Ignition of Gas Mixtures by Electric Sparks of Different Durations
Abstract Generated abstract
This paper examines how the minimum spark ignition energy of a hydrogen air mixture depends on the duration of an electrical pulse. It discusses experimental observations showing a constant minimum energy over an intermediate pulse duration range, a sharp rise for shorter pulses, and a gradual increase for longer pulses. The proposed interpretation relates this behavior to nonequilibrium energy distribution in the discharge and to electrical activation mechanisms involving electron energy transfer, metastable and excited molecules, ions, vibrational excitation, relaxation, and dissociation. The paper argues that the optimum pulse duration corresponds approximately to the relaxation and dissociation time in the forming flame kernel, where energy input most effectively initiates self-sustaining combustion.
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IGNITION OF GAS MIXTURES BY ELECTRIC SPARKS OF DIFFERENT DURATIONS
V. A. Marasanov
Spark ignition of gases is characterized by the fact that the bulk of the combustible medium (gas) remains cold, while heating occurs only in one small-volume region of the medium. Schematically, this process may be represented as consisting of two phases. In the first phase of ignition, the system receives from the spark a certain amount of activation energy, which leads to the reacting system overcoming a certain energy barrier, after which the development of the combustion reaction becomes possible. The character of the chemical processes occurring in the electric discharge depends on the physical parameters of the discharge. Therefore, the factors determining the critical conditions of ignition must be connected with the properties of the ignition source. The next phase is the propagation of the flame from the primary kernel throughout the entire volume; here the propagation velocity no longer depends on the parameters of the spark.
Experimental investigations that have been carried out on the dependence of the minimum ignition energy \((E_{\min})\) of a gas mixture \((6\%\,H_2 + \text{air})\) on the duration of the electrical pulse \((\tau)\) have shown that it has a rather peculiar character. In the time interval \(10^{-5}\)–\(10^{-4}\) sec, \(E_{\min}\) remains unchanged and equal to 0.52 mJ. When \(\tau\) is decreased, the ignition energy increases rather sharply, and at \(\tau = 1\ \mu\text{sec}\) differs from the minimum value by more than an order of magnitude; in the time interval \(10^{-4}\)–\(4 \cdot 10^{-4}\) sec, a smooth increase is observed.
Such a peculiar dependence of the ignition energy on \(\tau\) is apparently connected with the nonequilibrium distribution of the discharge energy and with the specific features of the electrical activation of the gas mixture.
It is of interest to consider the mechanism by which active particles appear in the discharge. It is believed that the energy supplied to the discharge in the first stage of its transformation is concentrated chiefly in the electron gas, constituting a certain reserve of the free energy of the discharge. The energy of the electron gas is transferred in collisions of electrons with molecules. A direct indication of the mechanism of “electronic activation” is the first order of reaction observed in the discharge for a wide variety of reactions.
When energy is transferred by electrons, metastable molecules are formed in the first stage; upon new collisions with electrons, these molecules can be excited to higher energy levels, carrying the energy reserve necessary for this process. This process may proceed all the way to their ionization. At the same time, in an electric discharge the kinetic energy of the electrons increases until, ultimately, inelastic collisions become possible, leading to the excitation of molecular electrons, which may lead to their ionization. Electronically excited molecules and ions, colliding with normal molecules, transfer to them the energy of their excitation by collisions of the second kind.
As a result of collisions of the second kind, the energy of the electric discharge is used to bring about in the system vibrationally excited molecules. The described mechanism of the activation process may be called “energy catalysis” in the discharge, because the agents that produce activation—electronically excited molecules and ions—accelerate the course of the reaction by transferring energy through the conversion of the energy of the electric field into the energy of vibrational excitation of the molecules. If, in some process, the vibrational level of a diatomic molecule is excited close to the energy level of the separated atoms, then dissociation can readily occur. In addition, excitation of vibrational levels manifests itself in a lowering of the activation energy of the chemical reaction. Dissociation can be considered by assuming that the process of vibrational relaxation has already ended. The process of vibrational relaxation and dissociation of the molecules of the initial substance directly precedes the chemical reaction.
Apparently, the relaxation time of vibrations and dissociation of molecules in the forming flame kernel is the optimum time of injection of electrical energy into the forming kernel, capable of causing the spread of the combustion reaction throughout the entire volume. This time gives the minimum ignition energy of the gas mixture.