How does powder granulation actually effect burn rate?

[M. De Land]

I got to thinking on this a bit after remembering seeing very old black powder cartridges where the BP had actually become a solid mass and still they would fire.
I understand the principle of larger grains having more air space between them which means volume is changed. This means smaller granulation allows more powder in the same area because of less air space between them hence more fuel in the explosion, increased pressure and thus velocity.
Apparently the air space around the granulation has little to do with the powder burn rate as the potassium nitrate within the powder is the oxygen source.
Now lets change the scenario to pan powder in a flint lock. It is obvious that finer grains light faster than does courser grains but why. It would seem that larger grains would have more surface area and air space between them to allow the flint sparks to light the fire but the opposite occurs in that fine grains light and burn faster.
I'm wondering if the finer powder lights easier because of the graphite coating difference between large grains and small grains. I had thought that usually the smaller grain size of black powder is from the grain fracture that is sieve out and separated from the larger sizes that have been graphite coated. Is this incorrect?
If so than the small grain fractures would not have the inhibition of a graphite coat which I think is non-flammable in solid or balk form, no?
Also, getting back to the solid mass of black powder in old shells or cartridges exploding with little difficulty, makes me think that the containment has a great deal to do with how the powder burns as opposed the the free state of say the same powder in a pan charge.
Any thoughts on the subject as it occurs to me that I don't really know much about the dynamics of how black powder burns?
Doing some thinking out loud is all.

 

[Britsmoothy]

Oh no, not again

 

[sawyer04]

Deep thinker

 

[arcticap]

Now lets change the scenario to pan powder in a flint lock. It is obvious that finer grains light faster than does courser grains but why. It would seem that larger grains would have more surface area and air space between them to allow the flint sparks to light the fire but the opposite occurs in that fine grains light and burn faster.
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I would disagree that larger grains have more surface area than smaller grains.
As you seemed to say, because of the air space, the smaller granulation may have more actual powder in the same space compared to a larger granulation.

If a person squeezes a bunch of small grains of powder and forms a large clump out of it, then what happens to the surface area of all the small grains?
I think that some of the small grains lose some of their surface area.

While there may be an exception for every rule as far as comparing different brands of powders, or powders from different makers,
I think that generally smaller grains means that it will have more surface area than an equal volume of larger grains.
And I think that's why the smaller granulation will produce a faster burn rate and more velocity.

In theory, different granulations should have about the same amount of graphite per pound.
But companies also make black powders without graphite, but use a different labeling system for it.

 

[Carbon 6]

I would disagree that larger grains have more surface area than smaller grains.
As you seemed to say, because of the air space, the smaller granulation may have more actual powder in the same space compared to a larger granulation.

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Smaller granules also have more collective surface area in a given volume. This means that smaller granules will catch a spark easier and they will deflagrate faster.

 

[yonderin]

From what I was taught; A pound of BP will have the same total energy, regardless of the grain size. It's how fast that energy can be released that changes with the granulation size. Smaller grains are consumed faster and therefore release their total energy faster.

 

[M. De Land]

Good discussion guys! The differing thoughts help me to widen the scope of my thoughts on the matter.

 

[Tom A Hawk]

From what I was taught; A pound of BP will have the same total energy, regardless of the grain size. It's how fast that energy can be released that changes with the granulation size. Smaller grains are consumed faster and therefore release their total energy faster.

That works for me. Also more powder per volume.

 

[Tom A Hawk]

Smaller granules also have more collective surface area in a given volume. This means that smaller granules will catch a spark easier and they will deflagrate faster.

That reminds me of the grainy looking guy in Walmart the other day who kept deflagrating...

 

[hanshi]

Yep, small grains, more surface area. Yonderin also had a good explanation of energy release.

 

[Baxter]

I'm with Yonderin on this one. "Containment" also has some effect on explosive force, I believe.

 

[Loyalist Dave]

That reminds me of the grainy looking guy in Walmart the other day who kept deflagrating.

Deflagrating OR do you mean, flatulating?



LD

 

[Tom A Hawk]

Its a bad sign when a joke has to be explained...

 

[Carbon 6]

Its a bad sign when a joke has to be explained...

Or the sign of a bad joke.

 

[RAEDWALD]

All things being equal, small grains will have a greater collective surface area than large ones for the same weight of charge. Thus the charge burns faster which results in a faster rise to a peak pressure. The larger grains will burn for a longer time so distributing their energy over both a longer period and in a further expanding space as the load will have moved further up the barrel by the time the gas has expended. The extremes are small pistols (say .32 calibre) which may cope with a 4F charge and a large cannon (say 6" diameter) which will blow itself up with a 1F charge. Hence coarse cannon powder is sold to hobbyist cannon users and large 19th century military cannon used grains aptly termed 'pebble powder' for ones which were over an inch in diameter. Complicating it all is the heat energy which continues to expand the gasses even after they have ceased to be produced. The Swiss Aubonne black powder factory makes it's sporting powder with slightly more nitre than the classic chemical proportions used by ordinary powder makers to include more oxygen to enhance the heat expansion rather than just gas production on deflagration.

It is very hard to isolate one factor in black powder deflagration in use in such a complex material. I could also mention density, glazing (as opposed to the detrimental use of graphite for aesthetic marketing purposes) purity, wood and non wood types, creosotes and temperature of firing, pressure and length of incorporation and gas encapsulation, I will stop there as life is too short. The role of sulphur is still a field of study even today.

The ultimate grain is, of course, the solid black powder pellet which was coming into military rifle use just as smokeless powders were beginning and the shape of the pellet (in rod form for modern centre fire @8mm bore cartridges) was critical for formulating the shape and peaks of the pressure curve from ignition to expulsion and that technology became that of modern solid fuel rockets.

Note well that substituting modern solid rocket fuels for black powder in any form in a gun is not a sound idea for the backyard experimenter at all. No exceptions. Even if you are very clever and definitely not if you are just stupid and brave.

Even black powder pellets are a risk outside industrial testing. The British .303" cartridge was designed for smokeless powder but production of that was not ready when the Lee Metford rifles began production so a solid black powder pellet was used for a short while instead. The peak pressure of the black powder cartridge was noticeably greater than the replacement cordite smokeless powder cartridges and this was after extensive research and development into the black powder pellet. It is easy to make one with a sharp spike of pressure that will exceed the strength of the action.

 

[Loyalist Dave]

You can check this yourself....
Burn is burn when it comes to the principal of surface area and burn rate.
So find a seasoned piece of firewood, the next time you have a campfire. Find a piece of seasoned wood that's been cut on the ends but is still full round. Split it in roughly in half. Now this won't be a perfect 50/50 split but it won't matter to the demonstration. Set one half aside, then split the other half into 4 pieces. Now place the half you saved and the four pieces that you split-up onto the same fire and see which burn down the quickest, the half, or the four split pieces. Same fire, same piece of the tree, and although not perfectly divided, the difference between the half and the four pieces will be pretty dramatic. Also note which, the half or the four pieces produces highest flame. It's quite crude, but you will note the smaller bits burn a lot faster.

LD

 

[Carbon 6]

You can check this yourself....
Burn is burn when it comes to the principal of surface area and burn rate.

LD

In our case increased surface area translates to smaller particle size which increases heat transfer and combustion. The campfire analogy doesn't work well because campfires don't supply their own oxygen like BP does.

 

[Loyalist Dave]

So you dont think the smaller pieces of wood = smaller "particles" than the much larger, single "half" of wood, and thus illustrates the point, regarless of where the Oxygen comes from? So If you made pellets of carbon and sulfur and omitted the potassium nitrate, then combusted them in an oxygen environment, the smaller particles wouldn't be consumed faster? Wood burns by releasing gas from its surface, more surface = more fuel and at a faster rate...at least I was taught that.

LD

 

[Bo T]

In our case increased surface area translates to smaller particle size which increases heat transfer and combustion. The campfire analogy doesn't work well because campfires don't supply their own oxygen like BP does.

The example does work well to demonstrate the effect of surface area on burn rate. The BP still needs to reach a kindling temperature to combust.

 

[Carbon 6]

It's just a confusing analogy. It confused me anyway, but know I understand what you are saying. I thought you were saying something different.

We're good.