Model Experiment And Numerical Simulation Of Jet Tunnel Ventilation In Urban Tunnels

The longitudinal ventilation mode with jet fan is widely used in road tunnel ventilation system because of its simplicity and economy. In this ventilation system the polluting gases inside tunnel are driven to the tunnel portal by the pressure-rise effect and speed-up effect of jet fans, then the polluting gases are discharged out from the tunnel portal. In this paper a tunnel ventilation model experiment had been carried out based on the similar principles. By the means of combaining the model experiment and numerical simulation, the airflow inside tunnel had been studied. And then, the reliability of numerical simulation had been verified by compareing the calculated value with the experimental value. Moreover, the3D numerical simulations were established based on the size of a road tunnel to study the velocity distribution and pressure distribution inside tunnel under different series modes, as well as the diffusion of polluting gases outside tunnel portal under different conditions. According to the study results, the best series mode had been picked out and the scope of contaminated area outside tunnel portal had been confirmed, which will provide reference opinions for the ventilation design inside tunnel and the municipal planning around tunnel portal. The following are the main conclusion:1. According to the experimental data, it can be concluded that there was regularity for the velocity variation trends. In the middle line of model tunnel, the velocity decreased with the distance from fan outlet increased. At the position of0.08m away from the middle line of model tunnel, as the distance from fan outlet increased, the velocity first increased then decreased. At the position of0.16m away from the middle line of model tunnel, as the distance from fan outlet increased, the velocity changed little and fluctuated within a certain scope.2. Comparing the calculated value and the experimental value, it can be known that the velocities variation trends were the same, and the difference in magnitude was little.3. The3D steady turbulence models were established by FLUENT to study the velocity distribution and pressure distribution under4series modes. The best series mode had been picked out by comparing the4operational conditions. Therefore, it can be concluded that the ventilation performance depends not only on the series mode but also on the number of fans in series. While no matter what series mode it is, it’s more conductive to enhancing the velocity and pressure inside tunnel when the distance between adjacent groups of fans is short.4. The3D models were established by FLUENT to study the polluting gas diffusion outside tunnel portal in windless condition and windy condition. From the results of simulation it can be known that the polluting gases diffused forward when the surrounding was windless, while the polluting gases diffused along the wind direction when the surrounding was windy. When the surrounding was windy, the polluting gases diffused toward high altitude more slowly than that in the windless condition. About40m high from the ground, the mass fraction of CO had met the specification requirements. However, the diffusion speed toward sides and forward depended on the wind direction outside tunnel portal.