Studies On Fire Development Characteristics In Shaft And Adjacent Space Of Buildings Under External Wind Conditions

In recent years,more and more high-rise buildings have been constructed with the economic development all over the world.At the same time,the catastrophic building fires frequently occur.The building fire safety has attracted more and more attentions,especially for the high-rise building.Statistics have shown that about 85%of victims in building fires were caused by hot and toxic smoke,such as carbon monoxide.In buildings,there are a lot of vertical shafts such as stairwells,elevator shafts and ventilation ducts.During fires,when the fire-induced smoke enters these vertical shafts,the special phenomenon of stack effect may occur.Once the stack effect occurs,it can spread very fast when the smoke enters these stairwells.The stairwell will be full of smoke quickly that makes difficult for the occupants to evacuate and threaten the safety of upper residents.At the same time,a lot of fresh air will be sucked into the building which could make the small fire become larger and may ignite the surrounding combustible and expand the burning area.Buildings are usually in a windy environment.The wind pressure will be formed when there exits wind around the building.The coupled effect of external wind and stack effect will have effect on the fire development and smoke flow characteristics in buildings.In this dissertation,the experiments were conducted in a 1/3th scale stairwell model and a 1/6th scale shaft model.The methods of literature researches,theoretical analyses and experimental studies were combined and adopted to investigate the fire development characteristics in shaft and adjacent space of buildings under external wind conditions.A set of experiments was conducted in a 1/3th scale building model to study the influence of stack effect on flame shapes and heat feedback.Results show that the heat pressure induced by smoke in the stairwell increases with the heat release rate(HRR)and the height of opened window.It is found that the non-dimensional air supplement velocity is proportional to 1/3 power of the product of HRR and opening height and the relationship between the non-dimensional air supplement velocity,HRR and opening height was built.The upper average smoke temperatures in the fire room and atria decrease with opening height increasing for the same HRR and they increase with HRR increasing for the same opening height.The upper average smoke temperatures in the fire room are higher than that in the atria for the smaller HRR.With the increase of HRR,the upper average smoke temperatures in the fire room are lower than that in the atria.The non-dimensional mean flame length is also proportional to the 2/5 power of non-dimensional HRR,but shows smaller than the Heskestad's equation.For the propane porous gas burner,whose HRR is independent of the heat feedback to the burner surface,the average flame tilt angle increases with HRR,but the increment becomes relatively small for the larger HRR.The conduction and radiation heat feedbacks received by the pool increase with the opening height increasing,but the conduction heat feedback remains almost invariant for the higher opening height.With the opening height increasing,the air supplement velocity and the burning rate increase,the convection heat feedback fraction remains almost invariant which indicates that it is the dominating one that influences the combustion process most.It is consisted with the results of References that the convection heat feedback fraction changes little when the Froude number is larger than 1.0.The conduction heat feedback fraction slightly decreases and the radiation heat feedback fraction increases with the increase of the strength of stack effect.A set of experiments was conducted in a 1/6th scale building model to study the influence of positive wind on fire development characteristics in a long passage connected to a shaft.Results show that for certain pool size,there is a critical wind velocity.When the wind velocity is lower than the critical value,the flame tilts towards the outdoor at early stage.The tilting angle gradually decreases and the flame eventually tilts towards the shaft due to the stack effect.When the wind velocity is higher than the critical value,the flame remains tilting towards the outdoor during the whole experiment.At the quasi-stable stage,when the wind velocity is lower than the critical value,the flame of the heptane pool fire towards the shaft and its tilting angle is approximately 71° which is surprisingly independent of the pool size and the positive wind velocity.It indicates that a dynamic equilibrium among the inertial force induced by stack effect and the buoyancy induced by fire is established.The flame tilting angle of the methanol pool fire decreases with the wind velocity increasing.When the wind velocity is higher than the critical value,the flame tilting towards the outdoor,the absolute flame tilt angles of the heptane and methanol pool fires at the quasi-steady stage increase with the wind velocity.The mean flame length of the heptane pool fire gradually decreases with increasing wind velocity and reaches a minimum when the external wind velocity approaches the critical value.When the wind velocity is higher than the critical value,the flame remains tilting towards the outdoor during the whole experiment and the mean flame length at the quasi-steady stage increases with the wind velocity.A non-dimensional number R is proposed to determine the flame tilting direction and its critical value is determined as 0.041 for the heptane pool fire and 0.085 for the methanol pool fire based on the experimental data.While the external wind velocity is lower than the critical velocity,the burning rate of the heptane pool fire at the quasi-steady stage does not change with the wind velocity.As the external wind velocity approaches the critical velocity,the burning rates of heptane and methanol pool fires at the quasi-steady stage decrease as the heat feedback from the boundaries is significantly reduced.Once the external wind velocity exceeds the critical velocity,the burning rates of heptane and methanol pool fires increase with wind velocity.The burning rate of the heptane pool fire is approximately 2 times of the one in open space under the same wind velocity.Moreover,it is found that the average temperature rise inside the shaft has power-law relationship with burning rate while the stack effect takes place,otherwise it remains as the ambient temperature.A set of experiments was conducted in a 1/6th scale building model to study the influence of lateral wind on fire development characteristics in a long passage connected to a shaft.Results show that there exits gas flow from bottom to top in shaft before the fire occurs,namely,a gas flow phenomenon which is induced by the pressure difference formed by the lateral wind acting at the top vent of the shaft.Once fire occurs,the smoke flow and the combustion will be influenced by the coupled effect of the gas flow induced by the lateral wind and stack effect.The effect of lateral wind on the heptane pool fire mainly reflects at the stage from the ignition to the steady burning.For the same pool size,with the increase of lateral wind velocity,the air supplement velocity at the door and the burning rate of the heptane pool fire increase,the steady burning stage starts early and the total burning time decreases.At the quasi-steady stage,with the increase of the lateral wind velocity,the air supplement velocity at the door increases,but the burning rate of the heptane pool fire has no noticeable change which indicates that the promoting effect and suppression effect are cancelled out with the air supplement velocity increasing.The burning rate of the methanol pool fire has no noticeable change firstly and then increases with the lateral wind velocity.It indicates that the promoting effect and suppression effect are cancelled out with smaller air supplement velocity and the promoting effect plays a leading role in the burning with larger air supplement velocity.The mean flame length of the heptane pool fire decreases with the increase of the lateral wind velocity,but the average flame tilt angle does not vary obviously because the thermal buoyancy and horizontal inertial force reach a dynamic equilibrium.The mean flame length and average flame tilt angle of the methanol pool fire decrease with the lateral wind velocity increasing.