Laser-Induced Fluorescence for Temperature and Density of CN and CO in the Boundary Layer of Graphite Ablation in an Inductively-Coupled Plasma Torch Plume

Laser-Induced Fluorescence (LIF) is used to study the cyano radical (CN) and carbon monoxide (CO) in the reacting boundary layer of an ablating carbon graphite sample exposed to high temperature air at atmospheric pressure. The CN radical is an important contributor to the radiative heat transfer experienced by a hypersonic vehicle, and there is uncertainty in the literature regarding the chemical kinetic parameters of its formation. CO, on the other hand, is the dominant product of surface oxidation for carbon ablators. The air plume is generated using the 50 kW Inductively-Coupled Plasma (ICP) torch at The University of Texas at Austin, which operates at a temperature of about 6000 K and delivers a cold-wall heat flux of approximately 130 W/cm2. Quantitative LIF of CO was obtained using a photoionization-limited (PI-limited), two-photon LIF technique, while the CN LIF results are purely qualitative. The CO mole fraction peaked at 0.25 approximately 100 micrometers from the graphite surface and decreased monotonically away from the sample to less than 0.05 at a distance of 3 mm. The temperature profile in the boundary layer – obtained from a two-line intensity ratio – varied from the freestream temperature of 6000 K about 1.5 mm from the sample to 3200 K about 100 micrometers away. From the qualitative CN LIF, we see that the dominant mechanism of CN production appears to be in the gas-phase, with relatively little CN LIF signal coming from near the surface.