High Temperature and Pressure Measurements from TDLAS Through the Application of 2nd Derivative Fitting and the Aggregate Boltzmann Method

Abstract

This work expands Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurement and data processing techniques. Specifically, it introduces new data acquisition and data reduction techniques that improve scanned direct absorption spectroscopy measurements.

Current data acquisition techniques are limited in temporal and spatial resolution. Some of these limitations are resolved by extending measurements from a single point to a plane by the introduction of a novel instrumentation method, the Linear Array Camera (LAC). The use of an LAC allows for simultaneous imaging of absorption spectra on multiple points, from which the spatial distribution of pressure, temperature and species fraction can be inferred. Example measurements are presented to demonstrate the capability of a planar measurement approach. Some practical challenges are identified, and corrections are given for common design problems, as well as models to help with the design of an LAC system.

TDLAS is typically used to generate measurements of temperatures, pressures and species fraction. Their accuracy is limited by errors introduced in the measurement of the absorption profile and by the accuracy of the spectral parameters used to related absorbance to the thermochemical state. Some of these limitations are overcome by the introduction of a novel application of the second derivative fitting of the absorption spectrum, the wavenumber-domain equivalent of Wavelength Modulation Spectroscopy (WMS), a Fourier domain technique. Because the WMS technique requires fast acquisition, it is impractical for use with state-of-the-art LAC systems, which have limited scan rates. The second derivative reconstruction method allows for insensitivity to baseline errors and increases sensitivity to small variations in the spectral absorbance, which allow for more accurate measurement of temperatures and vastly improve the measurement of pressure through spectral fitting.

The second derivative scheme, a spectral fitting technique, is dependent upon the accurate knowledge of the spectral parameters, which are typically taken from a spectral database. One common database is the HITRAN spectral database, which lists parameters for the spectral transitions and includes collisional parameters for air and self broadening. In actual applications, such as in combustion environments, the collisional partners are frequently far more numerous than air and self collisions. The lack of these parameters can introduce errors into the reconstructed measurements. This can be addressed through the use of the Boltzmann Plot method, which makes use of the integrated area under a number of isolated transitions to negate the dependence on accurate broadening parameters. To overcome the necessity for isolated features an extension is introduced, referred to as the Aggregate Boltzmann Plot method, based on the concept of aggregate spectrum within a Boltzmann Plot framework to expand its applicability to high pressure and temperature conditions where spectral blending has prevented the application of the traditional Boltzmann Plot method.

The Aggregate Boltzmann Plot requires accurate measures of spectral profiles, which are affected by baseline errors resolvable by second derivative fitting. Therefore, through the combination of second derivative fitting and the Aggregate Boltzmann Plot method, a robust measurement technique is obtained. This technique applies the second derivative fitting to obtain baseline insensitive spectral profiles, which are then applied in the Aggregate Boltzmann Plot method to mitigate the effects of missing broadening parameters. This results in a robust measurement technique capable to generate accurate measurements under conditions that were previously inaccessible by traditional approaches based on scanned direct absorption methods.