A Study of Fire Durability for a Road Tunnel: Comparing CFD and Simple Analytical Models

摘要

The durability of various typical tunnel sections in the event of a prescribed 100udMW fire has been assessed. Cast-iron sections, pre-cast concrete sections and in-situ concreteudcut and cover sections are all considered to be part of a 1 km long road tunnel. An analysis ofudthe tunnel constructions and surrounding geology (based on a real tunnel) has led to theudestimation of failure temperatures for the structural elements, internal cladding systems, jetudfans and their fixings. A commercial computational fluid dynamics (CFD) code was used toudsimulate various fire scenarios and calculate the times to failure of tunnel elements.udSimulations were carried out for fires in different locations for the three section types. Inudparallel to the CFD study, an analytical model was devised to predict gas temperatures in theudtunnel. Both models used the same input variables and general assumptions and greatudattention was given to establish the highest possible accuracy for all input variables andudgeneral assumptions. Comparing the predicted gas phase temperatures shows that there isudless than a 20% difference between the complex CFD and the simple analytical model; this isudwell within the bounds of uncertainty inherent in either model and to the input parameters.udUsing both sets of gas phase temperatures, a detailed heat transfer study was carried out toudcalculate the temperature evolution of each of the tunnel elements. The differences in gasudtemperatures between the two modelling methods did not alter the conclusions regarding theudtime to failure of any tunnel elements. It is found that fire durability can be better analyzed byudseparating the fire environment into two zones, a near field close to the flames, whereudaccuracy is defined by the assumptions, and a far field where the precision of the results isudlinked to the modelling method. This approach allows establishing that, for this particularudcase, failure of structural elements can only occur in the near field. This study shows that theuddetail of the calculations needs to be consistent with the accuracy of the input parameters andudassumptions. Although CFD models can give highly detailed results, the implied accuracy ofudthe results is defined by the assumptions inherent in the model setup, thus, there is theudpotential of a very costly and refined computation that leads to results of comparableudaccuracy to simple, less costly, models.