Tomographic reconstruction of the OH*-chemiluminescence distribution in premixed and diffusion flames

A fast tomographic reconstruction device has been developed to detect the two-dimensional distribution of the chemiluminescence of OH* in the reaction zones of flames. In the set-up, special emphasis was placed on the applicability of the technique to turbulent flames. A spatial resolution of the system, < 1-2 mm, and an exposure time of 100-200 mu s are required to resolve the chemiluminescence signal of OH* originating from the folded flame front of a turbulent flame. The tomographic system is realised by a set of 10 Kepler-telescopes surrounding the investigated object from different angles. The chemiluminescence signal collected by each telescope is imaged onto the face surface of an optical cable. The latter consists of 90 single fibres, which are arranged equidistantly from each other, in a single row. The opposite ends of these 10 cables/900 fibres are merged together in a light collector, which images the output signal of all the fibres onto the entrance window of an intensified CCD-camera. The telescopes are used to adapt the size of the investigated object to the size of the image intensifier of the CCD-camera. Additionally, an iris aperture positioned in the focal plane of the objective lens of the telescope guarantees, that only parallel rays can pass the system. The signal information of these 90 parallel rays obtained under 10 different angles is used for the tomographic reconstruction. The aperture of the iris inside the telescope limits the optical resolution of the telescopes, as well as the light intensity transmitted through the telescope. A series of single shot experiments were carried out in various laminar diffusion and specially-shaped premixed flat flames to investigate the limits of the apparatus and the tomographic algorithm with respect to the exposure times as well as geometric resolution. In these experiments reasonable temporal and spatial resolution of the instantaneous OH* distributions are obtained at exposure times down to 100 mu s so that this technique can be successfully applied to turbulent flames.