Near-infrared diode laser absorption diagnostics for temperature and species in engines

Engine researchers desire spatially and temporally-resolved measurements of temperature and gas species concentrations in the harsh combustion environments associated with modern propulsion and internal combustion devices. The potential benefits of real-time diagnostics are improved engine performance and reduced pollutant emissions. Water vapor has been identified as an attractive species for in situ engine measurements, as it is a major combustion product that may be spectroscopically probed by tunable laser sources.; Near-infrared, fiber-coupled, distributed feedback (DFB) lasers in the 1.31 to 1.47 micron spectral region have been selected to probe water vapor transitions (2nu2 and nu1 + nu3 bands). Direct absorption methods and wavelength modulation absorption spectroscopy have been developed and used to measure temperature and species concentrations in several full-scale engine test facilities as well as in laboratory validation experiments in heated optical cells at low and high pressures.; Several measurement campaigns are described in this work, including a SCRAMJET facility, a high-pressure gas turbine sector rig, and a multi-tube high-repetition-rate pulsed detonation engine (PDE). All three tests have demonstrated the feasibility of making diode-laser measurements in engine facilities with minimal intrusion. These measurement campaigns have provided realistic conditions that have facilitated sensor improvements in optical hardware as well as spectroscopic techniques.; In particular, wavelength modulation spectroscopy (WMS) with second harmonic detection (2f) has been explored as a means to improve temperature measurements in both low-pressure gases with isolated spectral features, as well as in gases with congested spectra, such as pressure-broadened water vapor. Experiments and analysis, with water vapor at atmospheric pressures and below, show that 2f ratio thermometry may be performed over large temperature ranges with minimal calibration and simple interpretation, provided that laser modulation depths are optimized. 2f methods not only provide benefits in dealing with weakly-absorbing features, but also demonstrate advantages for diagnostics with broadened and blended spectral features. Experiments and simulations with pressure-broadened water vapor, at pressures up to 20 atm, demonstrate the viability of this strategy. In these situations, 2f signal strengths depend critically upon laser modulation depths, which have been increased to values above those typically employed for WMS with diode lasers.