Numerical study on thermal environment in a long-distance underground power tunnel with tangential heat transfer and delay-effect

To prevent air overheating, the designed ventilation rate of long-distance underground power tunnels is calculated using the hottest month meteorological data, causing high energy consumption, so the ventilation system has energy conservation potential. Moreover, existing standards assume that heat transfer in the soil is one-dimensional steady-state without temperature delay-effect, which also leads to inaccuracies. Therefore, heat transfer in the soil around the tunnel is investigated by three-dimensional simulations considering the delay-effect. The results show tangential heat transfer along the tunnel wall and adjacent soil is significant, and the delay-effect greatly affects heat absorption by the soil. To integrate these impacts into the design, multiple nonlinear regression analysis is employed to develop equations for predicting air temperature at the exit and soil heat transfer intensity. These equations can be used to determine the maximum ventilation length and the required flow rates for months when air temperature is below 40 °C. The predictions show that the required ventilation volume during non-hottest months is at least 43 % less than that for the hottest month.