Properties of plasmas generated by electrical breakdown in flames

Abstract

Properties of electrical discharge in flames and influence of plasma electrons on gas neutrals are investigated by making use of the ionization cross section of air. An analytical expression of air ionization rate is obtained from tabulated data of the ionization cross section of oxygen and nitrogen, and is compared with air ionization rate measured with the applied electric field. The influence of gas temperature on electrical discharge properties is investigated by making use of electron energy gain in the electric field. Electrical breakdown occurs whenever ionization of neutrals dominates the electron attachment of oxygen molecules. It is found that the breakdown electric field in flames is inversely proportional to the flame temperature Tg, thereby easily generating plasmas in flames. A swarm of low-energy electrons in flames would allow a significant population of electronically excited states of flame molecules to be formed. The analysis shows that the electronic excitation of flame molecules may also considerably reduce the breakdown field. Plasma electrons generate atomic oxygen by the electron attachment of oxygen molecules in high-pressure flames. An example calculation shows that more than 63% of oxygen molecules are converted into atoms within 760 ms dwelling time for the plasma with density of np=1013 cm−3 and temperature of Te≈2.5 eV. Oxygen atoms are the most reactive radicals in flames for material oxidation.