The maxima limiting all practical effects of the movement of flame ions in electric fields are shown to depend on the current densities available. The theory of the electric field and space charge distributions inside and outside the flame is developed, checked experimentally, and used to deduce such maxima. Two factors are identified as limiting current densities; the rate of ion generation per unit flame area and the space charge-induced breakdown at the electrodes. The latter is shown to be ultimately limiting and the theory is used to calculate numerical values for all practical maxima. The former is limiting only in some flat laminar flames parallel to electrodes, but it leads to a method of measuring rates of ion generation in flames. The method is developed experimentally on the basis of the theory and applied to a series of hydrocarbon/air and hydrogen/hydrocarbon/air flames. As an example of its use, the results are applied to calculations of activation energies and orders of the ion-forming process.
Field‐based experiments were conducted to evaluate the fate and infectivity of the entomopathogenic fungus Metarhizium anisopliae var. acridum (Deuteromycotina: Hyphomycetes) in grasshopper cadavers in the Sahel. Unlike uninfected cadavers, which were rapidly scavenged, those infected with the fungus persisted in the environment for a number of weeks. The environmental factor most associated with cadaver disappearance was rainfall. The high environmental humidity associated with rainfall was also required for sporulation of the fungus on host cadavers, although the likelihood of sporulation differed between microsites. Characteristics of the infection profile from infective cadavers were investigated by the sequential exposure of uninfected hosts to sporulating cadavers in field cages. This experiment revealed that cadavers remained infective for > 30 days, with the net infectivity changing through time. The most likely explanation for these changes is climatic influences on both the fungus and host. High humidity was not required for infection. A measurement of the transmission coefficient between healthy hosts and sporulating cadavers in the field was obtained at a realistic density of infectious cadavers. This revealed a figure of 0.45 m2 day−1. Overall, these experiments show that following host death, M. anisopliae var. acridum can be persistent in the environment, sporulate on host cadavers and reinfect new hosts at a realistically low field density, although at least in arid or semi‐arid areas, rainfall may be critical to the horizontal transmission of this pathogen.
The analysis is concerned with the steady motion of dc arcs which are struck between a central cathode and the walls of a coaxial, cylindrical enclosure. There is an axial flow of gas and a steady, cylindrically symmetrical, applied magnetic field with both axial and radial components. It is shown that, under conditions such that the arc motion is controlled by the column rather than the electrode interactions, the motion can be described by two simultaneous differential equations. The boundary conditions are also considered. Solutions are obtained for several special cases in which the gas flow velocity and/or the radial component of the field are small, but not negligible, quantities. Such solutions are directly relevant to arc heater applications, for example. From the analysis it is clear that very great control of the arc position and rotation within the enclosure can be obtained with even the simplest field geometry, namely a constant axial magnetic field.
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.
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