Simulation of back-layering length in tunnel fire with vertical shafts

• Back-layering length is researched theoretically and experimentally.
• Effects of shaft dimension, amount and the distance on back-layering length are researched.
• Prediction formula for calculating back-layering length is proposed based on fire plume theory.

It is known that costs to install and operate tunnels with vertical shafts are lower than the conventional tunnel. However, currently, there is no universally accepted theory to calculate and assess the smoke exhausting efficiency when there is a fire in this new type of tunnel. In this paper, theoretical analysis is carried out to predict the back-layering length of fire in the tunnel with vertical shafts. Firstly, a theoretical model is proposed based on fire plume theory. Secondly, Computational Fluid Dynamics (CFD) is used to research the effect of shaft geometry on back-layering length. Thirdly, the calculation results from the proposed model are compared with both experimental data and simulation results. The comparison results show that the proposed theoretical model can be used to predict the back-layering length of fires in tunnel with vertical shafts.