Wavelength Modulation Spectroscopy for Temperature and Species Concentration in the Plume of a Supersonic Nozzle (Conference Paper with Briefing Charts)

We measure the plume temperature and species concentration profiles of a heated mixture of ammonia and steam flowing through a miniature converging-diverging nozzle area ratio 51, exit diameter 4.57 mm. The nozzle operating conditions are intended to simulate the plume of a thruster system operating in vacuum environments. Wavelength modulation spectroscopy WMS is a laser absorption spectroscopy technique that allows for quantitative, time-resolved, sensitive diagnostics of gaseous flows. The goal of the measurements is to assess the potential of WMS as a diagnostic technique for characterizing this type of system. The measurements use three multiplexed fiber-coupled tunable diode lasers two for measuring temperature and H2O concentration and one for measuring NH3concentration. The multiplexed laser beam is spatially translated across the plume on a two-axis stage. Using Abel transforms, we are able to successfully resolve the structure of the under-expanded plume despite absorption signals as low as 9e-4. As a result, WMS demonstrates excellent promise as a sensitive diagnostic tool for resolving the profile of such plumes.