Development of a Single-Ended Laser-Absorption Spectroscopy Sensor for High-Temperature Gases
Abstract
Tunable diode-laser-absorption spectroscopy (TDLAS) sensors have been one widely-used laser-diagnostic technique that offers great potential for non-intrusive, time-resolved and multi-parameter sensing in combustion systems. These sensors have been used for performance testing, model validation and feedback control of combustors. During operation, monochromatic laser light with a specific wavelength is transmitted through the test gas and collected on a photodetector. Gas conditions such as temperature and composition are then inferred by comparing the measured amount of light that is absorbed with that predicted by spectroscopic models. In this thesis, the design and demonstration of a compact single-ended laser-absorption spectroscopy (SE-LAS) sensor for measuring temperature and water in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture which uses a single lens to provide single-ended, alignment-free measurements of gas properties in a combustor without windows. The sensor is demonstrated to be capable of sustained operation at temperatures up to at least 625 K and is capable of withstanding direct exposure to high-temperature (around 1000 K) flame gases for long durations (at least 30 min) without compromising measurement quality. The sensor employs a fiber bundle and a 6-mm diameter AR-coated lens mounted in a 1/8" NPT-threaded stainless-steel body to collect laser light that is backscattered off native surfaces (e.g., a combustor wall). Distributed-feedback (DFB) tunable diode lasers (TDLs) with a wavelength near 1392 nm and 1343 nm were used to interrogate well-characterized water absorption transitions using wavelength-modulation spectroscopy (WMS) techniques. The sensor is demonstrated with measurements of gas temperature and water mole fraction in a propane-air burner with a measurement bandwidth up to 25 kHz. In addition, this work presents an improved wavelength-modulation-spectroscopy spectral-fitting technique which reduces computational time by a factor of 100 compared to previously developed techniques.
Degree
M.S.M.E.
Advisors
Goldenstein, Purdue University.
Subject Area
Mechanical engineering
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