Deflagration-to-Detonation Transition of DME/NH₃ Mixtures in a Microchannel: A Computational and Experimental Investigation

Abstract

Ammonia is one of the most promising carbon-free energy carriers, but its slow reaction kinetics, long ignition delay time, and narrow flammability range make direct detonation initiation difficult and limit its use in pressure-gain combustion systems. Reactive co-fuels offer a possible way to overcome these challenges. However, the deflagration-to-detonation transition (DDT) behavior of ammonia-dimethyl ether (NH₃/DME) mixtures has not yet been experimentally investigated, despite DME’s well-established role as a strong low-temperature ignition promoter. This thesis addresses that gap through a combined computational and experimental study of NH₃/DME/O₂/Ar mixtures in a microchannel. NASA CEA and Cantera are first used together to evaluate different fuel compositions based on detonability, ignitability, and equilibrium emissions, narrowing a broad design space into a practical experimental matrix. To support reliable diagnostics across multiple observation windows, an automated traversal system was developed to maintain optical alignment during imaging. Flame propagation and wave-structure evolution were then captured using synchronized high-speed imaging and shadowgraphy. Results at ϕ = 1.0 show repeatable DDT in the 90/10 and 80/20 DME/NH₃ blends, while the 75/25 composition marks the practical ignition limit of the current experimental configuration. Compositions exceeding approximately 25% ammonia failed to ignite under the tested conditions. Across all ignitable blends, increasing ammonia content progressively delays both first-stage ignition and DDT onset, while also lowering peak flame-tip velocity. Despite this reduction in reactivity, successful transition is still consistently achieved in the ignitable cases. This demonstrates that ammonia can reliably undergo the full DDT process when mixed with DME. These results show that DME acts as an effective enabling co-fuel, offsetting ammonia’s reactivity limitations and bringing meaningful NH₃ fractions into the detonable regime. Extending this work to fuel-rich NH₃/DME mixtures, where computational analysis predicts higher detonation velocities and potentially earlier DDT onset, is a promising next step for accessing stronger detonation regimes and even higher NH₃ fractions. Overall , this thesis establishes NH₃/DME blends as a promising mixture for detonation-based combustion and supports ammonia’s role as a practical carbon-free fuel for pressure-gain propulsion and power systems.