Full-scale experiments on fire smoke spreading respectively under natural and hybrid ventilation in a real urban road tunnel with shafts

•Three 1.6 m (length) × 1.6 m (width) diesel oil pool fire tests were conducted in a real URTS;•Much smoke was exhausted of shafts, but backflow occurred at those away from the fire sources;•Fans contributed to the smoke discharge of non-fanned shafts close tightly to the fanned-shafts;•Inhibition coefficient fi is proposed to evaluate the fans influence on the non-fanned shafts’ flow. In the past two decades, some shallow-buried urban road tunnels with shafts (URTS) have been built and operated in large cities in China, and successfully released the traffic congestion. However, uncertainties of fire smoke exhausting and individual safety have still been worried. Here, an idea of installing and operating the axial exhaust fans inside the unit shafts near to the fire source is proposed, then a hybrid ventilation is formed that combines both natural and mechanical ventilation. Three sets of full-scale fire experiments were carried out on a real URTS in Nanjing China. A 1.6 m (length) × 1.6 m (width) pan was placed on the ground in middle of the 264 m longest buried section. Diesel oil was used as the fire source. Fire 1 was conducted in a hot summer under pure natural ventilation without fan working. Fires 2 and 3 were conducted in a cold winter respectively without and with three SHANGFENG HTF-Ⅱ-5.5C fans working. Smoke parameters of temperature, velocity and CO concentration were recorded by monitors respectively under the ceiling, inside the shafts, on the shaft tops and at the shaft bottoms. Inhibition coefficient fi is proposed to evaluate the influence of the fans working on the smoke flow of those non-fanned shafts. Results show that the dual power-law exponent function provided a better fit for the ceiling temperature decay, and the decay coefficients ranged from 0.042 to 0.069 with the sortation: Fire 2 > Fire 3 > Fire 1. On top openings of the shafts, parameters reached up to 47 °C in temperature, 4.6 m/s in velocity, and 120 ppm in CO concentration. Mass flow rates of the unit shafts differed in range of 0.24 ∼ 2.03 kg/(s·m2) that decreased away from the fire sources in Fires 1 and 2, but increased away from the fanned-shafts in Fire 3. Wang model provides a better similar result to that under measurement. The coefficients fi ranged from −0.5 to 1.29. The fan operations did promote the smoke discharge of the non-fanned shafts close tightly to the fanned shafts, but reduce the discharge some far away from the fanned shafts. This study provides on-s