Lydia Wermer

Schlieren imaging investigation of successive laser-induced breakdowns in atmospheric-pressure air

Fast Schlieren imaging was performed to visualize the interactions between previously produced laser breakdown and a subsequent laser pulse. A pair of laser pulses was used to generate successive breakdowns in the quiescent standard air, and the interval between the pulses was varied from 50 ns to 100 μs to experimentally simulate various laser repetition rates. The incident laser energies ranged from 5 mJ to 31 mJ, and the energy absorbed by the breakdown of the second laser pulse was quantified by measuring the energies before and after the breakdown. The results indicate that the second laser pulse coupled to the background gas and produced a second laser breakdown only when the pulse interval was shorter than 250 ns or longer than 15 μs. For the shorter pulse intervals, the second breakdown occurred at the edge of the first breakdown region along the laser beam path, and its effect on the perturbation of the density field was found to be small. On the other hand, for the longer pulse intervals, the second breakdown occurred at the lens focal point, and the density field perturbations caused by the first and second breakdowns seemed to interact with each other inducing the Richtmyer-Mechkov instability. As a result, more significant turbulence in the density field was observed after successive laser pulse breakdowns than was observed following a single breakdown.

Comparison of Unstart Induced by Mass Addition and Heat Release

The unstart phenomena in a model scramjet with the freestream of both low and high enthalpy Mach 4.5 flow conditions at an arc-heated hypersonic wind tunnel are investigated. Then, the unstart phenomena induced by a nitrogen or ethylene jet at low and high enthalpy conditions are compared. The nitrogen or ethylene jet pressurize downstream by mass addition and heat release from combustion. High-speed schlieren at the jet and the lip of the scramjet model inlet and high resonance frequency surface pressure measurements are used to capture flow features during an unstart process. In both conditions, the similar transient behavior of unstart shockwave system spawned by the flow choking and quasi-steady state of the unstart shockwave system at localized favorable pressure gradient are observed. In combustion driven unstart process, severe oscillatory flow motions of the jet and the unstart shockwave at the lip of the model inlet are observed. On the other hand, the unstarted flow motions triggered by mass addition remains relatively steady after the completion of the unstart process. The discrepancies between the unstart processes induced by the nitrogen and the ethylene jet are explained by flow choking analysis.

Poster: Schlieren imaging of laser-induced ignition and flame propagation by single and successive laser pulses

Schlieren images are shown of laser-induced ignition by a single laser pulse and by successive laser pulses separated by 600 μs. Laser-induced sparks were produced over a nozzle with 4 m/s stoichiometric methane-air flow. Images were taken perpendicular to the laser beam path. Initially the ignition area for successive laser pulses is much smaller than that of a single laser pulse, but as the two laser pulses interact, they produce a turbulent flame structure which increases flame propagation speed. At 1 ms after the first pulse, the successive laser pulse flame propagation has extended further downstream than single laser pulse flame propagation. Acknowledgements This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1106756 500 μs 700 μs 1 ms 1.5 ms 2 ms 5 ms 3 ms Schlieren Imaging of Laser-induced Ignition and flame propagation by single and successive laser pulses Lydia Wermer1, Moon Soo Bak2, Seong-kyun Im1 1Worcester Polytechnic Institute, 2Sungkyunwan University, South Korea Single laser pulse 23 mJ energy Successive laser pulses 600 μs between pulses Laser energy: 1st pulse = 8 mJ 2nd pulse = 15 mJ Time after first breakdown Laser-induced plasma Light source High speed camera Pre-mixed methane-air mixture N d :Y A G L a s e r Experimental Setup Nozzle exit 27.6 mm 51.5 mm Hot plume expansion from first laser pulse Second laser pulse Interaction between first laser pulse flame front and the second laser pulse Flame front Flame front Hot plume expansion from single laser pulse