Determination of Impact of Heat Soak After Engine Shutdown on
Inclusive Dates: 01/01/01 - Current
Background - The preponderance of fuel-deposition research has been conducted under steady-state operating conditions in which fuel is flowing for long periods of time through various heated components such as long tubes and fuel nozzles. During rapid engine shutdown, as commonly occurs during operation of Navy aircraft and Army tanks, fuel is trapped or pooled within the nozzles and feed lines or it may dribble down the face of a nozzle. This fuel is subsequently exposed to heating loads, known as heat soak, which originates from the retained heat in the engine hot sections. Also, these fuel collections are exposed to a surrounding atmosphere with significant oxygen content that further promotes deposit formation. The development of deposition models for heat-soak conditions has received little attention, in part, because the severity of the problem has only recently been established by SwRI during nozzle-fouling studies associated with engine shutdown and startup cycles. The lack of a heat-soak deposition model greatly diminishes the prospects for developing a comprehensive thermal stability test method that is applicable to the full range of operating conditions of a turbojet engine. Furthermore, there is no recognized test for deposit propensity under pooling conditions, and it is not known if the industry standard Jet Fuel Thermal Oxidation Tester (JFTOT) can appropriately address this issue.
Approach - Mass conversion fractions (the fraction of fuel sample converted to a deposit) obtained experimentally are compared to several candidate kinetic reaction rate models to identify mass conversion mechanisms over temperature ranges typical of heat-soak conditions. Correlating factors include fuel composition, physical properties, and thermal stability (breakpoint temperature as determined by JFTOT results). Also, fuels with contaminants like "red dye" will be included in the study. The goal of this study is to better understand the deposition mechanisms and to establish whether the JFTOT is adequate for determining the propensity of fuels to form deposits under heat-soak conditions or if a new test method must be developed.
Accomplishments - Representative jet fuels have been subjected to heat-stress conditions representative of heat soak. Deposit formation on flat surfaces has been studied by a unique employment of thermogravimetric analysis (TGA) equipment with heating rates from 5 to 100 °C per minute. Deposition as a function of surface temperature has shown unexpected results for some fuels. An expected pronounced deposition peak was not apparent for some fuels. One possible explanation is related to the short reaction time available for the temperature profiles employed. Deposit formation for a diesel fuel, a fuel with larger fractions of high molecular weight hydrocarbon, exhibited a pronounced deposition peak. Also, this work verifies that deposition under heat-soak conditions as reported by other SwRI researchers does occur.