Influence of the water spray particle diameter and flow rate on the smoke flow and the back-layering in tunnels with longitudinal ventilation

When there is a fire in a tunnel, the heat released by the combustion is difficult to dissipate and the temperature inside the tunnel would rise rapidly. Longitudinal ventilation (LV) has been commonly adopted as the smoke control strategy in tunnel fires, which is essential to safeguard the tunnel structure and the trapped people. Besides the smoke control method by LV, the water mist system (WMS) can provide a powerful cooling effect for the high-temperature smoke in the tunnel. This article aims to investigate the impact of WMS on the smoke back-layering (BL) length with LV and the mutual effect process between the smoke spread and the WMS. The findings show that the BL length is restricted more effectively by the combined action of water mist and LV. Upon activation of the WMS, the water mist’s effect inhibits the smoke’s ability to spread upstream and causes the smoke layer to settle down, which disturbs the stability of the smoke layer. An unstable layer of smoke is more likely to be blown downstream by longitudinal ventilation. All water mist particle diameters show a significant reduction in smoke back-layering with an increased flow rate. The BL length would gradually reduce as the particle diameter is decreased. The tendency of flow rate and particle diameter on the BL length shows similar effects to the critical velocity ( Vcr). The Vcr is reduced with the rising flow rate and increased with expanding particle diameter.