Analytical and numerical study on natural ventilation performance in single- and gable-slope city tunnels

Natural ventilation plays an important role in reducing air pollution caused by idle automobile exhaust in traffic-blocking city tunnels, especially in sloping ones. In order to investigate the effects of natural ventilation in both single- and gable-slope city tunnels, a set of simplified analytical models was proposed in this work based on the equations of continuity, energy, and pressure, and the theoretical values of airflow temperature and velocity were obtained by iterative calculations. The influences of thermal pressure, tunnel slope, tunnel geometry, and shaft on tunnel ventilation performance were discussed. In the single-slope-shaftless, single-slope-single-shaft, and gable-slope-single-shaft tunnels, the calculated ranges of average temperature and velocity of airflow were obtained and analyzed, respectively. Moreover, the computational fluid dynamic (CFD) method employing the software FLUENT was selected to simulate the temperature and airflow velocity distributions in different types of tilted city tunnels. The variation tendencies of both analytical and CFD results were consistent. The iterated values of airflow temperature and velocity fitted the simulation results well. Results in this work can provide a reference for the optimal design and management of natural ventilation system and contribute to improving the air quality and controlling the pollutant concentration in single and gable sloping city tunnels.