Modeling effective heat transfer and ventilation in deeply buried underground tunnels

Underground traffic tunnels serve as entry and exit buildings for structures underground spaces. Most ventilation and air-conditioning systems in underground spaces rely on traffic tunnels for cooling or preheating to save energy. To analyze the cooling or preheating performance of the traffic tunnel, a combination of the energy conservation law, boundary layer theory, and field measurement was used to establish a simple heat transfer model of the deeply buried underground tunnel for smooth and rough conditions. Moreover, the dimensionless analysis method was used to perform a dimensionless analysis of the results. Field tests found that the air temperature in the traffic tunnel harmonically fluctuates from 20.2 °C to 29.4 °C at the entrance with time. The fluctuation amplitude decreases along the tunnel, and the air temperature difference between the peaks and troughs is only 1.2 °C at the outlet. In summer, the air temperature decreases and increases exponentially from 29.4 °C to 23.4 °C and from 20.2 °C to 22.2 °C along the tunnel during the day and night, respectively. The convective heat transfer coefficients are 2.31 W m−2 K−1, 4.05 W m−2 K−1, and 3.32 W m−2 K−1 under smooth, rough, and test conditions, respectively. Dimensionlessizing the air temperature found that the established model is a good description of the exponential decay of air temperature. Combined with the heat transfer models, the effective heat transfer analysis showed that the effective and optimal heat transfer lengths of the underground traffic tunnel were 1545 m and 628 m, respectively. The established the heat transfer model that calculated the temperature distribution of underground traffic tunnels provides important theoretical support for underground traffic tunnels as natural air conditioners, which have important economic and environmental benefits. •Simple heat transfer models were established for smooth and rough conditions.•Air temperature in the traffic tunnel harmonically fluctuates with time.•Dimensionless air temperature decays exponentially along the tunnel.•Effective heat transfer analysis was carried out for underground traffic tunnel.