The traditional design fires commonly considered in structural fire engineering, like the standard fire and Eurocode parametric fires, were developed several decades ago based on experimental compartments smaller than 100 m2 in floor area. These experiments led to the inherent assumption of flashover in design fires and that the temperatures and burning conditions are uniform in the whole of the compartment, regardless of its size. However, modern office buildings often have much larger open-plan floor areas (e.g. the Shard in London has a floor area of 1600 m2) where non-uniform fire conditions are likely to occur. This paper presents observations from a large-scale fire experiment x-ONE conducted inside a concrete farm building in Poland. The objective of x-ONE was to capture experimentally a natural fire inside a large and open plan compartment. With an open-plan floor area of 380 m2, x-ONE is the largest compartment fire experiment carried out to date. The fire was ignited at one end of the compartment and allowed to spread across a continuous wood crib (fuel load ~ 370 MJ/m2). A travelling fire with clear leading and trailing edges was observed spreading along 29 m of the compartment length. The flame spread rate was not constant but accelerated with time from 3 mm/s to 167 mm/s resulting in a gradually changing fire size. The fire travelled across the compartment and burned out at the far end 25 min after ignition. Flashover was not observed. The thermocouples and cameras installed along the fire path show clear near-field and far-field regions, indicating highly non-uniform spatial temperatures and burning within the compartment. The fire dynamics observed during this experiment are completely different to the fire dynamics reported in small scale compartments in previous literature and to the assumptions made in traditional design fires for structural design. This highlights the need for further research and experiments in large compartments to understand the fire dynamics and continue improving the safe design of modern buildings.

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a b s t r a c tMultiscale modelling of tunnel fires that uses a coupled 3D (fire area) and 1D (the rest of the tunnel) model is seen as the solution to the numerical problem of the large domains associated with long tunnels. The present study demonstrates the feasibility of the implementation of this method in FDS version 6.0, a widely used fire-specific, open source CFD software. Furthermore, it compares the reduction in simulation time given by multiscale modelling with the one given by the use of multiple processor calculation. This was done using a 1200 m long tunnel with a rectangular cross-section as a demonstration case. The multiscale implementation consisted of placing a 30 MW fire in the centre of a 400 m long 3D domain, along with two 400 m long 1D ducts on each side of it, that were again bounded by two nodes each. A fixed volume flow was defined in the upstream duct and the two models were coupled directly. The feasibility analysis showed a difference of only 2% in temperature results from the published reference work that was performed with Ansys Fluent . The reduction in simulation time was significantly larger when using multiscale modelling than when performing multiple processor calculation (97% faster when using a single mesh and multiscale modelling; only 46% faster when using the full tunnel and multiple meshes). In summary, it was found that multiscale modelling with FDS v.6.0 is feasible, and the combination of multiple meshes and multiscale modelling was established as the most efficient method for reduction of the calculation times while still maintaining accurate results. Still, some unphysical flow oscillations were predicted by FDS v.6.0 and such results must be treated carefully.

Natural materials like wood are increasingly used in the construction industry, making the understanding of their ignition and burning behaviour in fires crucial. The state of the art of wood flammability is based mostly on studies at constant heating. However, accidental fires are better represented by transient heating. Here, we study the piloted ignition and burning of medium density fibreboard (MDF) under transient irradiation. Experiments are conducted in a Fire Propagation Apparatus under parabolic heat flux pulses with peak irradiation ranging from 30 to 40 kW/m 2 and time-to-peak irradiation from 160 to 480 s. The experimental results reveal that the critical conditions for ignition of fibreboard vary over wide ranges: mass flux between 4.9 to 7.4 g/m 2-s, surface temperature between 276 to 298°C, and heat flux between 29 to 40 kW/m 2. Flameout conditions are studied as well, with observations of when it leads either to extinction or to smouldering combustion. We explored the experiments further with a one-dimensional pyrolysis model in Gpyro and show that predictions are accurate. Assuming a non-uniform density profile (a realistic assumption) improves the predictions in comparison to a uniform density profile by increasing the mass loss rate by 12%, decreasing the temperatures by 45%, and increasing the ignition time by 20 s. These results further support previous findings that a single critical condition for igntion or flameout established under constant irradiation does not hold under transient irradiation which indicates that ignition and extinction theories need improvements.