Wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

In this work, a practical implementation of calibration-free wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments is presented. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection-current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are: (1) normalization of the 2f signal by the first-harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral absorption model that is used to compare with measured 1f-normalized, WMS-2 f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements.;The calibration-free WMS technique is applied to two harsh environments---internal combustion (IC) engines via a modified spark plug for measurements of temperature and H2O concentration, and a ground-test scramjet combustor during operation at Mach 5-equivalent flight speed for measurements of temperature, H2O, and CO2.;To develop the sensor for IC engine operation, careful laboratory measurements of direct-absorption and 2f spectra of H2O and CO2 at high pressure were performed to validate the spectral absorption models used to infer gas properties. The direct-absorption spectra confirm that the Lorentzian line shape model traditionally used for pressure-broadened absorption transitions is not sufficiently accurate to model spectra at high density (greater than a few amagat) due to the breakdown of the instantaneous collision assumption inherent to the Lorentzian model. Empirical corrections to the line shape model suggested by past researchers were found to reduce error in the spectral models; however, a well-validated model for non-Lorentzian effects on high-pressure CO2 and H2O absorption still does not exist that delivers the low uncertainty levels necessary for accurate gas property sensing. Alternatively, the 2f spectra are shown to be at least a factor of four less-influenced by these non-Lorentzian effects, eliminating the need in most cases for corrections to the 2f spectral absorption simulations.;The resulting WMS sensor for IC engines is capable of accurate, crank angle-resolved measurements of temperature and H2O concentration during the compression stroke of IC engines at a rate of 15 kHz. The temperature is determined from the ratio of absorption for two transitions of water vapor in the intake-gas mixture, and the H2O concentration is determined from this inferred temperature and the absorption for one of the transitions. The measurements sample a short-path region (6 mm) of the in-cylinder gases near the spark plug. The accuracy of the sensor was validated in a static cell, giving RMS errors of less than 3% in temperature and less than 3.6% in H2O concentration over a wide range of conditions. Measurements performed in unfired and fired engine cylinders illustrate the potential of this sensor for investigating a range of difficult-to-model trends in current and proposed IC engine combustion schemes.;The WMS sensor is extended for measurements of temperature, H2O, and CO2 in the harsh, supersonic flow of a scramjet combustor. A comparison of direct-absorption and WMS measurements shows a factor of 4 increase in signal-to-noise ratio with WMS for measurements of weakly-absorbing CO2 in the supersonic flow. Two-dimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H 2O using a simple method that accounts for the influence of line-of-sight (LOS) non-uniformity on the absorption measurements. The comparisons show the ability of the LOS technique to gain useful information about multi-dimensional CFD models. In addition, fluctuations in temperature non-uniformity along the laser LOS are found to precede backpressure-induced unstart, a harmful condition which produces catastrophic failure to scramjets in flight. Though the precise cause of the fluctuations remains unknown, the detection method shows promise for use in control schemes to avoid back pressure-induced unstart in scramjets.;Overall, these demonstrations show that accurate, non-intrusive, high-bandwidth measurements can be made in harsh environments using the calibration-free WMS technique, and open the door to practical implementation of WMS in a variety of new environments that were previously too difficult for TDL absorption sensing. (Abstract shortened by UMI.)...