Effects of flame structure and flame strain on the growth region of carbon nanotubes in counter-flow diffusion flame

Abstract

The improvement of the carbon nanotube (CNT) synthesis control in flames requires the understanding of the effects of flame structure towards the catalytic growth. A preliminary prediction of growth region in different flame configuration is useful to efficiently improve the synthesis process. A growth rate model based on nickel catalyst that is coupled with a flame model based on computational fluid dynamics (CFD) is employed to predict the synthesized CNT length at different regions within the methane-ethylene diffusion flame with counter-flow configuration. At particle scale, the previously developed particle-scale model was successfully validated against a CNT length measurement in carbon vapour deposition (CVD) experiment. At flame scale, a satisfactory agreement was achieved between the predicted and the measured temperature along the flame centreline. The multi-scale model successfully predicts the growth region of CNT on the rich side of the reaction zone where the temperature range of 1200 K to 1500 K with methane mass fraction of 0.05 (nominal) provides a suitable growth environment for the CNT. Compared to the experimental observation, the predicted region of high growth is accurate within 1 mm. Despite the increase in temperature and carbon precursor concentration in opposing directions of the growth region, the growth rate reduces. The main finding is that the increase in flame strain rate results in a flat growth region which is favourable for efficient scaled-up production. The flat growth region results from the dominance of the inertial effects over the buoyancy effects on the reacting flow field. At high strain rate, the width of high temperature region within the shear layer reduces and the growth region is shifted towards the fuel side in response to the shift in the high temperature location towards the same direction. Interestingly, the parabolic trend of CNT growth rate within the counter diffusion flame is dictated by the temperature distribution while the spatial distribution of high yield region is determined by the flame structure.