Development of Time Resolved Laser Imaging Techniques for Studies of Turbulent Reacting Flows

Abstract: In the present thesis the development and application of a novel laser diagnostic system for high speed spectroscopic imaging of turbulent combustion phenomena is presented. The system is capable of recording sequences of up to 8 images with a separation between consecutive images as short as microseconds. The system allows a number of established laser diagnostic techniques to be used, extending them into the time resolved regime. For the first time it has thus become possible to observe the influence of fluid motion or reaction chemistry on flame structure in a direct, time resolved, fashion. The high speed diagnostic system has been used to study flame species and fuel distributions with a high temporal resolution in jet flames, combustion cells and in IC engines. In jet flames various flame instability mechanisms, like local flame extinction and flame lift-off have been studied. Large scale structures have been observed and tracked in time, using time resolved planar laser induced fluorescence (PLIF) imaging of the OH radical. Individual extinction phenomena have been studied in further detail by simultaneous measurements of the velocity field at the flame front, allowing correlations between flow and flame structures to be made. Vortices impinging on the flame front from the fuel side were found to be the main extinction mechanism, and the time scale of the extinction process could be estimated. High speed imaging has also been applied to study novel combustion engine concepts, like the homogeneous charge compression ignition (HCCI) engine. True single-cycle resolved measurements allows the evolution of single fuel injection, ignition or combustion events to be followed in time, and cycle-to-cycle variations of complex phenomena to be studied. In an HCCI engine the appearance and growth of multiple auto-ignition kernels was observed. Following ignition the fuel was found to be consumed gradually, at different rates in different regions, and not through propagating flame fronts as is the case in spark ignition engines. By rapidly displacing the eight laser beams through a measurement volume using a scanning mirror, three-dimensional (3-D) measurements also become possible. This technique has been demonstrated in both flames and engines, allowing flame topology or 3-D concentrations gradients to be studied. In addition to the experimental work, advanced image processing routines have also been developed and applied for automatic data enhancement and analysis.