A quick science question. Fan into a funnel?

Lazy-Ferret · 06-10-2015, 09:44

Picture the following bit of kit...

A 12v computer fan, 80mm diameter that can shift 50 litres of air per min, sealed into an 80mm tube.

If I could measure the air pressure at the output end of the tube, I am sure there would be a small increase.

If I then blocked the the output off so the air has nowhere to flow to, and measured the air pressure, I am pretty sure it must have increased in comparison to the ambient air pressure, or would it? Part of me wonders if it would just be the same, and with nowhere for the air to go, so no movement, the fan would effectively choke, and do nothing.

Now if the pressure has actually increased, what are the deciding factors on how much it has increased by? If I got a bigger diameter fan, or one that can shift higher volumes of air, could I increase the pressure further?

Based on that, if instead of blocking the end, I fed it into a funnel with a 15mm exit, what would come out of the thin end of the funnel, would I still get the same pressure I got with the end blocked, would the funnel effectively increase the pressure? My logic says the area of the 80mm tube is roughly 5000mm, and the area of the funnel output is roughly 180mm, so you should get a 28 fold increase.

Here's the reason for the questions.

I have a camera mounted on the back of the car, and when it is wet, the negative pressure caused by the flat back of the T2 causes the rain and spray to by sucked up behind the car, which ends up leaving the camera lens covered in drops of water. I was thinking of taking a small pipe to the camera lens, that can blow air across the lens. so the water dissipates. To do this, I could build something like I have talked about above, fix it inside the rear door of the T2, and run a small air pipe to the camera, which would not only stop the water, but maybe the dust in the summer.

Lastly, if I took a 100mm fan and put it at the entrance to the funnel, and then a bit further into the funnel I put a 50mm fan, would this help or hinder things?

06-10-2015, 09:44	#1
Lazy-Ferret Senior Member Join Date: Jun 2012 Location: Sevenoaks, Kent Vehicle: Terrano 2.7TDi SE Touring Posts: 5,221	A quick science question. Fan into a funnel? Picture the following bit of kit... A 12v computer fan, 80mm diameter that can shift 50 litres of air per min, sealed into an 80mm tube. If I could measure the air pressure at the output end of the tube, I am sure there would be a small increase. If I then blocked the the output off so the air has nowhere to flow to, and measured the air pressure, I am pretty sure it must have increased in comparison to the ambient air pressure, or would it? Part of me wonders if it would just be the same, and with nowhere for the air to go, so no movement, the fan would effectively choke, and do nothing. Now if the pressure has actually increased, what are the deciding factors on how much it has increased by? If I got a bigger diameter fan, or one that can shift higher volumes of air, could I increase the pressure further? Based on that, if instead of blocking the end, I fed it into a funnel with a 15mm exit, what would come out of the thin end of the funnel, would I still get the same pressure I got with the end blocked, would the funnel effectively increase the pressure? My logic says the area of the 80mm tube is roughly 5000mm, and the area of the funnel output is roughly 180mm, so you should get a 28 fold increase. Here's the reason for the questions. I have a camera mounted on the back of the car, and when it is wet, the negative pressure caused by the flat back of the T2 causes the rain and spray to by sucked up behind the car, which ends up leaving the camera lens covered in drops of water. I was thinking of taking a small pipe to the camera lens, that can blow air across the lens. so the water dissipates. To do this, I could build something like I have talked about above, fix it inside the rear door of the T2, and run a small air pipe to the camera, which would not only stop the water, but maybe the dust in the summer. Lastly, if I took a 100mm fan and put it at the entrance to the funnel, and then a bit further into the funnel I put a 50mm fan, would this help or hinder things?