Comparison on ignition and combustion of AlMgB composite fuel and B fuel

Abstract

A comparison on ignition and combustion characteristics of AlMgB composite fuel and B fuel was performed in the stagnation O2/N2 mixture gases. A reduction of the ignition temperature from 700 °C (B) to 640 °C (AlMgB) in pure oxygen was found. Two ignition modes were distinctively identified, one was the gas-phase ignition with a green flame at high oxygen concentrations (60%–100%), the other was the surface ignition without a flame at low oxygen concentrations (20%–40%). At a constant oxygen concentration, ignition delay of AlMgB was shorter than that of B fuel. With increasing power density, the heating rate increased, resulting in a reduction of ignition delay, thus the difference of ignition delay between AlMgB and B fuels gradually narrowed. During combustion, the flame appearance, temperature, emission spectra, and the weight gain were measured. The results showed the occurrence of gas-phase combustion and surface combustion. The time-resolved emission spectra revealed that BO2 generated in the gas phases. At high oxygen concentrations, the gas-phase combustion demonstrated a yellow-green-incandescent multilayer flame structure. It was found that the peak temperatures of AlMgB were lower, the microexplosion was more frequent, the burn time was shorter, and the consumption of oxygen was less, compared with the counterpart B. With decreasing oxygen concentration, the peak combustion temperature reduced, the time to reach peak temperatures increased, and the weight gain decreased. The gas-phase combustion of AlMgB composite fuel transmitted more quickly to the surface combustion than B fuel. Finally, the ignition and combustion mechanism of AlMgB and B fuels was discussed, laser power density and oxygen concentration play crucial roles in determining the gas-phase or the surface ignition and combustion modes.