Tunnel fire

Application Challenge 4-04 © copyright ERCOFTAC 2004

Overview of Tests

The fire tests undertaken in the Memorial Tunnel involve different size fires and different ventilation regimes. The fire sizes were approximately selected by using a combination of steel pans having different area, each filled with water topped by a layer of fuel oil.

For each of the tests, different ventilation options were selected. For the longitudinal ventilation tests, jet fans spaced at intervals along the tunnel length were activated at certain times within each test. The activation of the jet fans enabled an assessment to be made of their effectiveness in controlling the level of back-layering from the fires.

Measurements of temperature, air velocity and gas concentrations (e.g. CO) were made using instruments mounted on measurement ‘trees’ or ‘loops’, each tree positioned at a certain distance down the tunnel. The data from these measuring instruments was recorded at one second intervals throughout the duration of each fire test. Extensive data at selected times is available on the Memorial Tunnel CD-ROM, which is available from http://www.tunnelfire.com.

NAME	GNDPs		PDPs (problem definition parameters)		MPs (measured parameters)
	Re	Fr	Fire size	Jet fans	Detailed data	DOAPs
TEST 615B (jet fan fire test)	To $10^{6}+$		$\approx$ 100MW	Variable operation	T,V,CO conc	T,V,CO conc

Table EXP-A Summary description of Test Case 615B

	MP1 Velocity $(ms^{-1})$	MP2 Temp $(^{0}F)$	MP3 CO conc (ppm)	DOAPs, or other miscellaneous data
TEST 615B				Bulk Airflow (cfm)

Table EXP-B Summary description of all measured parameters and available datafiles

Test case 615B

Description of Experiment

The main parameters which define the experiment are the fire heat release rate and the ventilation flowrates occurring as a result of the activation of the jet fans. Prior to the ignition of the fuel oil for this test, measurements of air temperature and velocity were made to establish the initial flow conditions within the tunnel.

Fire Source

The test undertaken in the tunnel involves the burning of No 2 fuel oil in steel pans, located 238m from the south portal. Based on engineering estimates, a $48ft^{2}$ ( $4.5m^{2}$ ) exposed fuel surface area produces a nominal heat release rate of approximately 10 MW. Four pans of different sizes allow different total heat release rates to be selected. For the 100 MW fire case chosen for this study, the 20, 30 and 50 MW pans were used. The fire pans were set to approximately 30 in (0.76 m) from the tunnel floor and were filled with 6 in (0.15 m) of water, on which the measured supply of fuel oil floated. Delivery of the low sulphur No 2 fuel oil was remotely controlled from the control trailer. The fuel oil consumption was measured with instrumentation during the tests, allowing the transient variation in fire load to be calculated. For Test 615B, the fire load curve was:

Jet Fans

For the longitudinal flow tests, 24 reversible axial flow jet fans were installed in the tunnel in groups of three, nearly equally spaced along the tunnel length. The groups were arranged in a triangular fashion and suspended from the roof of the tunnel. Each fan was equipped with a 56 kW (75 hp) motor, rated to deliver 43 m3/s (91,000 cfm) at an exit velocity of 34.2 m/s. The fans were designed to withstand temperatures up to 299 ºC (570 ºF). For each of the tests a different operating sequence was specified for the fans. The location of each group of 3 fans is shown in the diagram below. Note that the fire site is between the 6th and 7th group of fans from the north portal.

The groups of jet fans were located at the following distances down the tunnel from the north portal:

Jet Fan Group	Distance from North Portal (m)
1	95
2	190
3	285
4	379
5	474
6	569
7	664
8	758

For Test 615B (100 MW fire) which is being considered in this study, the fan operation was as follows:

Jet Fan(s) Running	Period of Fan Operation
	Real Time (hr:min:sec)	Elapsed Time (min:sec)
None	11:51:13 - 11:53:13	0:00 - 2:00
1 ,3 ,4 ,6 ,7 ,9	11:53:13 - 12:05:13	2:00 - 14:00
1, 3, 4, 6, 9	12:05:13 - 12:13:13	14:00 - 22:00
1, 3, 4, 6, 7, 9	12:13:13 - 12:17:06	22:00 - 25:53
None	12:17:06 - 12:17:16	25:53 - 26:03
5, 8, 11 (rev)	12:17:16 - 12:25:32	26:03 - 34:19
5, 8, 11, 14 (rev)	12:25:32 - 12:26:24	34:19 - 35:11
5, 8, 11, 13, 14, 15 (rev)	12:26:24 - 12:34:35	35:11 - 43:22
None	12:34:35 - End of Test	43:22 - End of Test

In the plan view of the tunnel fans above, fans 1, 3, 4, 6, 7 and 9 are the outermost two fans in each group of fans for the first three groups from the north portal. Fans 5, 8, 11 and 14 are the central fans in fan groups 2 – 5 from the north portal. Fans 13, 14 and 15 are the fans in the 5th group from the north portal. Initially the fans blow down towards the south portal. From 26 minutes into the test, fans closer to the fire source reverse to draw smoke back towards the north portal.

Measurement Instrumentation

Values of temperature, air velocity and gas concentrations were measured at various locations down the length of the tunnel. The measurements were taken from instruments installed on instrument trees (or ‘loops’).

The measurement of air temperature was undertaken by the use of thermocouples at 15 cross-sections (loops) down the length of the tunnel. The accuracy of the thermocouples was ±0.75% and temperatures over the range 0 ºC to 1371 ºC (32 ºF to 2500 ºF) were capable of being measured. Additional temperature measurements were taken 15 m (50 ft) outside each tunnel portal.

The determination of air velocity was accomplished through the use of differential pressure instrumentation designed to measure very low pressure ranges. Bi-directional pitot tubes were used. The air temperatures in the vicinity of the pitot tubes and the ambient barometric pressure were combined with the measured differential pressure to calculate the air velocity.

Gas samplers attached to the instrument trees drew gas to gas analysers located in the electrical equipment rooms. Gases were analysed for Carbon Monoxide (CO), Carbon Dioxide (CO2) and total Hydrocarbon content (THC).

Two towers located outside the north and south portals were used for meteorological instrumentation, providing data on ambient dry and wet bulb air temperatures, barometric pressure, wind speed and direction.

In total there were approximately 1400 instrument sensing points. Readings were taken from each instrument once every second during the test. Therefore up to 3 million data points of information were recorded for a single test.

The locations and configuration of the instrument trees are shown on the next two pages.

Temperature measurements were made in ºF. Velocities were measured in feet per minute (fpm). Carbon monoxide measurements are presented as parts per million (ppm) values.

The test results are given on CD-ROM and include the following:

· Test Summary: A summary of the test, fire size, jet fan sequence, etc.

· Point-In-Time Graphics: Contours of temperature and air velocity are given at each instrument tree location, in addition to the bulk airflow values and smoke profile. The smoke profile illustrates the approximate location of the smoke boundary and the level of visibility along the tunnel.

These graphics are given every minute to 6 minutes, then every 2 minutes thereafter.

· Time History Graphs: The raw velocity data is given at 7 or 8 levels on the centre line of each instrument tree. The temperature data is similarly given at 7 or 8 levels of each instrument tree and represents the average temperature measured at each level. Most of the instrument trees have 8 levels, apart from those at the ends of the tunnel underneath the fan rooms, which have 7 (loops 202 and 214). Carbon monoxide measurements are given at 3 levels on each instrument tree.

Graphs are given for each instrument tree location, showing the variation in temperature, air velocity and carbon monoxide concentration with time throughout the duration of the test.

Boundary Data

Ambient temperature data are available for the tests and the initial air velocity in the tunnel before the start of the test. No external wind data is available and hence it is not clear whether the initial velocity is due to the wind or the chimney effect of the sloping tunnel.

The temperature, velocity and carbon monoxide concentration was measured at each of the instrument tree locations, as described in the previous section. No turbulence measurements were made.

Measurement Errors

For the temperature measurements, the thermocouples have an accuracy of ± 0.75%.

Due to the heavy layers of insulation which were applied to the instrument tubing and wiring, along with the instrument trees supporting the instrumentation, the thickness of this insulation greatly magnified the physical dimensions of the equipment it was meant to protect. The consequence of this was a reduction in the cross-sectional area of the tunnel, hence affecting the accurate measurement of air velocity.

In order to minimise the impact of the insulation on the measurements obtained from the instrumentation, cold airflow tests were undertaken, involving the use of vane anemometers positioned on a test rake at the same lateral and elevational coordinates as the pitot tubes on the instrument trees. Longitudinal airflow past the test rake and instrument trees was generated using the tunnel jet fans. The measurements obtained from the test rake and instrument trees were then compared, in order to obtain a suitable ratio for each pitot tube on the instrument trees. These ratios were then used in order to correct the measured velocity data obtained during the fire tests.

Measured Data

The measured data is viewable in the following files.

QP615B.dat File containing raw bulk airflow data.

VP615B.dat File containing raw velocity data.

TP615B.dat File containing raw temperature data.

CO615B.dat File containing raw carbon monoxide concentration data.

HRR615B.dat File containing raw fire heat release data.

References

1.Memorial Tunnel Fire Ventilation Test Program, Test Report, Bechtel/Parsons Brinckerhoff for the Massachusetts Highway Department, Nov 1995, with CD-ROM.

2.Website: http://www.fhwa.dot.gov/bridge/tunnel/tunres2.htm..Federal Highway Administration.

3.Website: www.tunnelfire.com. Tunnel fire simulation software, with particular reference to the Memorial Tunnel tests.

Contributors: Nicholas Waterson - Mott MacDonald Ltd

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