*1.4. Aim of This Work*

The goal of the present work is to develop a laser-absorption sensor for simultaneous CO, CO2, and H2O measurements in the exhaust of a gas turbine combustor test rig. The instrument is an extension of a single-wavelength transmitter/receiver sensor used for preliminary measurement of CO concentrations in the exhaust flow of a single full-scale gas turbine burner located at the Siemens Energy test center in Ludwigsfelde (Germany) [15,16]. Challenges include the high pressure and temperature inside the combustion zone and the high temperature and acoustic noise in the ambient humid gases near the test rig. The high temperature and pressure in the combustor exhaust produce a complex absorption

spectrum with a paucity of isolated lines, where most transitions are strongly overlapped by pressure broadening. The overlap of the fundamental absorption bands of CO and CO2 exacerbates this problem. The acoustic noise (up to 120 dB) exceeds the operating limits of typical electronic devices, and the sensor must be packaged to protect electronics and optics. The high ambient temperature (up to 60 ◦C) also exceeds typical electronics and optical equipment specifications, and the sensor package is thus assembled on watercooled breadboards. The packaged sensor must be small enough for easy transport with installation by two operators with only minor adjustment for optical path alignment at the test site.

While our previous work [15,16] focused on the challenging measurement environment associated with gas turbine combustor testing and solved issues such as optical access and thermal and acoustic isolation of the measurement equipment, the focus in this work is on enhancing the measurement capabilities of the sensor. In the present work, a combined NIR/MIR laser-based absorption spectrometer is described for the simultaneous measurement of line-of-sight-averaged gas-phase CO, CO2, and H2O in combustion exhaust, and temperature is inferred from the ratio of two H2O absorption transitions. The sender unit has a small footprint due to separately placing electronics and optical parts on two stacked temperature-controlled boards and mounting inside a N2-purged box to avoid background water vapor absorption. The instrument uses two MIR lasers near 2059.91 cm−<sup>1</sup> (4.8546 μm, centered around the P(20) CO transition) and 2190.02 cm−<sup>1</sup> (4.5562 μm, centered around the R(12) CO transition), previously explored by Spearrin et al. [11]. Simultaneously, two NIR diode lasers probe two water vapor transitions (7185.59 and 6806.03 cm−1) with differing temperature sensitivities previously used by Goldenstein et al. [7,8]. The beam delivery can be either free-space or via optical fibers compatible with the various wavelengths emitted by the installed laser sources. The receiver optics and detectors are also mounted in a metal box purged with an expanding flow of dry nitrogen for keeping the interior cool and free of water vapor.
