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Fire Safety Journal 27 (1996) 335-341 © 1997 Elsevier Science Limited

Printed in Northern Ireland. All rights reserved 0379-7112/96/$15.00

P I I : S 0 3 7 9 - 7 1 1 2 ( 9 6 ) 0 0 0 6 1 - 6

Short Communication

Smoke Control in Sloping Tunnels

G. T. A tk in son" & Y. W u b

" Health and Safety Laboratory, Harpur Hill, Buxton, Derbyshire SK17 9JN, UK h Department of Mechanical and Process Engineering, University of Sheffield,

Sheffield S1 3JD, UK

(Received 20 May 1996; revised version received 4 July 1996; accepted 22 July 1996)

A B S T R A C T The critical velocity to prevent upstream smoke flow in the event of a tunnel fire is an important part of the design of emergency ventilation systems. For tunnels with a downhill slope the critical velocity is somewhat greater than for the corresponding horizontal tunnel. This note presents experimental results from a study involving model tunnels with slopes between 0 and 10 degrees. A general slope correction factor is derived from these results. ~ 11997 Elsevier Science Ltd. All rights reserved.

1 I N T R O D U C T I O N

This note concerns the specification of the longitudinal ventilation necessary to prevent backflow of combustion products from fires in tunnels with downhill slopes between 0 ° and 10 °. The ventilation velocity required to prevent backflow past a specified point is referred to as the critical velocity. These results extend the work of Oka and Atkinson I on smoke control in horizontal tunnels.

The control of smoke flow during a tunnel fire is often an important part of emergency planning. The fire safety of many tunnel systems depends on the operators ' ability to keep evacuation routes clear of smoke using emergency, reversible ventilation.

The dependence of the critical velocity on tunnel gradient has not been systematically investigated previously. Current design methods for smoke control systems are based on results from a study of methane-rich roof layers by Bakke and Leach. 2 Equat ion (1) shows the correction factor

335

336 G. T. Atkinson, Y. Wu

used in the US Depar tment of Transport Subway Environment Simulation Program to predict the critical velocity in sloping tunnels: 3

V(o~) = V(0).[1 + 0.0374.o~ ''-~] (1)

where ~ is the tangent of the angle of slope expressed as a percentage and V(0) is the critical velocity in a corresponding horizontal tunnel.

In the methane layering experiments of Bakke and Leach, buoyant fluid was released at roof level; this contrasts with a tunnel fire, where some buoyancy is generated in the lower part of the tunnel and the fire plume suffers a large deflection before impinging on the roof. One consequence of this is that, for horizontal tunnels where the tunnel height and width are similar, the velocity required to prevent backflow from a small source of buoyancy is around five times greater if the source is at roof level than if it is near the floor. In principle, a quite different pattern of variation of critical velocity with slope could be expected for the two flows.

2 E X P E R I M E N T A L

The experimental methods used were similar to those described by Oka and Atkinson. ~ A model tunnel was used with a height of 244mm. The internal cross-section was the BS 227 colliery arch, which comprises a semicircular head on walls splayed out at 7 ° . All of the critical velocities reported are based on measurements of volume flux, i.e. they are values averaged over the tunnel section.

Propane was used as a fuel. This was introduced via a 100 mm diameter porous bed burner with its top surface set flush with the tunnel floor. The propane flow rate was varied between 2 and 101/min, corresponding to fires of between 15 and 75 MW in a tunnel of diameter - 5 m. This scaling is discussed in Oka and Atkinson.X The outflow velocity of propane varied between 0.4 and 2cm/s . This is very low compared with the tunnel ventilation velocity. The results should closely correspond to the low- m o m e n t u m limit that is characteristic of fire sources.

It was shown by Oka and Atkinson that the key variable in determining the critical velocity (for this tunnel shape) is Q* (see eqn (2)). For Q* less than -0 .12 the critical velocity depends o n Q,J/3; for values of Q* > 0.12 the critical velocity is independent of Q*. The fire sizes used in the current study bracket the transitional value of Q*.

Q Q* = (2)

po Cp Togl/2 HS/2

Smoke control in sloping tunnels 337

where Q is the convective heat release rate (kW), Po is the density of the inflow (kg/m3), Cp is the heat capacity of the inflow at constant pressure (kJ/kg K), To is the absolute temperature of the inflow (K), g is the gravitational acceleration (m/s 2) and H is the tunnel height (m).

In addition to the instrumentation described by Oka and Atkinson, I two rakes of eight thermocouples were used to measure the temperature profiles at distances equal to eight and 19 tunnel heights downstream.

3 RESULTS

The ventilation velocities required to control the length of the backed up flow to 1, 3, 5 and 10 times the tunnel height are shown in Fig. 1. These results can be used to obtain a good estimate of the ventilation flow needed to eliminate any back flow past the fire--which is the usual definition of the critical velocity. The result of extrapolation back to zero tunnel heights is shown in Fig. 2, together with the SES predictions based on methane layering experiments.

Figure 3 illustrates how the downstream temperature profiles vary with the angle of inclination of the tunnel in a typical case.

4 DISCUSSION

The effect of slope on critical velocity appears to be modest for fires above and below the transitional value of Q*. Smaller proportional changes in ventilation flow are required to compensate for changes in slope than for buoyant roof layers. Presumably this is because-- in the case of a tunnel--increases in ventilation flow act both on the front of the layer and on the plume throughout the depth of the tunnel.

The current results suggest the following correction factor to the critical velocity for downhill slopes in the range 0 to 10°:

V(O) = V(O).[1 + 0.014.0] (3)

where 0 is the tunnel slope in degrees. This is equivalent to

V(a) = V(0).[1 + 0.014.tan-'(a/100)] (4)

where the angle is measured in degrees and ot is the tangent of the angle of slope, expressed as a percentage. This new correction factor is also shown in Fig. 2.

338 G. T. Atkinson, }1. Wu

0.85

0.75

E . o.65t _o >~ 0.55i ._g ¢e 0.45 t" ¢1 > 0.35 1

! 025 t

o

ii

I1 tunnel height I

i *

i/ ii ~ 11

0.85

2 4 6 8 10 Tunnel slope (degrees)

~5 tunnel heigl~is~

12

0.75

g >, 0.65 ~-

o >= 0.55 g =

.~ 0.45

~ 0 . 3 5 ! i

0.25 ~-- . . . . . . . 0 2 4 6 8 10 12

Tunnel slope (degrees)

[3 tunnel heights]

0"65 i . . . . . I ~o.75 l E t ' 065 * [ '~ 0.55 ' / li i/ il q C I

.~ 0.45

;o25L > 0.35 i

t j ~ ~ l

0 2 4 6 8 10 12 Tunne l s lope (degrees)

110 tunnel heights]

0.85 --

~ 0.7

>~ 0.55 c O

~ ~ ~ ' ' ' ~ 0.4 ~ • . ' ' •

0.25 ~ . . . . . 0 2 4 6 8 10 12

Tunnel slope (degrees)

Fig. 1. Critical velocity to prevent back layering to various distances upstream. © 101/min propane, Q*= 0.44; A 5 l/min propane, Q*= 0.22; [] 21/min propane, Q*= 0.088. Open symbols mark a velocity that did not lead to backing-up to the specified

extent; filled symbols mark a velocity that did.

The variation of critical velocity shown in Fig. 1 is curious. For small backed-up layers the critical velocity increases steadily with the tunnel slope. For a backed-up layer of length equal to 10 tunnel heights, the critical velocity increases markedly when the slope is changed f rom 0 ° to 2 ° , but fur ther increases in angle produce little fur ther change in critical velocity. Whatever the explanation for these observations, the results of Figs 1-3 could provide a useful check on the accuracy of computat ional methods that are increasingly being used in the analysis of tunnel fires.

Smoke control in sloping tunnels 339

1.4

t- O N 1.3

o r- O 1.2 Q)

1.1

"~ 1 o

~ 0,9 I n

0

~J J J

f ~ J b

0 , 8 _ _ h _ _ l t _ _ i _ _

0 2 4 6 8 10 12 Tunnel slope (degrees)

Fig. 2. Measurements of critical velocity to prevent any backing-up. Critical velocities are plotted relative to the measured value for a horizontal tunnel. • 10 l/min propane, Q* = 0.44; © 5 l/min propane, Q* = 0.22; + 2 l/min propane, Q* = 0.088. Lines plotted: a, SES prediction, derived from methane layering experiments; b, eqn (3), based on current

work.

The fact that the critical velocity varies little for values of Q* between 0.2 and 0.4 indicates that the 'chimney effect ' -- the effect of density differences along the length of the tunnel downstream--is of little importance in determining the critical velocity. It is of course crucial in calculating the back-pressure that the ventilation system will be required to overcome to establish the critical velocity.

The measurements of temperature profiles show that the tunnel slope has a marked effect on the rate of mixing downstream. This may be significant, for example, in assessing the rate of heat loss from the flow and the extent to which buoyancy forces due to a fire on a downhill slope add to the flow resistance.

Any upstream hot layer is also rapidly mixed downwards if there is any downhill slope. This may be crucial to life safety if the ventilation flow falls significantly below the critical value for some reason and there is temporary loss of smoke control. The existence of clear air along an escape path under the backed-up smoke layer will be very sensitive to any downhill inclination.

3 4 0 G. T. Atkinson, Y. Wu

e- ._m

E

> 0

--n

Q)

120 - -

100 m - ~ ,

80 a f - -

60

4 0

2 0

~ J

• 0 ° • 2 ° • 4 °

O 6 °

<> 8 ° ,,, 10 °

i -m i

50 100 150 200 250

Distance below tunnel roof (mm)

0 6~ " 0 v

e- ._o ..Q

E t~

> 0

O .

E I -

80

60

40

20

0 0 250

"11 . . "~,

50 100 150 200

Distance below tunnel roof (mm)

Fig. 3. Downstream temperature profiles for tunnel slopes between O ° and 10°: a, eight tunnel heights downstream; b, 19 tunnel heights downstream. In all cases, fuel supply

2 1/min propane (Q* = 0.088), longitudinal ventilation velocity 0.375 m/s.

5 C O N C L U S I O N S

The results suggest the fol lowing expressions for the critical velocity in tunnels with the cross-section studied that slope downhill with an angle between 0 ° and 10°:

V , ~ , = [ g H ] ~ / z . V * a x . [ Q * / 1 . 2 ] " 3 . [ l + O . 0 1 4 . 0 ] for Q * < O . 1 2 (5)

Vcrit = [gH]l/2.V*max . [ 1 + 0 . 0 1 4 - 0 ] for Q * > 0 . 1 2 (6)

where Q*, H, g and 0 are as defined above. The values taken by Vm~ax for

Smoke control in sloping tunnels" 341

different fire sources with and without tunnel blockages are discussed by Oka and Atkinson? For a fire on the floor of a tunnel without significant blockage, the value of V*ax varies from --0.35 for a fire that is much narrower than the tunnel to 0.31 for a fire that extends to the full width of the tunnel.

A C K N O W L E D G E M E N T S

The authors would like to thank Mr D. Bagshaw and Mr E. Belfield of HSL for their work in building the equipment used in this work and Mr P. James and Mr J. Stoddard for help in carrying out the experiments.

R E F E R E N C E S

1. Oka, Y. & Atkinson, G. T., Control of smoke flow in tunnel fires. Fire Safety J., 25(4) (1995) 305-322.

2, Bakke, P. & Leach, S. J., Turbulent diffusion of a buoyant layer at a wall. Appl. Sci. Res., 15 (1965) 97-136.

3. Parsons Brinkerhoff Ouade and Douglas, Inc., Subway Environment Design Handbook, Vol. II, Subway Environment Simulation (SES) Computer Pro- gram Version 3 Part 1: User's Manual 2. Prepared in draft for the US Department of Transportation, 1980.