Smoke propagation velocity and maximum temperature in tunnel fires with varying heat release rate

In the early stage of tunnel fires, the heat release rate (HRR) typically increases with time following a t2 function. However, the impact of varying HRR on smoke flow properties remains relatively underexplored. This study primarily investigates the governing mechanisms of smoke propagation velocity and maximum temperature rise in tunnel t2-fires. A simple theoretical analysis is first conducted to identify the key parameters influencing smoke flow properties. A series of large eddy simulations and brine-water experiments is then performed to verify the analysis. Smoke propagation is characterized by three flow velocities: the propagation velocity of the front uf, the average flow velocity within the smoke layer uave, and the maximum flow velocity in the smoke layer umax. For a specific cross section, uave is approximately equal to uf, while umax is approximately 1.46uf. When the boundary heat loss is negligible, uf is approximately 0.71b01/3 for tunnel fires with constant HRR, while it is approximately 0.58b01/3 for t2-fires, where b0 is the transient source buoyancy flux per unit width. Considering boundary heat loss, uf exponentially decreases with the propagation distance Lp under constant HRR, while uf initially increases and then decreases with Lp for t2-fires. The transient maximum temperature rise of a specific cross section is proportional to B2/3, where B is the local total buoyancy flux. After the steady state is achieved, the maximum temperature rise along different cross sections exhibits an exponential decay with distance. The study may offer valuable insights into the smoke flow dynamics in tunnel fires.