SUPERCRITICAL FUEL DEPOSITION MECHANISMS

Jet fuel is the primary coolant used in high-speed aircraft. A decrease in surface deposition (fouling) is often seen as jet fuels and pure hydrocarbons are heated above approximately 370 degrees C, as measured by the wetted wall or film temperature. This temperature is near the critical temperature of most jet fuels. Two explanations have been offered for this decrease in deposition under supercritical conditions. One explanation is that the decrease reflects the temperature where hydroperoxide precursors to solids formation are depleted by thermal decomposition, interrupting the radical chain reactions forming solids. Another explanation is that the solvent properties of the fuel become enhanced under supercritical conditions, with the surface deposition reduced essentially by keeping the solids in solution. In single-tube heat exchanger tests with pure hydrocarbons and jet fuels of widely varying critical temperature, it was found that the deposition decrease is insensitive to fuel critical temperature but very sensitive to residence time/heating rate, indicating that the deposition decrease is a fuel chemistry effect rather than an effect of the supercritical nature of the fuel.