Experimental research on the mixture mechanism of polluted and fresh air at the portal of small-space road tunnels

•A testing ventilation system of close tunnel model was established.•The polluted wind of adjacent tunnel portals was mutually mixed under the different working conditions.•A model among interactional degree between tunnel airflow velocities and space between tunnels was established.•Two kinds of reducing ventilation influences (inlet and exhaust) engineering technical measures were proposed. To provide drivers and passengers with an adequate supply of oxygen and to dilute toxic gases emitted from vehicles to safe concentrations, ventilation systems, such as jet fans and exhaust shafts, are needed. To examine the design features of a bifurcate tunnel on Hurongxi highway, a testing ventilation system of close tunnel model was established. This system consists of two model tunnels, a fan, a CO storage tank, a pressure maintaining valve, a flow meter, an anemograph, a CO concentration detector, and so on. Different working conditions were compared and analyzed in terms of the mutual mixing of the polluted wind of adjacent tunnel portals. A relational model of the interactions between the tunnel ventilation, inlet and exhaust velocities, and space between tunnels was established. Meanwhile, two types of technical engineering measures to reduce ventilation influences (inlet and exhaust) were proposed.