The turbulent flame structure and flame length of fully-modulated diffusion flames was examined over a range of pulsing frequencies, injection flow rates, and duty-cycles. An injection system employing an electronically-controlled solenoid value was used to discharge puffs of unheated natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure. For small injected volumes and short injection times, compact, puff-like structures with a short flame length were generated. More elongated "cigar-shaped" structures, with a longer flame length closer to that of steady-state flames, resulted from longer injection times and larger injected volumes. An injection parameter which characterizes the transition from puff-like to cigar-shaped flame behavior is presemed. For puffs, an increase in duty-cycle generally led to an increase in flame length. This increase was less for cigar-shaped flames. The downstream location of the puff-like flame structures increased roughly with time to the 1/2 power, in agreement with buoyant thermals and starting jets.
The turbulent flame structure and flame length of fully-modulated diffusion flames was examined experimentally over a range of pulsing frequencies, injection flow rates, and duty-cycles. A pulsed injection system employing an electronically-controlled solenoid value was used to discharge puffs of unheated natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure, depending on the injection conditions. For small injection volumes and short injection times, compact, puff-like structures were generated. More elongated "cigar-shaped" structures resulted from longer injection times and larger injection volumes. A scaling parameter which characterizes the transition from pufflike to cigar-shaped flame behavior is presented. The flame length for widely-spaced puffs was seen to scale with the cube root of the injection volume; this scaling with injection volume became considerably weaker with the transition to cigar-shaped flames. For puffs, an increase in duty-cycle generally lead to an increase in flame length. This increase was less for cigar-shaped and transitional flames. The transition from discrete-puff to interacting puff behavior is characterized in terms of a scaling parameter developed previously.
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