Laser-Induced Plasma Ignition Experiments in a Direct-Connect Supersonic Combustor at Mach 3
Not provided.
Engineering↗
Engineering topics
Elliott, Gregory S.
Publications and source records attributed to Elliott, Gregory S..
A study of velocity, temperature, and density in the plasma generated by laser-induced breakdowns
The paper presents velocity measurements of shock-induced flow field, leading to vortex generation and plasma deformation in air. Femtosecond-laser electronic excitation tagging (FLEET) velocimetry was performed at the times Δt = 3, 20, 50, and 100 µs post laser-induced breakdown. Emissions over Δt = 3–10 µs showed the propagations of the initially elliptic shock, transitioning into a spherical front. The shock emanated along the laser axis causes the flow outward, and then the pressure gradient generated by the rarefaction wave drives the inward flow at later moments, with the velocity magnitude approaching a steady-state value of 40 m s -1 . Temporal velocity evolution was compared with non-self-similar solutions behind the propagating shock, which are sensitive to the size of the energy deposition, and the use of the measured initial plasma diameter reproduced the experiment. There establishes a region of uniform velocity around 35 m s -1 in the air-flow running through the plasma, which triggers the roll-up of the plasma surface by a large-scale vortex, providing the detail of flow field evolution from the shock propagation to the plasma deformation. A collective Thomson scattering and hybrid fs/ps pure rotational coherent anti-Stokes Raman scattering (CARS) were also performed to gain insight into the high-temperature plasma. An effective electron–ion recombination rate of 2 x 10 -12 cm 3 s -1 was measured at Δt = 0.5–10 µs, during a dynamic plasma expansion and compression. When the shock resides close to the plasma at Δt = 0.5–1 µs, the temperature distributions were found to follow the similarity law.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Laser-induced non-equilibrium plasma kernel dynamics
A non-equilibrium model for laser generated plasmas is validated and used to investigate plasma kernel dynamics. Breakdowns in quiescent air at different ambient conditions (e.g. pressure) for different laser parameters (e.g. power density and beam wavelength) are considered. The hydrodynamics is described based on the chemically reactive Navier–Stokes equations where non-equilibrium effects are accounted for using a three-temperature model. Laser-plasma interactions over nano-second time-scales are modeled with a radiative transfer equation (RTE), including both multiphoton ionization (MPI) and inverse Bremsstrahlung (IB). Validation is conducted by comparing absorbed energy and plasma emission with the corresponding experimental measurements, and by observations of the structure of the breakdown. The laser generated plasma exhibits a two-lobe structure, developing both in the forward and backward directions. An analysis of the dynamics suggests that the kernel evolution results from a radiation-driven wave that is: (i) triggered by MPI, (ii) sustained by energy deposition in IB interactions, which compensates for the energy lost by free-electrons in ionizing collisions, and (iii) guided by both MPI and ionization by electron impact (IE).
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Molecular filter-based diagnostics in high speed flows
The use of iodine molecular filters in nonintrusive planar velocimetry methods is examined. Detailed absorption profiles are obtained to highlight the effects that determine the profile shape. It is shown that pressure broadening induced by the presence of a nonabsorbing vapor can be utilized to significantly change the slopes bounding the absorbing region while remaining in the optically-thick regime.
Elliott, Gregory S.↗
Study of compressible mixing layers using filtered Rayleigh scattering based visualizations
Filtered Rayleigh scattering-based flow visualizations of compressible mixing layers are reported. The lower compressibility case (Mc = 0.51) displays well-defined roller-type spanwise structures and streamwise streaks. The structures of the high compressibility case (Mc = 0.86) are more 3D and oblique.
Elliott, Gregory S.↗
A study of compressible mixing layers using filtered Rayleigh scattering
High Reynolds number compressible planar free shear layers were studied using a planar laser visualization technique. Two convective Mach numbers, M(c) = 0.51 and 0.86, were studied in the developing and fully developed regions. The structures in the M(c) = 0.51 case were characterized by 2D core and roller regions, similar to subsonic shear layers. Also for the M(c) = 0.51 case, plan views in the developing region showed the existence of streamwise streaks, possibly indicating the presence of organized streamwise vorticity. The M(c) = 0.86 flow was much less organized than the lower convective Mach number case and highly three dimensional.
Elliott, Gregory S.↗