VENTILATION SYSTEM DESIGN FOR A ROADWAY TUNNEL IN ACAPULCO

MEXICO

Objective of the investigation:

Determine the number and arrangement of jet fans to be installed in the Acapulco Tunnel that will ensure an air quality that meets international standards for vehicle occupants.

•Cement coating finish

•Results comparison with specialized design programs

•axial and transversal distribution of pollutants

Acapulco Roadway TunnelLenght: 3 km Time savings during congestion: 1 h to 10 min.

PHOENICS

GENERAL EQUATION

S

xxxu

t kkk

k

0 = x

U

i

i

S + U + x

p

1 - =

x

U U Ui2 t

ij

ij i

S + C

= x

C U C

2

C

t

jj

k c = =

2t

t

For carbon monoxide (CO) concentration:

Vehicle emissions represented as a line source of5.5 ppm/(m3s) per lane, corresponds to the designvehicular density of 1,381 pv/h per lane

(pv := passenger vehicle)

Emission factor of fCO = 1 m3/(h.pv)(U.S. emission factor standard for 1989 was 0.6 m3/(h.pv)

and 2.1 m3/(h.pv) in 1967)

Preliminary calculations show that 10 air volume changesper hour would allow sufficient dilution of the CO gas so that the maximum concentration would be 150 ppm.

This is a flow rate of 716 m3/s.

Jet-fan characteristics:

•45 kW rating jet-fans have a thrust of 2.86 kN

•Air density of 1.19 kga/m3

•Jet area at the exit of 2.084 m2

•Exit velocity of 34 m/s.

Acapulco Tunnel geometrical characteristics:

•length of 2,947 m

•cross sectional area of 87.44 m2

•hydraulic diameter of 9.66 m.

PERSPECTIVAS

R6.15

2.3

0.65

0.20

12.3

Tunnel dimensions in m

Jet fan arrangement for 24 units of 45 kW rating

Grid distribution in the transverse plane

RESULTS

Cement finished tunnel walls with a surface roughnessof 3 mm, there was an improved performance for 24 jet-fans with 3 fans in the cross section as CO concentrations were lower than 100 ppm in the last 500 m of the tunnel, with a low rate of 760 m3/s.

Momentum source and sink that is due to the vehicledrag in the three tunnel lanes.An increase in wall-roughness was also considered due to tunnel appurtenances such as signs, cable ducts, for which a friction factor of 0.025 was implemented that correspondsto an equivalent wall roughness of 2.5 cm.

In this case the flow rate obtained with 24 jet-fans, 2 in the cross section, was 469 m3/s, a 37% reduction. However, the maximum CO concentration was 148 ppm near ground level at 22 m from the exit portal, which is under the 150 ppm design limit.

RESULTS

The momentum sink for the two counterflow lanes was22.25 kN, while the momentum source for theconcurrent lane was 3.2 kN.

If the tunnel is unidirectional (i.e. the three lanes arewith concurrent traffic/air flow) then for the same power input there would be a 50 % increase in the flow rate and a 20 % decrease in the maximum CO concentration.

CONCLUSIONS and RECOMMENDATIONS

A fairly straightforward model that calculates the requirednumber of jet-fans for an axially ventilated roadway tunnelhas been implemented in PHOENICS. The optimum designcan be investigated based on the spatially calculated carbonmonoxide concentration distribution.

The overall effect of vehicle drag for countercurrent trafficlanes is considerable; and although it was herein simplifiedby «moving plates» the results are conservative thus providing a «safety factor» for the maximum ground levelCO concentrations allowed.

The calculations derived form an integral approach arebased on a very rough estimate of the overall friction factor, and the drag coefficient that is experimentally derived for one vehicle. The overall result would tendto overestimate the required number of jet-fans.

CONCLUSIONS and RECOMMENDATIONS

The intermittent nature of vehicle drag effect should befurther investigated to have a more realistic mixing effectof the ground level vehicle emissions, and for the fan power requirements for a design airflow rate.

The interaction of the wind on the tunnel portals and trafficinduced airflows should also be further investigated in order to derive worst-case scenarios.

The present model can also be used to investigate a jet-fan operational policy in case of a fire inside the tunnel to ensure maximum safety for trapped vehicle passenger evacuation as well as for fire fighters.