Abstract
Temporal evolution of an ignition kernel in a spark ignited turbulent hydrogen–air mixing layer is studied using the large eddy simulation approach with an implicit treatment of the reaction source terms. The applied numerical code is based on a high-order compact difference approximation combined with a weighted essentially non-oscillatory scheme, which provide an accurate resolution of the small scale phenomena. The spark is modelled by an energy deposition model coupled with the enthalpy equation. Since the fuel and oxidiser streams move in the opposite directions the flow is dominated by shear stresses and vortical structures. The ignition follows three different scenarios depending on the spark location. An interaction between the developing flame kernel and large turbulent structure is analysed starting from the energy transfer up to a fully reacting state. The size and shape of the flames are correlated with the initial ignition scenario. A 3D visualisation of the transient position of the flame kernel shows its different spatio-temporal behaviour. It is observed that the flame can stay close to the initial position and spread equally in all directions or it can move far from the initial location and follow the evolving flow field. In the latter scenario two separate sub-stages are convincingly identified. Concerning the ignition probability (P_{ign}), when the spark is located in the immediate vicinity of the mixing layer, the P_{ign} on the fuel-rich side is higher. On the other hand, when the ignition occurs far from the mixing layer, the P_{ign} is significantly larger on the lean side. Unlike the local composition the impact of the strain rate of the velocity field was found to have very limited impact on the P_{ign}.
Highlights
The complete understanding of the turbulent flame ignition is extremely important both from the fundamental as well as practical point of view
The paper presented LES studies of spark ignition process in the temporally evolving mixing layer formed between the hydrogen/nitrogen mixture and air
We focused on the flames following two different ignition scenarios (No (1) and No (2)) that depend on the localization of the spark with respect to the vortex structures rolling-up in the mixing layer
Summary
The complete understanding of the turbulent flame ignition is extremely important both from the fundamental as well as practical point of view. Turbulence and Combustion (2020) 105:807–835 and combustion initiation through an externally introduced energy source (e.g. an electric spark, laser, plasma). Further development of these devices is a challenging task and it requires more experimental and numerical works on the ignition process and flame kernel evolution. A considerable amount of research in this field has been done to date but no univocal conclusions have been found Most of these studies focus on spark ignition of jets (Birch et al 1981; Smith et al 1988; Ahmed and Mastorakos 2006; McCraw et al 2007; Lacaze et al 2009; Chen et al 2017; Zhang et al 2019), counter-flows (Ahmed et al 2007) and mixing layers (Chakraborty et al 2007; Chakraborty and Mastorakos 2008; Ahmed and Mastorakos 2016; EidiAttarZade et al 2017; Wawrzak and Tyliszczak 2018, 2019). We focus only on some selected problems that are crucial for the present work
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