Wind Effect on Smoke Movement in a Building

Wind Effect on Smoke Movement in a Building

Wind effect on smoke movement in building

Literature review

Smoke, as it was described in this course program, is usually a mixture of gases and hot vapor liberated during the burning process along with the condensation matter, entrained air, and unburned decomposition1. Therefore, when investigating the motion of smoke in a building as a result of air flow or wind, it is paramount to divide a structure into two regions which are; cool and hot smoke zones.

Hot smoke zone; refers to those regions within a building where smoke temperature is high to the degree that the inherent buoyancy of smoke will cause the smoke to rise upwards whilst wind (less polluted air) flows to occupy the lower portions of the room. In normal circumstances, this condition is normally blatant within the room where the fire originated. In this scenario, the buoyant forces induced by the fire plays a greater role than wind in determining the motion of smoke into adjacent rooms and the corridor.
Cool smoke zone; are those areas in a building where the different forms of heat transfer and mixing cut down the buoyant effects induced by fire. In this scenario, wind plays a vital role in determining the motion of smoke around the building.

Countless researches have being conducted by scholars and fire engineers on the effect of wind on the motion of smoke in a fire-involved building due to the potential threat that smoke bears. Smoke, intrinsic in all the unwanted fires in buildings, is a dangerous product of combustion. It has the potential of causing vital influences on the protection of property, life safety, and the fire suppression practices deployed in a given building. During certain fire incidences, the volume of smoke can be so great to the extent of occupying the entire building and also obscuring visibility at the street level making it even difficult to spot the fire-involved building2. Notably, in other incidences, the volume of smoke being generated might not be too much but its danger to life possibly will not be totally diminished due to the influence of wind. As such, wind action is an important feature while studying smoke motion. In this regard, short and tall buildings behave rather differently to the mass of air moving over the roof and the four sides of the building. Normally, the windward wall of a building is usually subjected to inward pressure whilst the leeward side and the two side walls are usually subjected to suction or outward pressure as a result of air pressure distribution caused by wind moving over and around the structure2. Thus during structural design, engineers usually consider pressure and wind mappings on the walls as the key parameters in optimizing the master plan so as to make the structure fire safety. Diagram 1 is a clear illustration of a wind mapping around a building.

Diagram 1 Wind Mapping

Diagram 1 clearly shows that positive pressure occurs on the wall that faces the wind (the windward side) while negative pressure occurs on the opposite wall (the leeward side) as well as the parallel walls. A smaller suction is produced at the bottom part of the leeward side due to the large but slow moving eddy experienced at the leeward façade as wind flows over the structure. Generally, the phenomenon usually results in suction inside the building which causes large flow of air into the building via openings such as doors and windows. Correspondingly, the wind induced pressure difference around a building influences airflow via the external openings hence determining the motion of smoke in the event of an unwanted fire.

Diagram 2

Diagram 2 above is an illustration of a fire incident. Positive pressure created by wind is on the left side and suction is on the opposite side just like in diagram 1. However, in this scenario, the structure has an opening at the windward side near the ceiling. At the leeward side, the opening is located near the floor. If there exist null or little thermal difference between the inside and the outside of the building, wind will flow into the room through the higher opening at the left and leave via the lower opening, that is, the movement of air will be downward. However, this might not be the case in the event of an inferno. According to research, buoyancy due to fire will cause air and smoke to rise upwards and thus wind will provide an opposing effect on heat buoyancy3. In such a scenario, thermal buoyancy and the magnitude of wind will compete within the fire room and eventually the stronger parameter will determine the direction of movement of smoke. In case the velocity of wind exceeds the critical Froude number (critical wind speed), wind will cause the smoke inside the room to move downwards and if the wind’s speed is lower than the critical Froude number, wind will of course move upwards due to thermal buoyancy.

Based on studies focusing on fire behavior and smoke motion in buildings, it has been discovered that ambient wind plays an important role in smoke dispersion behavior. At higher altitudes, there is that critical velocity of wind beyond which the direction of the motion of smoke is largely determined by wind and not buoyancy. Remarkably, beyond the critical speed wind action can overwhelm mechanical ventilation routines in extracting smoke from a fire-involved building4. In addition, ambient wind plays a complex role in determining smoke temperature which affects buoyancy and movement of smoke. Per se, the number and location of external openings alongside the direction and magnitude of wind are key parameters when determining the movement of smoke within a building.

According to Klote and Nelson (1997), short, wide structures usually cause a large amount of wind to flow over the roof with equally less wind moving along the sides while tall narrow buildings usually cause large amount of wind to move along the path offering least resistance which is along the sides of the building whilst allowing least amount of wind to flow over the top5.

Primarily, the different velocities of these motions of wind are the ones which usually influence the directions and amount of air pressures on a building hence influencing even the movement of smoke in the event of an unwanted fire.

The velocities and directions of wind over and around a structure might vary depending on two influences, and they are; ground effect and structures.

Ground effect; during a normal scenario, where there is a usual arrangement of terrain and structure, turbulence as well as friction, occurring as wind moves over the ground, usually result in lowest wind speed at the ground level. The velocity of the wind usually increases as altitude increases.

Primarily, surface roughness at the air-ground boundary layer can help in coming up with the vertical profile of the average wind velocity, approximated using the power-law expression;

6

Where (z0) is the provided meteorological reference of wind velocity recorded at the standard height (z0) which is 10 meters above the ground level6.

Within the urban zones of big cities such as Hong Kong, it is estimated that the mean wind velocity at 32 m above the ground level is usually around 6 m/s and the maximum velocity normally exceed 40m/s. This implies that the pressure at the top of the building can build up to 240 Pa. during maximum wind. The higher momentum of wind will definitely dominate the smoke dispersion and flow behavior.

Structures; natural features (like trees) , man-made features, and buildings have the potential of inducing localized effects which can in turn decrease or increase the speed of wind and also alter the direction of wind.

Wind velocity can have considerable influences on the motion of smoke in the event of a fire. The speed plays a critical role in influencing air pressures as well as suctions which inherently modify the movement of air within a building7. For instance, negative air pressure at the roof of a tall building can result in an aspirating effect at the vertical shaft left open at the roof level. Such a situation can result in the observed draft exceeding the theoretical draft hence altering the anticipated movement of smoke within the building. It is important to note that variations in air density as a result of stack effect usually play a critical part in determining the pressure gradients around a building. Consequently, these pressure gradients can influence the wind speed in a building hence influencing the ventilation of a building as well as the movement of smoke during combustion.

It is therefore clear, that the action of wind determines airflow patterns around a building during fire and can cause smoke to spread from the fire-origin into other regions of a building. Furthermore, pressure difference around the building shell determine the air flow rate and as such they it is a key aspect that structural designers and fire safety engineers consider while determining the apt mechanical and natural ventilation systems to deploy in a building so as to make it fire safety.

When investigating smoke motion as a result of the action of wind, it is also important to take into consideration the height of a building. The higher a building is the more the speed of wind at the top. Wind usually experience intense mixing within the air-ground boundary layer making the direction and velocity of wind to remain almost constant within the layer despite the slight variations in height.

In the modern world, ventilation assessments in buildings are usually conducted to discover the thermally contented environment with the appropriate indoor air quality. To predict, how the ventilation of a building will perform during normal circumstances and in the event of an unwanted fire, most engineers usually deploy Computational Fluid Dynamics (C.F.D) in addition to the wind/ pressure distribution mappings8. C.F.D can provide useful information with regards to movement of air inside and outside a building even before construction commences. Considering air flow parameters (speed and direction of wind) is very important and has offered the CFD method a considerable success in structures.

In the recent times, ventilation alongside its related disciplines has become an integral part in wind engineering. It is possible to conduct a ventilation assessment by using CFD modeling, which is theoretical, or wind tunnel investigation, which is experimental. Wind-induced (natural) ventilation system is normally preferred by many developers than the forced ventilation systems9. Since many CFD softwares have been developed alongside other energy simulation softwares for buildings, it has now become quite an easy task to determine the possible effects that the different speeds of ambient wind could have on smoke motion during fires.

During the archaic times, when people chose a location to dwell at, majority were mindful of the availability of water and therefore most of the earlier developments were within the valley areas. However, in the contemporary times it has become way much easier to decide the location, orientation, and site of a building based on the environmental as well as the local geographical conditions. Wind loading nowadays usually plays an important role when deciding the site and location of a building. In a scenario whereby there are two buildings side by side and separated by a small gap, during wind flow, there will pressure build up in the gap that will cause the sides of the building to experience higher wind loads hence causing positive pressure while the other sides undergo suction.

Diagram 3 Vortex Effect

As illustrated in Diagram 3 above, when wind is flowing over a high-rise building, a vortex effect is usually created due to the downward flow at the front face. The vortex intensity can be affected by the upcoming winds. Therefore, high wind velocities will cause a higher interface layer. Due to this wind-induced action, it is imperative to close the external openings located on the leeward side so as to control the movement of smoke within the building. Leaving many external openings open during a fire outbreak can easily create a higher smoke layer which will remain stagnant hence reducing visibility within the room making fire control more challenging. In Diagram 3, the speed of wind at the reverse direction just above the ground level might have 150 % of the reference wind velocity. Therefore, it is important to install openings near the floor in such a building so that smoke can be driven out of the building faster. Computational Fluid Dynamics has made it possible to foresee such scenarios hence making it possible for architects and engineers to determine the most appropriate location to install ventilations and openings so as to control the movement of smoke during a fire outbreak10. Therefore by deploying the CFD analysis, it is possible to have the suitable information (convective co-efficient, solar radiation intensity, and local wind speeds) for deciding on the best natural and mechanical ventilation.

An alternative method to the CFD that has for a long time been deployed in studying the effect of wind on smoke motion in a building is the wind tunnel method. It is an experimental approach that consists of a passage that is tubular and the specimen mounted at the middle11. To create windy conditions, air is usually allowed to flow past the specimen using powerful fan systems. Using suitable sensors, it can then be possible to determine the positive pressure and suction region around a structure hence predict the likely effect of wind on smoke motion in the even t of a fire.

Diagram 4 Wind Tunnel Testing

Diagram 4 above clearly illustrates the wind tunnel investigation. A model of buildings was placed at the middle of the tunnel and by creating the windy conditions that exist at the actual site, one can easily investigate the possible effects that wind will have on the movement of smoke within the buildings incase fire breaks out. The approach was deployed on man-made structures from as early as the nineteenth century in studying buildings that were very tall hence exposing a larger surface to the wind. Notably, this method was used in developing the building codes in most cities in developed countries. However, with the invention of super fast digital computers and advances in the field of CFD, the rate of using wind tunnel investigation in assessing a building is relatively low. But since one cannot rely solely on CFD modeling, most developers and architects usually deploy CFD modeling and wind tunnel testing12.

Primarily, the effect of wind on a building depends on the velocity and direction of wind, the condition of the neighboring terrain, including the shielding effect caused by the adjacent buildings, and the height and shape of a building. This can however be complicated by the spatial varying nature of wind.

Diagram 5 Wind Speed Profile

Diagram 5 above shows a detailed speed profile of wind on a terrain that is assumed to be flat with no shielding at all. It is possible to develop the profile using the CFD modeling. In the above illustration, the mean wind velocity was assumed to be 15 miles per hour (typical wind speed in most big cities) at 33 feet above the ground level. The building is a high rise building and the wind pressure coefficients were assumed to be 0.8 velocity head at the windward side and -0.6 velocity head at both the side walls and the leeward side. It was assumed that the leakage areas were uniformly distributed within the building’s perimeter. Normally, under such an ideal condition for a square building, the leakage areas at the windward side are usually a quarter of the total outside walls leakage areas.

Under such simplified conditions, in diagram 5 with the same leakage areas, it is possible to predict the resultant pattern of airflow. As anticipated, air will flow into the building through the windward side and flow out of the building via the leeward and the parallel sides. Since the total inlet area is less than the gross outlet area, the pressure that will be created within the building will be a negative one as that at the leeward side13. Since the speed of wind increases as height increases, the values of suction or negative pressures within the building will also increase with height. As such, air will tend to flow upwards within the building under normal circumstances as air from outside will tend to flow into the building via the parallel and the leeward sides. However, during fires, the rate of the motion of smoke is usually a function of the air flow rate, and also there is always an instantaneous mixing of air and smoke within the flow paths.

Therefore, using the CFD modeling, it is possible to determine the relative influence of different factors, including the speed and direction of wind, in determining the motion of smoke within a building hence decide the most appropriate ventilation system that one can deploy in a building. Since wind enters into a building through the wind ward side, it is critical to decide on the best size of openings on the windward side so as to control smoke movement. It is usually expected that larger openings at the windward side will cause smoke to move upwards from the fire origin. In case there is fire within the vertical shaft, the high temperature as a result of the inferno will cause air to move upwards and allow downward movement of air within all the other shafts. Such a scenario can easily cause the re-circulation of air and smoke through the heated shaft and this will result in the flowing of smoke into the entire building. The CFD modeling approach makes it possible to vary the thermal conditions in the building hence project the manner which air will flow.

Present studies emphasize on implementing measures aimed at controlling smoke motion so as to offer safety to the occupants in a fire-involved building and limit the damages that smoke could cause incase fire breaks out. Computational Fluid Dynamics has made it possible for architects and engineers to evaluate the different ventilation systems and recommend the most appropriate to deploy on different type of buildings ranging from the high rise buildings to the short structures.

References

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2Yang, J., Yang, Y. and Chen, Y. (2012) ‘Numerical simulation of smoke movement influence to evacuation in a high-rise residential building fire’, Procedia Engineering, 45, pp. 727–734. doi: 10.1016/j.proeng.2012.08.231.

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