Hoytman wrote: ↑Fri. Jul. 24, 2020 10:46 am
This will be a good one for Isayre, McGiever, others interested...
Does anyone have any idea if air speed, and/or flow (I say that because they can likely be different, but I don't know), can be increased without mechanically inducing it? Likely not, but I thought I'd ask.
Sounds like a silly question, but I'm thinking of this in regard to stoves and chimney's, mainly a stove or outdoor fire pit. Obviously, if this could be done, then there would be no need of electrical/mechanical draft inducers, but I'm hoping I am wrong.
I'm wondering if creating a vortex, like in some of the rocket stoves, induces better air speed as heat increases? This is what I'm driving at. However, I'm also thinking along the lines of the cold intake side as well as exhaust.
It could be that a small or large fan is the only way to do this.
When I was a kid I remember this little toy I had. It was a short 1"-2" piece of plastic tubing about 1/2" inside diameter. You could barely blow on it to turn this little super-duper-dyno-whoppin' little thing-a-ma-bob inside of it that made this little whizzing noise....zzzzzzzzzz. Sort of reminded me a jet engine...and I don't know anything about jet engines or turbines, but they are interesting and fascinating.
Seems like back when I worked maintenance in a machine shop, that some of the machinists told me that air moving through a hole can be increased slightly by putting a bevel on both the hole entrance and exit point....countersinking it is the word I'm looking for. Also, I remember that there's a space around the outer edge of the hole where air has some "resistance"...maybe that's the word I'm looking for to describe it???
There has to be some sort of applied physics here, but that is one course I never took and know little about. Very little. I just know of very basic things and I'd likely even get those wrong.
https://www.google.com/search?ei=aPEaX5XQJI24tAat ... CAw&uact=5
I am wondering if somehow these thoughts can be applied as well.
https://physics.stackexchange.com/questions/13030 ... an-airfoil
Interesting questions that have been kicked around on here in some threads.
Your comparing "naturally aspirated" to forced induction (fan-fed). Naturally aspirated just means no mechanical help, only natural differences in pressure does all the work. With a stationary object like a stove the driving force is the pressure difference we call "draft", or some call it "vacuum" or "suction", but technically it is known as "pressure drop". However, you rarely hear that term used outside of auto fuel/air intake system designers. But all are the same thing. You can find out more by looking up those other terms.
And how well it does that pressure drop is mostly do to temperature difference of the gas (air) outside the stove/chimney system verses the lighter hot exhaust inside the stove/chimney system. The hotter the exhaust, the faster it will flow because of greater pressure difference. Trying to increase that can be done by reducing flow resistance into, through the fuel bed, and then out of the system. But that does not give as much gains as temperature difference if the system is already decent. Where you'd see the biggest change is on differences of coal size, as anyone who has tried different sized coal in their stove likely has learned.
But all that is limited. To get greater gains takes mechanically increasing the pressure difference. BTW to get more out of the coal fired boilers of steam ships they went to using ducted fan systems for pressurizing the air in boiler rooms. So, even with the best, expensive, naturally aspirated designs, they have their limits. And that's why they have super charges and turbos on engines. However, I doubt there is enough energy in stove exhaust to drive a stove turbo if anyone is thinking of hot-rodding their 3.5 liter Glenwood.
That bevel, or rounding of an edge of a hole, that a gas is to flow through, reduces what is known as ''edge effect". It's one of the elements that goes into carburetor and intake system designs. You see it in the trumpet shaped openings at the top of carburetor throttle bores. Edge effect is caused by turbulence created drag at the very edge of a hole (only within a few thousands of an inch) as air tries to flow in and gets mixed with air not wanting to move right at the edge. Because the effect is always close to the edge and about the same size, it's not much of a factor on large openings as small ones. I learned about this when designing a redistribution plate for a carburetor/turbocharger box when designing a turbo system for my 72 AMX. Needed the plate with 1/4 inch holes to redirect and even out air/fuel flow only coming into one the side of the box and having to make a tight 90 degree turn to go down into the top of the intake manifold. Without the redistribution plate most of the fuel vapor in the air/fuel mix would be thrown to one side of the intake manifold plenum area and thus feeding four cylinders too rich, leaving the other four cylinders too lean. Even though that air/fuel mix was being boosted to higher pressure/force by the turbo, calculations showed that I had to add at least 25% more holes to compensate for the total square area of openings in the plate lost to edge effect drag just to flow the same CFM as without the plate.
The Rocket stoves "rocket" because they are burning a hot fast burning fuel - wood kindling. Fill them with coal and see if they still rocket. You'd see the same if you had a stove like those of us that have antique stoves that burn both wood and coal. As you know, wood burns too fast to get as long and steady of a fire as coal can in the same stove.
As for why an airfoil works, look up Bernoulli affect and then think about if any of that applies to stoves.
Paul
