10.2514/6.2020-0299">
 

Validation and Data Processing of a Fixed Wavelength Absorption Spectroscopy System for High-Data-Rate Combustion Measurements

Document Type

Conference Proceeding

Publication Date

1-6-2020

Abstract

Different versions of Laser Absorption Spectroscopy (LAS) are used to measure species concentration, temperature, and pressure in combustion environments. Most LAS techniques rely on sweeping a single laser across a spectral region of interest, but fixed-wavelength lasers offer certain advantages in signal-to-noise ratio and data acquisition rate. The fixed wavelength absorption spectroscopy (FWAS) system used in this study employs multiple lasers which are stabilized at specific frequencies of interest, encoded using acousto-optic modulators, and multiplexed before being pitched through a test gas and onto a detector. The signals are then demodulated, and the attenuated intensities of each beam are then used to determine the absorption induced by the gas species. The molecular absorption of light through a gas can be temperature and pressure sensitive if the right interrogation wavelengths are selected. This means that values of absorption or absorption spectra can be mapped directly to the thermodynamic state of a gas species. This can be done by comparing absorption data taken from experiments to reference data made available through a large database. Another approach is to use experimental data in conjunction with tabulated spectroscopic parameters to analytically determine thermodynamic properties. A new FWAS system has been developed at AFIT/AFRL to study the combustion processes of several propulsion devices. To process the raw data taken from FWAS experiments, a new data processing technique was developed by the authors. The technique imports, organizes, and manipulates data taken from FWAS to obtain values of absorption at fixed wavelength locations within the absorption spectrum of the interrogated species. The interrogation wavelengths were selected to lie on or near temperature sensitive absorption features known to be present in the H2O spectrum at low and high temperatures. Data from the HITRAN/HITEMP database was then used along with the experimental data in a ratiometric approach to obtain instantaneous values of temperature for the species. Comparisons between the temperature measurements obtained through experimentation and the theoretical adiabatic flame temperatures were made. Poor agreement between experimental results and theory was found when attempting to apply the technique to a range of flame conditions. A resolution mismatch between the spectroscopic database and wavelength measurements of the interrogation beam used in experiments is thought to be a major source of this error. Known errors in the standard HITEMP2010 database might be an amplifier of this disagreement as well. The FWAS experiment, data processing methods and difficulties, as well as possible future applications will be discussed in the following sections.

Comments

The full conference paper is available from AIAA via subscription or purchase using the DOI link below.   AIAA Paper 2020-0299 Conference Session: Spectroscopic Techniques I

Source Publication

AIAA Scitech 2020 Forum (ISBN 978-1-62410-595-1)

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