Biomedical Engineering Reference
In-Depth Information
velocity, but at speeds significantly less than the speed of sound, which are
generally considered to be below the area of interest for study of particle
deposition on substrates. It is believed that the valve in this system did not
open fast enough for the requirements of this application. Experimental
velocity data from this device demonstrated asymptotic behavior well below
supersonic speeds. When compared to the analytical model, experimental
velocity values were much lower than those predicted analytically. The
analytical model does not account for many factors present in this pneumatic
device that were not present in the case of the vacuum cannon, including drag
from air resistance and throttling effects from the valve not opening fast
enough. The supersonic pneumatic gun was developed to operate at higher
pressures with the intent of reaching supersonic velocities through a larger
pressure differential. The diaphragm valve design was chosen due to its simple
design and extremely fast opening time, allowing for near instantaneous
pressure rise in the barrel. Unlike the subsonic device, velocity data recorded
from the supersonic gun was well above the maximum velocity predicted by
the first order analytical model. This is attributed to the rapid increase in
temperature in the barrel caused by the sharp pressure rise, and would allow
for speeds in the barrel to reach supersonic velocities relative to the open
atmosphere. However, no instrumentation was in place to confirm this idea.
The analytical model neglects temperature variations, and for its initial
purpose of predicting speeds in the vacuum cannon with its relatively low-
pressure gradient, this assumption is more realistic. Unfortunately, a very
limited number of substrate specimens were examined after impact. All four
samples recorded were placed at a distance of 3.0 inches from the exit of the
barrel, therefore this standoff distance was not included as a variable.
5.
C
ONCLUSIONS
The first order analytical model accurately predicted velocity inside the
barrel of the vacuum cannon. Sensors used in the collection of this
experimental data proved reliable and capable of being used in the smaller
pneumatic gun with faster projectiles. The supersonic pneumatic gun proved
capable of accelerating projectiles up to 1488 ft/s, thought to be adequate for
the investigation of particle deposition through high-speed impacts. The
impacted substrates were examined under the microscope - sample 1 shows a
small percentage of particles bonding with the substrate; the other samples
only show signs of erosion of the substrate. Hardness testing and XRF analysis
