Critical review of the common methods to determine the buoyancy-chimney effects in longitudinally-ventilated traffic tunnel fires

Determining the buoyancy-chimney effects (hot smoke stack effects) in the case of a fire in a traffic tunnel with a slope is of key importance in the process of sizing the ventilation system and smoke control. Given the complexity of the unsteady turbulent heated flow of air and combustion products mixture and the transient heat transfer by convection, radiation and tunnel wall conduction, various approximate methods, standards, guidelines, and numerical approaches are used, giving different values for the gas temperature distribution and pressure differences. Depending on the fire heat release rate, tunnel slope, and the length of the fire-affected zone, the differences in pressure can even reach values of over 100%, strongly affecting the ventilation sizing. To evaluate the reliability of available methods, a critical review of various methods is given, and a comparison analysis carried out in the case of a typical unidirectional-traffic tunnel equipped with a longitudinal ventilation system. The temperature field, the pressure rise, and the required number of fan units, required to provide the critical velocity for smoke control, are analyzed.