-2 0 2 4 6 8 10 12
0 50 100 150 200
time (nanoseconds)
spark current (amps)
time (nanoseconds)
spark charge (nC)
0 50 100 150 200
0 50 100 150 200
(a) (b)
Figure 2.11: (a) Current waveform from an example spark and (b) spark charge versus time obtained by numerically integrating the current waveform
VI, and the oscilloscope(s) were all set to wait for a trigger. In the short-spark ignition tests, the “fireset” key was turned to supply power to the high-voltage power supply and the “fire” button on the control panel was pressed to trigger the function generator. The function generator output initiated the high-voltage ramp, and when the breakdown voltage was reached a spark was induced between the electrodes. The spark current triggered the faster oscilloscope and a falling TTL signal from the first oscilloscope triggered the second oscilloscope recording the breakdown voltage and the delay generator. The delay generator then opened the high-voltage relay to disconnect the power supply from the capacitor to prevent multiple sparks. Signals from the delay generator also triggered the LabVIEW VI and Phantom camera.
In the variable-length spark ignition tests, the “fireset” was again used to turn on the high-voltage power supply and the “arm” switch was held up for approximately three seconds to close the high-voltage relay and charge the capacitor. After the
“arm” switch was released, the motorized stage was activated using a computer to step in the grounded electrode. When the breakdown distance was reached and the spark induced, the spark current once again triggered the fast oscilloscope. A falling TTL signal from the oscilloscope then triggered the LabVIEW VI and the delay generator, which in turn triggered the camera and the linear stage motor to turn off. In all tests, following the spark and triggering sequence, the schlieren video was saved and the current waveform was imported to a computer using a GPIB program.
The post-shot pressure, peak pressure, and test result (ignition or no ignition) was recorded on the checklist and the vessel was evacuated.
Chapter 3
Results & Analysis: Short,
Fixed-Length Spark Ignition Tests
1
3.1 Flammable Test Mixtures
In assessing the ignition threat to fuel tank vapor spaces due to lightning strikes on air- craft, the industry refers to the SAE Aerospace Recommended Practice 5416 Aircraft Lightning Test Methods (International, 2005), and the European equivalent ED 105 Lightning Testing Document. The recommended method for testing ignition sources is to use a flammable test mixture consisting of 5% hydrogen, 12% oxygen, and 83%
argon by volume. This mixture has been selected to meet the requirement that the flammable mixture has a 90% or greater probability of ignition with a 200 µJ voltage spark source. The foundation of this work is published in the DOT/FAA/CT94/74 Aircraft Fuel System Lightning Protection Design and Qualification Test Procedures Development (Administration,1994). The mixture recommended by the SAE in ARP 5416 is deliberately close to the lean flammability limit. Using mixtures so close to the lean flammability limit to determine incendivity creates a serious problem due to the difficulty of defining ignition limits in these situations. Britton (2002) has discussed this issue in regards to standardized testing for determining flammability limits and the disparity between the results of various test methods. He pointed out
1Significant portions of this chapter were also presented in Kwon et al.(2007) andBane et al.
(2009).
the difficulty of defining a combustion event, even when the pressure rise is measured, for near-limit cases for which only a narrow cone of the reactants is burned, producing a very small pressure rise. This same issue has been identified in this work while using mixtures with less than 6% hydrogen, which also have the added complication of the unusual behavior of flames in lean hydrogen mixtures. These issues are described in more detail in Section 3.2.
Flames in near-limit hydrogen-oxygen-diluent mixtures are a special case (Ron- ney, 1990). The high mass diffusivity of hydrogen molecules in the reactant mixture enables combustion to take place for extremely lean mixtures with very low flame temperatures as compared to hydrocarbon fuels near the flammability limit (Coward and Jones, 1952). The low temperature results in very low flame speeds, and the flames are sensitive to fluid motion (e.g., turbulence), flame stretching due to mo- tion associated with the buoyant rise of the hot combustion products (Lamoureux et al., 2003, Kumar, 1985), and radiation losses (Kusharin et al., 1996). As a con- sequence, the extent of combustion and resulting pressure rise are very sensitive to the experimental setup, as discussed in (Cashdollar et al., 2000). This behavior has been known since the earliest studies on hydrogen flammability (Coward and Jones, 1952) and leads to the substantial difference between the lower flammability limits for
“upward” flame propagation (4% H2) and “downward” flame propagation (8% H2) in hydrogen-air mixtures (Coward and Jones, 1952). This issue has been extensively studied in the context of nuclear safety and the potential for hydrogen explosions following loss-of-coolant accidents.
Motivated by the testing standards, the first flammable mixture that was consid- ered in this work was the the ARP-recommended mixture of 5% H2, 12% O2, 83% Ar.
To investigate the effect of small changes in the composition on ignition for very lean mixtures, two additional mixtures where the hydrogen concentration was increased by just 1% were also considered. Therefore, in addition to the 5% hydrogen mixture recommended by the SAE, tests were performed in a 6% H2, 12% O2, 82% Ar mixture and in a 7% H2, 12% O2, 81% Ar mixture.