Evolution patterns of the multiwavelength average particle size and particle number concentration of smoke aerosols under low-pressure conditions

Aircraft operations at high-altitude airports and during cruise phases are subjected to low-pressure environments, which significantly alter the physical and optical properties of smoke aerosols generated via cargo combustion, presenting severe challenges for conventional detection systems. Existing research has predominantly focused on macroscopic combustion parameters, while the evolutionary patterns of key microscopic parameters, such as the Sauter mean diameter (SMD) and particle number concentration, remain underexplored. This study aims to systematically elucidate the mechanisms underlying the evolution of multiwavelength smoke aerosol characteristics under low-pressure conditions, thereby providing theoretical support for enhancing the reliability of aviation-smoke detection.

This study established an integrated experimental platform based on a full-scale, dynamic-pressure and temperature-controlled chamber, capable of simulating a pressure range of 10–101 kPa. A triple-wavelength laser detection system—comprising three independent laser–detector pairs at 0.450 μm (blue), 0.532 μm (green), and 1.064 μm (infrared)—was designed and implemented. The optical path for each wavelength was independently calibrated, and the actual optical path lengths were precisely determined. The SMD and particle number concentration were retrieved using the multiwavelength extinction method and the Beer–Lambert law, combined with Mie scattering theory. Four representative fuels—beech wood, corrugated paper, n-heptane, and polyurethane—were selected to simulate typical cargo-compartment fire scenarios under smoldering and flaming conditions. Experiments were systematically conducted at three pressure levels: 90, 70, and 50 kPa. For each pressure condition, optical data were recorded at 1 s intervals across 1 500 repetitions and each set of experiments was repeated three times to ensure reproducibility. With a broad spectral span from blue to infrared wavelengths, the system provided enhanced sensitivity to particle size variations within the typical smoke aerosol range of 0.100–1.000 μm. The introduction of the third wavelength (green) served as an independent constraint, effectively reducing the common inversion multiplicity problem encountered in single-or dual-wavelength systems.

This study systematically revealed, to the best of our knowledge, for the first time, the differential responses of smoke parameters to pressure variations across combustion modes. 1) Under smoldering conditions, the SMD slightly increased with decreasing pressure (beech: 0.359→0.376 μm; paper: 0.292→0.318 μm), with variations being only < 0.020 μm, showing remarkable size stability. In contrast, flaming aerosols showed significant SMD reduction; n-heptane aerosols exhibited substantial variations exceeding 0.200 μm, while polyurethane aerosols varied < 0.050 μm, indicating higher pressure sensitivity for pure hydrocarbon fuels. 2) Regarding the particle concentration: smoldering smoke displayed a nonmonotonic trend (initial increase followed by decrease), inversely correlated with the optical power; the n-heptane flaming concentration continuously increased with decreasing pressure, whereas the polyurethane concentration decreased due to oxygen-limitation–induced pyrolysis suppression. 3) Method validation confirmed that dispersion values were < 10% for the triple-wavelength system, considerably enhancing the reliability of the particle size and concentration measurements under low-pressure conditions.

The evolution of smoke aerosol size and concentration in low-pressure environments is strongly governed by the combustion mode and fuel characteristics. Smoldering smoke exhibits notable size stability, whereas flaming smoke demonstrates significant pressure sensitivity, with fuel volatility and chemical structure being key influencing factors. The triple-wavelength extinction method, through multiwavelength collaborative constraints, effectively addresses the technical challenges of aerosol characterization at low pressures, providing crucial methodological support and a theoretical foundation for optimizing next-generation aviation smoke detection systems.