Gas dynamic regimes observed in dual-pulse laser ignition

Abstract

A numerical study of the gas dynamics induced by a dual-pulse laser pre-ionization plasma used in laser ignition applications is presented herein. Past experimental observations have revealed important differences in the gas dynamics of the dual-pulse plasma, generated by overlapping a pair of ultraviolet (UV) and near-infrared (NIR) pulses, as compared to those of single-pulse (typically near-infrared) laser breakdown that has been more commonly studied. The simulation results reported here show that the dual-pulse pre-ionization scheme can lead to various gas dynamic regimes depending on the axial offset of the focal points of the two beams along the optical axis. If the UV and NIR pulses are perfectly overlapped (no offset) the energy deposition is uniform along the optical axis, leading to the formation of a toroidal structure in the post-discharge cooling phase. Alternatively, if the NIR energy addition pulse is focused with a small axial offset, (i.e., upstream or downstream of the UV pre-ionization pulse by ~0.5 mm) then an asymmetric torus forms that exhibits a third lobe which propagates away from the main kernel (to the side where the NIR was focused). Finally, if the two beams are focused with a larger offset of ~2.5 mm (weaker coupling), then another regime with a fourth lobe can arise. The four main flow regimes revealed from the model are in agreement with OH* experimental chemiluminescence images. These flow regimes influence the development of the early flame kernels, therefore playing an important role in practical laser ignition applications.