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Parameters affecting the performance of emergency ventilation
strategies in a roadway tunnel
http://www.nrc-cnrc.gc.ca/irc
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4th International Symposium on Tunnel Safety and Security (ISTSS), Frankfurt,
Germany, March 17-20, 2010, pp. 528-532
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Parameters Affecting the Performance of Emergency
Ventilation Strategies in a Roadway Tunnel
Ahmed Kashef
National Research Council of Canada Ottawa, Canada
ABSTRACT
A research study was conducted at the National Research Council of Canada to evaluate the effect of different parameters on the performance of emergency ventilation systems in the event of a fire in a section of a road tunnel. The parameters included: tunnel cross-section width and height, tunnel slope, fire size and location, meteorological conditions, and modes of fan operation. The study aimed at assessing the ability of in-place emergency ventilation strategies to control smoke spread and minimize its impact on tunnel users using both numerical and experimental approaches. Four field fire experiments and eight numerical simulations were conducted. Based on the study results, recommendations were made to optimize the ventilation strategies in the tunnel section. This article describes the conducted in-situ fire tests and presents test measurements used to provide the initial and boundary conditions for the numerical simulations.
KEYWORDS: Emergency ventilation strategies, visibility, smoke backlayering, field fire testing. INTRODUCTION
The provision of a safe egress route that is free of smoke and hot gases is the main objective for a tunnel emergency ventilation system (EVS). Methods of controlling smoke in a tunnel using EVS include longitudinal airflow, smoke extraction and smoke dilution. In the event of a fire, the
phenomenon of “backlayering” should be controlled. The backlayering phenomenon is the situation in which smoke moves against the airflow provided by the EVS upstream of the fire, creating a danger environment to both tunnel users and emergency responders. Many parameters influence the smoke flow and its stratification. These include: heat release rate (fire size), tunnel length, cross-section and grade, traffic flow, meteorological conditions and fire protection systems. Establishing airflow requirements for roadway tunnels and consequently the capacity of the EVS is a challenging task due to the difficulty of controlling many variables (e.g. changes in traffic patterns and
modifications to the tunnel).
The investigated tunnel section was 305 m long and ran from west to east. The section was composed of existing tunnel and a new part, 232 m long, which extended up to the east portal and ran under a newly constructed building. The new tunnel section had 5 lanes with traffic flow from west to east. The clear height of the tunnel changes as the floor of the tunnel had an up-slope from west to east (Figure 1). The section was characterized with large cross-beams which obstructed airflow from wall vents and ducts.
Three sets of fans were used to provide ventilation in the tunnel section. The first set (SET I) was located in the existing tunnel section and provided fresh air through ceiling vents. The new section had longitudinal ventilation scheme that included two sets of reversible fans (SET II and SET III). SET II and SET III supplied/exhausted air through wall louvers and ducts, respectively.
For a fixed fire size and due to the slope of the tunnel section, the critical velocity changes along the tunnel length. Figure 2 shows the change of the critical velocity along tunnel length for different fire sizes. In general, the critical velocity decreased towards the east portal from its maximum values at the junction of old and new tunnel at the maximum ceiling height. EXPERIMENTAL STUDY
Four full-scale fire tests, corresponding to four
emergency ventilation scenarios, were conducted using a clean-burning, compact, portable propane burner that
generated minimal visible smoke and was capable of producing up to 5 MW of heat output simulating a small vehicle [1]. The burner was equipped with a control system for immediate shutdown (Figure 3). Artificial smoke was added for visualization purposes. Fire sizes of 2, 4, 3, and 4 MW were used depending on the ventilation scenario to limit temperature at tunnel ceiling to below 100°C.
Temperature and optical smoke density (SOD) measurements were recorded at three locations (L1, L2 and L3) with 26 points of measurements. Upstream was defined in the context of this article as the direction from which the traffic entered the tunnel. A thermocouple tree of 12 thermocouples (TCs) was installed at the fire location, L1. Two other thermocouples trees, each containing 7 TCs, were installed at L2 and L3. SOD values were measured at 4 locations at L2 and L3. Air speeds were measured at fire location and at several tunnel locations.
No backlayering was observed during the tests, except for Test 1 and Test 4, where some smoke was observed close to the interface section between the old and new sections. However, the fire size was limited to 4 MW. The tests revealed strong turbulent flow conditions around the three beams as a result of the ventilation flows impacting on them. Moreover, small airflow speeds were recorded at the interface section due to large pressure losses in the
fans plenum. The in-situ fire tests concluded that the ventilation scenario with the three sets of fans operating in supply mode, was the most effective scenario in eliminating smoke. Maximum SOD values were measured close to the ceiling, indicating that most of the smoke travelled close to the ceiling.
NUMERICAL STUDY
The study employed the Fire Dynamic Simulator (FDS) Computational Fluid Dynamics (CFD) model [2] for the simulation of the behavior of the smoke originating from fires in the tunnel section. Large fire sizes were not feasible for testing in the tunnel due to the risk of
damage to the tunnel and its support systems. Therefore, a 20 MW [3] gasoline pool fire (equivalent to a bus or truck on fire) was used to conduct all numerical simulations. It was assumed that the
gasoline fuel (C16H34) was being burned with constant yields of CO (0.131) and Soot (0.360).
CFD simulations of the EVS in the supply mode
To investigate the performance of EVS operating in supply mode during large fires, a ventilation scenario, with the three sets of fans operating in supply mode, was used in the numerical simulations
Figure 2. Critical velocity along tunnel
Figure 3. Propane burner system
(a) UFANS
(b) UBEAMS Figure 4. Velocity vectors
of four fire locations. One at the interface section “UFANS”; under the second beam “UBEAMS”; and at distances of 172 m “DIST 172” and 247 m “DIST 247”from west.
UFANS: Figure 4a shows the resulting velocity vector field due to the fire. The figure shows that a small area of reversed flow exists under the beams. The resulting smoke surface is shown in Figure 5a and the associated visibility is shown in Figure 6. The figures indicate that there was no backlayering in the old section of the tunnel and that all the smoke moved towards the east portal in the same traffic direction. In
this case, SET I has created an air curtain that prevented smoke from moving upstream of the fire. UBEAMS: Figure 4b shows the resulting
velocity vector field due to the fire. The flow field was modified due to the fire and the area under the beams had areas of reversed flow with the velocity being less than 3.0 m/s. Figures 5b and 6 indicate that backlayering extended to a distance of ~20 m upstream of the fire after which visibility was regained.
DIST 172: Figures 5c and 6 indicate that backlayering extended to a distance of ~70 m upstream of fire.
DIST 247: In this case, the average airflow velocity at fire was predicted to be ~2 m/s
(greater than the critical velocity to prevent smoke backlayering, 1.7 m/s). Figures 5d and 6 show that backlayering extended to a short distance of ~30 m upstream of fire. In this region, the average visibility dropped to ~15 m.
CFD Simulations of the EVS in the exhaust mode
As demonstrated in the previous section, operating the EVS in the supply mode produced untenable environment conditions upstream of the fire for UBEAMS and DIST 172. With the high ceiling at the interface section, the possibility of keeping smoke and hot gases at high elevations was investigated by operating the two sets of fans “SET 2” and “SET 3” in the exhaust mode for four fire locations: UFANS, UBEAMS, DIST 172 and DIST 247.
“UFANS”: The resulting smoke surface and the associated visibility are shown in Figures 7a and 9. The smoke and hot gases were kept at a level above the three beams and a 23 m visibility or higher was maintained upstream of the fire.
“UBEAMS”: Fresh air was drawn to the fire location from upstream and downstream of the fire. In this case the ceiling jet flow was mainly directed towards the exhaust fans as a result of high exhaust capacity. The smoke and hot gases (Figures 7b and 8) were kept at a level above the three beams and a 20 m visibility or higher was maintained upstream of the fire.
“DIST 172”: Fresh air was drawn to the fire from both directions with the larger portion being drawn from upstream of the fire. In general, the ceiling jet flow movement was decelerated due to the ceiling grid of beams. The cross-sectional area of the tunnel decreased towards the east portal.
Figure 5. Smoke spread – supply mode
As a result, the airflow accelerated as it moved downstream of the fire. Upstream of fire, smoke layer was maintained above the three beams with a visibility of 20 m (Figures 7c and 8).
Downstream of the fire, the visibility dropped to less than 10 m.
“DIST 247”: Fresh air was mainly drawn to the fire location from upstream of the fire.
Downstream of the fire, airflow was primarily directed towards the east portal. At the interface section and due to the remote position of the fire from this location, most of the exhausted airflow was drawn from the old tunnel section. This, in turn, limited the ability to remove smoke
(Figures 7d and 8) and as a result the smoke layer descended below the three beams. The average visibility downstream of the fire was lower than 15 m.
CONCLUSIONS AND RECOMMENDATIONS The performance of EVS in the supply mode was significantly affected by large pressure losses in the fans plenum and the large beams that obstructed the airflow and created strong turbulent flow conditions. The backlayering phenomenon was predicted for two fire locations close to the fans location. At locations further away from the exhaust fan where the airflow speed was greater than the critical velocity, the supply mode may be used to maintain tenable conditions upstream of the fire. To improve the performance of the ventilation fans in the supply mode, it was recommended to reduce airflow losses in the fans’ plenum and to direct the airflow in such a way as to avoid impacting on
the large beams and losing momentum, e.g. using ducts that would direct better the flow away from the beams. Another possible remedial measure was to install jet fans under the beams to enhance the airflow in this section.
With the high ceiling of this section of the tunnel, the possibility of keeping smoke and hot gases at high elevations was investigated by operating the EVS in the exhaust mode. In this case, the activated ventilation scenarios were able to maintain a tenable environment in the tunnel upstream of the fire up to the fire source being located at 172 m from west. At locations further away from the exhaust fan, the exhaust mode was not effective in maintaining tenable conditions in the tunnel section.
REFERENCE LIST
1. Kashef, A., Benichou, N., and Lougheed, G. “Simulation of Fires in the New Section under the
Palais Des Congrès of the Way B in the East Direction of the Ville-Marie Tunnel”, National Research Council Canada, Report B4540.1, 2005.
2. McGrattan, K. B., Baum, H. R., Rehm, R. G., Forney, G. P., Floyd, J. E., Hostikka, S., “Fire
Dynamics Simulator (Version 4), Technical Reference Guide (Technical Report NISTIR 6783)”, National Institute of Standards and Technology, Gaithersburg, Maryland, 2003.
3. National Fire Protection Association, NFPA 502 Standard for Road Tunnels, Bridges, and Other
Limited Access Highways, NFPA, Quincy, MA, 2008.
Figure 7. Smoke spread – exhaust mode
