| In the modern design of a high speed supersonic aircraft (>March 5), jet fuel itself is used as the primary coolant on board and the fuel temperature can reach as high as 540°C. Under such thermal stressing conditions, the jet fuel will degradate and a small portion of the fuel will convert to carbonaceous solid deposit on the metallic surfaces of the fuel system, resulting in potential malfunction of the fuel system. An even greater concern is the effects of these metal surfaces, for an active surface could catalyze the process of the formation of the deposit. However, there have been no systematic studies in the literature concerning the effects of metal surfaces on the formation of carbonaceous, deposit from jet fuels. It is the surface effects on deposit formation that constitute the principal objective of the current investigation.;The approach adopted in this work was first to form the carbonaceous deposit under "controlled" conditions, and then characterize the carbonaceous deposit by microscopic techniques such as scanning electron microscopy, transmission electron microscopy, polarized light optical microscopy, and spectroscopic techniques such as Fourie transform infrared spectroscopy, Raman Scattering spectroscopy, and other techniques, and finally to use the information generated from the deposit characterization to elucidate the mechanisms of the deposit formation and the surface effects on deposit formation.;In this work, different metal surfaces such as nickel, titanium, copper, stainless steel, and gold in the form of TEM grids were subject to initial screening experiments using n-dodecane and Jet A fuel in a tubing bomb batch reactor. Three types of growth modes were found on these surfaces, resulting from a strong surface interaction represented by nickel, an intermediate surface interaction by copper, and a weak surface interaction represented by stainless steel surfaces. These nickel, copper, and stainless steel surfaces were selected for more detailed study.;On the deposit formation could be from dodecane, Norpar 13 (a mixture of C12--C14 n-alkanes), and JP-8 fuel in a batch or a flow reactor. Through characterization of the deposit and the interface of metal/deposit, it is found that metal surfaces range from catalytic to inactive on deposit formation under the conditions studied. Nickel surface catalyzes carbon deposition through the formation of bulk-diffused filamentous carbon. In contrast, copper surface catalyzes carbon deposition through the formation of surface-diffused fibrous carbon. These surface effects are also found to be related to the composition of the fuels and reaction conditions. The relevance of the results from lab tests to those from actual engine tests will also be briefly discussed. |
