The issues of plasma initiation of combustion were intensively studied by many researchers (see, e.g., the review [1]). However, there are only few results on combustion initiation in a flow by means of a plasma jet and practically absent results on combustion initiation by means of a high-enthalpy supersonic plasma jet. The interaction of a supersonic plasma jet with a flow is extremely complicated and was not studied in sufficient detail [2]. The jet boundary in a submerged space (tangential discontinuity) is a streamline along which the pressure remains constant and equal to the ambient pressure in the case of exhaustion into a quiescent medium. In the region of the first “barrel,” a comparatively small part of the total mass flow of the gas in the jet (less than 20%) passes through the Mach disk, even if the central shock (Mach disk) has a rather large size. The main part of the gas moves over the periphery. A thin gas layer with a comparatively high density is formed near the jet boundary.
Additional investigations of injection of a supersonic plasma jet generated by a rail gun into a submerged space were performed to understand the result obtained and to develop an adequate mathematical model.
A scheme of the experimental setup [3] is shown in Figure 1. The processes occurring in 350-mm-diameter 400-mm-high sealed cylindrical reactor chamber 1were monitored through optical window 3 with a clear aperture of 100 mm.
Fig. 1: Scheme of the experimental setup: (1) cylindrical chamber, (2) Marshall gun, (3) optical window, (4) capacitor bank, (5) controlled switch gap, (6) plasma, and (7) pumping out.
Typical streak images taken with two mutually perpendicular orientations of the slits are shown in Figures 2a, b. Figure 2a shows a longitudinal streak image taken along the axis of the plasma flow, while figure 2b shows a transverse streak image taken in one of the transverse cross sections located consecutively along the axis.
Fig. 2: Streak images a – longitudinal, b – transverse.
In the experiment described above, an equilibrium methane plasma flow with a velocity of ~ 10 km/s, temperature of ~ 10,000 K, and pressure of several tens of megapascals is formed at the exit of the rail gun channel. The exhausting jet is strongly under expanded; as it follows from the processing of the photographic records of the process, the jet rapidly expands with a mean velocity u equal to 4.51, 8.05 and 10.0 km/s at a distance of 15, 30, and 45 mm from the channel exit, respectively. Owing to radiation and rapid expansion, the temperature at the jet boundary decreases to Т~4000 K (the gas is still luminous). Let us estimate the parameters of the mixing region on the interface between the jet and the ambient medium (methane), where the temperature is Т0 = 300 K. The estimate of the mixing region based on temperature and diffusion equation are and , where -the mean free path, -the characteristic size of the mixing region and .Thus, in the mixing region, and the nonequilibrium state in terms of the translational temperature of particles should be taken into account.
The study was performed within the framework of the model described in [4], which implies the nonequilibrium state in terms of the temperature of groups of particles. Such models are often used in studying kinetic schemes and finding details of individual mechanisms and a possibility of kinetic scheme reduction. The reacting system in this model is considered in the two-temperature approximation with the following temperature groups: “cold” particles including the species of the initial methane-air mixture and the products of its decomposition and “hot” particles including hydrogen and carbon atoms of the recombining methane plasma.
The kinetic scheme proposed in [5] was taken as a basis for investigating combustion of the methane-air mixture. The upper boundary of the confidence interval of temperatures does not exceed 3000-4000 K for the majority of rate constants of elementary reactions of hydrocarbon combustion [6]. This fact determines the level of admissible temperatures of the initial reagents.
Below are the results of calculations for СН4:О2 = 1:1, Th0 =3000.К, Ph0 = 0.3MPa
Fig. 3.Temperature change over time. Fig. 4. Substances involved in the burning of Hydrogen
Fig. 5. Concentrations of radicals
Fig.6. Substances involved in the combustion of methane.
Fig.7. Concentration of "Hot" hydrogen and carbon atoms, carbon dioxide.
Fig. 8.The pressure change over time.Ph - pressure "hot" components, P - "cold".
Despite of short-duration process of the combustion and a low level of a heat release, the possibility of ignition of hydrocarbon fuels has been confirmed at the high speed of the flow in channel.
The calculation has allowed to determine the nature and conditions of ignition and to understand the reasons of the fast discontinuance of combustion process at artificial initiation.
At plasma initiation of combustion, the kinetic mechanism is very sensitive to the model completeness of elementary reactions with participation of components of plasma and its elementary composition. Dynamics of process of the combustion initiation appreciably was formed by the initial stage, in which reactions with participation of "hot" particles are prevailing. Probably, at appropriate selection of plasma-forming gas, combustion initiation can occur. For example, if plasma of noble gas is used.
References
1. StarikovskaiaS.M..“Plasma assisted ignition and combustion.”Journal of Physics D: Applied Physics. 39.2006. R265–R299.
2. Lukyanov, G.A. 1985. “Supersonic plasma jet.”Mashinostroenie. (in Russian)
3. Golubov A.I., Katsnelson, S.S., Pozdnyakov, G.A “Interaction of a high-enthalpy plasma jet with surfaces chemically active media.”IEEE transactions on plasma science, vol. 38, No 8, 2010,pp. 1840-1849.
4. Katsnelson, S.S., Pozdnyakov, G.A. “Initiation of combustion in a hydrogen-oxygen mixture under the action of a low-energy strong-current electron beam.”Combustion, Explosion, and Shock Waves, vol. 43,No. 2, 2007, pp. 132-138.
5. Dautov N.G., Starik A.M. 1997 “On the Problem of Choosing a Kinetic Scheme for the Homogeneous Reaction of Methane with Air.”Kinetics and Catalysis, vol. 38, No 2, 1997, pp. 185-208.
6. Edited by W. C. Gardiner Jr. “Combustion chemistry.”Springer-Verlag. New York Inc.1984.
