Effects of cryogenic temperature on premixed hydrogen/air flame propagation in a closed channel |
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| Institution: | 1. State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China;2. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, United States;3. Center for Combustion Energy, Tsinghua University, Beijing 100084, China;1. Lehrstuhl für Thermodynamik, Technische Universität München, Garching, Germany;2. Institute for Advanced Study, Technische Universität München, Garching, Germany;3. Institute of Energy Systems and Fluid-Engineering, Zürich Univerity of Applied Sciences, Winterthur, Switzerland;1. State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China;2. School of Astronautics, Beihang University, Beijing 100191, China |
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| Abstract: | As a carbon-free fuel, hydrogen has received significant attention recently since it can help enable low-carbon-economy. Hydrogen has very broad flammability range and very low minimum ignition energy, and thereby there are severe safety concerns for hydrogen transportation and utilization. Cryo-compressed hydrogen is popularly used in practice. Therefore, it is necessary to investigate the combustion properties of hydrogen at extremely low or cryogenic temperatures. This study aims to assess and interpret the effects of cryogenic temperature on premixed hydrogen/air flame propagation and acceleration in a thin closed channel. Different initial temperatures ranging from normal temperature (T0 = 300 K) to cryogenic temperature (T0 = 100 K) are considered. Both one- and two-dimensional hydrogen/air flames are investigated through transient simulations considering detailed chemistry and transport. It is found that when the initial temperature decreases from T0 = 300 K to T0 = 100 K, the expansion ratio and equilibrium pressure both increase substantially while the laminar flame speeds relative to unburned and burned gasses decrease moderately. The one-dimensional flame propagation is determined by laminar flame speed and thereby the combustion duration increases as the initial temperature decreases. However, the opposite trend is found to happen to two-dimensional flame propagation, which is mainly controlled by the flame surface area increase due to the no-slip side wall constraint and flame instability. Based on the change in flame surface area, three stages including the initial acceleration, steady burning and rapid acceleration are identified and investigated. It is demonstrated that the large expansion ratio and high pressure rise at cryogenic temperatures can significantly increase the flame surface area in early stage and promote both Darrieus-Landau instability (hydrodynamic instability) and Rayleigh-Taylor instability in later stage. These two instabilities can substantially increase the flame surface area and thereby accelerate flame propagation in hydrogen/air mixtures at cryogenic temperatures. The present study provides useful insights into the fundamental physics of hydrogen flames at extremely low temperatures, and is closely related to hydrogen safety. |
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