Measurement of nonuniform temperature distributions using line-of-sight absorption spectroscopy

A laboratory demonstration is reported for two measurement strategies to determine nonuniform temperature distributions in combustion gases using line-of-sight absorption spectroscopy. These strategies rely on measurements of multiple absorption transitions of a single species, each with unique temperature dependence. The first strategy, called profile fitting, mathematically fits the observed absorption measurements constrained with a postulated temperature distribution. The second strategy, called temperature binning, determines the temperature probability density function along the line of sight using prescribed temperature bins. The wavelength-multiplexed sensor concepts and the mathematical representations are first explored in detail. The measurements of a two-zone temperature/mole-fraction (T/X-H2O) distribution with a wavelength-multiplexed scheme for near-infrared transitions of water vapor are then. presented to illustrate the fundamental concepts and investigate the sensor performance. The measured two-zone T/X-H2O distribution is composed of a 25.4 cm hot-flame zone (T approximate to 1500 K, X-H2O approximate to 10%) and a cold-room-air zone (T approximate to 300 K, X-H2O approximate to 2 %) of about the same length. The experimental results demonstrate that a nonuniform temperature distribution can be characterized with either strategy. The measurement accuracy will increase with the number of transitions, and also with the use of optimally chosen transitions. The experimental results also confirm that use of known physical constraints to reduce the number of degrees of freedom improves the interpretation of the measurement results and thus the sensor performance.