Experimental and Numerical Investigation of Early Phase of Laser Ignition Under Stoichiometric and Lean Conditions

Forced ignition, the initiation of combustion processes by rapid and localized introduction of energy, is central to the successful operation of many combustion systems. It is therefore of interest to investigate this process, starting from the introduction of energy to the emergence of self-sustained flame or the quenching of an otherwise initialized flame kernel. Since the process is highly non-equilibrium and involves various complex kinetic phenomena, it is important to understand the key aspects that control failed or successful ignition. Detailed studies of the early phases of the ignition process can lead to knowledge of more general characteristics of the problem so that reduced models of the ignition process can be developed. These reduced versions can be used in less costly computational studies to assess various ignition events. This paper reports an experimental and numerical investigations of the early phase of laser ignition. The gas mixtures, air, methane/N₂ and methane/air are considered to bring out the effect of heat release on the early flow field. The mixtures are studied at three different energy levels and the Jones blast wave theory is used to deduce the energy responsible for the development of the attendant shock waves. This energy is also used to specify initial conditions for the simulations of air and methane/air processes. Additionally, interferometry is used to resolve the density field within the plasma kernel. For the methane/air simulation two chemical models are used, a global reaction model supplemented by an ignition model and a two-step mechanism. The sensitivity of the simulations to the initial geometry of the laser spark is also investigated. The blast wave and interferometry results show that in the reacting methane/air mixture the resulting shock wave is strengthened by early heat release. It is also shown that the shock wave trajectory is not strongly affected by the initial spark geometry, but it has an impact on the velocity field and on the distribution of thermodynamic properties.