Experimental study on oscillating behaviour in a small-scale compartment fire

Fire Safety Journal 20 (1993) 377-384

Short Communication Experimental Study on Oscillating Behaviour in a Small-Scale Compartment Fire Kwang Ill. Kim, Hideo Ohtani & Yoichi Uehara Department of Safety Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan (Received 6 June 1989; revised version received 15 February 1992; accepted 4 March 1992) ABSTRACT To investigate fire behaviour in a compartment, the effect of ventilation factor and fuel surface area was studied experimentally by using methanol as a fuel. A limit of stable combustion appeared at a small ventilation factor. As the fuel surface became larger, the stable combustion limit increased. Oscillating combustion was observed in both an unstable combustion region and a stable combustion region near the boundary of these regions. In the unstable combustion region, extinction followed the oscillating combustion. On the other hand, in the stable region the oscillating combustion lasted until the fuel was exhausted. The period of the oscillating combustion was 1.0-1.5 s in the stable combustion region. INTRODUCTION Results of studies which have b e e n c o n d u c t e d on c o m p a r t m e n t fires have so far indicated that the burning rate d e p e n d s strongly on the ventilation factor: AH~a, 1 first identified by K a w a g o e . 2 T h o m a s et al. 3 later showed that there was a limit to ventilation control at high ventilation factors when fuel surface area b e c a m e rate controlling. Bullen 4 subsequently s h o w e d h o w the burning rate and the ventilation mechanism could be d e c o u p l e d to allow non-cellulosic fires to be modelled. L a t e r w o r k by T a k e d a and Akita 5 found that for a certain range of ventilation factors and fuel surface areas, oscillating combustion could be observed. H o w e v e r , the position of the oscillating 377 Fire Safety Journal 0379-7112/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

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K. L Kim, H. Ohtani, Y. Uehara

combustion region in the ventilation control regime has not been fully clarified. In this study, experiments were conducted to investigate the oscillating combustion p h e n o m e n a in a small-scale compartment.

EXPERIMENTAL APPARATUS AND PROCEDURE

The apparatus used in these experiments is shown in Fig. 1. It consisted of a small-scale compartment, a fuel tray, and a fuel supply system. The compartment was a cubic body of 30 cm side length, and made of 5 m m thick asbestos slates. The single door-like opening was set in a side wall. The opening was 15 cm wide and its variable height could be changed to vary the ventilation factor. The fuel trays were made of l mm thick stainless-steel plate. Five sizes of tray, 7-5 x 7 . 5 c m , 15 x 15 cm, 7-9 x 28.8 cm, 15 x 28.8 cm, 27.8 x 28.8 cm surface area and 5 cm deep were used to vary the fuel surface area. Methanol (Junsei Kagaku Co., Ltd, purity 99.9%) was used as the fuel. It was fed from a fuel vessel by using the siphon tube to keep the liquid surface level in the tray constant. The mass burning rate of fuel was evaluated by measuring the weight of the fuel vessel continuously. The temperatures at upper, middle and lower points along the centre line of the compartment, at the wall surface, and at upper and lower points in the opening, were measured by C h r o m e l - A l u m e l thermocouples with a wire diameter of 100/~m.

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Oscillating behaviour in a compartment fire

379

EXPERIMENTAL RESULTS AND DISCUSSION

Unstable combustion region and stable combustion region The correlation between the measured burning rate, R, and the ventilation factor, A H ~/2, for various fuel surface areas is shown in Fig. 2. In general, as the fuel surface area was increased, the burning rate was also increased. In the region corresponding to small ventilation factors, the fuel could be ignited, but combustion was not sustained. This is referred to as an unstable combustion region. Beyond a certain ventilation factor, combustion continued until the fuel was exhausted. This region is called a stable combustion region in the following section. The lower limit of stable combustion was found to d e p e n d linearly on the fuel surface area. As the fuel surface area became larger, the stable combustion limit became larger. For the smallest tray (56cm2), the limit was A H ~/2= 0.001 13 m 5/2 and the mass burning rate at the limit was - 5 g/min. For the largest tray (801 cm2), the limit was 0.0755 m 5/2 and the mass burning rate was - 7 2 g/min. From the mass burning rate and the fuel surface area, the surface regression rate could be evaluated if the density of methanol was assumed to be constant, 0-793 g/cm 3. The evaluated surface regression rate was 1.1 m m / m i n for both limiting cases. This value is just a little larger than the surface regression rate 140

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(d) Fig. 3---conM. (c) Oscillating combustion continued until fuel was exhausted (fuel surface area 225 cm 2, A H ~ = 0.004 69 m5'2), and (d) stable combustion continued until fuel was exhausted (fuel surface area = 432 cm 2, A H 1;2 -- 0-005 35 m5;2).

for pool-burning of methanol in the open. This fact was considered to suggest that under the stable combustion limit the opening could not offer enough ventilation for fuel burning. The above regression rate, namely mass burning rate per unit surface was considered to be the minimum requirement for sustaining combustion. Temperature histories Figure 3 shows temperature histories for various cases. The apparent time lags between each temperature trace were caused by displacement

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K. I. Kim, H. Ohtani, Y. Uehara

of recording pens. In the case shown in Fig. 3(a), the ventilation factor was below the stable combustion limit. Temperature histories show that the fuel was ignited, but the flame soon extinguished. In the case shown in Fig. 3(b), the ventilation factor was below, but close to, the stable combustion limit. Temperature histories show oscillating behaviour, but in this case, ventilation was not enough to sustain a fully developed fire. The flame oscillated four times before extinction. As shown in this figure, the gas temperature in the compartment became higher as the oscillation repeated. The temperature of the fuel tray increased and too much vapour came to be emitted. Consequently, the unstable balance between air supply and fuel vapour supply was broken and extinction occurred. This behaviour is called unstable oscillating combustion. In the case shown in Fig. 3(c), the ventilation factor was a little larger than the stable combustion limit. Oscillating combustion p h e n o m e n a were also observed in this figure. However, the air supply was large enough to sustain a fully developed fire. Therefore, no extinction occurred in this case. This behaviour is called stable oscillating combustion. In the case shown in Fig. 3(d), the ventilation factor was larger than the stable combustion limit. A stable, fully developed fire was attained in this case. These smooth temperature histories show that a stable flow field was established in the compartment.

Oscillating combustion From temperature histories like those shown in Fig. 3, the ventilationcontrolled regime could be divided into two main regions and two inner regions (Fig. 4). Characteristic oscillating combustion was observed in both of the inner regions. At the stable combustion limit it was considered that the rate of supply of fuel vapour was the m i n i m u m for sustained combustion. In the vicinity of the limit, the vapour supply is just a little higher than the minimum requirement. It can sustain stable combustion. However, combustion is not strong for forming a stable flow field in the compartment. There may be hydrodynamic instability observed in the compartment. The above consideration suggested that the oscillation period of the unstable oscillating combustion was relatively long and that of the stable oscillating combustion was relatively short. In the case of the unstable oscillating combustion, the oscillation period depends on time required for storing enough fuel vapour for strong combustion. The amount of fuel vapour depends on the air supply rate, i.e. the ventilation factor. Therefore, the oscillation period was considered to depend on the ventilation factor. Figure 5 shows the relationship

383

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between oscillation period and ventilation factor. It is clear that the oscillation period becomes shorter as the ventilation factor increased. In the case of the stable oscillating combustion, there was no appreciable dependence observed between the oscillation period and the ventilation factor. The oscillation period was 1-0-1-5 s in this case. If the oscillation was caused by hydrodynamic instability, inflow air rate, opening condition, etc., must affect the oscillation. That is, the

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K. L Kim, H. Ohtani, Y. Uehara

ventilation factor must affect the oscillation. However, variation of the observed oscillation period was so small that no fruitful conclusion could be obtained.

CONCLUSIONS Small-scale experiments indicate that the ventilation-controlled regime of a compartment fire can be divided into two main regions (stable and unstable), with two inner regions, which show oscillating combustion near the stable combustion limit. The oscillation period in the unstable combustion region depends largely on the ventilation factor.

REFERENCES 1. Drysdale, D., An Introduction to Fire Dynamics. John Wiley & Sons, New York, 1986. 2. Kawagoe, K., Fire Behaviour in Rooms. BRI Report No. 27, Building Research Institute, Japan, 1958. 3. Thomas, P. H., Heselden, A. J. M. & Law, M., Fully Developed Compartment Fires: Two Kinds of Behaviour. Fire Research Technical Paper No. 18, HMSO, London, UK, 1967. 4. Bullen, M. L., A combined overall and surface energy balance for fully developed ventilation controlled liquid fuel fires in compartments. Fire Res., 1 (1977) 171-85. 5. Takeda, H. & Akita, K., Critical phenomenon in compartment fires with liquid fuels. In Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 519.