Acoustically coupled combustion instabilities can result in large-scale, potentially catastrophic pressure oscillations in aerospace propulsion systems, including liquid rocket engines (LREs) and gas turbine engines. A fundamental understanding of the interactions among flow and flame hydrodynamics, acoustics, and reaction kinetics is essential to determining combustor stability and controlling combustion processes. This dissertation focuses on a range of fundamental experiments that can shed light on combustion instabilities and their dynamical signatures, including (1) exploration of the effects of nanoparticulate additives on liquid fuels during acoustically-coupled combustion and (2) the effects of acoustic excitation on gas phase combustion processes involving injector configurations of relevance to Air Force LRE systems, including single element, triple element, and coaxial jet injection systems. The experiments were conducted in a closed acoustic waveguide, enabling the creation of standing acoustic waves with different amplitudes and frequencies of excitation, with the ability to situate the reactive flow processes at different positions relative to pressure node (PN) or pressure antinode (PAN) locations. The experiments utilized various optical diagnostic methods in analyzing combustion dynamics, including phase-locked OH* chemiluminescence imaging, high speed visible imaging, and background-oriented Schlieren, in addition to localized measurements.
The first part of the present studies involved exploration of the effects of acoustic excitation on burning droplets of liquid ethanol loaded with reactive aluminum nanoparticles (nAl), in addition to nDodecane and Fischer-Tropsch (FT) synthetic fuel. In the presence of velocity perturbations, nAl-laden ethanol droplets were observed to burn for longer periods of time than in the absence of acoustics, likely resulting from suppression of particle agglomerate formation and expulsion. For a range of excitation amplitudes, increases in the droplet burning rate were observed with nAl addition, though not always at the highest loading concentration. Other benefits of nAl addition to ethanol included a delay in the transition to periodic partial extinction and reignition (PPER) of the flame for high amplitude acoustic excitation as well as an increase in the mean extinction strain rate for such flames, suggesting that such fuels could be more robust in an LRE environment. Limits on the particulate loading (to avoid fuel clogging) did present challenges, however, and this was observed more significantly for the sooting hydrocarbon liquid fuels, which themselves required a surfactant additive (Span80) to maintain a uniform dispersion of nanoparticles. While both Span80 and nAl particulates at various loading concentrations increased burning rates, somewhat differing trends in burning rates and droplet dynamics for the various nanofuels were found to result from complex phenomena associated with particle agglomeration and unstable droplet dynamics.
The second part of the present studies involved an extensive exploration of the dynamics of reactive jets of gaseous methane (CH4) injected into air within a closed acoustic waveguide under atmospheric conditions. These studies included single and multiple element gaseous fuel jet injection as well as coaxial injection in the acoustically resonant environment, where alternative excitation conditions, as well as flow conditions, were explored. Flame dynamics were quantified via phase-locked OH* chemiluminescence imaging, determining alterations in the Rayleigh index and temporal flame distortion, especially during sustained oscillatory combustion (SOC). But because of the initiation of periodic liftoff and reattachment (PLOR) of the flames at higher amplitude excitation, high speed visible imaging was required to enable time-resolved data, which were analyzed via proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Characteristic signatures associated with various types of flame response were then identified, including weakly oscillatory combustion, full-scale flame coupling, lock-in to excitation during SOC, and multi-mode flame dynamics via PLOR, preceding flame extinction and blowoff. Modal decomposition of flame dynamics via POD and associated phase portraits revealed important additional dynamical features and signatures characteristic of such transitions during flame-acoustic coupling. This detailed quantification for a broad parameter space provides the foundation for the development of future reduced order models (ROMs) based on topological features, with the ultimate benefit of robust future combustion control systems.
