EMISSION AND LASER-INDUCED FLUORESCENCE IMAGING METHODS IN EXPERIMENTAL COMBUSTION

Imaging methods provide new insights into many fundamental combustion processes. Many imaging techniques have been devised in recent years a nd applied to a range of experiments. One particularly useful method i s to seed the flow with oil particles and illuminate the domain of int erest with a planar sheet of laser light. The droplets evaporate and v anish when they pass through the flame. The light scattered by the par ticles may be imaged for example with a CCD camera or with high-speed cinematography to show the structure and dynamics of the flame front. This technique, sometimes called laser tomography, is based on Mie sca ttering. It provides essentially qualitative information on the geomet ry and motion of the flame front. Another valuable method relies on sp ontaneous emission imaging. In this method the light emitted by certai n radicals produced by the chemical reaction is detected by a camera a nd delivered to a computer for further processing. In some circumstanc es it is possible to deduce from this measurement the spatial distribu tion of heat release in the reactive flow. More quantitative data may be gathered with planar laser-induced fluorescence (PLIF) imaging. The reactive flow is illuminated with a planar laser sheet delivered by a tunable laser. The laser light excites the fluorescence of a species that is present in the flow, which is then detected with an intensifie d CCD camera. The data obtained in this way can be processed to obtain spatial measurements of the species concentration. The basic principl es, equipment requirements, and experimental aspects of these three im aging techniques are reviewed. Practical applications to turbulent fla mes are emphasized. It is shown that emission imaging applied to turbu lent ducted flames yields interesting information for modeling. A seco nd example of application is the ignition sequence of a multiple-injec tor combustor, of importance to modern cryogenic rocket engines. Emiss ion and PLIF imaging have been used to obtain data on the development of the initial flame kernel and on its propagation from the first inje ctor to the next. The images gathered in this experiment yield a uniqu e view on the flame patterns that lead to the final stabilization of t he reactive fronts. While current imaging methods are essentially qual itative, it is possible to deduce quantitative results from the data, and some of the present limitations may be overcome with more refined measurement procedures. These issues are analyzed, and future developm ents in this area are evaluated.