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Estimation of building ventilation on the heat release rate of fire in a room
Accepted Manuscript Estimation of building ventilation on the heat release rate of fire in a room Ran Gao, Zhiyu Fang, Angui Li, Congling Shi, Lunfei Che PII: DOI: Reference:
S1359-4311(16)32592-3 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.048 ATE 10197
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
3 November 2016 12 April 2017 13 April 2017
Please cite this article as: R. Gao, Z. Fang, A. Li, C. Shi, L. Che, Estimation of building ventilation on the heat release rate of fire in a room, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2017.04.048
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Estimation of building ventilation on the heat release rate of fire in a room Ran Gao1*, Zhiyu Fang1,Angui Li1, Congling Shi2Lunfei Che3 1
School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, P.R. China 2
3
China Academy of Safety Science and Technology, 100012, P.R. China
China Railway Siyuan Survey and Design Group Co., Ltd, Wuhan, Hubei, 430063, P.R. China
Abstract: Ventilation to the scene of afire in buildings is inevitable. The inflow of fresh air from outdoors may accelerate combustion and cause serious losses of property. However, the extent of this influence on fire remains unknown. The heat release rate (HRR) is an important factor that influences disaster intensity. In this study, the mass loss method was used to calculate the HRR of a combustible by testing the mass of the burning material mass with ventilation. We selected a full-scale room to determine the influence of ventilation. Different factors that affect HRR, such as inlet velocity, jet range, and air change rate (ACH), were considered. Preliminary results suggested that the HRR of a fire would not change significantly under different ACH conditions with indirect air supply. When the distance of the air supply was shorter than 1.5 m, a maximum difference of 1.8 times was found between indirect and direct air supply. The HRR with direct air supply is the function of air velocity and jet range. This function was used to quantify the effect of direct air supply on HRR. Keywords: Fire hazard; Ventilation; ACH; Air supply; HRR; Building fire
* Corresponding author. Tel.: +86 13629284215; fax: +86 29 82205958. E-mail address: [email protected] (R. Gao). 1
1.Introduction Urban construction has rapidly developed worldwide in recent years[1]. However, unavoidable events, such as fire disasters, frequently occur in urban construction. Fire disasters can cause deaths and economic losses[2–3]. Previous studies have shown that smoke is the major cause of death in a fire[4–6]. The primary method for controlling smoke is sufficient ventilation for buildings through smoke extraction systems[7]. ①At present, pressurized air supply technology is widely used to prevent smoke. This technology supplies fresh air from outdoors. ②In terms of smoke extraction, natural makeup air is necessary. This method can also mechanically introduce fresh air from outdoors to achieve mass conservation. ③During the initial stage of a fire disaster, ventilation and air conditioning systems are not turned off. Fresh air is then introduced from outdoors to the scene of the fire. ④Air will blow into the room in a high-rise building when windows are broken during fire disasters. On the basis of these four points, introducing fresh air from outdoors to the scene of afire is inevitable in an actual fire disaster. Fresh air from outdoors may influence the fire source in two aspects[8–10].First, fresh air can induce combustion because of the high oxygen concentration. Second, fresh air can suppress combustion because of the low temperature. However, whether fresh air can induce or suppress combustion, as well as to what extent it can enhance or reduce fire intensity, remains unclear. Therefore, the influence of air supply on fire intensity, which is represented by the heat release rate (HRR) in a building fire disaster, must be investigated. Moreover, future studies can address whether these influences should be considered in engineering design or the ways in which
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these effects can be addressed. HRR is the main parameter used to describe fire intensity. Numerous parameters influence HRR, such as ignition locations, water spray, fire source materials, and fuel pan diameters [11–14]. However, few researchers have focused on building ventilation. Carvel(2001) estimated the effect of forced longitudinal ventilation on the HRR of fires in tunnels. The behavior of cars and vehicles bearing heavy loads during fire accidents with ventilation velocities of 2–10m/s was investigated[15]. Li(2012) analyzed the effect of forced longitudinal ventilation on the HRR of tunnel fires. This researcher tried many materials and found that relative HRR exhibited a linear relationship with air velocity[16]. Chen(2013) measured the HRR of a high-speed passenger rail car. The windows were opened one by one to control natural ventilation. The tested HRR curve was provided[17]. Yuan(2013) analyzed the combustion efficiencies and HRR of pool fires. The HRR changed with the different sizes of the ceiling opening[18]. Chen(2013)estimated the peak HRRof coach fire under different statuses of side window opening. The HRR was calculated[19]. Pretrel(2013) conducted a series of experiments to determine the HRR of hydrocarbon pool fires based on oxygen consumption(OC) and carbon dioxide generation calorimeter. The ventilation effect on the HRR between these two methods was analyzed qualitatively [20]. Lassus(2014) investigated fires in a compartment equipped with a mechanical ventilation system. As ventilation flow increased, HRR also increased significantly[21]. Recently, Li(2016) studied cross-sectional wind and longitudinal wind in tunnel fires. The effect of are latively low air velocity of 0.5–1.12m on HRR was investigated. The researcher found that ventilation could both increase and decrease HRR[22]. Chen(2017) performed several experiments in a 1/9 reduced-scale tunnel to investigate the effect of ventilation opening. The HRR with different sealing ratios of the natural ventilation opening was also studied [23].
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In our literature search, most of the studies considered air flow as uniform flow rather than jet flow. In reality, however, the majority of air flow is jet flow because it is blown from different kinds of tuyere(e.g., windows, diffusers, jets). Uniform flow and jet flow differ in terms of velocity attenuation. Velocity will decrease with the increase in distance between the fire source and the inlets in jet flow, whereas air velocity is constant in uniform flow. In addition, two kinds of air flow should be studied separately. These types of air flow directly or indirectly blow on the fire source. They are both observed in actual fire incidents. The former is more frequently observed because the area of the wall is generally larger than that of a tuyere. This paper describes our effort to determine the effect of ventilation on HRR. The mass loss method was used to calculate the HRR of a combustible by testing the mass of the burning material during the burning process. We selected a full-scale room to determine the influence of ventilation on HRR with direct or indirect air supply.
2.Research method Four methods can be used to study HRR: combustion replacement, insulated tank, OC, and mass loss. Thorntonand and Huggett (1917)reported that organic liquid and gas could release a constant amount of net heat in OC per unit mass when the plastic used in a building and other organic combustible solids exhibited perfect combustion. The value of the constant is E = l3.l MJ/kg[24–25].In a practical application, the error is less than 5% under this constant. On this basis of this law, if the OC mass in a combustion system is accurately measured, then the HRR of combustion can be calculated[26–27]. Combustion replacement can also be called isothermal retrieval. The basic principle is that a
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combustible can generate a temperature field when it is burning, and the replaced combustible (typically dimethylmethane) can also generate the same temperature field. Thus, the HRR of the replaced combustible can be regarded as the HRR of the combustible. The first device used to measure HRR in combustion replacement was invented by Thompson and Cousins in 1959 and called the
calorimeter for FM construction materials [28–29]. In the insulated tank method, the enthalpy change between inlet air and post-combustion smoke must be determined to obtain HRR. The typical experimental facility used is the Ohio State University (OSU) HRR test device (calorimeter).This device was designed by Smith in OSU in 1972. The equipment was improved by adding OC measurement. ASTM International(1983) described this improved equipment in detail[30]. OC is used to calculate HRR based on perfect combustion. However, incomplete combustion typically occurs in an actual fire disaster[31–32]. In the insulated tank method with OC measurement, all combustion products should be collected and sent to the smoke sampling device in a backward position via a smoke vent when the HRR of combustion is measured based on the OC principle. The entire smoke collection system must have sufficient power to collect all the smoke without allowing it to escape. The pulling force of the collection system itself will produce uncontrolled ventilation and influence the ventilation effect being achieved in this study. The combustion replacement method is required to test the temperature in a room as comparison information. However, ventilation will cause the indoor temperature to become uneven and increase the experimental error. Therefore, the mass loss method is used to calculate the HRR of a combustible by testing the mass of the burning material during the burning process. The equation is as follows:
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,
(1)
where Q is the HRR of the fire source, kW; M is the fuel mass loss rate, kg/s; and α is the combustion efficiency factor of the combustible. The perfect combustion value is 1. In general, the value is between 0.3 and 0.9. q is the average calorific value of fuel, kj/kg(the average calorific value of ethyl alcohol is 3×104 kJ/kg). The fuel mass loss rate is
m mi mi 1 ,
(2)
where mi is the mass indication on electronic scales that is expressed in kg when time turns to i; and mi+1 is the mass indication on electronic scales that is expressed in kg when time turns to i+1, and the unit of i is second(s). The average calorific value of fuel can be calculated usingthe following equation: n
q qi pi i 1
,
(3)
where qi is the ith kind of the calorific value of fuel, kJ/kg; pi is the ith kind of the proportion of all fuel; and n is the variety of fuel. The mass loss experimental system mainly includes an electronic scale (weighing sensor), a data collection system, and a computer system. The sketch of the system is shown in Fig. 1. Among the four test methods, the mass loss method is the only one that does not influence the indoor fluid field and the air supply equipment layout. Moreover, the structure of the equipment is simple, and data can be easily visualized. We adopted the mass loss method in this study. The schematic diagram of the experimental setup is presented in Fig. 1. The major equipment used in this experiment is as follows: two axial fans(power: 75 W, rotational speed: 2750 r/min, blast capacity: 330 m3/h); adjustable rotating stents; electronic scale with an accuracy of 0.1g; a data 6
collection system; a speed controller; a tachymeter (TSI, Inc.);and a square and bar-type lacquer tray. In the experiment, the air supply speed, fire source area, and efflux angle range from 0 m/s to 5m/s, 0.1 m2to 0.2 m2, and 20° to 60°, respectively. The inlet diameter is between 90mm and 200mm, the fuel is liquid alcohol, and the volume of the fuel is 200mL. 3.Results 3.1 Influence of indirect air supply on the HRR of fire The air change rate(ACH) is an important parameter for describing the indoor ventilation rate. It describes the updated speed in the entire fire source. When the air supply does not directly blow on the fire source, ACH is an important parameter that describes the amount of air supply in a room. Therefore, this study investigated the influence of different ACHs on the HRR of a fire disaster (indirect air supply to the fire source).We controlled the speed of the draught fan by using a blast regulator and adjusted the amount of air supply. The rotational speeds of the smoke extraction fan and the ventilation fan were always the same to maintain the balance of indoor pressure. The air supply inlet was adjusted to not blow directly on the fire source. During this time, the range of indoor ACH was adjusted to 1.3, 2.6, 3.9, 5.2, and 6.5ACH. Under different ACH conditions, the HRR of a fire disaster did not significantly change, as shown inFig. 2. This phenomenon could be attributed to the homogeneous diffusion of oxygen from fresh air in the room because air did not blow directly on the fire source. The supply rate of fresh oxygen around the fire source was relatively slower than that of oxygen blown directly on the fire source. The influence of air supply on the HRR of afire source may change when the area of the fire source changes. A high amount of oxygen does not only influence the HRRof a burning fire source but also affect fire speed. Thus, we designed another rectangular fire source (0.1 m × 1 m).We determined
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the influence of ventilation on the HRR in this area of the fire source. Under a rectangular fire source, the HRR of a fire disaster also exhibited no significant change under different indoor ACH conditions, as shown inFig. 3. 3.2 Influence of direct air supply on theHRR of fire
When a fire disaster is located below the air distributor, the door toward the corridor, and toward the windows, the air supply will blow directly on the fire source. This scenario is dangerous and must be avoided in a fire system design. However, given the unstable and abrupt nature of fire disasters, this condition cannot be prevented with certainty. The HRR of a fire disaster in such situation cannot be determined because it is unknown. As shown in Fig. 4, when the fire source faces the inlet, the flow speed of the inlet is 0 m/s to 5 m/s. A comfortable air conditioning system has indoor wind speed requirements. Thus, the air flow speed of the air supply inlet is less than 3 m/s. In a normal situation, the highest indoor wind speed is less than 3 m/s. In a corridor, however, the return air inlet and nearby windows may exhibit high wind speed. In this study, wind speed was increased to 5 m/s. As the speed of direct wind increased, the HRR of a fire disaster increased rapidly, and the fire disaster would not last for a long time, as shown inFig. 4. This phenomenon could be attributed to the oxygen content around the fire source increasing more rapidly than that under indirect wind. The added value of the oxygen supply was determined using the following equation: The volume of the room is 2.7 m × 2.7 m × 2.4 m = 17.5 m³, ACH is 6.5, and the ratio of oxygen in air is 21%. The original air content in the room is 17.5 m³ × 21% = 3.68 m³. The newly added oxygen amount is 17.5 m³ × 21% × 6.5 × 5 min ÷ 60 min = 1.99³.
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If the volume of the fire burning layer is V, then ①When wind in the air supply inlet does not blow directly on the fire source, OC in the fire source is Vo1 = (3.68 m³ + 1.99 m³)/17.5 m³ × V = 0.32 V. ②When wind in the air supply inlet blows directly on the fire source, OC in the fire source is Vo2 =(3.68 m³/17.5 m³ × V + 1.99 m³) = 0.21 V + 1.99. The value of V is difficult to calculate, but the height of the flame can be soused. In this experiment, the height of the flame was 120 mm. Thus, V = 0.12 m × 0.2 m × 0.2 m = 0.0048 m³. Then,Vo2/Vo1 = (0.21 × 0.0048 m³ + 1.99)/0.32 × 0.0048 m³ = 1296. Among the two working conditions, the oxygen supply of direct wind was nearly 1296 times that of indirect wind. Therefore, the HRR of a fire disaster significantly differed under these two working conditions. The relationship between the inlet and the fire source is also critical. In the design of heat ventilation and air-conditioning systems, an inlet is frequently set on the top or side wall; these inlets maintain a certain distance from the fire source. Evidently, when wind from the inlet blows on the fire source, the velocity of air supply becomes weak. In this situation, a considerable inlet distance will result in a low air supply of the inlet effect on the HRR of a fire disaster. The theoretical wind speed sent to the fire source at different distances can be calculated using the following equation[33]:
um 0.48 = S u0 0.147 d0 ,
(4)
where D is the width of the inlet of arbitrary point X on the inlet area, width of the inlet, m;um is the axis linear velocity of arbitrary point X on the inlet area, inlet axial velocity, m/s;S is the distance from
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the inlet to arbitrary point X on the inlet area, length of the inlet axis (axial lead), m; and a is the turbulivity. With an asymmetric inlet of contraction, a = 0.066–0.071. When turbulence was small, we chose as mall value; when turbulence was large, we chose a large value. With acylindrical inlet, a = 0.076–0.08. When turbulence was small, we chose as mall value; when turbulence was large, we chose a large value without dimension. u0 is the inlet wind speed, m/s; and d0 is the inlet diameter, typically d0 = 0.09 m. Thus, when S = 0.5–1.5m, the inlet speed of the wind around the fire sourceis2.43 m/s to 0.91m/s. Therefore, air supply distance significantly influences inlet air speed when sent to the fire source. In this research, when air supply distance was longer than 1.5 m, the difference of the HRR of the fire disaster between direct wind and indirect wind was insignificant, as shown in Fig.5. Thus, the influence of the inlet on the HRR of a fire disaster should not be considered when the distance from the inlet to the fire source was longer than 1.5 m or the inlet speed in the location of the fire source was below1 m/s. In buildings, floor height is higher than 2.7m(higher than 1.5m). Air velocity is below 5m/s with or without a ventilation system[34]. Thus, the influence of air supply on HRR can be disregarded if the fire source is not located under the inlet.
4. Discussion The effect of ventilation on HRR is quantified in this study. A mathematical equation is used to measure this change. A dimensionless HRR, Q+, is introduced, as shown in the following equation: ,
(5)
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where Q is the HRR under different conditions. Q0 is the HRR without any air supply because the ventilation system is shut down. When the air supply is blown indirectly,
is linear with the ACH of the room.A linearfitis
achieved, as shown in Fig.7. The linear fit result is shown as follows: .
(6)
When the air supply is blown directly, q is nonlinear with the velocity of air flow at the inlet (v) and the distance between the fire source and the inlet (x). When the velocity of the air flow at the inlet is 3m/s, the relationship between q and x is fit, as shown below: .
(7)
To use this equation, two assumptions are made. First, the upper limit in this equation is 0.69. This value is the dimensionless HRR with an x of 0.5m. It is the safe distance to prevent the inlet from burning. When x is sufficiently small, the fire source is assumed to be located in the initial stage of the air flow. In this stage, the velocity of the air flow at the core zone is constant. Therefore, as x decreases, the dimensionless HRR will approach the constant. This changing trend is also illustrated in Fig.4. The second assumption is that when x is sufficiently large, the effect of air flow on the dimensionless HRR will be reduced until the HRR is equal to that without air supply. The velocity of the air flow at the inlet changes when x is 0.5. The relationship between q and v is fit, as shown below: .
(8)
Good agreement is achieved when equations in previous studies are compared with the fitting equations, as shown inFig. 8. The change in HRR with different velocities is assumed to remain
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constant at various distances between the inlet and the fire source. Equations 7 and 8 can be combined as follows: (0.5m
(9)
5. Conclusion A full-scale room was selected to determine the influence of ventilation, particularly air supply. Different factors, such as air supply angle, blast capacity, and wind speed on HRR, were considered. When air supply does not blow directly on the fire source, the HRR of a fire disaster will exhibit no significant change under different ACH conditions. Therefore, if the air supply does not blow directly on the fire source, then the influence of air supply on the HRR of fire can be ignored. This conclusion is also established under different sizes of the fire source. When air supply does not blow directly on the fire source, the HRR of afire disaster increases rapidly as the speed of direct wind increases. The HRR with an inlet air velocity of 5m/s is 1.8 times higher than that with an inlet air velocity of 0m/s. When air supply distance is longer than 1.5 m, the difference on the HRR of a fire disaster between direct wind and indirect wind is insignificant. Thus, the influence of the inlet on the HRR of a fire disaster should no longer be considered when the distance from the inlet to the fire source is longer than 1.5 m or the inlet speed in the location of the fire source is below 1 m/s. The floor height in buildings is higher than 2.7m(higher than 1.5m). Air velocity is less than 5m/s with or without a ventilation system. Thus, the influence of air supply on HRR can be disregarded if the fire source is not located under the inlet.
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Figure caption Fig.1 Schematic diagram of the experimental setup Fig. 2 Comparison of HRR under different intensities of indirect air supply with a square fire source(0.2m ×0.2m) Fig.3 Comparison of HRR under different intensities of indirect air supply with a rectangular fire source(0.1m ×1 m) Fig.4 Comparison of HRR under different intensities of direct air supply with a rectangular fire source(0.1m ×1 m) Fig.5 Comparison of HRR at different distances between the inlet and the fire source with direct air supply Fig.6. Effect of ACH on the fire source with indirect air supply Fig.7. Effect of jet range on the fire source with direct air supply Fig.8. Effect of air velocity at the inlet on the fire source with direct air supply
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[25] Huggett C. Estimation of Rate of Heat Release by Means of Oxygen Consumption Measurements[J]. Fire and Materials, 1980, 4(2):61-65. [26] Janssens, Marc L. "Measuring rate of heat release by oxygen consumption." Fire technology 27.3 (1991): 234-249. [27] Ingason, Haukur, and Anders Lönnermark. "Heat release rates from heavy goods vehicle trailer fires in tunnels." Fire Safety Journal 40.7 (2005): 646-668. [28]Babrauskas, Vytenis, and Richard D. Peacock. "Heat release rate: the single most important variable in fire hazard." Fire safety journal 18.3 (1992): 255-272. [29]Thompson, N. J., and E. W. Cousins. "The FM construction materials calorimeter." NFPA Q 52 (1959): 186-192. [30]Bowles C Q, Schijve J. Fatigue mechanisms: advances in quantitative measurement of physical damage[J]. ASTM STP, 1983, 811(400): 60. [31]Janssens M L. Measuring rate of heat release by oxygen consumption[J]. Fire technology, 1991, 27(3): 234-249. [32]Ayala P, Cantizano A, Gutiérrez-Montes C, et al. Influence of atrium roof geometries on the numerical predictions of fire tests under natural ventilation conditions[J]. Energy and Buildings, 2013, 65: 382-390. [33]Rajaratnam N. Turbulent jets[M]. Elsevier, 1976. [34]Handbook A F. American society of heating, refrigerating and air-conditioning engineers[J]. Inc.: Atlanta, GA, USA, 2009. [35]Xi, Y. H., J. Mao, G. Bai, and J. W. Hu. "Safe velocity of on-fire train running in the tunnel." Tunnelling and Underground Space Technology 60 (2016): 210-223.
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[36]Li, Ying Zhen, Chuan Gang Fan, Haukur Ingason, Anders Lönnermark, and Jie Ji. "Effect of cross section and ventilation on heat release rates in tunnel fires." Tunnelling and Underground Space Technology 51 (2016): 414-423. [37]Roh, Jae Seong, Seung Shin Yang, and Hong Sun Ryou. "Tunnel fires: experiments on critical velocity and burning rate in pool fire during longitudinal ventilation." Journal of fire sciences 25, no. 2 (2007): 161-176. Table caption: Table1. Previous studies on HRR under different ventilation conditions
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24
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Table 1. Previous studies on HRR under different ventilation conditions
Studies Present study Xi (2016) Li (2016) Roh (2005)
Fuel
Fire source size (m)
Air Velocity (m/s)
Fire scenario
alcohol
0.2 × 0.2
0–5
room
n-heptane wood acetone and n-heptane
φ0.05 0.15 × 0.5 0.125 × 0.125; 0.085 × 0.085
0–3.54 0–1.2
train tunnel
[35] [36]
0–1.68
tunnel
[37]
Reference
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Highlights: >The effect of building ventilation on the HRR of fire hazard is Estimated >Function was conducted to quantify the effect of direct supply air on HRR. >Function was conducted to quantify the effect of ACH on HRR.
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S1359-4311(16)32592-3 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.048 ATE 10197
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
3 November 2016 12 April 2017 13 April 2017
Please cite this article as: R. Gao, Z. Fang, A. Li, C. Shi, L. Che, Estimation of building ventilation on the heat release rate of fire in a room, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2017.04.048
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Estimation of building ventilation on the heat release rate of fire in a room Ran Gao1*, Zhiyu Fang1,Angui Li1, Congling Shi2Lunfei Che3 1
School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, P.R. China 2
3
China Academy of Safety Science and Technology, 100012, P.R. China
China Railway Siyuan Survey and Design Group Co., Ltd, Wuhan, Hubei, 430063, P.R. China
Abstract: Ventilation to the scene of afire in buildings is inevitable. The inflow of fresh air from outdoors may accelerate combustion and cause serious losses of property. However, the extent of this influence on fire remains unknown. The heat release rate (HRR) is an important factor that influences disaster intensity. In this study, the mass loss method was used to calculate the HRR of a combustible by testing the mass of the burning material mass with ventilation. We selected a full-scale room to determine the influence of ventilation. Different factors that affect HRR, such as inlet velocity, jet range, and air change rate (ACH), were considered. Preliminary results suggested that the HRR of a fire would not change significantly under different ACH conditions with indirect air supply. When the distance of the air supply was shorter than 1.5 m, a maximum difference of 1.8 times was found between indirect and direct air supply. The HRR with direct air supply is the function of air velocity and jet range. This function was used to quantify the effect of direct air supply on HRR. Keywords: Fire hazard; Ventilation; ACH; Air supply; HRR; Building fire
* Corresponding author. Tel.: +86 13629284215; fax: +86 29 82205958. E-mail address: [email protected] (R. Gao). 1
1.Introduction Urban construction has rapidly developed worldwide in recent years[1]. However, unavoidable events, such as fire disasters, frequently occur in urban construction. Fire disasters can cause deaths and economic losses[2–3]. Previous studies have shown that smoke is the major cause of death in a fire[4–6]. The primary method for controlling smoke is sufficient ventilation for buildings through smoke extraction systems[7]. ①At present, pressurized air supply technology is widely used to prevent smoke. This technology supplies fresh air from outdoors. ②In terms of smoke extraction, natural makeup air is necessary. This method can also mechanically introduce fresh air from outdoors to achieve mass conservation. ③During the initial stage of a fire disaster, ventilation and air conditioning systems are not turned off. Fresh air is then introduced from outdoors to the scene of the fire. ④Air will blow into the room in a high-rise building when windows are broken during fire disasters. On the basis of these four points, introducing fresh air from outdoors to the scene of afire is inevitable in an actual fire disaster. Fresh air from outdoors may influence the fire source in two aspects[8–10].First, fresh air can induce combustion because of the high oxygen concentration. Second, fresh air can suppress combustion because of the low temperature. However, whether fresh air can induce or suppress combustion, as well as to what extent it can enhance or reduce fire intensity, remains unclear. Therefore, the influence of air supply on fire intensity, which is represented by the heat release rate (HRR) in a building fire disaster, must be investigated. Moreover, future studies can address whether these influences should be considered in engineering design or the ways in which
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these effects can be addressed. HRR is the main parameter used to describe fire intensity. Numerous parameters influence HRR, such as ignition locations, water spray, fire source materials, and fuel pan diameters [11–14]. However, few researchers have focused on building ventilation. Carvel(2001) estimated the effect of forced longitudinal ventilation on the HRR of fires in tunnels. The behavior of cars and vehicles bearing heavy loads during fire accidents with ventilation velocities of 2–10m/s was investigated[15]. Li(2012) analyzed the effect of forced longitudinal ventilation on the HRR of tunnel fires. This researcher tried many materials and found that relative HRR exhibited a linear relationship with air velocity[16]. Chen(2013) measured the HRR of a high-speed passenger rail car. The windows were opened one by one to control natural ventilation. The tested HRR curve was provided[17]. Yuan(2013) analyzed the combustion efficiencies and HRR of pool fires. The HRR changed with the different sizes of the ceiling opening[18]. Chen(2013)estimated the peak HRRof coach fire under different statuses of side window opening. The HRR was calculated[19]. Pretrel(2013) conducted a series of experiments to determine the HRR of hydrocarbon pool fires based on oxygen consumption(OC) and carbon dioxide generation calorimeter. The ventilation effect on the HRR between these two methods was analyzed qualitatively [20]. Lassus(2014) investigated fires in a compartment equipped with a mechanical ventilation system. As ventilation flow increased, HRR also increased significantly[21]. Recently, Li(2016) studied cross-sectional wind and longitudinal wind in tunnel fires. The effect of are latively low air velocity of 0.5–1.12m on HRR was investigated. The researcher found that ventilation could both increase and decrease HRR[22]. Chen(2017) performed several experiments in a 1/9 reduced-scale tunnel to investigate the effect of ventilation opening. The HRR with different sealing ratios of the natural ventilation opening was also studied [23].
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In our literature search, most of the studies considered air flow as uniform flow rather than jet flow. In reality, however, the majority of air flow is jet flow because it is blown from different kinds of tuyere(e.g., windows, diffusers, jets). Uniform flow and jet flow differ in terms of velocity attenuation. Velocity will decrease with the increase in distance between the fire source and the inlets in jet flow, whereas air velocity is constant in uniform flow. In addition, two kinds of air flow should be studied separately. These types of air flow directly or indirectly blow on the fire source. They are both observed in actual fire incidents. The former is more frequently observed because the area of the wall is generally larger than that of a tuyere. This paper describes our effort to determine the effect of ventilation on HRR. The mass loss method was used to calculate the HRR of a combustible by testing the mass of the burning material during the burning process. We selected a full-scale room to determine the influence of ventilation on HRR with direct or indirect air supply.
2.Research method Four methods can be used to study HRR: combustion replacement, insulated tank, OC, and mass loss. Thorntonand and Huggett (1917)reported that organic liquid and gas could release a constant amount of net heat in OC per unit mass when the plastic used in a building and other organic combustible solids exhibited perfect combustion. The value of the constant is E = l3.l MJ/kg[24–25].In a practical application, the error is less than 5% under this constant. On this basis of this law, if the OC mass in a combustion system is accurately measured, then the HRR of combustion can be calculated[26–27]. Combustion replacement can also be called isothermal retrieval. The basic principle is that a
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combustible can generate a temperature field when it is burning, and the replaced combustible (typically dimethylmethane) can also generate the same temperature field. Thus, the HRR of the replaced combustible can be regarded as the HRR of the combustible. The first device used to measure HRR in combustion replacement was invented by Thompson and Cousins in 1959 and called the
calorimeter for FM construction materials [28–29]. In the insulated tank method, the enthalpy change between inlet air and post-combustion smoke must be determined to obtain HRR. The typical experimental facility used is the Ohio State University (OSU) HRR test device (calorimeter).This device was designed by Smith in OSU in 1972. The equipment was improved by adding OC measurement. ASTM International(1983) described this improved equipment in detail[30]. OC is used to calculate HRR based on perfect combustion. However, incomplete combustion typically occurs in an actual fire disaster[31–32]. In the insulated tank method with OC measurement, all combustion products should be collected and sent to the smoke sampling device in a backward position via a smoke vent when the HRR of combustion is measured based on the OC principle. The entire smoke collection system must have sufficient power to collect all the smoke without allowing it to escape. The pulling force of the collection system itself will produce uncontrolled ventilation and influence the ventilation effect being achieved in this study. The combustion replacement method is required to test the temperature in a room as comparison information. However, ventilation will cause the indoor temperature to become uneven and increase the experimental error. Therefore, the mass loss method is used to calculate the HRR of a combustible by testing the mass of the burning material during the burning process. The equation is as follows:
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,
(1)
where Q is the HRR of the fire source, kW; M is the fuel mass loss rate, kg/s; and α is the combustion efficiency factor of the combustible. The perfect combustion value is 1. In general, the value is between 0.3 and 0.9. q is the average calorific value of fuel, kj/kg(the average calorific value of ethyl alcohol is 3×104 kJ/kg). The fuel mass loss rate is
m mi mi 1 ,
(2)
where mi is the mass indication on electronic scales that is expressed in kg when time turns to i; and mi+1 is the mass indication on electronic scales that is expressed in kg when time turns to i+1, and the unit of i is second(s). The average calorific value of fuel can be calculated usingthe following equation: n
q qi pi i 1
,
(3)
where qi is the ith kind of the calorific value of fuel, kJ/kg; pi is the ith kind of the proportion of all fuel; and n is the variety of fuel. The mass loss experimental system mainly includes an electronic scale (weighing sensor), a data collection system, and a computer system. The sketch of the system is shown in Fig. 1. Among the four test methods, the mass loss method is the only one that does not influence the indoor fluid field and the air supply equipment layout. Moreover, the structure of the equipment is simple, and data can be easily visualized. We adopted the mass loss method in this study. The schematic diagram of the experimental setup is presented in Fig. 1. The major equipment used in this experiment is as follows: two axial fans(power: 75 W, rotational speed: 2750 r/min, blast capacity: 330 m3/h); adjustable rotating stents; electronic scale with an accuracy of 0.1g; a data 6
collection system; a speed controller; a tachymeter (TSI, Inc.);and a square and bar-type lacquer tray. In the experiment, the air supply speed, fire source area, and efflux angle range from 0 m/s to 5m/s, 0.1 m2to 0.2 m2, and 20° to 60°, respectively. The inlet diameter is between 90mm and 200mm, the fuel is liquid alcohol, and the volume of the fuel is 200mL. 3.Results 3.1 Influence of indirect air supply on the HRR of fire The air change rate(ACH) is an important parameter for describing the indoor ventilation rate. It describes the updated speed in the entire fire source. When the air supply does not directly blow on the fire source, ACH is an important parameter that describes the amount of air supply in a room. Therefore, this study investigated the influence of different ACHs on the HRR of a fire disaster (indirect air supply to the fire source).We controlled the speed of the draught fan by using a blast regulator and adjusted the amount of air supply. The rotational speeds of the smoke extraction fan and the ventilation fan were always the same to maintain the balance of indoor pressure. The air supply inlet was adjusted to not blow directly on the fire source. During this time, the range of indoor ACH was adjusted to 1.3, 2.6, 3.9, 5.2, and 6.5ACH. Under different ACH conditions, the HRR of a fire disaster did not significantly change, as shown inFig. 2. This phenomenon could be attributed to the homogeneous diffusion of oxygen from fresh air in the room because air did not blow directly on the fire source. The supply rate of fresh oxygen around the fire source was relatively slower than that of oxygen blown directly on the fire source. The influence of air supply on the HRR of afire source may change when the area of the fire source changes. A high amount of oxygen does not only influence the HRRof a burning fire source but also affect fire speed. Thus, we designed another rectangular fire source (0.1 m × 1 m).We determined
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the influence of ventilation on the HRR in this area of the fire source. Under a rectangular fire source, the HRR of a fire disaster also exhibited no significant change under different indoor ACH conditions, as shown inFig. 3. 3.2 Influence of direct air supply on theHRR of fire
When a fire disaster is located below the air distributor, the door toward the corridor, and toward the windows, the air supply will blow directly on the fire source. This scenario is dangerous and must be avoided in a fire system design. However, given the unstable and abrupt nature of fire disasters, this condition cannot be prevented with certainty. The HRR of a fire disaster in such situation cannot be determined because it is unknown. As shown in Fig. 4, when the fire source faces the inlet, the flow speed of the inlet is 0 m/s to 5 m/s. A comfortable air conditioning system has indoor wind speed requirements. Thus, the air flow speed of the air supply inlet is less than 3 m/s. In a normal situation, the highest indoor wind speed is less than 3 m/s. In a corridor, however, the return air inlet and nearby windows may exhibit high wind speed. In this study, wind speed was increased to 5 m/s. As the speed of direct wind increased, the HRR of a fire disaster increased rapidly, and the fire disaster would not last for a long time, as shown inFig. 4. This phenomenon could be attributed to the oxygen content around the fire source increasing more rapidly than that under indirect wind. The added value of the oxygen supply was determined using the following equation: The volume of the room is 2.7 m × 2.7 m × 2.4 m = 17.5 m³, ACH is 6.5, and the ratio of oxygen in air is 21%. The original air content in the room is 17.5 m³ × 21% = 3.68 m³. The newly added oxygen amount is 17.5 m³ × 21% × 6.5 × 5 min ÷ 60 min = 1.99³.
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If the volume of the fire burning layer is V, then ①When wind in the air supply inlet does not blow directly on the fire source, OC in the fire source is Vo1 = (3.68 m³ + 1.99 m³)/17.5 m³ × V = 0.32 V. ②When wind in the air supply inlet blows directly on the fire source, OC in the fire source is Vo2 =(3.68 m³/17.5 m³ × V + 1.99 m³) = 0.21 V + 1.99. The value of V is difficult to calculate, but the height of the flame can be soused. In this experiment, the height of the flame was 120 mm. Thus, V = 0.12 m × 0.2 m × 0.2 m = 0.0048 m³. Then,Vo2/Vo1 = (0.21 × 0.0048 m³ + 1.99)/0.32 × 0.0048 m³ = 1296. Among the two working conditions, the oxygen supply of direct wind was nearly 1296 times that of indirect wind. Therefore, the HRR of a fire disaster significantly differed under these two working conditions. The relationship between the inlet and the fire source is also critical. In the design of heat ventilation and air-conditioning systems, an inlet is frequently set on the top or side wall; these inlets maintain a certain distance from the fire source. Evidently, when wind from the inlet blows on the fire source, the velocity of air supply becomes weak. In this situation, a considerable inlet distance will result in a low air supply of the inlet effect on the HRR of a fire disaster. The theoretical wind speed sent to the fire source at different distances can be calculated using the following equation[33]:
um 0.48 = S u0 0.147 d0 ,
(4)
where D is the width of the inlet of arbitrary point X on the inlet area, width of the inlet, m;um is the axis linear velocity of arbitrary point X on the inlet area, inlet axial velocity, m/s;S is the distance from
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the inlet to arbitrary point X on the inlet area, length of the inlet axis (axial lead), m; and a is the turbulivity. With an asymmetric inlet of contraction, a = 0.066–0.071. When turbulence was small, we chose as mall value; when turbulence was large, we chose a large value. With acylindrical inlet, a = 0.076–0.08. When turbulence was small, we chose as mall value; when turbulence was large, we chose a large value without dimension. u0 is the inlet wind speed, m/s; and d0 is the inlet diameter, typically d0 = 0.09 m. Thus, when S = 0.5–1.5m, the inlet speed of the wind around the fire sourceis2.43 m/s to 0.91m/s. Therefore, air supply distance significantly influences inlet air speed when sent to the fire source. In this research, when air supply distance was longer than 1.5 m, the difference of the HRR of the fire disaster between direct wind and indirect wind was insignificant, as shown in Fig.5. Thus, the influence of the inlet on the HRR of a fire disaster should not be considered when the distance from the inlet to the fire source was longer than 1.5 m or the inlet speed in the location of the fire source was below1 m/s. In buildings, floor height is higher than 2.7m(higher than 1.5m). Air velocity is below 5m/s with or without a ventilation system[34]. Thus, the influence of air supply on HRR can be disregarded if the fire source is not located under the inlet.
4. Discussion The effect of ventilation on HRR is quantified in this study. A mathematical equation is used to measure this change. A dimensionless HRR, Q+, is introduced, as shown in the following equation: ,
(5)
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where Q is the HRR under different conditions. Q0 is the HRR without any air supply because the ventilation system is shut down. When the air supply is blown indirectly,
is linear with the ACH of the room.A linearfitis
achieved, as shown in Fig.7. The linear fit result is shown as follows: .
(6)
When the air supply is blown directly, q is nonlinear with the velocity of air flow at the inlet (v) and the distance between the fire source and the inlet (x). When the velocity of the air flow at the inlet is 3m/s, the relationship between q and x is fit, as shown below: .
(7)
To use this equation, two assumptions are made. First, the upper limit in this equation is 0.69. This value is the dimensionless HRR with an x of 0.5m. It is the safe distance to prevent the inlet from burning. When x is sufficiently small, the fire source is assumed to be located in the initial stage of the air flow. In this stage, the velocity of the air flow at the core zone is constant. Therefore, as x decreases, the dimensionless HRR will approach the constant. This changing trend is also illustrated in Fig.4. The second assumption is that when x is sufficiently large, the effect of air flow on the dimensionless HRR will be reduced until the HRR is equal to that without air supply. The velocity of the air flow at the inlet changes when x is 0.5. The relationship between q and v is fit, as shown below: .
(8)
Good agreement is achieved when equations in previous studies are compared with the fitting equations, as shown inFig. 8. The change in HRR with different velocities is assumed to remain
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constant at various distances between the inlet and the fire source. Equations 7 and 8 can be combined as follows: (0.5m
(9)
5. Conclusion A full-scale room was selected to determine the influence of ventilation, particularly air supply. Different factors, such as air supply angle, blast capacity, and wind speed on HRR, were considered. When air supply does not blow directly on the fire source, the HRR of a fire disaster will exhibit no significant change under different ACH conditions. Therefore, if the air supply does not blow directly on the fire source, then the influence of air supply on the HRR of fire can be ignored. This conclusion is also established under different sizes of the fire source. When air supply does not blow directly on the fire source, the HRR of afire disaster increases rapidly as the speed of direct wind increases. The HRR with an inlet air velocity of 5m/s is 1.8 times higher than that with an inlet air velocity of 0m/s. When air supply distance is longer than 1.5 m, the difference on the HRR of a fire disaster between direct wind and indirect wind is insignificant. Thus, the influence of the inlet on the HRR of a fire disaster should no longer be considered when the distance from the inlet to the fire source is longer than 1.5 m or the inlet speed in the location of the fire source is below 1 m/s. The floor height in buildings is higher than 2.7m(higher than 1.5m). Air velocity is less than 5m/s with or without a ventilation system. Thus, the influence of air supply on HRR can be disregarded if the fire source is not located under the inlet.
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Figure caption Fig.1 Schematic diagram of the experimental setup Fig. 2 Comparison of HRR under different intensities of indirect air supply with a square fire source(0.2m ×0.2m) Fig.3 Comparison of HRR under different intensities of indirect air supply with a rectangular fire source(0.1m ×1 m) Fig.4 Comparison of HRR under different intensities of direct air supply with a rectangular fire source(0.1m ×1 m) Fig.5 Comparison of HRR at different distances between the inlet and the fire source with direct air supply Fig.6. Effect of ACH on the fire source with indirect air supply Fig.7. Effect of jet range on the fire source with direct air supply Fig.8. Effect of air velocity at the inlet on the fire source with direct air supply
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[36]Li, Ying Zhen, Chuan Gang Fan, Haukur Ingason, Anders Lönnermark, and Jie Ji. "Effect of cross section and ventilation on heat release rates in tunnel fires." Tunnelling and Underground Space Technology 51 (2016): 414-423. [37]Roh, Jae Seong, Seung Shin Yang, and Hong Sun Ryou. "Tunnel fires: experiments on critical velocity and burning rate in pool fire during longitudinal ventilation." Journal of fire sciences 25, no. 2 (2007): 161-176. Table caption: Table1. Previous studies on HRR under different ventilation conditions
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Table 1. Previous studies on HRR under different ventilation conditions
Studies Present study Xi (2016) Li (2016) Roh (2005)
Fuel
Fire source size (m)
Air Velocity (m/s)
Fire scenario
alcohol
0.2 × 0.2
0–5
room
n-heptane wood acetone and n-heptane
φ0.05 0.15 × 0.5 0.125 × 0.125; 0.085 × 0.085
0–3.54 0–1.2
train tunnel
[35] [36]
0–1.68
tunnel
[37]
Reference
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Highlights: >The effect of building ventilation on the HRR of fire hazard is Estimated >Function was conducted to quantify the effect of direct supply air on HRR. >Function was conducted to quantify the effect of ACH on HRR.
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