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Experimental research on flame revolution and precession of fire whirls
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Proceedings of the Combustion Institute xxx (2012) xxx–xxx
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Experimental research on flame revolution and precession of fire whirls Jiao Lei a, Naian Liu a,⇑, Jesse S. Lozano b, Linhe Zhang a, Zhihua Deng a, Kohyu Satoh a a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Mechanical Engineering Department, University of California, Riverside, USA
Abstract This paper presents the first experimental effort to explore the large scale 3-D flame instabilities of fire whirls, including the inclined flame revolution during the transition from a general pool fire to fire whirl, and the swirling flame precession in a quasi-steady fire whirl. The experimental medium-scale fire whirls were produced by a fixed-frame facility. Experimental observations indicate that flame revolution is an important flame instability during the formation of fire whirl, showing that the entire flame is inclined and revolves around the geometrical axis of symmetry with increasing angular velocity until the critical point, without the self-rotation of the flame. It is found that the inlet velocity fluctuates synchronously with the flame revolution. As soon as the fire whirl forms, the erect swirling flame starts to precess around the geometrical axis of symmetry. Analysis indicates that during flame precession the periodic fluctuations of inlet velocity disappear and a local annular external recirculation zone (ERZ) is produced outside the flame (vortex core), while the flow is upward inside. It is found that the inlet velocities are nearly constant within the continuous flame in order to maintain a stable generating eddy. A good linear correlation exists between the average inlet velocities and average ambient circulations for all fuel pan sizes. The precession frequency is relatively stable during one test. The frequencies of flame revolution and precession are both proportional to the average inlet velocity, and the corresponding Strouhal numbers are constants of 0.42 and 0.80, respectively. The flame revolves and precesses in the same direction as the self-rotation of the fire whirl flame in all tests. The flame revolution is related to the periodical fluctuations of inlet flow, while the flame precession is considered to be linked to the occurrence of ERZ in fire whirls. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Fire whirl; Flame revolution; Flame precession; Fixed-frame facility; External recirculation zone
1. Introduction Fire whirl is a special swirling diffusion flame that may occur in both urban and wildland areas with possibility of causing great damage to prop⇑ Corresponding author. Fax: +86 551 3601669.
E-mail address: [email protected] (N. Liu).
erty and life [1,2]. As indicated, the formation of a fire whirl depends on three essential conditions – a generating eddy, a fluid sink within the eddy, and some friction or drag offered to the movement of air in the ground boundary of the eddy [3]. In a fire whirl, the hot gas generated by the fire itself serves as the fluid sink which entrains the ambient air with angular momentum from the generating
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.06.126
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eddy to the flame (vortex core). As summarized by Byram and Martin [3], the different types of facilities which can be used to produce single steady fire whirls in laboratory are based on the concepts of a generating eddy and a fluid sink. The facilities can be generally classified into two types, depending on whether the generating eddy is imposed mechanically by a spinning screen (Emmons type [4,5]) or induced by the entrained air flowing through well-arranged spiral paths (fixed-frame type [6–9], fire-wall type [10]). Some quantitative experimental researches have been performed that focused on burning rates, flame plume temperature, velocity distributions, and radiation heat flux of fire whirls [2,4,7,8,11]. As usually observed in fire whirl experiments, a distinct difference between fire whirls and general pool fires is the presence of an obvious flame wander around the geometrical centerline in experimental fire whirls, which can induce large fluctuations of data, leading to great difficulties for the subsequent data analysis [4,8]. Emmons and Ying [4] reported that the flame wander involved amplitude of several flame diameters with a lower frequency when compared to the large turbulent eddies. They attributed this wander to some inherent flame instability since the facility was symmetrical and careful precautions were taken for turbulence suppression. Flame instability was also found by numerical simulations of fire whirls. Snegirev et al. [12] reported the periodic flame wander, but without further discussion. Recently Su et al. [13], using Fire Dynamics Simulation (FDS) for simulating fire whirls in a ship engine room, found that the calculated angular velocity of flame wander fit well with a cube polynomial function of the inlet velocity along the four walls and increased rapidly when the inlet velocity was over a critical value (6.58 m/s). However, no experimental data was presented to compare to the numerical results. It is doubtful that using FDS without code modification would accurately represent the complex interaction of combustion and strong rotation of a fire whirl. In appearance, the flame wander in a single quasi-steady fire whirl is characterized by its self rotation and the simultaneous rotation of the entire flame body around a certain geometrical axis (generally the central axis of the pool). Therefore, the flame wander should be more exactly termed flame precession. Before the formation of the steady fire whirl, the tilted flame would revolve around the geometrical central axis, but without self rotation. We call this phenomenon of flame instability as flame revolution, so as to distinguish it from flame precession. This paper presents the first effort to quantitatively explore the characteristics of flame instability in fire whirls. We utilized a fourwall fixed-frame facility for experiments. The
frequencies of flame revolution and flame precession were extracted from data of periodic temperature fluctuations at specific points, and the flow characteristics were obtained by comprehensive velocity measurements. The relationships between the instability frequencies and inlet velocity were revealed. The differences of precession in fire whirls and ordinary swirling jet flames were examined extensively. 2. Experimental The fixed-frame fire whirl facility, located in a large test hall, was a square enclosure made of tempered glass, with a base dimension of 2 m 2 m and a height of 15 m (Fig. 1). The channel was open at the top. Each channel wall had a uniform 20 cm wide vertical gap at its corner to ensure that the entrained air induced by the burning flame could enter the channel, thus imparting the rotational flow necessary for fire whirl formation. The base table (2 m 2 m) was made of pine wood with a round hole (60 cm in diameter and 10 cm in depth) in the center. The steel circular fuel pans (with diameters d of 10– 55 cm with increments of 5 cm, and a depth of 10 cm) were positioned in the center of the facility and placed such that their rims were flush with the top of the base table. The initial height of the liquid fuel (n-heptane, 97%) surface was 7 cm. A water layer was used in the larger fuel pans to maintain the initial fuel surface height constant and also ensure safety. The lip height has proven to have little effect on the burning rate of fire whirls [8]. The fuel mass versus time was recorded using an electronic balance with a precision of 0.1 g. The temperatures were measured using Type K (chromel–alumel) thermocouples (bead diameter: 1 mm). They were arranged in the radial direction in intervals of 1.5 cm from r = 30 to 30 cm Table 1 The experimental data in the present work. d (cm)
V in ci (m/s)
fri V in cs fps C Rfs Rps Qa (Hz) (m/s) (Hz) (m2/s) (cm) (cm) (kW)
10 15 20 25 30 35 40 45 50 55
0.31 0.52 0.58 0.61 0.71 0.77 0.92 0.89 1.03 1.13
0.07 0.11 0.12 0.13 0.16 0.17 0.20 0.20 0.21 0.24
0.28 0.52 0.67 0.78 0.87 1.08 1.19 1.35 1.48 1.64
0.14 0.25 0.28 0.32 0.41 0.46 0.48 0.51 0.55 0.62
1.49 2.00 2.67 3.21 3.78 4.60 5.08 5.83 6.31 6.81
4.0 5.6 7.6 8.9 10.7 12.2 13.6 15.0 15.7 16.9
1.5 2.4 2.7 2.8 3.4 3.7 3.6 4.0 4.6 5.1
22.3 62.4 102.6 156.1 240.8 343.4 441.5 566.4 669.0 789.4
a The heat release rates Q were calculated by assuming unity of combustion efficiency in the quasi-steady state, and the effective heat of combustion for heptane is 44.6 kJ/g.
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J. Lei et al. / Proceedings of the Combustion Institute xxx (2012) xxx–xxx (cm) -30 400
2.0m
-20
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Pitot tubes for tangential velocity measurement
20cm
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3 20
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Pitot tubes for axial velocity measurement
HWA
Liquid Fuel pool d
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1.9m
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2.65m 2.15m 2.0m
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Entrained air Corner gap
HWA for inlet velocity measurement
0.91m
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T0
T20
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0.15m
0.5m
Liquid Fuel, d=10,15,20,...,55cm Raidial thermocouples (Spacing of 1.5cm,T-20~T20) Wood base
Electronic balance
Fig. 1. Schematic for fire whirl experiments.
(T20 to T20) at a height z = 40 cm, which is significantly lower than the minimum flame height for a pool diameter d = 10 cm. The signal sampling frequency was 100 Hz. The inlet air velocities were measured using Hot-wire anemometers (HWA, Kanomax Inc.) which were positioned along the central axis of an inlet at seven different heights (0.15–3.15 m, with increments of 0.50 m). The HWA probes were set to record at a sampling frequency of 10 Hz. The tangential and vertical velocities were measured at several heights using calibrated temperature-compensated pitot tubes (Series 160S ‘S’ type, Dwyer Inst. Co.) and Type K thermocouples (bead diameter: 1 mm) arranged in the radial direction (0–25 cm, with increments of 5 cm). The circulation was calculated by multiplying the tangential velocity by the corresponding radius. Two video cameras were used to monitor the flame radius and phenomena of flame instability in horizontal and oblique views. The large test hall remained closed and the smoke exhaust system was turned off to reduce turbulence during tests. All the experimental data is summarized in Table 1. 3. Results and discussion 3.1. Phenomenon observations When the liquid fuel pools were ignited, at first the flame behaved like general buoyant pool fires
(Fig. 2a). Several seconds later, the entrained air from the four inlets arrived at the flame to induce the interaction with the combustion. The entire buoyant flame tilted and revolved around the geometrical central axis of the facility, without selfrotation (Fig. 2b and c). The angle between the flame and the level plane was decreasing and the angular speed of the flame revolution was increasing. Several loops later, the flame became nearly horizontal. Then the flame tip turned into vertical rapidly and started to spin around the flame axis. This self-rotation shortly extended downward to the fuel surface (Fig. 2d), which was consistent with observations of the formation of a concentrated vortex [14]. Finally, the entire flame became vertical with rapid rotation, which suggested the formation of the fire whirl. At the same time, besides the self-rotation, the entire flame continued to revolve around the geometrical central axis of the fuel pan, and the swirling flame axis was parallel to the geometrical central axis. This suggested that the flame or the vortex core was precessing around the central axis. The swirling flame was then observed to quickly approach the geometrical centerline with increasing precession frequency and decreasing precession radius. The flame precession then reached a quasi-steady state with a smaller precession radius (Fig. 2e). Here the precession radius (Rp) was defined as the distance between the flame axis and the geometrical central axis of the fuel pan. At any cross section, the preces-
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Fig. 2. Flame images of transition from a general pool fire to a fire whirl (d = 50 cm).
sion trajectory of the flame center was almost circular which was centered nearly in the geometrical center. The transition durations from general buoyant pool fires to fire whirls, which depends on the pan size, ranged from 800 s to 50 s for d = 10–55 cm. We also note that the directions of flame revolution and precession are the same as the rotation direction of the flame self-rotation (in clockwise direction in the present facility). 3.2. Radial temperature data In our previous work [8], the fire whirl was proved to be a highly stable combustion phenomenon in the vortex frame of reference. By crosscorrelation analysis, we found that the instantaneous centerline temperature signals at different heights had no phase delay and that the radial signals showed synchronous fluctuations. Therefore, the temperature fluctuations were mainly induced by flame precession rather than turbulent fluctuations. Typical time-averaged radial temperatures, reported previously in [8], involved small gradients in the regions far from the flame and within the flame core, while near the flame boundary
the temperature gradient was very large. Therefore, the fluctuations in the instantaneous temperature data by flame precession, in the radial direction, differ greatly at different measurement points. As shown in Fig. 3, the trajectory of the flame center is roughly assumed to be circular with a precession radius of Rp. O0 and O00 are the flame centers at t = t0 and t = t00 in a half cycle. R and Rf are the pan radius and flame radius respectively. The temperature fluctuations at point A and E should be relatively small as a result of always being located far from the flame and in the flame core respectively. However, alternately point C (or D) is periodically inside and outside the flame zone which induces large fluctuations in the instantaneous temperature data. Point B is located outside the flame outer boundary at t = t00 but close to it at t = t0 , where significant fluctuations are also observed although the maximum value is smaller than the flame temperature. The time interval between two adjacent temperature peaks represents one cycle of the flame precession. The frequencies of flame precession can then be extracted from the instantaneous temperature data near the outer flame boundary at point C (D, B). Similarly, the frequencies of flame revolution can also be obtained by the regular
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The inlet velocities in the vertical direction are nearly constant within the continuous flame for a specific fuel pan. Moreover, we found that the ambient circulations around the fire whirls do not change significantly with vertical heights. As shown in Fig. 5, for all fuel pan sizes, the average ambient circulation (C) and the average inlet centerline velocity ðV in cs Þ were fitted well by a linear correlation C ¼ 4:23V in cs , here the subscript “in” denotes inlet, “c” denotes centerline and “s” denotes the quasi-steady state. In the present facility, the average circulation at the inlet is Cin ¼ 2pV in , where V in is the average velocity across the inlet. Thus we have V in ¼ 0:67V in cs , which is a reasonable result due to the non-uniform velocity profile across the inlet.
Fig. 3. Schematic for flame precession and temperature measurements in the radial direction.
3.4. Characteristics of flame revolution and precession
As shown in Fig. 4, the variation of inlet velocity is consistent with the combustion behavior. After ignition, entrainment started and the inlet velocity began to increase. Later, as the entire flame tilted and revolved around the geometrical axis, the inlet velocities at different heights also fluctuated in phase. Large regular inlet velocity fluctuations disappeared whenever the swirling flame moved close to the centerline after the formation of the fire whirl. The inlet velocity increases due to the increases of burning rates and wall temperatures until reaching the quasisteady state. This is especially the case for fuel pans with large heat release rates. As the pool size increases, the inlet velocity also increases.
3.4.1. Flame revolution in the initial stage In the initial stage, the entire buoyant flames tilted and revolved around the geometrical axis without flame self-rotation. The instantaneous temperatures at two symmetrical points (T18, T18, r = ±27 cm) and the centerline inlet velocity (z = 0.15 m) are shown in Fig. 6 for d = 15 cm. We can see that the inlet velocity involves significant fluctuations by the effect of flame revolution. Any time interval between two peaks of T18 (T18) is divided equally by the inner peak of T18 (T18), which suggests a regular flame revolution. Also the time intervals between the adjacent peaks of the temperature are close to those of inlet velocity. Thus, the complex interaction between the buoyant pool fire and the entrained air flow produces the periodical fluctuations of inlet velocity and pressure fields, which in turn inclines and drives the flame to revolve around the geometrical axis of the facility. Moreover, the cycle period decreases (i.e., revolution frequency increases)
Fig. 4. Instantaneous data of centerline inlet velocity (d = 15 cm).
Fig. 5. Average ambient circulations versus average inlet centerline velocity in the quasi-steady state with linear fitting indicated.
temperature fluctuations at specific measurement points. This will be discussed later in detail. 3.3. Inlet velocity and ambient circulations
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until a critical value (e.g. the last three cycles in Fig. 7) which is favorable for the formation of fire whirl. This critical frequency for flame revolution should be a characteristic for specific fuel pan and facility (Table 1). In Fig. 8, it is found that the critical frequencies of flame revolution increase linearly with the average inlet centerline velocity for different pool sizes, and the data are correlated well by fri ¼ 0:21V in ci . For the subscript “ri” used here, “r” denotes flame revolution and “i” denotes the initial stage. Here, a Strouhal number is defined by St ¼ fri D=V in
ci
ð1Þ
similar to that used in cyclones [15,16], where D is the horizontal dimension of the facility. Thus St is a constant of 0.42 for the flame revolution in the present facility. 3.4.2. Flame precession in the quasi-steady state When the vortex formed at the inclined flame tip extends downward to the fuel surface, an erect fire whirl is finally established and the swirling flame (vortex core) then starts to precess around the geometrical axis of symmetry. As discussed in Section 3.2, this swirling flame precession can produce fluctuations in the instantaneous temperature data. The typical data at the different points are shown in Fig. 9 for d = 40 cm. Consistent to the above discussion, the temperatures at the flame center (T0) and far from the flame outer boundary (T13) are relatively steady. However, the temperatures adjacent to the flame (T7, T8, T7, T8) fluctuate largely and the signals at symmetry points are generally opposite in phase. This clearly confirms flame precession in an experimental sense. In Fig. 9, the time intervals between two adjacent peaks (troughs) of T7 or T8 (T7 or T8) are 2.42 s, 2.23 s, 1.64 s, 1.84 s, 2.89 s, 2.78 s, 1.81 s respectively, which do not differ signifi-
Fig. 6. Instantaneous temperatures of T18, T18 (r = ±27.0 cm) and the centerline inlet velocity at z = 0.15 m before fire whirl formation (d = 15 cm).
Fig. 7. Time intervals between adjacent peaks of T20, T20 and inlet velocity (z = 0.15 m) during flame revolution before the formation of fire whirl (d = 15 cm).
Fig. 8. Frequencies of flame revolution in the initial stage and flame precession in the quasi-steady state versus the average inlet velocity with linear fitting indicated.
cantly and show a relatively regular precession. The precession frequency is then determined to be 0.48 Hz by averaging the frequencies of these cycles. For each pool size, the precession frequency was found to remain relatively stable during one test. In the present study, we are interested in the swirling flame precession at the quasi-steady state with stable inlet velocity and burning rate (Table 1). In Fig. 8, the precession frequencies at the quasi-steady state are also plotted versus the inlet velocities for all fuel pan sizes. The frequencies of swirling flame precession are about 2–3 times larger than that of flame revolution in the initial stage for the same fuel pan size. Similarly, the data can be correlated by a linear function fps ¼ 0:40V in cs well and thus fps ¼ 0:095C, which has the same form as the
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is roughly balanced by the radial pressure gradient due to @p=@r ¼ qw2 =r
Fig. 9. Instantaneous temperatures of four symmetry points at r = ±10.5 cm (T7, T7) and r = ±12.0 cm (T8, T8). The temperature at r = 0 (T0) and r = 19.5 cm (T13) are also shown (z = 40 cm, d = 40 cm, Rf = 13.6 cm).
correlation fp ¼ C=ð4p2 R02 Þ ¼ 0:013C proposed by Murakami [17], Belousov and Gupta [15] and Alekseenko et al. [18] for cold swirling flows in a tube or chamber, where R0 is the radial distance from the inlet to the center of the facility (R0 = 1.4 m for the present facility). The large increase of the coefficient may be due to the combustion and configurations. For the subscript “ps” used here, “p” denotes flame precession and “s” denotes the quasi-steady state. The corresponding Strouhal number is about 0.80. The linear correlation is not consistent with the cube polynomial function suggested by Su et al. [13], which may be due to that the inlet velocity was specified artificially in their simulation. Furthermore, the flame radius and precession radius are obtained by image processing of sequential flame images. The ratio of the precession radius and flame radius in the quasi-steady state are around 0.30 for all fuel pans. Therefore, a relatively small precession range is found in the fixed-frame facility as compared to that in the rotating-screen facility [4]. 3.5. Mechanics of flame precession in fire whirls 3.5.1. Differences of precession in jet swirling flames and fire whirls All elements in the rotating-screen and fixedframe facilities are symmetrical. However, the flame instability exists all the time. In literature, the three dimensional (3-D) precessing vortex core (PVC) also occurs in strong jet swirling flames and has been studied extensively in the last 30 years [19]. Similarly to flame precession in fire whirls, it was established that the PVC is not a consequence of significant violations of the symmetry of facility. In a swirling jet, the centrifugal force
ð2Þ
where p and w are the pressure and tangential velocity respectively. As the swirling jet is ejected from the nozzle, an adverse axial pressure gradient is formed near the jet axis due to the quick expansion of swirling flow with decreasing tangential velocity. Providing the swirl strength is high enough, a critical point is reached when this adverse axial pressure gradient exceeds the kinetic energy of fluid in the axial direction and hence a reverse flow is induced on the axis between two stagnation points. That is, the vortex breakdown occurs and the central recirculation zone (CRZ) is not stable. The central vortex core is then displaced from and starts to precess around the axis of symmetry at a well-defined frequency. Therefore, in strong swirling jet flames the PVC is usually linked to vortex breakdown phenomena and the accompanying CRZ. It was found that the PVC frequency has magnitudes of 10–1000 Hz. When vortex breakdown occurs, the flame height decreases due to increased turbulent mixing and reaction rate. However, in fire whirls, the precession frequency is lower than 1 Hz in the present experiments, which is much lower than that of jet swirling flames. Moreover, flame stretching is always observed in fire whirls. Therefore, qualitatively different effects of rotation on combustion dynamics are presented. 3.5.2. Flow characteristics of fire whirls with flame precession As shown in Fig. 10, in the quasi-steady state, the centerline axial velocity of fire whirls is positive and increases with height in the continuous flame region, which indicates that CRZ is not formed and no vortex breakdown occurs. Additionally, the axial velocities versus radius at several heights are shown in Fig. 11. Apparently, the axial flux is concentrated inside the flame. However, the axial velocity decreases with radius and even becomes small negative values from r = 20 cm at z = 0.91 m outside the flame (Rf = 15.7 cm, d = 50 cm) where an annular external recirculation zone (ERZ) is located. As soon as the fire whirl is generated and the precession starts, the axial velocity at z = 0.91 m rapidly becomes negative and remains relatively stable while decreases slightly due to increase of burning rate and inlet velocity, which indicates a stable CRZ during the flame precession. This weak reverse flow was also observed in a vortex flow produced in a rotating cylinder filled with water [20] and in a tornado-like vortex produced in a rotating-screen facility [21]. The reverse flow is also related to the adverse axial pressure gradient and can extend to the outside region next
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Fig. 10. Centerline axial velocities versus axial distance (d = 50 cm).
Fig. 12. Axial distributions of circulation outside the flame (Rf = 15.7 cm, d = 50 cm).
flame precession of fire whirls is linked to the occurrence of ERZ. A mechanics may be as follows: due to the formation of ERZ, a strong shear layer will be created on the boundary between the core and the outer reverse flow. This shear layer is prone to the Kelvin–Helmholtz instability, which may generate asymmetry in the flow field. Then the 3-D flame precession comes to being. Further experimental and theoretical investigations are demanded on this topic in the future. 4. Conclusions
Fig. 11. Radial distributions of axial velocities at different heights (d = 50 cm).
to the boundary layer if the rotation is sufficiently strong [21]. As shown in Fig. 12, the circulation along the axial direction is not a constant exactly and is a minimum at z = 1.2 m at r = 20 cm and 25 cm for d = 50 cm. The resultant axial pressure drop is small and cannot induce the CRZ in the vortex core with high axial velocity. However, this pressure drop can exceed the small kinetic energy of fluid outside the vortex core, which gives rise to the ERZ. Ying and Chang [21] also observed the vortex wander accompanied by the ERZ in the rotating-screen facility similar to that used by Emmons and Ying for fire whirls [4]. As we know, fire whirls and tornado-like vortexes are similar in the sense of fluid mechanics because they are classified into the concentrated vortex [22]. Moreover, low frequency precession is also observed in a confined jet with very weak rotation accompanied by ERZ without the occurrence of CRZ. The asymmetry induced by ERZ causes the flow to precess about the axis of the device [19,23]. Therefore, there is a strong possibility that the swirling
The 3-D flame instabilities in fire whirls including the flame revolution and flame precession were distinguished and investigated by experimental means in a medium-scale fixed-frame facility. The major results are summarized as follows: 1. Before the formation of fire whirl, the flame revolves around the geometrical axis of symmetry with increasing angular velocity until the critical revolution frequency required for the formation of fire whirls is reached. 2. As soon as the fire whirl forms and swirling flame precession starts, an annular ERZ is found adjacent to the flame outer boundary, while CRZ and the vortex breakdown are not present. 3. The inlet velocity fluctuates synchronically with the flame revolution in the initial stage, which disappears when precession starts. The inlet velocities are nearly constant within the continuous flame in order to maintain a stable generating eddy. The average ambient circulations and the average inlet velocities can be fitted well linearly for all pool sizes in the quasi-steady state. 4. The precession frequency is relatively stable during one test. The frequencies of flame revolution and precession are both linearly
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dependent on the average inlet velocity. The corresponding Strouhal numbers are 0.42 and 0.80 respectively. 5. The directions of flame revolution and precession are always the same as the direction of self-rotation of the fire whirl flame. The flame revolution is caused by the periodical fluctuations of the inlet flow, while the flame precession is considered to be linked to the occurrence of ERZ in fire whirls.
Acknowledgements This work was sponsored by the National Natural Science Foundation of China under Grant 51120165001 and 51076148, and National Key Technology R&D Program under Grant 2011BAK07B01-02. Naian Liu was supported by “the Fundamental Research Funds for the Central Universities (No. WK2320000014).
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Proceedings of the Combustion Institute xxx (2012) xxx–xxx
Combustion Institute www.elsevier.com/locate/proci
Experimental research on flame revolution and precession of fire whirls Jiao Lei a, Naian Liu a,⇑, Jesse S. Lozano b, Linhe Zhang a, Zhihua Deng a, Kohyu Satoh a a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Mechanical Engineering Department, University of California, Riverside, USA
Abstract This paper presents the first experimental effort to explore the large scale 3-D flame instabilities of fire whirls, including the inclined flame revolution during the transition from a general pool fire to fire whirl, and the swirling flame precession in a quasi-steady fire whirl. The experimental medium-scale fire whirls were produced by a fixed-frame facility. Experimental observations indicate that flame revolution is an important flame instability during the formation of fire whirl, showing that the entire flame is inclined and revolves around the geometrical axis of symmetry with increasing angular velocity until the critical point, without the self-rotation of the flame. It is found that the inlet velocity fluctuates synchronously with the flame revolution. As soon as the fire whirl forms, the erect swirling flame starts to precess around the geometrical axis of symmetry. Analysis indicates that during flame precession the periodic fluctuations of inlet velocity disappear and a local annular external recirculation zone (ERZ) is produced outside the flame (vortex core), while the flow is upward inside. It is found that the inlet velocities are nearly constant within the continuous flame in order to maintain a stable generating eddy. A good linear correlation exists between the average inlet velocities and average ambient circulations for all fuel pan sizes. The precession frequency is relatively stable during one test. The frequencies of flame revolution and precession are both proportional to the average inlet velocity, and the corresponding Strouhal numbers are constants of 0.42 and 0.80, respectively. The flame revolves and precesses in the same direction as the self-rotation of the fire whirl flame in all tests. The flame revolution is related to the periodical fluctuations of inlet flow, while the flame precession is considered to be linked to the occurrence of ERZ in fire whirls. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Fire whirl; Flame revolution; Flame precession; Fixed-frame facility; External recirculation zone
1. Introduction Fire whirl is a special swirling diffusion flame that may occur in both urban and wildland areas with possibility of causing great damage to prop⇑ Corresponding author. Fax: +86 551 3601669.
E-mail address: [email protected] (N. Liu).
erty and life [1,2]. As indicated, the formation of a fire whirl depends on three essential conditions – a generating eddy, a fluid sink within the eddy, and some friction or drag offered to the movement of air in the ground boundary of the eddy [3]. In a fire whirl, the hot gas generated by the fire itself serves as the fluid sink which entrains the ambient air with angular momentum from the generating
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.06.126
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eddy to the flame (vortex core). As summarized by Byram and Martin [3], the different types of facilities which can be used to produce single steady fire whirls in laboratory are based on the concepts of a generating eddy and a fluid sink. The facilities can be generally classified into two types, depending on whether the generating eddy is imposed mechanically by a spinning screen (Emmons type [4,5]) or induced by the entrained air flowing through well-arranged spiral paths (fixed-frame type [6–9], fire-wall type [10]). Some quantitative experimental researches have been performed that focused on burning rates, flame plume temperature, velocity distributions, and radiation heat flux of fire whirls [2,4,7,8,11]. As usually observed in fire whirl experiments, a distinct difference between fire whirls and general pool fires is the presence of an obvious flame wander around the geometrical centerline in experimental fire whirls, which can induce large fluctuations of data, leading to great difficulties for the subsequent data analysis [4,8]. Emmons and Ying [4] reported that the flame wander involved amplitude of several flame diameters with a lower frequency when compared to the large turbulent eddies. They attributed this wander to some inherent flame instability since the facility was symmetrical and careful precautions were taken for turbulence suppression. Flame instability was also found by numerical simulations of fire whirls. Snegirev et al. [12] reported the periodic flame wander, but without further discussion. Recently Su et al. [13], using Fire Dynamics Simulation (FDS) for simulating fire whirls in a ship engine room, found that the calculated angular velocity of flame wander fit well with a cube polynomial function of the inlet velocity along the four walls and increased rapidly when the inlet velocity was over a critical value (6.58 m/s). However, no experimental data was presented to compare to the numerical results. It is doubtful that using FDS without code modification would accurately represent the complex interaction of combustion and strong rotation of a fire whirl. In appearance, the flame wander in a single quasi-steady fire whirl is characterized by its self rotation and the simultaneous rotation of the entire flame body around a certain geometrical axis (generally the central axis of the pool). Therefore, the flame wander should be more exactly termed flame precession. Before the formation of the steady fire whirl, the tilted flame would revolve around the geometrical central axis, but without self rotation. We call this phenomenon of flame instability as flame revolution, so as to distinguish it from flame precession. This paper presents the first effort to quantitatively explore the characteristics of flame instability in fire whirls. We utilized a fourwall fixed-frame facility for experiments. The
frequencies of flame revolution and flame precession were extracted from data of periodic temperature fluctuations at specific points, and the flow characteristics were obtained by comprehensive velocity measurements. The relationships between the instability frequencies and inlet velocity were revealed. The differences of precession in fire whirls and ordinary swirling jet flames were examined extensively. 2. Experimental The fixed-frame fire whirl facility, located in a large test hall, was a square enclosure made of tempered glass, with a base dimension of 2 m 2 m and a height of 15 m (Fig. 1). The channel was open at the top. Each channel wall had a uniform 20 cm wide vertical gap at its corner to ensure that the entrained air induced by the burning flame could enter the channel, thus imparting the rotational flow necessary for fire whirl formation. The base table (2 m 2 m) was made of pine wood with a round hole (60 cm in diameter and 10 cm in depth) in the center. The steel circular fuel pans (with diameters d of 10– 55 cm with increments of 5 cm, and a depth of 10 cm) were positioned in the center of the facility and placed such that their rims were flush with the top of the base table. The initial height of the liquid fuel (n-heptane, 97%) surface was 7 cm. A water layer was used in the larger fuel pans to maintain the initial fuel surface height constant and also ensure safety. The lip height has proven to have little effect on the burning rate of fire whirls [8]. The fuel mass versus time was recorded using an electronic balance with a precision of 0.1 g. The temperatures were measured using Type K (chromel–alumel) thermocouples (bead diameter: 1 mm). They were arranged in the radial direction in intervals of 1.5 cm from r = 30 to 30 cm Table 1 The experimental data in the present work. d (cm)
V in ci (m/s)
fri V in cs fps C Rfs Rps Qa (Hz) (m/s) (Hz) (m2/s) (cm) (cm) (kW)
10 15 20 25 30 35 40 45 50 55
0.31 0.52 0.58 0.61 0.71 0.77 0.92 0.89 1.03 1.13
0.07 0.11 0.12 0.13 0.16 0.17 0.20 0.20 0.21 0.24
0.28 0.52 0.67 0.78 0.87 1.08 1.19 1.35 1.48 1.64
0.14 0.25 0.28 0.32 0.41 0.46 0.48 0.51 0.55 0.62
1.49 2.00 2.67 3.21 3.78 4.60 5.08 5.83 6.31 6.81
4.0 5.6 7.6 8.9 10.7 12.2 13.6 15.0 15.7 16.9
1.5 2.4 2.7 2.8 3.4 3.7 3.6 4.0 4.6 5.1
22.3 62.4 102.6 156.1 240.8 343.4 441.5 566.4 669.0 789.4
a The heat release rates Q were calculated by assuming unity of combustion efficiency in the quasi-steady state, and the effective heat of combustion for heptane is 44.6 kJ/g.
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J. Lei et al. / Proceedings of the Combustion Institute xxx (2012) xxx–xxx (cm) -30 400
2.0m
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HWA
Liquid Fuel pool d
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Liquid Fuel, d=10,15,20,...,55cm Raidial thermocouples (Spacing of 1.5cm,T-20~T20) Wood base
Electronic balance
Fig. 1. Schematic for fire whirl experiments.
(T20 to T20) at a height z = 40 cm, which is significantly lower than the minimum flame height for a pool diameter d = 10 cm. The signal sampling frequency was 100 Hz. The inlet air velocities were measured using Hot-wire anemometers (HWA, Kanomax Inc.) which were positioned along the central axis of an inlet at seven different heights (0.15–3.15 m, with increments of 0.50 m). The HWA probes were set to record at a sampling frequency of 10 Hz. The tangential and vertical velocities were measured at several heights using calibrated temperature-compensated pitot tubes (Series 160S ‘S’ type, Dwyer Inst. Co.) and Type K thermocouples (bead diameter: 1 mm) arranged in the radial direction (0–25 cm, with increments of 5 cm). The circulation was calculated by multiplying the tangential velocity by the corresponding radius. Two video cameras were used to monitor the flame radius and phenomena of flame instability in horizontal and oblique views. The large test hall remained closed and the smoke exhaust system was turned off to reduce turbulence during tests. All the experimental data is summarized in Table 1. 3. Results and discussion 3.1. Phenomenon observations When the liquid fuel pools were ignited, at first the flame behaved like general buoyant pool fires
(Fig. 2a). Several seconds later, the entrained air from the four inlets arrived at the flame to induce the interaction with the combustion. The entire buoyant flame tilted and revolved around the geometrical central axis of the facility, without selfrotation (Fig. 2b and c). The angle between the flame and the level plane was decreasing and the angular speed of the flame revolution was increasing. Several loops later, the flame became nearly horizontal. Then the flame tip turned into vertical rapidly and started to spin around the flame axis. This self-rotation shortly extended downward to the fuel surface (Fig. 2d), which was consistent with observations of the formation of a concentrated vortex [14]. Finally, the entire flame became vertical with rapid rotation, which suggested the formation of the fire whirl. At the same time, besides the self-rotation, the entire flame continued to revolve around the geometrical central axis of the fuel pan, and the swirling flame axis was parallel to the geometrical central axis. This suggested that the flame or the vortex core was precessing around the central axis. The swirling flame was then observed to quickly approach the geometrical centerline with increasing precession frequency and decreasing precession radius. The flame precession then reached a quasi-steady state with a smaller precession radius (Fig. 2e). Here the precession radius (Rp) was defined as the distance between the flame axis and the geometrical central axis of the fuel pan. At any cross section, the preces-
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Fig. 2. Flame images of transition from a general pool fire to a fire whirl (d = 50 cm).
sion trajectory of the flame center was almost circular which was centered nearly in the geometrical center. The transition durations from general buoyant pool fires to fire whirls, which depends on the pan size, ranged from 800 s to 50 s for d = 10–55 cm. We also note that the directions of flame revolution and precession are the same as the rotation direction of the flame self-rotation (in clockwise direction in the present facility). 3.2. Radial temperature data In our previous work [8], the fire whirl was proved to be a highly stable combustion phenomenon in the vortex frame of reference. By crosscorrelation analysis, we found that the instantaneous centerline temperature signals at different heights had no phase delay and that the radial signals showed synchronous fluctuations. Therefore, the temperature fluctuations were mainly induced by flame precession rather than turbulent fluctuations. Typical time-averaged radial temperatures, reported previously in [8], involved small gradients in the regions far from the flame and within the flame core, while near the flame boundary
the temperature gradient was very large. Therefore, the fluctuations in the instantaneous temperature data by flame precession, in the radial direction, differ greatly at different measurement points. As shown in Fig. 3, the trajectory of the flame center is roughly assumed to be circular with a precession radius of Rp. O0 and O00 are the flame centers at t = t0 and t = t00 in a half cycle. R and Rf are the pan radius and flame radius respectively. The temperature fluctuations at point A and E should be relatively small as a result of always being located far from the flame and in the flame core respectively. However, alternately point C (or D) is periodically inside and outside the flame zone which induces large fluctuations in the instantaneous temperature data. Point B is located outside the flame outer boundary at t = t00 but close to it at t = t0 , where significant fluctuations are also observed although the maximum value is smaller than the flame temperature. The time interval between two adjacent temperature peaks represents one cycle of the flame precession. The frequencies of flame precession can then be extracted from the instantaneous temperature data near the outer flame boundary at point C (D, B). Similarly, the frequencies of flame revolution can also be obtained by the regular
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The inlet velocities in the vertical direction are nearly constant within the continuous flame for a specific fuel pan. Moreover, we found that the ambient circulations around the fire whirls do not change significantly with vertical heights. As shown in Fig. 5, for all fuel pan sizes, the average ambient circulation (C) and the average inlet centerline velocity ðV in cs Þ were fitted well by a linear correlation C ¼ 4:23V in cs , here the subscript “in” denotes inlet, “c” denotes centerline and “s” denotes the quasi-steady state. In the present facility, the average circulation at the inlet is Cin ¼ 2pV in , where V in is the average velocity across the inlet. Thus we have V in ¼ 0:67V in cs , which is a reasonable result due to the non-uniform velocity profile across the inlet.
Fig. 3. Schematic for flame precession and temperature measurements in the radial direction.
3.4. Characteristics of flame revolution and precession
As shown in Fig. 4, the variation of inlet velocity is consistent with the combustion behavior. After ignition, entrainment started and the inlet velocity began to increase. Later, as the entire flame tilted and revolved around the geometrical axis, the inlet velocities at different heights also fluctuated in phase. Large regular inlet velocity fluctuations disappeared whenever the swirling flame moved close to the centerline after the formation of the fire whirl. The inlet velocity increases due to the increases of burning rates and wall temperatures until reaching the quasisteady state. This is especially the case for fuel pans with large heat release rates. As the pool size increases, the inlet velocity also increases.
3.4.1. Flame revolution in the initial stage In the initial stage, the entire buoyant flames tilted and revolved around the geometrical axis without flame self-rotation. The instantaneous temperatures at two symmetrical points (T18, T18, r = ±27 cm) and the centerline inlet velocity (z = 0.15 m) are shown in Fig. 6 for d = 15 cm. We can see that the inlet velocity involves significant fluctuations by the effect of flame revolution. Any time interval between two peaks of T18 (T18) is divided equally by the inner peak of T18 (T18), which suggests a regular flame revolution. Also the time intervals between the adjacent peaks of the temperature are close to those of inlet velocity. Thus, the complex interaction between the buoyant pool fire and the entrained air flow produces the periodical fluctuations of inlet velocity and pressure fields, which in turn inclines and drives the flame to revolve around the geometrical axis of the facility. Moreover, the cycle period decreases (i.e., revolution frequency increases)
Fig. 4. Instantaneous data of centerline inlet velocity (d = 15 cm).
Fig. 5. Average ambient circulations versus average inlet centerline velocity in the quasi-steady state with linear fitting indicated.
temperature fluctuations at specific measurement points. This will be discussed later in detail. 3.3. Inlet velocity and ambient circulations
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until a critical value (e.g. the last three cycles in Fig. 7) which is favorable for the formation of fire whirl. This critical frequency for flame revolution should be a characteristic for specific fuel pan and facility (Table 1). In Fig. 8, it is found that the critical frequencies of flame revolution increase linearly with the average inlet centerline velocity for different pool sizes, and the data are correlated well by fri ¼ 0:21V in ci . For the subscript “ri” used here, “r” denotes flame revolution and “i” denotes the initial stage. Here, a Strouhal number is defined by St ¼ fri D=V in
ci
ð1Þ
similar to that used in cyclones [15,16], where D is the horizontal dimension of the facility. Thus St is a constant of 0.42 for the flame revolution in the present facility. 3.4.2. Flame precession in the quasi-steady state When the vortex formed at the inclined flame tip extends downward to the fuel surface, an erect fire whirl is finally established and the swirling flame (vortex core) then starts to precess around the geometrical axis of symmetry. As discussed in Section 3.2, this swirling flame precession can produce fluctuations in the instantaneous temperature data. The typical data at the different points are shown in Fig. 9 for d = 40 cm. Consistent to the above discussion, the temperatures at the flame center (T0) and far from the flame outer boundary (T13) are relatively steady. However, the temperatures adjacent to the flame (T7, T8, T7, T8) fluctuate largely and the signals at symmetry points are generally opposite in phase. This clearly confirms flame precession in an experimental sense. In Fig. 9, the time intervals between two adjacent peaks (troughs) of T7 or T8 (T7 or T8) are 2.42 s, 2.23 s, 1.64 s, 1.84 s, 2.89 s, 2.78 s, 1.81 s respectively, which do not differ signifi-
Fig. 6. Instantaneous temperatures of T18, T18 (r = ±27.0 cm) and the centerline inlet velocity at z = 0.15 m before fire whirl formation (d = 15 cm).
Fig. 7. Time intervals between adjacent peaks of T20, T20 and inlet velocity (z = 0.15 m) during flame revolution before the formation of fire whirl (d = 15 cm).
Fig. 8. Frequencies of flame revolution in the initial stage and flame precession in the quasi-steady state versus the average inlet velocity with linear fitting indicated.
cantly and show a relatively regular precession. The precession frequency is then determined to be 0.48 Hz by averaging the frequencies of these cycles. For each pool size, the precession frequency was found to remain relatively stable during one test. In the present study, we are interested in the swirling flame precession at the quasi-steady state with stable inlet velocity and burning rate (Table 1). In Fig. 8, the precession frequencies at the quasi-steady state are also plotted versus the inlet velocities for all fuel pan sizes. The frequencies of swirling flame precession are about 2–3 times larger than that of flame revolution in the initial stage for the same fuel pan size. Similarly, the data can be correlated by a linear function fps ¼ 0:40V in cs well and thus fps ¼ 0:095C, which has the same form as the
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is roughly balanced by the radial pressure gradient due to @p=@r ¼ qw2 =r
Fig. 9. Instantaneous temperatures of four symmetry points at r = ±10.5 cm (T7, T7) and r = ±12.0 cm (T8, T8). The temperature at r = 0 (T0) and r = 19.5 cm (T13) are also shown (z = 40 cm, d = 40 cm, Rf = 13.6 cm).
correlation fp ¼ C=ð4p2 R02 Þ ¼ 0:013C proposed by Murakami [17], Belousov and Gupta [15] and Alekseenko et al. [18] for cold swirling flows in a tube or chamber, where R0 is the radial distance from the inlet to the center of the facility (R0 = 1.4 m for the present facility). The large increase of the coefficient may be due to the combustion and configurations. For the subscript “ps” used here, “p” denotes flame precession and “s” denotes the quasi-steady state. The corresponding Strouhal number is about 0.80. The linear correlation is not consistent with the cube polynomial function suggested by Su et al. [13], which may be due to that the inlet velocity was specified artificially in their simulation. Furthermore, the flame radius and precession radius are obtained by image processing of sequential flame images. The ratio of the precession radius and flame radius in the quasi-steady state are around 0.30 for all fuel pans. Therefore, a relatively small precession range is found in the fixed-frame facility as compared to that in the rotating-screen facility [4]. 3.5. Mechanics of flame precession in fire whirls 3.5.1. Differences of precession in jet swirling flames and fire whirls All elements in the rotating-screen and fixedframe facilities are symmetrical. However, the flame instability exists all the time. In literature, the three dimensional (3-D) precessing vortex core (PVC) also occurs in strong jet swirling flames and has been studied extensively in the last 30 years [19]. Similarly to flame precession in fire whirls, it was established that the PVC is not a consequence of significant violations of the symmetry of facility. In a swirling jet, the centrifugal force
ð2Þ
where p and w are the pressure and tangential velocity respectively. As the swirling jet is ejected from the nozzle, an adverse axial pressure gradient is formed near the jet axis due to the quick expansion of swirling flow with decreasing tangential velocity. Providing the swirl strength is high enough, a critical point is reached when this adverse axial pressure gradient exceeds the kinetic energy of fluid in the axial direction and hence a reverse flow is induced on the axis between two stagnation points. That is, the vortex breakdown occurs and the central recirculation zone (CRZ) is not stable. The central vortex core is then displaced from and starts to precess around the axis of symmetry at a well-defined frequency. Therefore, in strong swirling jet flames the PVC is usually linked to vortex breakdown phenomena and the accompanying CRZ. It was found that the PVC frequency has magnitudes of 10–1000 Hz. When vortex breakdown occurs, the flame height decreases due to increased turbulent mixing and reaction rate. However, in fire whirls, the precession frequency is lower than 1 Hz in the present experiments, which is much lower than that of jet swirling flames. Moreover, flame stretching is always observed in fire whirls. Therefore, qualitatively different effects of rotation on combustion dynamics are presented. 3.5.2. Flow characteristics of fire whirls with flame precession As shown in Fig. 10, in the quasi-steady state, the centerline axial velocity of fire whirls is positive and increases with height in the continuous flame region, which indicates that CRZ is not formed and no vortex breakdown occurs. Additionally, the axial velocities versus radius at several heights are shown in Fig. 11. Apparently, the axial flux is concentrated inside the flame. However, the axial velocity decreases with radius and even becomes small negative values from r = 20 cm at z = 0.91 m outside the flame (Rf = 15.7 cm, d = 50 cm) where an annular external recirculation zone (ERZ) is located. As soon as the fire whirl is generated and the precession starts, the axial velocity at z = 0.91 m rapidly becomes negative and remains relatively stable while decreases slightly due to increase of burning rate and inlet velocity, which indicates a stable CRZ during the flame precession. This weak reverse flow was also observed in a vortex flow produced in a rotating cylinder filled with water [20] and in a tornado-like vortex produced in a rotating-screen facility [21]. The reverse flow is also related to the adverse axial pressure gradient and can extend to the outside region next
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Fig. 10. Centerline axial velocities versus axial distance (d = 50 cm).
Fig. 12. Axial distributions of circulation outside the flame (Rf = 15.7 cm, d = 50 cm).
flame precession of fire whirls is linked to the occurrence of ERZ. A mechanics may be as follows: due to the formation of ERZ, a strong shear layer will be created on the boundary between the core and the outer reverse flow. This shear layer is prone to the Kelvin–Helmholtz instability, which may generate asymmetry in the flow field. Then the 3-D flame precession comes to being. Further experimental and theoretical investigations are demanded on this topic in the future. 4. Conclusions
Fig. 11. Radial distributions of axial velocities at different heights (d = 50 cm).
to the boundary layer if the rotation is sufficiently strong [21]. As shown in Fig. 12, the circulation along the axial direction is not a constant exactly and is a minimum at z = 1.2 m at r = 20 cm and 25 cm for d = 50 cm. The resultant axial pressure drop is small and cannot induce the CRZ in the vortex core with high axial velocity. However, this pressure drop can exceed the small kinetic energy of fluid outside the vortex core, which gives rise to the ERZ. Ying and Chang [21] also observed the vortex wander accompanied by the ERZ in the rotating-screen facility similar to that used by Emmons and Ying for fire whirls [4]. As we know, fire whirls and tornado-like vortexes are similar in the sense of fluid mechanics because they are classified into the concentrated vortex [22]. Moreover, low frequency precession is also observed in a confined jet with very weak rotation accompanied by ERZ without the occurrence of CRZ. The asymmetry induced by ERZ causes the flow to precess about the axis of the device [19,23]. Therefore, there is a strong possibility that the swirling
The 3-D flame instabilities in fire whirls including the flame revolution and flame precession were distinguished and investigated by experimental means in a medium-scale fixed-frame facility. The major results are summarized as follows: 1. Before the formation of fire whirl, the flame revolves around the geometrical axis of symmetry with increasing angular velocity until the critical revolution frequency required for the formation of fire whirls is reached. 2. As soon as the fire whirl forms and swirling flame precession starts, an annular ERZ is found adjacent to the flame outer boundary, while CRZ and the vortex breakdown are not present. 3. The inlet velocity fluctuates synchronically with the flame revolution in the initial stage, which disappears when precession starts. The inlet velocities are nearly constant within the continuous flame in order to maintain a stable generating eddy. The average ambient circulations and the average inlet velocities can be fitted well linearly for all pool sizes in the quasi-steady state. 4. The precession frequency is relatively stable during one test. The frequencies of flame revolution and precession are both linearly
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dependent on the average inlet velocity. The corresponding Strouhal numbers are 0.42 and 0.80 respectively. 5. The directions of flame revolution and precession are always the same as the direction of self-rotation of the fire whirl flame. The flame revolution is caused by the periodical fluctuations of the inlet flow, while the flame precession is considered to be linked to the occurrence of ERZ in fire whirls.
Acknowledgements This work was sponsored by the National Natural Science Foundation of China under Grant 51120165001 and 51076148, and National Key Technology R&D Program under Grant 2011BAK07B01-02. Naian Liu was supported by “the Fundamental Research Funds for the Central Universities (No. WK2320000014).
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