Tongji University 90 READING REPORT Wind effects on smoke motion and temperature of ventilation-controlled fire in tall building Rakitin Oleg Student ID:1134954 Shanghai,2012
Tongji University READING REPORT «Wind effects on smoke motion and temperature of ventilation-controlled fire in tall building» Rakitin Oleg Student ID: 1134954 Shanghai, 2012
1.Introduction Strong ambient wind blowing on the high-rise building will affect fire dynamics and smoke dispersion behavior in the built spaces It has been observed experimentally that the ambient wind has two contradictory effects on the compartm nt fire:promoting re severity by ng the nre by ne le gases.Ihe cts the therma exte e eje from th ong win ue efficiently underthe of wind arch on fire high-rise buildings under wind effects is an essential step to provide a guideline for fire safety design of these buildings In fire safety codes of many countries,the wind effects on tall building fires have not received the desired attention.In Hong Kong's Building Codes,wind effect is considered in designing the refuge floors of high-rise building so as to prevent smoke accumulation in the refuge floors. In the field of ventilation,natural ventilation of buildings generated by temperature differences and by the wind receives much more attention and multiple steady state behavior was found in recent years. r tempe much highe r than the rat associated with 0 cnt fire beha papd te pera r wind action in tall building eroposine 2.Theoretical consideration on smoke motion In high-rise building fires,the location and number of openings of the compartment,together with ambient wind direction,determine the smoke motion in the compartment.The compartment has an opening near to ceiling in the left wall and another opening near to the floor in the opposite wall.The ambient wind blows from left to right in Fig.1. Fig.1.The compartment fire scenario of opposing wind with downward flow As we know,if there is a fire in the compartment,the buoyancy will drive the smoke and air 营 o an the inner pressure at Z- as. i N he pressu
1. Introduction Strong ambient wind blowing on the high-rise building will affect fire dynamics and smoke dispersion behavior in the built spaces. It has been observed experimentally that the ambient wind has two contradictory effects on the compartment fire: promoting fire severity by more oxygen supply and cooling the fire by heat removal and dilution of combustible gases. The ambient wind pressure also affects the thermal behavior of external flame ejected from the compartment window. Strong wind may greatly influence fire spread and smoke movement behavior in the buildings; for example, mechanical ventilation routines might not extract smoke efficiently under the action of wind. The research on compartment fire phenomena in high-rise buildings under wind effects is an essential step to provide a guideline for fire safety design of these buildings. In fire safety codes of many countries, the wind effects on tall building fires have not received the desired attention. In Hong Kong’s Building Codes, wind effect is considered in designing the refuge floors of high-rise building so as to prevent smoke accumulation in the refuge floors. In the field of ventilation, natural ventilation of buildings generated by temperature differences and by the wind receives much more attention and multiple steady state behavior was found in recent years. However, in the compartment fire, the indoor temperature is often much higher than the outdoor temperature and the heat release rate of fire is influenced by the air inflow. It appears that many basic questions associated with wind effects on compartment fire behavior need more detailed discussions. These two papers address some of these questions and tries to investigate Wind effects on smoke motion and temperature of ventilation-controlled fire under opposing wind action in tall building. 2. Theoretical consideration on smoke motion In high-rise building fires, the location and number of openings of the compartment, together with ambient wind direction, determine the smoke motion in the compartment. The compartment has an opening near to ceiling in the left wall and another opening near to the floor in the opposite wall. The ambient wind blows from left to right in Fig.1. Fig. 1. The compartment fire scenario of opposing wind with downward flow As we know, if there is a fire in the compartment, the buoyancy will drive the smoke and air to move upward. So the ambient wind has an opposite effect on thermal buoyancy. In Fig. 1, a vertical coordinate, z, is defined and its origin (z = 0) is set at the middle height of the lower vent. We denote the ambient pressure as Po and the inner pressure at z = 0 as Pi0. Now the pressure differences at two openings and the mass flow rate of gas through each opening can be expressed as:
Across windward vent: aPW=w2Cwv2-Pa(1-)sh-dP rt =CTATV2pa4Pw Across leeward vent: 1 4R=R-R=20a91V2+4P ring=CBABV2pgAPL Here m is the mass flow rate,A is the vent area and C is the flow coefficient which varies between 0.6 and 0.8 depending on the type of vent. Define -ldPw+lw-Cpu)v2-(1-gh It is obvious that the sign of determines the direction of smoke motion.If>0,the flow is downward as shown in Fig.I and if o,de/dr will be negative and the inside gas temperature decreases too.So=is one of the steady states which the inside gas temperature will finally reach.On the way to the steady state,the non-dimensional wind effect,y,acts as an adjustor as
Across windward vent: Across leeward vent: Here m is the mass flow rate, A is the vent area and C is the flow coefficient which varies between 0.6 and 0.8 depending on the type of vent. Define It is obvious that the sign of Q determines the direction of smoke motion. If Q > 0, the flow is downward as shown in Fig. 1 and if Q < 0, the flow is upward. So Q = 0 is the critical condition that no gas exchanges through the openings. Therefore a critical ambient wind speed appears spontaneously as In practice, it is often hoped that smoke moves upward, that is, Q < 0 and hence the wind speed is deemed to be less than Vcr. As an estimation, Vcr 3.6 h (Ta = 300 K, Tg = 900 K and Cp W - Cp L = 1). If h is estimated as 3 m, Vcr = 6.2 m/s. That is to say, only when the ambient wind speed is less than 6.2 m/s, smoke moves upward in the compartment. When the ambient wind speed passing over high-rise buildings is higher than this critical value, some measurements according to normal fire safety codes may be out of service and result in drastic losses. 3. Theoretical considerations on smoke temperature Smoke temperature is mainly related to fire power, air flow rate and heat loss from the compartment envelope. The compartment envelope is assumed to be at the same temperature as the smoke in the enclosure and the energy balance on the compartment is Where M is total thermal mass (including the air), Cp is specific heat of total thermal mass. hw, Aw are respectively the convective heat transfer coefficient and the wall area. The simplest case is that the walls are adiabatic It is obvious that only when , d / d will be non-negative and hence the inside gas temperature increases. And when , d / d will be negative and the inside gas temperature decreases to . So is one of the steady states which the inside gas temperature will finally reach. On the way to the steady state, the non-dimensional wind effect, , acts as an adjustor as
shown in Fig.2.The sharp tip occurs at=/+1,which is the critical moment that the direction of smoke motion is changing. 107 -28r9 4 810 12 Fig.2.Rising rate of smoke temperature in an adiabatic compartment under wind effect(r0.2.=10) When y=0,the temperature rising rate firstly increases then decreases to zero at = When the rate always decreases.In the cases of the curve of the rate firstly drops to zero at =0/0+1 and then climbs,finally also dropping to zero at =o.This indicates that the ambient wind has great influence on the smoke temperature rising If the walls are not adiabatic,it can be extrapolated from Fig.2 that the sharp tip of d/dr profile will intersect with zero line possibly.Fig.3 gives two examples with different B values. V+rP0+1)+-+1o--=0 2 a12 b 10 2 10 ure I ment under wind effect (r=.2 cordingly the re is a critical va alue of ambient wind speed.N we obtain two critica wind speeds, one of wl h determines the direction of smoke motion and the other the number of the stea states of smoke temperat ntilation-controlled conditio the ormalized smoke tem nally reac h nearly 3.4 when there is no ambient wind (=0).If the ambient wind begins to blow and the speed increases very slowly so that the quasi-steady state
shown in Fig. 2. The sharp tip occurs at / 1 , which is the critical moment that the direction of smoke motion is changing. Fig. 2. Rising rate of smoke temperature in an adiabatic compartment under wind effect (r = 0.2, 10 ) When 0, the temperature rising rate firstly increases then decreases to zero at . When 1, the rate always decreases. In the cases of 0 1, the curve of the rate firstly drops to zero at / 1 and then climbs, finally also dropping to zero at . This indicates that the ambient wind has great influence on the smoke temperature rising. If the walls are not adiabatic, it can be extrapolated from Fig. 2 that the sharp tip of d / d profile will intersect with zero line possibly. Fig. 3 gives two examples with different values. Fig. 3. Rising rate of smoke temperature in a non-adiabatic compartment under wind effect (r =0.2, 10 ). 0.3 (a) and 0.9 (b). Accordingly, there is a critical value of ambient wind speed. Now we obtain two critical wind speeds, one of which determines the direction of smoke motion and the other the number of the steady states of smoke temperature equation. Considering a compartment fire in ventilation-controlled conditions, the normalized smoke temperature will finally reach nearly 3.4 when there is no ambient wind ( 0). If the ambient wind begins to blow and the speed increases very slowly so that the quasi-steady state
an be ssmd the steady temperature will firstly deerease until the wind speed reaches Then it will jump from the red arc to the black curve and begin to increase along the black curve when the wind speed exceedsIt is obvious that there is a sudden transition from one steady wh 4.Conclusion In high-rise buildings,the compartment fire behavior and smoke movement will be influenced greatly by the ambient wind.The investigation the wind effect on smoke motion direction and temperature of ventilation-controlled fire in a two-vent compartment and some conclusions can be drawn as follows: In the opposing wind cases,the wind force and the thermal buoyancy compete in the fire speed the motion ambient wind eed exceeds the critical valde,win will drive smoke to move downward,otherwise smoke will move upward driven by thermal buoyancy wind has a oke.There is another nd s ature.Otherwise e are three steady states.tv o of which are stable and the the When wind speed increases slowly from zero or decreases slowly from a very high value to zero the steady smoke temr rature will iumn from one state to another at the position reached by the critical wind speed.The critical wind speed is affected by heat loss of the compartment walls. High-rise building fire is often influenced by the ambient wind.Study concerning fire behavior in the compartment of high-rise buildings in wind environment is needed for exploring some effective methods used for evaluation of compartment fire smoke movement and control. 5.Literature -Haixiang Chen,Naian Liu.Wanki Chow.Wind ffe tilation-controlle oud fireintwo-vent comartment Bulnnd irmn04 25 Yang Y-C Su C.H Hsieh T-I Chung K-C Wind effects systems for tall buildin
can be assumed, the steady temperature will firstly decrease until the wind speed reaches cr . Then it will jump from the red arc to the black curve and begin to increase along the black curve when the wind speed exceeds cr . It is obvious that there is a sudden transition from one steady temperature to another temperature across cr . On the contrary, if the wind speed decreases slowly from a higher value, the steady smoke temperature will also jump from the black curve to the red arc across cr . This is very interesting and indicates that the wind effect on steady smoke temperature is very complicated.When suppressing a compartment fire in ventilationcontrolled conditions, the value of Frcr2, together with Frcr, should be firstly assessed. 4. Conclusion In high-rise buildings, the compartment fire behavior and smoke movement will be influenced greatly by the ambient wind. The investigation the wind effect on smoke motion direction and temperature of ventilation-controlled fire in a two-vent compartment and some conclusions can be drawn as follows: - In the opposing wind cases, the wind force and the thermal buoyancy compete in the fire compartment. The stronger one will dominate the direction of smoke motion. A critical wind speed can be therefore defined. When the ambient wind speed exceeds the critical value, wind will drive smoke to move downward, otherwise smoke will move upward driven by thermal buoyancy. - Ambient wind has a complex influence on the steady temperature of smoke. There is another critical wind speed. When ambient wind speed exceeds this critical one, there is only one steady state of smoke temperature. Otherwise, there are three steady states, two of which are stable and the other unstable. When wind speed increases slowly from zero or decreases slowly from a very high value to zero, the steady smoke temperature will jump from one state to another at the position reached by the critical wind speed. The critical wind speed is affected by heat loss of the compartment walls. High-rise building fire is often influenced by the ambient wind. Study concerning fire behavior in the compartment of high-rise buildings in wind environment is needed for exploring some effective methods used for evaluation of compartment fire smoke movement and control. 5. Literature - Haixiang Chen, Naian Liu, Wanki Chow. Wind effects on smoke motion and temperature of ventilation-controlled fire in a two-vent compartment. Building and Environment 2009;44:2512– 2526. - Yang Y-C, Su C-H, Hsieh T-L, Chung K-C. Wind effects on performance of smoke exhaust systems for tall buildings. Journal of Applied Fire Science 2005;14:189–203