Development and application of advanced numerical techniques for the analysis of optical data of turbulent flames

Abstract

This work is focused on the development and application of advanced numerical decomposition techniques for the analysis of combustion through time-resolved, high resolution images from turbulent flames. Particularly, the main techniques developed are based on Proper Orthogonal Decomposition (POD), Independent Component Analysis (ICA), Optical Flow (OF), and Dynamic Mode Decomposition (DMD). First, POD and ICA are employed both to extract the dominant features of the flames, in terms of luminosity field, and for the analysis of cycle-to-cycle variations in optically accessible internal combustion engines. Modal decomposition allowed to analyze the cycle variations both in terms of global luminosity and morphological features of the luminosity field. Moreover, ICA has allowed to extract the independent spatial features of the flame, i.e. the diffusive combustion of fuel jets in a Diesel engine and the ignition and radial-like flame propagation in a port-fuel injection (PFI) spark ignition (SI) engine. The work reported here includes the first application of Optical Flow (OF) to 2D flame images. Particularly, with regard to flame images relative to an optically accessible PFI SI engine, OF has permitted to estimate and study the motion field of the flame front during the propagation, more precisely the burned gas front, from the spark to the chamber wall. POD has also been used for the analysis of OH* chemiluminescence and OH-PLIF images from ethanol, n-heptane, n-decane, and n-dodecane swirl-stabilised spray flames far from and close to blow-off, to examine how the flame is modified at extinction conditions and how the large-scale features of the blow-off process may be detected before complete extinction. The analysis has allowed to identify for each flame and each operating condition the dominant structures of the flame (POD modes). Moreover, the statistical behaviour of the flames were also analysed via the coherent (non-Gaussian) and incoherent (Gaussian) analysis. Coherent-incoherent analyses carried out on OH-PLIF measurement has shown that the flame front assumes behaviour less and less Gaussian as the blow-off is approached, independently from the fuel used. Finally, the thesis reports on the application of a relatively new decomposition method, the Dynamic Mode Decomposition, particle image velocimetry (PIV) measurements from a swirled jet flow. DMD has allowed to identify the features of the precessing vortex in terms of frequency, growth rate and morphology (spatial pattern described by the dynamic modes), and even allowed to identify the feature associated with a transient phenomenon.