Plasma-Assisted Stabilization of Lifted Non-Premixed Jet Flames

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Plasma-Assisted Stabilization of Lifted Non-Premixed Jet Flames Ying-Hao Liao, and Xiang-Hong Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03940 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Energy & Fuels

Plasma-Assisted Stabilization of Lifted NonPremixed Jet Flames

Ying-Hao Liao*,a and Xiang-Hong Zhaoa

aDepartment

of Mechanical Engineering, National Chiao Tung University, 1001 University Rd., Hsinchu 300, Taiwan

*Corresponding author. Address: Department of Mechanical Engineering, National Chiao Tung University, 1001 University Rd., Hsinchu 300, Taiwan. Tel: +886-3-5712121ext.55119. Fax: +886-35720634. E-mail: [email protected]

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Abstract The present study experimentally investigates the effects of plasma discharges on the stabilization of lifted non-premixed jet flames in a stream of co-flow air. The plasma discharge is produced on the sharp edge of the fuel nozzle exit, facilitating its impact on flame stabilization. It is observed that the application of plasma discharges has an impact on the enhancement of flame lift-off velocity, lift-off height and hysteresis phenomenon, and leads to plasma-attached flames, plasma-enhanced lifted flames and plasma-ineffective lifted flames, depending on flame lift-off conditions. A maximum enhancement of approximately 84% is observed for the flame lift-off velocity when the co-flow velocity is sufficiently low. As the co-flow velocity or the jet Reynolds number is low, the flame is anchored at the nozzle by the discharge. As the co-flow velocity or the jet Reynolds number is increased, the flame detaches but with a decrease in lift-off height compared to the flame without the discharge. If the co-flow velocity of the jet Reynolds number is continuously increased such that the flame lift-off height is beyond a critical value, the effect of plasma discharges diminishes and the flame lift-off height becomes comparable to that without plasma discharges. In flame hysteresis studies, both of the flame lift-off and reattachment velocities are increased with twofold enhancement when the discharge is present. Spectroscopic study shows that the emission characteristics, particularly for those between 400 and 800 nm, in flames are intensified with the presence of plasma discharge, consistent with the flame luminosity visualization, probably due to the reduced lift-off height that leads to reduced air premixing, resulting in increased soot formation.

Keywords: plasma-assisted combustion, lifted flames, lift-off height, lift-off velocity, flame hysteresis, flame stabilization


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1. Introduction Flame instability has been of the primary interest to the combustion society and an essential issue to the industry. The investigation of lifted turbulent jet flames is of specific interest for many industrial, commercial, and military applications due to the extensive use of turbulent jet flames in furnaces, burners, boilers, and turbine reactors. Of particular interest to these applications is the continuous improvement in burners capable of operation with various fuels that have different heating values, laminar flame speeds, and Schmidt numbers etc. Understanding how various flames behave in the near-nozzle region under different operation conditions allows and benefits the design of clean, stable, and efficient combustion systems. In general, lift-off phenomenon is not desired in many applications due to the fact that it could cause leakage of unburned fuel, noise generation, flame extinction and blowout, resulting in safety problems. Traditionally, several methods have been used to achieve stabilization in combustion systems. Methods that are generally seen are pilot flames,1 oxygen-rich co-flow,2 bluff body3 and swirling flow.4 Pilot flames and oxygen-rich co-flow have been demonstrated to enhance flame stability in laboratory scale flames, but both of these methods add considerable complexity to an overall industry scale system. Flow recirculation generated by bluff body and swirling flow can improve the stability of flames by transporting hot combustion products back to the injection plane. However, the resulting entrainment of high-temperature burned gas and the long residence time of flow in the reaction region could potentially result in an increase in NOx production. The application of plasma to combustion seems to be a promising approach for flame control and manipulation due to its unique capability in producing active species and heat, subsequently


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altering reacting rates and pathways, leading to an enhancement in combustion processes.5 In recent years, considerable studies have been conducted to show the potential of plasma for aerospace6 and automotive7 applications. In high-speed propulsion such as scramjet engines, recent studies using different plasma discharges have shown that plasma can enhance ignition,8 flame stabilization,9 and fuel/air mixing6 through chemical, thermal, and plasma-induced aerodynamic effects. Starikovskiy et al.10 has also demonstrated that a nanosecond pulsed discharge in pulsed detonation engine (PDE) led to a shorter ignition delay time, resulting in a more rapid deflagrationto-detonation transition. In laboratory experiment, plasma has also been demonstrated to successfully extend the flammability range,11 enhance the flame stability,12 and facilitate the flame ignition.13 Barbosa et al.11 employed a nanosecond pulsed discharge on the flammability of a lean premixed propane/air flame at atmosphere. The flammability regime of the flame was determined by imaging CH* chemiluminescence of the flame. It was clearly shown that without plasma, the flame extinction was observed at an equivalence ratio of Φ = 0.41, whereas the flammability, with plasma, was extended to Φ = 0.11. Vincent-Randonnier et al.12 investigated the effect of a coaxial DBD (dielectric-barrier discharge) on a laminar lifted methane jet flame for potential application of this technique to stabilization of supersonic combustion. They observed that the detachment height of a lifted flame was decreased with the plasma discharge. As the applied voltage was increased, the flame eventually was hooked by the plasma discharge, and this situation continued for a Reynolds number up to Rej ≈ 3600. Kim et al.14 reported a study on the application of repetitive nanosecond pulsed plasma discharges to the stabilization of a laminar lifted methane jet flame with a vitiated co-flow of temperature ranging from 855 to 975 K. It was found that the flame stability, characterized by the average lift-off height, was improved substantially by the presence of the discharge. In the absence of the discharge, no auto-ignited/sustainable flame was obtained until the temperature of the vitiated co-flow reached approximately 940 K. Another study conducted by Kim


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et al.15 examined three different types of plasma discharge to the stabilization of a lifted methane jet flame with a Reynolds number of Rej = 3000 – 8000. Three discharges employed in the study were single electrode AC corona discharge (SECD), DBD, and ultra-short pulsed repetitive discharge (USRD). It was found that the application of USRD led to the most significant improvement on flame stability enhancement. Kim et al. attributed this observation to the higher current density of USRD than that of SECD and DBD. Recently, a study conducted by Tang et al.16 applied an argon jet (with floating electrode) to methane diffusion flames. The group showed that the lift-off and reattachment velocities were strongly affected by the plasmas, and the difference between these two velocities as well as the flame hysteresis region (i.e. the flame lift-off velocity is different than the reattachment velocity) was reduced and eventually disappeared at least with the plasma employed in the study.16 Studies on flame stabilization, as those mentioned above, generally produced plasma discharges between the electrode and the flame base with the flame serving as the open ground. This arrangement is most effective when the jet Reynolds number is small (e.g. the flame base is close to the electrode) and the applied voltage is high. However, most practical combustion applications are operated at turbulent range, and the high voltage requirement simply imposes the inconvenience of plasma application on combustion. The present study proposes to produce a corona discharge on the sharp edge of the fuel nozzle exit, irrelevant to the jet Reynolds number, to stabilize lifted jet flames, fueled with methane, in turbulent regime. Of particular interest is the impact of corona discharges on the flame lift-off height and lift-off velocity under various flow conditions, including the jet Reynolds number and co-flow velocity. In addition, flame hysteresis behaviors, both with and without the plasma, are also discussed. Chemiluminescence imaging and spectroscopic study are performed to investigate the interactions between plasma discharges and flames. 2. Experimental Setup


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A schematic of the experimental setup is shown in Figure 1. The jet flame burner with a co-flow duct is capable of producing a jet flame with a wide range of Reynolds numbers, from laminar to turbulent. The burner made of Teflon is equipped with glass beads, a perforated plate and a ceramic honeycomb for flow straightening and uniform distribution. The fuel nozzle made of stainless steel is centered in the coaxial co-flow stream and has an inner diameter of D = 3.7 mm. The length of the fuel tube is at least 100D to ensure the fully developed velocity profile at the exit.17 The co-flow duct has an inner diameter of approximately 150 mm, sufficiently large to avoid disturbance from the ambient air. High-grade methane (99.95%) is used in the present study. The fuel flow is controlled and regulated with a mass flow controller (MKS Instruments). The nominal jet velocity and the nozzle diameter are used for the determination of the fuel jet Reynolds number, Rej. The co-flow oxidizer is room air, consisting of 21% O2 and 78% N2. The volumetric flow rates of co-flow air can be varied in a range between 100 and 400 LPM, corresponding to a velocity range of 9.4 to 37.6 cm/s. It has been long known that both the jet and co-flow velocities have significant impacts on flame lift-off and stabilization.18 These two controlling parameters as well as their interactions with plasma discharge are investigated and discussed in the present study. A schematic of the electrode arrangement and a representative image of the plasma discharge are shown in Figure 2a and 2b, respectively. The discharge is powered by an AC power supply with a peak voltage of approximately 15 kV and a typical frequency range of 20 – 60 kHz. The maximum output power of the power supply is 2 kW. The fuel tube is connected to the power terminal and a coaxial copper sheet wrapped on a quartz cylinder serves as the ground. The quartz cylinder has an inner diameter of 40 mm and is used to avoid sparks and current growth between electrodes.19 This type of electrode arrangement produces corona discharge, as shown in Figure 2b on the sharp edge of the fuel nozzle exit, facilitating the impact of discharge on fuel decomposition and thus flame stabilization. Other advantages of this arrangement include its simple setup, stable operation, and


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Energy & Fuels

feasible scalability from small laboratory reactors to large industrial applications. The current and voltage signals are measured with a Rogowski coil (IPC CM-100-MG, Ion Physical Corporation Inc.) and a high-voltage probe (Tektronix P6015A), and subsequently recorded by a digital oscilloscope. Images of flame are taken with a high resolution CCD camera (Prosilica GX, Allied Vision Technologies) with a frame rate of 100 fps and an exposure time of 10 ms for all cases. A total of 200 images are used for the lift-off height measurement. An ICCD camera (PI-MAX4, Princeton Instrument) equipped with a narrow band-pass optical filter (330 ± 10 nm) is used for air plasma visualization, particularly for the strong emission of N2 (C3Πu – B3Πg) transition.20 The optical emission diagnostics is performed with a spectrometer (USB2000+, Ocean Optics) and a focusing lens. The spectrometer has a detecting range of 200 – 850 nm and an optical resolution of 1.4 nm. 3. Results and Discussions 3.1. Effect of Co-Flow on Flame Lift-Off The velocity of co-flow oxidizer has been shown to have an impact on flame lift-off phenomenon.21 The effect of co-flow velocity on the flame lift-off velocity and lift-off height, both with and without plasma discharges, is shown in Figure 3a and 3b, respectively. The fuel in both cases is methane and the Reynolds number of the fuel jet in Figure 3b is fixed at Rej = 5000. For flames without plasma discharges, as the co-flow velocity is increased, the lift-off velocity decreases and the lift-off height increases, consistent with those reported by Terry and Lyons.21 The increased lift-off height with co-flow velocity is due to the jet boundary experiencing less shear and thus reduced mixing at a higher co-flow flow rate, resulting in a further downstream stabilization point where the local flow velocity can be balanced with the flame burning velocity. The application of plasma discharge to delay the lift-off velocity is evident, as shown in Figure 3a. The maximum enhancement in retarding lift-off velocity is approximately 84% (from 14 m/s to 25.8 m/s) with a co-flow velocity of 9.4 cm/s. The application of plasma discharge is seen to decrease the lift-off height, as shown in Figure 3b. When the co-flow velocity is sufficiently low, e.g.


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9.4 cm/s here, the flame attaches to the nozzle with the presence of plasma. Although plasma has an impact on the lift-off velocity and lift-off height, the enhancement is seen to decrease as the co-flow velocity or the lift-off height is increased. In general, the application of plasma discharge, at least in the present study, to the flame lift-off enhancement can be categorized into: plasma-attached flames, plasma-enhanced lifted flames and plasma-ineffective lifted flames. Details of these flames and the effect of discharges on flame lift-off will be discussed in the next section. Compared to Kim et al.’s study,22 the enhancement on the lift-off velocity was found to be approximately 125% when a DBD discharge was employed. The electrical properties of the discharge, e.g. frequency, power mode, pulse width etc., in the study were similar to those employed in the present study. The greater enhancement on the lift-off velocity in Kim et al.’s study is probably due to the location of the discharge placement. In the study of Kim et al.,22 the discharge was activated in the fuel stream and 46 mm downstream of the fuel nozzle exit. However, the corona discharge employed here is activated at the nozzle exit and in the co-flow air stream in which a higher flow rate leads to less energy density of discharge. If the flow rate of co-flow air is sufficiently high, a negligible impact of discharge on the lift-off velocity and thus the blow-off velocity could be expected. In light of Kim et al.’s another study,15 different discharges, due to the degree of discharge current density, led to different enhancement on flame stabilization. In the study, the flame stability limit, in terms of the maximum co-flow velocity for flame to blow-off, was extended to 8 times with USRD, followed by 50% for DBD and 20-30% for SECD. At a constant co-flow velocity, the enhancement on flame stability was found to be 130% and 35% for DBD and SECD, respectively. No comparison on flame stability enhancement was made by Kim et al. for USRD at a constant co-flow velocity. In the present study, the blow-off limit is not performed due to the limitation of instrument. However, the blow-off limit may presumably not be altered with the discharge employed here since the lift-off velocity with the presence of discharge, as shown in Figure 3a, eventually becomes comparable to


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that without discharge as the co-flow velocity is increased. This is probably due to the location where the discharge is activated. In Kim et al.’s study,15 the discharge was activated in the fuel stream and an increase in co-flow flow rate has a negligible effect on the discharge, as mentioned previously. As shown in the study of Kim et al., there was significant variation in plasma-generated species when the discharge was activated in different flow gases. Besides, the intensity of discharge activated in the fuel stream was not expected to be affected by the co-flow. Moreover, the three discharges in Kim et al.’s study15 are distinct from the one applied here. The corona discharge produced in the current study is powered by an AC power with a typical pulse width in µs, while discharges applied by Kim et al.15 have nanosecond duration. The difference in the electrical properties of discharge may lead to difference in the current density of discharge and subsequently result in dissimilar effects on flame stabilization. 3.2. Effect of Fuel Jet on Flame Lift-Off The jet Reynolds number is seen to have an impact on the flame lift-off height and the results are shown in Figure 4 for flames with and without plasma discharges. The co-flow velocity is 9.4 cm/s, equivalent to 100 LPM, in each case. The lift-off height is seen to increase as the Reynolds number is increased. As evidently seen in Figure 4, the plasma discharge has an impact on the flame lift-off height. Representative images of flames with and without plasma are shown in Figure 5a and 5b, respectively. Under certain conditions, the presence of plasma is seen to anchor the flame at the nozzle exit. For flames studied here, the application of plasma to decrease the lift-off height is effective until the jet Reynolds number reaches Rej = 6000. Beyond Rej = 6000, the plasma has no significant impact on the flame lift-off height, as shown in Figure 4. The diminished effect of plasma on lift-off height with the jet Reynolds number, i.e. the discharge becomes less effective as the lift-off height increases, can be expected. First, the corona discharge is produced on the nozzle exit where the electric field is strong and the degree of ionization is high.19 As the jet Reynolds number is increased, the flame base is further away from the nozzle, where both


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of the intensities of electric field and ionization are significantly reduced. Second, an increase in the Reynolds number implies an increase in particle flux and a decrease in residence time. A combination of these factors makes plasma less effective on the flame lift-off height at high Reynolds numbers. As shown in Figure 2, the discharge is activated in the co-flow air stream. The energy density, ε, defined as the power deposited into the plasma discharge divided by the gas flow rate, is one of the most important parameters used to characterize the discharge.23 The effect of the energy density on the variation of lift-off height is shown in Figure 6. Here, the energy density is controlled by varying the gas flow rate, while the deposited power is kept constant. Thus, the energy density increases when the gas flow is decreased. The values of ε = 60 and ε = 180 J/L correspond to a co-flow rate of 300 and 100 LPM, respectively. The parameter, ΔH/H, on the ordinate represents the variation in lift-off height due to the presence of plasma normalized by the lift-off height without plasma. As shown in Figure 6, the energy density affects the behaviors of flame lift-off. The normalized variation in lift-off height shifts toward low Reynolds number with decreased energy density, suggesting less effective flame stabilization with the lower energy density. The overall results in Figure 6 also show that the plasma becomes less effective on lowering lift-off height with increased Reynolds number, regardless of the energy density, consistent with the observation shown in Figure 4. The overall observation suggests that the plasma discharge is most effective when the flame liftoff height is less than approximately H/D = 10, where flames are plasma-attached flames since they are anchored at the nozzle by the plasma. These flames are generally with low co-flow velocities and low Reynolds numbers. A typical image for one of these flames is shown in Figure 7a. To observe the flame stabilization by plasma, chemiluminescence imaging of N2 2nd positive transition (C3Πu – B3Πg), centering at 337 nm,24 is performed with an ICCD and an optical filter and the result is shown in Figure 8. Note that the image is the average out of ten with a gate width of 100 ms. As


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shown in Figure 8, the discharge extends from the nozzle tip toward the flame base and the flame is simultaneous stabilized by the plasma discharge, accompanied by a sizzling noise. As the co-flow velocity or the jet Reynolds number is increased, flames detach from the nozzle but with a smaller lift-off height compared to flames without the discharge, as shown in Figure 7b. These flames are plasma-enhanced lifted flames and the flame stabilization is probably due to the electric field, rather than the discharge. The edge of these flames generally tilts, pointing toward the nozzle (see Figure 7b). Behaviors of these flames are very similar to those reported in Lee et al.’s study,25 in which the flame stabilization was enhanced with the presence of an AC electric field. If the co-flow velocity or the jet Reynolds number is further increased such that the lift-off height is beyond approximately H/D = 15, the application of plasma discharges and electric field becomes ineffective on the flame stabilization, due to the reduced electric field as mentioned previously, and the flame lift-off height remains comparable to that without the presence of plasma discharges, as shown in Figure 7c. Moreover, an increase in co-flow velocity accompanies a decrease in energy density, suggesting a greater quench of plasma discharge, resulting in diminished enhancement on the lift-off velocity with the co-flow velocity when the plasma is present. 3.3. Effect of Plasma on Flame Hysteresis The hysteresis in flame stabilization refers to the situation that the flame has dual stabilization positions, governed by the lift-off and reattachment velocities.26 The hysteresis behaviors of methane-air flames studied here are shown in Figure 9. The co-flow air is 100 LPM for all cases. For flames without plasma, the lift-off and reattachment velocities are approximately 14 and 12.1 m/s, respectively, and both values are almost doubly enhanced with the presence of plasma. The effect of plasma discharges on retarding flame lift-off and promoting flame reattachment is consistent with that on decreasing flame lift-off height. The enhancement on both lift-off and reattachment velocities is different than those observed in Vincent-Randonnier et al.’s12 and Tang et al.’s16 studies. Both studies have shown that, with the


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presence of plasma, the reattachment velocity was enhanced, however, the lift-off velocity was decreased. This difference in plasma effect on the lift-off velocity is probably due to the method of plasma production and the arrangement of electrode configuration. Both studies of VincentRandonnier et al. and Tang et al. employed a high-powered needle electrode centered in the fuel stream, and plasma discharges were generated between the electrode and flame when flame is close enough to the electrode. Vincent-Randonnier et al.12 proposed that the accelerated onset of flame lift-off was due to the acoustic disturbances due to the plasma discharge or ionic wind. In the present study, the discharge is produced on the sharp edge of the nozzle, and the induced ionic wind intends to flow radially toward the ground electrode, different than that in VincentRandonnier et al.’s and Tang et al.’s studies. This extra flow due to plasma alters the flow field in the near-nozzle region, and a change in upstream diffusion, thus a delay in flame lift-off, may be expected. The delay of flame lift-off due to the upstream diffusion was also observed in Lee et al.’s study25 with the application of a high-voltage AC electric field. 3.4. Spectroscopic Emission Study The emission spectra of plasma discharge, flame and plasma-stabilized flame at the nozzle exit, i.e. at an axial location of X/D = 0, are shown in Figure 10. The flame has a Reynolds number of Rej = 5000. Due to the low optical resolution of the spectrometer, the spectra data shown here are only for qualitative discussion, not meant for quantitative measurement. Similar to other atmospheric non-equilibrium air discharges, excited N2 spectra are dominant in corona discharge employed here. Due to the flame lift-off when the discharge is not present, no emission is observed at the nozzle exit. The emission intensity is significantly intensified when both the flame and the plasma are present. This observation is consistent with that reported in Cha et al.’s study27, due to the combination of high temperature, leading to an increase in the reduced electric field, and the abundant positive ions in flames, raising an electric current in unipolar electric fields, accelerates the generation of plasma discharge.


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Continuously, the spectroscopic study is performed along the jet centerline at different downstream locations for flames with Rej = 5000, and a representative result is presented in Figure 11. As reported in spectrum studies of flame emissions,28, 29 there are three emission peaks around 309 nm (OH, A2Σ+ – X2Π), 431 nm (CH, A2Δ – X2Π) and 517 nm (C2, A3Πg – X3Πu) in flames, regardless of the presence of plasma. The increase in emission intensity, particularly for emissions above 400 nm, with downstream locations is probably associated with the more complete reactions and the growth of soot as the fuel molecule travels more downstream. In the presence of plasma, the continuous emissions between 400 and 800 nm are found to intensify, compared to those without the presence of plasma. The intensified emissions are consistent with the flame luminosity visualization, as shown in Figure 5, and a similar observation was also reported in Ombrello et al.’s study.30 The increase in flame luminosity when the plasma is present is probably associated with the increased soot formation, due to the reduced lift-off height that leads to a decrease in air premixing, and thus results in intensified broadband emissions, particularly for downstream flames, as seen in Figure 9b. For plasma discharges, the mean energy of electrons is proportional to the reduced electric field, E/n (E: electric field; n: number density). Typically, the reduced electric field for an atmospheric air corona is around 50 – 200 Td.5 By considering a coaxial cylindrical electrode system:31 E(r) =

V r ln(r2 r1 )

(1)

where V (≈ 15 kV) is the applied voltage, r1 and r2 are the outer radii of the fuel nozzle and the quartz, respectively, and r is the radial distance from one point to the center of the fuel nozzle, a rough estimation of the reduced electric field strength in the present study is less than 100 Td. According to the study of Starikovskiy and Aleksandrov,32 most of the electron energy, for a reduced electric field within 110 Td, is used for the dissociation of mixture molecules through the quench of electronically excited nitrogen species. The main process is:33


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N2 (v = 0) + e → N2 (v > 0) + e

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(2)

This is consistent with observations shown in Figures 8 and 10. Subsequently, the vibrationally excited nitrogen molecule can transfer the vibrational quanta to oxygen molecules,33 and produce multiple oxygen containing species, including O, O3, O2(v), O(1D), O2(a1∆g) etc.34 Among these species, O3 is stable and has a relatively long lifetime that has been shown to have enhancement on the laminar burning velocity34 and the flammability limit.35 However, due to the small discharge volume and the abundant air flow, the concentration of ozone is low enough (below 10 ppm measured with a ozone meter, Model EST-1015H, Environmental Sensor Technology) such that it has a negligible impact on the flame stabilization. The next longest lived species is O2(a1∆g), and at 101.3 kPa and 300 K has a collisional lifetime of approximately 20 ms.34 Given this short lifetime and the co-flow velocity, the characteristic travelling distance of O2(a1∆g) before quench or recombination is at least one order of magnitude less than the flame lift-off height. Thus, the effect of O2(a1∆g) on the flame stabilization, at least in the present study, may be ignored. Other excited oxygen species could be taken into no account here due to the extremely short lifetime. 4. Conclusion An experimental study is carried out to investigate the impact of plasma discharges on the behaviors of lifted non-premixed jet flames fueled with methane in a stream of co-flow air. A corona discharge is produced around the tip edge of the nozzle to stabilize the flame. It is observed that the presence of plasma has an impact on retarding the lift-off velocity, decreasing the lift-off height and altering the flame hysteresis behaviors. The application of plasma discharges to flame stabilization leads to plasma-attached flames, plasma-enhanced lifted flames and plasma-ineffective lifted flames, depending on flame lift-off conditions. In general, the effect of plasma discharges diminishes as the flame lift-off height becomes large. For flame hysteresis, both of the lift-off and the reattachment velocities are enhanced due to the presence of plasma, and the enhancement is found to be twofold here. Spectroscopic study confirms the effect of plasma discharges on flame


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stabilization and shows that the broadband emissions between 400 and 800 nm downstream of the flame are intensified with the presence of plasma, presumably due to the increased soot formation, due to a decrease in the lift-off height.

Acknowledgements This research is fully supported by the Ministry of Science and Technology (MOST) in Taiwan through grant: MOST 105-2221-E-009-075.


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Leonov, S. B.; Yarantsev, D. A. Plasma Sources Sci. Technol. 2007, 16, 132-138.

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Mariani, A.; Foucher, F. Appl. Energy 2014, 122, 151-161.

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Leonov, S. B.; Yarantsev, D. A.; Napartovich, A. P.; Kochetov, I. V. IEEE Trans. Plasma

Sci. 2006, 34, 2514-2525. (9)

Kimura, I.; Aoki, H.; Kato, M. Combust. Flame 1981, 42, 297-305.

(10) Starikovskiy, A.; Aleksandrov, N.; Rakitin, A. Phil. Trans. R. Soc. A. 2012, 370, 740-774. (11) Barbosa, S.; Pilla, G.; Lacoste, D.; Scouflaire, P.; Ducruix, S.; Laux, C.; Veynante, D. 4th European Combustion Meeting (Vienna, Austria), 2009. (12) Vincent-Randonnier, A.; Larigaldie, S.; Magre, P.; Sabel’nikov, V. IEEE Trans. Plasma Sci. 2007, 35, 223-232. (13) Singleton, D.; Pendleton, S. J.; Gundersen, M. A. J. Phys. D: Appl. Phys. 2011, 44, 002001. (14) Kim, W.; Do, H.; Mungal, M. G.; Cappelli, M. A. IEEE Trans. Plasma Sci. 2008, 36, 28982904. (15) Kim, W.; Do, H.; Mungal, M. G.; Cappelli, M. A. IEEE. Trans. Plasma Sci. 2006, 34, 25452551. (16) Tang, J.; Wei, L.; Song, J.; Yu, D. Fuel 2016, 179, 362-367. (17) Won, S. H.; Cha, M. S.; Park, C. S.; Chung, S. H. Proc. Combust Inst. 2007, 31, 963-970. (18) Brown, C. D.; Watson, K. A.; Lyons, K. Flow Turbul. Combust. 1999, 62, 249-273. (19) Chen, J.; Davidson, J. H. Plasma Chem. Plasma Proc. 2002, 22, 199-224. (20) Machala, Z.; Janda, M.; Hensel, K.; Jedlovský , I.; Leš tinská , L.; Foltin, V.; Martisovitš , V.; Morvová , M. J. Mol. Spectrosc. 2007, 243, 194-201. (21) Terry, S. D.; Lyons, K. M. Combust. Sci. Technol. 2005, 177, 2091-2112.


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(22) Kim W.; Mungal, M. G.; Cappelli, M. A. 43rd AIAA Aerospace Science Meeting and Exhibit (Reno, NV), 2005. (23) Tang, J.; Zhao, W.; Duan Y. Plasma Sources Sci. Technol. 2011, 20, 045009. (24) Kim, Y.; Hong, H. G.; Cha, M. S.; Song, Y. H.; Kim, S. J. J. Adv. Oxid. Technol. 2003, 6, 17-22. (25) Lee, S. M.; Park, C. S.; Cha, M. S.; Chung, S. H. IEEE Trans. Plasma Sci. 2005, 33, 17031709. (26) Moore, N. J.; Terry, S. D.; Lyons, K. M. J. Energy Resour. Technol. 2011, 133, 022202-1. (27) Cha, M. S.; Lee, S. M.; Kim, K. T.; Chung, S. H. Combust. Flame 2005, 141, 438-447. (28) Gaydon, A. G. The Spectroscopy of Flames; New York: Wiley, 1957. (29) Sandrowitz, A. K.; Cooke, J. M.; Glumac, N. G. Appl. Spectrosc. 1998, 52, 658-662. (30) Ombrello, T.; Qin, X.; Ju, Y. 43rd AIAA Aerospace Sciences Meeting and Exhibit (Reno, Nevada), 2005. (31) Shiraishi, T.; Urushihara, T.; Gundersen, M. J. Phys. D: Appl. Phys. 2009, 42, 135208. (32) Starikovskiy, A.; Aleksandrov, N. Prog. Energy Combust. Sci. 2013, 39, 61-110. (33) Mintoussov, E. I.; Pancheshnyi, S. V.; Starikovskii, A. Y. 42nd AIAA Aerospace Science Meeting and Exhibit (Reno, NV), 2004. (34) Ombrello, T.; Won, S. H.; Ju, Y.; Williams, S. Combust. Flame 2010, 157, 1906-1915. (35) Vu, T. M.; Won, S. H.; Ombrello, T.; Cha, M. S. Combust. Flame 2014, 161, 917-926.


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Energy & Fuels

Figure 1. Schematic of the experimental setup.

(a)

(b)

Figure 2. (a) Schematic of the electrode arrangement. (b) A representative image of the plasma discharge. The white dash line in (b) indicates the edge of the nozzle.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b) Figure 3. The effect of co-flow velocity on (a) the lift-off velocity and (b) the lift-off height, both with and without the presence of plasma. Rej = 5000 in (b).


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Energy & Fuels

Figure 4. The effect of the jet Reynolds number on the flame lift-off height for flames with and without the discharge. The co-flow flow rate is 100 LPM, corresponding to a velocity of 9.7 cm/s.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

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(b)

Figure 5. Representative images of flame luminosity (a) without plasma and (b) with plasma. The fuel is methane and Rej = 5000 for both cases.


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Energy & Fuels

Figure 6. The effect of the plasma energy density on the normalized variation of the flame lift-off height.


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Without plasma

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With plasma

(a)

(b)

(c)

Figure 7. Representative images of (a) plasma-attached flames, (b) plasma-enhanced lifted flames and (c) plasma-ineffective lifted flames. Each type of flames without the discharge is shown for comparisons. Rej = 5000 for all cases and co-flow velocity of (a) 9.7, (b) 19.4 and (c) 38.8 cm/s.


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Energy & Fuels

Figure 8. Chemiluminescence images of the excited N2 transition (C3Πu – B3Πg) for plasma-attached flames. Rej = 5000 and co-flow velocity of 9.7 cm/s.


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(a)

(b) Figure 9. Hysteresis behaviors of methane flames (a) without plasma and (b) with plasma. The coflow flow rate is 100 LPM.


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Energy & Fuels

Figure 10. Emission spectra of plasma, flame and plasma-stabilized flame at the nozzle exit. The flame is a methane flame with Rej = 5000.


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(a)

(b) Figure 11. Emission spectra along the centerline of a methane flame with Rej = 5000 at different downstream locations. (a) Without the plasma discharge; (b) with the plasma discharge.


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Energy & Fuels


Plasma-Assisted Stabilization of Lifted Non-Premixed Jet Flames

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