Study on air entrainment coefficient of one-dimensional horizontal movement stage of tunnel fire smoke in top central exhaust

•Entrainment coefficient was obtained using theoretical analysis.•Both heat release rate and smoke extraction rate influence entrainment coefficient.•Entrainment coefficient is between 0.01 and 0.03 in central exhausting.

When tunnel fire smoke is exhausted using a top central exhausting system, the air entrainment at the smoke-air interface has a significant effect on the overall mass flux. Using theoretical analysis, the air entrainment coefficient in one-dimensional horizontal smoke movement stage in central exhausting was obtained. Both heat release rate and smoke extraction rate have effects on the horizontal entrainment coefficient. The experimental results show that the entrainment coefficient in central exhausting is between 0.01 and 0.03 and the horizontal entrainment coefficient increases to 0.0181 with increasing heat release rate and becomes relatively stable afterwards. The entrainment coefficient increases with the smoke extraction rate increasing as the smoke extraction process will enhance the smoke momentum in the tunnel.