Emission and laser-induced fluorescence imaging methods in experimental combustion

Abstract

Imaging methods provide new insights into many fundamental combustion processes. Many imaging techniques have been devised in recent years and applied to a range of experiments. One particularly useful method is to seed the flow with oil particles and illuminate the domain of interest with a planar sheet of laser light. The droplets evaporate and vanish when they pass through the flame. The light scattered by the particles 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 scattering. It provides essentially qualitative information on the geometry and motion of the flame front. Another valuable method relies on spontaneous emission imaging. In this method the light emitted by certain radicals produced by the chemical reaction is detected by a camera and delivered to a computer for further processing. In some circumstances it is possible to deduce from this measurement the spatial distribution 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 intensified CCD camera. The data obtained in this way can be processed to obtain spatial measurements of the species concentration. The basic principles, equipment requirements, and experimental aspects of these three imaging techniques are reviewed. Practical applications to turbulent flames are emphasized. It is shown that emission imaging applied to turbulent ducted flames yields interesting information for modeling. A second example of application is the ignition sequence of a multiple-injector combustor, of importance to modern cryogenic rocket engines. Emission and PLIF imaging have been used to obtain data on the development of the initial flame kernel and on its propagation from the first injector to the next. The images gathered in this experiment yield a unique view on the flame patterns that lead to the final stabilization of the reactive fronts. While current imaging methods are essentially qualitative, 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 developments in this area are evaluated.