Oscillatory cool flame combustion behavior of submillimeter sized n-alkane droplet under near limit conditions |
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| Institution: | 1. Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA;2. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA;1. CERFACS, 42 Avenue Gaspard Coriolis, Toulouse Cedex 1 31057, France;2. Safran Helicopter Engines, Bordes 64511, France;1. Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan;2. Faculty of Engineering, Kyushu University, Japan;3. Japan Aerospace Exploration Agency, Japan;4. IHI Inspection & Instrumentation Co., Ltd., Japan;1. SKLTCS, Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China;2. School of Power Engineering, Chongqing University, Chongqing 400044, China;1. College of Industrial Technology, Nihon University 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan;2. Graduate School of Industrial Technology, Nihon University 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan;3. Graduate School of Engineering, Hokkaido University Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan;4. Central Research Institute of Electric Power Industry 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan |
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| Abstract: | This paper reports simulation results of oscillatory cool flame burning of an isolated, submillimeter sized n-heptane (n-C7H16) droplet in a selectively ozone (O3) seeded nitrogen-oxygen (N2-O2) environments at atmospheric pressure. An evolutionary one-dimensional droplet combustion code encompassing relevant physics and detailed chemistry was employed to explore the roles of low-temperature chemistry, O3 seeding, and dynamic flame structure on burning behaviors. For XO2= 21% and a range of selective ozone seeding, near-quasi-steady cool flame burning is achieved directly (without requiring hot flame initiation and radiative extinction). Under low oxygen index conditions, but with significant O3 seeding (XO3 = 5%), a nearly quasi-steady cool flame is initially established that then transitions to a dynamically oscillating cool flame burning mode which continues until the droplet is completely consumed. It is found that the oscillation occurs as result of a initial depletion of fuel vapor-oxidizer layer evolving near the droplet surface and its dynamic re-establishment through liquid vaporization and vapor/oxidizer transport. A kinetic analysis indicates that the dynamic competition between the reaction classes- (a) degenerate chain branching and (b) chain termination/propagation - along with continuous fuel and oxygen leakage through the flame location contributes to an oscillatory burning phenomena of ever-increasing amplitude. Analysis based on single full-cycle of oscillatory burning shows that the reaction progression matrices (evolution of heat and species) for QOOH?chain propagation/termination reactions (here, Q = C7H14-) directly scales with the gas phase temperature field. On the contrary, the QOOH?degenerate branching reactions undergoes three distinct stages within the same oscillatory cycle. The coupled flame dynamics and kinetics suggest that in the oscillatory burning mode, kinetic processes dynamically cross through conditions characterizing the negative temperature coefficient (NTC) turnover temperature, separating low temperature and NTC kinetic regimes. In addition, a parametric study is conducted to determine the role of O3 seeding level on the observed oscillation phenomena. |
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