Plasma-assisted combustion in a miniature microwave plasma torch applicator

Abstract

Summary form only given. A compact microwave plasma torch has been designed and experimentally evaluated for operation in both plasma-only and plasma-assisted combustion modes. The torch is designed to be light and to operate at atmospheric pressure with a torch discharge size of less than 1 mm in diameter. The potential applications of the torch are for materials synthesis, material cutting and welding, and various surface treatments. The operation of the torch in a combustion mode with hydrocarbon gases burning with oxygen or air is investigated with microwave power applied to modify the combustion process. The objective of this investigation is to quantify the changes in the combustion process as microwave power is applied to create or intensify the discharge. This torch employs an open ended coaxial structure of 12 mm outer diameter with the discharge located at the tip of the center conductor that is 5 mm in diameter. Microwave power at 2.45 GHz is coupled into the torch applicator at power levels of 10's to 100's of watts. The discharge is formed at atmospheric pressure where the feed gas flows through a nozzle hole of 200-500 mum diameter located at the end of the applicator center conductor. Two nozzle configurations studied include a water-cooled brass nozzle and a ceramic nozzle. The microwave plasma torch is experimentally evaluated with a variety of feed gas mixtures including argon, mixtures of argon with hydrogen, argon with nitrogen, methane and oxygen, and selected other hydrocarbon gases mixed with oxygen and air. This torch maintains discharges over a wide range of flows from diffusional flow for gentle surface processing to high velocity flow approaching supersonic velocities. Diagnostic measurements performed include (1) gas temperature measured by optical emission spectroscopy (OES), (2) discharge power densities and, (3) discharge volume and size. These measurements are performed versus absorbed microwave power, gas flow rate and gas mixture composition. By using spatially resolved OES measurements, the temperature profile of the discharge is determined. The discharge size for absorbed microwave powers of 10-30 watts using a plasma-only mode of operation (i.e. argon) is 0.4-0.5 mm in diameter and 2-4 mm long for a nozzle diameter of 0.25 mm. When the torch is operated in a plasma-assisted combustion mode the flame (discharge) volume and intensity increase as the microwave power is applied to the flame. Data quantifying the influence of microwave power on the flame will be presented.