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The King's Cross fire: Results and analysis from the scale model tests
Fire Safety Journal 18 (1992) 83-103
The King's Cross Fire: Results and Analysis from the Scale Model Tests* K. Moodie & S. F. Jagger Fire and Process Hazards Section, Explosion and Flame Laboratory, Research and Laboratory Services Division, Health and Safety Executive, Harper Hill, Buxton, Derbyshire SK17 9JN, UK
A BSTRA CT A series of fire development trials carried out on one-tenth and one-third scale models of the King's Cross escalator shaft ticket hall are described. The construction and instrumentation of these facilities are considered and an analysis of the principal data is presentod. The implications of the measurements for scaling considerations are discussed, and possible full-scale, ]ire development scenarios are outlined.
1 INTRODUCTION The computer simulations of the King's Cross fire, carried out under contract to the Health and Safety Executive (HSE) by A E R E Harwell, t.2 suggested that the flow in the region of a heat source in the channel of the escalator did not rise vertically upwards, but instead was confined by the updraught in the channel which dominated the flow structure. A parametric study of the escalator duct flow and of the relevant scaling criteria, carried out on our behalf by Cambridge Environmental Research Consultants, 3'4 showed that the up&aught and the consequent channelling of the flow were aerodynamical effects governed by the buoyancy flux of the heat source. They also showed that Froude number scaling, as discussed by De Ris, s would, in the circumstances, provide representative modelling of the parameters * Paper presented st the I. Mech. E. seminar 'The King'sCross Underground Fire: Fire Dynamics and the Organisation of Safety', 1st June 1989. 83
O 1991 Crown Copyright.
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governing the fire growth, and that modelling at ~-scale would provide results of value in indicati~.~ full-scale conditions. Thus, a series of scaie~ open-channel and model tests were undertaken in order to further understand the fire dynamics. The tests involved (a) 100-mm2-section open channels, (b) ~o-scale models of the Piccadilly shaft with partial roofing, (c) ~3-scale sections of open-channel escalator, and (d) a fully roofed model of the upper (passenger) section of the Piccadilly shaft connected to the front section of the ticket-hall complex. The principal results from these tests are considered in this paper. A fuller description of the tests themselves can be found in Refs 6-8.
2 OPEN-CHANNEL (100-mm2 CROSS-SECTION) TESTS The fire development was assessed on 2-m-long open channels, which were either of plywood construction lined with corrugated cardboard or
Fig. Is.
Flame front advancing up an open channel ahead of the burning region.
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of various types of plywood without any lining. The channels were inclined ~t 30° and set alight at a point near their bases. The tests were all conot~cted in still air conditions. In some tests, thermocouples were fixed at the top of the channel to record the gas temperatures as the fires progressed into the ticket-hall area. The progressions of the fires were recorded using video and still cameras. These were then used to obtain the velocities of the flame fronts as they progressed up the open channels. The results indicated that the flame front from a fully developed fire across the escalator advanced upwards at an exponentially increasing rate, emerging with velocities of about 2m/min. In all of the tests the advancing flame fronts were observed to remain virtually within the channel and to extend ahead of the visible pyrolysis front, the extent of which seemed to vary with the length of the burning zone. This is illustrated in the sequence of three photographs shown in Fig. 1, taken at 3-s intervals. They also illustrate the inherent nature of the flame structure when it is lying in the channel, namely to curve inwards towards the centre of the channel from the two vertical sides.
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Position of flame front 3 s later.
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Fig. 1¢. Position of flame front after another 3 s.
FIIg. 2. Sustained jet of flame from end of channel.
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When the flame fronts reached the end of the channels they emerged with sufficient momentum parallel to the channel to produce a sustained wall jet effect on any simulant horizontal ceiling in that region, as illustrated in Fig. 2.
3 PARTIALLY ROOFED CHANNEL (~o-SCALE) TESTS Some of the open-channel tests were repeated, but with the addition of a partially curved ceiling (90° segment) made from a thin ~etal sheet~ together with a longer test section of some 4 m. The escalator, facia and decking were constructed from corrugated cardboard mounted onto a plywood backing, the treads and risers being made to scale from 1-mm-thick cardboard. For some tests the ceiling was also lined with cardboard. In all tests a single source of ignition was used, some 2 m down from the top of the model and on the right-hand side of the models. Video records only were made of these tests, which again showed near exponentially increasing rates of fire growth, similar to those obtained from the open-channel tests. Numerical comparisons with equivalent open-channel results suggested that the roof was accelerating the rate of the fire growth. Of the three tests carried out with a cardboard lining on the roof, in only one did the cardboard catch fire prior to flames from the burning escalator arriving in the simulated ticket-hall area. This happened just prior to the flames reaching the top of the escalator channel, and did not influence their initial entry into the ticket hail. Some additional tests on a 0.4 m length of 100-mm2-section channel, lined with corrugated cardboard, showed that the flames did not lie down to the extent observed in longer channels, and as a consequence the rate of flame propagation was slower. A test on a 2-m-long, ~-scale, wood-lined channel gave a similar result. These two results suggested that a downstream channel length to channel width ratio of not less than 10:1 appeared to be necessary before a 'trench' effect was established.
4 OPEN-CHANNEL (~-SCALE) TESTS Three tests were carried out on 30°-inclined, X3-scale models of the Piccadilly tunnel escalator without roofing. The models were lined with wood attached to 0.5-mm-thick, metal backing sheets, which formed the treads and risers, the skirting board and the balustrades. The length
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of wooded section was approximately 2 m. However, for the second and third tests the test section was extended to 4 m by the addition of a 2-m-long, metal channel. The balustrades and skirting boards were lined with two layers of 6-mm-thick, hardwood-faced plywood. The treads and risers were similarly constructed, but, in addition, hardwood cleats were attached to the top of the treads. The fires were started at the base of the test piece, across the full section, by igniting wood cribs. A limited number of heat-flux, temperature and weight-loss measurements were made during these tests, together with video and stills records. The video record of the first test showed that the flames rose nearly vertically and did not lie down within the confines of the channel. Consequently, the flame front moved relatively slowly and took some 5 rain to reach the end of the channel. In the second and third tests, some 3-3-5 min after initiation and over a period of a few seconds, the flames switched from being near vertical to laying down in the trench. Less than I min later (20 s in the third test, for which the wood was drier) the flames had reached the end of the wooden section of channel, and within a further 30 s (both tests) flames were seen flickering out of the end of the metal channel. The total received heat flux as measured on a balustrade was in the order 60kW/m 2. The temperature measured in the centre of the channel was in excess of 600 °C, and the average heat output was some 90 kW per metre length of escalator.
5 MODEL TESTS (t-SCALE) WITH THE SHAFT AND TICKET-HALL COMPLEX A D D E D
$.1 General description Finally, a series of four fire growth tests were carried out on a length of escal~,~r that formed part of a ~-scale model of the passenger side of the Piccadilly shaft and the ticket-hall complex extending back to the ticket office. A 10-m-long section of escalator was modelled and placed at an angle of 30° inside the scaled shaft. The escalator was placed on the left-hand side of the shaft (when viewed from the bottom) in a position corresponding to that of escalator No. 4 of the Piccadilly shaft. At the rear of the ticket-hall model there was a plenum chamber, separated from the ticket hall by a partition, which simulated the blockage effect of the ticket office. From the plenum chamber, combustion products were exhausted via a single vertical stack.
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The tunnel ceiling was not painted for the first test, but for the second and third tests it was painted with two coats of white emulsion; however, these were not considered representative of the paint layers existing at King's Cross during the time of the incident. In the final test, the ceiling was painted to a London Underground specification intended to reproduce the surface spread of flame characteristics of the actual ceiling. During the experiments, detailed measurements were made of temperatures, weight loss, heat fluxes and gas velocities at various locations. In addition, the concentrations of oxygen, carbon dioxide and carbon monoxide were recorded continuously in the exhaust stack. Comprehensive photographic records of the tests, involving up to five video and five 'stills' 35-mm cameras, were also taken.
5.2 Constructional details The 3X-model was constructed upon an existing hillside on the Buxton site. This provided some degree of protection from the weather, and allowed for a more rigid scaffolding framework to be used. There was also the presence of a level area which could be used for the ticket-hall section of the replica. A scaffolding framework was used as the base for the model; a fiat steel floor was placed up the 30 ° slope and a tunnel of 1-mm-thick, galvanized steel sheeting erected over it. The tunnel was in an inverted U-shape, the radius of which being 1.15 m. The escalator section was placed at the appropriate height to give the same roof curvature as that in the Piccadilly shaft. External views of the completed model are shown in Figs 3 and 4. At the top of the escalator shaft a 'throat' section led into the ticket hall. This was constructed from steel sheeting; it was approximately 4 m wide and 3 m deep, with a height of 0.8 m. The escalator shaft was offset to the appropriate side of the ticket hall. At the rear of the ticket hall the wall had two airways built into it. These were the full height of the ticket hall and were square. They were positioned to correspond with the spaces on either side of the ticket o~ce at King's Cross. No attempt was made to model the ticket booths in the ticket hall. The plenum chamber, constructed behind the rear wall of the ticket hall, had an exhaust stack for the first two tests, some 3 m high and 0.5 m in diameter, t h e stack was changed to one with a 2 x 1 m, rectangular cross-section for the final two tests. The escalator model itself was identical in construction to the ~-scale sections already described, but with the addition of deckboards on
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Fig. 3. A general view of the model situation.
either side just below the tops of the balustrades and a vertical facia panel on the left-hand wall. The 10-m length of staircase represented about two-thirds of the scaled length of the full-size escalator. In the throat section the horizontal sections of all three escalators were replicated. The upper 7-m section of the escalator was independent of the lower
Fig. 4. The rear of the model, showing the ticket-hall covering the exhaust stack.
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Fig. Sa. A view of the escalator from the foot of the shaft. 3-m section, so that it could be mounted' on load cells and its weight determined prior to, and during, a fire. The facia panels were attached to the tunnel side walls and not to the weighted escalator section. On the right-hand side of the wooden escalator, the area corresponding to escalator No. 5 was covered over at the deckboard level with steel sheeting. The area corresponding to escalator No. 6 was left free
l~g, 5b, A view of the ticket hall looking towards the throat and shaft.
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as a walkway. A rail for a moving camera position was positioned along the centre line of the tunnel (subsequently removed after the first two tests). The view looking up the escalator tunnel is shown in Fig. 5a and that of the ticket hall area in Fig. 5b.
S.3 Experimental instrumentation A typical instrumentation layout is shown in Fig. 6. On the escalator section there were three main 'stations' for instrumentation. These were in fixed positions relative to the top of the inclined section of escalator, at 0.25, 2.5 and 4.75 m, respectively, from the top. Usually arrays of eight thermocnuples, Pitot-static tubes and pairs of heat-flux meters (radiant and total) were placed at each of these positions. In the ticket-hall area there were three thermocouple arrays. Additional thermocouples were also embedded into the balustrades or attached to the facia boards and ceiling from some tests. The Pitot-static tubes were placed at different locations for each test in order to measure air velocities in the shaft and in the exhaust stack. This also contained a gas sampling point for analysis of the combustion products. The inflow to the shaft was monitored using several hot-wire anemometers. A four-point, load-cell system was used to support and weigh the ?-m escalator section. All of the instrumentation was connected to a data logging and processing system based on a
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minicomputer (approximately 100 channels), which sampled all of the data at a rate of once per second.
5.4 Experimental procedure Once installed, the escalators were kept warm and dry and, about 1 h prior to a test, the moisture content of the balustrade panels was measured. The average levels were about 11%, with a 4% variability. The fires were started at the appropriate scaled distance clown from the top of the escalator. In the first two tests, five wood cribs placed across the escalator were used as an ignition source, but in the third a narrow layer of grease across one step was used instead. In the fourth test, narrow grease layers across half of two escalator steps were used. The computerised logging of the data was started at the same time as the fire, together with the 35-mm photographic and video equipment. The fires were extingushed a few minutes after flames first entered the ticket hall, in order to prevent excessive fire damage to the model. The fire damage was surveyed and photographed after each test, and in all cases had reproduced the salient features of the fire damage as described in Ref. 9.
$.$ Results snd discussion and results 5.5.1 Visual observations The visual records from the first three tests showed that, following ignition (at about the first 0.5-m marker), flame was soon established across the full width of the channel, with the flame front tending to remain low in the channel as the fire began to develop, as shown in Fig. 7. The fire then progressed up the escalator channel at an exponentially increasing rate of growth. The fire in the fourth test, because it was ignited on the right-hand side only, took longer to establish itself across the whole of the escalator section than in the previous three tests (some 9min), but thereafter it behaved in a similar manner. The rates of advance of the flame fronts, taken from the visual records of all four tests, are shown in Fig. 8. During the first two tests, ignition of the left-hand balustrade occurred within 2 min of the start of the fires. This was some 4 and 9 min, respectively, in the third and fourth tests. In all tests the left-hand facia boards caught fire some 20-40s later, at around the 1-2-m markers. At the same time the flame fronts in the channels were well ahead of this point, particularly so in the last test. In general, the flames on the burning facia remained nearly parallel to the escalator.
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(b) Fig. 7. The fire just after ignition of the left-hand side (test 4): view from (a) the foot of the shaft and (b) the right shaft camera.
However, flames from burning near the top of the escalator shaft appeared to corkscrew over the tunnel roof both before and after the flames emanating from the escalator channel had entered into the ticket hall. At all times, and in all of the tests, the flame fronts within the escalator channel remained ahead of the pyrolysis fronts on the
Results from the scale model tests
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balustrades, which were themselves ahead of the pyrolysis fronts on the facia boards. During each of the four tests, it took about 80, 55, 65 and 45 s, respectively, after ignition of the left-hand balustrades until a continuous ejection of flame into the ticket hall and across the ceiling began. The emergence of the jet of flame (test 4) as seen from the right and left side of the ticket office is shown in Fig. 9. This was preceded by a visible glow on the tunnel ceiling as the flame front progressed upwards and the occasional flickering of flames out of the shaft. In the final two tests the flickering of flames prior to continuous ejection lasted for around 9 s, compared with 20 s in the first two tests. 5.5.2 Air temperature measurements The vertical temperature profiles obtained on the centre lines at all three measuring stations showed that the peak temperatures were between 800 and 1000 °C within the escalator channel. The corresponding near-ceiling temperatures were lower in all cases, but increased progressively in magnitude towards the top of the escalator shaft. The results indicated that sharp temperature fronts were observed at all three measuring stations, which coincided with the visible flame fronts advancing up the channels. Some of these features are illustrated in Fig. 10, which shows, for all four tests and at the lowest measuring point, the maximum gas temperatures in the escalator channels and the gas temperatures 25 mm below the tunnel ceiling. Figure 10 also illustrates the degree of potential preheating that could occur at the tunnel ceiling prior to the arrival of the flame front. The temperature profiles
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(b) Fig. 9. The continuous ejection of flame into the booking hall, seen from (a) the left-hand side of the hall and (b) the angled window on the right-hand side of the hall.
confirmed that, as the flame fronts approached the ticket hall, they were advancing in some cases at speeds in excess of 8m/min, in agreement with the visual observations. The peak rates of temperature change at the top of the escalator were in the region of 40-60 °C/s, and these occurred within a matter of seconds following the continuous eruption of flames into the ticket hall.
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The temperature profiles taken near the exit of the horizontal section of the escalator channel showed similar peak temperatures to those observed within the sloping section. However, the positions of the peaks had moved to a region above the top of the channel. In general, these temperature fronts were more diffuse than at the measuring stations within the sloping sections of the tunnel and were also in accord with the counter-clockwise spread of the combustion products observed from the visual records. The temperature profiles on either side of the ticket office suggested that continuous thermally stratified layers were created below the ceiling following the arrival of the flame fronts in the ticket hall. Prior to this, the temperatures on either side of the ticket office built up gradually. The temperature rises were lower on the left side of the ticket office than on the right, indicating that the majority of the gas flows in each test were to the right side of the ticket office. Qualitatively, these were in agreement with the computer predictions given in Ref. 1. The rates of temperature rise on the left side of the ticket office, 0.2 m below the ceiling, were about 20 °C/s during the last two tests and were lower prior to and after the arrival of the flame fronts than at the top of the escalator. 5.5.3 Componenttemperatures The peak surface temperatures on either side of the sloping part of the balustrades were similar to those of the corresponding centre-line air
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temperatures in the escalator channels, and they can be used to infer the progress of the pyrolysis zone in the wood. This advanced at a similar rate up either side, although progress appeared to be quicker on the left than on the right during the development stage of the fire. The results also showed that the pyrolysis fronts always lagged behind the flame fronts, but to a progressively greater degree near the top of the escalator. When flames were halLway up the shaft the flame front was typically 1-2m ahead of the pyrolysis zone. The internal wood temperatures measured some 3 mm below the surface hallway down the escalator suggested that the pyrolysis frorJts would not penetrate to these depths prior to the flame fronts reaching the top of the escalator.
5.5.4 Heat fluxes The total heat fluxes received by the left-hand balustrades, facia panels and the tunnel ceiling were measured at various positions during each test. Typical measurements (from test 4) are shown in Fig. 11 for the positions near the top of the shaft shown in Fig. 6. The total heat flux to the ceiling was always much less than that received by the balustrades or facia panels immediately below. The peak, radiant heat fluxes to the ceiling and balustrades were roughly the same, about 25 k W / m 2. Thus, the substantial i~,crease in total heat flux to the balustrade and facia was 180
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due to an increase in convective heat flux, probably as a result of increased turbulence in the burning region, together with an increase in gas velocities especially near the top of the escalator. The total heat flux on the facia panel was about 10% less than that received by the balustrade. Such heat fluxes imply very rapid onset of pyrolysis in the presence of a flame, with typical preheat times of 10-20 s.
5.5.5 Stack exit conditions The gas analyses taken near the stack exit plane are shown in Fig. 12 for all but the third test~ during which no gas analysis was made. Thus, in general, the oxygen concentrations fell to about 10% by the time that the fires were extinguished, although, at the time that continuous ejection of flames into the ticket hall first began, the oxygen concentrations were typically 16% and the carbon-dioxide concentrations around 6%. The average stack exit temperatures were in the order of 400 °C, as measured mid-way up the stack where the combustion products were assumed to have fully mixed. There was no evidence of flames within the stack. The heat outputs of the fires as derived from the oxygen consumption suggested that they were typically about 0.6-0.8 MW at ~lq 8 /
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the time that the fires first entered the ticket hall, increasing to about !.5-2.0 MW just before the fires were extinguished. 5.5.6 Air velocity measurements The average air velocities measured in the trench below the fire zone were about 0-2-1.0 m/s, and increasing as the fires developed, although large-scale fluctuations were occasionally superimposed, typifying the atmospheric turbulence and gustiness prevailing during some of the tests. The escalator velocities above the fire zone, just below and parallel with the top of the handrails, were relatively low until the arrival of the flame front. The maximum velocities at the top of the escalator were between 2 and 4 m/s. However, the resultant velocity vectors may have been higher as the flows were corkscrewing at this cross-sectional position. The results also suggested that the trench velocities exceeded the ceiling velocities as the flame front traversed the measuring stations, but at the top of the shaft and in the throat of the shaft the ceiling velocities were the highest, suggesting that flames rose above the handrails in the upper parts of the shaft. The calculated, average mass flux up the shaft, based on the air velocities when flames first entered the ticket hall and just before the fires were extinguished, was about 1.3 kg/s. The corresponding mass flux up the trench was in the order of 0.03 kg/s. These figures suggest that the entrainment into the fires of air drawn up the trench represented a minority contribution; therefore, by implicatiotl, most entrainment was through the top surface of the fires.
6 PARAMETRIC ANALYSIS The data can be considered in the context of the Froude criteria for modelling firess and a parametric flow analysis of the type described in Ref. 3. The flow upstream of the fire is largely determined by the buoyancy flux, this being directly related to the heat release rate. Thus, measurements of this parameter, used in conjunction with simple plume models can infer expected levels of temperature both across and along the duct velocities. Such calculations largely confirm measurements of temperatures and flows observed during the tests. The Froude modelling technique permits some predictions of conditions at other scales. It is based on maintaining geometric similarity, the flow Froude number and the ratio of time-averaged mass burning rate
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to convective mass flux per unit area. On this basis temperature will be independent of scale, velocity will scale as L ~j2 (L is some characteristic dimension) and heat release rate should scale as L s~. However, since transient phenomena involving solid-state heat transfer do not obey the same scaling laws and Froude modelling violates proper scaling of radiation, convection and fuel bed geometry, difficulties are encountered in specifying times for fire development and the heat release rate is not easy to control. However, it is difficult to make progress unless the heat release rate is scaled as L sl2. Consequently, at emergence of flame into the ticket hall, the model, heat release rates suggest a fire with a heat output of 15-25 MW. Similarly, gas velocities at the top of the escalator extrapolated from the I-scale simulations are predicted to be in the range 8-12 m/s. Measured temperatures and their rates of rise are directly comparable. The progress of the fire on a larger scale is rather more difficult to predict, as this is dependent upon heat transfer from the preceding flames and hot gases, which is not properly accounted for in the model. The data suggest that convection is the dominant mode of heat transfer at the scale of the experiments. However, an examination of measurements at both ~- and 13-scale imply a linear dependence of radiative transfer with scale. This would mean that, even if convection were to be independent of model scale, then it would still be the principal, though less dominant, mode of heat transfer. There is some evidence to suggest that, since velocities scale with L 112, then so does the convective heat-transfer coefficient. Again, the experiments suggest a near-linear scale variation of convection. This is difficult to reconcile with other data determined on what are essentially smooth surfaces, and it i.,; ~ o r e probable that the measurements on the smallest models are strongly influenced by Reynolds number effects and do not form sufficient basis for extrapolation. Thus one possible way to make progress is to follow De Ris 5 and assume that the convective heat flux scales as L ~. Considering the fuel as thermally thick, an expression for the flame spread velocity (Vp) defined as the advance of the pyrolysis front, has been given by Green et aL : ~°
where p is the material density, cv the material specific heat, k thermal conductivity, T the firepoint temperature, T~ the ambient temperature, q the incident heat flux, and (xs - x v) the heated, unburnt fuel surface, which, for convection only, scales as L 2, since q---L 1/2. Therefore, it follows that the time (At) for the fire to spread between two equivalent
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points is inversely proportional to ~ e model scale, so that fire spread and development times at full-scale would be much reduced. Similarly, if the radiative flux scales with L, as can be tentatively implied from model data, then the times for fire spread will be further reduced, by a factor of up to 50%. Additionally, the enhanced flame stretch up the channel due to the growing fire will tend to accelerate the flame spread and reduce fire spread times. An initial appraisal of the lengths of the pyrolysis zones and heat release rates indicate that the length of such flames follow a power law relation with fuel burning rate with an index which, as would be expected from the nature of the situation, falls between that currently observed for a turbulent wall flame and that for a free diffusion flame from a horizontal fuel bed. The situation is somewhat analogous to the vertical spread of flame on a wall, a situation which, folMwing the treatment of Delichatsios, ~1 seems amenable to a relatively simp|e analysis. A similar approach for the trench effect, incorporating experimentally derived relationships for flame length, extent of the pyrolysis zone and heat transfer coefficients (for example, with heat release rates), shows some promise as a predictive tool; in addition, it gives the explicit dependence of the rate of fire spread on such parameters as material and channel geometry.
7 CONCLUSIONS
The experimental work described confirms that a mechanism for rapid fire spread exists, and it occurs primarily within the confines of the escalator trench. This is in accordance with the computational studies, 1 the modelling analysis3 and with evidence presented to the Inquiry both in June and August 1988. These scaled model tests provide both a qualitative picture of the fire spread mechanism and quantitative data from which extrapolations of the likely rate of fire spread at full-scale can be inferred, taking into account the increasing rates of radiative and convective heat transfer to the combustible material observed in the tests. They also provide basic data for modelling the rate of advance of the flame front based on flame spread up thermally thick inclined surfaces. The last test, in which the fire was started on the right-hand side, was in our opinion in good qualitative agreement with the events on the night, particularly with regard to the fire growth after the left-hand side of the escalator was alight.
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ACKNOWLEDGEMENTS The authors wish to acknowledge the considerable amount of assistance given by other members of the Research and Laboratory Services Division of the HSE in undertaking this investigation, in particular other members of the Fire and Process Hazards Section.
REFERENCES 1. Simcox, S., Wilkes, N. S. & Jones, I. P., Fire at King's Cross Underground Station, 18th November 1987: Numerical simulation of the buoyant flow and heat transfer. Report No. AERE-G 4677, Harwell Laboratory, Harwell, UK, 1988. 2. Simcox, S., Wilkes, N. S. & Jones, I. P., Fire at King's Cross Underground Station, 18th November 1987: Numerical simulation of the effect of train movements. Report No. AERE-G 4782, Harwell Laboratory, Harwell, UK, 1988. 3. Britter, R. E. & Linden, P. F., Fluid mechanics and physical modelling of King's Cross Fire. Report No. FM 17/88, Cambridge Environmental Research Consultants, Cambridge, UK, 1988. 4. Britter, R. E. & Linden, P. F., Fluid mechanics and physical modelling of King's Cross Fire: Flow in the booking hall. Report No. FM 19/88, Cambridge Environmental Research Constltants, Cambridge, UK, 1988. 5. De Ris, J., Modelling techniques for predictions of fires. In Applied Polymer Symposium No. 22, John Wiley and Sons, New York, 1973. 6. Moodie, K., Jagger, S. F., Bettis, R. J. & Beckett, H., Fire at King's Cross Underground Station, 18th November 1987. Part 11: Scale model fire growth tests. RLSD Incident Report No. IR/L/FR/88/22, Health and Safety Executive, Buxton, UK, 1988. 7. Jagger, S. F., Willoughby, D. B., Beckett, H., Bettis, R. J. & Moodie, K., Fire at King's Cross Underground Station, 18th November 1987. Part 17: Further one-third scale fire growth tests. RLSD Incident Report No. IR/L/FR/88/37, Health and Safety Executive, Buxton, UK, 1988. 8. Jagger, S. F., Beckett, H., Harper, P. J., Regan, N. J. & Willoughby, D. B., Fire at King's Cross Underground Station, 18th November 1987. Part 16: The LUL one-third scale fire growth test. RLSD Repayment Contract No. IR/L/FR/88/38, Health and Safety Executive, Buxton, UK, 1988. 9. Moodie, K. & Wharton, R. K., Fire at King's Cross Underground Station, 18th November 1987. Part 1: Damage assessment. RLSD Incident Report No. IR/L/FR/87/54, Health and Safety Executive, Buxton, UK, 1987. 10. Green, A. R., Apte, V. B. & Bilger, R. W., Flame spread along horizontal conveyor systems. In Proceedings of 23rd International Conference of Safety into Mines Research Institutes, US Bureau of Mines, Washington, DC, 1989 pp 127-36. 11. Delichatsios, M. A., Turbulent convective flows and burning on vertical walls. In 19th Symposium on Combustion. The Combustion Institute, Pittsburgh, Pennsylvania USA, 1982 pp 855-68.
The King's Cross Fire: Results and Analysis from the Scale Model Tests* K. Moodie & S. F. Jagger Fire and Process Hazards Section, Explosion and Flame Laboratory, Research and Laboratory Services Division, Health and Safety Executive, Harper Hill, Buxton, Derbyshire SK17 9JN, UK
A BSTRA CT A series of fire development trials carried out on one-tenth and one-third scale models of the King's Cross escalator shaft ticket hall are described. The construction and instrumentation of these facilities are considered and an analysis of the principal data is presentod. The implications of the measurements for scaling considerations are discussed, and possible full-scale, ]ire development scenarios are outlined.
1 INTRODUCTION The computer simulations of the King's Cross fire, carried out under contract to the Health and Safety Executive (HSE) by A E R E Harwell, t.2 suggested that the flow in the region of a heat source in the channel of the escalator did not rise vertically upwards, but instead was confined by the updraught in the channel which dominated the flow structure. A parametric study of the escalator duct flow and of the relevant scaling criteria, carried out on our behalf by Cambridge Environmental Research Consultants, 3'4 showed that the up&aught and the consequent channelling of the flow were aerodynamical effects governed by the buoyancy flux of the heat source. They also showed that Froude number scaling, as discussed by De Ris, s would, in the circumstances, provide representative modelling of the parameters * Paper presented st the I. Mech. E. seminar 'The King'sCross Underground Fire: Fire Dynamics and the Organisation of Safety', 1st June 1989. 83
O 1991 Crown Copyright.
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governing the fire growth, and that modelling at ~-scale would provide results of value in indicati~.~ full-scale conditions. Thus, a series of scaie~ open-channel and model tests were undertaken in order to further understand the fire dynamics. The tests involved (a) 100-mm2-section open channels, (b) ~o-scale models of the Piccadilly shaft with partial roofing, (c) ~3-scale sections of open-channel escalator, and (d) a fully roofed model of the upper (passenger) section of the Piccadilly shaft connected to the front section of the ticket-hall complex. The principal results from these tests are considered in this paper. A fuller description of the tests themselves can be found in Refs 6-8.
2 OPEN-CHANNEL (100-mm2 CROSS-SECTION) TESTS The fire development was assessed on 2-m-long open channels, which were either of plywood construction lined with corrugated cardboard or
Fig. Is.
Flame front advancing up an open channel ahead of the burning region.
Results from the scale model tests
85
of various types of plywood without any lining. The channels were inclined ~t 30° and set alight at a point near their bases. The tests were all conot~cted in still air conditions. In some tests, thermocouples were fixed at the top of the channel to record the gas temperatures as the fires progressed into the ticket-hall area. The progressions of the fires were recorded using video and still cameras. These were then used to obtain the velocities of the flame fronts as they progressed up the open channels. The results indicated that the flame front from a fully developed fire across the escalator advanced upwards at an exponentially increasing rate, emerging with velocities of about 2m/min. In all of the tests the advancing flame fronts were observed to remain virtually within the channel and to extend ahead of the visible pyrolysis front, the extent of which seemed to vary with the length of the burning zone. This is illustrated in the sequence of three photographs shown in Fig. 1, taken at 3-s intervals. They also illustrate the inherent nature of the flame structure when it is lying in the channel, namely to curve inwards towards the centre of the channel from the two vertical sides.
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Position of flame front 3 s later.
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K. Moodie, S. F. Jagger
Fig. 1¢. Position of flame front after another 3 s.
FIIg. 2. Sustained jet of flame from end of channel.
Results from the scale model tests
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When the flame fronts reached the end of the channels they emerged with sufficient momentum parallel to the channel to produce a sustained wall jet effect on any simulant horizontal ceiling in that region, as illustrated in Fig. 2.
3 PARTIALLY ROOFED CHANNEL (~o-SCALE) TESTS Some of the open-channel tests were repeated, but with the addition of a partially curved ceiling (90° segment) made from a thin ~etal sheet~ together with a longer test section of some 4 m. The escalator, facia and decking were constructed from corrugated cardboard mounted onto a plywood backing, the treads and risers being made to scale from 1-mm-thick cardboard. For some tests the ceiling was also lined with cardboard. In all tests a single source of ignition was used, some 2 m down from the top of the model and on the right-hand side of the models. Video records only were made of these tests, which again showed near exponentially increasing rates of fire growth, similar to those obtained from the open-channel tests. Numerical comparisons with equivalent open-channel results suggested that the roof was accelerating the rate of the fire growth. Of the three tests carried out with a cardboard lining on the roof, in only one did the cardboard catch fire prior to flames from the burning escalator arriving in the simulated ticket-hall area. This happened just prior to the flames reaching the top of the escalator channel, and did not influence their initial entry into the ticket hail. Some additional tests on a 0.4 m length of 100-mm2-section channel, lined with corrugated cardboard, showed that the flames did not lie down to the extent observed in longer channels, and as a consequence the rate of flame propagation was slower. A test on a 2-m-long, ~-scale, wood-lined channel gave a similar result. These two results suggested that a downstream channel length to channel width ratio of not less than 10:1 appeared to be necessary before a 'trench' effect was established.
4 OPEN-CHANNEL (~-SCALE) TESTS Three tests were carried out on 30°-inclined, X3-scale models of the Piccadilly tunnel escalator without roofing. The models were lined with wood attached to 0.5-mm-thick, metal backing sheets, which formed the treads and risers, the skirting board and the balustrades. The length
88
K. Moodie, S. F. Jagger
of wooded section was approximately 2 m. However, for the second and third tests the test section was extended to 4 m by the addition of a 2-m-long, metal channel. The balustrades and skirting boards were lined with two layers of 6-mm-thick, hardwood-faced plywood. The treads and risers were similarly constructed, but, in addition, hardwood cleats were attached to the top of the treads. The fires were started at the base of the test piece, across the full section, by igniting wood cribs. A limited number of heat-flux, temperature and weight-loss measurements were made during these tests, together with video and stills records. The video record of the first test showed that the flames rose nearly vertically and did not lie down within the confines of the channel. Consequently, the flame front moved relatively slowly and took some 5 rain to reach the end of the channel. In the second and third tests, some 3-3-5 min after initiation and over a period of a few seconds, the flames switched from being near vertical to laying down in the trench. Less than I min later (20 s in the third test, for which the wood was drier) the flames had reached the end of the wooden section of channel, and within a further 30 s (both tests) flames were seen flickering out of the end of the metal channel. The total received heat flux as measured on a balustrade was in the order 60kW/m 2. The temperature measured in the centre of the channel was in excess of 600 °C, and the average heat output was some 90 kW per metre length of escalator.
5 MODEL TESTS (t-SCALE) WITH THE SHAFT AND TICKET-HALL COMPLEX A D D E D
$.1 General description Finally, a series of four fire growth tests were carried out on a length of escal~,~r that formed part of a ~-scale model of the passenger side of the Piccadilly shaft and the ticket-hall complex extending back to the ticket office. A 10-m-long section of escalator was modelled and placed at an angle of 30° inside the scaled shaft. The escalator was placed on the left-hand side of the shaft (when viewed from the bottom) in a position corresponding to that of escalator No. 4 of the Piccadilly shaft. At the rear of the ticket-hall model there was a plenum chamber, separated from the ticket hall by a partition, which simulated the blockage effect of the ticket office. From the plenum chamber, combustion products were exhausted via a single vertical stack.
Results from the scale model tests
89
The tunnel ceiling was not painted for the first test, but for the second and third tests it was painted with two coats of white emulsion; however, these were not considered representative of the paint layers existing at King's Cross during the time of the incident. In the final test, the ceiling was painted to a London Underground specification intended to reproduce the surface spread of flame characteristics of the actual ceiling. During the experiments, detailed measurements were made of temperatures, weight loss, heat fluxes and gas velocities at various locations. In addition, the concentrations of oxygen, carbon dioxide and carbon monoxide were recorded continuously in the exhaust stack. Comprehensive photographic records of the tests, involving up to five video and five 'stills' 35-mm cameras, were also taken.
5.2 Constructional details The 3X-model was constructed upon an existing hillside on the Buxton site. This provided some degree of protection from the weather, and allowed for a more rigid scaffolding framework to be used. There was also the presence of a level area which could be used for the ticket-hall section of the replica. A scaffolding framework was used as the base for the model; a fiat steel floor was placed up the 30 ° slope and a tunnel of 1-mm-thick, galvanized steel sheeting erected over it. The tunnel was in an inverted U-shape, the radius of which being 1.15 m. The escalator section was placed at the appropriate height to give the same roof curvature as that in the Piccadilly shaft. External views of the completed model are shown in Figs 3 and 4. At the top of the escalator shaft a 'throat' section led into the ticket hall. This was constructed from steel sheeting; it was approximately 4 m wide and 3 m deep, with a height of 0.8 m. The escalator shaft was offset to the appropriate side of the ticket hall. At the rear of the ticket hall the wall had two airways built into it. These were the full height of the ticket hall and were square. They were positioned to correspond with the spaces on either side of the ticket o~ce at King's Cross. No attempt was made to model the ticket booths in the ticket hall. The plenum chamber, constructed behind the rear wall of the ticket hall, had an exhaust stack for the first two tests, some 3 m high and 0.5 m in diameter, t h e stack was changed to one with a 2 x 1 m, rectangular cross-section for the final two tests. The escalator model itself was identical in construction to the ~-scale sections already described, but with the addition of deckboards on
90
K. Moodie, S. F. Jagger
Fig. 3. A general view of the model situation.
either side just below the tops of the balustrades and a vertical facia panel on the left-hand wall. The 10-m length of staircase represented about two-thirds of the scaled length of the full-size escalator. In the throat section the horizontal sections of all three escalators were replicated. The upper 7-m section of the escalator was independent of the lower
Fig. 4. The rear of the model, showing the ticket-hall covering the exhaust stack.
Results from the scale model tests
91
Fig. Sa. A view of the escalator from the foot of the shaft. 3-m section, so that it could be mounted' on load cells and its weight determined prior to, and during, a fire. The facia panels were attached to the tunnel side walls and not to the weighted escalator section. On the right-hand side of the wooden escalator, the area corresponding to escalator No. 5 was covered over at the deckboard level with steel sheeting. The area corresponding to escalator No. 6 was left free
l~g, 5b, A view of the ticket hall looking towards the throat and shaft.
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K. Moodie, S. F. Jagger
as a walkway. A rail for a moving camera position was positioned along the centre line of the tunnel (subsequently removed after the first two tests). The view looking up the escalator tunnel is shown in Fig. 5a and that of the ticket hall area in Fig. 5b.
S.3 Experimental instrumentation A typical instrumentation layout is shown in Fig. 6. On the escalator section there were three main 'stations' for instrumentation. These were in fixed positions relative to the top of the inclined section of escalator, at 0.25, 2.5 and 4.75 m, respectively, from the top. Usually arrays of eight thermocnuples, Pitot-static tubes and pairs of heat-flux meters (radiant and total) were placed at each of these positions. In the ticket-hall area there were three thermocouple arrays. Additional thermocouples were also embedded into the balustrades or attached to the facia boards and ceiling from some tests. The Pitot-static tubes were placed at different locations for each test in order to measure air velocities in the shaft and in the exhaust stack. This also contained a gas sampling point for analysis of the combustion products. The inflow to the shaft was monitored using several hot-wire anemometers. A four-point, load-cell system was used to support and weigh the ?-m escalator section. All of the instrumentation was connected to a data logging and processing system based on a
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minicomputer (approximately 100 channels), which sampled all of the data at a rate of once per second.
5.4 Experimental procedure Once installed, the escalators were kept warm and dry and, about 1 h prior to a test, the moisture content of the balustrade panels was measured. The average levels were about 11%, with a 4% variability. The fires were started at the appropriate scaled distance clown from the top of the escalator. In the first two tests, five wood cribs placed across the escalator were used as an ignition source, but in the third a narrow layer of grease across one step was used instead. In the fourth test, narrow grease layers across half of two escalator steps were used. The computerised logging of the data was started at the same time as the fire, together with the 35-mm photographic and video equipment. The fires were extingushed a few minutes after flames first entered the ticket hall, in order to prevent excessive fire damage to the model. The fire damage was surveyed and photographed after each test, and in all cases had reproduced the salient features of the fire damage as described in Ref. 9.
$.$ Results snd discussion and results 5.5.1 Visual observations The visual records from the first three tests showed that, following ignition (at about the first 0.5-m marker), flame was soon established across the full width of the channel, with the flame front tending to remain low in the channel as the fire began to develop, as shown in Fig. 7. The fire then progressed up the escalator channel at an exponentially increasing rate of growth. The fire in the fourth test, because it was ignited on the right-hand side only, took longer to establish itself across the whole of the escalator section than in the previous three tests (some 9min), but thereafter it behaved in a similar manner. The rates of advance of the flame fronts, taken from the visual records of all four tests, are shown in Fig. 8. During the first two tests, ignition of the left-hand balustrade occurred within 2 min of the start of the fires. This was some 4 and 9 min, respectively, in the third and fourth tests. In all tests the left-hand facia boards caught fire some 20-40s later, at around the 1-2-m markers. At the same time the flame fronts in the channels were well ahead of this point, particularly so in the last test. In general, the flames on the burning facia remained nearly parallel to the escalator.
94
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(b) Fig. 7. The fire just after ignition of the left-hand side (test 4): view from (a) the foot of the shaft and (b) the right shaft camera.
However, flames from burning near the top of the escalator shaft appeared to corkscrew over the tunnel roof both before and after the flames emanating from the escalator channel had entered into the ticket hall. At all times, and in all of the tests, the flame fronts within the escalator channel remained ahead of the pyrolysis fronts on the
Results from the scale model tests
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balustrades, which were themselves ahead of the pyrolysis fronts on the facia boards. During each of the four tests, it took about 80, 55, 65 and 45 s, respectively, after ignition of the left-hand balustrades until a continuous ejection of flame into the ticket hall and across the ceiling began. The emergence of the jet of flame (test 4) as seen from the right and left side of the ticket office is shown in Fig. 9. This was preceded by a visible glow on the tunnel ceiling as the flame front progressed upwards and the occasional flickering of flames out of the shaft. In the final two tests the flickering of flames prior to continuous ejection lasted for around 9 s, compared with 20 s in the first two tests. 5.5.2 Air temperature measurements The vertical temperature profiles obtained on the centre lines at all three measuring stations showed that the peak temperatures were between 800 and 1000 °C within the escalator channel. The corresponding near-ceiling temperatures were lower in all cases, but increased progressively in magnitude towards the top of the escalator shaft. The results indicated that sharp temperature fronts were observed at all three measuring stations, which coincided with the visible flame fronts advancing up the channels. Some of these features are illustrated in Fig. 10, which shows, for all four tests and at the lowest measuring point, the maximum gas temperatures in the escalator channels and the gas temperatures 25 mm below the tunnel ceiling. Figure 10 also illustrates the degree of potential preheating that could occur at the tunnel ceiling prior to the arrival of the flame front. The temperature profiles
96
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(b) Fig. 9. The continuous ejection of flame into the booking hall, seen from (a) the left-hand side of the hall and (b) the angled window on the right-hand side of the hall.
confirmed that, as the flame fronts approached the ticket hall, they were advancing in some cases at speeds in excess of 8m/min, in agreement with the visual observations. The peak rates of temperature change at the top of the escalator were in the region of 40-60 °C/s, and these occurred within a matter of seconds following the continuous eruption of flames into the ticket hall.
Results from the scale model tests
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The temperature profiles taken near the exit of the horizontal section of the escalator channel showed similar peak temperatures to those observed within the sloping section. However, the positions of the peaks had moved to a region above the top of the channel. In general, these temperature fronts were more diffuse than at the measuring stations within the sloping sections of the tunnel and were also in accord with the counter-clockwise spread of the combustion products observed from the visual records. The temperature profiles on either side of the ticket office suggested that continuous thermally stratified layers were created below the ceiling following the arrival of the flame fronts in the ticket hall. Prior to this, the temperatures on either side of the ticket office built up gradually. The temperature rises were lower on the left side of the ticket office than on the right, indicating that the majority of the gas flows in each test were to the right side of the ticket office. Qualitatively, these were in agreement with the computer predictions given in Ref. 1. The rates of temperature rise on the left side of the ticket office, 0.2 m below the ceiling, were about 20 °C/s during the last two tests and were lower prior to and after the arrival of the flame fronts than at the top of the escalator. 5.5.3 Componenttemperatures The peak surface temperatures on either side of the sloping part of the balustrades were similar to those of the corresponding centre-line air
98
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temperatures in the escalator channels, and they can be used to infer the progress of the pyrolysis zone in the wood. This advanced at a similar rate up either side, although progress appeared to be quicker on the left than on the right during the development stage of the fire. The results also showed that the pyrolysis fronts always lagged behind the flame fronts, but to a progressively greater degree near the top of the escalator. When flames were halLway up the shaft the flame front was typically 1-2m ahead of the pyrolysis zone. The internal wood temperatures measured some 3 mm below the surface hallway down the escalator suggested that the pyrolysis frorJts would not penetrate to these depths prior to the flame fronts reaching the top of the escalator.
5.5.4 Heat fluxes The total heat fluxes received by the left-hand balustrades, facia panels and the tunnel ceiling were measured at various positions during each test. Typical measurements (from test 4) are shown in Fig. 11 for the positions near the top of the shaft shown in Fig. 6. The total heat flux to the ceiling was always much less than that received by the balustrades or facia panels immediately below. The peak, radiant heat fluxes to the ceiling and balustrades were roughly the same, about 25 k W / m 2. Thus, the substantial i~,crease in total heat flux to the balustrade and facia was 180
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due to an increase in convective heat flux, probably as a result of increased turbulence in the burning region, together with an increase in gas velocities especially near the top of the escalator. The total heat flux on the facia panel was about 10% less than that received by the balustrade. Such heat fluxes imply very rapid onset of pyrolysis in the presence of a flame, with typical preheat times of 10-20 s.
5.5.5 Stack exit conditions The gas analyses taken near the stack exit plane are shown in Fig. 12 for all but the third test~ during which no gas analysis was made. Thus, in general, the oxygen concentrations fell to about 10% by the time that the fires were extinguished, although, at the time that continuous ejection of flames into the ticket hall first began, the oxygen concentrations were typically 16% and the carbon-dioxide concentrations around 6%. The average stack exit temperatures were in the order of 400 °C, as measured mid-way up the stack where the combustion products were assumed to have fully mixed. There was no evidence of flames within the stack. The heat outputs of the fires as derived from the oxygen consumption suggested that they were typically about 0.6-0.8 MW at ~lq 8 /
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the time that the fires first entered the ticket hall, increasing to about !.5-2.0 MW just before the fires were extinguished. 5.5.6 Air velocity measurements The average air velocities measured in the trench below the fire zone were about 0-2-1.0 m/s, and increasing as the fires developed, although large-scale fluctuations were occasionally superimposed, typifying the atmospheric turbulence and gustiness prevailing during some of the tests. The escalator velocities above the fire zone, just below and parallel with the top of the handrails, were relatively low until the arrival of the flame front. The maximum velocities at the top of the escalator were between 2 and 4 m/s. However, the resultant velocity vectors may have been higher as the flows were corkscrewing at this cross-sectional position. The results also suggested that the trench velocities exceeded the ceiling velocities as the flame front traversed the measuring stations, but at the top of the shaft and in the throat of the shaft the ceiling velocities were the highest, suggesting that flames rose above the handrails in the upper parts of the shaft. The calculated, average mass flux up the shaft, based on the air velocities when flames first entered the ticket hall and just before the fires were extinguished, was about 1.3 kg/s. The corresponding mass flux up the trench was in the order of 0.03 kg/s. These figures suggest that the entrainment into the fires of air drawn up the trench represented a minority contribution; therefore, by implicatiotl, most entrainment was through the top surface of the fires.
6 PARAMETRIC ANALYSIS The data can be considered in the context of the Froude criteria for modelling firess and a parametric flow analysis of the type described in Ref. 3. The flow upstream of the fire is largely determined by the buoyancy flux, this being directly related to the heat release rate. Thus, measurements of this parameter, used in conjunction with simple plume models can infer expected levels of temperature both across and along the duct velocities. Such calculations largely confirm measurements of temperatures and flows observed during the tests. The Froude modelling technique permits some predictions of conditions at other scales. It is based on maintaining geometric similarity, the flow Froude number and the ratio of time-averaged mass burning rate
Results from the scale model tests
101
to convective mass flux per unit area. On this basis temperature will be independent of scale, velocity will scale as L ~j2 (L is some characteristic dimension) and heat release rate should scale as L s~. However, since transient phenomena involving solid-state heat transfer do not obey the same scaling laws and Froude modelling violates proper scaling of radiation, convection and fuel bed geometry, difficulties are encountered in specifying times for fire development and the heat release rate is not easy to control. However, it is difficult to make progress unless the heat release rate is scaled as L sl2. Consequently, at emergence of flame into the ticket hall, the model, heat release rates suggest a fire with a heat output of 15-25 MW. Similarly, gas velocities at the top of the escalator extrapolated from the I-scale simulations are predicted to be in the range 8-12 m/s. Measured temperatures and their rates of rise are directly comparable. The progress of the fire on a larger scale is rather more difficult to predict, as this is dependent upon heat transfer from the preceding flames and hot gases, which is not properly accounted for in the model. The data suggest that convection is the dominant mode of heat transfer at the scale of the experiments. However, an examination of measurements at both ~- and 13-scale imply a linear dependence of radiative transfer with scale. This would mean that, even if convection were to be independent of model scale, then it would still be the principal, though less dominant, mode of heat transfer. There is some evidence to suggest that, since velocities scale with L 112, then so does the convective heat-transfer coefficient. Again, the experiments suggest a near-linear scale variation of convection. This is difficult to reconcile with other data determined on what are essentially smooth surfaces, and it i.,; ~ o r e probable that the measurements on the smallest models are strongly influenced by Reynolds number effects and do not form sufficient basis for extrapolation. Thus one possible way to make progress is to follow De Ris 5 and assume that the convective heat flux scales as L ~. Considering the fuel as thermally thick, an expression for the flame spread velocity (Vp) defined as the advance of the pyrolysis front, has been given by Green et aL : ~°
where p is the material density, cv the material specific heat, k thermal conductivity, T the firepoint temperature, T~ the ambient temperature, q the incident heat flux, and (xs - x v) the heated, unburnt fuel surface, which, for convection only, scales as L 2, since q---L 1/2. Therefore, it follows that the time (At) for the fire to spread between two equivalent
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K. Moodie, S. F. Jagger
points is inversely proportional to ~ e model scale, so that fire spread and development times at full-scale would be much reduced. Similarly, if the radiative flux scales with L, as can be tentatively implied from model data, then the times for fire spread will be further reduced, by a factor of up to 50%. Additionally, the enhanced flame stretch up the channel due to the growing fire will tend to accelerate the flame spread and reduce fire spread times. An initial appraisal of the lengths of the pyrolysis zones and heat release rates indicate that the length of such flames follow a power law relation with fuel burning rate with an index which, as would be expected from the nature of the situation, falls between that currently observed for a turbulent wall flame and that for a free diffusion flame from a horizontal fuel bed. The situation is somewhat analogous to the vertical spread of flame on a wall, a situation which, folMwing the treatment of Delichatsios, ~1 seems amenable to a relatively simp|e analysis. A similar approach for the trench effect, incorporating experimentally derived relationships for flame length, extent of the pyrolysis zone and heat transfer coefficients (for example, with heat release rates), shows some promise as a predictive tool; in addition, it gives the explicit dependence of the rate of fire spread on such parameters as material and channel geometry.
7 CONCLUSIONS
The experimental work described confirms that a mechanism for rapid fire spread exists, and it occurs primarily within the confines of the escalator trench. This is in accordance with the computational studies, 1 the modelling analysis3 and with evidence presented to the Inquiry both in June and August 1988. These scaled model tests provide both a qualitative picture of the fire spread mechanism and quantitative data from which extrapolations of the likely rate of fire spread at full-scale can be inferred, taking into account the increasing rates of radiative and convective heat transfer to the combustible material observed in the tests. They also provide basic data for modelling the rate of advance of the flame front based on flame spread up thermally thick inclined surfaces. The last test, in which the fire was started on the right-hand side, was in our opinion in good qualitative agreement with the events on the night, particularly with regard to the fire growth after the left-hand side of the escalator was alight.
Resultsfrom the scale model tests
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ACKNOWLEDGEMENTS The authors wish to acknowledge the considerable amount of assistance given by other members of the Research and Laboratory Services Division of the HSE in undertaking this investigation, in particular other members of the Fire and Process Hazards Section.
REFERENCES 1. Simcox, S., Wilkes, N. S. & Jones, I. P., Fire at King's Cross Underground Station, 18th November 1987: Numerical simulation of the buoyant flow and heat transfer. Report No. AERE-G 4677, Harwell Laboratory, Harwell, UK, 1988. 2. Simcox, S., Wilkes, N. S. & Jones, I. P., Fire at King's Cross Underground Station, 18th November 1987: Numerical simulation of the effect of train movements. Report No. AERE-G 4782, Harwell Laboratory, Harwell, UK, 1988. 3. Britter, R. E. & Linden, P. F., Fluid mechanics and physical modelling of King's Cross Fire. Report No. FM 17/88, Cambridge Environmental Research Consultants, Cambridge, UK, 1988. 4. Britter, R. E. & Linden, P. F., Fluid mechanics and physical modelling of King's Cross Fire: Flow in the booking hall. Report No. FM 19/88, Cambridge Environmental Research Constltants, Cambridge, UK, 1988. 5. De Ris, J., Modelling techniques for predictions of fires. In Applied Polymer Symposium No. 22, John Wiley and Sons, New York, 1973. 6. Moodie, K., Jagger, S. F., Bettis, R. J. & Beckett, H., Fire at King's Cross Underground Station, 18th November 1987. Part 11: Scale model fire growth tests. RLSD Incident Report No. IR/L/FR/88/22, Health and Safety Executive, Buxton, UK, 1988. 7. Jagger, S. F., Willoughby, D. B., Beckett, H., Bettis, R. J. & Moodie, K., Fire at King's Cross Underground Station, 18th November 1987. Part 17: Further one-third scale fire growth tests. RLSD Incident Report No. IR/L/FR/88/37, Health and Safety Executive, Buxton, UK, 1988. 8. Jagger, S. F., Beckett, H., Harper, P. J., Regan, N. J. & Willoughby, D. B., Fire at King's Cross Underground Station, 18th November 1987. Part 16: The LUL one-third scale fire growth test. RLSD Repayment Contract No. IR/L/FR/88/38, Health and Safety Executive, Buxton, UK, 1988. 9. Moodie, K. & Wharton, R. K., Fire at King's Cross Underground Station, 18th November 1987. Part 1: Damage assessment. RLSD Incident Report No. IR/L/FR/87/54, Health and Safety Executive, Buxton, UK, 1987. 10. Green, A. R., Apte, V. B. & Bilger, R. W., Flame spread along horizontal conveyor systems. In Proceedings of 23rd International Conference of Safety into Mines Research Institutes, US Bureau of Mines, Washington, DC, 1989 pp 127-36. 11. Delichatsios, M. A., Turbulent convective flows and burning on vertical walls. In 19th Symposium on Combustion. The Combustion Institute, Pittsburgh, Pennsylvania USA, 1982 pp 855-68.