Taking Shiziyang tunnel as the background

the numerical simulation of fire was carried out under the conditions of different fire sources power

different ventilation velocities

different slopes and different equivalent diameters.The change law of backflow distance of smoke in the tunnel was analyzed

and the data of simulation was fitted.The result indicated that there are line relationships between backflow distance of smoke and natural logarithm of ventilation velocity

fire source power and tunnel equivalent diameter

increasing or decreasing.The impact of slope on backflow distance of smoke weakened with the increase of ventilation velocity.And then

relational expression between backflow distance of smoke and fire source power

ventilation velocity

and equivalent diameter was proposed based on analysis of change laws of backflow distances of smoke

and backflow distance formula was built up through data fitting.The formulas’ rationality and advantage were analyzed.