      
Water Arc Explosion Experiment:
D. M. Marett (2007)
Performing experiments can often involve dangers and hazards. The author assumes no responsibility or liability for injury or damage, to persons or property, resulting from use or misuse of the information provided on these pages. Introduction Interest
in water arc launchers was popularized by a series of papers by Peter
and Neal Graneau in the 1990's. As was explained in their 1998 paper in
the Journal of Plasma Physics (with G. Hathaway), the device appears to
work by an electrical discharge exploding a collection of fog droplets
in a water slug resting inside a barrel, launching the water at high
speed like a
projectile. The experiment described here uses a straightforward setup
to explore this phenomenon.
The water arc launcher was constructed as shown below. A spark gap was used so that the capacitor bank would charge up to the desired voltage, and then the electrical discharge on breakdown would follow a path through the water projectile. Fig. 1:
The capacitor is 50 x 35uF, 33VDC in series. The caps are photoflash capacitors. A sheet of Plexiglas was put in front of the device as a shield. The capacitors were charged up with a 0-20 KV high voltage power supply prior to firing. A small amount of water (1-2 mL) is pipetted into the barrel. The electrical circuit is completed from ground, through the screw, to the water slug, through the aluminum barrel, to the spark gap, that then connects to the H.V. side of the capacitor bank at the moment that an arc occurs in the gap.
Experimental Results: At 11.7kV, it took several arcs with a quiet snap before the water exploded with a loud bang (about the 5th time). It was not so loud as to be painful, though. It blew the bibulous paper off to a height of a couple of feet and put a hole through one sheet of it. It was soaked with water. Interestingly, the rubber stopper was not moved, nor did it appear damaged. It still continued to hold the water in. The stopper was press fit into the barrel, and was held in the breach by the pressure from the insulator, connected to the table below. Fig. 2: Picture of the Experimental Setup.
Fig.3: Aerial view of the Setup:
Fig. 4: Detail of the barrel.
Rough Approximation of the Work in the Water Arc Explosion: A ruler with a balance weight of 17g (one end balance on stand) was put on a pivot and placed with its end over the barrel. The height of the arc of the ruler was measured. First Successful explosion: approximately 20cm height @ 10.5kV Second successful explosion: arced completely over – approximately 50cm @ 11kV Joules of work = 0.017Kg (x 9.8m/s2) x 0.05m =
0.1666 ml:namespace prefix = st1 ns =
"urn:schemas-microsoft-com:office:smarttags" /> Energy in cap = ½ cv2 = 0.5 x 0.7uF x (11,000)2 = 42.35 J Efficiency = 0.2% Third successful explosion: ruler detached and flew to average height of 25cm on scale. Joules of work = 0.027Kg (x 9.8m/s2) x 0.05m =
0.2646 Energy in cap = ½ cv2 = 0.5 x 0.7uF x (11,000)2 = 42.35 J Efficiency = 0.156% A new spark gap (14kV) was fashioned on March 26th, 2007 using rounded electrodes. This gap has a little less of a tendency to corona prior to discharge than the wire-point gap. Capacitance is now 0.625uF, 18,500VDC Working voltage: 14kV Third Successful Explosion: ~ 1.5mL of distilled H20 This was about 0.33 meter toss of 27 gram weight = 0.087 J Input = 61 J Thus 0.143% Efficiency The last few explosions at 14kV always empty the chamber. Fourth successful explosion: Used 2mL of distilled water. This was a 0.65 meter toss of 28 grams (a bit wet) = 0.178 J Input 61 J = 0.292% Efficiency Strangely, the rubber stopper inside the barrel has actually moved upwards, to the extent that there is now a gap of about 3mm between the bottom screw end and the insulator below. Could this be due to bounce or is the recoil force actually in the wrong direction? In this run, a 105 g weight (an inverted aluminum ½ dome) was attempted. It lifted only 2 cm (0.02 m) This is only ~0.021 J of potential energy. On March 28th,
2007, I performed 3 more runs and videoed the effect using a webcam,
and also to tested to see if the movement of the rubber stopper up was
due to bounce or due to some unknown force. These are shown
in: Water
Arc Video 1. 2 and 3
On examining the stopper position without the penny blocking its downward movement, it could be seen in each case that the stopper had been pushed down by the blast, not up. On the final shot, the stopper was pushed almost all the way out of the barrel. Thus the upward movement was due to bounce. Conclusions: The average kinetic energy of the water projectile, based on its ability to lift objects, was around 0.1% to 0.3% of the input energy. This is in keeping with the efficiency levels reported in the Graneau's book "Newtonian Electrodynamics" - Electrodynamics of Arc Explosions P. 206. The efficiency was measured at 0.42% for a weight of 1.597Kg thrown to a height of 0.975 m using 8uF at 30Kv (3600J). So our results are consistent with the efficiencies reported by the Graneau brothers.
References:
1) George Hathaway, Peter Graneau, Neal Graneau, "Solar Energy Liberation from Water by Electric Arcs". Journal of Plasma Physics Vol. 60, Part 4, pp.775-786, 1998.
|
