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OPERATIONAL CHARACTERISTICS OF A PLASMA TORCH IN A SUPERSONIC CROSS FLOW

S. P. Kuo* and Daniel Bivolaru

Department of Electrical & Computer Engineering, Polytechnic University Six Metrotech Center, Brooklyn, NY 11201

Campbell D. Carterà and Lance Jacobsen§

Air Force Research Laboratory, AFRL/PRA Wright-Patterson AFB, OH 45433

Skip Williams **

Air Force Research Laboratory, AFRL/VSBXT Hanscom AFB, MA 01731

Abstract

Application of torch plasma, which is intended as an ignition aid within a scramjet engine, is studied. The plasma jet generated by a torch module is described by detailing the voltage-current characteristics and through imaging of its plume in a quiescent environment and in a supersonic crossflow. This torch system, with its high voltage discharge, can be operated in periodic (60-Hz) or pulsed modes, depending on the power supply used. In the 60-Hz mode, the capacitors are charged at the line frequency of 60 Hz, resulting in a cyclical discharge. In this mode, the cycle energy is up to 25 J. However, this energy is mainly limited by the power handling capability of the power supply. In the pulsed mode, the torch can deliver up to 100 J in each pulse. Within the Mach-2.5 supersonic flow, which approximates the scramjet-engine startup condition, the penetration height and the volume of torch plume into the crossflow vs. gas supply pressure and plasma energy are determined.

Introduction

The development of the scramjet propulsion system1-3 is an essential part of the development of hypersonic aircraft and long-range (greater than 750

* Professor of Electrical Engineering Research Scientist, Member AIAA àAerospace Engineer, AIAA Associate Fellow § NRC Research Associate, Member AIAA **Research Physicist, Member AIAA Copyright © 2003 by the American Institute of

Aeronautics and Astronautics, Inc. All rights reserved.

miles) scramjet-powered air-to-surface missiles with Mach-8 cruise capability.4 For the hydrocarbon-fueled scramjet in a typical startup scenario, cold liquid JP-7 is injected into a Mach-2 air crossflow (having a static temperature of ~ 500 K); under these conditions, the fuel-air mixture will not auto-ignite. Instead, some ignition aid—for example a cavity flameholder in conjunction with some mechanism to achieve a downstream pressure rise—is necessary to initiate main-duct combustion. With sufficient downstream pressure rise, a shock front will propagate upstream of the region for heat release. The heat release from combustion will ma intain the pre-combustion shock front, while subsonic conditions in the mixing and combustion region favor stable combustion and flameholding.

Of course, even though the device operates as a ramjet under startup conditions (i.e., subsonic flow downstream of the pre-combustion shock) the residence time through the combustion region is short, of order 1 ms. Within scramjet test facilities, the typical mechanisms for achieving the required downstream pressure rise (and stable combustion) are the so-called aero-throttle, where a “slug” of gas is injected in the downstream region, and the heat release from the pyrophoric gas silane (SiH4). Indeed, silane injection into the combustor is current mechanism by which the X43A scramjet vehicle is started. Both of these approaches, however, have their disadvantages: for example, the aero-throttle approach may not allow re-lighting attempts and silane poses obvious safety risks. Thus, an alternative approach is desired. This is the primary motivation for the development of our plasma torch.

41st Aerospace Sciences Meeting and Exhibit6-9 January 2003, Reno, Nevada

AIAA 2003-1190

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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For the purpose of developing techniques to reduce the ignition delay time and increase the rate of combustion of hydrocarbon fuels, Williams et al.5 have carried out kinetics computations to study the effect of ionization on hydrocarbon-air combustion chemistry. The models being developed—which include both the normal neutral-neutral reactions and ion-neutral reactions—focus primarily on the development of plasma-based ignition and combustion enhancement techniques for scramjet combustors. The results computed over the 900-1500 K temperature range show that the ignition delay time can be reduced significantly (three order of magnitude over the 900-1500 K temperature range) by increasing the initial temperature of fuel-air mixture.

Moreover, detailed kinetics modeling also shows a significant decrease in ignition delay in the presence of initial ionization—in the form of a H3O+/NO+/e− plasma—at levels of ionization mole fractions greater than 10−6. The ignition delay time is decreased most significantly at low temperatures. Indeed, the computational results suggest that even larger effects may be observed at the low temperatures encountered under engine startup. Nevertheless, to make use of the high-temperature torch effluent, which may include quantities of radicals, ions, and electrons, it is necessary to project this gas into the engine in such a way that it readily mixes with a fuel-air stream. Poor penetration of the torch plume into the combustor, and/or improper placement of each torch—that is, more than one torch may be required—will limit its effectiveness.

The unique features of the plasma torch developed in the present effort make it well suited for the purpose of ignition in a scramjet engine. These features include the following.

1. the compact size. It can be easily mounted to the combustor wall and requires no water cooling.

2. flexible design. It can deliver high peak powers (and pulse/cycle energy) in 60-Hz or pulsed modes. Furthermore, it can deliver high mass flow rates due to the large annular flow area.

3. high mass flow operation. It can configured to deliver 10s of grams of feedstock per second.

4. durability. It can be run for long periods with an air feedstock.

5. high-voltage operation. Rather than running at high current, the torch runs at high voltage, which allows greater penetration of the arc into the combustor and reduces the power loss to the electrodes (leading to longer electrode life).

The characteristics of the plasma torch produced in the quiescent environment as well as in a supersonic flow are described in the present work.

Plasma Torch Module and Power Supplies

The torch was fabricated by remodeling components from two commercially available spark plugs and adding a tungsten rod as the central electrode. A surface-gap spark plug, which has a concentric electrode pair, is used as the frame. It is chosen because of its concentric electrode structure and relatively long screw thread giving room for a gas-feed chamber. For torch operation, a gas flow between the electrodes is necessary. Thus, the original electrode insulator, which fills the space between the cathode and anode, is replaced with a new one taken from a different spark plug (having a smaller outer diameter). Holes are drilled through the base of the plug to pass gas into the region between the electrodes. Of course, this design requires that the torch be screwed into a plenum chamber that would supply the feedstock gas (possibly to multiple torches). Moreover, the central electrode (anode) set in the new ceramic insulator is replaced by a solid 0.24-cm diameter tungsten rod, which is held in place concentrically with the cathode by the new insulator and axially by a set-screw in the anode terminal post. Of course, the relatively high melting point of tungsten is desirable in the high-temperature environment of the arc. Shown in Fig. 1is a photograph of the torch module. The construction procedure and components of the module have been described in detail in the previous work,6 from which a US patent7 has been awarded.

60-Hz Operation

Two types of power supply are available in our laboratory to operate the torch module. One is a 60-Hz source, which sustains the discharge periodically in 60 Hz. Such produced plasma will be termed “60-Hz torch plasma” in the following. This power source8 includes 1) a power transformer with a turn ratio of 1:25 to step up the line voltage of 120 V from a wall outlet to 3 kV, 2) capacitors of C = 3 µF in series with the electrodes, and 3) a serially connected diode (made of 4 diodes, connected in parallel and each having 15 kV and 750 mA rating) and resistor (R = 4 kΩ) placed in parallel to the electrodes to further step up the peak voltage. The series resistor is used to protect the diode by preventing the charging current of the capacitor from exceeding the specification (750 mA) of each diode and to regulate the time constant of discharge.

In one half cycle when the diode is forward biased, the capacitor is charging, which reduces the available voltage for the discharge in the torch module. However, since the time constant RC = 12 ms is longer than the half period 8.5 ms of the ac input, the discharge can still be initiated during this half cycle; (even though the discharge has lower current and voltage than the corresponding ones in the other half cycle). During this

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Fig. 1. A photo of the plasma torch module.

other half period, the diode is reversed biased and the charged capacitor increases considerably the available voltage and current for the discharge in the torch module. The torch energy (i.e., the thermal energy carried by torch plasma) in each cycle varies with the gas supply pressure p0. The dependence measured in the pressure range from 1.36 atm to 7.82 atm is presented in Fig. 2.

As shown the dependence has a maximum at the gas supply pressure p0 = 6.12 atm, where the plasma energy is 25.6 J. The increasing dependence of the plasma energy on the flow rate in the region of low gas supply pressure (i.e., p0 < 6.12 atm) is realizable because the supplied gas flow works to increase the transit time of charge particles by keeping the discharge away from the shortest (direct) path between two electrodes. As the flow rate increases, the transit time loss of charge particles is reduced and thus the plasma energy increases. However, when the flow rate becomes too high (i.e., p0 > 6.12 atm), the mobilities of charge particles crossing the flow becomes significantly affected by the flow. In such a way that torch energy decrease with increasing pressure. It is noted in Fig. 2 that there is a significant plasma energy drop at p0 = 4.08 atm. This unexpected result may be explained as follows. Schlieren images indicate that a transition from subsonic to supersonic flow at the exit of the module occurs near p0 = 3.4 atm, which was identified by sudden appearance of the shock structure at exit of the torch nozzle in the schlieren image of the flowfield. After the transition, the flow becomes underexpanded. The At p0 = 4.08 atm, the low pressure region in the flow that favors gas breakdown is narrow in the flow direction and close to the exit of the module. Thus the discharge channel is narrow and the transit times of charge particles are small. Consequently, the plasma energy is reduced. As the pressure is further increased, this low-pressure region extends rapidly outward from the exit of the module so that the discharge can again appear in a larger region.

1 2 3 4 5 6 7 8

p0 (a tm )

0

1 0

2 0

3 0

E(J

)

Fig. 2. Dependence of the plasma energy in one cycle on the gas supply pressure.

Presented in Fig. 3 is the operational power function

in the optimal situation with p0 = 6.12 atm. As shown in each half cycle, the discharge lasts for about 7 ms. In each cycle, the one having a larger peak of about 3.8 kW is the power function of the discharge with the diode reverse biased. In the forward bias phase, the peak power is reduced to 2 kW. The average power of the torch exceeds 1.5 kW. It is noted that the added diode in the circuit reduces the voltage requirement of the transformer output for reliable discharge.

As a consequence of the high-voltage nature of the discharge, the arc loop can be many times the distance between the distance between the anode and cathode. This is illustrated with the instantaneous image in Fig. 4, which was recorded with an intensified CCD camera (a Roper Scientific PIMAX). The distance between the cathode (at the center of the base line) and anode spot (on the left-hand side of the cathode) is about 7.5 mm, whereas the length of the arc loop is about 5 cm. Such an extended arc loop increases the path length of the charge particles in the discharge by more than 15 times the direct path length from the cathode to the anode spot.

0 10 20 30 40

t (ms)

0

1

2

3

4

5

P(k

W)

Fig. 3. Power function of 60 Hz discharge; the gas supply pressure of the torch module is 6.12 atm.

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0 10 20 30 40 50 60 70

mm

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Fig. 4. A planar image of torch plasma taken by an ultra fast CCD camera with 10 ns exposure to laser-induced fluorescence from NO molecules.

It has the advantages of improving 1) the lifetime of plasma (by reducing the transit time loss), 2) the lifetime of the electrodes (by reducing charge particles’ kinetic energy before hitting electrodes), and 3) the conversion of electrical energy to plasma energy. Note also that images such as this one indicate that while the length of the arc loop is not sensitive to the flow rate, the width of the loop becomes narrower as the flow rate increases.

Also shown in Fig. 4 is laser-induced fluorescence from nitric oxide, NO, molecules (the emission in the background). NO is produced within the torch plume in the region where the hot torch gas —particularly that gas near the arc—mixes with air. For these measurements, nitrogen with pressure of 1.7 atm was supplied to the torch module; thus, NO is formed primarily near the cathode-portion of the arc loop. Here, we employed a Nd:YAG-pumped dye laser system to generate laser radiation at 226 nm and couple to a transition in the δ(0,0) band of NO. No effort was made to ensure that the arc was in the plane of the laser sheet.

Pulsed Operation The other power supply available is a dc pulsed

discharge source, which uses a RC circuit for charging and discharging (see Fig. 5). A very energetic torch plasma, albeit one with a low repetition rate, can be generated.

In the circuit, when the ballasting resistance R2 is set to zero, the peak power of the discharge can exceed 1 MW; however, the power function has a pulse length of about 400 µs. This ballasting resistor connected in series with the torch works to regulate the discharging current; though this does indeed change the pulse

Fig. 5. RC circuit of a dc pulse discharge source. duration, it also changes the pulse shape (as described below). The current pulse length is proportional to R2C, the time constant of the capacitor discharge, and thus can be adjusted by adjusting the R2 value. This is partially effective, however, as one can only prolong the pulse length over a limited range and has no control of the pulse shape. Shown in Fig. 6a is a power function obtained by connecting a resistor of R2 = 26 Ω in series with the torch. This power function has a peak of about 300 kW and a pulse length of about 800 µs, which is very close to the time constant R2C = 728 µs. The difference is accountable from the effective resistance of the discharge. The resistive component of the V-I characteristic of the discharge can be modeled mathematically by a function9

V = C1tanh α1I – C2tanh α2I (1)

where constant parameters C1, C2, α1, α2 (with C1 > C2 and α1 > α2) are determined experimentally. Thus the instantaneous resistance Rt(t) of the torch is given by

Rt(t) = dV/dI = C1α1sech2 α1I – C2α2sech2 α2I (2)

When the discharge is in the arc mode, α1I and α2I >>1, and thus (2) is reduced to

Rt(t) ≅ R0 exp(-2α1I) (3)

where R0 = 4 C1α1. As R2 is increased to 250 Ω , now the power

function shown in Fig. 6b consists of two parts: an initial part with a large peak for the ignition of the discharge and a subsequent near-constant low-power part, which maintains the discharge. Again, the time constant R2C = 7 ms gives a reasonably good estimate on the pulse length, though the actual value turns out to be about 10 ms. The relative difference between these

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(a)-0. 5 0 0.5 1 1. 5 2 2 .5

t (ms)

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20 0

40 0

P(k

W)

(b)-5 0 5 1 0 1 5

t (ms)

-5

0

5

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Fig. 6. Power functions of pulsed dc discharges with no flow in the background; gas supply pressure of the torch module is 2.72 atm. (a) R2 = 26 Ω and (b) R2 = 250 Ω.

two values is larger than that in the previous case because as shown by Eq. (3), the effective resistance of the arc discharge increases nonlinearly with the decrease of the discharge current. In Fig. 6b, the power level of the constant part is about 2.5 kW. The repetition rate of the discharge is mainly determined by the capability of the dc power supply; in our case, it was about two per second. The power level of the constant part of the pulse can be increased by increasing the capacitance of the RC circuit: for example, by increasing the capacitance from 28 µF to 140 µF and using 75 Ω ballasting resistor, the discharge can maintain a power output of 5 kW for about 16 ms. The energy contained in this portion of the pulse is about 60 J. The initial portion of the pulse has a peak power of about 200 kW, and the corresponding energy is about 40 J (over a span of slightly less than 0.8 ms). The total energy in the pulse is about 100J, which is double the pulse energy obtained from using 28 µF capacitor.

Plasma-Torch Flowfield

As described in the preceding section, each torch module has the size of a conventional spark plug and is in the form of a modular unit. In the following section, we discuss the performance of the plasma produced by this torch module in a supersonic crossflow and in a quiescent environment, for comparative purposes. Measurements consist of video images of the torch emission and of the flowfield schlieren (in the case of

injection into the supersonic flow). We note that due to the limited framing rate, 30 frames per second, these images represent a temporal average during the frame time. Thus, one does not freeze the arc-loop structure as was done with the intensified CCD (Fig. 4). This is true regardless of whether one is viewing the 60-Hz or pulsed discharge.

Experiments were conducted in the test section, measuring 38 cm × 38 cm, of a supersonic blow-down wind tunnel. The upstream flow had a flow speed 570 m/s, a static temperature T1 = 135 K, and a pressure P1 = 1.8×104 N/m2 (about 0.20 atm). These conditions approximate the scramjet startup conditions listed earlier, though the temperature and pressure are somewhat low (e.g., the static temperature for engine startup is about 500 K). The torch plume is injected normally into the supersonic flow, and the performance of torch plasma in terms of its height and shape in the supersonic flow is studied. In experiments, the air supply pressure is varied from 1.7 to 9.2 atm.

We first investigate the 60-Hz torch plasma in the wind tunnel by running with the 60-Hz power supply. Presented in Figs. 7a and b are two airglow images of the plasma torch produced under the conditions of no crossflow and 4.1 atm of air pressure supplied to the gas chamber of the torch module. These figures show the typical shapes of the plasma torch in each half cycle. Figures 7c and d show the torch emission—again with each image showing the torch behavior the half cycle—in the Mach-2.5 crossflow; clearly, the supersonic flow causes significant deformation in the shape of plasma torch. A comparison with Figs. 7a and b indicates that the penetration height of the torch is reduced significantly as the plume is swept downstream by the high-speed flow; nevertheless, the torch plume can still penetrate into the supersonic crossflow by more than 1 cm and also extends downstream about 1 cm, based on these emission images. A bow shock wave is also generated in front of the torch (since the torch acts an obstruction to the oncoming flow), as observed by the image presented in Fig. 8. This, of course, is typical behavior for a jet injected normally in a supersonic crossflow.

We next study the torch operation in the supersonic flow using the high-power pulsed power supply. Shown in Figs. 9a and b are two airglow images of torch plasmas, corresponding to (a) supersonic crossflow and (b) a quiescent environment; in both cases the supply pressure was 2.7 atm. As shown in the figure, the (penetration) height of the torchis again reduced considerably by the wind tunnel crossflow. Comparing with that shown in Fig. 7c, obtained in the case of higher gas supply pressure but lower power, the one shown in Fig. 9b extends about five times as far in the downstream direction and has a slightly larger penetration depth into the crossflow.

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(a)

(c)

(b)

(d)

Fig. 7. Airglow images of ac torch plasmas (a) and (b) no crossflow in the background, and (c) and (d) in the Mach-2.5 crossflow. In the insert of (a), d x = dy = 11.4 mm define the horizontal and vertical scales of the four photos, respectively.

0 2 0 40 60 8 0 1 00mm

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60

80

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Fig. 8. Shadow image of the flow; an oblique shock wave is generated in front of the torch.

Clearly, the increased discharge power produces a plasma having a larger volume, which is also evident in comparing Figs. 7a and 9a. To increase torch penetration height in the wind tunnel, the air supply pressure was increased to 9.2 atm. The resulting schlieren image is shown in Fig. 10. An oblique shock

wave is also generated in front of the torch as shown in this schlieren image. The voltage and current of the discharge as well as the shape and dimension of torch plasma vary with the torch flow rate and the crossflow condition. The results show that in addition to increasing the flow rate, one can increase the torch power to improve the penetration of the plasma into the crossflow. Because of the slow framing rate of the video camera, a time assembly approach was adopted to reconstruct such a time dependent behavior, i.e., assuming that the discharge was repeatable if the background was not perturbed. When the discharge does not exactly synchronize with the recording, the camera can catch different intervals of the discharge to reveal information on the temporal variation of the discharge. A set of results obtained by this method is presented in Fig. 11 to show how a pulsed torch decays in a supersonic flow. As shown, torch maintains a near constant penetration height of about 1 cm, while its horizontal extent is decreasing. This is because 1) the gas flow rate is constant and 2) during the decay the discharge voltage decreases much more slowly than does the current.

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0 20 40 60 80 100 120mm

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(a)

(b) Fig. 9. Sideview of airglow images of pulsed dc torch plasmas (a) in a quiescent environment and (b) in a supersonic crossflow (about 100 off the sideview line); the field of view in (b) is estimated to be 9.5 cm x 6 cm; the gas supply pressure of the torch module is 2.72 atm.

0 1 0 20 30 4 0m m

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Fig. 10. Schlieren image of pulsed dc torch plasma in a Mach 2.5 crossflow; the backpressure of the torch is 9.2 atm.

(a)

(b)

(c)

(d) Fig. 11. Time evolution of torch plasma produced by a pulsed dc discharge; It is a time assembling of the results from a sequence of similar discharges not synchronized with the recording, the field of view and the gas supply pressure of the torch module are the same as those of Fig. 9(b).

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Conclusions

We have designed and assembled a modular plasma torch—which alone, or in an assembly of torches —is intended as an ignition aid for a hydrocarbon-fueled scramjet engine. This torch design is based on components fro m standard spark plugs, but is modified for inclusion of a gas feedstock supply. Two principal advantages of this torch design are that it is compact, resembling a conventional spark plug, and it is durable, running for long periods of time on an air feedstock.

The peak and average power of the 60-Hz torch plasma generated by the 60-Hz ac power supply are 3.8 kW and 1.5 kW, respectively. Penetration of such a torch plasma into a Mach 2.5 crossflow by about 1 cm has been demonstrated. Furthermore, the cycle energy is 25 J. In the pulsed mode, the torch can deliver 100 J over a span of 16 ms, while the peak power is about 200 kW. Furthermore, in the pulsed mode of operation, the plasma penetrates 50% deeper into the Mach 2.5 crossflow and extends much farther in the flow direction than it does 60-Hz-mode.

Acknowledgements

We would like to acknowledge Dr. Thomas Jackson, Air Force Research Laboratory (AFRL/PRA), at Wright-Patterson AFB, OH, for the valuable discussions. We are also grateful to Mr. Lester Orlick for assisting us to run the wind tunnel.

This work was supported by Air Force Office of Scientific Research Grant AFOSR-F49620-01-1-0392.

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