Figure 1 Westbound Saccardo nozzle and inlet duct physical flow model
Good ventilation is vital to modern transportation tunnel design, particularly for ensuring proper environmental health and safety.
How does a design firm ensure minimum energy requirements and adequate ventilation at the same time?
Jacobs Engineering Group is currently under contract with the Washington State Department of Transportation (WSDOT) to provide design services for fire and life safety systems improvements within the Mount Baker and Mercer Island tunnels along I-90 in the Seattle metro area. One of these tunnels is the Mercer Island Lid, officially the Luther Burbank Lid (Lid), a half-mile concrete and steel structure that covers a section of Interstate 90. The Lid also happens to be home to a landscaped park, “Park on the Lid” (Figure 1).
The ventilation concepts for the eastbound and westbound lanes call for converting the existing transverse ventilation systems to a longitudinal design. The design includes use of quasi-two dimensional Saccardo nozzles just inside the entrances to entrain fresh air and introduce it at a high velocity into the tunnels. Also, single large exhaust ports will extract air from just inside the exit of each tunnel.
Jacobs performed CFD modeling to optimize the nozzle designs for both the East-bound and West-bound passages, and contracted Alden to complete a physical flow modeling study of the nozzle designs in order to verify the pressure loss coefficients determined by CFD analysis.
For reasons of practicality, the high aspect ratio nozzles were tested in “slice” models (not the full width). The scale of the models (1:6) and the width of the slice (24 inches) were set such that wall effects were minimal with respect to their contribution to the nozzle pressure loss (Figure 2).
Figure 2 The “Mercer Island Lid”
Root mean square velocity variation measured at the nozzle outlet was found to be very uniform (3% or less) for both nozzles, and the loss coefficients were very low, and in agreement with the Jacobs CFD models, providing confidence in moving forward with construction.