Burning of aluminum particles assisted by selective energy coupling with a microwave plasma torch

Aiming at the concept of metal fuel in external-combustion engines or powder-fueled ramjets, this work designs a microwave plasma actuator to achieve fast ignition and stable combustion of micron-sized aluminum particles. The compact design of the ¼-λ resonant cavity and high-quality factor (QF) allow the microwave actuator to generate a stable air plasma torch with a power consumption of approximately 120 W. Spectral fitting of the excited nitrogen molecules suggests a vibrational temperature exceeding 4000 K and a rotational temperature above 2000 K. Under cold conditions, particle image velocimetry (PIV) is used to map the velocity profile of two-phase flows and illuminate aluminum particles to estimate their number density. The plasma torch can ignite both well-dispersed aluminum particles and dense powder jets with a number density of up to 50 per cubic millimeter. At lower particle number densities, a single burning particle sustained in the microwave field exhibits cyan emission due to intense Al atomic spectra and AlO molecular bands. The Al atomic signal is expected to be generated from Al plasma, and fitting AlO spectra in the B-X (Δv = -1) band yields a high temperature of ∼4280 K. This indicates selective coupling of microwave energy with the condensed phase and localized aluminum flames, due to superfocusing of microwave radiation on extreme subwavelength scales. When the particle number density is increased, the burning powder jet produces a strong luminance and an estimated thermal power close to 1 kW, while the fitted temperatures of condensed phase and AlO gas phase are equivalent to be ∼3300 K. The flame of the dense particle jet exhibits a structure where a cyan plasma core is enveloped by a large-scale yellow flame column. Finally, the microwave power is increased to 150 W to facilitate a CO₂ plasma torch, which preliminarily enables stable carbon-negative combustion of aluminum particles with CO₂ and ambient air.