| ||STATEMENT OF INTENT: Establish a design space for compact augmentors
|| Objective: ||Establish a design space for compact augmentors that have supercritical fuel injection and/or do not use flameholding devices due to the rapid reaction rates at advanced augmentor conditions.
|| Description: ||Conventional augmentor systems rely on cooled flameholding devices to stabilize the combustion process. These devices are typically bluff bodies that are positioned in the hot flowpath. Due to high inlet temperature, the fuel injection sites are located on the flameholder, very close to the recirculation zone of the flameholder. This provides a means to inject the fuel into the flowpath before it can autoignite and cause damage to the flameholder. Due to close coupling of the fuel and flameholder, static and dynamic stability of the system are a real challenge. Conventional flameholding systems carry with them a debit to the engine cycle. Aerodynamic flameholders in the wake of the engine require cooling air, which results in net losses to the engine performance. Further aerodynamic flameholders produce a pressure drop in the cycle, which also robs the cycle of performance.
In a turbojet engine operating at supersonic speeds, the inlet conditions of the augmentor are so severe that large amounts of cooling would be required to cool aerodynamic flameholders. Further, augmentors operating at high Mach (ram) conditions at supersonic cruise conditions have short reaction times, making flameholding devices possibly unfeasible, because the high temperatures in this system nearly ensure autoignition of the fuel regardless of proximity of the fuel injection to the flameholder. These systems will also operate at such high temperatures and pressures that the fuel may be supercritical as it is injected into the augmentor.
Due to the limitations of aerodynamic flameholders in high-cruise-Mach-number systems and the state of the fuel as it enters the augmentor, novel flameholding concepts are required. Since the engine conditions are near the autoignition limit, and the fuel is typically supercritical, flameholding concepts that take advantage of these phenomena may prove to be useful. Operating at these conditions, however, means that the flame must rely on turbulent mixing to assure that the correct fuel/air mixture is within the flammability limits. Due to the flame fluctuations from these processes, acoustic instabilities can arise from these phenomena. As a result, combustion instabilities (SCREECH) in autoignition-mode augmentors may pose a serious design challenge. Future systems that operate at high Mach conditions are well outside the design space currently known by designers. Therefore, understanding the physical processes and generating the design space for these augmentor systems are needed.
|| ||PHASE I: Determine the feasibility of eliminating conventional augmentor flameholding devices and relying on autoignition for flame stabilization. Identify advanced concepts. Develop detailed plan to demonstrate advanced augmentor concept and map out design space.
|| || ||PHASE II: Conduct experiments in relevant environments that demonstrate key technical parameters. Explore the sensitivity of parameters such as supercritical fuel injection, autoignition, mixing, and augmentor geometry on the augmentor performance. Design, fabricate, and test novel flameholding concepts and compare results to baseline flameholding designs.
|| ||DUAL USE COMMERCIALIZATION: Military application: Models and design concepts generated in the Phase II effort can be validated for engine conditions and transitioned to military gas turbine OEMs for incorporation into augmentor design systems. Commercial application: Potential commercial applications include supersonic business jet and flameholding concepts for interturbine burner designs used in ground-based power and commercial propulsion systems.
|| References: ||1. Culick, FEC, “Combustion Instabilities in Liquid-Fueled Propulsion Systems – An Overview,” AGARD CP-450 pp. 1-73, 1989.
2. Glassman, I., Combustion, Academic Press, pp. 296-308, 1987.
3. Doungthip, T., Ervin, J.S., Williams, T.F., and Bento, J., "Studies of Injection of Jet Fuel at Supercritical Conditions," <i>Ind. Eng. Chern. Res.</i>, Vol. 41, pp.5856-5866, 2002.
4. Edwards, T. and Zabarnick, S., "Supercritical Fuel Deposition Mechanisms," <i>Ind. Eng. Chern. Res.</i>, Vol. 32, pp. 3117-3122, 1993.
5. Lai, M., Oljaca, M., Lubarsky, E., Shcherbik, D., Bibik, A., and Menon, S., Controllable Injection for Supercritical Combustion, AIAA 2004-3383, 40th AIAAlASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, FL. July 12-14,2004.
|Keywords: ||augmentor, combustion, static stability, dynamic stability, stability modeling, flameholding, supercritical fuel injection, fuel injection|