Control of smoke flow in tunnel fires

ELSEVIER

PII:

Fire Safety Journal 25 (1995) 305-322 Copyright (~ 1996 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0379-7112/95/$09.50 S0379-7112(96)00007-0

C o n t r o l o f S m o k e F l o w in T u n n e l Fires

Yasushi Oka a & Graham T. Atkinson "Department of Safety Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan bHealth and Safety Laboratory, Health and Safety Executive, Harpur Hill, Buxton, Derbyshire SK17 9JN, UK (Received 3 October 1995; revised version received 15 December 1995; accepted 9 January 1996)

ABSTRACT This paper concerns the specification of the longitudinal ventilation necessary to prevent upstream movement of combustion products in a tunnel fire. Experiments carried out in a model tunnel have revealed significant limitations on the utility of existing empirical expressions for the critical velocity. Simple formulae with a wider range o f applicability are presented. The method of scaling model results has been tested by comparison with large-scale test data. The effects of changes in the shape, size and location of the fire on the critical velocity have been investigated. Copyright © 1996 Elsevier Science Ltd.

1 INTRODUCTION The control of smoke flow during a tunnel fire is often an important part of e m e r g e n c y planning. The Channel Tunnel is an example of a tunnel system whose fire safety d e p e n d s on the operators' ability to keep evacuation routes clear of smoke using emergency, reversible ventilation. The basic scientific problem is the specification of the longitudinal ventilation rate necessary to prevent upstream m o v e m e n t of combustion products in a fire. T h e r e have been a n u m b e r of experimental and theoretical studies of this basic problem. 1-7 Previous research work has recently been reviewed by Lea et al. ~ The results most widely used to aid design are those of T h o m a s t'2 and Heselden. 3 Essentially, these authors suggest that the critical ventilation velocity d e p e n d s on the cube root of the heat release per unit width of the tunnel. This 305

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Y. Oka, G. T. Atkinson

conclusion is supported by dimensional analysis and by some experimental results. It is important to note, however, that the above dimensional arguments are only valid where the size of the flames is significantly smaller than the height of the tunnel and that the experimental fires used were also of this type. Unfortunately, experimental vehicle fires have shown that very large rates of heat release are possible. ~-12 For example, a single heavy goods vehicle carrying furniture can reach a rate of heat release in excess of 100 MW. ~3 Such a fire will produce flames with a length much greater than the height of most tunnels, and it is clearly important to determine the variation of critical velocity for such fires. A recent series of fire trials performed at the Health and Safety Laboratory (HSL), Buxton, UK 14"~5 showed that, as the fire size increases to the point where the flame length exceeds the tunnel height, the critical velocity becomes much more weakly dependent on the heat release rate than predicted by the one-third power law. This finding has been supported by computational work at the same laboratory, j~ The objective of this work was to accurately and systematically measure critical velocities for a model tunnel fire and to test the scaling procedures used in applying such results to full-scale fires. A large range of fire sizes was used, including flames with lengths much smaller and much larger than the tunnel height.

2 EXPERIMENTAL PROCEDURE To allow direct testing of scaling relationships, the model tunnel cross-section matched that of the full-scale fire gallery at HSL Buxton, used for the trials mentioned above. 14~5 The internal section, shown in Fig. l(b), is the BS 227 colliery arch, comprising a semicircular head on walls splayed out at 7 ° to the vertical. The majority of the model was formed from perspex (PMMA) with a thickness of 6.25 mm. Parts of the tunnel closest to the fire were made from 18 SWG (1.25 mm thick) stainless steel. Metal sections of the tunnel were cooled using a water spray throughout the tests. The height from the floor to the apex of the roof was 244 mm and the width at floor level was 274 mm. The cross-sectional area was 0-0569 m z. The total length of the tunnel was around 15 m. The inlet flow was channelled through a straight 4 in. (101-6 mm) steel pipe fitted with an orifice plate [Fig. l(a)]. The construction of the orifice plate and the inlet conditions were in accordance with BS 1042. The flow was driven by a jet of compressed air acting as a m o m e n t u m

307

Smoke flow in tunnel fires

(a) upstream thermocouples

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pump. Relatively coarse adjustments in the ventilation rate could be made by varying the compressed air supply. Fine adjustments in velocity, by amounts as low as 0.3%, were possible using an iris, partially obstructing the inlet to the m o m e n t u m pump. Orifice plate pressures were measured and recorded using a differential pressure transducer and data logger and, separately, using an inclined spirit manometer. The calibration of the latter was traceable to the UK national standard and the results quoted are all derived from this source. The logging of data provided a convenient means of matching changes in the imposed ventilation to observed changes in the temperature upstream caused by 'backing up' of the combustion products. The orifice plate provided a measure of the total volumetric flow. The velocities reported were calculated by dividing the volumetric flow by the model's cross-sectional area. The upstream flow of hot gases was detected using K-type stainless steel sheathed thermocouples with a diameter of 0-25 mm. The thermocouples were fixed 10 mm below the roof at distances equal to

Y. Oka, G. T. Atkinson

308

TABLE 1 B u r n e r Size and Location B u r n e r no. Burner Burner Burner Burner Burner Burner Burner Burner Burner

1 2 3 4 5 6 7 8 9

Dimension

Location

10 cm x 10 cm 4 cm x 14 cm 4 c m x 14 cm 4 m m x 236 m m 7 cm × 8 cm 7 cm x 8 cm 7 cm x 8 cm 7 cm x 8 cm 7 c m x 8 cm

Centre Centre Centre Centre Centre Beside wall 130 m m height 130 m m height 130 m m height

Notes

L o n g e r side set across tunnel width L o n g e r side set along tunnel axis L o n g e r side set across tunnel width L o n g e r side set along tunnel axis B u r n e r set 10 m m from wall 49 cm e blockage (9%) 121-4 cm 2 blockage (21%) 212-4 cm 2 blockage (37%)

1, 3, 5 and 10 tunnel heights upstream of the fire [Fig. l(a)]. The progress of a hot layer upstream to engulf a thermocouple was obvious from temperature records. Convective heating of the thermocouples could in all cases be easily and unambiguously differentiated from radiative effects, which were small and did not vary significantly during small changes in ventilation. The specification of burner employed is given in Table 1. Propane gas was used as the fuel. The flow rate was varied between 0-3 and 20 I/min. Using the scaling procedures described below, these fire sizes correspond to fires of between approximately 2 and 150 MW in a tunnel with a diameter of around 5 m. The conditions under which hot upstream flow is driven by a major vehicle fire can therefore be investigated. Preliminary tests showed that pre-heating of the tunnel roof upstream caused significant increases in the critical velocity. This is due to the formation of a hot layer below the roof, as heat is transferred from the heated tunnel to the inflow. The fluid density and dynamic head are reduced in the upper part of the inflow, reducing its effectiveness to prevent 'backing up'. This effect may be of some practical importance as emergency ventilation systems may be required to clear smoke from tunnels after the tunnel walls have been significantly warmed. In this paper, however, care was taken to ensure that the upstream section of the tunnel and the inflow were at the same temperature. For each fire source used, the critical velocity to prevent 'backing up' to 1, 3, 5 and 10 tunnel heights was separately determined. Figure 2 shows typical results. The velocity to prevent 'backing up' to other distances in the range 0-10 tunnel heights can be obtained by curve fitting to data of this kind. Where 'the critical velocity' is referred to

Smoke flow in tunnel fires

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below without qualification, a velocity to prevent any 'backing up' past the fire is implied, i.e. a value obtained from the measured data by extrapolation to zero tunnel height.

3 SCALING The technique used to scale model results for comparison with larger-scale tests or for use in the design of tunnel ventilation systems is known as 'Froude' scaling. The aim of the scaling method is that, given values of heat release and inlet velocity in a model tunnel which produce a flame shape and temperature field of interest (e.g. the onset of 'backing up'), the scaling method should identify the values of heat release and velocity that would produce a similar flow in the full-scale tunnel. The method can be conveniently described in two stages.

3.1 Scaling of heat release rate The model and full-scale heat release rates Q,,,,,j~, and Q,.. .....,~. are related by eqn (1) ~7 L model Qm,,dc, -- ( ~[~dl ~I~_' ")~5/,(1) QluII sculc

where tt,,,,,u~j and tt, u, ..... ,~ are the respective length scales, in this case the heights of the model and full-scale tunnels. It is relatively easy to see that the scaling must have this form if one considers the low velocity

310

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limit. In this case, dimensional arguments suggest that the distance from the fire at which the temperature takes a given value is proportional to

Q 2/5. 3.2 Scaling of velocity The model and full-scale inlet velocities V,,,,jc~ and Vfu,.... ~c are related by eqn (2). TM

Vmodel

__(Lmodel~

1/2

The form of this scaling relation and a theoretical basis for the results described below are discussed in Appendix A.

4 RESULTS Figure 3(a) shows combinations of heat release and velocity which lead to 'backing up' to 3 tunnel heights in the model and close to this value at the full scale. The values of heat release and velocity have been divided by the length scale to the powers 5/2 and 1/2 respectively--in line with Froude scaling. The fact that the loci of these two sets of points are near coincident means that Froude scaling is a useful means of applying measurements of critical velocity in the model to the larger scale. Figure 3(b) and (c) show similar comparisons for 'backing up' to around 5 and 10 tunnel heights. To make the comparisons shown in Fig. 3, it was necessary to find the average velocity in the full-scale tunnel. This was not measured in the full-scale tests, but had to be estimated from the velocity measured near the centroid of the tunnel as: V~...... ~ = 0"757V,.~,,~r,,id

The value of this coefficient (which depends on the tunnel section) was obtained by making simultaneous measurements of volumetric flow and velocity near the centre of the tunnel in the model. Figure 4 shows how the critical velocity necessary to prevent 'backing up" to various distances varies with heat release rate in the model experiments with burner 1. Also shown is Thomas's empirical formula for the critical velocity:

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312

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where g is the gravitational acceleration, Q is the rate of heat release, H is the tunnel height, p. is the density of the inflow, T, is the inlet temperature, Cp is the heat capacity of the inflow at constant pressure, and A is the cross-sectional area of the tunnel. Figure 4 shows that for low rates of heat release, the critical velocity does vary as the cube root of the heat release rate. At higher rates of heat release, the dependence on the heat release rate falls off rapidly. These results can be more easily applied to different scales if plotted as the dimensionless variables suggested by Froude scaling (Fig. 5): Q*=

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The results suggest the following replacement for the Thomas relationship (for this tunnel section and burner): V* = 0.35(0.124) ,/3(Q.),/3 for Q* < 0-124

(4)

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adjustments to these formulae to be expected when changes are made to the burner's size, shape and location or a blockage is introduced into the tunnel near the fire. In all cases, the variation of the critical velocity with heat release is similar. This fact is illustrated in Figs 6(a) and (b). Both plots show critical velocities determined for a range of different burners used. In Fig. 6(a), Q* and V* are plotted directly. In Fig. 6(b), values of V* divided by the m a x i m u m value of V* recorded for each burner are plotted against Q*. The data coincide quite closely, suggesting the following general expression for the critical velocity: V* = V*ax(0-12)-'/3(Q*) "s for Q* < 0"12

(5)

V* = V*ax for Q * > 0-12 The values of V%x for all of the burners used are given in Table 2.

5 E F F E C T S OF B U R N E R G E O M E T R Y A N D T U N N E L BLOCKAGE Figure 7 shows the effect of burner shape on the critical velocity. Burners 2 and 3 have the same rectangular shape with their longer sides set across and along the axis, respectively. The critical velocity is lower for the burner that occupies the larger proportion of the width of the tunnel. This effect is more p r o n o u n c e d with burner 4 which is a line burner occupying close to the full tunnel width.

314

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The explanation for this effect is that the flame represents a substantial soft blockage to longitudinal ventilation. The velocity is accelerated in the vicinity of the fire. The larger the fraction of the tunnel width occupied by the fire, the larger the blockage and consequent acceleration. Acceleration of the air flow around the fire also leads to much larger flame tilt angles. For example, for burner 1 at the critical velocity with a fuel supply of 5 l/min, the predicted tilt

Smoke flow in tunnel fires

315

TABLE 2 Value of V*.x for all Burners Used

V*max

Burner no.

1 2 3 4 5 6 7 8 9

Burner Burner Burner Burner Burner Burner Burner Burner Burner

0.354 0.318 0-352 0.312 0.353 0.357 0.383 0.326 0-22

angle from the vertical for an unconfined flame is 52°. ~v Photographs taken in a transparent tunnel section showed that in the model tunnel, the tilt angle was around 78 °. Figure 8 shows the effect of further changes in burner location. Comparing results for burners 5 and 6 shows that moving the fire from the centre of the tunnel to close to the tunnel side wall has little effect on the critical velocity. Raising the burner surface to 130 mm above the tunnel floor produces a significant increase in the critical velocity. Presumably this is because the vertical distance over which the plume is deflected is reduced and the acceleration of the inflow caused by the fire is reduced. There is a small decrease in the value of Q* at which the

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316

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(a) Burner 7

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in a vehicle fire. In a tunnel of this shape, a vehicle with a height of around half the tunnel height, occupying around 12% of the tunnel cross-section, should cause a decrease of around 15% in the critical velocity. If the vehicle occupies 32% of the tunnel cross-section, the critical velocity should be reduced by 40-45%.

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318

Y. Oka, G. T. Atkinson

6 CONCLUSIONS 1.

2.

3.

4.

5. 6.

The measurements of critical velocity obtained with the model tunnel can be used to obtain good estimates of the critical conditions in larger tunnels if Froude scaling is applied. Model experiments have potential as a general means of investigating the problem of the backing up of smoke in tunnel fires. Further work to investigate the effects of tunnels with a width much greater than their height and tunnels at a slope would be of particular interest. Established correlations which assume critical velocities to be proportional to the cube root of heat release for all values of the heat release rate are in error where the flame length is similar to or greater than the tunnel height. New formulae have been developed to predict the critical velocity. These are simple in form and can be applied to small and large fires. The fire geometry has a relatively minor effect on the critical velocity. If the fire occupies a large proportion of the width of the tunnel, the critical velocities are reduced. Solid blockages near the fire produce marked decreases in the critical velocity. Froude scaling is particularly suited to the problem of backing up, which depends on the local balance of mean pressure and buoyancy forces close to the fire. Tunnel fire problems which depend critically on mixing processes, e.g. the development of extended upstream or downstream layers, will require different scaling procedures because other quantities, especially those controlling heat transfer are involved.

ACKNOWLEDGEMENT Y. O. would like to thank the British Council for their financial support and Dr B. Thomson, Prof. T. Ogawa and Prof. Y. Uehara for giving the opportunity to study abroad. The authors would like to thank Mr D. R. Bagshaw and Mr E. Belfield for their help in setting up the rig, Dr C. J. Lea and Dr S. F. Jagger for their useful suggestions and Mr A. J. Deall for his experimental help.

Smoke flow in tunnel fires

319

A P P E N D I X : A T H E O R E T I C A L BASIS F O R T H E F R O U D E S C A L I N G OF V E L O C I T Y A N D T H E G E N E R A L F O R M OF THE RESULTS In the Froude scaling method, model and full-scale inlet velocities are related as follows: Vmoac, ( Hmoa~, ) '/2 (A1) ~'~llscal~

\ H f u . scale /

It is possible to determine this relation purely from dimensional arguments, given that the heat release is to be scaled as H 5'2. A simple mechanistic analysis gives the same result and some insight into the variation of the critical velocity with heat release rate. Consider a fire in a tunnel with a very small longitudinal flow. The plume is very slightly deflected. The static pressure difference between floor and roof level above the fire is different to that far from the fire because of the buoyancy of the fire gases. This buoyancy head is Apbouyancy =

fo"

Ap(z)g dz

(A2)

where z is the vertical distance from the floor. Away from the turning region close to the roof (which only extends to a small proportion of the total tunnel depth), the form of the density difference can be estimated using results for a free plume. 19 The density difference takes a particularly simple form if the fire size is very small or very large. For a very small fire where the flame length is a small fraction of the tunnel height,

Q 2/3 Ap ~ -Z5/3 The buoyancy head becomes [from eqn (A2)]: Apbouyancy OC

,, 02,3

~

Q2/3 Apbouyancy OCH2/-~

dz (A3)

In Froude scaling, the heat release is scaled as Q oc H 5/2, so eqn (A3) becomes: Apbouyancy OCH (A4) for small Q.

320

Y. Oka, G. T. Atkinson

For a very large fire with continuous flaming to roof level, the temperature is roughly constant above the fire and the density differences becomes a constant value, Apf, for all z and Q. The buoyancy head becomes Apbouyaney ~c fOH Apf d z

(A5)

Apbouyancy~CH

for large Q. The plume deflection caused by a small longitudinal flow depends on the ratio between the buoyancy head and dynamic head of the longitudinal flow impinging on the plume. If this ratio is the same in model and full-scale systems a similar (small) plume deflection is to be expected. The condition for similar flows in model and full-scale is therefore

Substituting

APdyBamic] : I APdy..... ie APbuoyancyJ model L~ ] fullscale APdynami c = poV2/2 and Apbuoyancy ~ H (large

(A6) or small Q):

[G. scale

H ] model or

(A7) for large or small Q. This is the velocity scaling relationship in Froude modelling. Equations (A3) and (A5) also give some insight into the form of the variation of critical velocity with heat release. Consider the ventilation velocity required to deflect the plume through some small critical angle. The ratio of the dynamic head of the inflow to the buoyancy head above the fire must exceed a critical value Ccr~,:

[ Apdynamic] Apbuoyancy~ c,i,ical > Ccrit

For small fires this becomes (using eqn (A3) and APdy..... i, =

[

V2H 2/3]

(A8)

poV2/2)

Smoke flow in tunnel fires

321

or [Oq

1/3

V > C1/2crit|~'~l LHJ

(A9)

for small Q. The critical velocity to exceed a given deflection angle is proportional to Q1/3 for small Q. For large fires, eqn (A5) gives Apbuoyancy OCH (and independent of Q). Substituting into eqn (A8) gives the condition that deflection will exceed a critical value V2 - - > Ccrit (A10) H for large Q: The critical velocity is independent of Q for large Q. These two results--eqns (A9) and ( A 1 0 ) - - m i r r o r the observed variation with Q of the critical velocity to prevent backing up. Of course, such simple arguments cannot be used to predict the velocity used to produce the large plume deflection necessary to prevent backing up. Nor can they give the variation of critical velocity at intermediate values of Q. These problems require C F D or experimental solutions. The result presented in this paper--Fig. 6 for e x a m p l e - - s h o w that, in fact, the transition between the low and high heat release regimes occurs over a remarkably small range of values of Q.

REFERENCES 1. Thomas, P. H., The movement of buoyant fluid against a stream and venting of underground fires. Fire Research Note, No. 351, Fire Research Station, Watford, UK, 1958. 2. Thomas, P. H., The movement of smoke in horizontal passages against an air flow, Fire Research Note, No. 723, Fire Research Station, Watford, UK, 1968. 3. Heselden, A. J. M., Studies of fire and smoke behaviour relevant to tunnels. Paper J1, 2nd Int. Syrup. on Aerodynamics and Ventilation of Vehicle Tunnels, Cambridge, UK, 1976. 4. Vantelow, J. P., Guelzim, A., Quach, D., Son, D. K., Gabay, D. & Dallest, D., Investigation of fire-induced smoke movement in tunnels and stations, an application to the Paris Metro. 3rd Int. Symp. on Fire Safety Science. Hemisphere, New York, 1991, pp. 907-18. 5. Kwack, E. Y. & Bankston, C. P., On the reverse flow ceiling jet in pool fire-ventilation cross-flow interactions in a simulated aircraft cabin interior. Heat and Mass Transfer in Fires, AIAA/ASME Thermophysics and Heat Transfer Conference, HTD, Vol. 141, 1990. 6. Danziger, N. H. & Kennedy, W. D., Longitudinal ventilation analysis for Glenwood Canyon Tunnel, 4th Int. Symp. on the Aerodynamics and Ventilation of Vehicle Tunnels, York, UK, 1982, pp. 169-86.

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