measurements of air flow, temperature...

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International Confe rence „Tunnel Safety and Ventilation“ 2004, Graz

MEASUREMENTS OF AIR FLOW, TEMPERATURE DIFFERENCES

AND PRESSURE DIFFERENCES IN ROAD TUNNELS

Steinemann U., Zumsteg F.

US+FZ Beratende Ingenieure, CH-8832 Wollerau P. Wildi

Tiefbauamt Kanton Graubünden, CH-7000 Chur

ABSTRACT

Natural forces caused by buoyancy, wind and barometric pressure differences can strongly influence the design and operation of ventilation systems for road tunnels. The field measurements described in this paper indicate concrete values for these influences.

Key words: tunnel, ventilation, design, pressure difference, buoyancy, barometric pressure, temperature, field test

1. INTRODUCTION

The knowledge of natural forces in a road tunnel is an essential base for the design of the ventilation system. Relevant natural forces can be buoyancy, wind and barometric pressure differences between the portals. Buoyancy plays only a role when the two portals are not on the same level above sea (positive or negative grade), relevant barometric pressure differences only occur in long tunnels which connect different meteorological regions.

The natural forces can support or act against the desired direction of air flow in a tunnel. The Swiss guideline Ventilation of Road Tunnels (FEDRO 2003) requires that in addition to effects from the incident the ventilation is able to cope with an unfavourable yet realistic combination of the following pressure differences:

• Pressure differences caused by buoyancy: 95 %-value of the hourly values of a year • Pressure differences caused by wind: Dynamic pressure of the average speed of winds

directed to the portal • Barometric pressure difference: 95 %-value of the hourly values of a year

The designer of a new tunnel has to assume values for the three effects. Whereas information about wind speed and direction is in most of the cases easily available, data about barometric pressure differences and buoyancy are more difficult to find. This paper describes the findings of measurement campaigns undertaken in two tunnels in Switzerland.

2. THEORETICAL BACKGROUND

2.1. Density of air

The density of air can be calculated as follows:

TpTp⋅

⋅⋅=0

00ρρ

? Density in kg/m3 at p and T ?0 Density at normalised conditions = 1.293 kg/m3 p Barometric pressure in mbar p0 Barometric pressure at normalised conditions = 1’013.2 mbar T Temperature in K T0 Temperature at normalised conditions = 273 K

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With the typical conditions in a road tunnel, the influence of the relative humidity to the density of air is less than 1 % and not relevant.

2.2. Pressure differences caused by buoyancy

The pressure difference in a tunnel caused by buoyancy can be approximated by

100/)( LNLgp Tie ⋅⋅−≅∆ ρρ

∆p Pressure difference in Pa (+ = updraught, - = downdraught) ?e Density of exterior air in kg/m3 ?i Density of air inside the tunnel in kg/m3 g Gravity in m/s2 (9.81) LT Length of tunnel in m LN Gradient in %

2.3. Air flow in a tunnel

In a tunnel with constant cross section, no traffic and no mechanical ventilation the air flow is given by

dtdv

LvDL

p ⋅⋅+⋅⋅+Σ+=∆ ρρλζ 2

2)1(

∆p Pressure difference in Pa Sζ Sum of resistances (related to v) λ Friction coefficient L Length of tunnel D Hydraulic diameter of tunnel cross section ? Density of the air in the tunnel v Velocity of the air in the tunnel in m/s t Time in s

3. MEASUREMENTS IN THE GOTSCHNATUNNEL

3.1. Description of test site

The measurements in the Gotschnatunnel were made in the construction phase of the tunnel between December 2001 and October 2002. In this period the drift of the tunne l was already finished and the work on the tunnel lining was under way. The cross section of the tunnel varied according to the progress of work. In addition the varying installations and the need to protect the tunnel in winter time against low temperatures caused strongly varying obstacles for the air flow in the tunnel. During the whole measurement campaign the tunnel was ventilated naturally. The characteristic data of the Gotschnatunnel are:

Tunnel with two lanes in one tube Bi-directional traffic Height above sea level 1’155 m (mean value) Total length 4’200 m Grade, averaged 4.6 % (200 m vertical difference of portal height) Cross section of finished tunnel 46.8 m2 (under the false ceiling) Hydraulic diameter 6.7 m Semi- transverse ventilation system with smoke extraction in ducts above a false ceiling, extraction-dampers every 70 m in the ceiling, jet fans

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3.2. Measurement of temperatures

The temperatures were measured with NTC-thermistors, the results were recorded with small loggers. For the measurement of the exterior temperature the device was protected against solar radiation. In the campaign we have placed 6 fixed probes in the tunnel and 2 probes in the environment close to the portals. In addition we measured several temperature profiles along the tunnel by hand for better understanding of the temperature distribution along the tunnel.

Figur 1: Setting of measuring device for temperature in the Gotschnatunnel

Figure 2: Setting measuring devices for air velocity in the Gotschnatunnel (see chapter 3.3)

The results of the temperature measurements in the Gotschnatunnel are summarized in figure 3. The campaign covered very cold as well as very warm situations in the range of -15°C to +30°C. In winter time the average tunnel temperature stayed in the range of 7 to 10°C. During 75 % of the time over a year, the air in the tunnel is warmer than in the environment, which causes an updraught. The rest of the time it is cooler in the tunnel, causing a downdraught.

Forces due to wind or to barometric pressure differences are not important at the Gotschnatunnel. It has to be noted, that the temperature inside the tunnel could change after the opening of the tunnel. We assume though that the average temperature will change only slightly compared with the results reported here.

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0

5

10

15

20

25

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Exterior temperature in °C (average of the two portals)

Tunn

elte

mpe

ratu

re in

°C

(ave

rage

ove

r tun

nel l

engh

t)

period 1a from 19.12.2001 to 13.1.2002 (tunnel closed)

period 1b from 14.1.2002 to 12.2.2002 (tunnel partly open)

period 2 from 14.6.2002 to 8.8.2002 (tunnel open)


Figure 3: Relation between exterior temperature and average temperature through the Gotschnatunnel

From the temperatures in figure 3 the buoyancy- induced pressure differences acting on the tunnel air follow according to chapter 2.2. The result is given in figure 4.

-100

-50

0

50

100

150

200

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34


Buoy

ancy

in P

a

period 1a from 19.12.2001 to 13.1.2002 (tunnel closed)

period 1b from 14.1.2002 to 12.2.2002 (tunnel partly open)



Figure 4: Relation between exterior temperature and buoyancy in the Gotschnatunnel

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3.3. Measurement of air velocities

For the measurement of the air velocity in the Gotschnatunnel, we have used two pitot tubes from Schiltknecht which are designed for the use in road tunnels (Steinemann U. and Zumsteg F. 2002). These devices have produced good results in a comparison study in the Gotthard road tunnel. Due to the progress of work in the tunnel, it was only possible to measure the air velocity in the periods 2 and 3 from June 14, 2002 to October 10, 2002. The installation is shown in figure 2.

The results of the measurements of the air velocities in the Gotschnatunnel are displayed in figure 5.

-3

-2

-1

0

1

2

3

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32


Air v

eloc

ity (u

pdra

ught

=pos

itiv)

in m

/s

period 2 from 14.6.2002 to 8.8.2002 (tunnel open)period 3 from 24.8.2002 to 10.10.2002 (tunnel open)

Figure 5: Relation between exterior temperature and air velocity in the Gotschnatunnel

3.4. Consequences

The results of the described measurements led to a re-design of the jet fan thrust in order to fulfill the requirements of the Swiss guideline described in chapter 1. The determinating case was found to be the combination of the exterior temperature of -11°C and the tunnel temperature of +10°C. To cope with the resulting buoyancy force, the Gotschnatunnel will be equipped with 24 uni-directional jet fans with a thrust of 640/270 N and an electrical power of 22 kW each. It is planned to open the tunnel for traffic in 2004.

4. MASUREMENTS IN THE GOTTHARD ROAD TUNNEL

4.1. Description of test site

The measurements of temperature and pressure at Gotthard road tunnel are made under normal operation with cross ventilation. The campaign is still ongoing. The characteristics of the Gotthard road tunnel are:

Tunnel with two lanes in one tube Bi-directional traffic

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Height above sea level 1’120 m (mean value) Total length 16’872 m incl. cut and cover section north of 550 m Grade, averaged 0.4 % (60 m vertical difference of portal height) Cross section of the tunnel 40.5 m2 (under the false ceiling) Hydraulic diameter 6.0 m Fully transverse ventilation system with fresh air and exhaust air in ducts above an intermediate ceiling, extraction-dampers every 96 m in the ceiling

4.2. Measurement of temperatures

On February 4, 2004 we measured the temperature distribution along the tunnel by driving four times through the tunnel with a constant speed of 70 km/h and gathering the temperature data in time steps with a fast responding NTC-thermistor. The results are shown in figure 6. All four passages led to very similar results and show an average temperature in the Gotthard road tunnel of 23.2°C (incl. cut and cover section north). The temperature-valleys are caused by the vertical shafts, through which the ventilation system brings cold air into the tunnel. The exterior temperature in Göschenen (north portal) was around 8°C, in Airolo (south portal) around 12°C.

0

5

10

15

20

25

30

35

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Distance from main portal Göschenen in km

Tem

per

atu

re in

°C

test 1: N => S test 2: S => Ntest 3: N => S test 4: S => Nvertical shaft tunnel incl. Vortunnel

Bäz

berg

Hos

pent

al

Gus

pisb

ach

Mot

to d

i Den

tro

Airo

lo

Gös

chen

en

Figure 6: Temperature distribution in the Gotthard road tunnel on February 4, 2004

4.3. Measurement of pressure differences between portals

The Gotthard road tunnel crosses the Alps, which can lead to significant differences of the barometric pressure between the two portals. To measure the relevant pressure difference is demanding, because it is the difference of two high absolute values which is relatively small. However, with extremely high performance measuring devices and after a long period with device adjustments the pressure measurements are reliable.

The measurements of the pressure differences between the two portals have started on July 24, 2003 and will last for one year at least. Until now the most relevant pressure differences have been measured in the period from December 22 to 26, 2003. The effective 1-hour pressure difference corrected with the height reached a maximum value of 3’700 Pa.

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855

865

875

885

895

905

915

20.12.03 22.12.03 24.12.03 26.12.03 28.12.03 30.12.03

Bar

omet

ric p

ress

ure

in h

Pa

portal south corrected to the height of portal north

portal north

Figure 7: Period with extreme barometric pressure differences between the portals of the Gotthard road tunnel during the presently running campaign

4.4. Consequences and ongoing work

The measurements show, that the buoyancy and the barometric pressure differences can cause significant pressure differences between the portals of the Gotthard road tunnel. The resulting longitudinal flow needs to be controlled, especially in case of a fire in the tunnel. The work is going on to define and implement proper strategies to early detect such conditions and control their consequences rapidly in the case of an incident. ACKNOWLEDGEMENT The measurements in the Gotschnatunnel have been financed by the Tiefbauamt Graubünden, the measurements in the Gotthard road tunnel by the Tiefbauämter Uri and Tessin. All measurements were only possible with the patient support of Hans Mayer of Gabathuler AG, Diessenhofen Switzerland. REFERENCES

• Swiss Federal Road Administration FEDRO 2003: Guideline for the Ventilation of Road Tunnels – Choice of System, Design and Equipment, Draft of December 19, 2003 (in German and French)

• Steinemann U. and Zumsteg F. 2004.: A28a Prättigauerstrasse Umfahrung Klosters – Gotschnatunnel – Strömungs- und Temperaturmessungen im Bauzustand, Schlussbericht US 90-23-17, Januar 2004 (Final report about the measurements in the Gotschnatunnel, in German)

• Steinemann U. and Zumsteg F. 2002: Vergleichende Luftgeschwindigkeitsmessungen im Fahrraum des Gotthard-Strassentunnels, Bericht vom 29. Mai 2002 (Final report about comparative measurements of air velocities in the Gotthard road tunnel, in German)

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