How Can a Flame Burn in a Closed Space Without a Fan?

[rice421]

Hi Everyone, Hope this is the right section of the forum.

I need some help to design a small tube heater for a specific application. I have attached a crude picture to show what i need. The flame needs to travel through a 3/4 inch pipe about 7 feet to heat the water. The problem is that i don't want to use any power so i can't use a blower or fan. I hope to use the venturi effect or some other way to get the flame to suck in enough air to burn hot and reliably with no power. Not sure how I am going to ignite it yet but that should be the easy part.

Any ideas on how to get this flame to burn in a closed space with no fan??
Thanks in advance for your help

 

[Baluncore]

You should round the corners of the tube so as not to obstruct flow.
Your flue should be vertical and tall in order to draw well.

Why does the flame need to burn along the length of the tube ?
Surely, the hot combustion products are sufficient to heat water ?

In this situation, I would also consider a counterflow heat exchanger.
http://en.wikipedia.org/wiki/Heat_exchanger
http://en.wikipedia.org/wiki/Countercurrent_exchange

 

[rice421]

Baluncore, Thank you for the reply. The 90's are rounded, the drawing is crude. good idea about the flue, i will make it vertical and as high as possible.

The flame does not need to be the entire length of the pipe, But the idea is to heat the water in a matter of seconds so wasting some propane is no problem if it decreases the time needed. The reason for the tube and not a coil or some other design is because of the application. the tube will just barely fit in there.

The problem i am looking for help to solve is getting the propane to burn good in the long tube with no fan. If the tube was say 2 feet, there would be no problem. I will look into the heat exchanger you mentioned tonight. i figured there must be some kind of fancy venturi setup that would miraculously draw a ridiculous amount of air, or some other phenomena...

 

[Baluncore]

The Bernoulli effect of the gas jet will draw air into the tube. Getting the air to fuel mix right is probably best done by using the burner from an existing commercial product that uses the same fuel source.

A slight trumpet shaped opening at the top of the flue will also improve flow.