Fiber-coupled optical probe for laser absorption diagnostics in shock tube experiments with high concentrations of non-monatomic species

Abstract

A novel fiber-coupled optical probe mounted in the shock tube end wall was developed for in-situ laser absorption measurements in experiments subject to non-ideal side-wall boundary layer effects that occur with high concentrations of non-monatomic species. The opto-mechanical design involves a custom-machined shock tube end wall with optical ports and port plugs that allow easy integration of sapphire rods as waveguides to shock tube facilities. In this first application, a quantum cascade laser (QCL) targeting a CO<inf>2</inf> transition near 4.2μm was free-space fiber-coupled with a single-mode fiber to deliver light through the sapphire waveguides across the uniform-property region in the core of shock-heated gases and away from the shock tube side walls. This hardware design is complemented by a scanned-wavelength direct absorption (scanned DA) optical sensing scheme, enabling minimally invasive, sensitive and time-resolved measurements of CO<inf>2</inf> in the post-reflected-shock region at 100 kHz. The end-wall probe with an optical path length of 7.6 cm, was first validated in a highly diluted, non-reacting environment against conventional cross-tube measurements (14.12 cm tube diameter). The results revealed excellent agreement between the two methods with minimal perturbations to the shock conditions from the sapphire rods. High-concentration CO<inf>2</inf> and CH<inf>4</inf> experiments were then conducted to demonstrate the probe's potential to study high-concentration spectroscopy as well as high-temperature pyrolysis of high-concentration methane mixtures where conventional cross-tube sensing scheme is sub-optimal. The measurements presented demonstrate a new shock tube diagnostic tool that will enable shock tube studies under conditions where non-ideal side wall effects, (e.g., due to reflected-shock bifurcation that occurs with non-monatomic species), are present.