Tunable Diode Laser Absorption Sensor Applications to Aeropropulsion Testing Ronald K. Hanson and Jay B. Jeffries Department of Mechanical Engineering, Stanford University, Stanford, CA, 94305, USA Ronald K Hanson

Mark G. Allen Physical Sciences Inc, Andover, MA, 01810, USA

ABSTRACT Compact, wavelength-tunable diode-laser (TDL) technology, based on telecommunication-type devices operating in the visible and near-IR portion of the spectrum, provide a robust tool for spectroscopic measurements in large-scale applications. Using well-understood and quantitative absorption spectroscopy, techniques for line-of-sight integrated measurements of gas temperature, velocity, pressure, density, and concentration of individual constituent species have been developed. Integrated, autonomous sensors have also been developed for continuous, real-time monitoring of these parameters in large-scale test facilities and on flight test vehicles. This presentation will briefly describe the optical physics that forms the basis for the measurement techniques and the typical sensor architectures employed. Example applications will be presented, including: full-scale SCRAMjet test articles, full-scale gas-turbine combustor sector test stands, pulse-detonation test engines, production-type gas turbine engines, and vehicle installations for flight testing.

1. BACKGROUND AND MOTIVATION Interest in cleaner-burning and higher-performance combustion and propulsion systems has motivated the development of advanced laser-based measurement techniques. These combustion and propulsion diagnostics have been the subject of several comprehensive reviews,[1-4] and include sophisticated species-selective imaging techniques like planar laser-induced fluorescence and Raman scattering, which are suitable for laboratory and ground-test measurements. Laser-based diagnostics also include optical extinction or absorption methods,[5-7] which can provide real-time measurements of important propulsion parameters including temperature, species concentration, pressure, and velocity. In addition to their use for ground-test measurements in reduced-scale or full-scale systems, these absorption techniques can provide sufficiently rapid feedback to have the potential for combustion and propulsion control applications. These laser absorption sensors are the focus of this paper. Laser absorption is a relatively mature technique that may be executed using ultraviolet, visible or infrared lasers, and important results have been obtained in all these spectral domains. However, over the last decade, there has been a growing emphasis on the use of near-infrared (NIR, i.e., the wavelength interval from approximately 700 nm to 3 microns) diode laser sources developed for use in the telecommunication and optical storage industries. These lasers can be quite economical and extremely robust, and can generally be coupled to optical fibers to allow transmission of light to and from the measurement location, e.g. stationary combustors or the engine of a propulsion system. Unfortunately, lasers are not available at all wavelengths of interest, although access over much of the wavelength range from ~750 nm to ~2.3 microns is improving and RTO-MP-AVT-124

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TUNABLE DIODE LASER ABSORPTION SENSOR APPLICATIONS is sufficient for many applications. The relatively low cost and robustness of tunable diode laser (TDL) sensors, combined with fast response and relative simplicity of operation and data interpretation, have led to rapid progress in application of these sensors to practical combustors, and have illustrated unique potential for control applications. The TDL sensors discussed here are based on spectrally-resolved absorption.[5-7] Variations of this non-invasive measurement strategy allow sensing of multiple flowfield parameters including temperature, species concentration, pressure and velocity. Absorption is a line-of-sight (LOS) method, and utilizes either fixed-wavelength or rapidly tuned, variable-wavelength laser sources. Typically a continuous wave (cw) or quasi-cw laser is used as the light source for absorption. It is increasingly common to use multiple lasers for simultaneous absorption measurements;[8] we term this approach “wavelength-multiplexing,” consistent with terminology for telecommunications. The underlying theory for absorption is well understood, though quantitative measurements require knowledge of spectroscopic and collisional parameters that may be poorly known, and these quantitative spectroscopic parameters are often the target of complementary studies in our laboratories. After a short introduction to the measurement fundamentals, we describe the use of laser absorption for thermometry, fuel concentration measurement, and mass flux monitoring for aeroengine ground-test applications and in addition discuss the potential for in-flight sensors in modern-propulsion systems.

2. MEASUREMENT FUNDAMENTALS TDL sensors are based on absorption of the wavelength–tuned laser intensity as the beam propagates across the measurement path. This absorption is described by the well-known Beer–Lambert relation: Iν = Iν ,o exp [− S (T ) φ (ν −ν o ) n l] = Iν ,o exp(−kν l)

(1)

where I< is the monochromatic laser intensity at frequency