WO2010090545A1

WO2010090545A1 - Formation method of high enthalpy gas jet based on pulse gas discharge

Info

Publication number: WO2010090545A1
Application number: PCT/RU2009/000051
Authority: WO
Inventors: Andrey Yurievich Starikovsky
Original assignee: Neq Lab Holding Inc.
Priority date: 2009-02-05
Filing date: 2009-02-05
Publication date: 2010-08-12

Abstract

To broaden the method application range, in particular to ensure application of the method in gas turbine engines and burners to initiate and intensify combustion of fuel-air mixture, plasma-forming gas is sent into a discharge gap with a pulse gas discharge excited in nanosecond duration range by applying high-voltage pulses to discharge gap electrodes at significant gap overvoltage (by 2-10 times).

Description

Formation method of High Enthalpy Gas Jet based on Pulse Gas Discharge

Application Area of Invention

The invention refers to plasma engineering and may be applied for combustion and intensification of chemical processes in air-fuel mixture in any type of combustion engines including, but not limited to, afterburners, combustion chambers in detonation engines, jet engines and gas turbine engines, as well as in energy burners and reformers.

Prior Art

Plasma lighting techniques are known to be designed to improve efficiency of combustion of air-fuel mixture in combustion engine chambers. Fir that purpose, various physical processes are used to generate plasma.

In patent claim DE 10037536, F02P23/04, 2002, nonequilibrium high- frequency discharge plasma is used to initiate combustion. In patents RU 2099584, F02P15/00, 1997 and US 6883507, F02P15/10, 123/506, 2005 corona discharge is used for same purposes. The flaw of the said techniques is relatively low electric field intensity in the field of main energy input thus resulting in energy efficiency losses during their use.

Plasma jet sources are known, namely, arc-discharge based plasma guns, used both for lighting and flame stabilization in pulverized-coal fired boilers and for other technological purposes: metal plating, welding or cutting, etc. (RU2111098, 1998; RU2211257, 2003).

Among flaws of the known plasma gun designs are short operational life and high energy consumption due to the use of arc discharge resulting in overheating of electrode system, erosion and breakage. This flaw was partly reduced in a plasma gun with stabilized weak- current arc discharge (RU 2111098, 1998; RU 2112635, 1997; EP 919317 Al, 1999; US 6087616, 2000; US 6156994, 2000; AU736916). This invention uses AC transfer discharge mode with current stabilization to achieve high dielectric field intensity in the discharge channel and reduce thermal load and erosion while maintaining high temperature of output plasma jet. To solve this problem, a DC inverter power source with flat drain-current characteristic is used. In this case electric discharge is stabilized at typical voltage approx. 1 kV/cm at IA current, which contributes high energy to plasma channel at low electrode wear. This invention ensures steady operation of a plasma gun in the selected range of parameters.

The plasma gun described above has a limited use range since it is not designed for operation together with combustible mixtures, can be operated with additional eternal electrode only and does not allow variations of ambient pressure in 1-30 atm range characteristic for combustion chambers.

Appearance of fast-switchable powerful transistors and thyristors enabled development of various discharge combustion systems. As opposed to output pulses in spark coil ignition systems which by themselves are slower (typically, build-up time about 60-200 μs) and longer (as a rule 1-2 ms), capacitive-discharge systems ensure quicker build-up time (1-50 μs) and short duration (5-500 μs). Ignition systems with quick pulse build-up are less susceptible to misfires because of failing ignition plugs. Contemporary inductive-discharge and capacitive-discharge systems ensure 5-100 mJ electric energy per pulse in 20,000-30,000 V peak output voltage range. Modern approaches to improvement of combustion are aimed at empirical optimization of electrode geometry, orientation and location inside combustion chambers, and at increased duration and/or spatial distribution of plasma core. Known improved ignition systems generally work at high energy levels in the range of 60 mJ to several J's per pulse. Such systems can provide one long-lasting glow discharge or low-current arc discharge, or a sequence of several short discharges with effective ignition core duration of 2 to 10 ms. The best spatial distribution ^! of the core is most frequently achieved if a wider discharge gap is used. This requires an ignition system able to constantly receive higher voltage required for gap breakdown. For that purpose, patent US 4677960 uses voltage pulse doubling circuit at the discharge gap.

To initiate combustion, patent US 4402036 uses pinch-effect with highly ionized plasma pinched by self-magnetic field. In the process plasma is generated with very high temperature and high energy density (the parameter values not optimal for ignition) which results in heavy electrode wear.

These problems have been solved with a combustion start system according to patent US4589398; the system optimizes both mixture ignition (firing) and combustion processes, and improves efficiency of discharge-to-fuel energy transfer. The system employs hard ignition discharge in gas. The term "hard ignition discharge" is used by the patent authors as referring to operation mode where inductance and resistance of a discharge circuit are low enough so that current magnitude and energy emission rate in the very discharge channel during breakdown phase were sufficiently controlled with resistance of the channel.

Such extreme operation mode is described with high efficiency (80-95%) of transfer of energy pulse energy initially stored within the first half-period of the discharge current cycle during various transient processes connected with formation and expansion of gas discharge. The authors note that in hard discharge mode within breakdown phase of the discharge (generally within first tens of nanoseconds) the largest portion of pulse energy is emitted, thus ensuring maximum power transfer from the control circuit until load impedance of the discharge channel rapidly drops. The hard discharge process has a very stable nature and is able to ensure steady engine operation when ultra-lean mixtures are used.

The said method is take for a prototype and is implemented as follows. To a discharge gap formed by electrodes high voltage (about 20% higher than breakdown voltage of the discharge gap) is applied from an external source with inductance L, capacity C and active resistance R. In this case to implement a hard discharge the following condition shall be met:

where: tm - time point when current rise rate is maximum (nanoseconds);

R_1n - resistance of a discharge channel at time t_m, Ohm;

G - capacity, nanofarad;

L - inductance, nanohenry;

I_g - length of dishrag gap, cm; Wo- total electric energy of the system.

Out of experimental data given in cited literature one can obtain the following experimental approximation for channel formation time: t_m ^ (236/E_o)(P/Po)^m, where t_m - time point at which current rise rate is the highest, nanoseconds,

EQ -^breakdown field intensity, kV/cm;

PfP o — "gas pressure in the gap vs. atmospheric air pressure" relationship.

The prototype gives the parameters for a hard discharge mode in the open air. The discharge has a pronounced aperiodic nature with 20-50 ns current pulse time and 10-20 ns pulse front time. As shown by experimental results with very hard discharges in the open air conditions in a linear gap at low overvoltage (for which E₀ is approx. 25 kV/cm), under these conditions criteria optimal for ensuring a critically decaying aperiodic discharge are, approximately:

Clg « (Clg)max « 840 pF*cm JVIg ^«{Byig)inax ^« 260 mJ/cm

The said method establishes requirements to parameters of capacitive energy storage devices and to parameters of a circuit forming voltage within a discharge gap depending on discharge gap value and in-gap pressure necessary to implement aperiodic discharge mode. The flaw of the method is that it does not account for discharge development times in case of varying gas pressures and temperatures, is confined to conditions for internal combustion engines, and rigidly connects pulse rise duration and its total duration. Besides, wavefront steepness are yet too low to prevent damage top electrodes.

Disclosure of Invention

The problem solved by the invention is to improve efficiency of energy transfer from external electric circuit to gas with allowance for such variations in process conditions as gas pressure, temperature and density. The suggested method also solves another problem, that is ensuring its broader application, in particular, its use in gas-turbine engines and burners to initiate and intensify combustion of fuel-air mixture owing to account made of various conditions under which φe said process takes place, such as uniform filling by plasma of a discharge gap and increased gas dissociation in the discharge. Recently this problem has become especially vital for development of various plasma-chemical devices based on transfer of electric energy to gas (plasma guns, plasma-fuel burners, etc.).

To solve the said problem, a high-enthalpy gas jet (as in the prototype) is created on the basis on an impulse gas discharge in the nanosecond duration range by applying high impulse voltage to a discharge gap. A nanosecond-long high-voltage discharge (providing strong overvoltage and high pressure) is developed according to a wave mechanism. As distinct from the prototype, the discharge is performed at high (by 2-

10 times) overvoltage.

Increased overvoltage results in quality changes in the process behavior, namely, it changes the discharge type. No pulse-periodic gas discharge at high and extra-high overvoltage has been known in the prior art, in particular, the prototype uses a discharge at 10-20% overvoltage.

It was determined in experiments that best results for the method claimed are achieved at the following amplitude U (B), impulse front rise time Tf (c) and pulse duration τ(c):

3 -10^~!4 > U/(L xn) > iσ¹⁵ (1) RC < τ_fr < lO^'23 x L² xn / U (2)

4-10⁷/ n < T_pui_se < 10¹¹L RZn, (3) with pulse recurrent rate (c^"1) selected in the range:

(Tpuif >f > VZL, (4) where: L - interelectrode gap width (cm), n - molecular density in the unit of volume of a discharge section (cm^"3).

R — feeding line resistance (Ohm),

C - capacity of the discharge gap (F)₃

V- gas ϊlow velocity in the discharge gap (cm/s).

For ambient conditions similar to those of the prototype, these relationships provide a shorter impulse with sufficiently less rise time not related to total pulse duration. Besides, the given relationships help to calculate process parameters for various conditions, while the relationships which determine performance of. the method according to the prototype do not allow for gas pressure and temperature.

Reduction of pulse rise time increases discharge uniformity, while increased pulse duration intensifies gas dissociation in a discharge which, combined, improves efficiency of combustion initiation in case of equal energy inputs.

The method enables simple arid easy adjustment of plasma jet temperature by controlling pulse recurrence rate in the above range, Thus it provides high efficiency of energy transfer at high E/N values, which, given exponential relationship between gas dissociation / ionization rate (caused with electron impacts) and reduced field intensity, ensures best total performance efficiency of respective devices.

Brief Description of Drawings

Fig. 1 gives a diagram illustrating development stages of pulse-periodic charges.

Fig. 2 - areas of various breakdown mechanisms. Fig. 3 shows possible design of a discharge plasma gun. Fig. 4 shows the schematic of a test installation for analysis of discharge development dynamics.

Fig. 5 - voltage waveform.

Fig. 6 shows relationship between streamer speed and discharge gap length for a linear flow range. Fig. 7 provides current amplitude values in the range shown on Fig. 6.

Fig. 8 (a-f) gives time profiles of discharge voltage, current and power for various discharge gap lengths.

Fig. 9 gives a schematic of one of the locations a discharge gap may have in the combustion chamber of jet-turbine jets.

Fig. 10 shows general appearance of a plasma jet generated according to the invention at atmospheric pressure.

Fig. 11 - curves of efficiency of the transfer of electric impulses to gas depending on pulse recurrence rate at various pulse duration.

Invention Embodiment

In case of breakdowns according to Townsend model, volume charge of a single electron avalanche is so small that it does not distort electric field in the gap. Additionally, self-sustained discharge conditions is satisfied, i.e. as a result of secondary processes at the cathode owing to development of a single electron avalanche and subsequent current of positively charged ions to the cathodes, or to photoelectric emission, at least one electron must appear giving rise to the following electron avalanche. Speaking about discharge development according to the Townsend mechanism, one generally distinguishes several stages: avalanche generation phase when current rise is due to development of electron avalanches which follow one another (at this stage concentration of charged particles does not exceed IO¹¹ cm^"3); propagation of ionization waves accompanied with a glow front and balancing concentration of charged particles along the gap length (at this stage concentration of charged particles grows by a factor of 100 and amounts to 10 cm^" ) and overall discharge combustion phase.

The phase of development of ionization waves in the glow discharge was studied both experimentally by recording discharge emissions and by numerical simulation. In the process of one-dimensional simulation of glow discharge development I hydrogen at 500 mm Hg pressure, inter-electrode gap 2 cm, field intensity 19.84 kV/cm and overvoltage 0.2%, continuity equations for electron and ion components were solved together with Poisson equation and electric circuit equation. Initial ' number of initiating electrons was 10², secondary emission rate was γ = 8.34 * 10^"4, while electron multiplication factor was μ = γ(exp(α d — I)) = 1.111. It was proved that within 30 μs after electric field is "turned on", the field practically experiences no distortion with spatial charges. Afterwards, the field starts to weaken close to the anode and strengthen in other areas. In 30 μs intensification of electric field close to cathode is observed leading to propagation of cathode-directed ionization wave. Propagation velocity of ionization wave was 2.8 * 10⁷ cm/s which exceeds by 5 times draft velocity of electrons in the external applied field. After one or more ionization waves pass, space glow discharge is fired.

Based on processing results for breakdown in hydrogen at 460 mm Hg pressure, 2 cm gap length and reduced electric field 23.5 V/(cm Hg), time-base circuit was built for Streamer breakdown glow (as shown on Fig. 1). A primary avalanche is developed at the cathode with draft velocity of 8.5 * 10⁶ cm/s. In 180 ns total charge carriers in the avalanche achieve critical value, and anode (AS) and cathode (CS) streamers start to propagate towards electrodes of the discharge system. After the gap is bridged with the streamer channel a number of ionization Waves (IWs) is excited equalizing conductance along the whole channel length. Velocity of subsequent waves exceeds velocity of previous ones and may reach velocities of the order of 10⁹ cm/s. Fig. 1 thermal ionization front is denoted as TIF.

Sometimes ionization wave is meant as propagation of a streamer in general, i.e. invasion of the plasma channel into a weak external field area, on the assumption that radius of the invading streamer channel is much less than its length, and the structure of a streamer head changes with time slowly enough sot that its quazi-stationary change can be mentioned. The main processes determining propagation of such waves are ionization and electron drift in the field ensuring exposure of ions, creation of a spatial charge and redistribution of the electric field redistribution.

Propagation of the streamer as an ionization wave can be described at sufficiently rough approximation as a system of equations including continuity equations for electrons and ions and Poisson equation for electric fields. P. The term hereinafter referred to as "breakdown" denotes processes of creation of charged particles in within the gap and propagation of the ionized area from one electrode to another when constant voltage or pulse voltage are applied to electrodes of the discharge system. In this case one can distinguish three physically different breakdown types in middle pressure range (from fractions to hundreds mm Hg).

If voltage is applied to electrodes within gas-filled gap and then was gradually increased, glow discharge will fire in the gap at certain voltage. This discharge develops according to Townsend breakdown mechanism which is determined in the first line by ionizing efficiency and secondary emission from cathode surfaces. In general, Townsend breakdown diffusely covers the whole discharge gap.

In case of pulse breakdown, the gap may withstand overvoltage well in excess of breakdown voltage. In fact, in these conditions overvoltage K=U¹ fU _& (U_br - breakdown voltage) beside parameter pd (p — pressure, d - inter- electrode gap length) determines a breakdown mechanism. If overvoltage values amount to tens of percents, volume charge of a single electron avalanche increases to such extent that a field inside the avalanche matches eternal field and, while fields in the avalanche head and tail appear intensified. As a result breakdown develops according to streamer mechanism, i.e. low-conductivity and small-diameter formations are propagated towards one of the electrodes (or to both) at velocity of 10 - 10 cm/s. A curve dividing breakdown development areas according to Townsend and streamer mechanisms is reproduced on Fig. 2, where TD stands for Townsend discharge area. In case of higher overvoltages (hundreds of percents) breakdown again acquires diffuse glow nature but for different physical reasons: at sufficiently high intensity values of reduced electric field in the breakdown front a part of electrons will pass to continuous acceleration mode (so-called "runaway of electrons") contributing to pre-ionization homogeneous (by volume) at breakdown front. In this case breakdown will develop from a high-voltage electrode to a low-voltage one at typical velocity of several cm/ns or higher.

One should note that no distinct border line exists between a streamer and a spatially homogeneous nanosecond breakdown. If Townsend breakdown is distinguished by available secondary emission from the cathode, main elementary processes responsible for development of both streamer and nanosecond breakdowns¹ are photo ionization of gas, pre-ionization with fast electrons (in case of sufficiently high intensity fields) and ionization with electron impact behind breakdown front. A horizontal line (Fig. 2) denotes electron runaway threshold. The threshold is determined based on electron energy balance. For a nonrelativistic electron, the force of its deceleration in vacuum is determined by molecular density of gas N₀, total electrons in a molecule Z, their kinetic energy ε = mv /2 and average energy of inelastic losses I. If electric field exceeds critical value E_crit = F_m/e, electron starts to continuously acquire energy as it moves along x axis. On the basis on analysis of dependence of energy losses per path unit on electron energy, availability of top border was shown for parameter E/N for streamer mechanism of gas breakdown. As evaluation of critical field intensity value, E_crit— (l/f) (d ε / dx) ^m _tot was suggested where (d ε/ dx) ^m _tot - peak of electron energy loss curve, /- factor of field strengthening close to a special charge. In this case, for atmospheric density nitrogen we obtain E_crit~ 300 kV/cm.

Thus, at discharge formation stage high electric fields contribute to increased plasma homogeneity and increase volume of discharge-excited gas. In general case, the methods is implemented as follows. Discharge gap with length L is made with an outer electrode 1 and inner electrode 2 separated with an isolator (Fig. 3). Outer cylindrical electrode 1 has a channel 4 to feed plasma-generating gas. Here one should note that electrode geometry may be either planar or any other shape. Fig. 3 illustrates only one of possible method implementations. Outer electrode 1 is grounded, while pulse voltage from a power source is applied to the inner electrode.

For generation of high pulsed voltages, e.g. a generator may be used based on high-speed semi-conductor keys. (An example of implementation of such generator with the following parameters: voltage rise rate at the pulse front — 5-150 kV/ns depending on required pulse properties; pulse duration may be set in the range 1 - 100 ns; output voltage may vary in the range 10-150 kV at load "50 Ohm.

Depending on gas composition in the discharge gap and gas pressure, which determined gas concentration n in the discharge gap, and using the condition

3-iσ¹⁴ > U/(L xn) > iσ¹⁵, determines the range of possible values of voltage amplitude U.

This condition is obtained from constraints for reduced electric field intensity necessary for effective ionization with electron impact: 100 Td < E/n < 3000 Td. Thus we get: E/n = U/(L*n), and

3 -10^'14 > U [V] /(L[cm] xn[cm ³]) > 10^'15 Rise time of high voltage pulse front T_f [s] is limited with the condition:

RC < τ_f < 10^~23 x L² x n / U, This condition (on the side of low time values) is obtained from limitations for frequency at which discharge cell capacity shunts a discharge. From here we get:

RC < Tf On the side of high time values the limitation is derived from a condition for voltage rise in the gap. Voltage rise rate shall be sufficient so that by the time the gap is bridged field intensity within the gap fell into "effective" range. That is, time of operating voltage rise limited with condition (1) is less than transit ^"tini'e of ionization wave through gap L. τ = LZV, where V ~ 7 -10²¹Uf(Yi-L) (if evaluated by drift velocity in maximum field), and V ~ 5-10²³U/(n-L) (if evaluated by wave front transit velocity in maximum field). Taking that velocity in the beginning of the process is 0:

Tf < 10-10^~24n ^■ L²/U[s] = W⁸L [s] High voltage pulse duration τ_puι_se [s] is limited with condition:

4-10⁷/n < τ_pulse < W¹¹L RZn

This condition (on the side of low time values) is obtained from limitations for frequency of ionizing impacts: /~ 1-10⁹ s^'!mm Hg^'1 at EZn = 3

I. kTd. It is obvious that minimum application time for such field cannot be les than typical ionization time τ = IZf= 10^'9 s-mm Hg = 10^"9 -4- 10^]% [s] = 4-10⁷/n [s] The right limit is determined from overheating degree of a discharge channel: vC_pAT = U² /Rτ_puls Let us assume that discharge channel is a cylinder with typical radius r, while channel radius is determined by its ionizing expansion in strong field:

EJn = 120 Td = U/ (n*r) hence r = UZ(n* E,/n) And, finally: τ _puk = 10¹¹L RZn [S]

Pulse recurrence rate/ [s^'1] is limited with condition:

where U- high voltage pulse amplitude, [V]; L — inter-electron gap size, [cm], n — molecular concentration in volume unit of a discharge section, [cm^" ]. R — feeding line resistance [Ohm],

C - discharge gap capacity [F], V- gas flow velocity in the discharge section, [cm/s]. Example of implementation of the claimed method in air discharge mode from a extended electrode. Experimental plant ensuring possibility of complex diagnostics of electric discharges developing in streamer discharge form in an overstressed gap is comprised from a generator of high-voltage nanosecond pulses, a system to monitor electric discharge parameters and spectroscopic plasma diagnostic system in a streamer channel. Overall schematic_' of the experimental plant is given on Fig. 4. High voltage generators used in operation are connected via oil-filled connector to wave line 5 assembled from cable PK-50-24-13, length 60 m. In 30 meters from the discharge unit into the cable braiding a shunt backward current shunt circuit 6 is cut enabling to control parameters of current pulses from the both generators. Thus, the method provides simultaneous use of use of various high- voltage impulse generators with control of electric pulse parameters. A discharge section is a conventional version of a geometry "needle (high-voltage electrode 7) - plane (low-voltage electrode 8)" enabling adjustment of inter- electrode gap in the range of 0 to 30 cm. Low voltage electrode 8 is made of aluminum in form of a disc 8 mm thick and 550 mm diameter. High-voltage electrode 7 is made of brass in cone form 300 mm long with 2 mm radius of rounding at the cone top. The structure of the discharge unit helps to adjust discharge gap length and its location relative to base surfaces so that it is not necessary to readjust optical recording paths. This is achieved by shifting top and low electrode assemblies along vertical guides 9, with fixing any intermediate position with threaded stops 10. In conductor lines of the discharge section, gaps are provided to tie in current shunts 11 of direct and return current loops enabling recording of incident and elapsed voltage pulses. For independent current control, in the discharge gap on a dielectric part of the electrode system a broadband Rogovski coil 12 is installed.

As a high-voltage impulse generator, a Marx four-step generator is used. Voltage rise rate at the pulse front is 0.1-1.5 kV/ns depending on generator voltage. Total energy emitted during the pulse depends on its duration adjustable in the range of 100 ns ÷ 1 μs.

To study properties of a single streamer discharge, voltage impulse generator made according to rotating chopper design in pulse-periodic mode. The study was performed in the standard needle-plane geometry using absolute emission spectroscopy methods at atmospheric pressure.

Voltage impulse (fig. 5) fed from the generator via a 50 Ohm cable line 5 was registered with backward current shunt loop 6. Fig. 5 shows an incident pulse (positive polarity). Amplitude of incident pulse was U_max= 9 kV, half- height duration

ns and rise rate τ_in<rQ5 ns. Relative volume registration of active particles in the discharge significantly depends on the size of an inter-electrode gap and resulting average electric field in the gap. Here, as was shown by tests in spatial resolution mode, active particles in electron-excited condition is mainly registered on the streamer head and in directly adjacent areas. As shown on Fig. 6, streamer velocity increases as electrode gap reduces and varies (for our pulse parameters and test plant geometry) in the range of (2- 3.5) 10⁷ cm/s. In the interval 6-12 mm of the inter-electrode gap one observes both streamer and spark discharges, since in this range time of spark channel generation is comparable with high- voltage pulse duration.

Dynamic behavior of voltage, current, power and total discharge energy in case of positive and negative polarity of pulse voltage is shown below. Fig. 7 demonstrates current amplitude values in the given range. As seen from the Figure, minimum gap size at which streamer (weak-current) discharge form is 7 mm at both polarities of high-voltage impulses, which corresponds with gap- average field E = U_max/L - 19 kV/cm. Maximum gap length at which a spark breakdown can happen, strongly depends on pulse recurrence rate. Thus, at 50 Hz pulse recurrence rate this value is 9 mm, which corresponds to average breakdown field 15 kV/cm; however, already at 1.2 kHz frequency limit gap size increased to 12 mm, i.e. breakdown field was 11 kV/cm.

A shift in the minimum gap size for implementation of only streamer discharges is related to accumulation of active particles and partly with locale gap heating as pulse recurrence rate was increased. Fig. 8 (a - f) provided curves illustrating dynamic behavior of voltage, current, power and total discharge energy at positive polarity of voltage, pulse recurrence*τate of 12 kHz, maximum voltage in cable line 8 kV, high- voltage pulse duration (half-height) 75 ns and gap length of 2 ÷50 mm. Figs. 8a and 8b show curves for current 12, voltage 13, in-gap power 14, and total in-gap energy 15 within a high-voltage pulse for a gap with length L = 2 mm. Similarly, Figs. 8c and 8d shows curves for current 16, voltage 17, power 18 and total energy 19 for a gap with length L = I mm, and Figures 8e and 8f - demonstrate curves of the said parameters (current 20, voltage 21, power 22 and total energy 23) for discharge gap length L = 20 mm. A conclusion based on results of the studies performed was that an increase irr electric pulse, duration is limited with a transit to a spark discharge form with low discharge gap resistance. This effect radically limits maximum efficiency of energy input into a discharge gap when using higher duration impulses and DC current discharges. Thus it is shown that to ensure maximum efficiency of energy transfer from a high-voltage pulse to gas, one must limit pulse duration and voltage rise rate within a gap.

To provide an illustrative method implementation example, let us give a selection of parameters for a plasma jet generator operating according to the above principle. Plasma gun geometry: L = 0.2 cm, gas pumping speed V^~ 10 m/s, gas concentration «=5*1 O¹⁹ cm^"3, electric capacity of the gap 1 pF. For the said values amplitude U [V] of a discharge is limited with the condition: 3-10^"14.> U/{Lxή)> 10^"15 *7> 10kV, C/<300kV

We can take any value within the range; let us assume e.g. that the generator ensures £/=30 kV at internal resistance 50 Ohm.

Rise rate for high-voltage pulse front T/[s] is limited with the condition:

RC< τ_f<l0-²⁴xL²xn/U 5*10^"π < ^<6*10^"9

That is, the generator should ensure voltage rise rate at 6 ns max. High- voltage pulse duration τ_puι_se [s] is limited with the condition:

4-l0^I/n<τ_puι_se<l0^uLR/n

8-10-¹³< T_pH/5e<2*10^"8 Maximum pulse duration is 20 ns.

Pulse recurrence rate f, [s^"1] is limited with the condition:

(W¹ >/ > V/L

50 MHz >/> 5 IdHk

Thus, it is possible to control plasma gun power (output temperature of gas jet) across almost 4 orders of magnitude - from grades to several thousand grades, since it is proportional to pulse frequency.

The method claimed can find its practical application in gas turbine engines and burners used to initiate and intensify combustion of fuel-air mixture (Figs-, 9 and 10). Here pos. 24, 25 and 26 stand for flame tubes of the combustion chamber; pos. 27, 28, 29 stand for plasma-fuel burners. Fig. 10 shows general appearance of a plasma jet generated by the plasma gun at atmospheric pressure. In this case oxidant (air) flow enters the combustion chamber after it is compressed with a compressor unit. In the combustion chamber air flow is mixed with fuel and enters flame tubes 24, 25, 26. (As a rule, stoichiometric ratio "fuel/oxidant" is well within the range of 0.25÷4, though it is limited with this range). Efficient firing of gas and maintaining combustion in low-power mode is ensured by injecting a high-enthalpy gas jet via nozzles 27, 28 and 29 from the plasma gun, in which high efficiency of transferring electric pulse energy to gas is ensured by optimization of high- voltage pulse as to relationships suggested above. Fig. 11 demonstrates dependence of efficiency of transferring electric pulse energy to gas in case of various pulse durations (13 ns and 7 ns) and pulse recurrence rate.

Claims

Invention Claims

1. A method of generation of a high-enthalpy gas jet; the said method involving injection of plasma-forming gas into a discharge gap and exciting of pulse gas discharge in nanosecond pulse duration range by applying to discharge gap electrodes of high- voltage impulses, distinctive in that fact that a discharge is performed at significant gap overvoltage (by 2-10 times).

2. The method of Claim I₃ distinctive in the fact that amplitude U [V], pulse front rise rate i_f [s] and pulse duration τ_puι_se [s] are selected from relationships: -

3-iσ¹⁴ > U/(L xn) > ia¹⁵ RC < τ_f < iσ²³ x L² x n / υ 4-10⁷/n < τ_pulse < 10¹¹L RJn,

While pulse recurrence rate/ [$^''] is selected in the range:

(τ_Puisf >f > VfL, where: L - inter-electrode gap size, [cm], n - molecular concentration in volume unit of the discharge section,

[cm^"3].

R - feeder resistance [Ohm],

C - discharge gap capacity [F], V- velocity of gas flow in the discharge gap, [cm/s].

3. The method Claim I₅ distinctive in the fact plasma jet temperature is adjusted by controlling pulse recurrence rate within the specified range.