[american institute of aeronautics and astronautics 46th aiaa aerospace sciences meeting and exhibit...

American Institute of Aeronautics and Astronautics

1

Experiments on Plasma-Assisted Combustion in M=2 Hot Test-Bed PWT-50H

Sergey B. Leonov*, Constantin V. Savelkin† and Dmitry A. Yarantsev‡ Joint Institute for High Temperature RAS, Moscow, 125412 Russia,

Campbell Carter§ Air Force Research Laboratory, AFRL/RZAS, Wright-Patterson AFB, OH, US,

and

Valery N. Sermanov**, Michail A. Starodubtsev†† TsAGI, Zhukovsky, Moscow region, Russia

The paper considers the results of several years’ efforts in field of Plasma-Assisted Combustion in high-speed airflow. Electrical discharge generated straight in M=2 flow is applied for different geometric configurations: cavity, backwise wallstep, and on a plane wall. Hydrogen and ethylene ignition and flameholding are demonstrated experimentally at air temperature T0=300-670K. Parametric dependences of the flameholding effect on fuel feeding, gas temperature, geometry, discharge power, etc. are described. The effect of “cold” combustion is shown under lean and rich mixture. The physical mechanism of combustion breakdown is discussed. The experiments are supported by 3D Navier-Stokes simulations.

I. Introduction he technique based on electrical discharge generation in a predefined zone of the flowfield has a practical potential for flow/flight control generally, and for combustion control particularly. Several topics are of the

most interest in frames of “Plasma-Assisted Combustion”: fuel ignition under non-optimal conditions, mixing of reactive components, flameholding, instabilities suppression, completeness augmentation, etc. The idea of this work is to study the phenomena of plasma-induced ignition and flameholding at direct fuel injection into high-speed low-temperature airflow due to multi-electrode quasi-DC electrical discharge generated straight in zone of potential combustion. Several relevant problems of high-speed combustion (such as shortening of combustor, reduction of radiative and dissociative loss, pollution control, mixing, and others) could be mitigated significantly in case of successful plasma application [1-3].

Recently interest in the field of plasma-assisted combustion has grown, as it is one of the most promising methods for acceleration of the ignition, enhancement of the chemistry, and intensification of the mixing. Accordingly, the number of publications has quickly risen; refs. [4-16] are small part only, which gives some representation about the scale of the overall effort. The physical mechanisms of an air-fuel composition processing by electrical discharge for the combustion enhancement are well described last years [17]. Two domains are the most important: 1) molecular kinetics modification (heating, plasma-chemical excitation, active particles generation); and 2) the flow structure control to create local zones of intensive mixing and extended time of the mixture residence.

Fig. 1 below shows conceptually several possible schemes of the Plasma-Assisted Combustion in the lab-scale tests. Two schemes in the first column are quite typical. The first scheme of premixed composition ignition is used

* DSc, Head of Plasma Aerodynamics Lab, AIAA Associate Fellow, [email protected] † Senior Engineer ‡ PhD Student, Plasma Aerodynamics Lab. Research Staff § PhD, Senior Aerospace Engineer, AIAA Associate Fellow ** DSc, Head of Lab, Dep#1 †† PhD, Research Staff

T

46th AIAA Aerospace Sciences Meeting and Exhibit7 - 10 January 2008, Reno, Nevada

AIAA 2008-1359

Copyright © 2008 by S. Leonov. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


2

in model tests and at theoretical analysis of non-equilibrium plasma effect on the flame propagation. The second scheme presents so called “plasma torch” generation for the fuel ignition. Torch location can be different: torch upward, downward, in a cavity, and so on. We consider that such a scheme is less effective due to high power loss and the need to achieve adequate fuel-air mixing before plasma assistance can take place. Two other schemes were supposed to avoid the heat loss: (1) the electrical discharge generation just in place of interaction with fuel, and (2) using a small amount of extra fuel as an additive to feedstock gas to increase thermal power of a plasma torch in place of interaction with fuel (active pilot flame). The first approach was tested in particular in our works and has shown promising results. An improved scheme is considered in this paper as a candidate for flameholding over a plane wall. It has to be noted that not only those schemes were tested. Numbers of interesting works were published, especially under radio-frequency, RF, and microwave, MW, types of the plasma excitation.

Figure 1. Experimental schemes of the plasma assistance in high-speed combustion. Red color is a fuel

injection, blue – air flow, green – electrical power supply.

At the same time most works in this field describe lab-scale experiments with premixed air-fuel composition or theoretical studies. Such a position is understandable overall, since full-scale experiments are expensive, complicated, and dangerous. The experimental facility PWT-50H is designed as an alternative to full-scale continuous supersonic combustor experiments in accordance with the following requirements: flow parameters are close to typical for combustors; operation time in steady-state mode is much greater than the characteristic gasdynamic time; test section dimensions are much greater than thickness of boundary layer; the total fueling is small enough to be non-hazardous but large enough to observe the physical and chemical effects; the number of runs is in the range of 10-100 per day; special arrangement can be made for plasma generation and diagnostics. Generally speaking, it is a sample of “low-cost” experimental science.

In 2003 a new method of flameholding in supersonic combustor based on generation of plasma induced local unsteady separation near plane wall has been announced in ref. [18]. Here, plasma formation near surface in airflow influences the near-wall layer itself, and in particular, leads to a local flow separation. That aerodynamic configuration can be utilized instead of mechanical arrangements, especially, under non-optimal operation mode of the combustor. The flame stabilization mode could occur at relatively low level of extra energy deposition. The experimental demonstration of the effect was published in [19]. This paper presents at the first time parametrical experimental results in frames of that method development. A main advantage of the scheme of direct flameholding by plasma is considered as a more stable operation in comparison with the discharge integrated to mechanical flameholder.

II. Test Facility Description The experiments were conducted in a short-duration blowdown wind tunnel PWT-50H with a closed test section

Y×Z=72×60mm at initial Mach number M=2.0 and static pressure Pst=100-250Torr. More detailed description can be found in [19-20]. The experimental facility is equipped with a schlieren system, high-speed video camera, fast line-scan camera, IR camera, set of fast-response pressure transducers, spectroscopic system, photo-sensors, current-voltage sensors, chemical station and a set of control-measurement devices. Data acquisition system includes PC-based individual devices for pressure distribution records, video cameras control, spectroscopic and operation parameters’ records, electrical parameters record, etc. The technical parameters of facility PWT-50H are shown in


3

the table 1 below in two columns: parameters of PWT-50 without heater and maximal parameters at the heater operation. The principal scheme is presented in Fig.2.

Table 1. Technical parameters of the facility PWT-50H. PWT-50, 20.05.06 Current state, 20.12.07 Test section cross-dimensions, mm 60*72 60*72 Mach Number 0.1-0.9; 2 2 Maximal stagnation pressure, Bar 1.0 1.8 Stagnation temperature, K 300 300-670 Maximal air mass flow rate, kg/s <1.0 0.5-1.0 Maximal fuel mass flow rate, g/s <5 0.1-8 Operation time, s <1 0.3-0.5 Steady stage duration, s 0.2-0.5 0.15 Typical power of heater, kW - 700 Maximal thermal power with fuel combustion, kW 300 1000 Maximal power of discharge, kW 10 20 Runs per 8 hours, range 10-80 10-30 Lifetime, runs between services 500 100

Figure 2. Experimental facility PWT-50H. General layout.

The experimental modeling of high-speed combustor requires heating the ingoing gas as it occurs in inlet. For this purpose two-stages heater is applied. The concept-design of the air heater includes arc chamber and burning chamber (fore-chamber of main facility in our case). Maximal quasi-continuous thermal power of the heater is about 700kW. Thermal loss is significant in honeycomb due to a short operation; resulting stagnation temperature of air measured as of T0≤670K in continuous mode (t>100ms) and up to T0=1100K in pulse mode (t≈10ms). Extra oxygen addition is used for compensation of burned amount. Operation mode is examined by the procedure included P0 and V measurements, record of electrical parameters of the heater, flame luminosity, and chemical composition of the exhaust gases.

The Fig.3 presents schemes of electrodes’ arrangement. They consist of insertions made of refractory insulating material. The insertions are flush-mounted as well as electrodes themselves. Number of electrodes N=3-14 can be varied test per test depending on the specific task. This arrangement allows a quite flexible operation. The discharge operates with the following typical parameters: time of generation – tpl=50-200ms; current through each electrode – Ipl1=1-5A; typical electrical input power – Wpl=3-10kW that is equivalent to a mean gas temperature elevation less than ΔTav=20K. The fuel injectors are installed directly on the bottom wall of the test section as it is shown in Fig.3. Minimal portion at the single pulse is about Δm=1mg. Maximal fuel mass flow rate is about Gf=8g/sec. The fuel orifices number can be adjusted for each test. Numerous experiments were conducted at different conditions: subsonic and supersonic flow; plane wall, backwise wallstep, long cavity (l/d=6.5), and short cavity (l/d=3.3); different pressure; different power release at both polarities; gas temperature variation; etc.


4

Figure 3. Basic schemes of test geometry and the electrodes arrangement. The discharge dynamics and parameters in flow were measured carefully [21]. Usually plasma channels follow

the edge of separation zone, but in the case of combustion this zone occurs rather unstable. The field’s magnitude in supersonic flow is higher significantly than in the case of subsonic flow or ambient air. Due to the field elevation (E/n≈5×10-16Vcm2) the electron temperature has to be increased from Te=0.8ev to Te=1.5ev approximately. The maximal plasma temperature was measured by the fitting of experimental and calculated spectra of the N2 second positive system (0-0, 337.1nm) and of the N2

+ first negative system (which has bands 2-0, 3-1, 4-2 in this spectral area). By the analysis of the N2 spectra the rotational temperature has occurred to be the same for different conditions, its value is Trot=3.0±0.2kK. Temperature measurements by the N2

+ spectrum give the following results: Trot=4±0.5kK; Tvib=5±0.5kK.

A power release by the discharge is self-adjusted depending on interaction with injected fuel. The initial value can be regulated by number of electrodes switched on or by current variation. The first method allows to keep the reduced electrical field constant. Typical record is shown in Fig.4. It contains three phases: discharge only, discharge-fuel interaction; and combustion. The power release is calculated for each phase separately.

Figure 4. Volt-ampere-power record for the typical run. Typical discharge images at 7 electrodes for two

cases: injection off/on. Two methods were utilized mainly to study the discharge effect on flow structure: pressure measurements and

schlieren visualization with exposure t=0.1μs. Summarizing the discharge effect on pressure distribution in cavity and behind wallstep it should be considered that, as a rule, the pressure rises noticeably just near discharge zone and its distribution occurs more smooth in a cavity as a whole. The discharge effect on flow structure lies in an intensive turbulization of gas in interaction area at simultaneous slight increase of the separation zone volume. As was shown previously [10-21] a flash of radiation is not obvious evidence of the fuel combustion in case of plasma assistance. The most reliable way to recognize namely “combustion” is a vast pressure rise in separation zone.


5

III. Experimental Data

3.1. Hydrogen combustion in cavity and behind wallstep The hydrogen combustion in cavity takes place, if the power deposition to the discharge Wpl>1kW. If the

discharge was turned off, the combustion in cavity is ceased. Increasing of hydrogen flow rate over stoichiometric ratio pushes the combustion above the cavity, if the power deposition is not less than Wpl=3kW. When the thermal power of combustion grew more than to Wfuel=20kW a thermal choking of the duct occurred. The discharge switching off leads to immediate extinction of the hydrogen flame in a free stream, but the combustion in the cavity can be continued unstably. The test on ignition and flameholding of hydrogen behind backwise wallstep demonstrates practically the same values of energetic threshold. Two differences are observed: no discharge means no combustion at all; and wider limits of fuel mass flow-rate before the choking.

The Fig.5 presents typical experimental data on the pressure distribution due to the hydrogen ignition by the discharge in the cavity and behind wallstep in comparison with results of numerical simulation. The estimation on the base of chemical analysis of exhaust gases and due to comparison with 3D Navier-Stocks simulations gives the value of combustion completeness in the cavity’s configuration η≈0.9 and behind wallstep η>0.7 .

Figure 5. Pressure redistribution in cavity and behind wallstep due to discharge generation and injected

hydrogen combustion.

Figure 6. Schlieren photos of the discharge interaction with injected hydrogen behind wallstep. Fuel mass

flow rate Gf=0; 0.4g/s; and 0.8g/s correspondingly.


6

The Fig.6 demonstrates the comparison of result of hydrogen combustion for a fuel mass flow rate of GH2=0.4g/s and GH2=0.8-0.9g/s. The reactions were very intensive with transition to M=1 flow in the second case. The flow regime is changed in the upstream zone as well. It can be seen how the flow disturbances produced by combustion occupied almost the entire duct. Switching off the discharge destroys these regimes immediately. The tests were made at two values of stagnation temperature T0=300K and T0=500K. As there were considered the combustion is more unstable in case of elevated temperature than for cold airflow. The graph of the pressure elevation due to the combustion depending on the fuel mass flow-rate is presented in Fig.7 for cold flow and for heated air both. More detail consideration of the temperature effect is done below.

Figure 7. Modes of hydrogen combustion behind wallstep due to plasma support. Temperature 300K, 500K.

3.2. Ethylene flameholding at fuel mass flow-rate variation This section presents the experimental data on the ethylene ignition and flameholding by electrical discharge at gas temperature T0=293K and initial Mach number of the flow M=1.95 (M=1.8 in section of interaction). Pressure tabs located along the whole duct on the top and bottom walls both. Three points were selected as the most representative: Pst1 – static pressure just upstream the upper line of electrodes, Pst2 – static pressure just behind the wallstep, and Pst9 – static pressure 90mm downstream the wallstep. The data were normalized on the static pressure Pst10 received by sensor located on top wall upstream the area of interaction. That pressure is quite conservative and insensitive to combustion (as will be considered below the ethylene combustion never chokes the duct under described conditions because of the mechanism of instability). The computational analysis performed preliminary gives the value of air mass flow-rate through the separation zone behind wallstep about G1air≈9g/s. The discharge generation increases this value by 20-30%. The combustion being started change the circulation structure in separation zone significantly. So it can be expected the optimal ethylene mass flow rate about Gfuel≈0.6-1g/s. The experiments were conducted at fuel injection in a range Gfuel≈0.1-6g/s.

Figure 8. Pressure increase dependence on ethylene mass flow-rate in three characteristic points.


7

The result at a glance is shown in Fig.8. As previously for the hydrogen three main modes can be recognized: linear zone; zone of saturation; and combustion breakdown at rich mixture. An estimated line of 100% combustion completeness is shown as well for the point of measurement Pst2. The highest level of the completeness can be achieved in zone from relatively lean mixture to stoichiometric ratio, G≈0.5-1g/s. At higher level of injection the fuel (and combustion front) penetrates upstream the wallstep. The pressure increases near electrodes and the combustion completeness drops. Finally the instability prevents reaction. The schlieren picture and pressure record are shown in Fig.9a-c for three typical cases: linear zone, zone of saturation, and rich mixture.

Figure 9a. The typical schlieren image and pressure record for Gfuel≈0.56g/s.

Figure 9b. The typical schlieren image and pressure record for Gfuel≈1.4g/s. T0≈293K.

Figure 9c. The typical schlieren image and pressure record for Gfuel≈2.8g/s. T0≈293K.

Oscillograms prove a high level of interaction in the last case, high value of the discharge voltage. It seems

that some increase in the pressure just behind the wallstep can be explained by power release in the discharge. It can be supposed that the extra power release at very lean and rich mixtures occurs due to partial fuel oxidation by the discharge’s products. We call this mode conditionally as “cold” combustion.

3.3. Ethylene flameholding depending on air temperature. As it was considered previously (section 3.1), in the most cases the effect of the discharge on injected fuel at

heated air (T0=500K) is less than under the cold airflow. In this series of experiments the temperature range was extended with the same result. An extra increase of the discharge power allows to obtain the combustion at elevated


8

temperature. The result of measurements is presented below for the constant value of the fuel mass flow-rate GC2H4=0.9g/s. The graphs in Fig.10 show the pressure rise in three characteristic points in dependence on the gas temperature of the flow and the discharge power. It can be considered that increase of the air temperature requires the increase of the discharge power for the proper combustion. It is clear that the relative power release (Wpl/Htot) in this case is less in comparison with a cold flow. From the other hand the discharge-fuel interaction is actually observed based on voltage data. A current explanation of that effect is in significant modification of the flow structure in separation zone and acceleration of the gas exchange.

Figure 10. Normalized pressure rise vs gas temperature at different discharge power.

Figure 11. Time delay of the ethylene combustion vs gas temperature.

In a certain respect a time delay of the combustion after the fuel injection start gives a better understanding what happens: flow velocity increase in separation zone at temperature evaluation requires more time for an active species accumulation, and sequentially more time for a complete combustion. The graph is presented in Fig.11.

3.4. Ethylene flameholding at the discharge power variation This section presents the experimental data on the ethylene ignition and flameholding by electrical discharge at gas temperature T0=293K and initial Mach number of the flow M=1.95 at different value of electrical power deposition. To conserve the discharge main parameters we reduced the power by means of sequential switching off the electrodes, i.e. 7, 5, 3 or 1 pairs of electrodes were turned on during the run.

The combustion intensity reduced when the power deposition was decreased. In the case Wpl<3kW the combustion was absent independently on the fuel mass flow-rate. The pressure effect and delay time vs power release are presented in Figs.12, 13. Well seen that the effect of flameholding has a power threshold.

Figure 12. Pressure increase vs power deposition in discharge.

Figure 13. Time delay of the ethylene combustion vs power deposition.


9

3.5. Specific properties of electrical discharge at combustion.

An effective resistance of the discharge channel increases significantly at the interaction with an injected fuel and under the combustion extra. A sample of recalculation on the base of volt-ampere measurement is presented in Fig.14 for a single plasma filament. The resistance of the channel increases in 3times under the combustion in average. The level of the voltage and resistance fluctuation is about 40% with respect of average values. Spectra of the voltage oscillations are shown in Fig.15 for the discharge only and for the combustion. Well seen that in case of combustion a main tone shifts to a higher frequency. This fact may be a result of the gas faster movement in separation zone at combustion.

Figure 14. Plasma filament’s resistance behavior during the run.

Figure 15. FFT spectra of the voltage oscillations in cases discharge only and combustion. The reason of the discharge voltage increase at combustion is not fully clear currently. The observation of the

discharge images shows the effect of plasma filament “splitting”. Due to a real exposure was limited by 30us it can be actual splitting or fast movement: in both cases the voltage might increase. To recognize actual plasma behavior the line-scan camera was applied for visualization (6us of exposure of each line, 20us line rate). The results of observation are presented in Fig.16 for 1pair of electrodes. In this case there was only “cold” combustion mode.

Figure 16. Data of line-scan camera under 2 electrodes configuration. Discharge only, discharge+fuel

injection, and “cold” combustion modes. It can be considered that for 2electrodes configuration (single plasma filament) an unsteady “splitting” can be

observed under the combustion, but seldom. The velocity of the plasma filament movement reach a value Vfil=100m/s. In case of multi-electrodes configuration it is difficult to recognize the effect of splitting. The picture is very unstable and inhomogeneous. The electrical current is shared between electrodes located in the same line. But it is clear that the discharge filaments move in zone of interaction very quickly. As result the effective length of the filament occurs higher or the crossed media occurs denser. It seems that this is only a reason of the voltage increase.


10

3.6. Flameholding on a plane wall.

Electrical discharge was excited transversally on plane wall by scheme “anode-cathode-anode-cathode-anode” in 3, 5, or 7 electrodes configuration. As usually the discharge looks as longitudinal plumes from each electrode due to fast movement of the discharges’ loops in flow. The photo is shown in Fig.17 for 7-electrodes configuration. Power release regulation was realized by the discharge current variation in a range Ipl=1-10A. The increase of the current in about 10 times affects on the discharge’s voltage reducing it, which is resulted in rise of the power in about 2 times only. Such a method also leads to some variation in reduced electrical field that may be important further for understanding of some features of interaction. At rather low current the discharge occurs unstable, at too high current the electrodes’ erosion is significant.

A subsonic area or separation zone is generated near the surface downstream the plasma area. Accurate analysis of the data of pressure measurements leads to conclusion that the total pressure near the wall decreases and the static pressure is practically constant or increases, when the effect of plasma-induced separation takes place. At the same time the stagnation pressure at the axis of the duct increases slightly. In case of large power input the “stagnation” pressure near the wall becomes lower than “static” pressure. It means that a circulative flow near the wall with reverse direction of the flow velocity vector is observed.

Figure 17. Discharge photos in M=2 flow without fuel injection (left) and at ethylene injection.

Fuel was injected through 5 orifices located just opposite electrodes in 15mm downstream each one. The fuel

injection was started prior the discharge and was switched off after it. Typically the fuel injection continued 10-20ms longer than the discharge operation to observe a flame extinguishing.

Figure 18. Schlieren pictures in four cases: 1 – no combustion (effect of discharge at Wpl=Wmax=6kW), 2 –

moderate fuel feeding and small discharge power (flame front locates behind inclination), 3 – “optimal” flameholding (no choking, intensive combustion), 4 – M<1 transition in the duct.

The fuel ignition and flameholding were obtained experimentally for the hydrogen and the ethylene injection

both. Primary ignition took place behind the inclination (no separation initially). Further a flame front propagated forestream to the discharge location on position, which depended on fuel mass flowrate and the discharge power release. The data below illustrate details of plasma-fuel-flow interaction: schlieren images in Fig.18; and data on pressure distribution in Fig.19. In a range of GH2=0.4-0.6g/s the flame position is sensitive to the fuel feeding rate and discharge power. Two characteristic points were chosen as the most representative: Pst1 – static pressure just upstream electrodes, and PW2 – static pressure at the mid of inclined plane. The graphs in Fig.20 show pressure rise


11

in these points in dependence on fuel feeding and the discharge power. The same data are presented in Fig.21 for the ethylene at Wpl≥4kW, when the combustion is detected. Well seen that the ethylene is more difficult to be ignited by the discharge.

Figure 19. Static pressure distribution in dependence on hydrogen feeding at constant discharge power.

Figure 20. Pressure increase in two characteristic points of the duct at hydrogen combustion.

Figure 21. Pressure increase in two characteristic points at ignition and flameholding of ethylene.

The analysis of experimental data has shown that a main steering parameter of the discharge effect on the

ignition and flameholding is the total power Wpl deposited. At the same time some influence of the power density and of nonequilibrium level was revealed too. There was possible to keep the total power and to vary a number of electrodes switched on. Reducing electrodes quantity to 3 ones leads to some decrease in power threshold of ignition. The penalty of that is sufficiently 3D character of the flame. The effect of reduced electrical field increasing in the discharge (electron temperature consequently) can be recognized in Fig.20, where the less power Wpl=3.4kW gave a bit better result in comparison with the higher power Wpl=4kW. At least we have no another reasonable explanation currently.


12

IV. Simulation of fuel ignition behind wallstep at elevated temperature A main objective of the computational analysis was the recognition of reason of effectiveness decrease at gas temperature increase. The same effect has been observed in simulations. Currently we are not sure than the physical nature of the effect in simulations and in experiment is the same. At least it can be one of the possible explanations.

In frames of 3-D NS simulation of the fuel ignition and flameholding behind wallstep the analysis was fulfilled for gas temperature T0=300K, 500K, and 700K. The scheme and the method of calculations were presented previously [19]. Here valuable results and comments are presented only.

A main result of simulation of hydrogen-air combustion is the effect of completeness reduction at the gas temperature increase. The figures below Fig.22a,b,c show the dependence of the pressure rise and combustion completeness on fuel mass flow-rate at three different gas temperatures and constant value of the discharge power Wpl=5kW.

Figure 22. Pressure rise and combustion completeness as the result of modeling in comparison with

experimental data. M0=2, T0=300, 500, and 700K, W=5kW. The model didn’t predict a thermal choking of the duct and instability of the combustion at rich mixture.

Moreover the combustion completeness occurs a bit higher in experiment than under modeling. At T0=500K and T0=700K the combustion completeness calculated is much lower than at T0=300K. The analysis has shown that the formal reason of this effect is in acceleration of the flow circulation in separation zone with sequential reduction of the gas temperature and concentration of the atomic oxygen O. The illustration is shown in Fig.23. As it can be recognized the maximal gas temperature decreases from 4240K down to 2320K at the gas temperature increase. Appropriate mass fraction of O is reduced from 2.4×10-2 to <10-10.

Figure 23. Temperature distribution without fuel injection at different gas temperatures.

V. Effectiveness and capability The principal question of a technology is the level of its efficiency. The energetic type of criterion can be taken

for the rough characterization of the ignition efficiency. The most important figure of this approach is the power of self-ignition Wsi, which is the calculated power that has to be released to the gas for predefined induction time of thermal ignition. The physical criterion suggests that the thermal power of self-ignition should be related to a measured power threshold (electrical) of ignition or flame stabilization:

pl

si

WPTW ),,(

10τη = ,

where Wsi is calculated on the base of thermal equilibrium approach for a residence time of gas in freestream or in the fixed separation zone.

The Fig.24 shows the calculated thermal limits of self-ignition for hydrogen and ethylene. An admissible residence time is defined by a reasonable combustor’s length in case of freestream and by the time of gas circulation in the case of fixed separation zone. The limits for model test are shown in the Fig.24 as well. For the ethylene test


13

the temperature of selfignition is T0=1150K approximately. The value of self-ignition power can be estimated by simple expression:

)(1 0TTcGW sipairsi −××≈ , where Gair1 is defined by the geometric configuration. In a wallstep based flameholder the simulation gives Gair1≈10g/s in our case. By such a way the calculated power at ethylene fueling into wallstep configuration is Wsi≈8.5kW for T0=293K and Wsi≈6.5kW for T0=500K. An appropriate effectiveness of plasma ignition of ethylene behind wallstep is shown in the table 2 below in respect of flow through the separation zone and in respect of the flow in a whole duct. An energetic threshold was measured for GC2H4=0.8g/s with the criteria of unstable combustion (breakdown) probability A≈0.5.

Figure 24. Self-ignition limits of gaseous fuels at 1Bar.

Table 2. Estimations of the effectiveness of plasma technique for the high-speed combustion support. T0=293K T0=500K Threshold of stable plasma-assisted combustion 2.5kW 4kW Heat of selfignition behind wallstep 8.5kW 6.5kW Heat of selfignition in respect of whole flow 680kW 520kW Effectiveness for separation zone, Gair=10g/s 3.4 1.6 Effectiveness for whole duct, Gair=0.8kg/s 270 130

The energetic threshold measured of the ignition and flameholding by the discharge near plane wall can be

compared with the data for the ignition in separated zone and in a shear layer behind backwise wallstep. The statistically averaged results are shown in the table 3 below.

Table 3. Energetic threshold of the flameholding measured.

H2 (T0=300K)

C2H4 (T0=300K)

C2H4 (T0=500K)

C2H4 (T0=650K)

Threshold of ignition in cavity and behind wallstep 1kW 2.5kW 4kW

Threshold of flameholding in shear layer over wallstep <3kW 3.5kW ≈5kW ≈10kW

Threshold of flameholding over plane wall <3kW 4.5kW >5kW >8kW

VI. Conclusion The main physical mechanisms of the plasma effect are described as follows: they are not only heating of the

gas and the enormously high value of radical deposition by nonequilibrium plasma, but also the flow structure regulation. The maximum effect at minimal power deposition can be realized under in-situ generation, nonuniform discharge structure and nonequilibrium composition. Such an approach was demonstrated in different geometrical configurations, including the fuel ignition over a plane wall of supersonic duct.

The energetic threshold for hydrogen ignition in the separated zone was about Wpl=1kW, while the value required for plasma-assisted combustion in the shear layer was 3 times higher for our experimental conditions. The


14

thermal effect of the fuel combustion was up to Wf=100kW. The estimated completeness was η>0,9 for hydrogen fueling and η>0,8 for ethylene fueling. The effectiveness of the plasma technique is estimated as large as η1>100 for a whole duct flowrate.

Two main regimes of interaction were considered: “cold” combustion and “normal” combustion. In the first case only a small amount of the fuel were oxidized. Current understanding is that the “cold” combustion occurs at the chemical reactions with excited species in air after the discharge processing and did not transform to the “normal” combustion mode.

The plasma technique performance for the ignition and flameholding in the zone of fixed separation is limited by flow instability, especially under rich mixture. Extra power release due to chemical reactions causes a significant increase of the gas exchange between main flow and separated zone and the combustion breakdown. Vise versa to that the flameholding on a plane wall demonstrates a stable operation in wide range of parameters.

The experiments were carried out on the plasma-assisted combustion of ethylene in cold air and under the heater operation at temperatures T0=300-670K. It was considered the reducing of intensity of the fuel combustion at increase of the gas temperature under the experimental conditions. A working hypothesis is that an increase of the temperature leads to intensification of the gas circulation in separation zone and gas exchange between separated zone and main flow.

The analysis of the combustion effectiveness in different conditions and the discharge features under the combustion was made. Optimal parameters of interaction were defined to achieve the completeness close to 100%. Two main peculiarities of the ethylene combustion under plasma assistance can be noted: there were no regimes with thermal choking of the duct; and, no discharge means no combustion.

Acknowledgments In 2005-2007 the work was funded by EOARD through ISTC project #3057p (special thanks to Dr. J. Tishkov

and Dr. S. Surampudi).

References 1. E. Curran, and S. Murthy, -“Scramjet Propulsion”, Purdue University, 2001, Progress in Astronautics and Aeronautics. 2. Kuznetsov V.R., Sabel’nikov V.A. “Turbulence and Combustion”. New York: Hemisphere (1990). 3. P. K. Tretyakov “The problems of combustion at supersonic flow”, West-East High Speed Flow Field Conference, 19-22

November, 2007, Moscow, Russia 4. L. Jacobsen, C. Carter, R. Baurie, T. Jackson “Plasma-Assisted Ignition in Scramjet”, AIAA-2003-0871, 41st AIAA

Aerospace Meeting and Exhibit, 6-9 January, Reno, NV, 2003. 5. Takita K. Ignition and Flame-Holding by Oxygen, Nitrogen and Argon Plasma Torches in Supersonic Airflow, Combustion

and Flame. 2002. V. 128. P. 301. 6. P. Bletzinger, B.N.Ganguly, D. VanWie, A. Garscadden, “Plasmas in high speed aerodynamics”, Topical review, J. Phys. D:

Appl. Phys., 38 (2005), R33-R57 7. Starikovskaia S.M. Plasma Assisted Ignition and Combustion, J.Phys. D: Appl. Phys. 2006. V. 39. R265. 8. Shibkov V.M., Chernikov A.P., Ershov A.P. et al. Ignition of the Supersonic Propane-air Mixture with the Help of the

Surface Discharge, Proceedings of the 5th Workshop on Magnetoplasma Aerodynamics for Aerospace Application, Moscow, 2003. pp. 162-165.

9. N.A. Popov, “Influence of Nonequilibrium Excitation on H2-O2 Mixtures Ignition”, Termophysics of High Temperatures (rus), 2007, v.45, #2, pp.296-315

10. Leonov, S. B., Yarantsev, D. A., Napartovich, A. P., Kochetov, I. V. “Plasma-Assisted Combustion of Gaseous Fuel in Supersonic Duct”, Plasma Science, IEEE Transactions on, 2006, Volume: 34, Issue: 6, pp.2514-2525

11. Ainan Bao, Guofeng Lou, Munetake Nishihara, Igor V. Adamovich “On the Mechanism of Ignition of Premixed CO-Air and Hydrocarbon-Air Flows by Nonequilibrium RF Plasma”, AIAA-2005-1197, 43rd AIAA Aerospace Meeting and Exhibit, 10-13 January, Reno, NV, 2005.

12. I. Matveev, S. Matveeva, A. Gutsol, A. Fridman “Non-Equilibrium Plasma Igniters and Pilots for Aerospace Application”, AIAA-2005-1191, 43rd AIAA Aerospace Meeting and Exhibit, 10-13 January, Reno, NV, 2005.


15

13. P. Magre, V. Sabel’nikov, D. Teixeira, A. Vincent-Randonnier “Effect of a Dielectric Barrier Discharge on the Stabilization of a Methane-Air Diffusion Flame”, ISABE-2005-1147, 17th International Symposium on Air-Breathing Engines, Munich, Germany, 4-9 September 2005.

14. V. A. Vinogradov, A. F. Alexandrov, I. B. Timofeev, and I. I. Esakov, “The effects of plasma formations on ignition and combustion,” AIAA-2004-1356, 42nd AIAA Aerospace Meeting and Exhibit, 5-8 January, Reno, NV, 2004.

15. J. Liu, F. Wang, L. Lee, N. Theiss, P. Ronney, M. Gundersen, “Effect of Discharge Energy and Cavity Geometry on Flame Ignition by Transient Plasma”, AIAA-2004-1011, 42nd AIAA Aerospace Meeting and Exhibit, 5-8 January, Reno, NV, 2004.

16. S. Williams, S. Popovic, L. Vuskovic, C. D. Carter, L. Jacobsen, S. P. Kuo, D. Bivolaru, S. Corera, M. Kahandawala, and S. Sidhu, “Model and igniter development for plasma assisted combustion,” AIAA-2004-1012, 42nd AIAA Aerospace Meeting and Exhibit, 5-8 January, Reno, NV, 2004.

17. S. Leonov, “Gasdynamic Phenomena Concomitant with Plasma-Assisted Combustion Experiments” Proceedings of the 7th Workshop on Magnetoplasma Aerodynamics for Aerospace Application, Moscow, IVTAN, April 2007.

18. S. Leonov, V. Bityurin, K. Savelkin, D. Yarantsev "Progress in Investigation for Plasma Control of Duct-Driven Flows.” AIAA-2003-0699 41th AIAA Aerospace Meeting and Exhibit, 6-10 January, Reno, NV, 2003.

19. S. Leonov, C. Carter, M. Starodubtsev, D. Yarantsev, “Mechanisms of Fuel Ignition by Electrical Discharge in High-Speed Flow”, Paper AIAA-2006-7908, 14th AIAA/AHI Space Planes and Hypersonic Systems and Technologies Conference, Canberra, Australia, Nov. 6-9, 2006

20. I. Kochetov, S. Leonov, A. Napartovich, D. Yarantsev “Plasma-Assisted Ignition and Flameholding in High-Speed Flow”, Paper AIAA-2006-0563, 44th AIAA Aerospace Sciences Meeting & Exhibit, January 2006/ Reno, NV.

21. D.A. Yarantsev, S.B. Leonov, K.V. Savelkin, “Passive optical spectroscopy of plasma assisted combustion of ethylene in high speed flow”, 31st International Symposium on Combustion, University of Heidelberg, Germany, August 6-11, 2006.

[american institute of aeronautics and astronautics 46th aiaa aerospace sciences meeting and exhibit...

Documents