Simultaneous OH-PLIF and schlieren imaging of flame...

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Simultaneous OH-PLIF and schlieren imaging of flame propagation in an obstacle-laden channel with H2/O2 and H2/air mixtures

Lorenz R. Boeck1,*, Gabriel Ciccarelli2 1: Graduate Aerospace Laboratories, California Institute of Technology, Pasadena, CA 91125, USA

2: Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON K7L3N6, CANADA * Correspondent author: [email protected]

Keywords: Explosions, Flame instabilities, Hydrogen, OH-PLIF, Schlieren

ABSTRACT

Flame propagation across a single obstacle in a closed channel has been studied by means of simultaneous single-image OH-PLIF and high-speed schlieren visualization. The test mixtures are stoichiometric H2/O2 at an initial pressure of 8 kPa and lean H2/air (Φ=0.5) at an initial pressure of 100 kPa. The OH-PLIF technique overcomes the problem of line-of-sight integration associated with schlieren visualization and thus enables the investigation of flame topology. For H2/O2, a smooth (laminar) flame front is observed upstream of the obstacle. Downstream, the flame interacts with vortices being shed off the obstacle corners and appears wrinkled in the obstacle recirculation zones. At the channel centerline, the flame front remains smooth. For the H2/air experiments, the macroscopic flow field, vortex-flame interactions and flame front instability interact producing a highly complex flame topology. Cells along the flame front are dynamically destroyed and re-established. OH-PLIF is particularly beneficial to diagnose such complex flames.

1. Introduction The potential for an explosion following the accidental release of a gaseous fuel represents a major hazard for industries that produce, transport, store or process explosive gases. In contrast to more common fires, the hazard is associated with high explosion overpressure rather than high combustion temperature. As early as 1881, Berthelot and Vieille [1] and Mallard and Le Chatelier [6] observed flame acceleration in gaseous explosions in tubes; the process of flame acceleration has been investigated in great detail ever since. The recent review by Ciccarelli and Dorofeev [4] summarizes the current state-of-knowledge and illustrates the complexity of the phenomenon. In the case of closed-end ignition in a tube, the flame propagation process involves several phases where different fluid dynamic phenomena affect the structure of the explosion front. Shortly after ignition, a laminar flame front propagates into the fresh mixture. During this phase


the flame may be wrinkled by hydrodynamic Landau-Darrieus instability. At the same time, diffusive-thermal effects will either stabilize or destabilize it, depending on the mixture Lewis number. The result will be a smooth flame front if the mixture is diffusive-thermally stable, or a cellular flame in the case of diffusive-thermal unstable. Flow is generated ahead of the flame as a result of the thermal expansion of the combustion products. This unburnt gas flow interacts with solid boundaries. At sufficiently high flow velocity, or Reynolds number, turbulence is generated in regions such as wall boundary layers, or by vortex shedding off obstacle edges. The flame interacts with these vortices increasing the volumetric burning rate, manifesting itself as an acceleration of the leading-edge of the flame. While the general physical mechanism of flame acceleration is believed to be understood, the current challenge is the development of models (analytical and numerical) capable of predicting explosions in real-world scenarios on an industrial scale. Difficulties are encountered due to the wide range of spatial and temporal scales that need to be modeled. Due to these demanding requirements, predictive capabilities of available codes are insufficient, especially when turbulence and shock-flame interactions are important. The need for experimental validation data is evident. Conventional techniques measuring flame speed and overpressure have been sufficient for the validation of analytical models and lumped parameter codes in the past. Nowadays, the thorough validation of numerical simulations requires detailed information on flow and flame dynamics. Advanced optical techniques can help to obtain such data, resolving, for instance, turbulence-flame interaction and the effect of flame and flow instabilities during flame acceleration. Schlieren imaging, the most common means of optical explosion diagnostics, highlights spatial density gradients, i.e., those associated with a flame. The main drawback is line-of-sight integration, which hinders the unambiguous characterization of three-dimensional flames as stressed by Ivanov et al. [5]. Planar laser-induced fluorescence imaging of hydroxyl radicals (OH-PLIF) overcomes this limitation and allows for direct visualization of the flame topology. Simultaneous acquisition of OH-PLIF and schlieren images makes it possible to interpret flame topology that cannot be identified solely based on schlieren images. This paper presents results from simultaneous OH-PLIF and schlieren imaging of flame propagation in a square obstacle-laden channel in stoichiometric H2/O2 mixture and lean H2/air mixture (Φ=0.5) at initial pressures of 8 kPa and 100 kPa, respectively. All experiments were performed at an initial temperature of 295 K. Stoichiometric H2/O2 mixture at low pressure is diffusive-thermally stable, flame wrinkling arises only due to turbulence-flame interactions. By


contrast, lean H2/air at higher pressure exhibits strong flame wrinkling and cellularity due to instabilities. 2. Experimental setup Experiments were performed in a closed, square cross-section channel, see Fig. 1. The cross-section and length were 0.0762x0.0762 m and 0.4953 m, respectively. A single obstacle was located at x=0.1524 m. The obstacle consisted of an upper and lower wall mounted fence-type quartz obstacle, each with a height of 0.0191 m and a thickness of 0.0127 m, extending across the channel span-wise dimension. Thus, the obstacle blockage ratio tested was 50%. Side-wall windows permitted optical access. Mixtures were prepared in a separate mixing vessel using the method of partial pressures and filled into the pre-evacuated channel. A spark plug centered at the channel end wall (x=0 m) ignited the mixture.

Ignition quartz obstacle

0.07

62 m

0.4953 m0.1524 m

x OH-PLIF field-of-view

Fig. 1: Experimental setup

3. Diagnostics Two optical measurement techniques were applied simultaneously, single-image OH-PLIF and schlieren imaging recorded at 75,000 fps. The OH-PLIF system consisted of a Spectra Physics Quanta Ray PIV 400 pump laser, a Sirah Precision Scan dye laser with Rhodamie 6G as a dye and a Princeton Instruments PI-MAX image-intensified camera equipped with a UV-Nikkor 105mm f/4.5 camera lens with an interference filter, 312 nm central wavelength, FWHM=24 nm. OH radicals were excited at their Q1(6) transition, λ≈282.9 nm. The laser beam was expanded vertically with an f=-30 mm cylindrical lens and collimated with an f=500 mm spherical lens. The light sheet entered the channel through a vertical quartz window mounted on the channel end-plate (x=0.4953 m) on the span-wise centerline, covering the entire channel height. The obstacle was made from quartz glass capable of transmitting the OH-PLIF laser light sheet. The OH-PLIF camera was oriented at a horizontal angle of 75 degrees with respect to the channel axis in order to provide a clear path for the schlieren light beam passing at 90 degrees to the channel. Linear


target-based transformation of the OH-PLIF raw images was performed to correct for the image distortion due to the OH-PLIF camera viewing angle. The schlieren setup consisted of two 250 mm diameter concave mirrors, f=3.048 m, two planar turning mirrors, a constant white Xenon arc light source and a Photron SA-5 camera. The schlieren camera was directly triggered off the ignition signal. One OH-PLIF image was captured per experiment. Since the phenomenon is repeatable, varying the delay time between ignition and OH-PLIF image capture permitted generating coherent OH-PLIF image sequences from repeated experiments. 4. Results This section provides experimental results from two different gas mixtures, stoichiometric H2/O2 at an initial pressure of 8 kPa and lean H2/air (Φ=0.5) at 100 kPa. First, flame tip velocity measured by tracking the flame tip in the high-speed schlieren sequences, is shown in Fig. 2. The same general flame acceleration behavior is observed for both mixtures. The flame accelerates towards the obstacle, x


obstacle

x [m]0.05 0.1 0.15 0.2 0.25 0.3

v [m

/s]

0

100

200

300

400

500

600# 30# 31# 32

1 3 42

obstacle

x [m]0.05 0.1 0.15 0.2 0.25 0.3

v [m

/s]

0

10

20

30

40

50

60# 56# 57# 58

31 46

2

5

(a) (b) Fig. 2: Flame tip velocity from high-speed schlieren for three experiments, (a) stoichiometric H2/O2 mixture; (b) lean

H2/air mixture (Φ=0.5). The vertical lines represent the positon of the flame tip when the OH-PLIF images were taken

Figure 3 shows the image sequence from four experiments obtained by OH-PLIF in the stoichiometric H2/O2 mixture at 8 kPa initial pressure. The image numbers correspond to the numbers in Fig. 2 (a). Flames propagate from left to right. General characteristics of these OH-PLIF images are discussed first; (1) Due to a high combustion product temperature on the order of 3000 K the OH concentration remains high behind the flame front and OH-PLIF signal is collected from the entire region consisting of burnt gas. The end of the reaction zone can thus not be identified, but the leading edge of the reaction zone is well visible as a sharp increase in OH-PLIF signal. (2) The white superimposed rectangles in Fig. 3 correspond to the obstacle contour in the OH-PLIF measurement plane. The OH-PLIF camera viewing-angle of 75 degrees leads to a perspective view of the quartz glass obstacle. Obstacle edges can be seen as dark shadows, for example in Fig. 3, frames 3 and 4, partially obstructing the flame image upstream of the obstacle. (3) Two horizontal thin lines can be seen upstream of the obstacle in the burnt gas region. As the laser light sheet enters the field-of-view from the right, these lines are shadows emerging from the top face of each obstacle. They provide an indication of the light sheet collimation and wall-parallel orientation.


1

2

3

4

Fig. 3: OH-PLIF sequence from four experiments in stoichiometric H2/O2 mixture, p0=8 kPa, T0=295 K. Experiments #30; 35; 33; 31

Two distinct phases of flame propagation can be observed: (1) At first, the flame front before and after the obstacle remains smooth (laminar) and continuous, see frames 1 and 2. Due to the thermal expansion of hot combustion products in this closed-end tube, flow of unburnt gas is induced ahead of the flame, which interacts with the obstacle. It has been shown (both through numerical simulation and experiments) that vortices are shed off the upstream obstacle edges, and are convected downstream [3, 7]. In this first phase, the flame does not interact with these vortices and remains laminar: wrinkling of the flame front is not observed. The flame is diffusive-thermally stable, a cellular flame topology is not observed. (2) By contrast, frames 3 and 4 show distinct flame front wrinkling downstream of the obstacle (e.g. arrows in frame 3), which is due to vortex-flame interactions. Note that flame front wrinkling is only present in the upper and lower recirculation zones downstream of the obstacle. In the core region (along the channel centerline), the flame front remains smooth at all times since no significant turbulent motion is present in the gas directly ahead of the flame front. Frame 4 shows the consumption of mixture in the recirculation zones. This delayed turbulent combustion leads to the secondary flame acceleration shown in Fig. 2. Of note in frame 4 is the burnt pocket of gas in the recirculation zone just downstream of the bottom obstacle (see arrow). Part of the flame tip could be quenched due to the high recirculation flow velocity. This is important because numerical simulations often assume a continuous flame surface, i.e., wrinkled flamelet model.


Figure 4 shows the flame front contours extracted from frames 1-4 in Fig. 3. In the 2-D measurement plane, the flame front length can be determined from this data, which is representative of the flame front surface area of the 3-D flame. It can be seen that the flame front length increases by a factor of 5-6 across the obstacle. The flame surface enlargement is both due to a macroscopic effect associated with flow contraction and expansion through the obstacle opening; and also due to small-scale flame wrinkling due to vortex-flame interactions. Note that the flame front directly at the channel walls is not taken into account in this analysis.

L1 = 0.128 mL2 = 0.239 mL3 = 0.483 mL4 = 0.720 m

Fig. 4: Flame front contours extracted from Fig. 3, frames 1-4

Figure 5 shows a representative high-speed schlieren sequence for a H2/O2 mixture. Schlieren visualization can add to the OH-PLIF results since it can identify compression and shock waves generated as a result of flame acceleration. However, the sequence shown in Fig. 5 does not show any indication of such waves. Thus, the flame is referred to as a slow flame (or deflagration), which propagates due to diffusion of species and heat in a flow that is subsonic and is not influenced by precursor shock waves and their interaction with the flame. We discuss faster regimes of flame propagation in [2]. The laminar flame shape in the first four images looks identical to that observed in the OH-PLIF images. The largest difference between the OH-PLIF and schlieren corresponds to the visualization of the flame entrainment into the recirculation zone downstream of the obstacle. In Fig. 5, frame 6, the line-of-sight integration (inherent to the schlieren technique) makes it impossible to determine the nature of the turbulent combustion. Whereas, as discussed earlier, in the OH-PLIF image (Fig. 3, frame 4) it is clear that the flame surface remains largely intact (wrinkled flame regime).


Fig. 5: Schlieren sequence from experiment #30 in stoichiometric H2/O2 mixture, p0=8 kPa, T0=295 K

In contrast to the stoichiometric H2/O2 mixture at an initial pressure of 8 kPa, a lean mixture of H2 and air (equivalence ratio of 0.5) at a higher initial pressure of 100 kPa exhibits strong flame front instabilities. Figure 6 shows the OH-PLIF sequence, Fig. 7 presents the extracted flame fronts, and Fig. 8 includes representative high-speed schlieren images. The OH-PLIF signal intensity drops behind the flame front due to a lower combustion product temperature than in the case of the H2/O2 mixture, leading to a lower OH concentration in the burnt gas. For this mixture, a cellular flame topology can be observed in the OH-PLIF, already at the beginning of the sequence (frame 1). Cells along the flame front get stretched as the flame tip passes the obstacle opening (frame 2). Downstream of the obstacle, the flame interacts with the train of vortices shed from the obstacle edges (not observable in the images). An asymmetric flame tip is observed. Due to the slower flame tip and flow velocities, the time available for flame roll-up into the recirculation zones is longer than in the H2/O2 case. Roll-up begins in frame 3, with deep penetration of the flame into the recirculation zone by frame 4. The portions of the flame that become entrained in the recirculation zones remain intact, cellular, and separate from the very thin sliver of burnt gas that occupies the core region within the obstacle. In frame 5, the recirculation-zone flame tongues and the “core-flame” region in the obstacle opening interact. The convergence of the two recirculation-zone flame tongues pinch the core-flame region slightly downstream of the opening (see arrows). At the same time, the gas directly in the


obstacle opening is brought to rest. The core-flame region experiences the influence of flames and thermal expansion on both sides of the obstacle. This leads to an outward propagation of this flame section (flame “bump” in the obstacle opening). In frame 5, the flow in the obstacle opening is reversed, and the “bump” has been convected upstream and steepened further. In frame 6, the recirculation-flame tongues are on the verge of propagating through the obstacle opening in the upstream direction. The flame upstream of the obstacle has come to rest and consumes the remaining mixture at a very low rate. The leading flame tip on the channel axis remains relatively smooth for a few frames (3-5). By frame 6 the flame tip decelerates and the cells are re-established dynamically. The highly complex flame topology is a result of the macroscopic flow field including recirculation, vortex-flame interactions and flame front instabilities. Figure 7 shows the extracted flame fronts from Fig. 6, frames 1-5. Similar to the H2/O2 case, a strong increase in flame length across the obstacle can be seen. The more complete flame roll-up into the recirculation zones, as well as the intense flame instability causes a larger increase in flame length as compared to H2/O2, Fig. 4. The corresponding schlieren sequence is presented in Fig. 8. Especially in this case of highly unstable flames the benefit of a planar measurement like OH-PLIF is apparent. The cellular flame front observed in the first frames in Fig. 8 indicates that the flame front is globally three-dimensional, i.e., the flame tip is almost hemi-spherical in frames 1 and 2. The OH-PLIF images reveal that these perturbations do not extend to the inside the flame as the OH signal is very uniform inside the flame front. The absence of the cells in the schlieren images for the H2/O2 flame makes it impossible to assess the three-dimensionality of the flame front (which clearly must exist). The highly cellular flame surface observed in the schlieren image in Fig. 8, frame 6, makes it impossible to interpret effects downstream of the obstacle. However, the OH-PLIF images (frames 5 and 6) clearly show that the flame becomes entrained into the recirculation zone and eventually propagates upstream past the obstacle.


1

2

4

5

3 6 Fig. 6: OH-PLIF sequence from four experiments in lean H2/air mixture (Φ=0.5), p0=100 kPa, T0=295 K. Experiments

#56; 57; 58; 59; 60; 63

L1 = 0.110 mL2 = 0.172 mL3 = 0.322 mL4 = 0.732 mL5 = 1.014 m

Fig.7:Flame fronts extracted from Fig. 6, frames 1-5


Fig. 8: Schlieren sequence from experiment #56 in lean H2/air mixture (Φ=0.5), p0=100 kPa, T0=295 K

5. Conclusions This paper presents results from a high-speed combustion experiment employing simultaneous single-image OH-PLIF and high-speed schlieren video diagnostics to resolve flame front topology. The advantages of the OH-PLIF are discussed based on experiments performed with two mixtures: low-pressure stoichiometric H2/O2 and high-pressure lean H2/air. The flame in H2/O2 initially exhibits a smooth (laminar) flame front before it is wrinkled by vortex-flame interactions in the recirculation zones downstream of the obstacle. By contrast, the H2/air mixture shows distinct flame front cellularity. The highly complex flame topology is the result of the macroscopic flow field, vortex-flame interactions and flame front instability. The novel OH-PLIF images highlight flame topology that cannot be identified in schlieren images due to line-of-sight integration. Acknowledgements The authors are grateful to the National Research Council Canada for the loan of the intensified camera and lens, to Dr. Matthew Johnson from Carleton University for the loan of the lasers and optics, and to Adam Steinberg for use of the OH filter.


References [1] Berthelot, M. and Vieille, P. (1881). Sur la vitesse de propagation des phénomènes explosifs dans les gaz. CR Acad. Sci., Paris, 93, 18-22. [2] Boeck, L.R., Kellenberger, M., Rainsford, G. and Ciccarelli, G. (2016). Simultameous OH-PLIF and Schlieren Imaging of Flame Acceleration in an Obstacle-Laden Channel. P. Combust. Inst. [3] Boeck, L.R., Lapointe, S., Melguizo-Gavilanes, J. and Ciccarelli, G. (2016). Flame Propagation Across an Obstacle: OH-PLIF and 2-D Simulations with Detailed Chemistry. P. Combust. Inst. [4] Ciccarelli, G. and Dorofeev, S. (2008). Flame acceleration and transition to detonation in ducts. Prog. Energ. Combust., 34(4), 499-550. [5] Ivanov, M. F., Kiverin, A. D., Yakovenko, I. S. and Liberman, M. A. (2013). Hydrogen–oxygen flame acceleration and deflagration-to-detonation transition in three-dimensional rectangular channels with no-slip walls. Int. J. Hydrogen Energ., 38(36), 16427-16440. [6] Mallard, E. and Le Chatelier, H. (1881). Sur les vitesses de propagation de l’inflammation dans les mélanges gazeux explosifs. CR Ac. des Sc, 93, 145-148. [7] Johanson, C. and Ciccarelli, G. (2009). Visualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel. Combust. Flame, 156 (2), 405-416.

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