A quick science question. Fan into a funnel?

[Lazy-Ferret]

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?

[makeitfit]

Put some Rainex on the lens

(on the science side of things , I think you're confusing pressure and flow, either way , keep it simple)

What about fitting the camera inside the rear screen ?

[jims-terrano]

I think a certain Mr Whittle went there before you but he introduced paraffin into the equation which is fun.

Mind you your camera may melt if you did that

[Lazy-Ferret]

Originally Posted by makeitfit

Put some Rainex on the lens

(on the science side of things , I think you're confusing pressure and flow, either way , keep it simple)

What about fitting the camera inside the rear screen ?

I have tried Rainex, but because the lens is recessed, it builds up round the edges and still sits on it.

I think you are right about the airflow/pressure bit, that's why I put it on here, as I have got to the stage I have over thought it, and now have no idea. I have just discovered though that Fans and Blowers are not the same thing, and I need to use a blower before I even start to do anything else.

I can't put the camera inside the car, as it needs to look down, and the spare wheel gets in the way.

Originally Posted by jims-terrano

I think a certain Mr Whittle went there before you but he introduced paraffin into the equation which is fun.

Mind you your camera may melt if you did that

Ahhhh... so it dries as well... Might get an extra 5BHP if I make the jet so I can angle it...

[don simon]

What you need is something like a mini air knife and I don't know if something like that even exists.

[Lazy-Ferret]

Originally Posted by don simon

What you need is something like a mini air knife and I don't know if something like that even exists.

Would be handy, but like you say, I have not seen anything like that in my travels. For the nozzle, I was thinking of using a "spray" windscreen washer jet, as it should be pretty easy to mount it, and if the jet is too small, I can open it up fairly easily. For the air supply, an Airbed pump might be just the thing, and I have one of those somewhere I can have play with to see if it works in principle.

[Sweety]

As far as I know air speed should increase as the pipe reduces slowly in diameter but over a long distance there may be no gain at all & air flow might feed back towards the fan blades & slow them down a bit Have you tried making a little spoiler for the top of the camera sticking out maybe 3-4" past the lens which might cause a second negative pressure but this time further away from the camera lens

[Ogri]

Air behaves as a fluid - just like water - the only difference is the density. So it may help you to visualise what would happen with water under the same circumstances albeit with a tap or pump, rather than a fan.

My fluid mechanics is not up to the task of answering what size fan and nozzle you might need though

Good luck

[jims-terrano]

Originally Posted by Lazy-Ferret

Might get an extra 5BHP if I make the jet so I can angle it...

Fworrr vectored thrust now ya talking

[Lazy-Ferret]

Originally Posted by Sweety

As far as I know air speed should increase as the pipe reduces slowly in diameter but over a long distance there may be no gain at all & air flow might feed back towards the fan blades & slow them down a bit Have you tried making a little spoiler for the top of the camera sticking out maybe 3-4" past the lens which might cause a second negative pressure but this time further away from the camera lens

I think you are right, for every bit you gain, you loose because of other problems.

The problem is, the camera already sticks out, with just the rear towbar step sticking out about an inch further, so I don't really want to make it stick out any more. It does have a shade that goes over the top, that in theory should deflect air down past the lens, but fails. The problem is, if you bring the shade down any further, the IR LED's reflect off it at night, and shine back into the camera, making it look all fuzzy.

The trouble is, it's a great camera, has 2 lenses built into one unit, so one looks to the rear, and gives you a rear view mirror, the other looks down, and is great when reversing up close to something.

I might have to convince the wife the only thing to do is mount a compressor and tank under the back seat, and use a quick blast of compressed air for the camera... Ohhh... an now it's fitted I can pump my own tyres up and fit that train horn I have always wanted...

[jims-terrano]

Sounds daft but could you fit another washer nozzle from the wash wipe?

[emjaybee]

What about a bit of drain pipe to one side of the lens and training one of the ferrets to pop out and lick it clean?

I'm not helping really am I?

The wife's told me about that.



I'll go away now...

[Sweety]

Originally Posted by emjaybee

What about a bit of drain pipe to one side of the lens and training one of the ferrets to pop out and lick it clean?

what you really need is a micro wiper motor & blade I know it sound daft but am wondering if you can get a motor meant for hobbies etc that only rotates 90' or so the a thin bit of alloy to attach to the motor with a cut off bit of wiper blade mounted on it bit like headlight wipers but on a much smaller scale

[kitchenman]

Crazy idea here, you have a negative pressure at the lens, so how about fitting a tube over the lens vented at the lens end via a small pipe to somewhere at a more positive pressure, this would pass air over the lens and exit at the end of the tube

[Blocky10]

Originally Posted by kitchenman

Crazy idea here, you have a negative pressure at the lens, so how about fitting a tube over the lens vented at the lens end via a small pipe to somewhere at a more positive pressure, this would pass air over the lens and exit at the end of the tube

Kind of what I was thinking. Ram air... It only gets dirty when you are moving so a small air scoop blowing across the lens should keep it clear without the 12v fan complications

Ian