Trade Study of a Thermally Insulated, Nonuniform, Single-Zone Absorption Cell

Different chemical species have unique line spectra that contain information pertaining to their physical properties such as temperature, pressure, and species concentration. Line spectra must be calibrated with a high-temperature absorption cell in order to validate experimentally measured spectra with simulated spectra. The University of Tennessee Space Institute uses a high-temperature, single-zone absorption cell for spectral calibrations. Tunable diode laser absorption spectroscopy (TDLAS) is the diagnostic technique used in this research to measure temperature and species concentrations of CO and CO₂ mixtures in the absorption cell. However, TDLAS takes the path-average measurement of the line-of-sight, which is problematic because the absorption cell suffers from a nonuniform temperature distribution across the heated path. Thermal insulation is applied to the absorption cell in order to reduce that temperature gradient. The research presented in this thesis studies the effects that a reduced temperature gradient has on accuracy of the measured temperature and species concentration for a 3% CO/97% N₂ and 1% CO₂/99% N₂ mixture. Results show that insulation was effective at improving the overall temperature measurement accuracy of CO by reducing the percentage difference of measured to known values from 21% - 59% to 0.70% 8.6%; however, CO₂ temperature measurement accuracy only improved half of the time. Unfortunately, the measurement system was unable to produce meaningful results for CO species concentrations, but CO₂ species concentrations produced reasonable results that demonstrated an increase in percentage difference of measured to known values from 55% - 155% to 87% - 161% once the insulation was applied. These results indicate that insulation showed improvement in measurement accuracy for some cases, and that the data contained inconsistencies and non physical results. Therefore, a better correction system for the nonuniform temperature distribution is required.