UCLA BPPL - Gasdynamics in fireballs

It has been observed that a positively biased electrode which creates a fireball begins to move when it is free to do so. The electrode recoils from the fireball. The deflection does not remain in steady-state. It is not created by electric or magnetic forces. A second object, conductor or insulator, is also pushed away from a pulsed fireball. The effect has been traced to a flow of neutral gas away from the fireball.

Since the force is transient its effect can be amplified by suspending the deflected object as a gravitational pendulum and driving it resonantly with fireball pulses. This results in a large-amplitude excursion which has been captured in a video clip shown below.

fireball pendulum

Fig. 1. A one-sided electrode is suspended by a thin copper wire. The electrode is biased positively with pulses at the pendulum's resonance frequency. The expanding gas of the fireball pushes the electrode. Click image to open the movie of the fireball pendulum.

The properties of the gas flow have been studied by using a simple pendulum for diagnostic purpose. As the schematic setup of Fig. 2 shows it consists of a thin mica sheet, whose deflection is amplified by reflecting a laser beam from its surface which strikes two photodiodes ata distance R. The velocity of the mica sheet can be determined from the time lag between the diode signals and the geometry of the arrangement.


Schematic

Fig. 2. Schematic of the experiment and diagnostics for measuring the gas flow emerging from a pulsed fireball.

Figure 3 shows the electrical signals of the two photodiodes as the laser beam sweeps over them. First, the sequence of the pulses indicates that the mica sensor is pushed away from the fireball. Second, from the time delay between the diode signals the velocity is obtained. In Fig. 3b the velocity is displayed vs time, showing that the mica sensor is accelerated as the fireball is turned on. The mica mass and acceleration yields the force or gas pressure exerted on it.


Schematic

Fig. 3. Acceleration of the mica pendulum yields the force and gas pressure exerted on it.

Figure 4 shows that the gas expansion from a fireball is nearly isotropic and radially outward. The deflection of the mica sheet is reversed when the fireball is located on opposite sides of the sensor at the same radial distance.


Schematic

Fig. 4.Isotropic gas flow.

The gas expansion can be partially collimated by varying the shape of the fireball. As shown in Fig. 5 the fireball is formed in a glass cylinder with a recessed electrode. The cylindrical fireball pushes the gas axially outward. By rotating the fireball relative to the mica sensor the angular dependence of the gas velocity is obtained (Fig. 5a). It forms a jet of hot gas. When an object (a thin wire) is inserted into the gas jet it begins to glow indicating gas temperatures of at least 500 dec C. Such effects can lead to useful applications.


Schematic

Fig. 5. Directional gas flow with a fireball gun.

References