The theory of the ionic wind is developed for flame ions travelling towards electrodes of various configurations so that entrainment as well as main stream gas velocities can be predicted. It is shown that, by confining entrainment to specified regions, large flow velocities can be induced at the flame itself, where they can be used to modify a variety of combustion processes. Theoretical maximum values of the flow parameters are calculated for several configurations likely to be of practical use and these are compared with results of experiment. The experiments are designed to test the general theory and to determine to what extent the theoretically deduced maxima are altered by inevitable practical complications such as entrainment of hot gas, deposition of soot and other specks on the electrodes and convergence of lines of force on to individual strands of gauze-electrodes. The potentialities of varying parameters such as geometry, temperature, pressure and composition as well as super-imposing magnetic fields are also examined. A variety of practical examples is considered in the light of this theory. Experiments confirm that confined entrainment can be used to aerate diffusion flames in an accurately controllable manner without risking flash-back or requiring an air supply, metering and mixing systems. Similarly, it is demonstrated that combustion intensity can be increased by field-induced recirculation of hot products, thereby minimizing random turbulence and heat losses to the large obstacles usually employed for this purpose.
Previous studies have shown that carbon particles in flames, being all charged, can be manipulated by electric fields so as to control their residence time and hence size, as well as their rate of formation and all the parameters of deposition on the electrodes―mass, position and form of aggregate. To gain further insight into the fundamental processes, particularly those occurring during the very early (nucleation) stages, measurements of particle mobility and detailed size analyses are now added to those of current and mass deposition, when a variable potential is applied across seeded and unseeded flat, counter-flow diffusion flames. It is found that particles which account for the mass deposited have mobilities ranging from 10 –3 to 3 x 10 –2 cm 2 s –1 V –1 , depending on the applied potential. This allows their trajectories in a field to be calculated and also shows, when taken together with size measurements from electron micrographs, that each carries unit charge over practically the entire experimental range, the majority of the current being carried by smaller charge carriers. Among the conclusions are that both growth of carbon on flame ions and initially neutral growth followed by attachment-charging do occur. The rate of mass deposition is determined entirely by particles following the latter course because, in the presence of a field, the former have a very much shorter residence time available for growth in the pyrolysis zone― at high field strengths these times are indeed too short for growth to a measurable size. The theory of the growth of the larger, initially uncharged particles, is developed in an appendix. It is shown that they acquire charge predominantly by diffusion of ions, although thermionic emission can become important under certain conditions. All the trends recorded in this and in earlier work, including the variation of particle size, mass and number rate of collection are accounted for.
The possibility of varying at will the rate of burning of solid propellants, after ignition, by the use of electric fields, is considered. Two methods seem possible; varying the normal burning rate and varying the rate of flame spread over surfaces. The latter, which can be used to control the total consumption rate by varying the rate at which ‘internal area’ can be opened up, is shown to be by far the most promising. Ionic winds can be used to increase it by making the propellant one electrode, or decrease it by using an electrode contacting the flame, in an enclosed system, so as to maintain the propellant surface cool by a flow of entrained air. In simple systems at atmospheric pressure increases of about 200 fold and decreases of approx. 10 fold, with respect to the unperturbed value, are achieved. Theory indicates that larger effects should be possible at the higher pressures relevant to combustion in rockets.
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