As documented in numerous publications,1-8 hypersonic flight will require substantial cooling of engine and other vehicle components by the fuel. Some cooling can be achieved based just on the heat capacity of the fuel, however, for hypersonic flight this sensible cooling is inadequate, and it will be important to be able to absorb substantial additional energy by driving endothermic reactions in the fuel. Reactions such as dehydrogenation and cracking not only absorb substantial energy (hundreds of kJ/mole per bond broken), but also convert the fuel to a molecular mixture that should burn more efficiently, with greater energy release. An important limitation is the tendency of such reactions to form coke precursors such as alkynes and aromatics (including PAHs), leading to carbon deposition and eventual fuel system clogging. Use of catalysts improves performance by driving desired reactions selectively, and by lowering the temperature where endothermic cooling becomes significant, thereby increasing cooling capacity, and reducing undesirable thermal cracking (pyrolysis) which tends to generate coke precursors.
The focus of this basic research initiative project is to develop improved catalysts for endothermic fuel applications. Our approach begins with the premise that the endothermic fuel application is subject to very different cost and performance constraints compared to such industrial applications of cracking and dehydrogenation, thus catalyst optimization needs to be approached with a fresh perspective. We are focusing on understanding the relationships between catalyst electronic and geometric structure, fuel structure, and activity and selectivity for endothermic reactions under conditions relevant to cooling of air vehicles. We use this fundamental understanding to address three problems in endothermic cooling: Catalyst Discovery and Optimization, Fuel Structure Effects, and Minimizing Coke Deposition.
The work will involve five distinct types of experiments and three complementary theoretical activities, closely coupled to provide maximum insight. The focus is on catalysts immobilized on the fuel channel walls, rather than dissolved in the fuel, because this approach maximizes heat transfer, makes it practical to use expensive materials, and avoids deposition of suspended catalysts on the walls as the fuels transition to supercritical conditions.9,10
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