Study of a Flame Kernel Evolution in a Turbulent Mixing Layer Using LES with a Laminar Chemistry Model
Agnieszka Wawrzak, Artur Tyliszczak
Abstract
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 ( $$P_{ign}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>ign</mml:mi> </mml:mrow> </mml:msub> </mml:math> ), when the spark is located in the immediate vicinity of the mixing layer, the $$P_{ign}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>ign</mml:mi> </mml:mrow> </mml:msub> </mml:math> on the fuel-rich side is higher. On the other hand, when the ignition occurs far from the mixing layer, the $$P_{ign}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>ign</mml:mi> </mml:mrow> </mml:msub> </mml:math> 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 $$P_{ign}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>ign</mml:mi> </mml:mrow> </mml:msub> </mml:math> .