There are several mechanisms that affect a gas when using a discharge to initiate combustion or stabilize the flame. There are two thermal mechanisms: 1) gas heating due to energy release causes the acceleration of chemical reactions; 2) inhomogeneous gas heating generates flow perturbations, promoting turbulence and mixing. Non-thermal mechanisms include 3) the ionic wind effect (the momentum transfer from the electric field to the gas due to space charge); 4) ions and electrons drift in the electric field can cause additional fluxes of active radicals in gradient flows; 5) excitation, dissociation and ionization of the gas by e-impact leads to the production of nonequilibrium radicals and changes the kinetic mechanisms of ignition and combustion. These mechanisms combined together, or separately, can provide additional control of combustion, which is necessary for ultra-lean flames, high-speed flows, cold low-pressure conditions of high-altitude GTE relighting, detonation initiation in pulsed detonation engines, distributed ignition control in HCCI engines, and so on. The operation efficiency of a scramjet depends critically on the efficiency and completeness of the combustion process. Namely, a low efficiency of combustion restricts the upper boundary of the flight envelope. Plasma-assisted combustion is one of the very interesting possibilities for stabilizing ignition at high altitudes, and low dynamic pressures and temperatures. In addition, plasma-assisted ignition can stabilize the ultra-fast combustion required for M > 10 operations. Thus, the ultimate goal of using plasma technologies in hypersonic applications is the extension of an engine's stability region to low dynamic pressures (0.2-0.05 arm) and high Mach numbers (M = 10-20). The major difference between common combustion and plasma-assisted combustion is the extremely nonequilibrium excitation of the gas in the discharge. The external electric field accelerates electrons, and the energy exchange between electrons and the translational degrees of freedom of molecules is very slow because of the huge differences in masses. As a result of this feature, electron impact can transfer energy to the internal degrees of freedom of molecules only. If the rate of internal energy relaxation is not very high, the population distribution of the excited states of the molecules will differ greatly from the initial Boltzmann energy distribution. The overpopulation of excited states causes an increase in the system reactivity and facilitates ignition and flame propagation. From this point of view, the most important questions regarding plasma-stimulated chemistry concern the distribution of the discharge energy through the different degrees of freedom of molecules, the rate of system relaxation (thermalization) and the response of a chemically active system to this nonequilibrium excitation. Precise control of the direction of energy deposition in the plasma is possible for a relatively short time period when the multiplication of electrons (above the breakdown threshold) or recombination (below the threshold) do not have enough time to change the electron concentration and plasma conductivity significantly. When the plasma conductivity is not very high, it is possible to maintain a desired value of E in the inter-electrode gap. This approach, together in combination with a short high-voltage pulse and constant bias, allows for the selective and extremely nonequilibrium excitation of the gas. The duration of the critical high-voltage pulse depends on the gas parameters (density, composition), but for the practically important range of parameters, it is restricted to few nanoseconds. The main mechanisms of nonequilibrium gas excitation and their influence on the ignition and combustion were briefly discussed. Rotational excitation, vibrational excitation, electronic excitation, dissociation by electron impact and ionization were all analyzed, as well as the ways in which the selectivity of the gas excitation in the discharge can be controlled. Despite of some lack of knowledge of mechanism details, a nonequilibrium plasma demonstrates great potential for controlling ultra-lean, ultra-fast, low-temperature flames and is an extremely promising technology for a very wide range of applications.
展开▼
