Study of smoke backlayering during suppression in tunnels

•The sprinkler system is tested in conjunction with a longitudinal ventilation system in tunnels.•The effectiveness of the longitudinal ventilation system in smoke control was improved.•Smoke backlayering can be controlled with a velocity lower than the critical velocity.•A method to predict the backlayering when the sprinkler system is used was developed.

Full-scale suppression tests and Computational Fluid Dynamics (CFD) simulations were carried out to investigate the effect of Water-based Fixed Fire Fighting Systems (WFFFS) on the effectiveness of longitudinal ventilation systems in resisting smoke backlayering in tunnels. Test results show that WFFFS enables longitudinal ventilation systems to resist smoke backlayering with a lower velocity than the critical velocity for the same size of tunnel fire. Based on data obtained from the test program and the simulation program, a design method is proposed to estimate the velocity required to resist smoke backlayering in tunnels when WFFFS is active.