When the air in a bottle gets hot, it expands. If there is any water inside, it will boil and expand 1700 times, so this is a serious danger. Imploding is never an issue. Water vapor is already expanded, and will only expand more if heated.
Hi Glenn. I'm a little unsure, but from what I have gleened thus, vaporisation pressure only increases if there is a source of water. More water has to vaporise in order for the water vapour pressure to increase. In other words, I don't think that it is the water vapour which is expanding more and more from an increase in heat. I really need to do some more research on the precise behaviour of water vapour when heated.
When I put the first fire in it to dry out the core, this bottle got hot fast and the clay plug kept it from venting, so it blew out the back face with a loud pop/bang. Fortunately it didn't damage anything important.
Glad to hear no-one was hurt! But I wonder if the problem lies in liquid water having contaminated the bottle in someway, and does not necessarily come from air per se. I have only a vague understanding of these things, and have trawled a few web pages to try and learn a bit more, but as I understand it , a bottle filled with air at 0 c, at something like 15psi, will find that when it is heated to 100c, the pressure will increase by a third, to something like 20 psi. Can a sealed glass bottle keep up with these increases in pressure? One hopes so ... I've included a tantalising link below where the guy compresses the air in a wine bottle to 260psi!
Gas Pressure Increase with Temperature
In 1702, Amontons discovered a linear increase of P with T for air, and found P to increase about 33% from the freezing point of water to the boiling point of water.
That is to say, he discovered that if a container of air were to be sealed at 0°C, at ordinary atmospheric pressure of 15 pounds per square inch, and then heated to 100°C but kept at the same volume, the air would now exert a pressure of about 20 pounds per square inch on the sides of the container. (Of course, strictly speaking, the container will also have increased in size, that would lower the effect—but it’s a tiny correction, about ½% for copper, even less for steel and glass.)
Remarkably, Amontons discovered, if the gas were initially at a pressure of thirty pounds per square inch at 0°C, on heating to 100°C the pressure would go to about 40 pounds per square inch—so the percentage increase in pressure was the same for any initial pressure: on heating through 100°C, the pressure would always increase by about 33%.
Furthermore, the result turned out to be the same for different gases!
http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/ThermProps.htm
Like I say there are huge gaps in my understanding, and when I get some time, I would love to start another thread which goes into more detail about bottle/ tin can insulation.
A little microwave like you show might be small enough to heat with a 3" rocket... but you will need a bunch of mass and insulation around it, and you will need to cut an entrance in the bottom for the hot gases to enter, and make an insulating door. All in all, it seems like a lot more work to make an oven the way you are trying than to make the whole thing of clay/soil cob. The dense bricks would be the right kind of material for the oven floor.
No matter how well you insulate them, the heavy bricks will not work as well as cob or firebrick or insulating refractory. A tiny 3" system needs all the help it can get to work well.
I don't plan on any of the gases entering the oven chamber ~ they will move only around the outside, much like the one seen in the diagram below...
3-Inch Cans
How long are these going to last? Not very long. Off the top of my head I'm thinking aluminum melts around 1800 F and mild steel a little higher, a little below 2000 F. I've measured 1710 F at the entry of my burn chamber (6" system), and it is doubtful that is the hottest point. Under ideal conditions wood is said to burn at roughly 3400 F. The bottom line, is that I think the cans are doomed to fail, and fairly quickly from a heating use (as opposed to some of the flaming cans of death seem on picnic tables in some YouTube videos, heheh).
Doesn't this mean that for the 3-inch can idea to work as a model for space heating, a pourable / castable core material will have to be poured in around the tin can mock up? That would only be using the cans as the form, and letting them burn out is fine after the casting material has set up.
Hi Eric. I hope to use the cans late on the exhaust, where temperatures are lower, but would try to design something where they are easily replaced, or cast in something cheap like everyday concrete, rather than anything refactory.
Much thanks.
