Studies On Smoke Movement And Positive Pressure Ventilation Methods In Stairwell Of High-rise Buildings

In recent years,with the rapid development of economy,the large-scale cities in China have been expanding in size.There are more and more high-rise buildings and the high-rise buildings have more and more complex structures.Although high-rise buildings have the advantages of saving urban land and enriching space,they also have problems such as high fire hazard and it has difficulty in fighting and evacuation.There are many vertical passages in the high-rise buildings.After the fire broke out,it would become the main propagation channel of hot smoke,which caused the fire hazard area to expand,resulting in suffocation and casualties of people away from the fire floor.In the previous research,due to the limitations of experimental conditions,researchers used the experimental method of proportional model to explore the flow characteristic and temperature distribution of smoke in the stairwell in the event of fire,and the reaserch is lack of comparison with the full-scale stairwell experiment.The experimental results of the proportional model had certain reference significance,but still had limitations.Therefore,this thesis combined theoretical analysis,full-scale experiment and numerical simulation to explore the effects of different forms such as opening form,heat release rate and positon of fire on smoke flow and positive pressure ventilation system in the stairwell.The thesis is mamly composed of three parts.First,through the combination of full-scale experiment and numerical simulation,the flow characteristics and parameter distribution of smoke under different opening forms(height and open and closed state)were studied.The conclusions obtained from the study are as follows.As the height of the opening position increases,the temperature difference of smoke which is located in upper and lower floors of the opening position and the absolute value of the vertical velocity in the stairwell at the open floor are continuously decreasing,while the height of the neutral plane in the stairwell first increases and then decreases.The maximum area affected by the smoke and the concentration of CO is gradually increasing as the position of the opening increases.The opening and closing of the top and bottom openings affects fire behavior in the atria and the temperature distribution within the stairwell.When the top opening is closed,the attenuation coefficient of the temperature in the stairwell is twice larger than the top opening condition.When the bottom opening is opened,the attenuation coefficient of the temperature in the stairwell reduces.Secondly,the numerical simulation method was used to study the flow characteristics and parameter distribution of smoke in the stairwell under different fire posiiton(horizontal and vertical).The conclusions of the study are as follows.When the fire source is at the door,the temperature at the ceiling of the atria is lower than that when the fire source is at the wall.The temperature in the stairwell is arranged from the large to the small is the position at stairwell door,the positon at atria door,the positon in the center of atria,the positon by the side of wall which facing stairwell door and the positon by the side of wall which facing atria door.The trend of pressure distribution in the stairwell under different horizontal positions of fire sources is the same.The farther the fire source is away from the doorway,the lower the height of the neutral surface is.The temperature distribution under the ceiling of the atria and the position of the highest temperature will change as the height of fire source changes.The same fire source height,when the floor doors are all open,the temperature distribution of the atria is more uniform than the conditon when the door of 1th and 21th fire floors are open,and the temperature is lower.When the floor door is fully opened,the maximum temperature in the stairwell increases gradually with the increase of the fire source position,while the attenuation coefficient increases first and then decreases.At this time,the pressure distribution is basically not affected by the fire source position.When the door of 1th,21th and fire floors are open,the maximum temperature and maximum pressure in the stairwell are decreasing with the increase of the fire source position,and the temperature attenuation coefficient is also decreasing.By introducing the fire source height factor,the relationship between the maximum temperature and the fire source height factor under different opening states is proposed.Third,the numerical simulation method is used to study the effects of different influencing factors on the ventilation effect of the stairwell when the fire occurs.The conclusions obtained from the study are as follows.The volume of pressurized air supply has a great influence on the ability of the positive pressure ventilation system to block the intrusion of smoke.The larger the air supply volume,the stronger the ability to block the flow of smoke into the stairwell.By analyzing the temperature and pressure distribution of the stairwell,the air supply volume can be divided into three sections:the zone which hot pressure is not overcome,the zone which hot pressure is basically overcome,and the zone hot pressure is completely overcome.The mass flow rate of each air supply outlet in the stairwell increases linearly with the increase of the pressurized air supply volume,and the lower the height of the air supply outlet,the higher the growth rate.The position of open doors also affects the ventilation effect of the positive pressure ventilation system.As the height of the opening door increases,the temperature and the highest position that the smoke can reach in the stairwell both rises first and then decreases.The increase of the opening height will make increase the positive pressure at the fire floor increase,thereby improving the ability of the PPV system to resist the intrusion of smoke.When the upper floor is open,the mass flow rate of the air outlet in this area will be significantly increased,while the mass flow rate in the fire floor area will decrease as the opening height increases.The location of the fire source has little effect on the ventilation effect of the PPV system.In the case of a small volume of positive air supply,the positive pressure generated by the PPV system under different ignition sources cannot withstand the hot pressure generated by the stact effect.The fire floor divides the pressure curve into two parts:the lower part and the upper part.In the lower part,the pressure gradually increases as the height of the fire source increases,while the upper area decreases as the height of the fire source increases.