Development of an infrared absorption tomography instrument for temporally and spatially resolved fuel concentration measurements

Fuel-to-air ratio non-uniformities play a key role in the noise characteristics and nitrogen oxide emissions performance of lean premixed combustors. An instrument was developed to measure spatially and temporally resolved fuel concentration fields through line-of-sight hydrocarbon absorption of 3.39 mum laser radiation. An optical system was designed to deliver a collimated infrared laser sheet into the test section and the attenuated laser sheet power is recorded by a custom-built infrared detector array. Absorption data acquired at multiple radial and angular positions is combined using the principles of computed tomography to reconstruct the time-resolved concentration profiles. A tomography algorithm called the constrained conjugate gradient method was implemented and tested with an artificially generated Gaussian function that models the concentration distribution of a fuel jet. An attempt was made to experimentally validate the algorithm through measurements of known concentration fluctuations induced by a modulated axisymmetric methane jet in co-flowing air.