A novel supersonic injection scheme for laser induced breakdown ignition

The motivation for this work is the documentation of the initial results for a newly developed fuel injection scheme in supersonic combustion experiments. The first set of tests of the Laser Ignition Transverse axisymmetric (XPACC) injector (LIT-X) yield novel flowfield characteristics and combustion performance. These initial tests involved the use of Schlieren photography for flow visualization and OH chemiluminescence for flame visualization. Notable flowfield characteristics are a region of flow separation upstream of the jet and a recirculation zone downstream of the jet. The attendant mixing region between the supersonic freestream and these low speed flow regions were targeted for laser induced plasma coupled ignition, and the transient ignition kernel was tracked at the 25-95 μs timescale. For the breakdown locations tested, the injection scheme achieved 100% success at sustaining a flame at the injector for a hydrogen injection into pure oxygen crossflow and a 0% success for injection into an 80-20 mixture of nitrogen and oxygen. However, in the case of successful combustion, it was found that after the initial transient ignition kernel, full sustained combustion always resulted in combustor unstart. Thus, it can be said that successful ignition occurs in a supersonic environment, whereas steady-state operation occurs in a pseudo-ramjet mode.