Flow fields induced by longitudinal ventilation and water spray system in reduced-scale tunnel fires

Computational Fluid Dynamics (CFD) simulation results, obtained with FDS 6.0.1, are presented of reduced-scale tunnel fire tests with longitudinal mechanical ventilation and an activated water spray system. This is a sub-set of a more extensive data set, in the context of blocking fire-induced smoke tunnel tests conducted previously. As discussed in Sun et al. (2018), CFD can complement the experimental data, providing thermocouple tree temperature measurements, yielding insight into the turbulent flow field generated by the fire and the water spray. The CFD settings, decided upon on the basis of an extensive mesh sensitivity study in Sun et al. (2018), are applied here to illustrate the impact of longitudinal mechanical longitudinal ventilation in cases with and without the water spray system activated. Analysis of the mean flow and temperature fields provides significant additional insight into the impact of the combined longitudinal ventilation and the water spray system on the fire-induced smoke flow. The upward flow in between the water sprays in the absence of longitudinal ventilation is still observed, but it does not impinge onto the ceiling anymore: the horizontal momentum from the mechanical ventilation makes the 'pushing' force of the spray-induced downward flows impinging onto the floor less strong and moves them further downstream. Consequently, the smoke crosses the water spray system region. The smoke stratification is lost downstream o f the spray region. Upstream of the water sprays, and throughout the tunnel in the absence of the water spray system, a wavy pattern is obtained in the smoke layer. This is related to the Froude or Richardson number for the flows with the settings at hand. The presence of the water spray system downstream of the fire affects the flow and temperature fields up to the fire source.