Effects of Differential Diffusion on the Stabilization of Unsteady Lean Premixed Flames Behind a Bluff-Body
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Effects of Differential Diffusion on the Stabilization of Unsteady Lean Premixed Flames Behind a Bluff‑Body Yu Jeong Kim1 · Bok Jik Lee2 · Hong G. Im1 Received: 13 December 2019 / Accepted: 7 September 2020 © Springer Nature B.V. 2020
Abstract Two-dimensional direct numerical simulations were conducted to investigate the effects of differential diffusion on flame stabilization and blow-off dynamics of lean premixed hydrogen–air and syngas–air flames stabilized on a meso-scale bluff-body in a square channel. The unity Lewis number for all species was imposed to isolate the effects of differential diffusion. Four sets of simulation cases were conducted. Two different inflow temperature with unity Lewis number were applied to examine distinct levels of hydrodynamic instability. Each unity Lewis number case was compared with the non-unity Lewis number case to investigate how differential diffusion affects the overall flame responses, instabilities, and blow-off mechanism. For all cases, the overall flame dynamics were observed in several distinct modes as the inflow velocity approaches blow-off limit. One of the primary effects of unity Lewis number was an increased level of hydrodynamic instability due to the lower flame temperature and thus a lower density ratio. The lower gas temperature also led to a weakening of the re-ignition of the quenched local mixture by the product gas entrainment. The combined effects were manifested as suppression of the re-ignition events, leading to a revised conclusion that the ultimate blow-off behavior at high velocity conditions are mainly controlled by the onset of local extinction. Keywords Direct numerical simulation · Bluff-body · Lean premixed flames · Unity Lewis number · Stabilization mechanism · Blow-off
1 Introduction Bluff-bodies of various types have commonly been employed in premixed combustion systems in order to achieve flame stabilization over a wide range of operating conditions. The main idea is to create recirculation zones behind the solid object to increase the flow residence time and heat recuperation by the residual hot products. At highly turbulent * Hong G. Im [email protected] 1
Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal 23955‑6900, Kingdom of Saudi Arabia
2
Institute of Advanced Aerospace Technology, Seoul National University, Seoul 08826, Republic of Korea
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Vol.:(0123456789)
Flow, Turbulence and Combustion
conditions, however, the bluff-bodies also involve large fluctuations in flames and flow field such as unstable vortex shedding, which may lead to a total blow-off. Understanding the physical mechanism of flame stabilization and blow-off is important in order to ensure steady operation of the combustion devices. Due to the complexities of dynamics and many physical and chemical parameters involved, it is difficult to obtain a comprehensive understanding of the phenomena. Shanbhogue et al. (2009) reported a general review about lean blow-off process, scaling of blow-off, and dynamics
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