CAREER: Spatially Resolved, Continuous Monitoring of Transient Processes Using Novel Optical Sensors

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Intellectual merit The research plan is centered around two techniques, both of which employ a Ti:sapphire laser and new wavelength-scanning techniques. A larger portion of the effort is devoted to two-photon direct absorption of xenon. In this technique, rapidly tunable ultraviolet light is used to monitor a xenon absorption feature in much the same way tunable diode lasers have been used to monitor water absorption in the near infrared. However, because the two-photon absorption in xenon is a nonlinear process, it occurs only where laser irradiance is high. Thus, in a focused laser beam, the process occurs only near the beam focus and not along the entire beam path. This property is exploited to extend standard direct-absorption techniques to enable spatial resolution. Spatial resolution in a line, plane, or volume are accomplished by rapidly scanning the laser focus through space using galvanometers. The technique monitors one property: xenon number density. However, because xenon is inert and easy to seed in a variety of environments, xenon number-density information is much more useful than it might seem on the surface. For instance, whenever pressure is known, gas temperature can be calculated from the xenon number density. In addition, by seeding xenon into one of multiple fluids in a mixing experiment, the xenon can be used to track that fluid even through chemical reactions that change its composition. The second technique uses new wavelength-scanning approaches to monitor Raman scattering spectra. This technique complements the xenon technique by offering more property information (e.g., simultaneous concentrations of multiple species) at the cost of lower signal levels. Applications in liquid and spray systems relevant to combustion are described. Although combustion sensing is of primary interest, both techniques are expected to be valuable for sensing in other environments as well. Broader impact A research and teaching plan for the next five years is laid out, designed to build a firm foundation future efforts. The research plan extends new laser wavelength-scanning concepts developed by the PI to provide continuous, spatially resolved gas and liquid property measurements of an unprecedented nature. Such data are obtained in fundamental and applied systems relevant to combustion, with the goal of dramatically improving the understanding of the governing processes. The research inspires the inclusion of active learning techniques into the undergraduate thermal-fluid science curriculum. Balanced by a traditional lecture-based format, these techniques are intended to foster creative thinking and physical familiarity with course material. Concept inventories are used to guide and assess the active learning exercises. In addition, undergraduate researchers (including students of under-represented groups), a graduate class in optical techniques, and outreach activities are integrated with the research program.