A theoretical explanation of natural ventilation at roof openings in urban road tunnels

Abstract Natural ventilation through openings in the roof of a road tunnel is a green technique that saves energy and greatly improves the air quality inside the tunnel. However, even though multiple tunnels with natural ventilation have been built in China, the theoretical basis for designing and constructing such tunnels is incomplete and there is as yet no uniform and systematic understanding of the natural-ventilation mechanism at the roof openings. Aimed at these problems and based on nearly six years of research and experiments, this paper presents a theoretical explanation of the flow-field characteristics in a road tunnel and the natural ventilation at its roof openings, including (i) the distribution and governing equations for the flow in the tunnel, (ii) the energy governing equation and (iii) the equation governing the natural ventilation at the roof openings. The presented theoretical formulas are consistent with the measured data. The main conclusions are as follows. (1) The airflow in a road tunnel is drag flow resulting from vehicles driving in a restricted space. The flow develops in a bottom-up manner and tends to be stable. The flow velocity is distributed double-logarithmically with height in the tunnel. (2) Moving vehicles are energy sources for the airflow in a road tunnel. Some of the energy transferred from a moving vehicle to the air increases the airflow velocity, while the rest is consumed in turbulence. As the flow develops, the kinetic energy increment decreases gradually and tends to zero, while the energy dissipation decreases gradually and tends to be stable. (3) The air above a road tunnel with roof openings can be sucked into the tunnel by the airflow induced therein by the unidirectional continuous traffic flow. This creates airflow from exterior to interior, but there is no corresponding ‘breathing’ phenomenon. The air inlet velocity at the roof openings differs considerably along the tunnel. It is maximum near the tunnel entrance and decreases gradually along the tunnel, presenting a concave curve.