Biomedical Engineering Reference
In-Depth Information
using air as a propellant gas without the use of a CD nozzle, and observed inlet
pressures from the regulator not exceeding 1000 psi. This exceeds the
maximum velocity predicted by the analytical model by over 500 ft/s, a
substantial margin. It is thought that the analytical model fails to account for
local temperature increases in the barrel as the barrel pressure rapidly
increases. An increased local temperature would increase the speed of sound,
thereby also increasing the maximum velocity predicted by this model. This
effect may be negligible at the relatively low pressures encountered in the
vacuum cannon but may be a largely contributing factor in the supersonic
pneumatic gun, which creates causes extremely high pressure differentials in
comparison. Burst disc material consisted of purchased steel shim stock, .001‖
thickness, and household heavy duty aluminum foil, .0014‖ thickness. The
aluminum material was also tested by applying several layers, and the pressure
data is only approximate. The process for collecting velocity data was identical
to that described for the subsonic pneumatic gun. Table 7 shows results of the
various combinations of burst discs tested. It can be seen that increasing the
number of aluminum burst discs has diminishing returns in terms of projectile
velocity. A single steel shim disc produces substantially higher velocities,
approximately 240 ft/s more than five aluminum burst discs stacked together,
albeit at higher pressures.
Speed vs. Pressure
2800
2300
Analytical - Air
Analytical - Helium
1800
1300
800
300
0
200
400
600
800
1000
Inlet Pressure (psi)
Reproduced with permission. Copyright retained by Inderscience Publishers.
Figure 55. Using Analytical Model to Compare Different Propellant Gases.
With this test data for burst disc combinations, the single steel burst disc
was selected for final testing, based on the highest available speed without
surpassing 1000 psi, which is considered the safe pressure limit for the device.
