A Review of Theory and Practical Considerations of Tunable Diode Laser Absorption Spectroscopy Diagnostics

TL;DR

This paper reviews the principles, practical considerations, and recent advancements in Tunable Diode Laser Absorption Spectroscopy (TDLAS) for diagnostics in power and propulsion systems, emphasizing WMS techniques.

Contribution

It provides a comprehensive overview of TDLAS theory, practical implementation, and recent developments like calibration-free WMS, aiding sensor design and application in harsh environments.

Findings

Wavelength-modulation spectroscopy offers noise rejection benefits.

Calibration-free WMS models improve measurement accuracy.

Practical implementation challenges are identified and addressed.

Abstract

Tunable Diode Laser Absorption Spectroscopy (TDLAS) has emerged as a versatile and reliable diagnostic tool for measuring temperature, pressure, gas composition, and velocity in power generation and propulsion systems. This paper provides a comprehensive review of TDLAS principles and practical considerations for sensor design and implementation. The discussion begins with a mathematical introduction to the theory of gas absorption including: lineshape modeling and broadening mechanisms, quantitative measurements and challenges, and practical line selection rules. The analysis progresses to wavelength-modulation spectroscopy (WMS), highlighting its advantages in noise rejection and robustness in harsh environments. Furthermore, the calibration-free WMS model and the connection between WMS harmonics and lineshape derivatives is derived. Quantitative measurements through use of multiple harmonics is discussed and challenges surrounding measurement rate are presented. The end of the discussion focuses on practical aspects regarding the implementation of scanned-WMS sensors including laser characterization, background subtraction, and hardware debugging.