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Flow instability in laminar jet flames driven by alternating current electric fields
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Flow instability in laminar jet flames driven by alternating current electric fields Gyeong Taek Kim a, Dae Geun Park b, Min Suk Cha b,∗, Jeong Park a,∗, Suk Ho Chung b a Interdisciplinary b King
Program of Biomechanical Engineering, Pukyong National University, Busan, Republic of Korea Abdullah University of Science and Technology (KAUST), Clean Combustion Research Center (CCRC) and Physical Science and Engineering Division (PSE), Thuwal, Saudi Arabia Received 2 December 2015; accepted 8 September 2016 Available online xxx
Abstract The effect of electric fields on the instability of laminar nonpremixed jet flames was investigated experimentally by applying the alternating current (AC) to a jet nozzle. We aimed to elucidate the origin of the occurrence of twin-lifted jet flames in laminar jet flow configurations, which occurred when AC electric fields were applied. The results indicated that a twin-lifted jet flame originated from cold jet instability, caused by interactions between negative ions in the jet flow via electron attachment as O2 + e → O2 – when AC electric fields were applied. This was confirmed by conducting systematic, parametric experiment, which included changing gaseous component in jets and applying different polarity of direct current (DC) to the nozzle. Using two deflection plates installed in parallel with the jet stream, we found that only negative DC on the nozzle could charge oxygen molecules negatively. Meanwhile, the cold jet instability occurred only for oxygencontaining jets. A shedding frequency of jet stream due to AC driven instability showed a good correlation with applied AC frequency exhibiting a frequency doubling. However, for the applied AC frequencies over 80 Hz, the jet did not respond to the AC, indicating an existence of a minimum flow induction time in a dynamic response of negative ions to external AC fields. Detailed regime of the instability in terms of jet velocity, AC voltage and frequency was presented and discussed. Hypothesized mechanism to explain the instability was also proposed. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Electric field; Jet flow instability; Twin jet flame
1. Introduction
∗
Corresponding authors. Fax: +82 51 629 6126. E-mail addresses: [email protected] (M.S. Cha), [email protected] (J. Park).
For several decades, researchers have been interested in the electrical properties of flames and how they can be controlled by the application of electric fields [1–3]. The bidirectional electric body force that acts on negative and positive charge
http://dx.doi.org/10.1016/j.proci.2016.09.015 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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Fig. 1. Dramatic changes in flame morphology with applied electric fields for a nozzle diameter of 4.4 mm; the mole fraction of C3 H8 in nitrogen was 0.165, and the jet velocity was 1.5 m/s: (a) without an applied voltage and (b) applying 7 kV with an alternating current (AC) frequency of 26 Hz to a fuel nozzle.
carriers is a fundamental physical mechanism that occurs when a flame is subjected to an external electric field. Basically, this selective external force on the ions generated in a flame zone produces ionic wind, resulting in a dynamic response by the flame. The effects of external electric fields can be used to achieve various phenomenological and practical improvements in combustion characteristics [4–6]. Among the many flame configurations studied by applying electric fields, lifted jet flames have been studied extensively, due to their intrinsic stabilization features: a tribrachial flame comprising liftoff and blowout. Investigations for laminar jet flames under electric fields can provide validation data for multi-physics simulations as well as fundamental understanding of a controlling method for jets and flames. The jet configuration is also of practical importance in various burner systems. Both laminar and turbulent jet flames with alternating current (AC) have shown liftoff stability, indicating a significantly enhanced velocity range for a stable nozzle-attached flame [7–9]. An increase in the displacement speed of the lifted flame edge with AC and direct current (DC) electric fields has also been observed [10,11]. Most related studies have focused on the flame characteristics when electric fields are applied. While Won et al. [10] observed cold flow modification caused by electric fields; specifically, the breakup of a cold flow away from the jet nozzle, visualized by Schlieren imaging using an AC frequency of 60 Hz with a root mean square (rms) voltage of 12 kV. When a 26-Hz AC field with 7 kV (rms) was applied to a fuel nozzle in a preliminary experiment, as shown in Fig. 1, a nitrogen-diluted propane flame exhibited a drastic transition from a normal laminar-lifted flame with a typical tribrachial
Fig. 2. Schematic diagram of the experimental setup: (a) overall setup and (b) deflection plates used to detect the charged species.
structure at the edge to twin-lifted flames that appeared to be stabilized in separate branches. As will be shown later, the 26-Hz AC frequency was such that the visual variation of the flame was maximal. We determined that this effect originated from cold flow instabilities caused by the electric field, which indicates that the characterization of cold flows should be understood prior to investigating lifted flames in this situation. Note that combustion phenomenon is appreciably influenced by flow field. Although potential influence of electric fields on jet flow field was reported previously [10], the mechanism of flow instability caused by applied electric fields, which eventually affects flame behavior, has not been systematically reported yet. In the present study, we focused our attention on identification of the effect of electric fields on cold jets. Both AC and DC fields were considered; the cold jets were visualized and the flow field was quantified via particle image velocimetry (PIV). As a result, we identified the key parameters affecting the flow, in turn, influencing flame behavior and proposed a mechanism to explain the cold flow instability. 2. Experiment Figure 2 schematically illustrates the experimental setup. A nozzle was made of a 55-cm-long
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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stainless steel tube, with inner and outer diameters of 0.44 and 0.64 cm, respectively, which ensured fully developed parabolic velocity profiles at the nozzle exit. A coflow section (diameter: 9.4 cm) surrounded the nozzle, through which air or nitrogen was supplied depending on the experiment. To minimize external disturbances, we covered the coflow section with a quartz cylinder (length: 30 cm). To detect electrically charged species in the cold jet, the quartz cylinder was replaced with deflection plates connected to a DC power source (Fig. 2b). The gases tested were propane (>99.95%), nitrogen, oxygen, and air; flow rates were controlled using mass flow controllers. A power supply (Trek, 10/10B-HS) applied a high voltage to the jet nozzle. The voltage waveform was controlled with a function generator (NF, WF 1973) to produce a sinusoidal AC with arbitrary frequency (f ac ), or negative or positive DC. The applied AC voltage (rms Vac ) was up to 7 kV. Note that an upper stainless-steel-mesh electrode (diameter: 20 cm) was positioned 17.5 cm above the nozzle exit, on top of the quartz cylinder; this electrode was grounded. To measure electric current between the nozzle and the ground electrode, a current preamplifier (Stanford Research Systems, SR570), which was connected to the ground electrode, was used in conjunction with an oscilloscope (Tektronix, MSO 2024). Another DC power supply (Trek, 10/10B-HS) applied +10 kV to one of the deflection plates (width: 4 cm; thickness: 1 mm); the deflection plate had a 9.0 cm long vertical slit to enable visualization of the flow field by laser irradiation. A diode laser (LVI, VD-IVA) and continuous wave Ar-ion laser (Spectra-Physics, Stabilite 2017) illuminated seed particles (TiO2 particle size ∼0.2 μm) so that the flow field could be visualized with a digital camera (Sony, HDRCX560) and measured with a high-speed camera (Photron, MC2 and SA4). The frame rate of the high-speed camera was 2000 frames per second. Consecutive images were used to quantify the flow field using commercial software (La Vision, DAVIS).
3. Results and discussion To identify the mechanism influencing the twinlifted flames observed in Fig. 1b, the flow characteristics of jets are investigated. The existence of the twin-lifted flame with electric fields can be caused either by flame generated ions or by cold jet influenced by electric fields. However, for the twinlifted flame to exist, the flow field upstream from the lifted flame edge should be influenced by electric field. In this regard, we have systematically investigated cold flow to isolate the effect of flame generated ions under the influence of electric fields.
3
Fig. 3. Cold flow visualization for the flames shown in Fig. 1: (a) Vac = 0, (b) 3 kV, (c) 7 kV, and (d) 7 kV (top view of (c)) at f ac = 26 Hz.
3.1. Cold jet instability caused by alternating current (AC) Figure 3 illustrates the measured electric current in rms values together with cold flow visualization via Mie-scattering of seed particles. The flow condition corresponds to the case shown in Fig. 1. The mole fraction of propane (XC3H8 ) diluted in nitrogen was 0.165 in the fuel stream, and the jet velocity was U0 = 1.5 m/s. The AC frequency was fixed at 26 Hz. As shown in the graph, the electric current was irresponsive to the applied voltage up to 2.0 kV, then the current gradually increases from 0 to 0.58 μA in a range of 2.0< Vac ≤ 7.0 kV. This indicates that there must be a flow of electric charges in between the nozzle and the upper ground electrode. Note that the electric power at Vac = 7 kV was ∼4 mW. The visualization in Fig. 3a shows that the core of the cold jet is unaffected from laminar flow pattern without applied voltage. The flow pattern was unchanged until Vac = 2 kV where the electric current was practically zero. However, for higher applied voltages (>2.5 kV) being accompanied by electric currents, a jet breakup occurred at a certain height from the nozzle rim. For instance, when Vac = 3 kV with f ac = 26 Hz was applied to the nozzle, the main jet branched out into two separate streams (Fig. 3b), and for further increased applied voltage (Vac = 7 kV, Fig. 3c) a shorter breakup length and a wider separation angle could be observed. A top view of the horizontal cross-section of the jet over the breakup point with Vac = 7 kV is shown in Fig. 3d; the separation of the main jet was not axisymmetric exhibiting plane symmetry; thus, the twin-lifted flames shown in Fig. 1 were a result of the planar separation of the main jet. Note that the direction of jet separation in a vertical plane was
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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3.2. Origin of the instability
Fig. 4. Evidence of a negatively charged jet stream under Vac = 7 kV and f ac = 26 Hz: (a) without a deflection voltage and (b) with a deflection voltage of +10 kV applied to the plate on the right side.
randomly determined at the critical moment as Vac increased. The AC voltage applied to the nozzle caused significant modification in the jet flow even without a flame. This indicates that certain molecules respond to the applied electric field. Potential mechanisms to explain our observation include ionization of the flow by the energized nozzle, and reaction of the resulting ions to the electric field; and/or the migration of polar molecules subjected to a nonuniform region with a stronger field intensity. However, there are no polar molecules in the system of interest, because propane, nitrogen, and oxygen molecules in entrained air are all non-polar. It is necessary to confirm whether the flow is ionized by applying an AC. Because there is no light emission or sizzling noise from an electrical discharge, such as corona discharge, electron impact ionization yielding positive ions is unlikely to occur. Also, no discernible modifications of the cold flow were found when either negative or positive DCs were applied to the nozzle. This point will be clarified in Section 3.3. To investigate whether the flow was ionized, deflection plates with a separation of 60 mm were installed vertically 20 mm above the nozzle exit after removing the quartz cylinder (Fig. 2b). The positions of the deflection plates were determined from preliminary tests to avoid electrical discharges between the plates and the nozzle. Figure 4 shows that the results for this condition were the same as those shown in Fig. 3c (Xp = 0.165, U0 = 1.5 m/s, Vac = 7 kV, and f ac = 26 Hz). The vertical plate on the right side was subjected to +10 kV, while the other plate was grounded. With a DC field between the deflection plates (Fig. 4b), the flow field bent towards the positively charged plate; this was not the case without a DC field, as shown in Fig. 4a, indicating the presence of negatively charged molecules in the jet stream.
In proximity to the nozzle exit, the flow volume contained propane, nitrogen, and oxygen. The source of negative ions in the flow was most likely oxygen, due to its high electron affinity; i.e., electron attachment to oxygen resulting in negative O2 − ions [12]. To confirm the negatively charged species, the cold jet behavior of each gas was investigated without the deflection plates. To avoid interference from oxygen molecules, the coflow air was replaced with nitrogen. Figure 5 shows significant evidence for the origin of the instability that causes unstable behavior in the downstream flow. During the test of four different gases (propane, nitrogen, air, and oxygen) injected from the jet nozzle, the jet velocity was fixed at 1.5 m/s. A nitrogen coflow velocity of 0.25 m/s was maintained with Vac = 7 kV and f ac = 26 Hz. The propane and nitrogen flows were not significantly different in the presence of an AC field, while the jet breakup downstream of the propane jet appeared to be an intrinsic property of the heavier gas with coflow nitrogen (Fig. 5a and b) [13]. However, jet breakup was observed for the air and oxygen jet steams under an applied AC field (Fig. 5c and d). Taken together, these results indicate that oxygen is most likely responsible for jet instability. Finally, it can be shown that the oxygen molecules are charged in jet streams of air and oxygen in AC fields. Note that when we replaced the coflow with air, all of the tested gases demonstrated early jet breakup, implying the role of entrained oxygen in this phenomenon. Note that a previous study for aerodynamic control using dielectric barrier discharge [14] reported electron attachment to oxygen molecule is the most viable mechanism behind its observation. 3.3. Hypothesized mechanism for the AC-driven instability To find a viable physical mechanism to explain the instability, we first needed to clarify that DC fields applied to the nozzle do not cause any discernable instability. We investigated the flow characteristics of the air jet by applying both positive and negative DC fields to the nozzle (Fig. 6). When the nozzle was energized by 7 kV, no noticeable change was observed with or without the deflection voltage at +10 kV (Fig. 6a), indicating that oxygen cannot be charged with a positive electrode. However, when the nozzle was negatively charged (−7 kV) (Fig. 6b), the jet stream bent towards the higher potential deflection plate. No jet breakup or instability in the downstream flow was observed when negative DC fields were applied to the nozzle. In summary, oxygen molecules can be negatively charged only when the electrical potential applied to the nozzle is negative; however, this is not
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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Fig. 6. Effect of DC field on oxygen ionization, with a deflection voltage at 0 kV or 10 kV applied to the air jet: (a) Vdc = 7 kV and (b) Vdc = −7 kV to the nozzle.
Fig. 5. Effect of an AC field on the jet of (a) C3 H8 , (b) N2 , (c) air, and (d) O2 with nitrogen coflow.
sufficient to cause instability in the downstream flow, thus indicating the role of AC fields that change polarity. Field electron emission from the cathode, based on the theory of Fowler and Nordheim [15], supports the results shown in Fig. 6. When the nozzle tip is exposed to a high-intensity electric field with a negative potential, in which case the nozzle becomes a cathode, field-emitted electrons, which are not sufficiently energetic to break down air and cause discharge, tend to produce negative ions via electron attachment as O2 + e → O2 – . The resulting flow volume containing oxygen ions is subjected to a force caused by the electric field around the energized nozzle. Unlike an ionic wind due to the electric body force on the flame generated ions [16–19], because the present flow contains only negative ions, electric body force should result in unidirectional ionic wind, which accelerate or decelerate the main jet depending on the polarity of applied voltage. Although DC fields generate oxygen ions, homogenous electric body forces applied to the flow volume result in a situation that is equivalent to reduced buoyancy or a jet of gases with a lighter mass than air (Fig. 7a). We cannot foresee any visible modification of the jet streams when a DC field is applied. However, when an AC field is applied to the nozzle, as shown in Fig. 7b, during the half period of negative voltage, the oxygen ions produced are pushed from the nozzle. The period with a positive voltage does not contribute to ion generation; however, the nozzle exerts a
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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Fig. 7. Conceptual schematic diagram of the force acting on a negatively ionized flow volume due to an electric field produced by: (a) negative DC and (b) AC energized nozzles. In reality, the ion movement caused by electric field would be faster than flow convection.
pulling force on the ionized part of the flow volume. Therefore, this longitudinally fluctuating disturbance may trigger a hydrodynamic instability, resulting in early jet breakup and, thus, branching into two separate streams. Note that the paramagnetic effect of molecular oxygen under the AC induced magnetic field, which is much smaller than that of Earth (∼order of 10 μT) [6], on the cold jet instability should be negligible, because it requires very strong magnetic field (>0.1 T) to achieve a visible impact on flow field [20]. High-speed images of scattered seed particles, illuminated by an Ar-ion laser in the air jet at U0 = 1.5 m/s at Vac = 7 kV and f ac = 26 Hz (Supplementary Movie 1), illustrate the detailed flow motion at the point of breakup. The hydrodynamic instability generated in the proximity of the nozzle appears to propagate in the stream direction, exhibiting wavy motion in the jet core. At a certain location, periodic shedding of the stream occurs in two opposite directions, creating a plane of separation. Typical examples of measured flow fields with and without Vac = 7 kV at f ac = 26 Hz in Fig. 8 and Supplementary Movie 2 show a transverse fluctuation of the flow field and shedding of the flow volume into two streams. Although the seed particles are affected by electric fields based on previous experience, a visualization of the overall flow field can be achieved because the seed particles follow the main flow reasonably [19]. Based on the results of high-speed visualization, we realized that the transverse shedding of the main jet was also periodic. Thus, to elucidate this AC-driven cold jet instability, the shedding frequency of the main jet into two branches was analyzed. The typical downstream location of the breakup point was monitored using high-speed imaging of seed particles. A fast Fourier transform algorithm was used to calculate the representative frequency of the flow motion. Each result, from various f ac values, showed a single strong peak in the frequency domain. Figure 9 shows the results of the shedding frequency of the main air jet
Fig. 8. Quantification of the jet stream with U0 = 1.5 m/s using PIV: (a) original jet and (b) with Vac = 7 kV at f ac = 26 Hz.
Fig. 9. Shedding frequency of the main jet for various AC frequencies at Vac = 7 kV.
(U0 = 1.5 m/s) for various applied AC frequencies at fixed Vac = 7 kV. The shedding frequency shows a frequency-doubling phenomenon when the applied AC is in the f ac range of 10–70 Hz; for higher f ac , no flow modification was observed. Because the electric body force acts on the flow volume via sparse ionized oxygen molecules, there should be a typical characteristic time required for the volume to react to this force. Kono et al. [21] estimated a molecular collisional response time of 14 ms for air at 1500 K and 1 atm. However, in previous studies [22–25], the flame response times with transient DC electric fields were observed to be 0 (1) ms showing a significant gap with 14 ms. To provide scientifically sound explanation for the
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trode was removed. This can be attributed to the fact that the cold field emission is controlled by local field intensity at the nozzle surface, thus the location of the ground electrode has minimal effect on it unless the distance from the nozzle is close enough to influence near-nozzle field intensity. 4. Conclusions
Fig. 10. Overall mapping of flow characteristics in terms of Vac and U0 at fixed f ac = 26 Hz.
irresponsive behavior over 80 Hz, further numerical and experimental studies should be conducted elucidating fundamental interaction between electric field and charged species movement and resulted ionic wind generation. In addition, although we cannot fully explain the observed frequencydoubling phenomenon, it is obvious that the AC frequency has an important role in terms of the reported instability. Thus, our proposed hypothesis, illustrated in Fig. 7, provides a reasonable firstorder estimation of this effect. Figure 10 shows an overview of the AC-driven air jet instability, in terms of the applied AC voltage and mean jet velocity at a fixed f ac of 26 Hz. First, the AC-driven jet breakup is highly coupled with the jet velocity. The minimum U0 for the jet to be broken at Vac = 1 kV was 1.7 m/s; this value decreased monotonically as Vac increased, with U0 = 0.83 m/s at Vac = 7 kV. Thus, jet flows below the jet breakup velocity maintained an unspoiled laminar flow pattern, regardless of Vac . However, in the transient regime, the transition jet velocity from AC-driven branching flows at Vac = 1 kV was 2.79 m/s, while it was 3.59 m/s at Vac = 7 V; in this case, the transition flow was sprayed completely. Turbulence dominated the AC-driven instability. When the jet velocity was in the turbulent regime, the AC field caused no discernible flow modification. It should be noted that there was negligible difference in the result when the upper ground elec-
Motivated by an observation of twin-lifted flames in free jets by applying AC, the effect of electric fields on cold-flow jet instability was investigated experimentally to isolate the effect of electric field on flame generated ions. To elaborate on the reason behind a jet breakup applying AC, each component in a jet was tested separately by changing a polarity of applied high-voltage using DC. By virtue of two deflection plates in a downstream of a jet, we could identify that only oxygen molecules were responded, moving toward an anode in the deflection plates, when the nozzle was negatively charged, which can be explained with the cold field emission theory. However, the jet flow instability could be found only by applying AC to the nozzle, thus we hypothesized a mechanism to explain the cold jet instability based on a dynamic response of ionized portion of flow volume to an external harmonic excitation due to applied AC. Flow characteristics generated in the proximity of the nozzle rim applying AC was analyzed using a high-speed PIV technique. The shedding frequency of the main jet into two branches was examined. The jet instability occurred with applied frequencies up to 70 Hz, exhibiting a frequency doubling phenomenon in the shedding frequency, while a jet did not respond to the AC when the applied frequency was over 80 Hz indicating a minimum flow induction time required as most of dynamic systems have a such response time. The overall mapping of jet flow characteristics, such as laminar flow, breakdowns, and transient and turbulent flows, were addressed in terms of jet velocity, and applied AC voltage. As a result, the twin-lifted jet flame originated from jet flow instability; the instability was caused by interactions between negative ions in the jet flow volume and the nozzle under AC electric field application. Detailed clarification of frequency doubling and characterization of twin-lifted flames will be our future works.
Acknowledgment This work was supported by the project of Development of the Technology of Energy from KETEP in 2015–16, Science and Technology in 2015. DGP, MSC, and SHC were supported by the King Abdullah University of Science and Technology (KAUST).
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.proci.2016.09.015. References [1] J. Lawton, F.J. Weinberg, Electrical aspects of combustion, Clarendon Press Oxford, 1969. [2] F. Carleton, F. Weinberg, Nature 330 (1987) 635–636. [3] A.B. Fialkov, Prog. Energy Combust. Sci. 23 (1997) 399–528. [4] D.G. Park, B.C. Choi, M.S. Cha, S.H. Chung, Combust. Sci. Technol. 186 (2014) 644–656. [5] M.S. Cha, Y. Lee, IEEE Trans. Plasma Sci. 40 (2012) 3131–3138. [6] Y. Xiong, M.S. Cha, S.H. Chung, Proc. Combust. Inst. 35 (2015) 3513–3520. [7] M.K. Kim, S.H. Chung, H.H. Kim, Proc. Combust. Inst. 33 (2011) 1137–1144. [8] M.K. Kim, S.K. Ryu, S.H. Won, S.H. Chung, Combust. Flame 157 (2010) 17–24. [9] S.M. Lee, C.S. Park, M.S. Cha, S.H. Chung, Plasma Sci. IEEE Trans. 33 (2005) 1703–1709. [10] S.H. Won, M.S. Cha, C.S. Park, S.H. Chung, Proc. Combust. Inst. 31 (2007) 963–970. [11] S.H. Won, S.K. Ryu, M.K. Kim, M.S. Cha, S.H. Chung, Combust. Flame 152 (2008) 496–506.
[12] W.P. Allis, Motions of ions and electrons, Springer, 1956. [13] Y. Xiong, M.S. Cha, S.H. Chung, Proc. Combust. Inst. 35 (2015) 873–880. [14] W. Kim, H. Do, M.G. Mungal, M.A. Cappelli, Appl. Phys. Lett. 91 (2007) 181501. [15] R.H. Fowler, L. Nordheim, Proc. R. Soc. London A: Math. Phys. Eng. Sci. 119 (1928) 173–181. [16] A. Cessou, E. Varea, K. Criner, G. Godard, P. Vervisch, Exp. Fluids 52 (2012) 905–917. [17] F. Altendorfner, J. Kuhl, L. Zigan, A. Leipertz, Proc. Combust. Inst. 33 (2011) 3195–3201. [18] J. Schmidt, S. Kostka, A. Lynch, B. Ganguly, Exp. Fluids 51 (2011) 657–665. [19] D.G. Park, S.H. Chung, M.S. Cha, Combust. Flame 168 (2016) 138–146. [20] N.I. Wakayama, H. Ito, Y. Kuroda, O. Fujita, K. Ito, Combust. Flame 107 (1996) 187–192. [21] M. Kono, F.B. Carleton, A.R. Jones, F.J. Weinberg, Combust. Flame 78 (1989) 357–364. [22] J. Schmidt, B. Ganguly, Combust. Flame 160 (2013) 2820–2826. [23] J. Kuhl, G. Jovicic, L. Zigan, A. Leipertz, Proc. Combust. Inst. 34 (2013) 3303–3310. [24] J. Kuhl, G. Jovicic, L. Zigan, S. Will, A. Leipertz, Proc. Combust. Inst. 35 (2015) 3521–3528. [25] S. Marcum, B. Ganguly, Combust. Flame 143 (2005) 27–36.
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Proceedings of the Combustion Institute 000 (2016) 1–8 www.elsevier.com/locate/proci
Flow instability in laminar jet flames driven by alternating current electric fields Gyeong Taek Kim a, Dae Geun Park b, Min Suk Cha b,∗, Jeong Park a,∗, Suk Ho Chung b a Interdisciplinary b King
Program of Biomechanical Engineering, Pukyong National University, Busan, Republic of Korea Abdullah University of Science and Technology (KAUST), Clean Combustion Research Center (CCRC) and Physical Science and Engineering Division (PSE), Thuwal, Saudi Arabia Received 2 December 2015; accepted 8 September 2016 Available online xxx
Abstract The effect of electric fields on the instability of laminar nonpremixed jet flames was investigated experimentally by applying the alternating current (AC) to a jet nozzle. We aimed to elucidate the origin of the occurrence of twin-lifted jet flames in laminar jet flow configurations, which occurred when AC electric fields were applied. The results indicated that a twin-lifted jet flame originated from cold jet instability, caused by interactions between negative ions in the jet flow via electron attachment as O2 + e → O2 – when AC electric fields were applied. This was confirmed by conducting systematic, parametric experiment, which included changing gaseous component in jets and applying different polarity of direct current (DC) to the nozzle. Using two deflection plates installed in parallel with the jet stream, we found that only negative DC on the nozzle could charge oxygen molecules negatively. Meanwhile, the cold jet instability occurred only for oxygencontaining jets. A shedding frequency of jet stream due to AC driven instability showed a good correlation with applied AC frequency exhibiting a frequency doubling. However, for the applied AC frequencies over 80 Hz, the jet did not respond to the AC, indicating an existence of a minimum flow induction time in a dynamic response of negative ions to external AC fields. Detailed regime of the instability in terms of jet velocity, AC voltage and frequency was presented and discussed. Hypothesized mechanism to explain the instability was also proposed. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Electric field; Jet flow instability; Twin jet flame
1. Introduction
∗
Corresponding authors. Fax: +82 51 629 6126. E-mail addresses: [email protected] (M.S. Cha), [email protected] (J. Park).
For several decades, researchers have been interested in the electrical properties of flames and how they can be controlled by the application of electric fields [1–3]. The bidirectional electric body force that acts on negative and positive charge
http://dx.doi.org/10.1016/j.proci.2016.09.015 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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Fig. 1. Dramatic changes in flame morphology with applied electric fields for a nozzle diameter of 4.4 mm; the mole fraction of C3 H8 in nitrogen was 0.165, and the jet velocity was 1.5 m/s: (a) without an applied voltage and (b) applying 7 kV with an alternating current (AC) frequency of 26 Hz to a fuel nozzle.
carriers is a fundamental physical mechanism that occurs when a flame is subjected to an external electric field. Basically, this selective external force on the ions generated in a flame zone produces ionic wind, resulting in a dynamic response by the flame. The effects of external electric fields can be used to achieve various phenomenological and practical improvements in combustion characteristics [4–6]. Among the many flame configurations studied by applying electric fields, lifted jet flames have been studied extensively, due to their intrinsic stabilization features: a tribrachial flame comprising liftoff and blowout. Investigations for laminar jet flames under electric fields can provide validation data for multi-physics simulations as well as fundamental understanding of a controlling method for jets and flames. The jet configuration is also of practical importance in various burner systems. Both laminar and turbulent jet flames with alternating current (AC) have shown liftoff stability, indicating a significantly enhanced velocity range for a stable nozzle-attached flame [7–9]. An increase in the displacement speed of the lifted flame edge with AC and direct current (DC) electric fields has also been observed [10,11]. Most related studies have focused on the flame characteristics when electric fields are applied. While Won et al. [10] observed cold flow modification caused by electric fields; specifically, the breakup of a cold flow away from the jet nozzle, visualized by Schlieren imaging using an AC frequency of 60 Hz with a root mean square (rms) voltage of 12 kV. When a 26-Hz AC field with 7 kV (rms) was applied to a fuel nozzle in a preliminary experiment, as shown in Fig. 1, a nitrogen-diluted propane flame exhibited a drastic transition from a normal laminar-lifted flame with a typical tribrachial
Fig. 2. Schematic diagram of the experimental setup: (a) overall setup and (b) deflection plates used to detect the charged species.
structure at the edge to twin-lifted flames that appeared to be stabilized in separate branches. As will be shown later, the 26-Hz AC frequency was such that the visual variation of the flame was maximal. We determined that this effect originated from cold flow instabilities caused by the electric field, which indicates that the characterization of cold flows should be understood prior to investigating lifted flames in this situation. Note that combustion phenomenon is appreciably influenced by flow field. Although potential influence of electric fields on jet flow field was reported previously [10], the mechanism of flow instability caused by applied electric fields, which eventually affects flame behavior, has not been systematically reported yet. In the present study, we focused our attention on identification of the effect of electric fields on cold jets. Both AC and DC fields were considered; the cold jets were visualized and the flow field was quantified via particle image velocimetry (PIV). As a result, we identified the key parameters affecting the flow, in turn, influencing flame behavior and proposed a mechanism to explain the cold flow instability. 2. Experiment Figure 2 schematically illustrates the experimental setup. A nozzle was made of a 55-cm-long
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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stainless steel tube, with inner and outer diameters of 0.44 and 0.64 cm, respectively, which ensured fully developed parabolic velocity profiles at the nozzle exit. A coflow section (diameter: 9.4 cm) surrounded the nozzle, through which air or nitrogen was supplied depending on the experiment. To minimize external disturbances, we covered the coflow section with a quartz cylinder (length: 30 cm). To detect electrically charged species in the cold jet, the quartz cylinder was replaced with deflection plates connected to a DC power source (Fig. 2b). The gases tested were propane (>99.95%), nitrogen, oxygen, and air; flow rates were controlled using mass flow controllers. A power supply (Trek, 10/10B-HS) applied a high voltage to the jet nozzle. The voltage waveform was controlled with a function generator (NF, WF 1973) to produce a sinusoidal AC with arbitrary frequency (f ac ), or negative or positive DC. The applied AC voltage (rms Vac ) was up to 7 kV. Note that an upper stainless-steel-mesh electrode (diameter: 20 cm) was positioned 17.5 cm above the nozzle exit, on top of the quartz cylinder; this electrode was grounded. To measure electric current between the nozzle and the ground electrode, a current preamplifier (Stanford Research Systems, SR570), which was connected to the ground electrode, was used in conjunction with an oscilloscope (Tektronix, MSO 2024). Another DC power supply (Trek, 10/10B-HS) applied +10 kV to one of the deflection plates (width: 4 cm; thickness: 1 mm); the deflection plate had a 9.0 cm long vertical slit to enable visualization of the flow field by laser irradiation. A diode laser (LVI, VD-IVA) and continuous wave Ar-ion laser (Spectra-Physics, Stabilite 2017) illuminated seed particles (TiO2 particle size ∼0.2 μm) so that the flow field could be visualized with a digital camera (Sony, HDRCX560) and measured with a high-speed camera (Photron, MC2 and SA4). The frame rate of the high-speed camera was 2000 frames per second. Consecutive images were used to quantify the flow field using commercial software (La Vision, DAVIS).
3. Results and discussion To identify the mechanism influencing the twinlifted flames observed in Fig. 1b, the flow characteristics of jets are investigated. The existence of the twin-lifted flame with electric fields can be caused either by flame generated ions or by cold jet influenced by electric fields. However, for the twinlifted flame to exist, the flow field upstream from the lifted flame edge should be influenced by electric field. In this regard, we have systematically investigated cold flow to isolate the effect of flame generated ions under the influence of electric fields.
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Fig. 3. Cold flow visualization for the flames shown in Fig. 1: (a) Vac = 0, (b) 3 kV, (c) 7 kV, and (d) 7 kV (top view of (c)) at f ac = 26 Hz.
3.1. Cold jet instability caused by alternating current (AC) Figure 3 illustrates the measured electric current in rms values together with cold flow visualization via Mie-scattering of seed particles. The flow condition corresponds to the case shown in Fig. 1. The mole fraction of propane (XC3H8 ) diluted in nitrogen was 0.165 in the fuel stream, and the jet velocity was U0 = 1.5 m/s. The AC frequency was fixed at 26 Hz. As shown in the graph, the electric current was irresponsive to the applied voltage up to 2.0 kV, then the current gradually increases from 0 to 0.58 μA in a range of 2.0< Vac ≤ 7.0 kV. This indicates that there must be a flow of electric charges in between the nozzle and the upper ground electrode. Note that the electric power at Vac = 7 kV was ∼4 mW. The visualization in Fig. 3a shows that the core of the cold jet is unaffected from laminar flow pattern without applied voltage. The flow pattern was unchanged until Vac = 2 kV where the electric current was practically zero. However, for higher applied voltages (>2.5 kV) being accompanied by electric currents, a jet breakup occurred at a certain height from the nozzle rim. For instance, when Vac = 3 kV with f ac = 26 Hz was applied to the nozzle, the main jet branched out into two separate streams (Fig. 3b), and for further increased applied voltage (Vac = 7 kV, Fig. 3c) a shorter breakup length and a wider separation angle could be observed. A top view of the horizontal cross-section of the jet over the breakup point with Vac = 7 kV is shown in Fig. 3d; the separation of the main jet was not axisymmetric exhibiting plane symmetry; thus, the twin-lifted flames shown in Fig. 1 were a result of the planar separation of the main jet. Note that the direction of jet separation in a vertical plane was
Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015
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3.2. Origin of the instability
Fig. 4. Evidence of a negatively charged jet stream under Vac = 7 kV and f ac = 26 Hz: (a) without a deflection voltage and (b) with a deflection voltage of +10 kV applied to the plate on the right side.
randomly determined at the critical moment as Vac increased. The AC voltage applied to the nozzle caused significant modification in the jet flow even without a flame. This indicates that certain molecules respond to the applied electric field. Potential mechanisms to explain our observation include ionization of the flow by the energized nozzle, and reaction of the resulting ions to the electric field; and/or the migration of polar molecules subjected to a nonuniform region with a stronger field intensity. However, there are no polar molecules in the system of interest, because propane, nitrogen, and oxygen molecules in entrained air are all non-polar. It is necessary to confirm whether the flow is ionized by applying an AC. Because there is no light emission or sizzling noise from an electrical discharge, such as corona discharge, electron impact ionization yielding positive ions is unlikely to occur. Also, no discernible modifications of the cold flow were found when either negative or positive DCs were applied to the nozzle. This point will be clarified in Section 3.3. To investigate whether the flow was ionized, deflection plates with a separation of 60 mm were installed vertically 20 mm above the nozzle exit after removing the quartz cylinder (Fig. 2b). The positions of the deflection plates were determined from preliminary tests to avoid electrical discharges between the plates and the nozzle. Figure 4 shows that the results for this condition were the same as those shown in Fig. 3c (Xp = 0.165, U0 = 1.5 m/s, Vac = 7 kV, and f ac = 26 Hz). The vertical plate on the right side was subjected to +10 kV, while the other plate was grounded. With a DC field between the deflection plates (Fig. 4b), the flow field bent towards the positively charged plate; this was not the case without a DC field, as shown in Fig. 4a, indicating the presence of negatively charged molecules in the jet stream.
In proximity to the nozzle exit, the flow volume contained propane, nitrogen, and oxygen. The source of negative ions in the flow was most likely oxygen, due to its high electron affinity; i.e., electron attachment to oxygen resulting in negative O2 − ions [12]. To confirm the negatively charged species, the cold jet behavior of each gas was investigated without the deflection plates. To avoid interference from oxygen molecules, the coflow air was replaced with nitrogen. Figure 5 shows significant evidence for the origin of the instability that causes unstable behavior in the downstream flow. During the test of four different gases (propane, nitrogen, air, and oxygen) injected from the jet nozzle, the jet velocity was fixed at 1.5 m/s. A nitrogen coflow velocity of 0.25 m/s was maintained with Vac = 7 kV and f ac = 26 Hz. The propane and nitrogen flows were not significantly different in the presence of an AC field, while the jet breakup downstream of the propane jet appeared to be an intrinsic property of the heavier gas with coflow nitrogen (Fig. 5a and b) [13]. However, jet breakup was observed for the air and oxygen jet steams under an applied AC field (Fig. 5c and d). Taken together, these results indicate that oxygen is most likely responsible for jet instability. Finally, it can be shown that the oxygen molecules are charged in jet streams of air and oxygen in AC fields. Note that when we replaced the coflow with air, all of the tested gases demonstrated early jet breakup, implying the role of entrained oxygen in this phenomenon. Note that a previous study for aerodynamic control using dielectric barrier discharge [14] reported electron attachment to oxygen molecule is the most viable mechanism behind its observation. 3.3. Hypothesized mechanism for the AC-driven instability To find a viable physical mechanism to explain the instability, we first needed to clarify that DC fields applied to the nozzle do not cause any discernable instability. We investigated the flow characteristics of the air jet by applying both positive and negative DC fields to the nozzle (Fig. 6). When the nozzle was energized by 7 kV, no noticeable change was observed with or without the deflection voltage at +10 kV (Fig. 6a), indicating that oxygen cannot be charged with a positive electrode. However, when the nozzle was negatively charged (−7 kV) (Fig. 6b), the jet stream bent towards the higher potential deflection plate. No jet breakup or instability in the downstream flow was observed when negative DC fields were applied to the nozzle. In summary, oxygen molecules can be negatively charged only when the electrical potential applied to the nozzle is negative; however, this is not
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Fig. 6. Effect of DC field on oxygen ionization, with a deflection voltage at 0 kV or 10 kV applied to the air jet: (a) Vdc = 7 kV and (b) Vdc = −7 kV to the nozzle.
Fig. 5. Effect of an AC field on the jet of (a) C3 H8 , (b) N2 , (c) air, and (d) O2 with nitrogen coflow.
sufficient to cause instability in the downstream flow, thus indicating the role of AC fields that change polarity. Field electron emission from the cathode, based on the theory of Fowler and Nordheim [15], supports the results shown in Fig. 6. When the nozzle tip is exposed to a high-intensity electric field with a negative potential, in which case the nozzle becomes a cathode, field-emitted electrons, which are not sufficiently energetic to break down air and cause discharge, tend to produce negative ions via electron attachment as O2 + e → O2 – . The resulting flow volume containing oxygen ions is subjected to a force caused by the electric field around the energized nozzle. Unlike an ionic wind due to the electric body force on the flame generated ions [16–19], because the present flow contains only negative ions, electric body force should result in unidirectional ionic wind, which accelerate or decelerate the main jet depending on the polarity of applied voltage. Although DC fields generate oxygen ions, homogenous electric body forces applied to the flow volume result in a situation that is equivalent to reduced buoyancy or a jet of gases with a lighter mass than air (Fig. 7a). We cannot foresee any visible modification of the jet streams when a DC field is applied. However, when an AC field is applied to the nozzle, as shown in Fig. 7b, during the half period of negative voltage, the oxygen ions produced are pushed from the nozzle. The period with a positive voltage does not contribute to ion generation; however, the nozzle exerts a
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Fig. 7. Conceptual schematic diagram of the force acting on a negatively ionized flow volume due to an electric field produced by: (a) negative DC and (b) AC energized nozzles. In reality, the ion movement caused by electric field would be faster than flow convection.
pulling force on the ionized part of the flow volume. Therefore, this longitudinally fluctuating disturbance may trigger a hydrodynamic instability, resulting in early jet breakup and, thus, branching into two separate streams. Note that the paramagnetic effect of molecular oxygen under the AC induced magnetic field, which is much smaller than that of Earth (∼order of 10 μT) [6], on the cold jet instability should be negligible, because it requires very strong magnetic field (>0.1 T) to achieve a visible impact on flow field [20]. High-speed images of scattered seed particles, illuminated by an Ar-ion laser in the air jet at U0 = 1.5 m/s at Vac = 7 kV and f ac = 26 Hz (Supplementary Movie 1), illustrate the detailed flow motion at the point of breakup. The hydrodynamic instability generated in the proximity of the nozzle appears to propagate in the stream direction, exhibiting wavy motion in the jet core. At a certain location, periodic shedding of the stream occurs in two opposite directions, creating a plane of separation. Typical examples of measured flow fields with and without Vac = 7 kV at f ac = 26 Hz in Fig. 8 and Supplementary Movie 2 show a transverse fluctuation of the flow field and shedding of the flow volume into two streams. Although the seed particles are affected by electric fields based on previous experience, a visualization of the overall flow field can be achieved because the seed particles follow the main flow reasonably [19]. Based on the results of high-speed visualization, we realized that the transverse shedding of the main jet was also periodic. Thus, to elucidate this AC-driven cold jet instability, the shedding frequency of the main jet into two branches was analyzed. The typical downstream location of the breakup point was monitored using high-speed imaging of seed particles. A fast Fourier transform algorithm was used to calculate the representative frequency of the flow motion. Each result, from various f ac values, showed a single strong peak in the frequency domain. Figure 9 shows the results of the shedding frequency of the main air jet
Fig. 8. Quantification of the jet stream with U0 = 1.5 m/s using PIV: (a) original jet and (b) with Vac = 7 kV at f ac = 26 Hz.
Fig. 9. Shedding frequency of the main jet for various AC frequencies at Vac = 7 kV.
(U0 = 1.5 m/s) for various applied AC frequencies at fixed Vac = 7 kV. The shedding frequency shows a frequency-doubling phenomenon when the applied AC is in the f ac range of 10–70 Hz; for higher f ac , no flow modification was observed. Because the electric body force acts on the flow volume via sparse ionized oxygen molecules, there should be a typical characteristic time required for the volume to react to this force. Kono et al. [21] estimated a molecular collisional response time of 14 ms for air at 1500 K and 1 atm. However, in previous studies [22–25], the flame response times with transient DC electric fields were observed to be 0 (1) ms showing a significant gap with 14 ms. To provide scientifically sound explanation for the
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trode was removed. This can be attributed to the fact that the cold field emission is controlled by local field intensity at the nozzle surface, thus the location of the ground electrode has minimal effect on it unless the distance from the nozzle is close enough to influence near-nozzle field intensity. 4. Conclusions
Fig. 10. Overall mapping of flow characteristics in terms of Vac and U0 at fixed f ac = 26 Hz.
irresponsive behavior over 80 Hz, further numerical and experimental studies should be conducted elucidating fundamental interaction between electric field and charged species movement and resulted ionic wind generation. In addition, although we cannot fully explain the observed frequencydoubling phenomenon, it is obvious that the AC frequency has an important role in terms of the reported instability. Thus, our proposed hypothesis, illustrated in Fig. 7, provides a reasonable firstorder estimation of this effect. Figure 10 shows an overview of the AC-driven air jet instability, in terms of the applied AC voltage and mean jet velocity at a fixed f ac of 26 Hz. First, the AC-driven jet breakup is highly coupled with the jet velocity. The minimum U0 for the jet to be broken at Vac = 1 kV was 1.7 m/s; this value decreased monotonically as Vac increased, with U0 = 0.83 m/s at Vac = 7 kV. Thus, jet flows below the jet breakup velocity maintained an unspoiled laminar flow pattern, regardless of Vac . However, in the transient regime, the transition jet velocity from AC-driven branching flows at Vac = 1 kV was 2.79 m/s, while it was 3.59 m/s at Vac = 7 V; in this case, the transition flow was sprayed completely. Turbulence dominated the AC-driven instability. When the jet velocity was in the turbulent regime, the AC field caused no discernible flow modification. It should be noted that there was negligible difference in the result when the upper ground elec-
Motivated by an observation of twin-lifted flames in free jets by applying AC, the effect of electric fields on cold-flow jet instability was investigated experimentally to isolate the effect of electric field on flame generated ions. To elaborate on the reason behind a jet breakup applying AC, each component in a jet was tested separately by changing a polarity of applied high-voltage using DC. By virtue of two deflection plates in a downstream of a jet, we could identify that only oxygen molecules were responded, moving toward an anode in the deflection plates, when the nozzle was negatively charged, which can be explained with the cold field emission theory. However, the jet flow instability could be found only by applying AC to the nozzle, thus we hypothesized a mechanism to explain the cold jet instability based on a dynamic response of ionized portion of flow volume to an external harmonic excitation due to applied AC. Flow characteristics generated in the proximity of the nozzle rim applying AC was analyzed using a high-speed PIV technique. The shedding frequency of the main jet into two branches was examined. The jet instability occurred with applied frequencies up to 70 Hz, exhibiting a frequency doubling phenomenon in the shedding frequency, while a jet did not respond to the AC when the applied frequency was over 80 Hz indicating a minimum flow induction time required as most of dynamic systems have a such response time. The overall mapping of jet flow characteristics, such as laminar flow, breakdowns, and transient and turbulent flows, were addressed in terms of jet velocity, and applied AC voltage. As a result, the twin-lifted jet flame originated from jet flow instability; the instability was caused by interactions between negative ions in the jet flow volume and the nozzle under AC electric field application. Detailed clarification of frequency doubling and characterization of twin-lifted flames will be our future works.
Acknowledgment This work was supported by the project of Development of the Technology of Energy from KETEP in 2015–16, Science and Technology in 2015. DGP, MSC, and SHC were supported by the King Abdullah University of Science and Technology (KAUST).
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Please cite this article as: G.T. Kim et al., Flow instability in laminar jet flames driven by alternating current electric fields, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.09.015