Onset of internal fire whirl s in a room with ceiling vent s was studied numerically with Computational Fluid Dynamics . Two different ventilation arrangem ents were set up by varying the door height. Two fires of 0.12 MW and 0 .24 MW were considere d. Swirling air flow , change s in eddy size and whirling speed near to the fire source were simulated . CFD predictions indicate that air swirled up from the lower to the upper part of the room. Onset of an internal fire whirl depend s on the interaction between the indoor hot air and the outdoor cool air flows. Swirling motion result s when there is ventilation provision at the sidewa ll. Keywords : buildings, struct ures & design, fire engineering, thermal effects , numerical model 3 1. Intro duction Fire whirls in mass fires were observed several times including the 2007 forest fire in California , USA . As proposed by Emmons and Ying [1], flame whirling burns the fuels faster and more vigorously . There are also concerns [2] on inducing internal fire whirl s in atrium buildings storing large amount s of combustibles with natural smoke venting design . For a room with natural ventilation provided at the sidewalls, flame swirl ing would be different from fire whirls induced in bigger buildings or i n open atmosphere . Both experiment al and numerical studies on flame whirling due to natural ventilation were carried out by Satoh and Yang [3,4] . A kerosene pool fire of 0.12 m diame ter was burnt in a 0.7 m square shaf t model of height 1.8 m. The flame height was observed to stretch up to 1.7 m upon onsetting an internal fire whirl . The flame height was several times longer than the normal value while burning the pool fire in free space. Ventilation arrangement was identified to be a key factor. The a ir inlet and pool fire were placed symmetrically in that particular scenario . Stability of the fire whirl and the result ing swirlin g frequency depend on the symmetr y of ventilation out lets and change s in heat release rate. For rooms with natural ventilati on provisions such as a door opening , a ceiling vent or sidewall vents, an internal fire whirl can onset under some conditions [5]. A tall flame of much higher heat release rate s could be induced . It is necessary to study the hazard of such fire scenario s. Experimental studies on internal fire whirl s produced within two semi -circular glass shields were reported by Hassan et al. [6]. A shallow 5 cm propan ol pool fire was burnt at the centreline. Two gaps appeared on both sides with the two centr es of gla ss shields overlapp ed. Outside air 4 was entrained to burn more vigorously. A maximum air speed of 0.7 to 0.9 ms-1 was measured by particle image velocimetry . Lower air density of 0.4 to 0.7 kgm-3 was found at the upper part of the pool fire. The value w as much lower than 1.21 kgm-3 at the air inlet. O utside cold air was drawn in at 1.0 ms-1 due to t he pressure difference caused by the density difference . Experiments were carried ou t by Snegirev et al. [7] to investigat e the conditions for onsetting buoyant whirling f lame . A test room of size 2.77 m by 2.4 m with height 2.29 m was constructed of corrugated steel with a hardwood floor. A door of width 0.8 m and height 1.95 m was placed in the front wall at a distance 0.135 m away from the sidewall. A c eiling vent of size 0.8 by 0.9 m was located at the centre. A pool fire with commercial fuel gas of diameter 0.6 m and lip height 0.3 m was placed at the room centre. Room-scale whir ling flames were observed. The period between onset and destruction of the whirling core w as record ed. The reduction in burning time for a fixed amount of liquid fuel with fire whirls was also reported . Numerical simulations on inducing buoyant whirling flames in the experiments were carried out . Predicted results illustra ted that the rotational flow was induced by buoyant turbulent diffusion flame . Effects of the opening ventilation condition were discussed . Rotating flames were predict ed when the do orway was full y open. Flame swirling was not predicted when the incomin g airflow was deflected towards the axis . Burning rate for flame swirling would increase from 0.029 to 0.039 kgm-2s-1, compared with that from 0.017 to 0.027 kgm-2s-1 for non-whirling flames. Swirling of fire in the compartment was described in detail. Swirling patterns at different time s and correlation with heat release rates were derived . Numerical results indicate d that the Swirl number is proportional to the external circulatio n. 5 An internal fire whirl [5,8] can be set up readily by burnin g a smal l fire in vertical shafts with sidewall ventilation . In a compartment with a vent, motion of the fire plume is not stable with swirl ing observed at the side of the compartment in many experiments. An internal fire swirl might be induce d when vents along the symmetric room axis are provided. Internal fire whirls induced by a pool fire in a vertical shaft of height 15 m with different ventilation conditions were studied experimentally [9]. Ventilation was provided from the sidewall with a gap of different widths. Gasoline pool fires with diameters up to 0.46 m were put inside the shaft . Average flame height was found to be 1 to 4.5 m when whirling onset. Experimental studies indicate d that air supply to the upper part of the shaft is a key factor for initiat ing internal fire whirls. Oxygen is required to sustain combustion to generate heat for inducing whirling motions. Therefore, flame s will not whirl without adequate oxygen supply. As the combustion involve s many complicated intermediate chemical s, those reactions were not considered in this paper. The effect of the door opening and ceiling vent on the creation of internal fire whirl s was studied . Swirling of flame under natur al venti lation in small compartment s with ceiling vents and two different ventil ation arrangements were describe d in this paper. Ventilation provision at the sidewa ll is a key factor for the onset of internal fire whirl s with a ceiling vent. This point is demons trated in this paper by n umerical simulations in a normal room with ceil ing vent using Computational Fluid Dynamics (CFD) [10]. 6 2. An Extended Discussion As observed experimentally by Snegirev et al. [7] in studying a room fire , hot smoke moved out throu gh the ceiling vent and the upper part of the door opening. The flaming and the plume regions were not clear with swirling motion. The fuel burnt v ery vigorous ly after onset of whirling motion . The burning rate s were from 0.029 to 0.039 kgm-2s-1 for whi rling fires . Note that the burning rate was only lying from 0.017 to 0.0 27 kgm-2s-1 for non -whirling fires, increased by 70% for swirling fires. This process lasted for a period of 10 to 15 s until all the fuel was consumed. The swirling flame motion was observed to be determined by external air motion . Numerical simulatio ns were carried out by Snegirev et al . [7] in a room similar to their experiment al studie s. The CFD software Fire3D was used . Predicted CFD r esults were similar to th eir experimental observation. Transient whirling fires were predicted . The period with and without fire whirl would be reduc ed if the fuel supply rate increased. Increas ing the burning mass rate would reduce the oscillation period further . Note that the effect of whirling motion is not included in the turbulen ce model s in Fire3D . The Richardson number Ri was defined to include centrifugal acceleration of the mean flow explicitly in terms of the ratio square of the centrifugal acceleration, 2/c Vr with V being the angular velocity and cr the local radius of curvature of the streamlines of the mean flow, and the 7 corresponding turbulent quantit ies k and ε as: 22 2( / ) /c iVrR k = (1) The c oefficient C  appear ed [11] in the turbulent viscosity t: = /kC2 t (2) Value of C was modified by two adjustable constants minC and C w: min 2 i wmin *C)RC exp()C C( C   +− −= (3) The values used under whirling condition s were suggest ed by Snegirev et al. [7]. A single -step global reaction in complex fuel oxidation t o give carbon dioxide, water vapor, carbon monoxide and soot was used. The generation efficiencies for soot an d carbon monoxide were taken as constants in their studies . The chemical reaction rate is determined by the eddy break -up mode l. For vaporized kerosene, C 14H30, the enthalpy of formation is -387.5 kJ mol-1, the boiling temperature is 523 K, and the soot and carbon monoxide efficiencies are 0.04 and 0.007 respectively. The Monte Carlo statistical method was used to model thermal radiation 8 transfer in a gas-soot mixture. Numerical experiment s on varying the swirl number S under whirling condition s were carried out by Snegirev et al. [7] in a room under free boundary condition s. Two fire conditions with different fuel loss rate fuelm and heat release rate Q were used : -2 -1 fuel m = 0.02 kgm s Q 258 kW = (4) and -2 -1 fuel m = 0.04 kgm s Q 516 kW = (5) Results for non -whirling condition s were compared with those for whirling condition. Maximum values of V  were 0, 0.2 and 0.6 ms-1 respect ively under three major conditions. However, the simulated flame height did not increase with the whirling speed. The results would be improved by selecting appropriate turbulen ce model s with swirling . It was concluded that when the swirl number S excee ded 0.6, the flame height was shortened. Th is phenomenon was also observed in jet swirling. In simulatin g the whirling condition inside the room with air flowing in and out freely, a hot air 9 layer was predicte d. Hot air was flowing out through the ceil ing vent and the doorway. Fire whirls were induced and pass ed through the ceiling vent. Unstead y whirling motion with high temperature core was predicted and agreed with experiment al observations . The maximum value of vorticity component along the vertical axis z depends on the supply rate of different burning fuel. Faster burning rate and larger burning mass would give higher temperature and larger value of z. The predicted average period s between the formations of straight whirling flame were 17 s for fuelm of 0.02 kgm-2s-1 and 12 s for fuelm of 0.04 kgm-2s-1. Results are similar to the average period observed in their experiments with values from 10 to 15 s. Flame length and time-average mass burning rate in whir ling fires were found to be greater than those in non-whirli ng ones. 10 3. Numerical Experiment s To avoid having repeated cycles of onset and dissipation of fire whirls by Snegirev et al. [7], a modified room was used. This design would supply suffici ent air to the fire room. A room of length 3.6 m, width 2.4 m and height 2.4 m as in Fig. 1 was considered . The ceiling vent dimension s were changed from 0.8 m by 0.9 m to 1.4 m by 1.0 m. A doorway of width 0.8 m was made on the rear wall. Two differen t height s of 1.0 m and 2.0 m were used to supply adequate ai r. Effects of ventilation on the whirling speed and whirling paths were observed. The heat release rate was kept constant in the numerical simulation s. Modifying the room as in above way would give circular air motion . Since there was an additional opening at the sidewall, cool air drawn would give circular motion. As reported by Snegirev et al . [7], for rooms withou t a ceiling vent with the fire located at a corner, only smal l-scale whirls wou ld be created by a small fire with low heat release rate. This point can easily be studied by investigating the scenarios leading to onset of fire whirl by CFD. The study is not intended to compare the CFD results with those experiments b y Snegirev et al. [7], but to extend their experim ents. The h eat release rates in the two scenarios in this paper were much larger than th ose used by Snegirev et al. [7]. Two ventilation conditions V1 and V2 were provided by another door at the rear wall as in Fig. 1: 11  V1: Door of height 2 m  V2: Door of height 1 m Two p ool fires F1 and F2 of the same area of 0.5 m by 0.5 m but different heat release rate s were used:  F1: 0.24 MW and normal flame height 1.6 m in outside air  F2: 0.12 MW and normal flame he ight 0.8 m in outside air Fire whirls can be form ed under different ventilati on areas , different heat release rate s and different room size s. Numerical experiment s were carried out under these two ventilation conditions. The numerical input data used in this study was base d on the experimental studies reported by Snegirev et al . [7]. The CFD software PHOENICS v3.5 [12] was used with a Pentium personal computer . The software had been demonstrated years ago to be suitable for simulat ing building fire s [10]. A reasonably fin e grid size with a total number of computing cells 216,000 was used. There are 60 cells along the x, y and z directions as shown in Fig. 1. The coarse grid size is 3.6 m / 60 or 0.06 m, satisfying the convergence criterion worked out on thermal plume [13]. The time interval was 10 s. The total computing time f or 300 s (30 time steps, 10 s each ) was 20 hours to give converge d result s. Three boundary conditions were used in this study . • Bound ary conditions of wall surface: 12 The wall was c onsidered to be adiabatic with no slip . The l ogarithm law was used near the wall surface. Velocity on the wall is zero for the momentum equation. Wall function was used f or the enthalpy equation but not for solving the  equation . The diffusion coefficient of k was taken to be that of the momentum equation. • Inlet boundary condition s: Flow variables set to be input conditions with t urbulence intensity varying from 0.02 to 0.05% . • Free boundary conditions : At the free boundary, t he pressure is taken as the atmospheric value. The normal component of veloci ty is zero and fluid can flow in or out, depending on the estimated pressure differences . For inflow and outflow zone s, a zero variable gradient was set along the vertical bound ary plane. Pressu re was taken to be the atmospheric value at those positions . The v ariables k,  and T were taken as inflow values . The above boundary conditions and grid numbers w ere selected based on CFD simulations for similar p roblems . These exa mples had been justifi ed in the past by different experiment al studies [10]. Only the effect s of the door o pening , the ceiling vent and the heat release rate are focused 13 on in this paper . Intermediate chemical reactions in burning the fir e were not taken i nto account . Heat release rate was taken to be the most impo rtant input factor affecting the onset of fire whirls. Selected temperature contours are plotted in Fig. 2. Results for the smaller fire F2 and ventilation condition V1 with th e taller door are shown in Fig. 2a at the vertical plane for x at 1.8 m and 120 s. T he temperature distribution was observed to be stable at 120 s. The temperature distribution s for the bigger fire F1 and the same vent V1 at the same plane at 120 s and 240 s are shown in Figs. 2b and 2c. Velocity vectors at different heights y at 0.2 m, 0.6 m, 1.0 m, 1.6 m, 2.2 m and 2.8 m for the bigger fire F1 with large r vent V1 at 5 s, 60 s, 120 s, 180 s and 240 s are plotted in Figs. 3a to 3e. 14 4. Ventilation V1 with B ig Fire The followi ng are observed from the CFD predictions : • No whirling motion was formed before 5 s as in Fig. 3a. Only upward air flow due to buoyancy was observed . • Upward flow with whirling was formed at 5 s with the direction of air flow depend ent on the location of the side vent. Air swirled up from near the flame bottom through the ceiling vent. Air speed for swirling along the tangent ial direction in the compartment is less than 0.5 ms-1. At 5 s, the airflo w at the bottom side rotate d half a circl e in the compartment with a maximum tangent ial speed less th an 0.3 ms-1. • Stronger whirling flow near the flame bottom was formed a t 60 s as in Fig. 3b . The maximum tangent ial velocity in the rotation al direction is up to 1.0 ms-1. The up ward velocity at the compartment ceiling due to buoyancy is about 6.0 ms-1. This is the maximum predicted speed up to 300 s wh en the flow was steady. The max imum predicted speed was found at the ceiling vent. • Whirling motion was not induce d about the flame axis at 120 s as shown in Fig. 3c . Near the top of the compartment, the swirling motion centr e moved more towards one side of the compartment with the whirling size decrea sed. The overall vertical up ward speed increase d as shown in Fig. 3c due to the increase in buoyancy . Predicted re sults are different from 15 those at 60 s shown in Fig. 3b. The tangent ial whirling speed is smaller at the top than at the bottom of the compartment as shown in Fig. 3c. • As shown from the velocity vector s at 180 s in Fig. 3d, tempera ture in the com partment distributed more uniform ly. The tangential speed for rotation increased, especially at the bottom of the compartment . This is because of the high er flame temperature near the central bottom area and lowest incoming air temperature . Therefore, t he maximum speed for whirling was observed th ere. • Both the velocity and temperature became stable at 240 s as seen from Fig. 3e. Swirling flow at the bottom of the compartment was observed . Selecte d patterns on predicted streamlines at 5 s, 60 s, 120 s, 180 s and 240 s are shown in Fig. 4. Swirli ng motion depends on the air temperature at the lower part of the compartment. For example, the bigger fire F1 would give high air temperature. The air i n the up per part of the compartment would move up due to the temperature difference between the hot a ir moving at the centr e and the cool air at the ceiling vent. The p attern remain ed the same up to 240 s , demonstrating strong flow . 16 5. Comparison with Small Fire Streamline s with the smalle r fire F2 with ventilation V1 at 120 s and 240 s are shown i n Fig. 5 . Air circulated two time s before flowing out of the compartment at 120 s. Air temperature was not stable. This is because air temperature near the ceiling vent was lower than that for the bigger fire F1. Therefore, swirling motion was not indu ced. Note that air swirled many times before moving up to the ceiling for the bigger fire F1 as in Fig. 4. The three -dimension al streamline pattern at 240 s is shown in Fig. 5 b. Air at the uppe r compartment did not move out at the vent cent re after the second swirl . The swirling radius decreased. The whirling tangent ial speed s for both big and small fires F1 and F2 under ventilat ion condition V1 are plotted in Fig. 6 a. It is obser ved that the tangential speed s increase d with time . Comparing the scena rios F1 and F2 , the difference in the tangent ial speed also increased with time. The rotating tangent ial speed of the smaller fire is about 0.5 ms-1 at 180 s, lower than 1.2 ms-1 for the bigger fire. Variation of t he tangent ial speed with height y in the compartment at distance from the fire centr e of 0.2 m at 300 s is shown in Fig. 6. When the height in the compartment is higher t han 1.8 m, the tangent ial speed changes slightly. In fact, the ta ngent ial speed change d slightly with height under the smalle r fire. For the smaller fire F2, tangent ial speed only decrease d slightly from 0.6 ms-1 at y of 0.25 m to 0.4 ms-1 at y of 2.0 m. The tangent ial speed for the bigger fire F1 change d significant ly from 1.9 ms-1 at y of 0.2 ms-1 to 1.0 ms-1 at y of 2.0 m. 17 6. Ventilation Condition Variation of t angent ial speed with time at y of 0.2 m for the two venti lation conditions V1 and V2 is shown in Fig. 7a. There is a slight difference in the tangent ial speed of the whirling at the bottom for the two ventilation conditions . It is observed from Fig. 7b that when the height of the vent increased by two times , the compartment temperature at 300 s was stable . The max imum tangent ial speed of the whirling change d slightly. Despite the increas e in the vent height, t he tangent ial velocity decrease d by 20 % or 30 % with the height. In other words, the height change of the rear vent has no effect on the extent of decrease of the tangent ial velocity with height . The slope of the tangent ial velocity with height remain ed the same when the height of the rear vent increased two times. Variatio n of the tangent ial velocity with height for these two ventilation conditions remain parallel as shown in Fig. 7b. 18 7. Effect of Flame Height For the selected room with the same size as that used by Snegirev et al. [7], another door was constructed at the r ear wall. Two different ventilation conditions V1 and V2 were set up with different heights of door of 1 m and 2 m . The door width and also the size of the ceiling opening w ere the same as those reported [7]. With the heat source at the centre of the r oom, swirling was formed with cool air drawing in under natural ventilation. The flow velocity depends on the hea t release rate, flame bottom temperature and the outside air temperature. As the heat inside the room accumulated, the rotational speed also increased. Finally, when a steady state thermal balance between the heat inside the room and the cool air enterin g from outside was reached , a steady swirling motion due to the heat source was formed. Numerical simulations indicated that there were no pa rticular patterns for the flow. With the increas e in heat release rate and fire height, the rotation al speed alon g the tangential direction of the whirling at the bottom would increase. The extent of increas e changes with time. When temperature be comes stable in the compartment (after t = 240 s in this paper ), the maximum tangent ial velocity of the whirling is obse rved. The speed would no longer change with time. When temperature and velocity become stable, with the increas e in heat release rate and fi re 19 height, the tangent ial velocity of the whirling would increase significantly . In the simu lation for the smalle r fire F2 (0.12 MW and 0.8 m height), the max imum tangent ial velocity decreased from 0.6 ms-1 at 0.2 m to 0.3 ms-1 at 2.4 m. For the bigger fire F1 (0.24 MW and 1.6 m height ), the max imum tangent ial velocity decreased from 1.9 ms-1 at 0.2 m to 0.93 ms-1 at 2.4 m. The tangent ial velocity decreased by 50% with heig ht. When the heat release rate and height doubled , the max imum tangent ial velocit y changed from 0.6 ms-1 to 1.9 ms-1 at the height of 0.2 m; and changed from 0.3 ms-1 to 0.93 ms-1 at 2.4 m. The maximum tangent ial velocity of the rotation in the compartme nt increase d three times. When the fire height is lower than 0.8 m at the bottom of the compartment, the air whirling would move out of the ceiling vent after circulating twice . When the fire he ight is 1.6 m, the air at the same location w ould move out o f the ceiling vent after circulating less than 1.5 times . 20 8. Conclusion s Numeric al experiments on the onset of internal fire whirls were carried out based on some earlier studies by Sn egirev et al. [7]. The following conclusion s can be drawn from the above CFD simulation s: • The height of the rear vent has no effect on the swirling flow . • Upon the onset of fire whirl, air temperature be came stable in the compartment . • Increas ing the rear door vent height has no effect on the variation of rotati onal speed of the whirling with height . Swirling air flow is induced at the bottom of the co mpartment. Air flow at the up per part of the compartment is mainly upward moving through the ceiling ven t. These observations are useful to assess fire hazard for a room fire with different ventilation provisions. The possibility of fire whirls and the associated hazards should be further studi ed in compartments or spaces with a ceiling vent or side opening s, particularly for bigger fires with higher heat release rates. 21 9. Additional Comments The paper was pub lished as a journal paper [ 14]. With the p ractice of publisher, preprint i s allowed to upload at a website. There are furth er studi es on this topic [ 15-43] in the past 10 years. Projects by the authors funded by Research Grants Council of Hong Kong are: • Onsetting of internal fire whirls in buildings and associated safety provisions . • A study of internal fire whirl in vertical shafts wi th open roofs . • A study on electric and magnetic effects associated with an internal fire whirl in a vertical shaft . • A study of the hazardous consequences of fire whirls generated in a room. Ackno wledgement The work described in this pa per was supported by a grant from the Re search Grants Council of Hong Kong for the project “Onsetting of internal fire whirls in building s and associated safety provisions (PolyU 5149/06E)” with account code B -Q01E. 22