Natural Vs Forced Ventilation During Fires In Relatively Short
Road Tunnels
P. Ciambelli, M.G. Meo, P. Russo, S. Vaccaro
Dipartimento di Ingegneria Chimica e Alimentare - Università di Salerno – ITALY
1. Introduction
Modern life in developed countries is strongly based upon efficient and reliable goods and
people transportation systems, in which tunnels are a key element. Statistics suggest that in
the last decades traffic markedly increased and changed in composition (more combustible
and flammable goods), and consequently, the safety level in existing tunnels decreased and
the number of accidents evolving in fires increased as witnessed by recent major accidents
occurred in tunnels and resulting in loss of lives and severe damages to tunnel structures. This
have caught the attention on safety in road and rail tunnels especially in those countries, like
Italy, where there are about 50% of the EEC road tunnels and about 80% of goods is
nowadays transported by road, with a 30% increase with reference to the 2010 forecast [1].
Accidental fires may be prevented or at least their consequences may be mitigate by
upgrading existing tunnels and by equipping new tunnels with efficient fire protection
systems. Among these latter a key role is played by ventilation systems whose effect on the
consequences of a fire can be predicted by experimental tests or by reliable computer
simulations. The former are very expensive and difficult to realize and, generally, do not give
detailed gas flow and smoke movement patterns inside tunnels. In contrast, computer
simulations, especially when obtained by computational fluid dynamics (CFD) models,
provide a relatively cheap mean for gaining a good insight of the region of the fire and
prediction of the escape conditions for people involved in fires. Obviously, to be really
reliable for safety studies CFD models must be able to reproduce closely not only the overall
known behavior of fires in tunnel, but also measured values from controlled tunnel fire
experiments [2].
In this work a CFD code, JASMINE, was employed for the simulation of a gasoline pool fire
consequent to a spill from a road tanker involved in an accident in a 800 m long one-way road
tunnel situated on the A3 highway in Italy. The effect of natural and mechanical ventilations
on the conditions (safe or unsafe) occurring inside the tunnel after the fire was investigated.
2. Tunnel fire safety
2.1. Ventilation directives
Road tunnels may require ventilation either to dilute and remove pollutants during normal
traffic operation or for controlling smoke and hot gases during a fire emergency, to allow safe
evacuation and rescue. The fresh-air requirement depends on tunnel geometry, traffic volume
and composition, and specific motor-vehicle emissions.
In each country, safety standards are regulated by National Road Societies and Ministries. In
Italy, according to the ANAS directives (1999) for tunnel safety [3], only in particular cases
tunnels shorter than 1000 m have to be equipped with ventilation systems. In contrast,
European guidelines for the harmonization of safety facilities and procedures for the
development of the trans-European transport network [4], recommend the use of forced
ventilation systems for one-way road tunnels when longer than 500 m.
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2.2. Fire safety issues
The safety of tunnel users and rescuers is the main objective of the tunnel fire safety. Hot
smoke spreading in a tunnel as consequence of a fire can represent a serious hazard to which
people may be exposed. The main fire safety issues include:
♣ safe evacuation of people inside the tunnel and safe rescue operation;
♣ minimal effects on environment due to the release of combustion gases;
♣ minimal loss of structure property.
During a tunnel fire people life is threatened in a number of ways by the hot smoke spreading.
Indeed, survival aims are:
I) to keep pollutants and toxic species concentrations below dangerous values (i.e. IDLH is
50000 ppm for CO2 and 1500 ppm for CO);
II) to guarantee a minimum O2 concentration (17vol %) to allow breathing;
III) to control smoke and, then, visibility (illuminated signs should be discernible at 9.1 m);
IV) to provide survivable gas temperatures (i.e. not exceeding 60 °C).
In the case of a fire developing in a naturally ventilated road tunnel with length close to 1000
m, it appears difficult that the above limits could be matched, especially when atmospheric
stable conditions verify.
3. CFD
The computer model JASMINE (Analysis of Smoke Movement In Enclosures) was employed
in this study for the simulations of tunnel fire. The model, developed by FRS/BRE, uses a
CFD code to describe the heat and mass transfer processes associated with the dispersion of
combustion products from a fire. The processes of convection, diffusion and entrainment are
simulated by solution of the Navier-Strokes equations. The code includes the key processes of
buoyancy, convection, entrainment, turbulence, combustion, thermal radiation and boundary
heat transfer relevant to the movement of smoke. It is a finite-volume code using a Cartesian
grid and is based on the SIMPLEST pressure-correction procedure. The upwind discretisation
scheme is employed. Transient solutions are advanced by a first-order, fully implicit scheme.
Turbulent closure is provided by a standard, high Reynolds number, two-equation (k- ε )
model. Various physical sub-models are included for combustion and radiation processes, gas
phase properties (density and specific heat) and solid boundary heat transfer [5].
4. Simulations
4.1. Case study
Liquid fuels and LPG yield the major contribution to the risk deriving from goods
transportation on the road [6]. Among the possible incidental scenarios the one more likely to
occur for liquid fuels (like gasoline) road transportation is a pool fire subsequent to an
accidental release. Therefore, in this study a gasoline pool fire in a tunnel, consequent to a
spill from a road tanker involved in an accident, was assumed to occur.
The tunnel investigated is a two-tubes (each tube one-way) road tunnel located between
Pontecagnano and Salerno along the Southern Italy highway A3, which is the main highway
for goods transportation in Southern Italy. Such a tunnel is about 800 m long, with an arched
cross section of 12 m x 7 m and, according to Italian directives, it is equipped only with
natural ventilation.
The pool fire was assumed to be the consequence of gasoline release, lasting 10 min, from a
hole in a road tanker. The tanker (20 m3, 6 m long, 2 m in diameter) was assumed to be
stopped at a distance of about 400 m from the tunnel entrance. A 25 mm hole was supposed to
form at the rear side of the tank, from which about 720 kg of gasoline spilled out in 10 min,
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Italian Section of the Combustion Institute
generating an enlarging pool (10 mm in thickness) up to 10 x 10 m2. After the ignition, the
heat release rate from the pool fire rises rapidly (in 90 s) up to 170 MW, and remains constant
for about 10 min. For very large pools (>5-10 m in diameter) a 20% decrease in the burning
rate, and consequently in the HRR, is usually noted, attributed to poorer mixing, poorer
combustion and formation of a cold smoke layer above the pool surface [7]. Therefore, a
burning rate of 0.04 kg/m2s was assumed.
4.2. Ventilation systems investigated
Different runs were launched to investigate the influence of the presence of ventilation
systems on the temperature and gas composition in the tunnel after the fire. In particular, the
tunnel fire was simulated under the following conditions:
• natural ventilation, in case of stable or unstable atmospheric conditions (_P=0 or 0.05 mbar
between tunnel portals);
3
• longitudinal forced ventilation with 80, 60 and 40 m /s air flow rates, provided by two
ceiling axial fans close to the entrance portal.
Simulations with different ventilation flow rates were run to identify the critical airflow for
preventing hot gas backlayering. When the ventilation velocity is too low, smoke and hot
gases can move in the upstream direction against the ventilation air. The critical velocity is
the minimum air velocity required to avoid such a phenomenon. Its value depends on the
tunnel cross-sectional geometry and slope, and on the fire size. The critical velocity value has
become one of the main criteria for the design of tunnel ventilation systems, especially in case
of unidirectional tunnels, where it is likely that, after the fire starts, the traffic ahead of the fire
will proceed to the exit portal and the drivers of the involved vehicles will be located
upstream of the fire site.
5. Results
The results of the simulations yielded fire-induced temperatures, visibility, oxygen and carbon
dioxide concentrations in the presence or not of forced ventilation systems. The calculated
profiles at the average breathing height 2 m during fire evolution were compared to the safety
values of such variables, reported in the subsection 2.2. In order to know if people can safely
escape the fire, the presumed location of pedestrians in the tunnel was evaluated by
considering that the evacuation walking speed in a road tunnel under unfavorable conditions
can be assumed to be about 1.2 m/s [8], and taking also into account the time to perceive the
risk and the time to react and leave the vehicle. Therefore, the time for walking the maximum
distance of 400 m in the tunnel is about 5.5 min. In addition, since the escape is supposed to
start 90 s after ignition so the total time to run away can be assumed to be 7 min.
1200
30
Natural ventilation,
…P=0
Δ
Natural ventilation,
…P=0.05mbar
Δ
T max
t = 2 min
visibility distance (m)
Temperature (°C)
1600
Escape distance
800
400
a)
0
-400
-200
0
200
20
Escape distance
10
400
tunnel length - distance from fire (m)
Fig. 1
Natural ventilation,
…P=0
Δ
Natural ventilation,
…P=0.05mbar
Δ
visibility min
t = 2 min
b)
0
-400
-200
0
200
400
tunnel length - distance from fire (m)
Natural ventilations: profiles along the tunnel length, at tunnel centreline and 2 m
above the floor; t = 2 min. a) temperature; b) visibility.
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In both cases of natural ventilation, gas temperature (Fig. 1a) and visibility (Fig. 1b) are
beyond the survival values already at t = 2 min (i.e. 30 s after the escape beginning). This
occurs at the corresponding escape distances (at t = 2 min the location of pedestrians escaping
from tunnel centre is at 36 m from fire site), and as far as about 200 m along the tunnel length,
almost symmetrically on both fire sides. Conversely, O2 and CO2 concentrations, at the
breathing height and at the escape distance, remain within the safety limits during the entire
evacuation time. Therefore, in the presence of natural ventilation escaping people who
initially stands close to the fire location could not be able to safely reach the tunnel exit, either
for the relatively high temperatures either for the scarce visibility.
Comparison of simulation results in the presence of longitudinal ventilation at different air
flow rates were reported in Fig. 2. Results in terms of gas temperature (Fig. 2a) and visibility
(Fig. 2b) showed that the lower ventilation airflow (40 m3/s) is inefficient: the upper layer of
heated air and smoke flows in the upstream direction causing backlayering already 2 min after
ignition. Moreover, at the escape distance upwind the fire, gas temperature is about 180°C
and visibility distance is about 2.4 m, both outside the safety limits.
1600
30
40 m3/s
t = 2 min
1200
3
80 m /s
T max
800
Escape distance
400
a)
0
-400
-200
0
200
40 m3/s
20
80 m3/s
visibility min
Escape distance
10
b)
0
-400
400
-200
tunnel length - distance from fire (m)
Fig. 2
t = 2 min
60 m3/s
visibility distance (m)
Temperature (°C)
60 m3/s
0
200
400
tunnel lenght - distance from fire (m)
Longitudinal ventilations: profiles along the tunnel length, at tunnel centreline and
2 m above the floor; t = 2 min. a) temperature; b) visibility.
On the contrary, both longitudinal ventilations performed at higher air flow rates are able to
push smoke and hot gases in the downstream direction and, hence, to assure safe escape
conditions upstream the fire. But the greater airflow (80 m3/s) appear to be fruitlessly high,
decreasing gas temperature and smoke also downstream the fire (Fig. 2a) where all vehicles
can be assumed to escape the tunnel through the exit portal. Therefore, the critical airflow rate
able to prevent backlayering was identified to be about 60 m3/s.
7
7
t = 2 min
t = 2 min
6
a)
5
tunnel height (m)
tunnel height (m)
6
4
3
2
Longitudinal ventilation, 60 m /s
1
Natural ventilation, ΔP=0.05mbar
3
Longitudinal ventilation, 60 m 3/s
Natural ventilation, ΔP=0.05mbar
b)
visibility min
5
4
3
2
1
T max
0
0
0
Fig. 3
200
400
600
Temperature (°C)
800
1000
0
50
100
150
200
visibility distance (m)
250
300
Natural vs longitudinal ventilation: profiles along the tunnel height, at tunnel
centreline and at the upwind escape distance; t = 2 min. a) temperature; b) visibility.
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The results above are also confirmed by the comparisons between temperature and visibility
vertical profiles, obtained with natural and longitudinal ventilations at the upwind escape
distance traveled by pedestrians after 2 min from the fire start (36 m). Such comparisons,
reported in Fig. 3, clearly shows the ineffectiveness of natural ventilation, even in the case of
unstable atmospheric conditions, since in this case either the gas temperature or the visibility
distance are beyond safety limits already at t=2 min. On the contrary, the longitudinal
ventilation (60 m3/sec) is capable to assure safe conditions (in terms of both gas temperature
and visibility distance) not only at the breathing height, but on the whole tunnel section.
Furthermore, results reveal that safe conditions upwind the fire are maintained during the
entire time of tunnel evacuation, as shown in Fig. 4, where the time evolution of air
temperature and velocity patterns on the tunnel longitudinal symmetry plane in the presence
of the critical longitudinal ventilation are shown.
Fig. 4
t = 1 min
t = 2 min
t = 4 min
t = 6 min
Time evolution of temperature and velocity patterns on the tunnel middle plane with
longitudinal ventilation (60 m3/s).
10
1min
2min
3min
4min
5min
6min
7min
u1 (m/s)
6
2
-400
-200
0
200
400
-2
tunnel length - distance from fire (m)
Fig. 5
Air velocity profiles along the tunnel length inside of the escape time, at tunnel
centreline and 2 m from the floor; longitudinal ventilation with 60 m3/s airflow.
For what concerns the choice of the rate of the longitudinal ventilation, it worth noting that
excessive air flow rate is an un-useful energy waste and, in addition, high ventilation rates can
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represent an hazard for persons walking in such a stream. Escaping people under emergency
conditions can tolerate velocities as high as 11 m/s [9]. Longitudinal ventilation with 60 m3/s
air flow rate rather than 80 m3/s guarantees safer conditions in terms of tolerable velocity too,
as reported in Fig. 5.
6. Conclusions
A gasoline pool fire generated by an hypothetical accident, involving a gasoline tanker, that
could occur inside a real two-tubes (each tube one-way) tunnel along the Southern Italy
highway A3, between Pontecagnano and Salerno, was simulated by using JASMINE CFD
code, in the presence of various configurations of the tunnel ventilation system.
Simulations results allowed to assess the effectiveness of the ventilation system (natural or
forced) on the evolution of the hypothesised tunnel fire scenario, and the likelihood for the
passengers to safely escape the tunnel. In particular, results indicated that, for the
hypothesised accident, natural ventilation is not sufficient to assure safe conditions for escape
and rescue operations. On the contrary, as the traffic flow is unidirectional, the longitudinal
forced ventilation proved to be very effective, as it blows all smoke and hot gases in the
downstream direction (where all vehicles can be normally assumed to escape the tunnel
through the exit portal), so immediately creating along the whole tunnel portion upstream the
fire a safe route for evacuation and rescue.
The minimal (critical) ventilation airflow rate able to prevent backlayering was also identified
(60 m3/s). Such flow is high enough to allow safe escape and rescue operations and low
enough to avoid energy waste and comfortable air flow conditions for people inside the
tunnel.
Simulation results allowed to assess that, according to European guidelines, every road tunnel
used for flammable liquid transportation, even if shorter than 1000 m (i.e. the limit length for
Italian directives for tunnel safety), should be equipped with a proper emergency ventilation
system.
7. References
1.
2.
3.
4.
5.
6.
7.
8.
9.
Fabiano, B., Currò, F., Riverberi, A.P., Pastorino, R.: J. Loss Prev. Process Ind, 18:403
(2005).
Miles, S.D., Kumar, S., Andrews, R.D., First International Conference on Tunnel Fires
and One Day Seminar on Escape from Tunnels, Lyon, France, May, p. 159 (1999).
ANAS, Circolare dell’8/9/99 prot. no 7735, Direttive per la sicurezza della circolazione
nelle gallerie stradali (1999).
Decision No 1692/96/EC of the European Parliament and of the Council of 23 July 1996
on Community guidelines for the development of the trans-European transport network,
GU L 228 9/11/1996.
BRE Report, (2003).
Milazzo, M.F., Lisi, R., Maschio, G., Antonioni, G., Bonvicini, S., Spadoni, G.: J. Loss
Prev. Process Ind, 15:347 (2002).
Babrauskas V., Burning Rates - The SFPE Handbook of Fire Protection Engineering,
Second Edition. National Fire Protection Association, USA (1995).
Sabato, F.: Second Conference on Protection From Fire in Road and Rail Tunnels,
Rome, Italy, June, p. 19 (1999).
Kashef, A., Bénichou, N., Lougheed, G., Research Report: IRC-RR-141, (2003).
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