A numerical study is performed to give a quantitative description of the fire development in a horizontal model tunnel with a wind velocity, U-0, ranging from 0.5 to 2.5m/s. Controlling mechanisms of three-dimensional (3D) flow, combustion, soot production and radiation are coupled with a large eddy simulation (LES). The computed, time-averaged flame shape (length/height) and heat flux are compared with experimental data from a simplified model tunnel, and a good agreement is attained. Then, a numerical study on flame propagation over a liquid fuel (heptane) surface in a 3D model tunnel is performed. Predictions of the radiation flux, turbulence intensity and air entrainment velocity show an inverse dependence with wind velocity. For the wind velocity lower than 1.5 m/s, the extent of the flame is up to 9 times pyrolysis length due to lack of oxygen. As the wind velocity increases to 2m/s, the flame length is reduced to approximately 4 times the pyrolysis length because of the sufficient oxygen supply in the reacting zone and development of the counter-rotating vortex structure on the cross-section of the tunnel. The safety limit for which the heat flux is lower than 5 kW/m(2) downstream the fire is about 5 times the pyrolysis zone in length, practically insensitive to the wind velocity. (C) 2007 Elsevier Ltd. All rights reserved.