Studies On Effect Of Ambient Wind On Smoke Motion In Vertical Channels Of High-rise Buildings

With the rapid growth of China’s economy and accelerating process of urbanization,an incredible number of high-rise buildings have been constructed all over the nation to meet people’s demands for housing and producing.At the same time,frequent high-rise building fires have urged people to request higher demands for fire safety of high-rise buildings.Statistics have shown that majority of victims died of inhaling hot toxic gases during fires.The fire-induced smoke could spread along the vertical channel fairly quickly and then stack effect is formed.The stack effect can not only facilitate the spread of smoke to upper floors and rooms,but also boost the development of fires.In fact,high-rise buildings are usually in a windy environment when fire occurs and the direction and velocity of external wind could have distinguished impacts on mechanisms controlling smoke movement.Therefore,in this dissertation,experimental research and numerical study are combined together to study the ambient wind effect on smoke motion in vertical channels and fire behaviors in adjacent corridor.On top of that,the groove structure,as a kind of external semi-closed channel,is widely used in the design of high-rise buildings,of which the effect on smoke characteristics is investigated as well.Firstly,numerical simulation is adopted to study the influence of external cross wind on smoke rising characteristics in a full-scale stairwell.Heat release rates and wind velocity are varied.Results show that when wind flows into a stairwell through a broken or open window,for a given heat release rate,there must be a critical wind velocity to determine if the stack effect could be formed.That is,when external wind velocity exceeds this critical value,the stack effect would not occur.Compared to previous research,taking the thermal buoyancy of hot smoke into account,a criterion that could be used to determine the smoke movement(a non-dimensional number η)in a stairwell is proposed and the critical value is 0.1 or so.When the value of η is less than 0.1,the thermal buoyancy are able to overcome wind force.Thus stack effect could be formed gradually and smoke flows out of the stairwell through upper window.Otherwise the smoke flows out of the stairwell through the bottom door and the stairwell remain ambient temperature.Meanwhile,the velocity of cross wind has a negative effect on the rising of hot smoke.By incorporating the non-dimensional number η,an equation is proposed to predict the rising time of hot smoke affected by cross wind in the stairwell.The extended rise time for plume arriving at a given height might cause the occupants residing in upper floors unable to detect the occurrence of fire timely and increase the difficulty of evacuation.The supplement mass flow rate decreases with the wind velocity since the pressure difference near the door decreases.It is also found that the CO concentration in the staircase increases by 15%at maximum.For one thing,decreased upward velocity of smoke makes CO easier to accumulate in the staircase.For another thing,yield of CO increases due to worse ventilation condition when supplement air flow velocity at the door declines.Secondly,further investigation is carried out to study the effect of external side wind on high-rise building fires based on experimental data from our research group including air flow behaviors,combustion characteristics of methanol pools and smoke temperature in a 1/6 scaled corridor connected to a 6-floor shaft.On the basis of research on cross wind from previous literature and my work,the differences and similarities of how two types of wind impact high-rise building fires are studied.The external side wind(parallel to top window)leads to pressure attenuation inside building and induces air to flow inside through bottom door.Thus the flame tilts to shaft immediately,which is completely from that under no wind or cross wind conditions.In the experiments,the supplement air flow velocity through bottom door and the pressure difference greatly increase at high wind velocity while it remains proportional to 1/3 power of heat release rate,leading to faster spreading of smoke.Besides,the relationship between supplement air flow velocity,heat release rate,wind velocity and flame length is established.Compared to no wind and cross wind conditions,the burning rates of methanol pools are accelerated and the combustion is intensified.On one hand,more fresh air entrained through bottom door enhances the combustion and contributes to the temperature rise of smoke.On the other hand,the cooling effect could account for the following decrease of smoke temperature at high wind velocities.Therefore,the temperature is correlated with burning rate and side wind velocity and the proposed equation is also applicable to cross wind conditions.Finally,FDS simulation is adopted to study the study motion characteristics inside the external semi-closed channels as well.The temperature and spread velocity of smoke,the pressure and CO concentration distribution inside the external channel are studied by setting up a full-model groove structure and changing the aspect ratio from 0 to 4.Results show that the side walls can efficiently limit the air entrainment from sides and decrease the contact between hot smoke and surrounding air with the increasing of aspect ratio,which is similar to stack effect.Compared to condition without groove structure,the temperature attenuation slows down;the spread velocity of smoke increases,so does CO concentration.The effect of aspect ratio of groove structure on these three smoke characteristics manifests as the height increases.But as the aspect ratio is larger than 1,all three parameters begin to become stable gradually.The further increase of aspect ratio seems to have slight effect.This study suggests that when fire occurs in the external semi-closed channel in a high-rise building,smoke could spread faster under the influence that is similar to stack effect and it could keep relatively high temperature and CO concentration in high upper floors.