Experimental Investigation On The Effect Of Canyon Cross Wind Direction On Combustion And Smoke Movement Characteristics Of Tunnel Fires

With the development of building “the Belt and Road” and China’s strength in transportation,tunnel construction is more focused on mountain areas instead of plain.The toxic smoke and heat produced by fire in a tunnel is hard to be exhausted because of its long and narrow structure characteristics.These will cause the damage and destruction of tunnel lining and structure,as well as the death of poisoning and suffocation,which leads to disastrous consequences.Canyon is a common landform in mountain areas,in which the tunnel inevitably passes through deep canyons.When the air flows through the canyon from the open area,it will be accelerated into high-speed canyon cross wind due to the "channeling effect".The canyon cross wind direction is complex and variable because of the special terrain,landform,season,climate and pressure in mountain areas.When the canyon cross wind in different directions flows across the tunnel portal,the pressure distribution at the tunnel portal and inside the tunnel will be changed,which induces different flow fields in the tunnel and affects the behaviors of tunnel fires.In this paper,the canyon cross wind yaw angle was employed to represent the canyon cross wind direction.The characteristics of flow field,combustion and smoke movement of tunnel fires under the canyon cross wind yaw angle in the range of 45°-135° were investigated by theoretical analysis and reduced scale tests.The main contents are as follows:Firstly,the flow field in the tunnel induced by the canyon cross wind with different wind directions were studied by experimental measurement.Results showed that the complex and threedimensional flow fields inside the tunnel were formed by canyon cross wind,which presented different zone division of turbulence zone,unidirectional flow zone,and negative pressure zone with the boundaries of wind yaw angles 60° and 90°.The airflow was disordered and many vortices exist in the turbulence zone.The airflow was stable and towards the downstream in the unidirectional flow zone.The airflow was stable and towards the upstream in the negative pressure zone.Secondly,the effect of canyon cross wind direction on the burning rate and flame tilt characteristics of tunnel fires were investigated by reduced scale tests.The fire behaviors were mainly affected by the flow field near the fire source.The mass burning rate decreased with the increasing wind yaw angle when the wind yaw angle was in 45°-90°,but remained steady when the angle was in 90°-135°.A power function prediction model of the normalized mass burning rate was developed considering normalized fire source location.The flame tilt characteristics were significantly affected by the flow field.The variation law of the mean flame length and flame tilt angle in the turbulence zone,which was close to the upstream portal,was different from that in other zones because of the influence of turbulence and vortexes induced by the canyon cross wind.A prediction correlation between the flame tilt angle and the canyon cross wind yaw angle and speed was established according to the competition between the fire buoyancy and the induced airflow momentum.Thirdly,the effect of canyon cross wind direction on the maximum ceiling gas temperature and its position,smoke backlayering length and downstream smoke spread length of tunnel fires was investigated by reduced scale tests.The maximum ceiling gas temperature increased with the increasing wind yaw angle in the range of 45°-90°,but remained steady when the angle was in 90°-135°.A linear correlation of the normalized maximum ceiling gas temperature was established considering normalized canyon cross wind speed.The position of maximum ceiling gas temperature was different in both horizontal and longitudinal directions due to the canyon cross wind.The airflow barrier and double peak value phenomenon of ceiling gas temperature were formed by the canyon cross wind.Besides,when the wind yaw angle was in 45°-90°,the smoke back-layering was resisted and the downstream smoke spread was promoted by the canyon wind.However,the smoke back-layering was promoted and the downstream smoke spread was resisted due to the negative pressure induction when the angle was in 90°-135°.This paper responses to the Peaceful China initiative and is helpful to enrich the awareness of tunnel fire accident scenarios under the influence of complex airflow.This study can improve the tunnel fire theory in canyon wind conditions,and provide theoretical supports and reference for the fire protection system and evacuation rescue design of key transport corridors in high altitude mountain areas.There are 42 figures,4 tables and 92 references in this dissertation...