fan backwash

i dont really know what to call it, but anyways, i noticed that alot of the air instead of going into and out the sides of my heatsink, its coming right back up through the fan does anyone have any experience on how to fix this? I feel that this could be ALOT more efficient if it all would go right through the heatsink. Well, if anyone has any idea's it would be greatly apreciated.

Thanks
-f1

Are you sure you aren't feeling it sucking? This is pretty impossible.

Stick a piece of paper on it, it should stick from the suction. As ffats said, it's pretty impossible.

Are you using some kind of tapered duct with a steep angle of reduction? This would cause the blowback you describe.

That is called backflow, no it is far from impossible. It is trying to push more air through your HS than possible, if you are using an adjustable fan turn it down to where it isnt doing that any longer, you may actually get better temps.

If you arent using and adjustable fan(or want to run high speed) gut an old fan if you have one, and put it on the heatsink 'first' then attch the working fan on top, and it may reduce the backflow.(either that or it will improve teperatures anyway)

BTW, what HSF do you have?

The other thing you can do instead of turn the fan down is try reversing the fan so it blows up, sucking air from the heatsink rather than blowing t down.
If its something other than the proc HSF (I wouldnt try it there) you might try moving the fan to the side of the HS to blow perpendicular to the fins on the HS rather than stright down on them.

eobard said:

Are you using some kind of tapered duct with a steep angle of reduction? This would cause the blowback you describe.

Click to expand...

Nope no duct. ill try the paper thing. and if it is blowing out, ill try reversing it. Actually, having my volcano 9 suck air into the heatsink might be better becuase the smart fan 2 has a large hub, and then i could duct the hot air directly out of my case! Ill also try turning it down a little.

Backflow will always occur to some degree with axial fans moving air from an area of low pressure to an area of high pressure. Using the method pik4chu described will help decrease this, as any non restrictive ducting would. Also, thicker fans create less backflow for similar reasons.

Centrifugal blowers are much better than axial fans when pumping air against pressure... It is difficult for an axial fan to create much force.

Thanks! ill try what pik4chu said. and i noticed this not so great volcano 9 really cant push much air though it at all, and both sides where ait can come out are kinda blocked by capaciters on the mobo, so i guess that makes even more pressure, and probably explaines my new 50 degree load temp, becuase the air inside is just standing still or not much is moving though it. So i will try having it suck.

one thing, ive heard mixed veiws on stacking fans, im not going to, but lets say i threw another smt fan 2 on top of this one, i know that could restrict the amount of air that actually whent in, but wouldnt it shove more air in by keeping less of it escaping? becuase the 2nd fan wouldnt create more air going through, but it would add twice as much torque keeping the air flowing. what do you think?

Thanks everyone!
-Nick

You would likely get more airlflow yes, however it would be far less efficient.

Before I got my blower, I modded my case and put a case fan directly over the CPU HSF - I think there was maybe an inch or two of clearance between fans. The case fan considerably improved my temp readings - I think by something like 5 degrees C, but that was with a stock HS.

Results depend on a lot of variables though - if you wanted to test this you could cut a hole in a board of wood the size of the side of your case, and install a fan to see what the result was before actually hacking up your case. If you have the material handy it might be worth it.

"Before I got my blower, I modded my case and put a case fan directly over the CPU HSF - I think there was maybe an inch or two of clearance between fans. The case fan considerably improved my temp readings - I think by something like 5 degrees C, but that was with a stock HS."

I did the same thing and it does help alot!
But after i did this i was checking things out as to the air flow and noticed the fan i have in the back of my case (blowing thought the metal grid) is having a lot of "BACKFLOW"
Im currently waiting for a friend to let me use his dremel so i can cut the metal grid out in the center of the mounting holes so there will be not restrictions. As well im gonna do the same thing to the front case fan and grid!
I hope this will improve the air flow throught the case!

as for fan stacking Ive done some amatuer measurements and with two fans stacked it looks like you get about 25-40% more airflow than just one, assuming identical fans. so its a bit better I guess since it not any noisier and if fan size is limited.

math 2 fans that push say 20cfm (just example) then by stacking them youd get maybe 30cfm tops. of course you get more if you seperate them but is is assuming you dont ahve room to do that.

For blowing into low restriction areas such as cases, have 2 fans blowing in seperate holes (i.e. side-by-side, not stacked) for max airflow.

For high restriction areas such as heatsinks or maybe small cases with restricted air passages, stacking one fan on top of another helps produce more airflow by increasing the pressure behind the airflow.

This comes from an ATX design guide, but I can't find the link at the moment.

schismspeak wasn't talking about stacking two fans, he was talking about cutting the fan portion out of an old one, and using the remaining shroud between your working fan and your HS.
This would help some on your backflow problem, and help get air to the fins that are blocked by your fan's larger hub (as you said it had). Even if it doesn't solve the backflow, your temps would most likely drop because more fin area is exposed to the airstream.

Where I've seen "fan stacking" work the best is when you mount the two in a square tube (paper and cardboard work for testing)
leaving an inch or more between them. You can even add a couple of fins between them projecting into the airflow to help straighten the air column before it reaches the second fan.

Really, the watercoolers here wouldn't be caught dead without a shroud to improve efficiency, it's beyond me why the aircoolers don't do the same....It also helps with noise.

check and see if the heatsink is full of dust..........