Nikolay V. Landl

Glow-to-Spark Transitions in a Plasma System for Ignition and Combustion Control

This paper deals with the investigation of a new regime in a plasma ignition and flame control system as applied to air-hydrocarbon mixtures. The system is based on the design of a classical high-current arc plasmatron. Compared with a thermal plasmatron mode, the average discharge current in the described device has been decreased to about 0.1 A. Although average power dissipated in the discharge does not exceed 200 W and average gas temperature at the plasmatron exit for typical regimes of the discharge operation in air is less than 500 K, the device demonstrates reliable ignition and flame stabilization in a wide range of equivalence ratios. Physical mechanism of ignition is associated with the nonsteady-state properties of discharge. At a low current level, the discharge burns in a kind of glow mode, and because of the glow-to-spark transitions, the high-current nanosecond pulses are superimposed on the glow plasma background. Then, the spark discharge initiates the combustion process, which is efficiently sustained in the glow plasma.

Low-Current “Gliding Arc” in an Air Flow

This paper describes the result of the investigation of a gas discharge in a flow of air with electrode geometry typical for the so-called gliding arc. The feature characteristic of the experimental conditions is a rather low level of an average discharge current (of about 0.2 A). The discharge is initiated due to a spark breakdown in the narrow part of the gap. After that, the spark discharge is transformed into a kind of a glow discharge. At the subsequent stage, the plasma column travels in the gap under the effect of a gas flow, and the current is sustained in the regime of a normal glow discharge. The typical cathode-voltage-drop value in this regime is estimated to be about 300 V, an average electric field in the positive column plasma is (700-800) V/cm, and the neutral-particle temperature in the negative glow region is T ≈ 1100 K.

Nonsteady-State Gas-Discharge Processes in Plasmatron for Combustion Sustaining and Hydrocarbon Decomposition

This paper deals with the investigation of nonsteady-state discharge regimes in the plasmatron as applied to air-hydrocarbon mixtures. The electrode system is based on the design of a classical high-current arc plasmatron. Compared with a thermal plasmatron mode, the averaged discharge current in the described device has been decreased to about 0.2 A. Then, the discharge regime can be interpreted as a kind of glow discharge with the random transitions from glow to sparks. Two types of transitions have been observed: completed and non-completed transitions. Completed transition is accompanied by the appearance of a high-conductivity spark channel, and for non-completed transition, a low-conductivity diffuse channel arises. The discharge features have been investigated for plasmatrons with a long-length and with a short-length anode nozzle. The principal features of the nonsteady-state discharge behavior are the same for both designs. However, for the long nozzle, the discharge phenomena mainly proceed inside the anode cavity and for the short nozzle outside the cavity. Use of different designs of the plasmatron anode cavity offers to extend an area of plasmatron applications.

Plasma-Assisted Combustion System Based on Nonsteady-State Gas-Discharge Plasma Torch

This paper describes the experiments with the plasma-assisted combustion system as applied to gaseous hydrocarbons. The system is based on a nonsteady-state gas-discharge plasmatron with a low average current. One of the subjects of the investigations is to elucidate a correlation between the discharge burning regimes in the plasmatron and the properties of the torch flame in the combustion chamber. Depending on the gas-discharge regimes and plasmatron design, the conditions of complete hydrocarbon combustion and partial oxidation have been demonstrated. Aside from that, the data on testing a special power supply intentionally developed for nonsteady discharge powering are presented.

Propane Oxidation in a Plasma Torch of a Low-Current Nonsteady-State Plasmatron

This paper describes the plasma-assisted combustion system intended to generate a torch flame with a high power density per unit area. In the system, a kind of hybrid concept is proposed. A primary unit for combustion sustaining is a low-current nonsteady-state plasmatron with a low level of electric power. The plasmatron activates an air/hydrocarbon mixture and sustains the oxidation processes in the plasma torch. In turn, the heat power of the torch sustains the main burning process in the torch flame. The results of experiments on propane oxidation in the plasma torch of plasmatron in a wide range of equivalence ratio are presented. As applied to the combustion system design, the plasma torch can provide both the complete and the partial propane oxidation with syngas generation.

Transient Processes During Formation of a Steady-State Glow Discharge in Air

This paper describes the investigation of an atmospheric-pressure glow discharge in air at a current of 0.05-0.3 A. Before the glow discharge is established, a preliminary nonsteady temporal stage is available in the gap. The principal process, which governs with the nonsteady-state discharge behavior, is the glow-to-spark transition phenomenon. The transition is initiated due to the explosive emission instability in the near-cathode layer of glow-type discharge that results in a microexplosion of the cathode surface and appearing of a spark cathode spot. At a low current, the spot is extinguished, so that the discharge starts burning again in one of the glow modes. After that, a new act of transition occurs and so on. The preliminary nonsteady-state stage ensures two prerequisites. First, an effective gas pressure decreases to a low value. Second, due to microexplosions, conditioning of the cathode surface is provided. Both factors facilitate establishing the normal glow discharge.

High-Current Stages in a Low-Pressure Glow Discharge With Hollow Cathode

This paper presents an interpretation of the dense and superdense glow discharge stages in pseudospark switch geometry. The discharge is treated as a self-organizing system that is able to rearrange itself to provide the current requested by external electric circuit. The principal discharge regions in the glow stages are the hollow-cathode plasma, the positive column plasma, and the double electric layer that separates these plasma regions. A model that allows some quantitative estimates when applied to the hollow-cathode plasma is developed, in which a generalized secondary emission coefficient that considers an external emission current is introduced. The abrupt transition from dense glow stage to superdense glow stage occurs because of microexplosions at the cathode surface and appearing the metal vapor plasma. In terms of the model, this means an abrupt increase in the secondary emission coefficient. The comparison with the experiment is made for discharges in hydrogen and xenon at a current up to several kiloamperes and at a current rise time from several microseconds to hundreds of nanoseconds. The physical reasons for the current quenching effect that manifests itself at a decreased gas pressure are also discussed.

Low-Current Plasmatron as a Source of Nitrogen Oxide Molecules

This paper describes a usage of a low-current coaxial plasmatron for generation of nitrogen oxide molecules. Glow-type discharge in vortex air flow is sustained at an average current from 0.05 to 0.2 A that corresponds to an average discharge power from 65 to 160 W. The diameter of an exit nozzle of the plasmatron is of 0.5 cm, and the air flow is varied from 0.2 to 1.5 g/s. In such conditions, the discharge burns in nonsteady-state regime, when a sustainment of a plasma jet/plume and a plasma column in the plasmatron nozzle is accompanied by the spontaneous glow-to-spark transitions. Due to the special design of the anode nozzle, an efficient interaction of the air flow with the plasma plume and plasma column is provided. Typical contents of nitric monoxide in the output gas are of about several grams per cubic meter, and the cost for formation of one molecule is from 50 to 35 eV.

A Multi-Mode Plasma Pilot

This paper describes a new type of plasma ignition and flame control system. The main features of the plasmatron operation modes are extremely low average current (about 0.1 A) and low average power (less than 100 W). However, it demonstrates reliable ignition and flame stabilization in a wide range of equivalence ratios. Physical mechanism of ignition is associated with the non-steady state properties of discharge. At a low current level discharge burns in a kind of glow mode and because of the glow-to-spark transition the high-current nanosecond pulses are superimposed on the glow plasma background. Then spark discharge initiates combustion process, which is efficiently sustained in the glow plasma. The plasma pilot based on this principle is described.

Methane Oxidation in a Low-Current Nonsteady-State Plasmatron

This paper describes the results on methane oxidation in the plasma torch of low-current plasmatron at typical air expenditure of 0.1-0.55 g/s and at a flow velocity in a longitudinal direction up to 22 m/s. The discharge in a vortex gas flow burns in a glow regime with the spontaneous transitions from glow to spark. Due to special design of the plasmatron nozzle (with a ring groove at the inner surface of the nozzle), an efficient interaction of the gas flow with plasma column and the reproducible data on chemical gas composition in a combustion chamber are provided. An average discharge current in the plasmatron was varied from 0.05 to 0.2 A, which corresponded to an average power dissipated in the discharge from 60 to 150 W. A heat power due to fuel burning in the plasma torch was at a level of 1 kW. The data on chemical gas composition in the combustion chamber in a wide range of air excess coefficient α had been obtained. For the lean air-to-fuel compositions (that is for α > 1), the lower flammable limit was of α ≈ 3. In a regime of syngas generation, i.e., for the rich air-fuel mixtures, the upper flammable limit was of about α ≈ 0.55. It is demonstrated that both the low and the upper flammable limits depend on the discharge current.