This paper describes a novel laser diagnostic and its demonstration in a practical aero-propulsion engine (General Electric J85). The diagnostic technique, named hyperspectral tomography (HT), enables simultaneous 2-dimensional (2D) imaging of temperature and water-vapor concentration at 225 spatial grid points with a temporal response up to 50 kHz. To our knowledge, this is the first time that such sensing capabilities have been reported. This paper introduces the principles of the HT techniques, reports its operation and application in a J85 engine, and discusses its perspective for the study of high-speed reactive flows.
Line-of-sight diode-laser absorption techniques have been extended to enable temperature measurements in nonuniform-property flows. The sensing strategy for such flows exploits the broad wavelength-scanning abilities (>1.7 nm approximately 30 cm(-1)) of a vertical cavity surface-emitting laser (VCSEL) to interrogate multiple absorption transitions along a single line of sight. To demonstrate the strategy, a VCSEL-based sensor for oxygen gas temperature distributions was developed. A VCSEL beam was directed through paths containing atmospheric-pressure air with known (and relatively simple) temperature distributions in the 200-700 K range. The VCSEL was scanned over ten transitions in the R branch of the oxygen A band near 760 nm and optionally over six transitions in the P branch. Temperature distribution information can be inferred from these scans because the line strength of each probed transition has a unique temperature dependence; the measurement accuracy and resolution depend on the details of this temperature dependence and on the total number of lines scanned. The performance of the sensing strategy can be optimized and predicted theoretically. Because the sensor exhibits a fast time response (~30 ms) and can be adapted to probe a variety of species over a range of temperatures and pressures, it shows promise for industrial application.
A novel technique has been developed to obtain simultaneous tomographic images of temperature and species concentration based on hyperspectral absorption spectroscopy. The hyperspectral information enables several key advantages when compared to traditional tomography techniques based on limited spectral information. These advantages include a significant reduction in the number of required projection measurements, and an enhanced insensitivity to measurements/inversion uncertainties. These advantages greatly facilitate the practical implementation and application of the tomography technique. This paper reports the development of the technique, and the experimental demonstration of a prototype sensor in a near-adiabatic, atmospheric-pressure laboratory Hencken burner. The spatial and temporal resolution enabled by this new sensing technique is expected to resolve several key issues in practical combustion devices.
We present a novel method for low noise, high-speed, real-time spectroscopy to monitor molecular absorption spectra. The system is based on a rapidly swept, narrowband CW Fourier-domain mode-locked (FDML) laser source for spectral encoding in time and an optically time-multiplexed split-pulse data acquisition system for improved noise performance and sensitivity. An acquisition speed of ~100 kHz, a spectral resolution better than 0.1 nm over a wavelength range of ~1335-1373 nm and a relative noise level of ~5 mOD (~1% minimum detectable base-e absorbance) are achieved. The system is applied for crank-angle-resolved gas thermometry by H(2)O absorption spectroscopy in an engine motoring at 600 and 900 rpm with a precision of ~1%. Influences of various noise sources such as laser phase and intensity noise, trigger and synchronization jitter in the electronic detection system, and the accuracy of available H(2)O absorption databases are discussed.
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