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National Advisory Committee for Aeronautics, Report - Formation and Combustion of Smoke in Laminar Flames

The nature and formation of smoke and its combustion were
investigated. Smoke, which consists of tiny mesomorphous
crystals tightly packed into popcorn-ball—like particles that ag-
glomerate to give filaments, was found to contain about 5 percent
of the hydrogen originally present in the fuel.

Factors aflecting smoke formation were studied in both dizfu-
sionflames and prede Bunsen flames. The rates at which
hydrocarbon types could be burned without smoking decreased
in the approximate order: n-paraflins>isoparafins>cyclo-
paraffins Z olefins > cycloolefins 2 diolefins ~ alkynes > n—
alkylbenzenes. It is suggested that smoking tendency increases
with increasing stability of the carbon skeleton of the molecule,
as determined by relative bond strengths. Smoking tendencies
within the n—paraflin series increasedwith molecular weight, pre-
sumably because of changes in diffusion rates. Increasing the
laminar airflow around a flame decreased its smoking tendency,
but eventually a limiting smoke-free fuel rate was found for dif-
fusion flames and a limiting primary fuel-air ratio was found
for Bunsen flames. Moreover, the smoking-pointfuel—air ratio
of a premisedflame deprived of secondary air was only half that
of a similar flame burning in “st' ” air. Oxygen-enrichment
of the surrounding air, or substitution of argon for nitrogen in
this “air,” decreased smoking tendency for fuels of moderate
smoking tendency. For some fuels of greater smoking tendency,
however, moderate oxygen-enrichment or argon substitution (both
of which increase flame temperature) increased smoking tend—
ency. Increasing the fuel rate to a Bunsen flame decreased the
smoking-point fuel-air ratio, while preheating the fuel or fuel-
air mixture had no efl'ect. For difiusionjlames, smoking tend-
ency was linearly related to reciprocal pressure for the range }£
to 20 atmospheres, but for prmiaedflames pressure is reported
to have no ezfect; these facts again suggest the importance of
difi'usion rates.