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Detonation diffraction leads to either successful transmission of the detonation or quenching wherein the propagation mechanism is attenuated. The transmission behavior is governed by competing effects of energy release, curvature, and unsteadiness. There is a potentially unique critical diameter that will determine the diffraction outcome for every combustible mixture composition at each set of initial conditions. The critical diffraction diameter has been correlated to several detonation parameters to date; however, these correlations all have limitations. Analytical or quasi-analytical solutions to the diffraction problem, specifically those able to predict the critical diameter, are scarce. The present work develops several critical diameter models by uniting previous work on diffraction phenomena and the critical initiation energy problem. Curvature, decay rate, and energy-based models are established, and their critical diameter predictions are compared against a wide range of experimental critical diameter data. While detonation diffraction is a complex multifaceted phenomenon, a curvature-based one-dimensional model in this work is shown to accurately reproduce empirical critical diameter behavior at relatively low computational cost. 

The ammonia (NH3) and dimethyl ether (DME) mixture is a promising alternative fuel that offers the potential for cleaner combustion. This study presents an investigation of the autoignition-assisted flame speeds of stochiometric NH3/DME mixtures under conditions relevant to practical combustion systems. Experiments were conducted at pressures of 5 and 10 bar, gas temperatures ranging from 625 to 708 K, and three DME concentrations (10, 20, and 30%, mole fraction basis) in NH3/DME fuel mixtures using a rapid compression machine-flame (RCM-Flame) apparatus. For the majority of the autoignition experiments, first-stage ignition delay time was observed. Thus, the flame experiments were performed by igniting the spark both before and after the first-stage ignition delay time. The results are presented in terms of the Beta-Damköhler Number, defined as the ratio of spark ignition time to the first-stage ignition delay. The flame speed changes depending on the Beta-Damköhler Number, pressure, gas temperature, and DME concentration. The flame speed increases by increasing the temperature, decreasing the pressure, and increasing DME concentrations. However, the effect of Beta-Damköhler Number on flame speed is complicated: with 10% DME in the mixture, the flame speed is independent to Beta-Damköhler Number, and slight observed slight decrease of flame speed is due to the temperature drop during the post-compression period; with 20% DME in the mixture, at both pressures, the flame speed jumps after the first ignition delay (or Beta-Damköhler Number of one) , and remains constant before and after; similar behavior was observed with 30% DME in the mixture at 5 bar, however, at some temperatures, the flame speed increases at Beta-Damköhler Number of greater than one, and at 10 bar, the first ignition delay was short and flame speed was not measured at Beta-Damköhler Number of less than one. For all studied conditions, a linear trend was observed between burning velocity and stretch rate. Positive Markstein lengths were observed at most conditions, except for two specific gas temperatures (664 K at 5 bar and 671 K at 10 bar) with 30% DME, where negative Markstein lengths are found. One-dimensional laminar flame speed simulations agreed with measured data for Beta-Damköhler Numbers. less than one, but underpredicted the measured data at other conditions. 

The diffraction behavior of gaseous detonations through an abrupt area change is investigated using hydrogen-oxygen-nitrogen mixtures at initial pressures of 0.5 and 1.0 bar. Critical conditions are noted and detailed discussion of the differing diffraction behaviors is undertaken, supported by simultaneous Schlieren and direct photography imaging as well as pressure-based velocity measurements. The experiments reveal four distinct diffraction regimes. The subcritical outcome is characterized by transmission failure with the leading shock front decoupling from the reaction zone, seen predominantly at lower oxygen concentrations. At intermediate oxygen levels, reinitiation from reflected shock waves is consistently observed. The critical regime exhibits both subcritical and supercritical outcomes, with detonation reinitiation at the diffraction dome's head leading to localized implosions for the supercritical case. Supercritical outcomes demonstrate successful detonation transmission, maintaining the shock front and reaction zone coupling. The effects of initial conditions on the probability of successful detonation transition and diffraction are highlighted. With the use of simultaneous direct photography and Schlieren imaging techniques, previously unseen details of the detonation and diffraction processes are recorded and explained. 

Elevated temperature and pressure laminar flame speed measurements of propane and n-heptane fuel blends were conducted using a Rapid Compression Machine-Flame (RCM-Flame) apparatus. Herein, the lack of experimental flame speed data at simultaneously high temperatures and pressures akin to practical combustion conditions is addressed. The RCM-Flame apparatus is validated against a larger constant volume combustion chamber (CVCC) and simulations using a propane-nitrogen-oxygen mixture at ambient temperature and different pressures, demonstrating high fidelity. Further experiments with an n-heptane-nitrogen-helium-oxygen mixture reveal agreement between experimental and simulated flame speeds at semi-elevated, post-compression conditions. Trials with a propane-helium-oxygen mixture over varied temperatures and pressures demonstrate measured flame speeds falling between two kinetic mechanism simulations, maintaining the general trend. A power-law model correlating laminar flame speeds with elevated temperatures and pressures is developed for propane-helium-oxygen flames at a unity equivalence ratio. Overall, the kinetic mechanisms are shown to be able to predict flame speeds at elevated temperatures and pressures providing validation at conditions not yet explored in literature, optimistically advancing combustion research for practical applications. 

The autoignition characteristics of ammonia (NH3) and dimethyl ether (DME) blends were examined in this research project. The study investigates the autoignition characteristics by measuring ignition delay times across a range of gas temperatures from 621 to 725 K and at pressures of 5, 10, and 20 bar by using a rapid compression machine (RCM). Ignition delays of NH3/DME blends, with DME concentrations in the fuel mixture ranging from 0 to 50%, were measured, simulated, and compared with JP-8 and JP-5 fuel ignition delays. At a pressure of 20 bar, blends containing 30 and 50% DME concentrations exhibited ignition delay times similar to those of JP-8 and JP-5. Furthermore, the fuel blend with a 30% DME concentration showed similar ignition delays at the lower pressures of 5 and 10 bar. Several kinetic models were used to model the autoignition and compared with the measured data. Simulation results fairly matched the measured ignition delays. Through rigorous experimental verification, this comprehensive analysis evaluated the reliability of existing chemical models and paved the way for further studies on customized fuel blends, thereby contributing to the ongoing debate on sustainable energy alternatives.