Impact of Heat Soak After Engine
Shutdown on Fuel
Nozzle Deposits, 03-9234
Principal Investigator
James E. Johnson
Inclusive Dates: 01/01/01 - 07/01/02
Background - The preponderance of fuel-deposition research has been conducted under steady-state operating conditions under which fuel is flowing for long periods of time through various heated components such as long tubes and fuel nozzles. However, 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 the formation of deposits. 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 - The goal of this study is to understand deposition mechanisms better and to establish procedures for determining the propensity of fuels to form deposits under heat soak conditions. Fuels with known characteristics are subjected to heat stress conditions representative of engine heat soak environments. Mass conversion fractions (the fraction of fuel sample converted to a deposit) obtained experimentally are correlated with factors that include fuel composition, physical properties, thermal stability (breakpoint temperature as determined by JFTOT results), and the surface conditions where deposits are formed. Deposition experiments are also conducted with fuels containing "red dye" contaminants.
Accomplishments - The study of deposit formation on flat surfaces was initially accomplished by a unique employment of thermogravimetric analysis (TGA) equipment where pooled fuels were heated at rates from 5 to 100 °C per minute. Shortcomings with this experimental approach led to the development of a new test method that uses small metallic bars with a cup-like depression for pooling the fuel. These specimens, which can be machined from a variety of materials with differing surface finish, are known as micro-oxidation coupons. Deposition results with the coupons compared reasonably well with the results of other experimenters. It was found that surface roughness, surface temperature, and soak time play important roles in the conversion process. Also, coupon tests extend the range of test conditions of the JFTOT. The new method offers a potentially inexpensive and rapid test method that can be used for early stages in screening the effects of fuel additives or contaminates.
