Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive Krypton 85. We find that the general morphology of a positive streamer discharge in high purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding Krypton 85, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates. Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.Comment: 30 pages, 13 figure

Plasma supported combustion

Abstract Oxidation of molecular hydrogen and different hydrocarbons in stoichiometric mixtures with air and oxygen in the pulsed nanosecond discharges was studied at room temperature, and the detailed kinetics of the process has been numerically investigated. In the discharge afterglow, the reactions including electron-excited particles play a dominant role for the time up to 100 ns, ion-molecular reactions-for the time of microsecond range, and reactions including radicals mostly contribute for the time interval of several milliseconds. The principal role of processes with formation of excited components that support the development of the chain mechanism of oxidation has been shown. The spatial uniformity of the gas-mixture combustion initiated by a high-voltage nanosecond volume discharge is investigated at gas pressures of 0.3-2.4 atm and temperatures of 1000-2250 K. The self-ignition time and the time of discharge-induced ignition are determined. It is found that the discharge significantly (by 600 K) decreases the ignition temperature with very low energy in the discharge ($10 À2 J/cm 3 ). The influence of gas excitation by a pulsed nanosecond discharge with a high-voltage pulse amplitude up to 25 kV on the properties of a premixed propane-air flame has been investigated over a wide range of the equivalence ratios (0.4-5). It was experimentally found that the flameÕs blow-off velocity increased more than twice at a discharge energy input less than 1% of the burner power. Efficient production of active radicals under the action of a barrier discharge has been observed. The increase in the flameÕs propagation velocity is explained by the production of atomic oxygen in a discharge by the quenching of electronically excited molecular nitrogen N 2 and the dissociation of molecular oxygen on electron-impact. A numerical model has been developed, which describes the influence of pulsed electric discharges on the ignition, combustion, and flame propagation

Deflagration-to-detonation transition under initiation by high-voltage nanosecond discharges

Experimental study of detonation initiation by high-voltage pulsed gas discharges has been performed in three detonation tubes. It was shown in the experiments that distributed nanosecond discharge is significantly more efficient for detonation initiation than localized microsecond discharge of comparable energy. For a detailed experimental study of deflagration-to-detonation transition (DDT), a detonation tube with a single discharge cell and nanosecond initiation has been assembled. Optical observation of the discharge development, ignition, and DDT process was performed with an intensified charge-coupled device (ICCD) camera. A mechanism of detonation initiation by high-voltage nanosecond discharges has been proposed

Deflagration-to-Detonation transition under initiation by high-voltage nanosecond discharges

Experimental study of detonation initiation by high-voltage pulsed gas discharges has been performed in three detonation tubes. It was shown in the experiments that distributed nanosecond discharge is significantly more efficient for detonation initiation than localized microsecond discharge of comparable energy. For a detailed experimental study of deflagration-to-detonation transition (DDT), a detonation tube with a single discharge cell and nanosecond initiation has been assembled. Optical observation of the discharge development, ignition, and DDT process was performed with an intensified charge-coupled device (ICCD) camera. A mechanism of detonation initiation by high-voltage nanosecond discharges has been proposed

Effect of High-Voltage Pulsed Discharges on Deflagration to Detonation Transition

An experimental sutdy of ignition and detonation initiation by two different kinds of high-voltage pulsed gas discharge has been performed in two smooth detonation tubes. The experiments were carried out at pressures ranging from 0.15 to 1 bar in various gaseous stoichiometric mixtures. In the first setup, a distributed nonequilibrium nanosecond discharge was used for mixture excitation and ignition. In the second setup, a localized microsecond pulsed spark discharge with a stored energy of 14 J developed. The electrical parameters of the discharges, ignition delay time, flame front, and shock wave velocities were measured in the experiments. ..