Effects of structure and hydrodynamics on the sooting behavior of spherical microgravity diffusion flames

Sunderland, Peter B.

doi:10.1016/s0010-2180(02)00424-8

Cited by 47 publications

(38 citation statements)

References 18 publications

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“…Under normal gravity conditions, a SWNT forming carbon arc discharge looks very much like a sooting diffusion flame 38 as shown by the first still frame in Figure 5 taken from a video of a 2.1 second discharge from the capacitively powered mini arc in normal gravity. Measurements confirm the rapid development of convective flow with a velocity of several meters per second above the discharge.…”

Section: Resultsmentioning

confidence: 99%

Miniature Arcs for Synthesis of Carbon Nanotubes in Microgravity

Alford

Mason

Feikema

2006

44th AIAA Aerospace Sciences Meeting and Exhibit

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Although many methods are available for producing single-walled carbon nanotubes (SWNTs), the conventional carbon arc process remains the most popular due to its simplicity and large production rate. In the carbon arc, SWNTs are catalytically synthesized by rapidly evaporating a graphite anode impregnated with Ni/Y metal catalyst from which the nanotubes grow in an inert atmosphere. However, high temperatures inside the carbon arc generate strong buoyancy driven convection, and it is hypothesized that the non-uniform environment created by this flow has a large effect on the growth and morphology of the SWNTs. To study the effect of buoyancy on the arc process, a miniature carbon arc apparatus was developed to synthesize SWNTs in a microgravity environment substantially free from these strong convective flows. The reactor was operated for either 2.2-or 5-seconds during free-fall in the drop towers at the NASA Glenn Research Center. Two apparatus designs differing mainly in their production rate and power capacity were investigated. The first consisted of a miniaturized carbon arc employing a 1 mm diameter graphite anode and powered by a 0.54 F capacitor bank charged to 65 V. The second, larger apparatus employed a 4 mm diameter anode and was powered by a portable battery pack capable of providing in excess of 300 amps at 30 volts to the arc for the duration of a 5-second drop. Initial results indicated that transient heating is a very large effect in the short-duration drop tower carbon arcs, and thermal equilibrium of the arc plasma, buffer gas, and apparatus was not attained during the short microgravity periods. In addition, removal of the buoyant convection by the microgravity now allowed clear observation of large jets of evaporated carbon vapor streaming from the anode and mixing with the inert buffer gas. The initial mixing of these jets with the cold buffer gas combined with the thermal transient made it difficult to establish a uniform high temperature environment around the arc in the 2.1-to 5-second microgravity time interval, and even with a very high-powered arc, the arc region was cooler than in continuously operated arcs. Despite these difficulties, the miniature arc produced SWNTs in microgravity. However, given the large thermal transient to overcome, no dramatic difference in sample yield or composition was noted between normal gravity and 2.2-and 5-second long microgravity runs.

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Section: Resultsmentioning

confidence: 99%

Miniature Arcs for Synthesis of Carbon Nanotubes in Microgravity

Alford

Mason

Feikema

2006

44th AIAA Aerospace Sciences Meeting and Exhibit

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“…Further details of the apparatus are contained in Ref. 22. These experiments were conducted in microgravity in the NASA Glenn 2.2 s drop tower.…”

Section: Methodsmentioning

confidence: 99%

Effects of C/O Ratio and Temperature on Sooting Limits of Spherical Diffusion Flames

Lecoustre

Sunderland

Chao

et al. 2008

46th AIAA Aerospace Sciences Meeting and Exhibit

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Limiting conditions for soot particle inception in spherical diffusion flames were investigated numerically. The flames were modeled using a one-dimensional, time accurate diffusion flame code with detailed chemistry and transport and an optically thick radiation model. Seventeen normal and inverse flames were considered, covering a wide range of stoichiometric mixture fraction, adiabatic flame temperature, residence time and scalar dissipation rate. These flames were previously observed to reach their sooting limits after 2 s of microgravity. Sooting-limit diffusion flames with scalar dissipation rate lower than 2 s -1 were found to have temperatures near 1400 K where C/O = 0.51, whereas flames with greater scalar dissipation rate required increased temperatures. This finding was valid across a broad range of fuel and oxidizer compositions and convection directions.

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“…The experimental apparatus is described in detail in Sunderland et al [16,17]. The burner reactant flows from a storage tank through a solenoid valve, a metering valve, a mass flowmeter, and a second solenoid valve into the spherical burner.…”

Section: Methodsmentioning

confidence: 99%

“…Both normal and inverse flames were considered here. In normal flames the pressure vessel contained an atmosphere of oxidizer while in inverse flames the atmosphere contained fuel [16,17]. Various levels of nitrogen dilution were considered to obtain conditions where flames would ignite and then extinguish within the 2.2 s of available microgravity time.…”

Section: Methodsmentioning

confidence: 99%

Radiative extinction of gaseous spherical diffusion flames in microgravity

Santa

Chao

Sunderland

et al. 2007

Combustion and Flame

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Radiative extinction of spherical diffusion flames was investigated experimentally and numerically. The experiments involved microgravity spherical diffusion flames burning ethylene and propane at 0.98 bar. Both normal (fuel flowing into oxidizer) and inverse (oxidizer flowing into fuel) flames were studied, with nitrogen supplied to either the fuel or the oxygen. Flame conditions were chosen to ensure that the flames extinguished within the 2.2 s of available test time; thus extinction occurred during unsteady flame conditions. Diagnostics included color video and thin-filament pyrometry. The computations, which simulated flow from a porous sphere into a quiescent environment, included detailed chemistry, transport and radiation, and yielded transient results. Radiative extinction was observed experimentally and simulated numerically. Extinction time, peak temperature, and radiative loss fraction were found to be independent of flow rate except at very low flow rates. Radiative heat loss was dominated by the combustion products downstream of the flame and was found to scale with flame surface area, not volume. For large transient flames the heat release rate also scaled with surface area and thus the radiative loss fraction was largely independent of flow rate. Peak temperatures at extinction onset were about 1100 K, which is significantly lower than for kinetic extinction. One observation of this work is that while radiative heat losses can drive transient extinction, this is not because radiative losses are increasing with time (flame size) but rather because the heat release rate is falling off as the temperature drops.

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Effects of structure and hydrodynamics on the sooting behavior of spherical microgravity diffusion flames

Abstract: We have examined the sooting behavior of spherical microgravity diffusion flames burning ethylene at atmospheric pressure in the NASA Glenn 2.2-second drop tower.

Cited by 47 publications

References 18 publications

Miniature Arcs for Synthesis of Carbon Nanotubes in Microgravity

Miniature Arcs for Synthesis of Carbon Nanotubes in Microgravity

Effects of C/O Ratio and Temperature on Sooting Limits of Spherical Diffusion Flames

Radiative extinction of gaseous spherical diffusion flames in microgravity

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