The production of detrimental carbonaceous deposits in jet aircraft fuel systems results from the involvement of trace heteroatomic species in the autoxidation chain that occurs upon fuel heating. Although it has been known for many years that these sulfur-, nitrogen-, and oxygen-containing species contribute to the tendency of a fuel to form deposits, simple correlations have been unable to predict the oxidation rates or the deposit forming tendencies over a range of fuel samples. In the present work, a chemical kinetic mechanism developed previously is refined to include the roles of key fuel species classes, such as phenols, reactive sulfur species, dissolved metals, and hydroperoxides. The concentrations of these fuel species classes in the unreacted fuel samples are measured experimentally and used as an input to the mechanism. The resulting model is used to simulate autoxidation behavior observed over a range of fuel samples. The model includes simulation of the consumption of dissolved oxygen, as well as the formation and consumption of hydroperoxide species during thermal exposure. In addition, the chemical kinetic mechanism is employed with a global deposition submechanism in computational fluid dynamics (CFD) simulations of deposit formation occurring in nearisothermal as well as non-isothermal flowing environments. Experimental measurements of oxygen consumption, hydroperoxide formation, and deposition are performed for a set of seven fuels. Comparison with experimental measurements indicates that the methodology offers the ability to predict both oxidation and deposition rates in complex flow environments, such as aircraft fuel systems, using only measured chemical species class concentrations for the fuel of interest.
This paper will outline a steady flow control technique that augments the diffusion process within a stator passage via a continuous co-flowing secondary flow stream along the suction surface. The technique is similar to that used for flow vectoring in nozzles where a secondary flow stream is used to enhance the diffusion and vectoring of high speed jets. Diffusion factors in excess of 0.95 are simulated and the “penalty” for the secondary system is addressed with an availability and simple power analysis. Losses within the secondary flow stream were included in the availability analysis, but it did not account for losses within a delivery system of this secondary flow. This was accomplished through the ID power analysis which assessed this technique’s impact on the efficiency of an axial compression stage and the sensitivity of this efficiency to the secondary flow system’s efficiency. Also, a system level analysis is presented to assess the merits that may be realized in a notional engine with this type of flow control. Particularly, impacts on specific fuel consumption and thrust-to-weight ratio were addressed. A cascade experiment was performed to demonstrate the concept and was conducted in a blow-down cascade tunnel. Significant improvements in diffusion were qualitatively seen from the DPIV measurements despite limitations in achieving the desired secondary flow conditions.
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