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In this study, methane–ethylene jet diffusion flames modulated by acoustic excitation in an atmospheric environment were used to investigate the effects of acoustic excitation frequency and mixed fuel on nanomaterial formation. Acoustic output power was maintained at a constant value of 10 W, while the acoustic excitation frequency was varied (f = 0–90 Hz). The results show that the flame could not be stabilized on the port when the ethylene volume concentration (ΩE) was less than 40% at f = 10 Hz, or when ΩE = 0% (i.e., pure methane) at f = 90 Hz. The reason for this is that the flame had a low intensity and was extinguished by the entrained air due to acoustic modulation. Without acoustic excitation (f = 0 Hz), the flame was comprised of a single-layer structure for all values of ΩE, and almost no carbon nanomaterials were synthesized. However, with acoustic excitation, a double-layer flame structure was generated for frequencies close to both the natural flickering frequency and the acoustically resonant frequency. This double-layer flame structure provided a favorable flame environment for the fabrication of carbon nanomaterials. Consequently, the synthesis of carbon nano-onions was significantly enhanced by acoustic excitation near both the natural flickering frequency and the acoustically resonant frequency. At f = 20 Hz (near the natural flickering frequency) for 0% ≤ ΩE ≤ 100%, a quantity of carbon nano-onions (CNOs) piled like bunches of grapes was obtained as a result of improved mixing of the fuel with ambient air. High-density CNOs were also produced at f = 70 Hz (close to the acoustically resonant frequency) for 40% ≤ ΩE ≤ 100%. Furthermore, carbon nanotubes (CNTs) were synthesized only at 80 Hz for ΩE = 0%. The suitable temperature range for the synthesis of CNTs was slightly higher than that for the formation of CNOs (about 600 °C for CNTs; 510–600 °C for CNOs).

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It is important to identify the dominant factors for governing the growth of carbon nanotubes (CNTs) and nano-onions (CNOs). A diffusion flame of a gas mixture of methane-ethylene was used as the carbon and heat sources and Ni as the catalyst for the synthesis of CNTs and CNOs. The effects of CH4/C2H4 ratio in the fuel side and oxygen concentration in the oxidizer side for counterflow diffusion flames were investigated. It was found that oxygen concentration can greatly affect the morphologies of synthesized products with a threshold of 30% distinguishing the formation of CNO or CNT. CNOs were fabricated at higher oxygen concentrations (30%, 40%, 50%), and CNTs were synthesized only at lower oxygen concentrations (21%, 30%). The fuel composition has minor effects on the morphologies except for the threshold value of oxygen concentration (30%). More carbon sources are required for the synthesis of CNOs than for CNTs, but the temperature requirements are similar (1140-1160 K for CNTs, 1070-1160 K for CNOs). The nanostructures were synthesized as long as the fuel concentration is sufficiently high regardless of the oxygen concentration. Higher fabrication tendency was found for ethylene as fuel to form nanostructures than for methane.

Fire tests of fireproof and non-fireproof curtains were conducted to investigate the cooling performance of the proposed water film system. The experimental results showed that although commercial fireproof curtains without a water film system had good flame resistance, they had limited heat resistance. The maximum temperature on the unexposed surface of the tested commercial fireproof curtains without a water film system reached 693 °C, and the curtains failed in 30 min. In the cases of curtains with a water film system, the temperature of the unexposed surface was able to remain below 45 °C for the fireproof curtain and 55 °C for the non-fireproof curtain. The integrity of both curtains was conserved for the entire 60-min test duration. Using the proposed water film system, the heat resistance and fire integrity of curtains were greatly improved. ARTICLE HISTORY