Mid-IR laser absorption diagnostics for hydrocarbon vapor sensing in harsh environments
Abstract
Fuel/air stoichiometry is an important parameter in modern combustion devices because it has a profound influence on efficiency, power, and pollutant formation. As engine technologies continue to advance, diagnostics and sensors are becoming essential for studying fundamental combustion processes and characterizing performance of combustion-based engines. Optical-absorption diagnostics have been used previously to probe various species in these environments and to infer quantities such as concentration, temperature, pressure, and velocity. However, there have been only a limited number of demonstrations of optical diagnostics for hydrocarbon fuels. This thesis describes the development of mid-IR optical-absorption sensors for time-resolved measurements of hydrocarbon species to infer critical parameters such as concentration and temperature. These sensors provide the necessary sensitivity and time resolution for measurements in shock tubes, pulse detonation engines, and internal combustion engines. Different aspects of the research conducted are summarized below. An FTIR spectrometer is used to measure the temperature-dependent absorption spectra of a selection of hydrocarbon species and blended fuels in the ∼3.3 mum region of the fundamental C-H stretching vibration. This spectroscopic library provides the first high-temperature spectral information for many of the species studied and facilitates development of sensitive diagnostics for various applications. This unique database also enables modelling of the absorption spectra of blended fuels such as gasoline. An ethylene and propane diagnostic is designed for measuring fuel concentration in a pulse detonation engine using a fixed-wavelength helium-neon laser. Time-resolved measurements during fired tests of a repetitively pulsed engine reveal non-ideal cycle-to-cycle interactions that cause a substantial amount of fuel to leave the engine unburned. By quantifying the fuel loading and identifying the amount of unburned fuel, engine performance can be characterized and future engine designs can be improved to utilize all of the fuel supplied to the engine. Simultaneous measurement of absorption at two wavelengths is used as a basis for hydrocarbon detection in severe environments. A novel wavelength-tunable mid-IR laser is modified to rapidly switch between two wavelengths, improving the versatility of this laser system. The two-wavelength technique is then exploited to measure vapor concentration while rejecting interferences such as scattering from liquid droplets and absorption from other species. This two-wavelength laser is also used to simultaneously determine temperature and vapor concentration. These techniques, in combination with the library of temperature-dependent hydrocarbon spectra, lay the groundwork necessary to develop fuel diagnostics for laboratory experiments and tests in pulse detonation engines and internal combustion engines. The temperature-dependent spectroscopy of gasoline is examined to develop a sensor for fuel/air ratio in an internal combustion engine. A wavelength was selected for good sensitivity to gasoline concentration. A spectroscopic model is developed that uses the relative concentrations of five structural classes to predict the absorption spectrum of gasoline samples with varying composition. The model is tested on 21 samples of gasoline for temperatures ranging from 300 to 1200 K, showing good agreement between model and measurements over the entire temperature range. Finally, a two-wavelength diagnostic was developed to measure the post-evaporation temperature and n-dodecane concentration in an aerosol-laden shock tube. The experimental data validate a model which calculates the effects of shock-wave compression on a two-phase mixture. The measured post-shock temperature and vapor concentration compare favorably for gas-phase and aerosol experiments. The agreement between the two fuel-loading techniques verifies that this aerosol shock tube can be used to study hydrocarbon chemistry for low-vapor-pressure compounds. The diagnostics and techniques presented here illustrate the utility and some potential applications of mid-IR laser absorption diagnostics for combustion systems.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2007
- Bibcode:
- 2007PhDT.......189K