Outgassing Characteristics from Oxygen Dissolved in JP-8 During Depressurization

Abstract

Outgassing of oxygen from fuel is an important phenomenon affecting the ullage oxygen concentration of aircraft fuel tanks, which is a critical operating parameter associated with the risk of explosions due to hazards such as electric sparks or combat damage. Knowledge of the outgassing characteristics of specific fuels in their operational environment is important to fuel system designers. Although the factors affecting the evolution rate of dissolved gases from solution, such as pressure, temperature, and agitation, have been characterized to some extent, the effect of bubble generation on the bulk rate of outgassing to the ullage space has not been well described in the literature. The motivation for the present work was to study the effect of bubbles naturally induced by fuel pumps on the outgassing of JP-8 during depressurizations that are typical for climb and high-altitude cruise. An experimental fuel tank model was developed that included visualization windows, oxygen measurements in the fuel and ullage, and a vacuum system enabling controlled depressurization down to 0.1 atm. Significant differences between bubble-up and diffusion-driven outgassing are evident in the analysis of oxygen concentration trends with time under various test conditions. The ullage oxygen volume fraction is significantly increased during depressurization when agitation is accompanied by small bubbles, in comparison to quiet conditions. Ullage washing before depressurization affects the initial state but not the overall transient characteristics, regardless of whether agitation is applied.