Let me separate different points mentioned above.
1. Efficiency of cooling
You should
really keep this router in it's 'natural' position. Not only for all the right reasons
@heysoundude mentioned,
is this with the fan moving air into the case or to help exhaust it? I think your fan might be fighting the natural flow in this set up, or only cooling one side of the board.
I believe active cooling using the existing passive cooling design is key for this model -
Agreed, the router should be in its natural position...
Aside from the dust issue is there any substantiated argument for why the router should be in the upright ("natural") position when cooled with a fan?
As far as I understand, the only argument given here (by
@heysoundude and referenced by
@Hazel) - is based on the thought that the cooling design of the router is optimized for that position. I agree with the thought, - but that is for passive cooling, i.e. for the natural convection.
Once we turn on a significant airflow, we are in the
forced convection regime. The air flow patterns change significantly, and, while you are correct, in some cases you might be going against the original, natural convection air flow, that does not matter.
You can carefully design your yard irrigation system, with each sprinkler providing highly-optimized coverage and run-off for the specific tree, bush, or a flower bed. But, if you have a torrential rain that will flood your entire neighborhood, it doesn't matter what your original pattern and run-off was: you will have no dry spot left even if that rain was working against your sprinkler pattern.
Seeing
huge, 10-20 C temperature drops with a fan, I would say that the forced convection overpowers the natural convection by a huge factor: likely an order(s) of magnitude (in terms of the air flow velocity).
By the way, just to confirm this point, in my setup, I have compared the two orientations of the router, horizontal and vertical, both with the fan attached to the middle of the "back". I see no difference in the temperatures between the two positions.
So, I conclude:
there is no significant difference in the efficiency of cooling between the router standing up-right, and laying on its "face" (front side).
Even if there were a
slight variation in the temperature, say a degree Celcius, compared to ~20C drop in the CPU temperature, it is not a big problem you need to worry about if you have other constraints.
The situation
might be different if you are using a small (e.g. 40mm) and low-speed fan that might not be creating and overpowering the natural convection. I don't have any experience with such a fan, - but I would think that even with such a fan, the forced convection will still be stronger than the natural one.
2. Dust accumulation with the fan.
This is a valid concern, and I agree that the horizontal position of the router increases the potential for the dust to settle inside it due to the gravity.
However, one has to take into consideration the huge forced air flow created by the fan. That air flow does two things to the dust: (a) it sends the dust inside the unit, but (b) it also blasts it out with the air that needs to escape. Most people who cleaned their laptops (or other dust-collecting electronics) from the dust accumulated inside, used some method of blasting the dust from inside - with the compressed air, hair fan, or even a vacuum cleaner with the hose attached to the exhaust. With a strong air flow from a 120mm fan, you have the same effect. However, there are always "pockets" (usually in the corners) where the dust will be accumulated, and maybe even compacted with the strong air flow.
The balance between (a) and (b) depends on the size and position of your fan relative to the shape of the router.
In part for this reason, I decided against the dual-fan solution: I did not want to cover the entire back of the router. With the fan over the middle of RT-AC86U (it has a hole-less vertical portion with the label right in the middle) - with the middle, bearing part of the fan laying over that "plate", I see a good, healthy backward flow from the grilles on the outside of the fan and at the original "top" edge.
3. Other factors and constrains:
In my case, the shelf is not tall enough to accommodate the router standing, and I have tested that it is the optimum location for the signal.
4. Dust filters
Note the dustfilter attached to the fan to prevent most of the dust being blown into the router.
What are you using for the dust filter? I couldn't figure out from the photo: is that just the metal mesh, or is there some foam behind it?
I am considering something like this:
https://www.grainger.com/product/31CC65
I don't know if it would be easy to mount (insert) it between the fan and the grille of my infinity Multifan S3 and how significant would be the impact on the air flow.
5. Optimum fan position and the air flow pattern
The fan is over the WiFi chips since the CPU is mostly covered by the solid plastic behind the label. This setup result in substancial temperature drops, especially for the WiFi chips, with ambient temperature of 70F (21°C):
The air flow with the normal (i.e. 90 degree) incidence at that chip may or may not be optimum. In some cases blowing on one side of it could be better, if that results in the cool air flowing rapidly along the entire surface of the chip.
I have tested: in case of A
C-86U and my setup, blowing over the middle of the router's back produces better temperatures for both CPU and the radios. The difference in the temperatures between when the fan over the middle of the router and when pushed to the size of the router (aligned with the edge of the router) is 1-2 C, depending on the chip.
Whether it is due to a better flow pattern or due to the fact that that hole-less "flat panel" portion is slightly in the way of flow from the fan fins when the fan is aligned with the edge of the router (or both?!) - remains an unanswered question.
F:01
