Thermo-Fluid Diagnostic Methods using Laser-Induced Plasma and Ultrasound Wave

Abstract

We investigated the LIBS (laser-induced breakdown spectroscopy) characteristics in various phase conditions including single-phase (gas) and two-phase (liquid gas, solid gas) for thermo-fluid applications. For single-phase (gas) analysis, two-dimensional mapping of the LIBS signals from chemical species information was performed in propane flames with in situ diagnostics. Key combustion information, such as density, fuel concentration, and fuel/air equivalence ratio were provided by the LIBS measurement. For the two-phase spray flame, the simultaneous laser ignition and spectroscopy enabled rapidly determining the local equivalence ratio and condensed fuel concentration during the reaction. In parallel with the laser ignition, the equivalence ratio and droplet characteristics, such as concentration, size, and distribution of spray combustion, were simultaneously obtained for a feedback control system. For the two-phase aerosol analysis, LIBS detected solid carbon particulates in a flow system that was designed to replicate aircraft exhaust flow conditions. The detected signals from the emissions stream at velocities of up to 70 m/s showed that in-situ characterization of carbon particulates in the high-speed exhaust were proven feasible. The results obtained through this research will allow for more practical diagnostics in various applications such as biology, combustion and the environment. In addition, basic research was conducted for the thermo-fluid diagnostics using LIUS (laser-induced ultrasound) waves. We demonstrated that the reduced-graphene-oxide-coated thin aluminum film (rGO-Al) and rGO-coated polydimethylsiloxane (rGO-PDMS) were effective optoacoustic transmitters for generating high-pressure and high-frequency ultrasound waves. Under pulsed laser excitation, rGO-Al and rGO-PDMS transmitters generated enhanced optoacoustic pressure that was 64 and 76 times stronger, respectively, than Al alone. Promising optoacoustic wave generation was made possible by optimizing the thermoelasticity of metal, the PDMS film and the thermal conductivity of rGO in the proposed transmitter for laser-induced ultrasound applications. LIBS and LIUS have great advantages as analytical techniques, namely, real-time rapid analysis and stand-off detection capability, which are all material phases for diagnosing thermo-fluid phenomena.