Experimental Study of Sub-critical Velocity in Longitudinally Ventilated Tunnels

摘要

The critical velocity is the minimum velocity blowing towards the fire seat in tunnels to reduce the length of back-layering to zero, and it has been extensively adopted in the design of longitudinal smoke control in tunnels. A new concept, i.e., sub-critical velocity, is proposed in lieu of critical velocity for longitudinally ventilated tunnels. The sub-critical velocity allows the presence of back-layering upstream of the fire, provided that the back-layering is kept above the heads of motorists, thus posing no detrimental impact on them during evacuation. Experiments with propane fires with heat release rates of 3.7-25.5 kW were conducted in a 1/20 reduced-scale model tunnel. Both the lengths and the heights of back-layering at varying HRRs and longitudinal velocities were studied. Subsequently, the relationship of critical velocity and sub-critical velocity were developed. The results showed that sub-critical velocity is around 45-60% of the critical velocity. The adoption of sub-critical velocity in the design of longitudinally ventilated tunnels not only significantly reduces the cost of fans to drive the airflow, but also minimizes the impact of ventilation on heat release rates.

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