Characteristics of Ignition Energy and Pressure Rise of Ammonia/Oxygen/Inert-gas Mixtures by a Laser-Induced Breakdown Spark Ignition

The influences of incident energy magnitude and the thermal and transport properties of inert gases on the ignitability and combustion strength of ammonia/oxygen/inert-gas mixtures have been experimentally investigated. Laser-induced spark ignition was adopted because it allows easy adjustment of incident energy and generates plasma without requiring electrode contact. The influences of inert gas and incident energy on minimum ignition energy (MIE), flammable range, peak overpressure (Pmax), the time to reach Pmax after the laser was fired (tpmax), pressure increasing rate (dP/dt), and index of combustion strength (Kg value) were systematically investigated. The thermal diffusivity primarily influences ignition characteristics (MIE and flammable range), because a certain period of retention of a hot kernel immediately after the incident energy is provided is required to initiate ignition. Since incident energy compensates for the lacking energy needed to initiate the combustion reaction, its influence becomes more apparent in mixtures with high thermal diffusivity or large specific heat. On the other hand, specific heat primary influences the maximum pressure rise value and the flame temperature. If sufficient energy to sustain the combustion reaction is provided, thermal diffusivity contributes to the rapid increase in the pressure rise because it facilitates the quick transport of heat generated by the combustion reaction throughout the entire the premixed gas. The influence of inert gas on fluid-dynamic phenomena, such as the formation of a third lobe and transient flame velocity behavior, was also explained based on the aforementioned mechanism. The dependence of pressure rise on the equivalence ratio was closely related the adiabatic flame temperature for each composition. In summary, the key properties affecting the ignition and combustion characteristics are thermal diffusivity and specific heat. However, which of these physical properties has a more dominant influence depends on whether it affects ignitability or combustion strength.