Xinliang An

High speed engine gas thermometry by Fourier-domain mode-locked laser absorption spectroscopy.

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.

Validation of temperature imaging by H2O absorption spectroscopy using hyperspectral tomography in controlled experiments.

This paper describes a preliminary demonstration and validation of temperature imaging using hyperspectral H2O absorption tomography in controlled experiments. Fifteen wavelengths are monitored on each of 30 laser beams to reconstruct the temperature image in a 381 mm × 381 mm square room-temperature plane that contains a 102 mm × 102 mm square zone of lower or higher temperature. The hyperspectral tomography technique attempts to leverage multispectral information to enhance measurement fidelity. The experimental temperature images exhibit average accuracies of 2.3% or better, with pixel-by-pixel standard deviations of less than 1%. In addition, even when the internal zone is only 4 K cooler than the surroundings, its presence is still detectable; statistical analysis of the associated experimental image reveals a 98% confidence that the internal zone is in fact cooler than the surroundings.

Measurements of multiple gas parameters in a pulsed-detonation combustor using time-division-multiplexed Fourier-domain mode-locked lasers

Hyperspectral absorption spectroscopy is being used to monitor gas temperature, velocity, pressure, and H(2)O mole fraction in a research-grade pulsed-detonation combustor (PDC) at the Air Force Research Laboratory. The hyperspectral source employed is termed the TDM 3-FDML because it consists of three time-division-multiplexed (TDM) Fourier-domain mode-locked (FDML) lasers. This optical-fiber-based source monitors sufficient spectral information in the H(2)O absorption spectrum near 1350 nm to permit measurements over the wide range of conditions encountered throughout the PDC cycle. Doppler velocimetry based on absorption features is accomplished using a counterpropagating beam approach that is designed to minimize common-mode flow noise. The PDC in this study is operated in two configurations: one in which the combustion tube exhausts directly to the ambient environment and another in which it feeds an automotive-style turbocharger to assess the performance of a detonation-driven turbine. Because the enthalpy flow [kilojoule/second] is important in assessing the performance of the PDC in various configurations, it is calculated from the measured gas properties.

Demonstration of temperature imaging by H 2 O absorption spectroscopy using compressed sensing tomography

This paper introduces temperature imaging by total-variation-based compressed sensing (CS) tomography of H2O vapor absorption spectroscopy. A controlled laboratory setup is used to generate a constant two-dimensional temperature distribution in air (a roughly Gaussian temperature profile with a central temperature of 677 K). A wavelength-tunable laser beam is directed through the known distribution; the beam is translated and rotated using motorized stages to acquire complete absorption spectra in the 1330-1365 nm range at each of 64 beam locations and 60 view angles. Temperature reconstructions are compared to independent thermocouple measurements. Although the distribution studied is approximately axisymmetric, axisymmetry is not assumed and simulations show similar performance for arbitrary temperature distributions. We study the measurement error as a function of number of beams and view angles used in reconstruction to gauge the potential for application of CS in practical test articles where optical access is limited.

Simultaneous optimization method for absorption spectroscopy postprocessing

A simultaneous optimization method is proposed for absorption spectroscopy postprocessing. This method is particularly useful for thermometry measurements based on congested spectra, as commonly encountered in combustion applications of H2O absorption spectroscopy. A comparison test demonstrated that the simultaneous optimization method had greater accuracy, greater precision, and was more user-independent than the common step-wise postprocessing method previously used by the authors. The simultaneous optimization method was also used to process experimental data from an environmental chamber and a constant volume combustion chamber, producing results with errors on the order of only 1%.

Demonstration of High Speed Imaging in Practical Propulsion Systems using Hyperspectral Tomography

This paper reports the demonstration of a 30-laser-beam tomography system in a J85 turbojet engine for the imaging of temperature and water vapor concentration at an overall repetition rate of 50 kHz.

Optimized Wavelength Selection for Molecular Absorption Thermometry

A wavelength selection technique is applied to optimize temperature precision of absorption thermometry. The line-of-sight thermometry is analyzed by using an optimization algorithm. This method is also promising for use in 2D absorption tomography.

High-Bandwidth H2O Absorption Sensor for Measuring Pressure, Enthalpy, and Mass Flux in a Pulsed-Detonation Combustor

High-repetition-rate hyperspectral absorption spectroscopy was used to monitor gas temperature, pressure, velocity, and H2O mole fraction in a research-grade pulseddetonation engine (PDE) at the Air Force Research Laboratory. The hyperspectral source that was used is termed the TDM 3-FDML because it consists of three time-division multiplexed (TDM) Fourier-domain modelocked (FDML) lasers. This source monitors sufficient spectral information in the H2O absorption spectrum near 1350 nm to permit temperature measurements over the wide range of conditions encountered throughout the pulsed-detonation engine cycle. Absorption-feature Doppler velocimetry is accomplished using a novel counter-propagating beam approach designed to minimize common-mode flow noise.

Frequency-Dependent Lower-State Energy: A New Spectroscopic Parameter Useful for Designing Laser-Based Thermometers

A spectroscopic transition’s temperature sensitivity is characterized by its lower-state energy. Here, we define a related parameter that is not fundamental but is a continuous function of frequency and applies to broadened and blended spectra.