Longitudinal ventilation used in road tunnels requires control of pollutants at the portal exit, especially for tunnels in urban or residential areas.
The use of longitudinal tunnel ventilation in road tunnels is arguably the preferred method for modern tunnels, in particular for those classified as ‘long tunnels’, with lengths generally over 1500 metres (4921 feet). However, the use of longitudinal ventilation often brings with it the need to control portal emissions, especially for tunnels located in urban or residential areas.
In a longitudinal system the concentration of pollutants in the tunnel air generated by vehicles transiting the tunnel increases continuously from the entry portal to the exit portal (refer to Figure 1).
Ideally, the tunnel air would need to be captured prior to exiting the tunnel, and discharged with sufficient dispersion so as to meet ambient air quality limits at the nearest sensitive receptors. This typically requires the ability to control the tunnel airflow so that a net inflow of fresh air is able to be maintained through the exit portal against the traffic direction, generally at a steady-state inflow velocity of between 0.5 – 2.0 metres per second (1.7 – 6.6 feet per second), depending on the aerodynamic characteristics. Consequently, the installed capacity of the tunnel ventilation system may not be driven by air quality demands, but rather by the need to control the piston-driven airflow generated by vehicles.
Parameters such as the maximum design traffic speed and the proportion of heavy goods vehicles become more critical when designing for portal emission control. Figure 2 provides an overview of a typical longitudinal ventilation system with portal emission control and point extraction prior to the exit portal.
The dilution air that is required to maintain in-tunnel pollution limits is introduced into the tunnel through the entry portal(s) and extracted via the exhaust system, just prior to the exit portal(s). Depending on the traffic flow and tunnel length, the quantity of airflow generated by the piston effect can be in excess of this dilution air, which would need to be extracted by the exhaust system, with longer tunnels requiring greater exhaust capacity. Figure 2 schematically shows the location of the exhaust system within the context of the tunnel and associated ventilation equipment. The capacity of the exhaust system needs to be sufficient to capture both the main tunnel airflow and the additional fresh air inflow through the exit portal.
Aerodynamic efficiencies can be improved by augmenting the exhaust system capacity with jet fans within the tunnel to control tunnel and portal airflows (see Figure 3).
In a unidirectional tunnel the aerodynamic drag of the vehicles moving in one direction creates a piston effect which generates air flow in the direction of traffic. In a simple longitudinal ventilated tunnel without portal emission control, the piston effect is utilised with the jet fans augmenting the flow when required. When portal emissions are to be controlled, jet fans are often utilised to retard the traffic piston effect along the length of the tunnel, to reduce the required extraction system capacity just prior to the exit portal. The number of jet fans needed to operate at any one time is dependent upon a number of factors including the ventilation system exhaust rate, the traffic speed, the fleet vehicle profile, the tunnel geometry, and the capacity of the jet fans.
Generally, the relationship between the ventilation system exhaust rate and the number of jet fans required to maintain a portal inflow is an inverse one, such that higher exhaust rates require fewer jet fans to operate.
Designing for portal emission control can be complex due to the dynamic effects of real world traffic. The piston effect from moving vehicles often dominates the tunnel aerodynamics, particularly during peak vehicle flow at maximum design speed. The ventilation system often has to operate throughout the day in order to maintain control of portal emissions and therefore the system operating cost is an important consideration during the design phase.
Mid-tunnel exit ramps
Portal emission control becomes more complicated and challenging when mid-tunnel exit ramps are introduced into the tunnel alignment creating an additional exit portal. A dedicated exhaust point incorporated prior to the additional exit portal would require an additional exhaust plant and ventilation outlet near the exit ramp, or a complex arrangement of ductwork/adits, that connect the exhaust point to the main ventilation plant. Apart from additional cost implications, this has the potential to create unwanted environmental issues and community response.
The use of jet fans to control the airflow in the ramp should also be considered. The jet fans would be used to generate sufficient reverse airflow (against the flow of traffic) in the ramp to overcome the piston effect generated by the vehicles and induce a net positive inflow of air through the exit portal. Although controlling the ramp flows in this manner can introduce additional challenges for the control of the system as a whole, it is widely regarded as a practical and cost effective solution to an otherwise technically and environmentally challenging problem. This is particularly the case where peak traffic flow is limited to a small number of hours per day.
The ramp length must be sufficient to accommodate the installation of the minimum number of jet fans required to achieve the reverse flow. In addition, the introduction of airflow from the ramp back into the mainline tunnel would increase the concentration of pollutants and the minimum required exhaust capacity of the main ventilation plant, which would need to be accounted for. Additional jet fans may be required to operate within the mainline tunnel sections to augment the ramp airflow control.
Short-term dynamic effects
In order to ensure that portal emission control is maintained, it is also important to consider the short-term transient effects that fluctuations in the traffic volume, profile, or speed can have on the portal inflow condition. The tunnel exit section is typically short in length with the air within that section having a relatively low inertia making it susceptible to short-term fluctuations due to traffic. A short-term increase in the number of heavy goods vehicles travelling through the tunnel in a convoy could generate a piston effect sufficient to not only reduce the portal inflow but also create an outflow condition.
Short-term effects can be managed by maintaining a relatively high portal inflow condition, although this could result in relatively high operating costs. An inflow condition of approximately 1.0 metre per second (3.3 feet per second) should be sufficient for most cases; however, depending on the specific project design parameters, an inflow condition as low as 0.5 metres per second (1.7 feet per second) may also be acceptable.
A complex tunnel geometry also could affect portal emission control. The bifurcation of an exit ramp after the ventilation exhaust point (as shown in Figure 4) can make it difficult to control portal emissions, especially if the exhaust point is located at the side of the tunnel, rather than the top. The bifurcation creates two separate exit portals with a large tunnel cross section just prior to the bifurcation.
An inflow will need to be maintained for both exit ramps; however, due to interconnectivity, they can be particularly susceptible to short-term fluctuations in the volume of traffic exiting either ramp. Analysis has shown that a higher portal inflow condition of approximately 1.5 – 2.0 metres per second (4.7 – 6.6 feet per second) could be required in order to maintain an acceptable portal inflow condition for bifurcated exit ramps. The inflow of air through each exit portal can be achieved independently of the bifurcation geometry, the traffic flow, and the physical location of the exhaust point within the tunnel, provided that the mainline jet fans are utilised to assist in the control of the airflow. An increased portal inflow condition does, however, increase the fan capacities, which could be compensated for by reducing the cross-sectional area of the portal openings.
It has been shown that there are engineering solutions available for the control of emissions through tunnel exit portals. However, we recommend that a holistic approach be taken prior to committing to a portal emission control ventilation strategy and that it not be carried out in isolation from other disciplines.
The use of jet fans to control in-tunnel airflow and exhaust fans to extract the air from the tunnel is a relatively costly strategy, considering that the fans may be required to operate on a 24-hour basis, depending on the traffic and air quality limits. The decision to go ahead with portal emission control should be undertaken with support from air quality assessment, dispersion modelling, power demand, and future climate change projections on power cost, carbon emissions, community consultation, and whole-of-life assessment.
Relaxing portal emission control during off-peak, shoulder, and night time operation should also be considered, provided that portal emissions are monitored, ambient air quality limits strictly observed, and community consultation and subsequent buy-in is obtained.