Post by a***@gmail.comHi, I'm trying to do some academic tests on pressures produced by
black powder and I'm having some issues with the experiments that are
scaled down in size. They seem to produce much lower pressure values
than basic similitude would dictate, and I'm thinking they may be
seeing lower pressures because of diameter effect? But I'm such a BP
novice I don't even have a ballpark for what size scale this starts to
occur at for black powder. I've found some academic literature on burn
rate vs. pressure in the literature, but it's almost entirely in
Russian and therefore only marginally helpful to me.... any wisdom in
the group?
Rachel,
There are a number of effects at work to make small volumes of black
powder seem less powerful per unit-volume than larger volumes.
One of the more profound effects with small volumes in heat-conducting
test vessels is the cooling effect of the vessel on the gasses, as they
are released during combustion. The surface area in contact with those
hot gasses increases inversely as the cube of the vessel dimensions. Any
vessel acting as a "heat sink" will have a more profound effect, the
smaller the test volume.
There are other things: For instance, in a larger quantity, much of the
powder BEGINS to be involved in combustion at much higher ambient
pressure and temperature, thus accelerating its burn rate significantly.
(Though, it is true that BP doesn't have as steep a pressure-temperature-
burnrate gradient as do some other pyrotechnic compositions).
Rachel, there's nothing 'inherent' in black powder itself that makes a
smaller volume less energetic than a larger volume. Volume-per-volume, a
given BP should do the same amount of work... But how that work gets USED
changes depending upon the container, the ignition method, the measuring
methods, etc., etc., etc..
LLoyd