Abstract: A hot smoke test was carried out in a real subway tunnel to verify the validity of FDS simulation results. Three- dimension model of subway tunnel was established and effects of outdoor air design parameters on smoke back- layering length and critical velocity were investigated by FDS. Results of study showed that outdoor air design parameters have important influences on smoke back-layering length in subway tunnel fires. The smoke back-layering is longer when the design temperature of outdoor air is lower. The smoke back-layering is shorter when outdoor air design humidity is bigger. The smoke back- layering is shorter when outdoor atmospheric pressure is bigger.

Key words: outdoor air, air temperature, atmospheric pressure, air humidity, smoke back- layering flow, back- layering length, critical velocity