Jet diffusion flames are modulated acoustically either to improve combustion efficiency or to increase soot formation traditionally. In this study, a double-flame structure was generated with a periodic flow of an ethylene-nitrogen mixture by means of acoustic modulation to study carbon nanomaterial synthesis. The formation of carbon nano-onions (CNOs) was significantly enhanced by acoustic excitation near the natural flickering frequency and the acoustical resonant frequency. The suitable range of fuel concentration for synthesis was wider at these two frequencies than at other frequencies. Furthermore, carbon nanotubes instead of CNOs formed as the fuel concentration was decreased below 20%. The available heat and carbon source fluctuated in the radial direction, which increased the radial synthetical region. This study also revealed the relationship between formation of carbon nanomaterial and nitrogen dilution, which may extend the potential of acoustic modulation in the field of flame synthesis of carbon nanomaterials.
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