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Combustion and Flame

Pinch-off process of Burke–Schumann flame under acoustic excitation

Abstract

The pinch-off phenomenon in a flame refers to flame separation after reaching the critical frequency and amplitude under acoustic excitation. The pinch-off flame consists of the main flame attached to the nozzle and the detached pocket flame, and it is reported that more pollutants are discharged as the residence time of the pocket gas (i.e., the hot products) increases. This study focuses on the mechanism of the pinch-off phenomenon. The flame structure and flow field were analyzed by the simultaneous measurement of OH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV). Due to a lack of previous research on the necessary conditions for a pinch-off flame, this study mapped the conditions for pinch-off in terms of the forcing frequency and velocity perturbation intensity ( u ′ / u ¯ ) under acoustic excitation. The flame height in the pinch-off region was defined in two ways: the main flame height from the nozzle tip to the tip of the attached flame, and the total flame height including the detached pocket. Strouhal number was calculated based on these two definitions of flame height. In addition, the vortex flow and reverse flow under external forces were measured through analyzing the non-reacting and reacting flow fields. Flame deformation due to the entrainment of air in the vortex flow and the reverse flow was confirmed, and it was found to correlate with the pinch-off phenomenon. To analyze this observation quantitatively, time-dependent strain rate analysis was performed, and a high strain was confirmed to be present upon flame extinction. Therefore, these results demonstrate that pinch-off is correlated with the vortex flow and the reverse flow, and caused by a high strain rate acting on the flame surface.

Introduction

Acoustic excitation is used in a variety of applications such as pollutant reduction [1], [2], [3] and flame stabilization [4], [5], [6]. It is also a common tool for studying combustion instability [7], [8], [9], [10], which is known to be caused by the interaction of heat release perturbation, equivalence ratio perturbation, and acoustic excitation perturbation. Various excitation devices (e.g., loudspeakers and rotating disks) have been employed to perturb the flow and generate arbitrary unstable states in the combustor. In contrast, in non-premixed flames with dynamic characteristics, such as flame surface fluctuation, vortex shedding, flickering flame, and reverse flow behaviors have been reported.

Another characteristic of acoustic excitation is an enhanced mixing of fuel and air, which affects the pollutants formed during the combustion process. For example, Kim et al. [11] studied the relationship between the resonance frequency and the NOx concentration in a non-premixed flame. In all excited cases over a wide frequency range, they were observed a lower NOx emission than in the non-excited cases. In particular, a significant decrease in the NOx concentration was observed in the resonance frequency of the flame. These pollutant studies were conducted not only in laminar flames [12], [13], [14], [15] but also in turbulent flames [16], [17], [18], and upon considering the effects of increasing pressure [19], [20], [21]. The pinch-off flame tip is the main factor contributing to the generation of the polltant [22], and the residence time of the pocket flame separated from the main flame is longer than that of other normal flames. Since the soot growth time increases with increasing residence time of the flame, greater quantities of pollutants are emitted from the pocket flame [23]. Shaddix et al. [24,25] compared the soot volume fraction between a laminar steady-non-premixed flame and a pinch-off flame, and found that soot production was four times greater in the latter. Furthermore, a qualitative agreement was verified with computational studies [25], as well as between different sets of experimental results. Overall, soot growth occurs for a longer period in pinch-off flames compared to in steady flames, and that soot production also increases due to the higher temperature maintained in the hot product.

Previously suggested mechanisms of pinch-off include local flame extinction [26,27], phenomenon caused by vortex flow [28,29], and the inflow of air. Strawa et al. [26] examined the dynamic characteristics of jet diffusion flame with increasing pressure during acoustic excitation at 9 Hz. The flame breakup phenomenon was measured by schlieren, and the buoyancy-driven breakup process was followed near the nozzle outlet. In their analytical and numerical study of the flame response oscillation, Tyagi et al. [30] suggested that pinch-off occurs at a certain frequency and critical amplitude. Unfortunately, that study was limited to low frequencies (below 1 Hz), and a wider frequency range needs to be considered to identify the conditions (frequencies and amplitudes) of pinch-off. Magina [23] reported that frequency and amplitude are the key parameters that control the pinch-off behavior and performed a theoretical analysis of pinch-off according to the axial diffusion effect. After considering cases with/without the axial diffusion effect, it was confirmed that axial diffusion causes pinch-off over a narrower Strouhal number range. Gao et al. [28] studied the dynamic characteristics of flames excited at 100 Hz through simultaneous OH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements. Pinch-off flame of various amplitudes was confirmed at 100 Hz, and local pinch-off was suggested by the large vortex structure. However, acoustic excitation was applied only to the fuel supply line, and the fuel jet was accelerated by the influence of the Kelvin–Helmholtz (K–H) instability [11] under acoustic excitation.

In non-premixed flames, the flame structure is affected by the strain rate [31], and it has been suggested that a higher strain rate changes the flame surface area and increases the chemical reaction rate. Acoustic excitation affects the flame length, flame shape, and local flame extinction [32] due to enhanced mixing in the flame, and these physical characteristics can be analyzed in terms of the strain rate. Carrier et al. [31] suggested that the strain rate affects the flame in two ways. First, a high strain rate causes inflow of fresh air into the flame zone, and so the flame is first cooled and eventually quenched or extinguished. Secondly, a high strain rate extends the flame surface, which in turn increases the area in contact with the flame front as well as the chemical reaction rate. Donbar et al. [33] analyzed the strain rate along the CH reaction layer through simultaneous measurement of CH PLIF and PIV in a turbulent non-premixed flame. Various trends were observed, such as large curvatures, cusps, near-extinction, expansion due to entrained air, and increase in flame area due to vortex rollup. Kim et al. [11] performed simultaneous measurement of OH PLIF/PIV in turbulent non-premixed flames, and observed local flame extinction due to excessive strain rate in the high strain region. The authors suggested that local flame extinction is caused by two factors. The first is an increased diffusive influx of fuel by the internal vortex, and the second is air entrainment by the stretching motion of the external vortex. In previous studies [11,33], strain rate analysis was performed with regard to the flame contour derived from instantaneous data. However, the pinch-off behavior is a periodic phenomenon on a non-premixed flame, and so a time-resolved study is necessary to determine the mechanism. In this current investigation, strain rate analysis is performed with time-course measurements.

The specific aims of this study are analyzing and understanding the mechanism of pinch-off through examination of the flame response and the vortex-flame interactions in a laminar non-premixed flame (Burke–Schumann flame [32,34]) under acoustic excitation. The range of pinch-off in terms of forcing frequency and velocity perturbation intensity ( u ′ / u ¯ ) is confirmed, and the characteristics of the Strouhal number according to the flame height definition in the pinch-off flame region are identified. In addition, different trends are identified for the main flame height and the total flame height (i.e., with pocket) according to the velocity perturbation intensity. Furthermore, the vortical structure is measured in the non-reacting flow, the effects of the vortex flow and the reverse flow in the pinch-off flame in the reacting flow are investigated, and time-resolved strain rate and axial velocity analyses are performed.

Section snippets

Experimental setup and methodology

Pinch-off in a non-premixed flame by acoustic excitation was examined in a Burke–Schumann flame [32]-based combustor(Fig. 1(a)). The diameter of the fuel nozzle was 5 mm, the diameter of the air feeding line was 50 mm, and the interior of the combustor was a square column with a cross section of 50 mm × 50 mm. All sides of the combustor windows were equipped with quartz to block external air. Acoustic excitation through loudspeakers (Woofer, 8 inch, 100 W) and amplifier allowed us to

Flame response characteristics under various excitation frequencies

This study aims to measure the OH PLIF images under flow oscillation and to map the parameter range of pinch-off flame. Pinch-off is a local flame extinction phenomenon, in which the flame is separated under specific acoustic excitation conditions into a 'main flame' attached to the nozzle tip and a separate `pocket flame' [27,30] Tyagi et al. [30] suggested that pinch-off flame occurs at an appropriate excitation frequency and amplitude, although they only examined a limited frequency range of

Conclusions

Flame pinch-off is a flame separation phenomenon under an external force. It is of particular importance since pollutant discharge from a pinch-off flame is reported to be 4–5 times higher than that from a non-pinch-off flame due to a longer residence time in the detached pocket flame (i.e., the hot product). We analyzed the mechanism of pinch-off during acoustic excitation by varying the forcing frequency and velocity perturbation intensity. Simultaneous OH planar laser-induced fluorescence

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Advanced Research Center Program (NRF-2013R1A5A1073861), Ministry of Trade, Industry & Energy Industrial Technology Innovation Program (No.10067074), and Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017H1A2A1043206) and has been supported by the Institute of Advanced Aerospace Technology, Seoul National University.

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