Combined MW-DC Discharge in a High Speed Propane-Butane-Air Stream

Abstract

The mechanism of the gas -phase oxidation of various combustible gases, including hydrocarbons and hydrogen, has been thoroughly studied, with the emphasis on their ignition mechanism. The great majority of publications in this field have dealt with factors determining the induction per iod preceding the ignition event. In recent decades, there has also been much literature discussing the possibility of effectively controlling combustion processes by various physical means. Use of the gas discharge is one of such ways , promoting an intens ification of chain combustion of hydrocarbons. However, the ignition kinetics is not completely understood even for the rather simple model system hydrogen -oxygen under low -temperature gas -discharge plasma conditions, which are established at large values of the reduced electric field. Therefore, for a deeper insight in the physicochemical processes occurring in the low -temperature plasma initiation of the ignition of a combustible gas, the experimental study of the effect of a gas discharge on the ignition event should be fulfilled and accompanied by mathematical modeling. The study of the ignition and combustion of hydrogen -containing mixtures under low -temperature plasma conditions is of importance from various standpoints: it is necessary to carry out b oth fundamental research in the mechanism and kinetics of atom -molecule reactions in a strong electric field and an analysis of a variety of applied problems, including the optimization of plasma chemical processes. One important problem is to develop the physical principles of the burning of high speed flow of combustible gases. Under such conditions it is necessary to ensure a rapid space ignition of the high -velocity hydrocarbon flow. To do this, it is necessary to minimize the induction period. In our l aboratory, we initiate d ignition with dc discharges (either longitudinal or transverse to the flow), periodic pulsed discharges, and freely localized and surface microwave discharges. Initially, the effect of low temperature plasma on the combustion kineti cs of a gaseous fuel was experimentally studied for a propane -butane air flow with a Mach number of M=2. Our experimental setup consists of a cylindrical vacuum chamber with an inner diameter of 1 m and a length of 3 m, a high -pressure air receiver, a high -pressure propane -butane receiver, a system for mixing the propane -butane mixture with air, a system for producing a high speed propane -butane -air flow, an aerodynamic channel, a discharge section, different plasma generators, a pulsed high -voltage power s upply, a synchronization system, and a diagnostic system. The air flow rate can be varied between 25 and 100 g/s; the propane -butane flow rate, between 1 and 8 g/s. The basic part of this setup is the vacuum chamber, which serves to produce a high speed fl ow and is a reservoir for the exhaust gases and combustion products. The vacuum systems allows operation in a wide pressure range of p=10 -3 -10 3 torr . We used some types of a gas discharge for ignition: freely localized microwave discharge , surface microwav e discharge, direct current discharge, and pulsed transverse electrode discharge. The ignition of the high speed stream was detected as a glow in the aerodynamic channel downstream of the discharge section. No glow was observed when a gas discharge was gen erated in an air flow, when it was generated in a high speed propane -butane -air flow but its parameters (pulse duration, discharge current, electric field strength in the plasma, and the electric power deposited in the discharge) were inappropriate for ign ition, or when the mixture was far from stoichiometric. Induction time was simultaneously derived from different measurements: (1) the minimum pulse duration resulting in a glowing flame in the aerodynamic channel downstream of the discharge section; (2) the time taken by the intensity of the molecular band of the excited CH * radical (the (0;0) band due to the A 2 ��X 2 � transition), with an edge wavelength of �=431.5 nm, to achieve the maximum growth rate;