The dynamic motion of liquid in a container experiencing external forces is called slosh, and this poses a risk to dynamic, non-stationary fuel containers because of the violent mixing and droplet separation that is produced, leading to enhanced flammability effects. Fuel slosh can be characterized as one of two conditions: Subcritical or Supercritical.
ESI was commissioned to evaluate the circumstances necessary to produce these two conditions and how they may lead to flammability. All testing was conducted at the USAF 46th Aerospace Safety & Survivability Test Group’s Simulated Aircraft Fuel Tank Environment (SAFTE) Facility at Wright Patterson AFB.
The conditions that produce wave/wave interactions and hydraulic jumps represent the difference between subcritical and supercritical conditions.
- Subcritical conditions occur when waves travel in opposite directions
- Supercritical conditions occur when waves travel in a single direction
The hydraulic jump studies clearly demonstrated that there is a transitional region between subcritical to supercritical conditions that was related to the type of wave propagation and liquid/air interactions that occur, and this volatile mixing is believed to enhance fuel tank flammability.
Furthermore, a correlation relating the oscillation condition to the size and formation rate of hydraulic jumps in the fuel tanks was developed.
ESI was commissioned to determine the LOC for aviation kerosene fuel under static conditions, and then to determine the effect on the LOC by implementing the above different dynamic conditions.
It was found that for wave/wave interaction the locations at which wave interactions were observed and the height of the constructive interference of the two waves could be determined for varying amplitudes and frequencies of roll.
From previous studies it is known that there is a Limiting Oxygen Concentration (LOC) within fuel containers for air platforms and is typically of the order of 12% oxygen by volume for commercial applications, which has been well documented by the FAA and other organizations, primarily under stationary tank conditions.
Peak reaction pressure measured for various oxygen concentrations at three different fuel temperatures provided a baseline for dynamic ignition comparison, and demonstrated that the LOC is temperature dependent, with the 60 °C fuel temperature matching the FAA data suggesting a 12% limit under stationary conditions.
However, under supercritical sloshing conditions with a fuel temperature of 55 °C Ignition was observed below the 12% LOC limit at 11% Oxygen Concentration.
ESI recommended to the military that the current 9% LOC limit is more suitable and a move to the 12% commercial requirement is not advisable.