Combination of optical methods for studies in combustion and wave propagation

Abstract: In this thesis, different optical methods were combined for studies of combustion processes in gases and transient wave propagation in plates and water. Pulsed TV holography was used in all these projects. In three of the five appended papers, the pulsed TV holography technique was compared and combined with the schlieren or the Laser Doppler Vibrometry (LDV) technique. Three investigations of combustion processes were performed. A turbulent propane-air jet diffusion flame was investigated using pulsed TV holography and schlieren. From Large Eddy Simulations (LES) of a model describing the flame dynamics, a refractive index field of the process was calculated. The actual experiment was simulated by ray-tracing through the numerically calculated refractive index field. Comparing this result to the corresponding experimental data allowed the model to be validated. Further, the ignition of pre-mixed propane gas was studied with pulsed TV holography. The ignition of the gas was initiated by a focused laser pulse from the same laser used for the recording of the digital holograms. The position of the flame front at different time instants could be determined. In addition, differences in refractive index between a flowing gas and ambient air could be determined in situ. Finally, the propagation of a high- speed turbulent flame jet of a stoichiometric air-hydrogen gas mixture inside a rig was studied using schlieren and pulsed TV holography. The latter method was used to calibrate the schlieren images for quantitative measurements. A theory that takes tensile forces into account for anisotropic plates subjected to an impulsive load was expanded to include paper sheets subjected to tensile forces as in real papermaking machines. Matching this solution to experimentally obtained interferograms using pulsed TV holography showed that material parameters of the paper sheets, such as stiffness and anisotropy in various directions, could be determined from the recorded bending waves. Studies of propagating bending waves inan impact-loaded aluminium plate of water-filled box and acoustic waves in the water were performed using pulsed TV holography and LDV. The results indicate that both methods are supplementary in wave propagation measurements, since pulsed TV holography provides high spatial resolution and LDV gives high temporal resolution. In conclusion: To investigate complex processes, e.g. combustion, different methods are required. Numerical models must be validated by experiments for reliability, often with a combination of supplementary methods. An iterative process between experiments and numerical modelling will result in an increased understanding of the physical phenomena involved.