Experimental Research on Flame Revolution and Precession of Fire

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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

Available online 13 July 2012

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 erty and life [1,2]. As indicated, the formation of a fire whirl depends on three essential conditions – a Fire whirl is a special swirling diffusion flame generating eddy, a fluid sink within the eddy, and that may occur in both urban and wildland areas some friction or drag offered to the movement of with possibility of causing great damage to prop- air in the ground boundary of the eddy [3].Ina fire whirl, the hot gas generated by the fire itself ⇑ Corresponding author. Fax: +86 551 3601669. serves as the fluid sink which entrains the ambient E-mail address: [email protected] (N. Liu). 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 2608 J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615 eddy to the flame (vortex core). As summarized by frequencies of flame revolution and flame preces- Byram and Martin [3], the different types of facil- sion were extracted from data of periodic temper- ities which can be used to produce single steady ature fluctuations at specific points, and the flow fire whirls in laboratory are based on the concepts characteristics were obtained by comprehensive of a generating eddy and a fluid sink. The facilities velocity measurements. The relationships between can be generally classified into two types, depend- the instability frequencies and inlet velocity were ing on whether the generating eddy is imposed revealed. The differences of precession in fire mechanically by a spinning screen (Emmons type whirls and ordinary swirling jet flames were exam- [4,5]) or induced by the entrained air flowing ined extensively. through well-arranged spiral paths (fixed-frame type [6–9], fire-wall type [10]). Some quantitative experimental researches have been performed that 2. Experimental focused on burning rates, flame plume temperature, velocity distributions, and radiation heat flux The fixed-frame fire whirl facility, located in a of fire whirls [2,4,7,8,11]. large test hall, was a square enclosure made of As usually observed in fire whirl experiments, a tempered glass, with a base dimension of distinct difference between fire whirls and general 2m 2 m and a height of 15 m (Fig. 1). The pool fires is the presence of an obvious flame wan- channel was open at the top. Each channel wall der around the geometrical centerline in experi- had a uniform 20 cm wide vertical gap at its cor- mental fire whirls, which can induce large ner to ensure that the entrained air induced by fluctuations of data, leading to great difficulties the burning flame could enter the channel, thus for the subsequent data analysis [4,8]. Emmons imparting the rotational flow necessary for fire and Ying [4] reported that the flame wander whirl formation. The base table (2 m 2 m) was involved amplitude of several flame diameters made of pine wood with a round hole (60 cm in with a lower frequency when compared to the diameter and 10 cm in depth) in the center. The large turbulent eddies. They attributed this wan- steel circular fuel pans (with diameters d of 10– der to some inherent flame instability since the 55 cm with increments of 5 cm, and a depth of facility was symmetrical and careful precautions 10 cm) were positioned in the center of the facility were taken for turbulence suppression. and placed such that their rims were flush with the Flame instability was also found by numerical top of the base table. The initial height of the simulations of fire whirls. Snegirev et al. [12] liquid fuel (n-heptane, 97%) surface was 7 cm. A reported the periodic flame wander, but without water layer was used in the larger fuel pans to further discussion. Recently Su et al. [13], using maintain the initial fuel surface height constant Fire Dynamics Simulation (FDS) for simulating and also ensure safety. The lip height has proven fire whirls in a ship engine room, found that the to have little effect on the burning rate of fire calculated angular velocity of flame wander fit whirls [8]. The fuel mass versus time was recorded well with a cube polynomial function of the inlet using an electronic balance with a precision of velocity along the four walls and increased rapidly 0.1 g. when the inlet velocity was over a critical value The temperatures were measured using Type K (6.58 m/s). However, no experimental data was (chromel–alumel) thermocouples (bead diameter: presented to compare to the numerical results. It 1 mm). They were arranged in the radial direction is doubtful that using FDS without code modifica- in intervals of 1.5 cm from r = 30 to 30 cm tion would accurately represent the complex interaction of combustion and strong rotation of a fire Table 1 whirl. The experimental data in the present work. In appearance, the flame wander in a single a d V in ci fri V in cs fps C Rfs Rps Q quasi-steady fire whirl is characterized by its self (cm) (m/s) (Hz) (m/s) (Hz) (m2/s) (cm) (cm) (kW) rotation and the simultaneous rotation of the entire flame body around a certain geometrical 10 0.31 0.07 0.28 0.14 1.49 4.0 1.5 22.3 15 0.52 0.11 0.52 0.25 2.00 5.6 2.4 62.4 axis (generally the central axis of the pool). There- 20 0.58 0.12 0.67 0.28 2.67 7.6 2.7 102.6 fore, the flame wander should be more exactly 25 0.61 0.13 0.78 0.32 3.21 8.9 2.8 156.1 termed flame precession. Before the formation of 30 0.71 0.16 0.87 0.41 3.78 10.7 3.4 240.8 the steady fire whirl, the tilted flame would revolve 35 0.77 0.17 1.08 0.46 4.60 12.2 3.7 343.4 around the geometrical central axis, but without 40 0.92 0.20 1.19 0.48 5.08 13.6 3.6 441.5 self rotation. We call this phenomenon of flame 45 0.89 0.20 1.35 0.51 5.83 15.0 4.0 566.4 instability as flame revolution, so as to distinguish 50 1.03 0.21 1.48 0.55 6.31 15.7 4.6 669.0 it from flame precession. 55 1.13 0.24 1.64 0.62 6.81 16.9 5.1 789.4 This paper presents the first effort to a The heat release rates Q were calculated by assum- quantitatively explore the characteristics of flame ing unity of combustion efficiency in the quasi-steady instability in fire whirls. We utilized a four- state, and the effective heat of combustion for heptane is wall fixed-frame facility for experiments. The 44.6 kJ/g. J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615 2609

(cm) -30 -20 -10 0 10 20 30 2.0m 400 4.0m Pitot tubes for tangential Pitot tubes for axial 20cm HWA velocity measurement velocity measurement 350 3.15m 300 3.0m 3.0m 2.65m 250 2.6m Liquid 2.15m Fuel pool 200 1.9m 2.0m d 2.0m 1.65m 150 1.2m 1.15m Tempered glasses wall Tempered glasses 100 0.91m T T T Entrained air -20 0 20 0.65m Corner gap 50 0.5m HWA for inlet velocity measurement 0.17m 0.15m d Raidial thermocouples Liquid Fuel, =10,15,20,...,55cm (Spacing of 1.5cm,T ~T ) -20 20 Wood base

Electronic balance

Fig. 1. Schematic for ﬁre whirl experiments.

(T20 to T20) at a height z = 40 cm, which is sig- (Fig. 2a). Several seconds later, the entrained air nificantly lower than the minimum flame height from the four inlets arrived at the flame to induce for a pool diameter d = 10 cm. The signal sam- the interaction with the combustion. The entire pling frequency was 100 Hz. The inlet air veloci- buoyant flame tilted and revolved around the geo- ties were measured using Hot-wire anemometers metrical central axis of the facility, without self- (HWA, Kanomax Inc.) which were positioned rotation (Fig. 2b and c). The angle between the along the central axis of an inlet at seven different flame and the level plane was decreasing and the heights (0.15–3.15 m, with increments of 0.50 m). angular speed of the flame revolution was increas- The HWA probes were set to record at a sampling ing. Several loops later, the flame became nearly frequency of 10 Hz. The tangential and vertical horizontal. Then the flame tip turned into vertical velocities were measured at several heights using rapidly and started to spin around the flame axis. calibrated temperature-compensated pitot tubes This self-rotation shortly extended downward to (Series 160S ‘S’ type, Dwyer Inst. Co.) and Type the fuel surface (Fig. 2d), which was consistent K thermocouples (bead diameter: 1 mm) arranged with observations of the formation of a concen- in the radial direction (0–25 cm, with increments trated vortex [14]. Finally, the entire flame became of 5 cm). The circulation was calculated by multi- vertical with rapid rotation, which suggested the plying the tangential velocity by the correspond- formation of the fire whirl. ing radius. Two video cameras were used to At the same time, besides the self-rotation, the monitor the flame radius and phenomena of flame entire flame continued to revolve around the geo- instability in horizontal and oblique views. The metrical central axis of the fuel pan, and the large test hall remained closed and the smoke swirling flame axis was parallel to the geometri- exhaust system was turned off to reduce turbu- cal central axis. This suggested that the flame lence during tests. All the experimental data is or the vortex core was precessing around the cen- summarized in Table 1. tral axis. The swirling flame was then observed to quickly approach the geometrical centerline with increasing precession frequency and decreasing 3. Results and discussion precession radius. The flame precession then reached a quasi-steady state with a smaller pre- 3.1. Phenomenon observations cession radius (Fig. 2e). Here the precession radius (Rp) was defined as the distance between When the liquid fuel pools were ignited, at first the flame axis and the geometrical central axis the flame behaved like general buoyant pool fires of the fuel pan. At any cross section, the preces- 2610 J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615

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 the temperature gradient was very large. There- circular which was centered nearly in the geomet- fore, the fluctuations in the instantaneous temper- rical center. The transition durations from ature data by flame precession, in the radial general buoyant pool fires to fire whirls, which direction, differ greatly at different measurement depends on the pan size, ranged from 800 s to points. As shown in Fig. 3, the trajectory of the 50 s for d = 10–55 cm. We also note that the flame center is roughly assumed to be circular 0 00 directions of flame revolution and precession with a precession radius of Rp. O and O are are the same as the rotation direction of the the flame centers at t = t0 and t = t00 in a half cycle. flame self-rotation (in clockwise direction in the R and Rf are the pan radius and flame radius present facility). respectively. The temperature fluctuations at point A and E should be relatively small as a result of 3.2. Radial temperature data always being located far from the flame and in the flame core respectively. However, alternately In our previous work [8], the fire whirl was point C (or D) is periodically inside and outside proved to be a highly stable combustion phenom- the flame zone which induces large fluctuations enon in the vortex frame of reference. By cross- in the instantaneous temperature data. Point B correlation analysis, we found that the instanta- is located outside the flame outer boundary at neous centerline temperature signals at different t = t00 but close to it at t = t0, where significant heights had no phase delay and that the radial sig- fluctuations are also observed although the maxi- nals showed synchronous fluctuations. Therefore, mum value is smaller than the flame temperature. the temperature fluctuations were mainly induced The time interval between two adjacent tempera- by flame precession rather than turbulent ture peaks represents one cycle of the flame pre- fluctuations. cession. The frequencies of flame precession can Typical time-averaged radial temperatures, then be extracted from the instantaneous temper- reported previously in [8], involved small gradi- ature data near the outer flame boundary at point ents in the regions far from the flame and within C (D, B). Similarly, the frequencies of flame the flame core, while near the flame boundary revolution can also be obtained by the regular J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615 2611

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 csÞ 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.

3.4. Characteristics of flame revolution and Fig. 3. Schematic for flame precession and temperature precession measurements in the radial direction. 3.4.1. Flame revolution in the initial stage temperature fluctuations at specific measurement In the initial stage, the entire buoyant flames points. This will be discussed later in detail. tilted and revolved around the geometrical axis without flame self-rotation. The instantaneous 3.3. Inlet velocity and ambient circulations temperatures at two symmetrical points (T18, T18, r = ±27 cm) and the centerline inlet velocity As shown in Fig. 4, the variation of inlet veloc- (z = 0.15 m) are shown in Fig. 6 for d = 15 cm. ity is consistent with the combustion behavior. We can see that the inlet velocity involves signifi- After ignition, entrainment started and the inlet cant fluctuations by the effect of flame revolution. velocity began to increase. Later, as the entire Any time interval between two peaks of T18 (T18) flame tilted and revolved around the geometrical is divided equally by the inner peak of T18 (T18), axis, the inlet velocities at different heights also which suggests a regular flame revolution. Also fluctuated in phase. Large regular inlet velocity the time intervals between the adjacent peaks of fluctuations disappeared whenever the swirling the temperature are close to those of inlet velocity. flame moved close to the centerline after the for- Thus, the complex interaction between the buoy- mation of the fire whirl. The inlet velocity ant pool fire and the entrained air flow produces increases due to the increases of burning rates the periodical fluctuations of inlet velocity and and wall temperatures until reaching the quasi- pressure fields, which in turn inclines and drives steady state. This is especially the case for fuel the flame to revolve around the geometrical axis pans with large heat release rates. As the pool size of the facility. Moreover, the cycle period increases, the inlet velocity also increases. decreases (i.e., revolution frequency increases)

Fig. 5. Average ambient circulations versus average Fig. 4. Instantaneous data of centerline inlet velocity inlet centerline velocity in the quasi-steady state with (d = 15 cm). linear fitting indicated. 2612 J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615 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 friD=V in ci ð1Þ similar to that used in cyclones [15,16], where D is Fig. 7. Time intervals between adjacent peaks of T20, the horizontal dimension of the facility. Thus St is T20 and inlet velocity (z = 0.15 m) during flame revo- a constant of 0.42 for the flame revolution in the lution before the formation of fire whirl (d = 15 cm). 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 experimen- Fig. 8. Frequencies of flame revolution in the initial tal sense. In Fig. 9, the time intervals between two stage and flame precession in the quasi-steady state adjacent peaks (troughs) of T7 or T8 (T7 or T8) versus the average inlet velocity with linear fitting are 2.42 s, 2.23 s, 1.64 s, 1.84 s, 2.89 s, 2.78 s, indicated. 1.81 s respectively, which do not differ signifi-

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 Fig. 6. Instantaneous temperatures of T18, T18 (r = ±27.0 cm) and the centerline inlet velocity at function fps ¼ 0:40V in cs well and thus fps ¼ z = 0.15 m before fire whirl formation (d = 15 cm). 0:095C, which has the same form as the J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615 2613

is roughly balanced by the radial pressure gradient due to

@p=@r ¼ qw2=r 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 Fig. 9. Instantaneous temperatures of four symmetry reverse flow is induced on the axis between two points at r = ±10.5 cm (T7, T7) and r = ±12.0 cm (T8, stagnation points. That is, the vortex breakdown T8). The temperature at r =0 (T0) and r = 19.5 cm occurs and the central recirculation zone (CRZ) (T13) are also shown (z = 40 cm, d = 40 cm, is not stable. The central vortex core is then dis- Rf = 13.6 cm). placed from and starts to precess around the axis of symmetry at a well-defined frequency. There- fore, in strong swirling jet flames the PVC is usually linked to vortex breakdown phenomena and 2 02 the accompanying CRZ. It was found that the correlation fp ¼ C=ð4p R Þ¼0:013C proposed by Murakami [17], Belousov and Gupta [15] PVC frequency has magnitudes of 10–1000 Hz. and Alekseenko et al. [18] for cold swirling flows When vortex breakdown occurs, the flame height in a tube or chamber, where R0 is the radial dis- decreases due to increased turbulent mixing and tance from the inlet to the center of the facility reaction rate. However, in fire whirls, the preces- (R0 = 1.4 m for the present facility). The large sion frequency is lower than 1 Hz in the present increase of the coefficient may be due to the experiments, which is much lower than that of combustion and configurations. For the sub- jet swirling flames. Moreover, flame stretching is script “ps” used here, “p” denotes flame preces- always observed in fire whirls. Therefore, qualita- sion and “s” denotes the quasi-steady state. tively different effects of rotation on combustion The corresponding Strouhal number is about dynamics are presented. 0.80. The linear correlation is not consistent with the cube polynomial function suggested by Su 3.5.2. Flow characteristics of fire whirls with flame et al. [13], which may be due to that the precession inlet velocity was specified artificially in their As shown in Fig. 10, in the quasi-steady state, simulation. the centerline axial velocity of fire whirls is posi- Furthermore, the flame radius and precession tive and increases with height in the continuous radius are obtained by image processing of flame region, which indicates that CRZ is not sequential flame images. The ratio of the preces- formed and no vortex breakdown occurs. Addi- sion radius and flame radius in the quasi-steady tionally, the axial velocities versus radius at sev- state are around 0.30 for all fuel pans. Therefore, eral heights are shown in Fig. 11. Apparently, a relatively small precession range is found in the the axial flux is concentrated inside the flame. fixed-frame facility as compared to that in the However, the axial velocity decreases with radius rotating-screen facility [4]. and even becomes small negative values from r =20cm at z = 0.91 m outside the flame 3.5. Mechanics of flame precession in fire whirls (Rf = 15.7 cm, d = 50 cm) where an annular external recirculation zone (ERZ) is located. As soon 3.5.1. Differences of precession in jet swirling as the fire whirl is generated and the precession flames and fire whirls starts, the axial velocity at z = 0.91 m rapidly All elements in the rotating-screen and fixed- becomes negative and remains relatively stable frame facilities are symmetrical. However, the while decreases slightly due to increase of burning flame instability exists all the time. In literature, rate and inlet velocity, which indicates a stable the three dimensional (3-D) precessing vortex core CRZ during the flame precession. (PVC) also occurs in strong jet swirling flames and This weak reverse flow was also observed in a has been studied extensively in the last 30 years vortex flow produced in a rotating cylinder filled [19]. Similarly to flame precession in fire whirls, with water [20] and in a tornado-like vortex pro- it was established that the PVC is not a conse- duced in a rotating-screen facility [21]. The reverse quence of significant violations of the symmetry flow is also related to the adverse axial pressure of facility. In a swirling jet, the centrifugal force gradient and can extend to the outside region next 2614 J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615

Fig. 10. Centerline axial velocities versus axial distance Fig. 12. Axial distributions of circulation outside the (d = 50 cm). 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

The 3-D flame instabilities in fire whirls includ- Fig. 11. Radial distributions of axial velocities at ing the flame revolution and flame precession were different heights (d = 50 cm). distinguished and investigated by experimental means in a medium-scale fixed-frame facility. The major results are summarized as follows: to the boundary layer if the rotation is sufficiently strong [21]. As shown in Fig. 12, the circulation 1. Before the formation of fire whirl, the flame along the axial direction is not a constant exactly revolves around the geometrical axis of sym- and is a minimum at z = 1.2 m at r = 20 cm and metry with increasing angular velocity until 25 cm for d = 50 cm. The resultant axial pressure the critical revolution frequency required for drop is small and cannot induce the CRZ in the the formation of fire whirls is reached. vortex core with high axial velocity. However, this 2. As soon as the fire whirl forms and swirling pressure drop can exceed the small kinetic energy flame precession starts, an annular ERZ is of fluid outside the vortex core, which gives rise to found adjacent to the flame outer boundary, the ERZ. while CRZ and the vortex breakdown are not Ying and Chang [21] also observed the vortex present. wander accompanied by the ERZ in the rotat- 3. The inlet velocity fluctuates synchronically ing-screen facility similar to that used by Emmons with the flame revolution in the initial stage, and Ying for fire whirls [4]. As we know, fire which disappears when precession starts. The whirls and tornado-like vortexes are similar in inlet velocities are nearly constant within the the sense of fluid mechanics because they are clas- continuous flame in order to maintain a stable sified into the concentrated vortex [22]. Moreover, generating eddy. The average ambient circula- low frequency precession is also observed in a tions and the average inlet velocities can be confined jet with very weak rotation accompanied fitted well linearly for all pool sizes in the by ERZ without the occurrence of CRZ. The quasi-steady state. asymmetry induced by ERZ causes the flow to 4. The precession frequency is relatively stable precess about the axis of the device [19,23]. There- during one test. The frequencies of flame fore, there is a strong possibility that the swirling revolution and precession are both linearly J. Lei et al. / Proceedings of the Combustion Institute 34 (2013) 2607–2615 2615

dependent on the average inlet velocity. The [6] K. Saito, C.J. Cremers, Proc. Joint ASME/JSME corresponding Strouhal numbers are 0.42 and Fluids Eng. Conf. (1995) 220. 0.80 respectively. [7] K. Satoh, K.T. Yang, ASME Heat Trans. Div. 335 5. The directions of flame revolution and preces- (1996) 393–400. sion are always the same as the direction of [8] J. Lei, N.A. Liu, L.H. Zhang, et al., Proc. Combust. Inst. 33 (2011) 2407–2415. self-rotation of the fire whirl flame. The flame [9] K. Kuwana, S. Morishita, R. Dobashi, K.H. revolution is caused by the periodical fluctua- Chuah, K. Saito, Proc. Combust. Inst. 33 (2011) tions of the inlet flow, while the flame preces- 2425–2432. sion is considered to be linked to the [10] R. Zhou, Z.N. Wu, J. Fluid Mech. 583 (2007) 313– occurrence of ERZ in fire whirls. 345. [11] K.B. Zhou, N.A. Liu, K. Satoh, in: Proc. 10th Int. Symp. Fire Saf. Sci., Maryland, USA, 2011. [12] A.Y. Snegirev, J.A. Marsden, J. Francis, G.M. Acknowledgements Makhviladze, Int. J. Heat Mass Transfer 47 (2004) 2523–2539. This work was sponsored by the National [13] S.C. Su, L. Wang, Y.H. Nie, X. Gu, Saf. Sci. 50 (1) Natural Science Foundation of China under (2012) 12–18. Grant 51120165001 and 51076148, and National [14] L.M. Leslie, J. Fluid Mech. 48 (1) (1971) 1–21. Key Technology R&D Program under Grant [15] A.K. Gupta, D.G. Lilley, N. Syred, Swirl Flows, 2011BAK07B01-02. Naian Liu was supported by Abacus, Turnbridge Wells, 1984. “the Fundamental Research Funds for the [16] A. Hoekstra, J. Derksen, H. Van Den Akker, Chem. Eng. Sci. Central Universities (No. WK2320000014). 54 (1999) 2055–2065. [17] M. Murakami, J. Eng. Power Trans. ASME 83 (1961) 36–42. [18] S.V. Alekseenko, P.A. Kuibin, V.L. Okulov, Theory References of Concentrated Vortices: An Introduction, Springer, Berlin Heidelberg, 2007. [1] K. Kuwana, K. Sekimoto, K. Saito, F.A. Williams, [19] N. Syred, Prog. Energy Combust. Sci. 32 (2) (2006) Fire Saf. J. 43 (4) (2008) 252–257. 93–161. [2] S. Soma, K. Saito, Combust. Flame 86 (3) (1991) [20] J.S. Turner, D.K. Lilly, J. Atmos. Sci. 20 (5) (1963) 269–284. 468–471. [3] G.M. Byram, R.E. Martin, For. Sci. 16 (1970) 386– [21] S.J. Ying, C.C. Chang, J. Atmos. Sci. 27 (1) (1970) 399. 3–14. [4] H.W. Emmons, S.J. Ying, Proc. Combust. Inst. 11 [22] B.R. Morton, Fire Res. Abstr. Rev. 12 (1) (1970) 1– (1967) 475–488. 19. [5] K.H. Chuah, G. Kushida, Proc. Combust. Inst. 31 [23] G.J. Nathan, S.J. Hill, R.E. Luxton, J. Fluid Mech. (2007) 2599–2606. 370 (1998) 347–380.