Combustion and Flame 209 (2019) 353–356 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustfame Brief Communications A spark ignition scenario in a temporally evolving mixing layer Agnieszka Wawrzak, Artur Tyliszczak ∗ Institute of Thermal Machinery Czestochowa University of Technology, Al. Armii Krajowej 21, Czestochowa 42–201, Poland a r t i c l e i n f o Article history: Received 13 February 2019 Revised 30 July 2019 Accepted 31 July 2019 a b s t r a c t The paper presents the numerical studies on spark ignition in a turbulent mixing layer formed between a fuel stream (H 2 /N 2 ) and air flowing in the opposite directions. Compared to an ignition mechanism observed in stationary mixing layers or in premixed homogeneous mixtures studied previously by many authors, the present results show a significantly different flame formation process. At an early stage of the ignition process, the flame kernel, which is initially spherical, is strongly torn by shear stresses and vortical structures formed in a region of the mixing layer. Depending on the spark location with respect to a vortical structure three different ignition scenarios were identified: (i) the flame kernel is formed, but at the successive time instants, it is destroyed by vortices and eventually vanishes; (ii) the flame develops around the initial spark position; (iii) the flame develops being simultaneously transported along the mixing layer. In the former case the flame grows significantly faster and its volume is found to be even two times larger. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Compared to the knowledge on auto-ignition phenomena our understanding of spark ignition mechanisms is much less ad- vanced. Beside many years of study recent investigations on spark ignition still focus on its fundamental aspects and are mostly per- formed in simple flow configurations including jets [1–4], counter- flows [5] and mixing layers [6,7]. The main findings concerning the ignition phenomenon, discussion of its probabilistic nature and modelling aspects were presented in review papers by Mastorakos [8,9]. In general, it is known that the probability of finding flammable mixture (P f ) and the probability of successful flame kernel gener- ation (P ker ) are different and most often P ker < P f . Moreover, con- sidering the successful ignition as a four-step process: energy de- position → flame kernel formation → propagation → stabilization, it is known that its probability (P ign ) is lower than P f [8]. A study of P f , P ker and P ign , reported by Ahmed and Mastorakos [1] for a jet flow and Ahmed et al. [5] for a counter-flow configuration, clearly showed that even if the spark was initiated at favourable mix- ture conditions the flame could be quenched confirming the rule that P ign < P ker < P f . On the other hand, Ahmed et al. [5] observed that the successful ignitions can happen in theoretically impossible locations (P f = 0) thanks to a fast movement of hot products of the spark to the region with P f > 0. These findings show that the ∗ Corresponding author. E-mail addresses: atyl@imc.pcz.pl, arturtyliszczak@gmail.com (A. Tyliszczak). ignition process has definitely stochastic nature which is largely unpredictable and conditioned by flow regimes. On the one hand, placing the spark in the region where P f = 0 does not preclude generation of the flame development. On the other hand, the loca- tion of the spark at the point with P f > 0, where P ker > 0, does not guarantee the ignition. In a turbulent flow the flame kernel may be too weak to propagate against the vortices or may grow and propagate with the speed conditioned by a spark location. In this short paper we identify three significantly different ignition scenar- ios that can happen in turbulent flows and show their impact on the flame development. We consider a temporally evolving mixing layer dominated by shear stresses and strong vortical structures, which are typical phenomena in jet type fuel injectors and bluff body configurations. 2. Computational configuration The computational geometry is shown in Fig. 1. It is a rectangu- lar box which is periodic in the x and z-directions while its upper and lower sides at y = ±L y /2 are treated as the moving walls. The fuel (Y H 2 = 0.1, Y N 2 = 0.9, ξ ST = 0.225) flows in the upper area and air flows in the lower part. Their temperature is equal to 300 K and the initial velocity field is defined by a hyperbolic tangent func- tion as u(y) = U ∞ tanh(2y/δ ) where U ∞ is the free stream velocity and δ = 2U ∞ /|du/dy| max is the initial vorticity thickness assumed equal to δ = 0.5 mm. The shape factor for the assumed velocity profile is typical for a boundary layer in transitional regimes and is equal to H = 2.25. The Reynolds number is Re δ = U ∞ δ/ν air = 600, which corresponds to the jet flows at moderate speeds. In the https://doi.org/10.1016/j.combustflame.2019.07.045 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.