Pulsed Plasma Jet Research Articles

Adapting Continuous-Flow Plasma Jets for Intermittent Ignition in Gas Turbine Combustors

Abstract The effectiveness as pulsed igniters of plasma jets originally designed as continuous-flow radical generators, developed for flame stabilisation, is investigated with particular reference to ignition in aircraft gas turbines. It is found that these devices are able to ignite kerosene sprays in aircraft flame tubes more effectively than conventional high energy igniters, with either nitrogen or air as the plasma medium, in spite of the very low mean enthalpy of the plume and at the shortest of times attainable with a manual “on-off” switch. Further reductions in discharge times down to orders of 100ms permit the use of a compact capacitor-based power supply and this device was tested, at various levels of turbulence and pressures down to 1/4 atmospheric, using equipment at Leeds University. In tests with lean methane/air mixtures it proved to be as effective as the most incendiary fuel-wetted pulsed plasma jets in previous tests, in spite of the further dilution of the lean mixtures by the inert p...

Pulsed plasma jet igniters: Species measurements in methane combustion

We have measured species concentration histories in the plume of a pulsed plasma jet igniter during ignition and subsequent combustion of methane/oxygen/nitrogen and methane/oxygen/argon mixtures using molecular beam mass spectrometry. Equivalence ratios of 1.0, 0.6, and 0.4 were used in the igniter cavity and in the main combustion chamber to provide both combusting and noncombusting cases. Species measurements indicate that combustion initiated by plasma jets can be divided into three time periods: a very hot initial stage, where high temperature species formed both in the plasma cavity and in the turbulent plume dominate, a second region where entrainment continues along with the production of products in the plume but not in the bulk gas mixture, and finally flame propagation through the combustible mixture. High radical concentrations characterize, the first stage; N atoms peak at a mole fraction of 0.004, and OH reaches a value of 0.007 in the stoichiometric case. The second stage involves the reactions of intermediate products such as H 2 , NO, and CO along with the production of final, stable products. In the final stage, CO 2 and H 2 O concentrations rise to levels expected from complete combustion of the reactants. These measurements are used to explain how pulsed plasma jets enhance ignition in lean mixtures.

Liquid Propellants in Pulsed Plasma Jet Igniters

Abstract Pulsed plasma jets fed with liquid propellants (aqueous mixtures of nitrate salts of hydroxylamine and 4-carbon aliphatic amine) have been studied as high energy ignition plugs. Ultra high speed shadowgraphy applied to optically accessible plug model s have been used to observe details of electrical energy transfer to the propellant and the subsequent decomposition and ejection processes. Depending on the electrical variables and on the plug geometry, events corresponding to a variety of desired applications, ranging from slow ejection of propellant to very violent events associated with the emission of shock waves, can be generated at will. In additi on to displaying current and voltage transients. the volume relea sed by propellant decomposition is monitored and a meth od. based on streak photography, is developed which allows the energy giving rise to individual shock waves to be measured. It is shown that, at atmo spheric pressures and in the absence of confinement of the decomposing propella...

Structure of the pulsed plasma jet

The flow characteristics of a pulsed plasma jet have been studied using streak and conventional (time-resolved) photography of the luminous plasma jet. The nature and placement of the shock structure observed in these photographs is that expected of under-expanded jet flow. Direct measurements of the gas pressures at the nozzle of the cavity (made using an optical technique) are in good agreement with those calculated from the observed shock structure. A second set of pressure measurements, of the stagnation pressure throughout the jet, shows good agreement with the calculated pressure behind the bow shock which is located at the front of the expanding jet. On the basis of these results, a simple model of jet penetration is proposed in which the jet 'tunnels' its way into the dense surrounding air. The implications of this model, in view of its similarities to other well studied phenomena such as astrophysical jets, are discussed in terms of the turbulence created by the jet, a factor which could be most significant when such jets are used as igniters.

Pulsed Plasma Jet Igniters: Species Measurements in Nitrogen and Air

Abstract A molecular beam mass spectrometer was used to obtain quantitative data on the species present along the centerline of nitrogen and air plasma jets. Dilution of the jet as ambient gas is entrained into the turbulent plume was measured by probing an argon jet discharging into nitrogen. The gas from the jet igniter cavity is diluted by more than a factor of ten in the first 4 mm downstream of the orifice, with a further dilution of 40 percent over the next 8 mm of jet penetration. Comparison of peak mole fractions of reactive species in the jet with the dilution results indicate that the plasma species are not decaying at a rate solely due to dilution. The nitrogen plasma jet was probed for metastable nitrogen (N2*) and N atoms as a function of time at various distances from the jet orifice. Pteak mole fraction measurements demonstrate that N2* persists at distances farther downstream than N atoms. Metastable nitrogen concentrations are relatively constant from 4 to 14 mm, indicating that metastable nitrogen is produced in the plume. Nitrogen atoms, oxygen atoms, and nitric oxide were measured in an air plasma. Nitric oxide persists significantly longer than the atoms and has a peak mole fraction at the point where O and N decrease significantly. The O atom mole fraction of 0·02 is an order of magnitude higher than the N atom concentration, but the radical concentration in the air plasma is lower than that in pure N2. No evidence of metastable nitrogen was found in this system. The chemistry with respect to these results is discussed, both in terms of the kinetics of the system and calculated equilibrium concentrations.

Flow and pressure transients in a pulsed plasma jet

Simultaneous streak photographs of plasma luminosity within and without a pulsed plasma–jet cavity provide strong evidence that arc instabilities inside the cavity arc responsible for the well-known inhomogeneities in the luminous pattern of the jet. Pressure measurements obtained with a high-speed optical technique reveal that most of the gas is expelled from the cavity within about 30 μs and that the luminosity inhomogeneities do indeed correspond to substantial fluctuations in mass flow rather than just variations in temperature. It is concluded that such variations in How constitute a likely source of intense turbulence, and as such, form an important part of the igniter's known ability to enhance the rate of combustion in its vicinity.

Hydrogen atom distribution in pulsed plasma jet igniters

Hydrogen atom distribution in pulsed plasma jet igniters

Species and Temperature Measurements in a Pulsed Plasma Jet by Molecular Beam Mass Spectrometry

Abstract Molecular beam mass spectrometry has been used to determine temperature and species concentrations in the discharge plume of a pulsed plasma jet igniter. Laser schlieren photographs of the plume/sampling cone interaction reveal that the molecular beam sampling has negligible effect on jet fluid mechanics up to the time of impingement, and temperature and species measurements indicate that at later times the gas is sampled from within the plume. The temperature in an atmospheric pressure jet of nitrogen ranged from a maximum of 50,000 K to ambient temperature. Peak nitrogen atom concentration at the detector was found to range from 5 percent. 2 mm from the igniter, to less than 100 ppm at 14 mm.

The Initiation of Combustion by a Hot Turbulent Mixing Element

Abstract The ignition characteristics of the turbulent element produced by a pulsed plasma source are modelled using a simple Arrenhius reaction rate formulation and the entrainment hypothesis. It is shown that critical ignition conditions depend on the heat content of the combustible mixture and the ratio of the energy and momentum injected by the plasma source. The model indicates formation times of the order of milliseconds during which times extinguishing cases cannot easily be distinguished from combusting cases. During this time the chemical reactions are controlled by the turbulent mixing alone. This furnishes a tentative explanation of anomalously long ignition delay times sometimes observed with pulsed plasma jet ignition systems. Experimental observations confirm the broad features of the theoretical model, in particular the validity of the similarity assumptions and the scaling factors predicted for the element behavior.

Ignition and flame stabilization by plasma jets in fast gas streams

Pulsed and continuous flow plasma jets are used to achieve ignition and flame holding in fast flowing lean hydrocarbon/air streams. Small, steady-flow nitrogen plasma jets with electrical inputs of the order of 1 kW and sonic flow through their nozzles stabilize flames well below the lean limit of flammability for propane, in complete contrast to argon plasmas. Pulsed plasma jets of the surface discharge type also demonstrate the contribution of radicals to successful ignition of sub-limit propane air streams. This is achieved with plasma media ioncluding water and hydrocarbon fuel. The variation of the limit of incendividity with repeated discharges following a single wetting of the plug with the plasma medium shows very long lasting effects and contributes to the understanding of mechahisms. The potential practical applications of these observations are discussed.

A novel ignition device for the internal combustion engine

Although the recent rapid proliferation of research into plasma jet ignition1–16 has partly been prompted by the requirements of continuous combustion and pollutant removal17–25, the main motive has been to extend the lean operability limit of the internal combustion engine. This trend to leaner mixtures has been stimulated by the need to improve engine emission characteristics and efficiencies. Because optimum conditions occur near the lean misfire limit, a plug providing increased rates of flame propagation and more effective and reliable ignition has been sought; pulsed plasma jets offer much promise but use too much power. In these devices, the igniting jet is generated by explosive gas expulsion through an orifice, engendered by a short-duration arc discharge to which the plasma medium is exposed while confined by the walls of a cavity within the plug. Such plasma jets, and particularly those fed with specific plasma media, have proved capable of extending the flammable fuel–air mixture range both at the fuel lean and at the rich limit, and increase the speed at which flames propagate following ignition. This is thought to be due to the generation of turbulence and free radicals. We report here the development and testing of an ignition plug based on two sparks at the extremities of an internal cavity. This design combines much of the effectiveness of a plasma jet with the small power consumption, and consequently low rates of wear, of a conventional sparking plug.

Chemical activity and transport processes in the vicinity of a plasma jet igniter

Because of the well-known enhancement of combustion observed with plasma jet ignition, various measurements have been carried out upon the subsonic puff of highly turbulent gas ejected from a pulsed plasma jet igniter. “Spot” measurements of temperature show that the puff propagates with a sharp temperature front whose thickness is ∼ 10 −1 mm. Measurements of the flow velocity within the puff reveal that it is approximately equal to the front velocity of the puff and is thus capable of propagating both heat and particles from the igniter to the front of the puff. Finally measurements of the nitric oxide generated at various distances from the igniter when the plug is filled with N 2 or O 2 and is discharged into a vessel containing the complementary gas show that atomic nitrogen or oxygen is generated in amounts greatly in excess of equilibrium dissociation in a uniformly heated plasma. The production of NO extends several centimeters away from the igniter, where, in view of the low measured gas temperature, it can only be ascribed to the reaction between atomic nitrogen and molecular oxygen (or vice versa). Since such atoms are only created in the plasma within and close to the igniter, we conclude that they survive for appreciable times and distances and that radicals as well as heat are propagated by the internal flow processes.

Level population by recombination in a pulsed plasma jet in an He-Xe mixture

Measurements have been made on the spatial and time characteristics of the radiation from an He-Xe plasma jet emerging from a discharge channel through a nozzle with a speed of about 105 cm/sec. A recombination mechanism is indicated for the line excitation by the delay between the current pulse and the production of the radiation, as well as by the substantial differences in time course for the intensities of the atomic and ionic lines and by the increase in intensity with pressure. Measurements have been made of the electron temperature and concentration, and estimates have been made of the recombination coefficients, which reflect the performance of the collisional-radiative recombination in a decaying He-Xe plasma jet.

An experimental investigation of the effect of a plasma jet on a freely expanding methane-air flame

Pulsed plasma jet igniters have been shown to be effective in enhancing the flame speed of combustible mixtures of gases. The mechanism by which this enhancement is achieved is somewhat unclear, although the consensus of recent investigations is that the possible mechanism involves the inherent generation by the jet of uv radiation, radicals turbulance, or some combination of the above. This paper describes a series of measurements of flame speed in which ignition and velocity enhancement processes are separate and independent. With such a configuration it is possible to demonstrate that an enhancement in flame speed is achieved in the vicinity of the jet and that this enhancement is not necessarily related to some jet-related perturbation of the ignition process. For the strengths of mixtures investigated (approximately stoichiometric to φ = 0.67) turbulence generated by the plasma jet igniter is shown to dominate both ignition and flame propagation.

Plasma expulsion from the plasma jet igniter

Measurements of the plasma efflux from a pulsed plasma jet igniter show that, contrary to previous expectations, plasma leaves the jet cavity in a substantially supersonic condition. Associated measurements of the electric field of the arc within the cavity and of the time taken for plasma to be expelled from the cavity show, together with the efflux measurements, good agreement with established arc theory and with a theory of plasma expansion based on a model in which the jet cavity is considered to act as the high-pressure section of a conventional shock tube. The implications of this model to the efficiency of a plasma igniter are discussed.

Dynamics of supersonic plane plasma jets

Using a fast photorecorder (FPR), the process of flight and reflection from a wall of a plane pulsed plasma jet in a closed gas-filled cylindrical volume is investigated. It is shown that, in an electrical-discharge source, a plane jet is formed from a set of individual flames. The gasdynamic structure of an individual flame is investigated. From the change in jet velocity at the pressure discontinuity, the plasma temperature in the discharge channel and in the jet is estimated.

Recombination laser utilizing a plasma jet of helium and xenon

Stimulated emission was obtained at λ = 2.026 μ from Xe I in a pulsed helium-xenon plasma jet emerging from a discharge channel through a nozzle or a multinozzle grid at a velocity of ~105 cm/sec. A study was made of the spatial and temporal dependences of the output power as a function of the pressure and composition of the mixture, input energy, shape and duration of the current pulses. It was established that at sufficiently high helium pressures (pHe=100–300 Torr) the active levels were populated by a recombination mechanism.

Discreteness of pulsed plasma jets

Discreteness of pulsed plasma jets

Quasi-steady plasma acceleration

Applications of quasi-steady state plasma acceleration to development of pulsed plasma jet engines

Spectroscopic examination of a pulsed plasma jet in air and under vacuum

Spectroscopic examination of a pulsed plasma jet in air and under vacuum