SUPERCRITICAL FUEL DEPOSITION MECHANISMS

Jet fuel is the primary coolant used in high-speed aircraft. A decreas e 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 off ered for this decrease in deposition under supercritical conditions. O ne explanation is that the decrease reflects the temperature where hyd roperoxide precursors to solids formation are depleted by thermal deco mposition, interrupting the radical chain reactions forming solids. An other explanation is that the solvent properties of the fuel become en hanced under supercritical conditions, with the surface deposition red uced essentially by keeping the solids in solution. In single-tube hea t exchanger tests with pure hydrocarbons and jet fuels of widely varyi ng critical temperature, it was found that the deposition decrease is insensitive to fuel critical temperature but very sensitive to residen ce time/heating rate, indicating that the deposition decrease is a fue l chemistry effect rather than an effect of the supercritical nature o f the fuel.