A set of experiments was conducted to study the influence of external wind on flame shapes of n-heptane pool fires in a long passage connected to a shaft. The competitive effect led by external wind and stack effect was investigated. Results show that for certain pool size, there is a critical wind velocity. When the wind velocity is lower than the critical value, the flame tilts towards the outdoor at early stage. The tilting angle gradually decreases and the flame eventually tilts towards the shaft due to the stack effect. A dynamic equilibrium among the inertial force induced by stack effect and the buoyancy induced by fire is established. At this stage, the flame tilting angle towards the shaft is approximately 710 which is surprisingly independent of the pool size and the external wind velocity. The mean flame length gradually decreases with increasing wind velocity and reaches a minimum when the external wind velocity approaches the critical value. When the wind velocity is higher than the critical value, the flame remains tilting towards the outdoor during the whole experiment and the flame tilt angle and the mean flame length at the quasi-steady stage increase with the wind velocity. A non-dimensional number R, denoting the ratio of horizontal inertial force generated by external wind to vertical buoyancy force causing the stack effect, is proposed to determine the flame tilting direction and its critical value is determined as 0.041 based on the experimental data. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.