How does powder granulation actually effect burn rate?

[M. De Land]

Would a spark shower have a better chance of starting a fire landing in a pile of fine dry saw dust or courser saw dust of the same moisture content ,keeping in mind that the fine dust packs together tighter from gravity alone ? The sparks land on top but are less able to get down between the grains into the larger air space separating them. My guess is that the course grain saw dust would light easier, no? This is the same physical scenario that occurs in a pan charge of black powder. So why does the finer powder seem to light faster and easier in the pan? Each individual grain of course powder has more surface area exposed to ignition than does each individual grain of fine powder when laying in a pan on compressed.
I'm wondering possibly if starting the powder fire is a different dynamic of grain behavior as opposed to once it begins to burn.
I have read some folks use the same powder for pan as for charge and get fast reliable ignition.
This quite often with powder as course as 2F.

 

[Zonie]

While I agree with your statement, " Each individual grain of course powder has more surface area exposed to ignition than does each individual grain of fine powder when laying in a pan on compressed.", I don't agree with the idea that several smaller grainuales of powder have less surface area.

For instance if we have a 1 inch cube, it has a surface area of 6 square inches of exposed surface. (There are 6, 1 square inch surfaces on the cube.)

If we take that 1 inch cube and slice it in half, we end up with two 1" X 1" X 1/2" blocks and each block has a surface area of 2 square inches plus 4, 1/2 square inch surfaces for a total of 4 square inches per block but, because we now have two of these smaller blocks the total surface area of them combined is 2 X 4 = 8 square inches of exposed surface.

The bottom line is, more, finer grainuales of powder have more surface area than fewer course grainuales in a pile that is the same size.

 

[yonderin]

Would a spark shower have a better chance of starting a fire landing in a pile of fine dry saw dust or courser saw dust of the same moisture content ,keeping in mind that the fine dust packs together tighter from gravity alone ? The sparks land on top but are less able to get down between the grains into the larger air space separating them. My guess is that the course grain saw dust would light easier, no? This is the same physical scenario that occurs in a pan charge of black powder. So why does the finer powder seem to light faster and easier in the pan? Each individual grain of course powder has more surface area exposed to ignition than does each individual grain of fine powder when laying in a pan on compressed.
I'm wondering possibly if starting the powder fire is a different dynamic of grain behavior as opposed to once it begins to burn.
I have read some folks use the same powder for pan as for charge and get fast reliable ignition.
This quite often with powder as course as 2F.

I don't think too many of us mere mortals can detect the full difference in the speed of ignition of different size grains just by our perception. 2F or 3F provides reliable ignition and it saves having a second powder to fuss with.

Could be a good safety point too. With all the dry-balling that happens; what if you weren't paying attention and used 4F as the main charge? Maybe I should get rid of mine.

I did a calculation on the surface area of spheres; doubling the radius resulted in 4 times the surface area.

 

[Carbon 6]

I don't think too many of us mere mortals can detect the full difference in the speed of ignition of different size grains just by our perception. 2F or 3F provides reliable ignition and it saves having a second powder to fuss with.

I am not a "mere" mortal.

Could be a good safety point too. With all the dry-balling that happens; what if you weren't paying attention and used 4F as the main charge? Maybe I should get rid of mine.

That would be difficult (impossible) with a priming flask. and wouldn't result in any harm unless maybe you are a member of the Rambo Magnum squad.

I did a calculation on the surface area of spheres; doubling the radius resulted in 4 times the surface area.

Try that calculation again but envision it like this.

 

[M. De Land]

I don't think too many of us mere mortals can detect the full difference in the speed of ignition of different size grains just by our perception. 2F or 3F provides reliable ignition and it saves having a second powder to fuss with.

Could be a good safety point too. With all the dry-balling that happens; what if you weren't paying attention and used 4F as the main charge? Maybe I should get rid of mine.

I did a calculation on the surface area of spheres; doubling the radius resulted in 4 times the surface area.

No question about the surface area increase of many small grains and less large grains but I don't think that is the dynamic involved when were talking about a pan charge igniting.
My thinking is that all the grains, large or small are only presenting their top surface plus air space between, in a pan charge and that the larger grains with more area and irregularity on the top surface along with larger air gaps may actually present more opportunity for a sparks ignition potential.
Once ignition occurs there is no question fine grains burn faster but I'm just not convinced any more that they actually light more efficiently.

 

[Carbon 6]

Heat transfer from a spark to a granule is better with smaller granules than with large ones.

 

[M. De Land]

Heat transfer from a spark to a granule is better with smaller granules than with large ones.

Once the explosion begins then the individual grains , large or small, apparently are moving away from each other by each grains energy release. This is when the individual grains burn faster and because there is less air space between them at the beginning the weight and thus greater amount of powder is present to feed the explosion. Not only do the smaller grains burn faster there is actually more powder present in the same space when volume measured.
Now going back to a level pan of course grain powder, before ignition occurs there is more individual grain surface area on top and more air between them for a spark to start the fire.
Heat transfer to a single grain of 2 F with more air around it would seem to be arguably more able to occur than with the tighter packed surface of say 4F were the heat transfer would be to multiple grains laying closely together.
I guess what I'm wondering about is does the whole grain of powder ignite when the heat transfer elevates it to the flash point by conduction or does the chemical mix of sulfur, potassium nitrate and charcoal , present in the same amount throughout the grain, only need to make contact with the spark from the frizzen to produce ignition. My guess is that the spark only need to touch a portion of the grain to start the fire it does not have to transfer heat enough to raise the whole grain to ignition temperature. A small or large piece of wood does not ignite simultaneously, it is a progressive heat transfer.
If the later is true than larger grain size may be more efficient at starting a pan fire.
The reason the smaller grains work better in a pan may be because that once burning there overall energy release is greater because of increased density and thus more fuel in the same space.

 

[Carbon 6]

Heat transfer from a spark to a granule is better with smaller granules than with large ones.

I was referring to catching sparks, or initial ignition.

 

[M. De Land]

I was referring to catching sparks, or initial ignition.

Me too!

 

[Carbon 6]

I guess what I'm wondering about is does the whole grain of powder ignite when the heat transfer elevates it to the flash point by conduction or does the chemical mix of sulfur, potassium nitrate and charcoal , present in the same amount throughout the grain, only need to make contact with the spark from the frizzen to produce ignition. My guess is that the spark only need to touch a portion of the grain to start the fire it does not have to transfer heat enough to raise the whole grain to ignition temperature. Just like a small or large piece of wood does not ignite simultaneously, it is a progressive heat transfer.
If the later is true than larger grain size may be more efficient at starting a pan fire.
The reason the smaller grains work better in a pan may be because that once burning there overall energy release is greater because of increased density and thus more fuel in the same space.

I think you missed one piece to the puzzle. In a pan, the powder burns un-contained. This is different than powder that is contained.

In simplest terms, more surface area amounts to more sparks making contact with the powder.

I think the difference between large and small granules are best observed in small locks.

 

[Carbon 6]

Me too!

Hadn't read that far when I posted.

 

[Kansas Jake]

This discussion reminds me of the old story of a teacher filling a container with rocks and asking the students if it was full. They answered yes Then the teacher added gravel as she shook the container. Is it full, "Yes answered the students." Next she got a cup of sand and added it to the container, and you know how it goes, followed by a glass of water.

It would seem to me a finer grain powder would result in more powder given a set volume. Now I suspect it doesn't have a huge effect on a given charge.

 

[RedFeather]

Parts read more like the old joke "If it takes one man one hour to dig a hole, how long will it take one man to dig half a hole?'

Sometime back, I read about experiments Rodman made compressing black powder into cakes with honeycomb patterns. He reached some serious pressure levels. Makes you wonder just what pressure those old dried, compressed charges might produce?

 

[Carbon 6]

Sometime back, I read about experiments Rodman made compressing black powder into cakes with honeycomb patterns. He reached some serious pressure levels. Makes you wonder just what pressure those old dried, compressed charges might produce?

Interesting, I suspect the honeycomb structure led to the higher pressures.
I've also read where different "mixtures" developed higher pressures, some nearing those of smokeless powder. There was a lot of experimentation, fumbling, bumbling, and failures done to get to where we are today.

 

[RedFeather]

I read where Rodman, experimenting post-CW, got some hellacious pressures. If I find the reference, I will post it. He was working with the huge "soda bottle" shore battery guns. Years ago (actually, the 1976 Bicentennial, to date my old bones), I visited the old Arts and Industries Building of the Smithdonian down on the Mall In DC. They had a couple of those huge Rodmans, complete with almost basketball-sized round shot. Enough to say, I wanted one!!!

 

[sussexmuzllodr]

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.

If a Comet hits Jupiter vs. a small moon like Titan well uh....

SM

 

[bang]

Air space does not play in that much. BP carries its own oxidizer. That's why it is packed solid, no voids. Burn rate, from some reading, has a lot to do with amount of time it takes for burn of a grain and how many grains are consumed per that time and number of grains. Larger grains take longer and less number of grains per volume than finer grains. Of course your dealing with a time measurement that's in the plank moments. All Black Powder will basically explode in open air. But 4f burns more grains at one time than 3f or 4f because ther are more actual grains in same volume. Or something like that. The research study is a mess to read. I think it's because the differences in time are so small it seems futile to care. I was always told you use larger grain in shotguns and 54 cal and up for more of a push, mid grain in 50 cal and down, mid or fine in 50 cal pistol and down. Main thing is use what works best but in suitable volume that provides proper operation of the firearm.

 

[Carbon 6]

Air space does not play in that much. BP carries its own oxidizer. That's why it is packed solid, no voids.

The focus is on the space between granules, not the air or oxygen in it.. Space affect burn rate.
I agree with everything else you said.

 

[bang]

The focus is on the space between granules, not the air or oxygen in it.. Space affect burn rate.
I agree with everything else you said.

Larger the grain the more air space. Point was air space does not change burn rate it only changes number of grains in same volume per grain size.

 

[Carbon 6]

Larger the grain the more air space. Point was air space does not change burn rate it only changes number of grains in same volume per grain size.

The volume in a pan is not confined.
Space=distance=time.