spookmineer wrote:Shingoshi wrote:Wings with narrow chords move air across them with greater speed. That's because they have the lowest resistance of wings with wider chords.
There are no curves in the fins of a heatsink. Locally, the air across (above?) a wing moves faster than the air below it, but this has a function: to create lift. There is no special geometry in fins, the speed will be the same at either side, but will decrease because of drag if fins are longer.
With sailplanes it makes sense to reduce drag, high aspect ratio wings are more efficient, because the lift/drag ratio is higher.
Maybe that's why heatsinks like the TRUE are performing so well, with their high aspect ratio fins
Shingoshi wrote:Laminar flow is essential to whether the wing is likely to stall at higher angles of attack.
Usually, the angle of attack in a heatsink is zero. Fans are mounted perpendicular to the fins of the heatsink.
Shingoshi wrote:Wider spacing of the fins would create greater turbulence. More turbulence = lower airspeed.
If so, the biggest turbulence would happen if the fins were miles apart: if only 1 fin would be on a heatsink. I don't think a single fin would create more turbulence than a bunch of them.
Also, just 1 fin would mean the least obstruction to airflow, so this will remain almost the same. With lots of fins, I can imagine the airspeed to drop.
Shingoshi wrote:The wider the fins, the lower the airspeed. So again, wider fins don't work.
Do you mean wider fins as in thicker, or wider fin spacing as in the distance between them?
Turbulent air will also be
more noisy, and that is what Neil pointed out: "Closely spaced fins = noisier HSF", and as you say, "increasing the depth of the fins would be less efficient".
It is less efficient, up to the point where making fins deeper to increase total area, will cause air to become turbulent, which is what we don't want.
Very good points raised here!
1. I thought about the absence of complex geometry on fins. You're right, the air moves with equal speed on both sides of each fin. Mostly, I was concerned with the chord of the fins. Specifically, that they are narrow. I looked at the TRUE to see what you meant. Interestingly, the TRUE seems to have a wider chord, based on the images alone.
1a. From the description of the TRUE:
The Ultra-120 utilizes the winglet design helping to minimize air resistance and maximize wind air flow.
2. The point I was making about laminar flow, wasn't well defined. Maybe I shouldn't have raised it.
3. Fin spacing: No point in irrelevant exaggeration here. We were talking about a finite number of fins in a relatively small region (the intake area). The intake area of the heatsink isn't (likely) going to be increased. And I think everyone here expected that (intake) region to divided by a smaller number of fins, to obtain a wider spacing. The expected trade-off, was the need to increase the depth of the fins to maintain the same surface area of each fin.
4. I meant wider, as in the spacing between the fins. Sorry about the lack of clarity. I meant that if you increase the spacing between the fins (with surfaces equally smooth), there might or should be an increase in the turbulence.
5. I think the fins spacing would have more to do with pitch than volume. I think that if you blow air at a constant rate through tubes of different width, the narrower tube produces a higher pitch, while the wider tube produces a lower pitch. So I think the sound we be different in terms of pitch (frequency), not volume (decibels). Although, I guess the volume could also be lower from the wider tube, due to a greater drop in air pressure. (see below)
One thing I do know. The wider spacing between the fins would require greater pressure to maintain the air velocity. Which we couldn't do with identical fans. So two heatsinks with identical fans pushing air through each heatsink (one heatsink with narrow spacing and the other heatsink with wider spacing), the wider spacing would (should) have a greater drop in air pressure, between the intake and exhaust ends. And that greater pressure drop would reduce the airspeed of the exhaust. So the adjacent components wouldn't be cooled as effectively. But I could be wrong.
Ah crap! There are too many things to consider here! Would the heatsink with narrower fin spacing and a possibly higher pitch, be an indication of higher efficiency? Wouldn't lower efficiency mean higher temperatures? Do you want to trade off higher efficiency for lower noise?
Considering that the AXP-140 is sold as a passive cooler, with the option of active cooling (with a fan), I think the fin spacing was chosen accordingly. The less distance the air has to move to reach the edge of the fin, the quicker that air can escape the heatsink.
Ah, forget it!
Xavian-Anderson Macpherson
Shingoshi
