cm to make the cone, because more than that would make the laptop too uncomfortable to use. What do you think about that? Do you think it would be worth it to make a 1,5cm tall cone? Or better use that space to separate the fan from the surface(such as a desk) where it rests to improve airflow?

 

[lumineZ]

Making a 15mm cone is no use. Makes no sense.

Then you would be better to raise the laptop like you are saying instead.

 

[Solandri]

The fan is 200mm. How wide is your duct outlet going to be? More than likely 15 mm isn't enough room. If you contract the duct too sharply, it basically becomes a brick wall that the fan fights against. You want the contraction to be gradual, a 45 degree angle at most, to reduce the amount of air deflected by the angled duct walls, and to give the air time to speed up. A 45 degree angle at 15mm length will only give you an outlet diameter of 170mm, which simply isn't worth it.

 

[amadeok]

The fan is 200mm. How wide is your duct outlet going to be? More than likely 15 mm isn't enough room. If you contract the duct too sharply, it basically becomes a brick wall that the fan fights against. You want the contraction to be gradual, a 45 degree angle at most, to reduce the amount of air deflected by the angled duct walls, and to give the air time to speed up. A 45 degree angle at 15mm length will only give you an outlet diameter of 170mm, which simply isn't worth it.

I see. This is a picture of the laptop without the bottom cover: https://i1.wp.com/laptopmedia.com/wp-content/uploads/2017/07/IMG_20170620_183719.jpg?resize=680%2C383
As you can see the heatpipes go extend from one side to the other passing on top of the cpu and gpu which are in the center, the cpu in the left and the gpu on the right. I was thinking that maybe ideally at least 1cm should be kept between the fan and the heat pipes as well as 1cm between the fan and the where it rests on, to promote airflow. The back would still be higher than the front, to make it confortable to use, and to add some more space between the fan and the desk. Instead of a cone, i was thinking of using that 1cm space to redirect the airflow with a design like this:
https://ibb.co/gk4pZS
https://ibb.co/hoQk8n
https://ibb.co/e79pZS
https://ibb.co/jBECon
The idea would be to take advantage of the airflow by redirecting it to the rest of the heat pipes, and then letting it exaust through the sides. The big square in the middle would be the 200mm fan, and the two trapezes to its sides would be the redirectors.

 

[Solandri]

You may be overthinking the problem. An owner of the gaming laptop I have (Gigabyte P34W) simply cut out the plastic grilles beneath the intake fans, and replaced them speaker fabric. The improved airflow resulted in a 10 C drop in temps. The drawback of course being that he has to be careful not to poke anything into the fabric when moving or using the laptop.

But if you do wish to go with a box contraption, make sure to get a static pressure fan. These are designed to generate a pressure differential (in your case, higher pressure inside the box). A regular airflow fan is designed simply to move air - if that air hits an obstruction (like the top of your box) the fan loses most of its effectiveness.

https://superiorgamingtech.com/air-flow-vs-static-pressure-fans-choosing-between-the-two

 

[amadeok]

You may be overthinking the problem. An owner of the gaming laptop I have (Gigabyte P34W) simply cut out the plastic grilles beneath the intake fans, and replaced them speaker fabric. The improved airflow resulted in a 10 C drop in temps. The drawback of course being that he has to be careful not to poke anything into the fabric when moving or using the laptop.

But if you do wish to go with a box contraption, make sure to get a static pressure fan. These are designed to generate a pressure differential (in your case, higher pressure inside the box). A regular airflow fan is designed simply to move air - if that air hits an obstruction (like the top of your box) the fan loses most of its effectiveness.

https://superiorgamingtech.com/air-flow-vs-static-pressure-fans-choosing-between-the-two

I thanks thats very insightful. In that case ill go with the nf-a14 140mm fan. Ill probably post the results here when i do the experiment.