Ignition by electrical discharges
Project 5 is focused on the experimental examination of ignition by electrical discharges with respect to safety assessment in hazardous areas. In the first period, two important types of electrical discharges – non-equilibrium plasmas due to streamer discharges and thermal plasmas – shall be examined.
The energy of an examined electrical discharge is always near the minimum ignition energy. Using optical emission spectroscopy, detailed information about the temporal evolution of atomic oxygen and rotational and vibrational temperatures of molecular nitrogen can be yielded. Time and spatially resolved laser-induced fluorescence measurements of OH radicals will be performed yielding four subsequent images during one single ignition event. Hence, detailed information about flame kernel growth and flame velocities can be derived. In the case of ignition by non-equilibrium plasmas detailed examination of atomic oxygen using two-photon absorption laser-induced fluorescence will be used to examine the temporal evolution of this radical in the ignition volume. All these experiments are necessary a) in the case of thermal plasmas to determine the thermal losses towards the electrodes and b) in the case of non-equilibrium plasmas to develop a kinetic scheme of the chemical processes by streamer discharges.
Along with numerical simulations of these processes useful information about the ignition process induced by electrical discharges will be yielded.
Scientists
| Project leader |  Dr.-Ing. Detlev Markus |
| Researcher |  Stefan Essmann |
Publication single view
Title: |
Investigation of the ignition by repetitive streamer discharges using time-resolved OH LIF measurements |
|---|---|
| Author(s): | , , , and |
| Year: | 2011 |
| Month: | July |
| Day: | 24 |
| Book title: | 23rd International Colloquium on the Dynamics of Explosions and Reactive Systems |
| Conference place: | Irvine, Calif. |
| Conference date: | 24-29, July, 2011 |
| Abstract: | Non-equilibrium plasmas like in streamer discharges have a significant influence on ignition and combustion processes. The formation of electronically excited species using non-equilibrium plasmas can enhance burning velocities and reduce ignition delay times or autoignition temperatures, respectively. The promotion of the ignition and combustion processes is mainly explained by the production of atomic oxygen either by quenching electronically excited N2 or by the electron impact dissociation of O2. Streamer discharges occur at atmospheric pressures as the initial process of the temporal evolution of an electrical breakdown. Considering AC voltage with high frequency, streamer discharges occur only at the peak voltage. Depending on the electrode configuration, the discharge time is possibly too short for a complete breakdown within the electrode distance. Therefore, a spark breakdown can not occur. While a single streamer discharge has only minor influence on the combustible gas mixture concerning gas temperature and mixture composition, repetitive streamer discharges can ignite combustible/air mixtures. This has to be avoided in hazardous areas. The ignition of combustible/air mixtures by electrical sparks, which has been examined experimentally and numerically in detail, is mainly dominated by the local heating of the gas mixture. However, for a detailed understanding of the ignition by streamer discharges, it is essential to examine additionally the production of radicals in the non-equilibrium plasma and the accumulation of energy considering repetitive streamer discharges. A numerical simulation of the ignition processes requires more detailed experimental information. Therefore, this work presents experimental investigations of the ignition of H2/air mixtures by repetitive streamer discharges at atmospheric pressure using a rod/plane electrode configuration with alternating current at high frequency (f = 740 kHz). Time-resolved high speed imaging of laser induced fluorescence (LIF) of OH radicals after the ignition of different H2/air mixtures in the mixture range from 10 Vol.% to 40 Vol.% H2 in air is performed to examine the temporal flame kernel growth after ignition. The experimental results are compared to numerical simulations using a one-dimensional approximation, which will be used in future work to yield detailed information about the energy densities inside the ignition volume. |