Theoretical and numerical study on the heat extraction of shafts in a naturally ventilated urban highway tunnel fire under ambient wind environment

Shafts are widely used in naturally ventilated urban highway tunnels, and their heat extraction in fires is highly susceptible to external wind conditions, yet research on their heat removal characteristics under relevant conditions remains inadequate. This study combines theoretical analysis with numerical simulations to explore the heat removal of vertical shafts in a naturally ventilated urban highway tunnel under ambient wind environment. The findings indicate that heat removal in shafts can be divided into three stages: (I) plug-holing, (II) stable smoke layer, and (III) downdraught. By non-dimensionalizing the environmental wind speed, the inflection points of heat extraction are determined: when v_wind^⁎⁎ = 3, heat dissipation enters stage II, and when v_wind^⁎⁎ = 6, heat dissipation enters stage III. When downdraught does not occur, v_wind^⁎⁎ = 4.8 corresponds to the most unfavorable point for heat extraction. For the same other conditions, the smaller the fire source power, the more pronounced the suppressive effect of environmental wind on shaft heat removal. A predictive model for internal temperature and heat extraction of the vertical shaft is developed, and the predicted results align well with the simulation data. This study provides valuable insights into the operation of smoke extraction shafts under ambient wind conditions.