Electric - field - induced flame speed modification

A rather large amount of works has been reported on the various effects of electric field on a wide variety of flames. However, the mechanisms responsible for the dramatic field-induced effects on the size and shape of the inner cone of some hydrocarbon flames are still not well understood. Bradley provides a good overview of this subject.

Electric - field - induced modifications of flame geometry have been known for some time, the first report being published by Chattock in 1889. Since then, many aspects of the effects of externally applied fields have been investigated. For example, increases in flame blow-off flow rate due to the application of external fields in both diffusion and premixed methane-air flames have been measured. Similar increases in flame stability have also been observed at fields sufficient to produce a corona discharge within the flame. Improved heat transfer to solid surfaces due to externally applied fields has been reported. Increased burning velocities for hydrocarbon flames in direct current (DC) fields and elevated temperatures for flames in microwave fields have been similarly established. It has also been shown that soot formation in diffusion flame is diminished by application of DC external field. Flame extinction limits in premixed flame are also perturbed by DC fields. The most extensive efforts to construct and test theoretical model sufficient to predict the maximum practical effects of electric fields on flames are contained in the publications by Lawton and co-workers. Recently, a kinetic model simulating the effects of electric fields in premixed methane flames has been used to simulate microgravity in small diffusion flames.

The effects of pulsed and continuous direct current electric fields on the reaction zones of premixed propane/air flames have been investigated using several types of experimental measurements. All observed effects on the flame are dependent on the applied voltage polarity, indicating that negatively charged flame species do not play a role in the perturbation of the reaction zone. Experiments characterize the electric-field-induced modifications of the shape and size of the inner cone, and the concomitant changes in the temperature profiles of flames with equivalence ratios between 0.7 and 0.8 are also reported. High-speed two-dimensional imaging of the flame response to a pulsed direct current voltage shows that the unperturbed conical flame front (laminar flow) is driven into wrinkled laminar flame (cellular) geometry on a time scale of the order of 5 ms. Temperature distributions derived from pyrometry measurements in flames perturbed by continuous direct current field show similar large changes in the reaction zone geometry, with no change in maximum flame temperature. All measurements are consistent with the observed flame perturbations being a fluid mechanical response to the applied field brought about by forcing positive flame ions counter to the flow. The resulting electric pressure decreases. The observed increases in flame speed and the flame front trend toward turbulence can be described in terms of the wrinkling of flame front and concomitant increase in reaction sheet area. This effect is a potentially attractive means of controlling flame fluid mechanical characteristics. The observed effects require a minimal input of electrical power due to the better electric field coupling in the present experiments to the previous studies.

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