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Experimental study of interlayer effect induced by building facade curtain wall on downward flame spread behavior of polyurethane
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Research Paper
Experimental study of interlayer effect induced by building facade curtain wall on downward flame spread behavior of polyurethane Xin Maa, Ran Tub, Weiguang Anc, Li Xud, Shengfeng Luoe, Jingwei Wanga, Fei Tangf,
⁎
a
College of Environment and Energy Engineering, Anhui Jianzhu University, Hefei, Anhui 230022, China College of Mechanical Engineering and Automation, Huaqiao University, Xiamen, Fujian 361021, China c Jiangsu Key Laboratory of Fire Safety in Urban Underground Space, China University of Mining and Technology, Xuzhou 221116, China d College of Civil Science and Engineering, Yangzhou University, Yangzhou, Jiangsu 225127, China e Department of Engineering Physics, Tsinghua University, Beijing 100091, China f School of Automotive and Transportation Engineering, Hefei University of Technology, Hefei, Anhui 230009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Curtain wall Interlayer effect Polyurethane material Melting-dripping Building fire safety
This paper presents an experimental investigation on interlayer effect induced by curtain wall, parallel to the building exterior facade, on thermal and burning behavior of insulation material flexible polyurethane (FPU). Results show that, downward flames propagation behavior along the interlayer was determined by the coupling effects including chimney effect, constraint effect and related heat transfer variation induced by curtain wall. Firstly, comparing to the occasionally melting-dripping phenomenon without curtain wall, continuous meltingdripping liquids were generated subject to positively heat feedback by internal heat radiation to the incomplete ember layer at later combustion stage, which is more distinct for narrow curtain wall spacing. Secondly based on the analysis of heat feedback and the dripping degree, the non-monotonous relation between mass loss and spacing D was interpreted. Thirdly, flame physical variations e.g., height and stretching induced by complex turbulence were discussed with plate flow theory. Furthermore, hot gas temperature field along board surface centerline and sideline were compared. Due to the insufficient combustion and cooling of upwards air entrainment, temperature along centerline was found to be lower generally, which is different from the free burning. In addition, analysis on radiation flux development with soot blockage effect and also flame spread velocity was proposed.
1. Introduction Modern designs for the structure of building exterior facades tend to be diversified as a result of requirements related to lighting or aesthetics. At the same time, curtain walls are usually installed parallel to the building exterior facade. Due to the complex effects, the issue of facade flame spread has been incorporated in building fire safety engineering design guidelines [1]. As an example, the facade flame spreading characteristics associated with building curtain wall can complicate fire rescue operations in high-rise buildings, where a vertical channel is formed between the curtain wall and the building cladding. In history, e.g., London Grenfell Tower fire in 2017, the gap between cladding and curtain wall may act as chimney when combustible insulation materials burn in fire accident. This fire scenario is different from single facade fires due to curtain wall configurations significantly affect the air entrainment, flow field and also the heat and mass transfer. The interlayer structure, especially for the two facades ⁎
confined fire burning or spreading is truly a key factor for fire development [2–6]. On the other hand, insulation materials are widely used on the cladding of high-rise buildings assemblies as a popular energy-saving method. Flexible polyurethane (FPU) foam, is regarded as a kind of highly representative polymer material with good portability and thermal insulation performance. However, some kinds of FPU foam are combustible and thermoplastic-like by manufacture process, which would be melt partially and ignited easily by external heat flux. Furthermore, due to the quick flame spread velocity in FPU burning, the potential risk of fire hazard and challenges in fire prevention could never be ignored. Our previous studies [7–9] have shown that the downward flame spreading of the FPU foam board can be considered as a small, narrow pool fire on the top of the solid foam. However, the coupled factors determining the fire hazards of FPU under curtain wall structure condition are actually quite complicated, and the unique “melting-dripping” behavior of FPU during combustion can represent a
Corresponding author. E-mail address: [email protected] (F. Tang).
https://doi.org/10.1016/j.applthermaleng.2019.114694 Received 7 September 2019; Received in revised form 6 November 2019; Accepted 16 November 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xin Ma, et al., Applied Thermal Engineering, https://doi.org/10.1016/j.applthermaleng.2019.114694
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Nomenclature
B cp D Dc k ṁ ṁ ″ md mi ṁ t″ q̇ q¯″̇ q̇w″, rad q̇f″, conv ″ , cond q̇FPU ″ , rad q̇ref q̇c″, rad q̇p″
Tf − max Tig T∞ Uw,v u∞ v V δp ρ ΔHg τs
spalding number specific heat of fuel (J/g·K) distance between facade and curtain wall (cm) a critical spacing about 10 (cm) thermal conductivity of solid fuel (W/m·K) burning rate (g/s) burning rate per area (g/s·m2) dripping mass of the fuel (g) initial mass of the FPU foam board (g) mass transfer caused by shear flow (g) heat flux rate (W/s) averaged heat flux (W/m2) radiation from the curtain wall (W/m2) convection in the interlayer space (W/m2) thermal conduction inside FPU (W/m2) flame radiation reflection by curtain wall (W/m2) radiation from the curtain wall itself (W/m2) preheating heat flux to pyrolysis zone (W/m2)
peak temperature of each thermocouple (K) ignition temperature (K) ambient temperature (K) velocity of vertical air entrainment (m/s) free-stream velocity (m/s) kinematic viscosity or momentum diffusivity (Pa⋅s) flame spread velocity (m/s) thermal penetration length (cm) density (kg/m3) enthalpy changes (kJ/kg) viscosity coefficient multiplied by the derivative of velocity
Subscripts
f h p v
challenge when designing fire protection systems for buildings. Except for the flame spread along the interlayer, previous works have also been reported on the building facade flame ejected from the opening of the enclosure room, which was then restricted by curtain wall. Window ejected flame may further induce the special form of fire spreading between the interlayer of curtain wall in some special cases. Lee and Delichatsios [10] investigated the constraint effect of an opposite facing wall (opposite to the opening of the compartment facade). Tang et al. [11] revealed the flame height evolution with a sloping facing wall constraint at various angles. Lu et al. [12] further studied the merging behavior of the two facade flames from two parallel openings. Although the influence of external wall on compartment fires is being studied over the past decades, little literature concerning the influence of curtain wall on the building facade cladding materials' combustion behavior. Curtain wall is a typical structure in many high-rise buildings. The semi-enclosed restricted space formed by curtain wall will directly affect the thermal feedback, flame morphology, hot smoke flow, air entrainment and other characteristics in the combustion process. An [13] investigated the effects of parallel walls on the downward flame
fire or flame horizontal component pyrolysis vertical component
spreading characteristics of insulation materials extruded polystyrene (XPS) foam. The average flame height and maximum flame temperature were found to initially become lower and then increase with increasing spacing due to the coupling effects induced walls. The theoretical relationship related to diffusion flame spreading were extended by Kurosaki [14] based on the introduction of vertical downward flame spreading along two parallel paper sheets, and different conclusions were obtained. The experimental and theoretical results showed that convective heat transfer is dominant in the case of a narrow space between the burning sheets of paper, and this factor controls the flame spreading rate. In contrast, radiation from the opposite flame and embers plays an important role in controlling the flame spreading rate in the case of wider spacing. In this paper, comparative bench-scale experiments were conducted to investigate the vertical downward flame spread behavior of FPU foam board in conjunction with variable curtain wall spacing. Characteristics including flame shape, spread velocity, heat transfer, etc. were interpreted and analyzed. Following the introduction, the experimental setup is depicted in Section 2, the experimental results are presented and correlated in Section 3, and finally the conclusions are
Fig. 1. Experimental setup for FPU board downward burning behavior with various curtain wall conditions. 2
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slower flame spread in the middle and thus an inverted V structure. This phenomenon can be explained by Gollner’s boundary layer theory [15], which is based on the relationship between mass, momentum and heat transfer in the boundary layer on the fuel surface. The associated equation is
summarized in Section 4. 2. Materials and methods All the experiments in this study were carried out on a new experimental setup, as depicted in Fig. 1. Fire scenario was simulated by combining a facade wall fire and an opposite parallel wall representing a curtain wall. In this work, transparent fire-retardant glass was adopted to simulate the curtain wall and makes it easier to observe the entire downward flame spread behavior from front view. This experimental facility mainly consists of three parts, which are: (1) downward flame spread system, (2) curtain wall system, and (3) measurement systems. The flame spread experimental apparatus consisted of an electric balance produced by Sartorius Co. Ltd, with an accuracy of 0.01 g to monitor changes in the fuel mass, an FPU board holder, sensors, and a dripping slot which can collect the mass of dripping liquids generated by melting FPU. A sample of FPU foam board (2 cm thick, 80 cm long, 20 cm wide) was mounted on an insulating gypsum board on the FPU board holder. The FPU board was ignited by a long horizontal electric heating wire to achieve linear ignition and downward flame spreading. Two digital video recorders (SONY, FDR-AX100E) were positioned at front and lateral side of flame, respectively. To capture more details about flame spread behaviors and flame structures, two sequences of thermocouples were used and situated approximately 2 mm above the board surface with an interval of 10 cm. The accuracy of K-type ultra-fine thermocouple is 0.1 °C with its maximum value of measured temperature is 1000 °C. An array of five thermocouples (T0–T4) was located along centerline of the FPU surface and another array was located along lateral side (T5–T9). Reference lines were drawn at increments of 10 cm on board to allow visual analysis of the flame dispersion. As illustrated in Fig. 1, the distance or the spacing D between external facade and a parallel curtain wall could be adjusted, and six spaces were selected as D = 4.5 cm , 6.0 cm , 8.5 cm , 11 cm , 14 cm and D = ∞ (no curtain wall). Water-cooling radiometers (STT-25–50-R/WF, Tu Xin Inc. with accuracy of 0.01 kW/m2) were employed to measure the variation of radiant heat flux along the surface of the thermal insulation board during the process of fire spreading in our experiments. We used radiometer to observe and record the developing trend of radiation in the interlayer by adjusting the distance D between the curtain wall and board. The sensors of radiometers were placed near the surface of the board, and the data terminal recorded the amount of radiation on the surface during the spreading of the fire at a frequency of 1 s. All tests were conducted at constant initial air temperature and humidity (22 ± 2.0 °C, 55 ± 4%) relative humidity. All temperature and fuel mass data were recorded at a frequency of 1 Hz. Each test was repeated at least three times until results were confirmed to be reproducible. The parameters of the FPU foam board and curtain wall selected for experiments are listed in Table 1.
ṁ ′ ′ τs = 2/3 l u∞ v 2/3 D ln(1 + B )
(1)
In the middle of the board, the entrainment of the ambient air by the flame was limited and it is difficult to maintain its original combustion because of the restriction effect of curtain wall. Thus, the spread velocity in the middle of the board is slower than both sides relatively. At the same time, it can be seen that local small scale melting drops, caused by the thermoplastic behavior of the material, will occur randomly in the spreading process (Fig. 2b), which is also an important cause of secondary fire point. The frequency of melting drops will increase in later stage of flame spreading by heat accumulation inside interlayer between curtain wall and fuel board. In fire accidents, dripping fuel will generate high temperature flammable material detached from the original location, creating multiple fires scene by dripping downward, and induce thermal hazard to surrounding people, properties or adjacent buildings, finally complicating fire rescue efforts. As mentioned above, the entrainment and heat transfer in the combustion process will change significantly when curtain wall existed was shown in Fig. 3. The penetration depth is defined to judge if the materials are thermally thin or thermally thick, which can be expressed by δ ≈ αs τ [16], where α s is the thermal diffusivity of the sample materials and τ means the characteristic time for solid sample to be exposed to the heat from the gas phase. As the FPU board used in our experiment is thermally thin solid, the heat balance equation of the combustion rate could be [17],
V ≈ (q¯″̇ )2
4δp πkρcp (Tig − T∞)2
(2)
with the attenuation of thermal conduction and convection heat transfer along the surface of board, most of the heat exchange is concentrated in an area δp , that is, the thermal penetration length. Further, q¯″̇ represents the density of average heat flux which received in the main area of pyrolysis and preheating δp . When the curtain wall existed, heat balance becomes,
″̇ , cond ⎧ q¯″̇ = qẇ″, rad + qf″̇ , conv + qFPU ⎨ ″ ″ ″ ̇ ̇ ̇ qw, rad = qref , rad + qc, rad ⎩
(3)
The main heat transfer in FPU thermal penetration zone for vertical fire propagation includes radiation from the curtain wall q̇w″, rad , convection in the interlayer space q̇f″, conv , and thermal conduction inside ″ , cond . For the first term, the overall heat feedback of the curtain FPU q̇FPU wall constituted two parts. Although the direct thermal radiation heat feedback from flame were negligible for vertical flame propagation, the
3. Results and discussion Table 1 Properties of the FPU foam and curtain wall used for tests.
3.1. Flame morphology and spreading behavior
Fuel material: FPU foam board
The burning of FPU foam is the burning of splitting small molecule liquid chain in essence, and the final combustion process is the burning of liquid vapor in the pyrolysis zone which is extremely similar to that of pool fire. Due to the upward parallel airflow in the interlayer or chimney space, the flame length and morphology are different with that without curtain wall condition, which will be compared and analyzed later. A typical morphological downward combustion scenario taking curtain wall spacing D = 6 cm condition as an example was depicted in Fig. 2a. During the downward spread of the flame, the updraft air entrainment velocity in the middle of the FPU foam will be higher than that at both sides due to air flow from side and bottom, resulting in a
Average element structure Density (kg/m3) Specific heat (KJ/kg. K) Heat conductivity coefficient (W/m⋅K) Pyrolysis temperature (°C) Heat of combustion (MJ/kg)
CH1.8O0.30N0.05 41.5 1.5 ~0.037 ~440 30
Curtain wall material: Fire-resistant glass Dimension (Height × Width × Thickness, cm) Density (103 kg/m3) Transmittance (%)
3
100 × 40 × 0.3 2.7 87–91
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represents a steady flame propagation process. However, when we compare the average burning rate in the quasi-steady spread stage, complex relationship was found between mass loss and spacing D , which is clearer in Fig. 4(b). When the curtain wall spacing D = 4.5 cm , the mass loss rate showed peak value. At the same time, it can be seen that although the general trend of combustion rate decreases with the increase of D , but it is not monotonous. The reason is that several thermal feedbacks in Eq. (3) also have competition with the increase of D . Firstly as D increases, q̇w″, rad will become larger and then smaller due to the coupling effect of radiation perspective coefficient and curtain wall distance variation. Secondly, q̇f″, conv also becomes more complicated due to the change of upward airflow. In addition, for thermoplastic materials, the melting droplet effect will take away considerable mass and related heat, and thus reduce the combustion rate (this effect is also suggested to first become large and then small with the increase of D , which is the main reason for the steep drop in D = 6.0 cm , 8.5 cm in Fig. 4(b)). For a better understanding, the melting droplet effect is specifically measured depicted in Table 2. m The extent of FPU's melting-dripping behavior η , defined as η = md , i is introduced to study the melting and dripping effects under building curtain wall condition, where mi is the initial mass of the FPU foam board (~210 g), md is the dripping mass of fuel that collected by dripping slot and measured when each case finished. As depicted in Table 2, it can be observed that without curtain wall condition, the mass of dripping fuel was lowest which was consistent with the phenomenon discribed. Meanwhile, the mass of dripping fuel seems to reach the maximum for D = 6–8.5 cm, which may be attributed to two competitive effects of the curtain wall. One of the effects is that the curtain wall can promote temperature field since extra heat feedback from the curtain wall to FPU. Moreover, the formed chimney due to
Fig. 2. Downward flame propagation behavior of melting-dripping behavior of FPU at spacing D = 6 cm .
Fig. 3. Schematic diagram of heat transfer and air entrainment.
″ , rad by curtain wall would further enhance flame radiation reflection q̇ref the burning process. Meanwhile, the radiation q̇c″, rad will be emitted by curtain wall itself after heated by flame. In addition, as most of building curtain walls (including our experiments) are made of transparent glass materials, the reflected radiation is inversely correlated with the light transmittance of curtain wall. It should be noted that the overall temperature field of the interlayer space will increase as the spreading process, which means q̇f″, conv will increase, but on the other hand, rising entrainment flow at the bottom (enhanced cooling) would also increase. So there is no obvious acceleration of the fire spread in the experimental observations due to the two opposite effects, analyzed later.
3.2. Comparison of burning rate Fig. 4 presents a comparison of mass loss data variation during downward flame spread in conjunction with curtain wall spacing. The mass loss vs. time was approximately linear as shown in Fig. 4a which
Fig. 4. (a) The mass loss as a function of time (at various curtain wall distances) and (b) the burning rates as a function of D for FPU boards. 4
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Table 2 Melting-Dripping degree of FPU at various curtain wall spacing. D (cm)
4.5
6.0
8.5
11.0
14.0
∞
η (%)
30
37.9
36.1
17.6
11.6
8.4
curtain wall could also acclerate the ambient air cooling, leading to incomplete combustion and decrease of the heat release rate. Moreover, the former effect was weakend and latter effect was strengthed approximately with the increase spacing, i.e., former effect is dominant for the smaller spacing, whereas the latter is decisive for the larger spacing. Therefore, the complex trend in Fig. 4 is actually determined by coupled heat transfer changes and droplet effect.
3.3. Flame height Average flame height at quasi-steady spread stage with different curtain wall spacing D were acquired by the corresponding image series recorded by the CCD camera with intermittency of 0.5 as shown in Fig. 5. CCD high-speed camera was used to collect the video information of flame morphology, and Zukoski's [18] probability method of 50% flame spot was adopted to obtain the average flame height under each spacing D condition through the self-compiled MATLAB program. The increasing of the flame height in middle stage leads to larger chimney effect and stretching effect induced by the curtain wall, causing further increase of the flame height. Subsequently, the flame height becomes dropping in later stage as most of the molten FPU is consumed and the fuel supply decreases. The average flame heights under different test conditions are presented in Fig. 6, which indicates that the average flame height first rises and then drops with the enlarged spacing D with a critical turning point, i.e. Dc (~10 cm in this work). The variation of the average flame height is significant as D ⩽ Dc while indistinctive change is observed for
Fig. 6. Flame height variation with the curtain wall spacing D .
Fig. 5. Flame intermittency contour and determination of flame height (I = 0.5), (a) D = 6cm ; (a) D = 11cm . 5
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spacing effect on temperature is not obvious but it does also show nonmonotonicity (as stated in the paper). Hot gas or plume temperature is mainly decided by the combustion reaction process, so we think that the effect of spacing D works through the entrainment and air cooling effect, although this effect is limited. Meanwhile as shown in Fig. 10, D = 4.5 cm , 8.5 cm (i.e., in the case of a curtain wall), the radiation intensity of the upper part will be slightly greater than the lower part, the reason is that a large number of smoke gathered during fire development by FPU material burning, so the carbon soot generated the blocking effect of radiation intensity [21]. It is more apparent in the later development stage, and the peak value of the radiometer at the bottom is lower. Instead, without the curtain wall, lower part peak becomes bigger than the upper part. This is because the fire spread to the bottom in a fully developed state (fully developed status), regardless of its height and burning rate is relatively early larger and more stable. The phenomenon observed here is practical for building fire prevention and control.Fig. 11. In addition, the spreading velocity should be positively correlated with the burning rate theoretically
D > Dc . The apparent reason involved is explained as follows. The presence of the induces the chimney effect and the restriction of air entrainment. Firstly for D ⩽ Dc , the chimney effect induced by curtain wall is significant, leading to obvious flame stretching phenomenon. But for very small D condition, the incomplete combustion by insufficient air supply and related heat release rate decrease would reduce the flame height. Secondly for D > Dc , the chimney effect may be weakened as the spacing increases and no longer the dominant factor. A slowly decreasing flame height vs. D would be observed by the decaying stretching effect. A deeper disccusion on flame morphology is given based on classical fire dynamics and plate flow. According to the theory of Thomas [19], flame stretching effect is positively relative to burning rate and homodromous wind speed. However, the air flow between the curtain wall and the plate is a complex state due to influence of combustion and boundary layer. As shown in Fig. 7, the upward air entrainment first presents a semi-laminar flow state at the entrance of the interlayer, and then transit to turbulent flow after a distance. In classical theory, Re number of flat plate is used to represent this transition in flow Re =
D ·Uw, v·ρ μ
ṁ ∝
(4)
When Re is less than a critical value (the ideal value without combustion is about 2000), μ is the viscosity coefficient, the boundary flow is dominated by laminar, but this critical value will also change with spacing D .
Rec = f (D), and Rec
increases with D
q̇ V ρ̃ ΔHg + ρcp (Tig − T∞)
(6)
Comparing Fig. 4 and the following 11, the variation trend is similar as Eq. (6) shows. When curtain wall spacing D is small, the curtain wall and gypsum board are heated by the flame at the same time, the heat received at unburned area is relatively large, leading to the faster flame spreading rate. With the increasing D , the internal boundary layer disturbance intensifies and the combustion instability increases, which may weaken both the temperature and the continuous flame zone. At the same time, due to the decreasing heat from the flame by reflection of the opposite curtain wall, this additional thermal feedback effect would reduce rapidly. Meanwhile, as illustrated above, as spacing D become larger, the combustion becomes more sufficient, and the flame spreading velocity tend to maintain a stable level.
(5)
It should be noted that, the larger D is, the smaller the turbulence disturbance of the bottom entrainment to the flame will be. This is further confirmed by the results of the FPU pulsation frequency change of the flame shown in Fig. 8. Therefore, considering the coupling effect of upward entrainment and turbulence intensity (the stronger the turbulence in the flame area, the weaker the stretching effect of updraft), the overall flame height shown in Fig. 6 is not surprising. 3.4. Temperature field and flame spreading velocity
4. Conclusions
Flame and hot plume temperature field is key factor for heat transfer [20–23], mass loss rate, flame spread rate, etc. The temperature by two sequences of thermocouples located at the central and lateral side under curtain wall spacing D = 6 cm conditions are depicted in Fig. 9. The Tf − max (the peak temperature measured by each thermocouple location) were analyzed here. It is shown that the temperature at lateral was higher than central condition generally. With the curtain wall spacing D variation, the temperature field of the middle and both sides will be affected. It is mainly subjected to the changed air entrainment manner. For small spacing with limited upward air supplied, the air ventilation from lateral side was strengthened and controlled the combustion sufficiency. This phenomenon was also verified by An [24] which simulated the vortex field by using CFD method. In An's study, when the curtain wall spacing become narrower, the external large-scale vortex structure was pulled and splited due to lateral air entrainment force, small inverted vortex occurred near the centerline, and finally lead to the temperature along the centerline derceased and higher temperature area appears at both sides that consistent with our experiment results. Further, for enlarged spacing D with sufficient air supply, the flame mainly developed along the vertical direction and heat accumulates between the two plates, so the middle temperature could be increased slightly. When the spacing D is large enough, two plates constained structure become to a open veneer structure, and with the invalid curtain wall effect, the temperature along centerline and lateral side would become similar. Fig. 9 shows a typical result of hot gas temperature field development near the plate surface, that is, the temperature on the outside is higher than the center slightly. According to our measurements, the
The influence of parallel curtain wall on the characteristics of downward flame spread over FPU foam board was investigated in this work by series of laboratory-scale experiments. Further, the flame leading edge shape, mass loss rate, flame height, temperature field, radiation heat flux and flame spread rate were obtained and analyzed based on heat transfer mechanism and fire dynamics. The conclusions are as following:
Fig. 7. The state of interplate flow changes with the increase of D . 6
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Fig. 11. Average flame spreading rate at different curtain wall spacing.
Fig. 8. Average flame plusation frequency with increasing spacing.
effects by curtain wall configuration. Also, phenomenology explaination was proposed based on the analysis of heat feedback mechanisms and turbulent flow characteristics. (3) The average flame height was found to first drop and then increase with the enlarged spacing by a critical spacing, i.e. Dc . The variation of the average flame height is significant as D ⩽ Dc while indistinctive change is observed for D > Dc due to the combined chimney and restriction effects induced by changes in the curtain wall spacing. (4) The temperature at lateral was higher than that of at central condition in general, which is related to the air entrainment supply and the sufficient degree in burning. Also, the development of radiation in interlayer zone was disccussed. The results of this work should be helpful with respect to the fire hazard assessment and safe design of adjacent building facades. Declaration of Competing Interest Fig. 9. Temperatures field at the central and lateral side under the condition of curtain wall spacing D = 6 cm .
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was supported by the National Key Research and Development Program of China (No. 2017YFC0803300), National Natural Science Foundation of China (No. 51776060 & No. 51606215 & No. 51606002), Key Research and Development Plan of Anhui Province, Granted (201904a07020070, 201904a07020072), Anhui Provincial Natural Science Foundation of China (Grant No. 1908085QE250), Opening Fund of State Key Laboratory of Fire Science (Grant No. HZ2019-KF13). The authors gratefully acknowledge all these supports. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114694.
Fig. 10. Radiation heat flux of curtain wall spacing D = 4.5 cm, 8.5 cm, ∞.
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(1) The constraint effect of curtain wall would made the lateral side air entrainment weaken, and delay the phenomenon of gradient inverted “V” shape of flame leading edge. Furthermore, the meltingdripping phenomenon was more distinct due to the heat accumulation under narrow spacing condition which could complicate fire rescue. (2) The mass loss showed non-monotonous relationship with curtain wall spacing attributed to the competition between the multiple
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental study of interlayer effect induced by building facade curtain wall on downward flame spread behavior of polyurethane Xin Maa, Ran Tub, Weiguang Anc, Li Xud, Shengfeng Luoe, Jingwei Wanga, Fei Tangf,
⁎
a
College of Environment and Energy Engineering, Anhui Jianzhu University, Hefei, Anhui 230022, China College of Mechanical Engineering and Automation, Huaqiao University, Xiamen, Fujian 361021, China c Jiangsu Key Laboratory of Fire Safety in Urban Underground Space, China University of Mining and Technology, Xuzhou 221116, China d College of Civil Science and Engineering, Yangzhou University, Yangzhou, Jiangsu 225127, China e Department of Engineering Physics, Tsinghua University, Beijing 100091, China f School of Automotive and Transportation Engineering, Hefei University of Technology, Hefei, Anhui 230009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Curtain wall Interlayer effect Polyurethane material Melting-dripping Building fire safety
This paper presents an experimental investigation on interlayer effect induced by curtain wall, parallel to the building exterior facade, on thermal and burning behavior of insulation material flexible polyurethane (FPU). Results show that, downward flames propagation behavior along the interlayer was determined by the coupling effects including chimney effect, constraint effect and related heat transfer variation induced by curtain wall. Firstly, comparing to the occasionally melting-dripping phenomenon without curtain wall, continuous meltingdripping liquids were generated subject to positively heat feedback by internal heat radiation to the incomplete ember layer at later combustion stage, which is more distinct for narrow curtain wall spacing. Secondly based on the analysis of heat feedback and the dripping degree, the non-monotonous relation between mass loss and spacing D was interpreted. Thirdly, flame physical variations e.g., height and stretching induced by complex turbulence were discussed with plate flow theory. Furthermore, hot gas temperature field along board surface centerline and sideline were compared. Due to the insufficient combustion and cooling of upwards air entrainment, temperature along centerline was found to be lower generally, which is different from the free burning. In addition, analysis on radiation flux development with soot blockage effect and also flame spread velocity was proposed.
1. Introduction Modern designs for the structure of building exterior facades tend to be diversified as a result of requirements related to lighting or aesthetics. At the same time, curtain walls are usually installed parallel to the building exterior facade. Due to the complex effects, the issue of facade flame spread has been incorporated in building fire safety engineering design guidelines [1]. As an example, the facade flame spreading characteristics associated with building curtain wall can complicate fire rescue operations in high-rise buildings, where a vertical channel is formed between the curtain wall and the building cladding. In history, e.g., London Grenfell Tower fire in 2017, the gap between cladding and curtain wall may act as chimney when combustible insulation materials burn in fire accident. This fire scenario is different from single facade fires due to curtain wall configurations significantly affect the air entrainment, flow field and also the heat and mass transfer. The interlayer structure, especially for the two facades ⁎
confined fire burning or spreading is truly a key factor for fire development [2–6]. On the other hand, insulation materials are widely used on the cladding of high-rise buildings assemblies as a popular energy-saving method. Flexible polyurethane (FPU) foam, is regarded as a kind of highly representative polymer material with good portability and thermal insulation performance. However, some kinds of FPU foam are combustible and thermoplastic-like by manufacture process, which would be melt partially and ignited easily by external heat flux. Furthermore, due to the quick flame spread velocity in FPU burning, the potential risk of fire hazard and challenges in fire prevention could never be ignored. Our previous studies [7–9] have shown that the downward flame spreading of the FPU foam board can be considered as a small, narrow pool fire on the top of the solid foam. However, the coupled factors determining the fire hazards of FPU under curtain wall structure condition are actually quite complicated, and the unique “melting-dripping” behavior of FPU during combustion can represent a
Corresponding author. E-mail address: [email protected] (F. Tang).
https://doi.org/10.1016/j.applthermaleng.2019.114694 Received 7 September 2019; Received in revised form 6 November 2019; Accepted 16 November 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xin Ma, et al., Applied Thermal Engineering, https://doi.org/10.1016/j.applthermaleng.2019.114694
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Nomenclature
B cp D Dc k ṁ ṁ ″ md mi ṁ t″ q̇ q¯″̇ q̇w″, rad q̇f″, conv ″ , cond q̇FPU ″ , rad q̇ref q̇c″, rad q̇p″
Tf − max Tig T∞ Uw,v u∞ v V δp ρ ΔHg τs
spalding number specific heat of fuel (J/g·K) distance between facade and curtain wall (cm) a critical spacing about 10 (cm) thermal conductivity of solid fuel (W/m·K) burning rate (g/s) burning rate per area (g/s·m2) dripping mass of the fuel (g) initial mass of the FPU foam board (g) mass transfer caused by shear flow (g) heat flux rate (W/s) averaged heat flux (W/m2) radiation from the curtain wall (W/m2) convection in the interlayer space (W/m2) thermal conduction inside FPU (W/m2) flame radiation reflection by curtain wall (W/m2) radiation from the curtain wall itself (W/m2) preheating heat flux to pyrolysis zone (W/m2)
peak temperature of each thermocouple (K) ignition temperature (K) ambient temperature (K) velocity of vertical air entrainment (m/s) free-stream velocity (m/s) kinematic viscosity or momentum diffusivity (Pa⋅s) flame spread velocity (m/s) thermal penetration length (cm) density (kg/m3) enthalpy changes (kJ/kg) viscosity coefficient multiplied by the derivative of velocity
Subscripts
f h p v
challenge when designing fire protection systems for buildings. Except for the flame spread along the interlayer, previous works have also been reported on the building facade flame ejected from the opening of the enclosure room, which was then restricted by curtain wall. Window ejected flame may further induce the special form of fire spreading between the interlayer of curtain wall in some special cases. Lee and Delichatsios [10] investigated the constraint effect of an opposite facing wall (opposite to the opening of the compartment facade). Tang et al. [11] revealed the flame height evolution with a sloping facing wall constraint at various angles. Lu et al. [12] further studied the merging behavior of the two facade flames from two parallel openings. Although the influence of external wall on compartment fires is being studied over the past decades, little literature concerning the influence of curtain wall on the building facade cladding materials' combustion behavior. Curtain wall is a typical structure in many high-rise buildings. The semi-enclosed restricted space formed by curtain wall will directly affect the thermal feedback, flame morphology, hot smoke flow, air entrainment and other characteristics in the combustion process. An [13] investigated the effects of parallel walls on the downward flame
fire or flame horizontal component pyrolysis vertical component
spreading characteristics of insulation materials extruded polystyrene (XPS) foam. The average flame height and maximum flame temperature were found to initially become lower and then increase with increasing spacing due to the coupling effects induced walls. The theoretical relationship related to diffusion flame spreading were extended by Kurosaki [14] based on the introduction of vertical downward flame spreading along two parallel paper sheets, and different conclusions were obtained. The experimental and theoretical results showed that convective heat transfer is dominant in the case of a narrow space between the burning sheets of paper, and this factor controls the flame spreading rate. In contrast, radiation from the opposite flame and embers plays an important role in controlling the flame spreading rate in the case of wider spacing. In this paper, comparative bench-scale experiments were conducted to investigate the vertical downward flame spread behavior of FPU foam board in conjunction with variable curtain wall spacing. Characteristics including flame shape, spread velocity, heat transfer, etc. were interpreted and analyzed. Following the introduction, the experimental setup is depicted in Section 2, the experimental results are presented and correlated in Section 3, and finally the conclusions are
Fig. 1. Experimental setup for FPU board downward burning behavior with various curtain wall conditions. 2
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slower flame spread in the middle and thus an inverted V structure. This phenomenon can be explained by Gollner’s boundary layer theory [15], which is based on the relationship between mass, momentum and heat transfer in the boundary layer on the fuel surface. The associated equation is
summarized in Section 4. 2. Materials and methods All the experiments in this study were carried out on a new experimental setup, as depicted in Fig. 1. Fire scenario was simulated by combining a facade wall fire and an opposite parallel wall representing a curtain wall. In this work, transparent fire-retardant glass was adopted to simulate the curtain wall and makes it easier to observe the entire downward flame spread behavior from front view. This experimental facility mainly consists of three parts, which are: (1) downward flame spread system, (2) curtain wall system, and (3) measurement systems. The flame spread experimental apparatus consisted of an electric balance produced by Sartorius Co. Ltd, with an accuracy of 0.01 g to monitor changes in the fuel mass, an FPU board holder, sensors, and a dripping slot which can collect the mass of dripping liquids generated by melting FPU. A sample of FPU foam board (2 cm thick, 80 cm long, 20 cm wide) was mounted on an insulating gypsum board on the FPU board holder. The FPU board was ignited by a long horizontal electric heating wire to achieve linear ignition and downward flame spreading. Two digital video recorders (SONY, FDR-AX100E) were positioned at front and lateral side of flame, respectively. To capture more details about flame spread behaviors and flame structures, two sequences of thermocouples were used and situated approximately 2 mm above the board surface with an interval of 10 cm. The accuracy of K-type ultra-fine thermocouple is 0.1 °C with its maximum value of measured temperature is 1000 °C. An array of five thermocouples (T0–T4) was located along centerline of the FPU surface and another array was located along lateral side (T5–T9). Reference lines were drawn at increments of 10 cm on board to allow visual analysis of the flame dispersion. As illustrated in Fig. 1, the distance or the spacing D between external facade and a parallel curtain wall could be adjusted, and six spaces were selected as D = 4.5 cm , 6.0 cm , 8.5 cm , 11 cm , 14 cm and D = ∞ (no curtain wall). Water-cooling radiometers (STT-25–50-R/WF, Tu Xin Inc. with accuracy of 0.01 kW/m2) were employed to measure the variation of radiant heat flux along the surface of the thermal insulation board during the process of fire spreading in our experiments. We used radiometer to observe and record the developing trend of radiation in the interlayer by adjusting the distance D between the curtain wall and board. The sensors of radiometers were placed near the surface of the board, and the data terminal recorded the amount of radiation on the surface during the spreading of the fire at a frequency of 1 s. All tests were conducted at constant initial air temperature and humidity (22 ± 2.0 °C, 55 ± 4%) relative humidity. All temperature and fuel mass data were recorded at a frequency of 1 Hz. Each test was repeated at least three times until results were confirmed to be reproducible. The parameters of the FPU foam board and curtain wall selected for experiments are listed in Table 1.
ṁ ′ ′ τs = 2/3 l u∞ v 2/3 D ln(1 + B )
(1)
In the middle of the board, the entrainment of the ambient air by the flame was limited and it is difficult to maintain its original combustion because of the restriction effect of curtain wall. Thus, the spread velocity in the middle of the board is slower than both sides relatively. At the same time, it can be seen that local small scale melting drops, caused by the thermoplastic behavior of the material, will occur randomly in the spreading process (Fig. 2b), which is also an important cause of secondary fire point. The frequency of melting drops will increase in later stage of flame spreading by heat accumulation inside interlayer between curtain wall and fuel board. In fire accidents, dripping fuel will generate high temperature flammable material detached from the original location, creating multiple fires scene by dripping downward, and induce thermal hazard to surrounding people, properties or adjacent buildings, finally complicating fire rescue efforts. As mentioned above, the entrainment and heat transfer in the combustion process will change significantly when curtain wall existed was shown in Fig. 3. The penetration depth is defined to judge if the materials are thermally thin or thermally thick, which can be expressed by δ ≈ αs τ [16], where α s is the thermal diffusivity of the sample materials and τ means the characteristic time for solid sample to be exposed to the heat from the gas phase. As the FPU board used in our experiment is thermally thin solid, the heat balance equation of the combustion rate could be [17],
V ≈ (q¯″̇ )2
4δp πkρcp (Tig − T∞)2
(2)
with the attenuation of thermal conduction and convection heat transfer along the surface of board, most of the heat exchange is concentrated in an area δp , that is, the thermal penetration length. Further, q¯″̇ represents the density of average heat flux which received in the main area of pyrolysis and preheating δp . When the curtain wall existed, heat balance becomes,
″̇ , cond ⎧ q¯″̇ = qẇ″, rad + qf″̇ , conv + qFPU ⎨ ″ ″ ″ ̇ ̇ ̇ qw, rad = qref , rad + qc, rad ⎩
(3)
The main heat transfer in FPU thermal penetration zone for vertical fire propagation includes radiation from the curtain wall q̇w″, rad , convection in the interlayer space q̇f″, conv , and thermal conduction inside ″ , cond . For the first term, the overall heat feedback of the curtain FPU q̇FPU wall constituted two parts. Although the direct thermal radiation heat feedback from flame were negligible for vertical flame propagation, the
3. Results and discussion Table 1 Properties of the FPU foam and curtain wall used for tests.
3.1. Flame morphology and spreading behavior
Fuel material: FPU foam board
The burning of FPU foam is the burning of splitting small molecule liquid chain in essence, and the final combustion process is the burning of liquid vapor in the pyrolysis zone which is extremely similar to that of pool fire. Due to the upward parallel airflow in the interlayer or chimney space, the flame length and morphology are different with that without curtain wall condition, which will be compared and analyzed later. A typical morphological downward combustion scenario taking curtain wall spacing D = 6 cm condition as an example was depicted in Fig. 2a. During the downward spread of the flame, the updraft air entrainment velocity in the middle of the FPU foam will be higher than that at both sides due to air flow from side and bottom, resulting in a
Average element structure Density (kg/m3) Specific heat (KJ/kg. K) Heat conductivity coefficient (W/m⋅K) Pyrolysis temperature (°C) Heat of combustion (MJ/kg)
CH1.8O0.30N0.05 41.5 1.5 ~0.037 ~440 30
Curtain wall material: Fire-resistant glass Dimension (Height × Width × Thickness, cm) Density (103 kg/m3) Transmittance (%)
3
100 × 40 × 0.3 2.7 87–91
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represents a steady flame propagation process. However, when we compare the average burning rate in the quasi-steady spread stage, complex relationship was found between mass loss and spacing D , which is clearer in Fig. 4(b). When the curtain wall spacing D = 4.5 cm , the mass loss rate showed peak value. At the same time, it can be seen that although the general trend of combustion rate decreases with the increase of D , but it is not monotonous. The reason is that several thermal feedbacks in Eq. (3) also have competition with the increase of D . Firstly as D increases, q̇w″, rad will become larger and then smaller due to the coupling effect of radiation perspective coefficient and curtain wall distance variation. Secondly, q̇f″, conv also becomes more complicated due to the change of upward airflow. In addition, for thermoplastic materials, the melting droplet effect will take away considerable mass and related heat, and thus reduce the combustion rate (this effect is also suggested to first become large and then small with the increase of D , which is the main reason for the steep drop in D = 6.0 cm , 8.5 cm in Fig. 4(b)). For a better understanding, the melting droplet effect is specifically measured depicted in Table 2. m The extent of FPU's melting-dripping behavior η , defined as η = md , i is introduced to study the melting and dripping effects under building curtain wall condition, where mi is the initial mass of the FPU foam board (~210 g), md is the dripping mass of fuel that collected by dripping slot and measured when each case finished. As depicted in Table 2, it can be observed that without curtain wall condition, the mass of dripping fuel was lowest which was consistent with the phenomenon discribed. Meanwhile, the mass of dripping fuel seems to reach the maximum for D = 6–8.5 cm, which may be attributed to two competitive effects of the curtain wall. One of the effects is that the curtain wall can promote temperature field since extra heat feedback from the curtain wall to FPU. Moreover, the formed chimney due to
Fig. 2. Downward flame propagation behavior of melting-dripping behavior of FPU at spacing D = 6 cm .
Fig. 3. Schematic diagram of heat transfer and air entrainment.
″ , rad by curtain wall would further enhance flame radiation reflection q̇ref the burning process. Meanwhile, the radiation q̇c″, rad will be emitted by curtain wall itself after heated by flame. In addition, as most of building curtain walls (including our experiments) are made of transparent glass materials, the reflected radiation is inversely correlated with the light transmittance of curtain wall. It should be noted that the overall temperature field of the interlayer space will increase as the spreading process, which means q̇f″, conv will increase, but on the other hand, rising entrainment flow at the bottom (enhanced cooling) would also increase. So there is no obvious acceleration of the fire spread in the experimental observations due to the two opposite effects, analyzed later.
3.2. Comparison of burning rate Fig. 4 presents a comparison of mass loss data variation during downward flame spread in conjunction with curtain wall spacing. The mass loss vs. time was approximately linear as shown in Fig. 4a which
Fig. 4. (a) The mass loss as a function of time (at various curtain wall distances) and (b) the burning rates as a function of D for FPU boards. 4
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Table 2 Melting-Dripping degree of FPU at various curtain wall spacing. D (cm)
4.5
6.0
8.5
11.0
14.0
∞
η (%)
30
37.9
36.1
17.6
11.6
8.4
curtain wall could also acclerate the ambient air cooling, leading to incomplete combustion and decrease of the heat release rate. Moreover, the former effect was weakend and latter effect was strengthed approximately with the increase spacing, i.e., former effect is dominant for the smaller spacing, whereas the latter is decisive for the larger spacing. Therefore, the complex trend in Fig. 4 is actually determined by coupled heat transfer changes and droplet effect.
3.3. Flame height Average flame height at quasi-steady spread stage with different curtain wall spacing D were acquired by the corresponding image series recorded by the CCD camera with intermittency of 0.5 as shown in Fig. 5. CCD high-speed camera was used to collect the video information of flame morphology, and Zukoski's [18] probability method of 50% flame spot was adopted to obtain the average flame height under each spacing D condition through the self-compiled MATLAB program. The increasing of the flame height in middle stage leads to larger chimney effect and stretching effect induced by the curtain wall, causing further increase of the flame height. Subsequently, the flame height becomes dropping in later stage as most of the molten FPU is consumed and the fuel supply decreases. The average flame heights under different test conditions are presented in Fig. 6, which indicates that the average flame height first rises and then drops with the enlarged spacing D with a critical turning point, i.e. Dc (~10 cm in this work). The variation of the average flame height is significant as D ⩽ Dc while indistinctive change is observed for
Fig. 6. Flame height variation with the curtain wall spacing D .
Fig. 5. Flame intermittency contour and determination of flame height (I = 0.5), (a) D = 6cm ; (a) D = 11cm . 5
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spacing effect on temperature is not obvious but it does also show nonmonotonicity (as stated in the paper). Hot gas or plume temperature is mainly decided by the combustion reaction process, so we think that the effect of spacing D works through the entrainment and air cooling effect, although this effect is limited. Meanwhile as shown in Fig. 10, D = 4.5 cm , 8.5 cm (i.e., in the case of a curtain wall), the radiation intensity of the upper part will be slightly greater than the lower part, the reason is that a large number of smoke gathered during fire development by FPU material burning, so the carbon soot generated the blocking effect of radiation intensity [21]. It is more apparent in the later development stage, and the peak value of the radiometer at the bottom is lower. Instead, without the curtain wall, lower part peak becomes bigger than the upper part. This is because the fire spread to the bottom in a fully developed state (fully developed status), regardless of its height and burning rate is relatively early larger and more stable. The phenomenon observed here is practical for building fire prevention and control.Fig. 11. In addition, the spreading velocity should be positively correlated with the burning rate theoretically
D > Dc . The apparent reason involved is explained as follows. The presence of the induces the chimney effect and the restriction of air entrainment. Firstly for D ⩽ Dc , the chimney effect induced by curtain wall is significant, leading to obvious flame stretching phenomenon. But for very small D condition, the incomplete combustion by insufficient air supply and related heat release rate decrease would reduce the flame height. Secondly for D > Dc , the chimney effect may be weakened as the spacing increases and no longer the dominant factor. A slowly decreasing flame height vs. D would be observed by the decaying stretching effect. A deeper disccusion on flame morphology is given based on classical fire dynamics and plate flow. According to the theory of Thomas [19], flame stretching effect is positively relative to burning rate and homodromous wind speed. However, the air flow between the curtain wall and the plate is a complex state due to influence of combustion and boundary layer. As shown in Fig. 7, the upward air entrainment first presents a semi-laminar flow state at the entrance of the interlayer, and then transit to turbulent flow after a distance. In classical theory, Re number of flat plate is used to represent this transition in flow Re =
D ·Uw, v·ρ μ
ṁ ∝
(4)
When Re is less than a critical value (the ideal value without combustion is about 2000), μ is the viscosity coefficient, the boundary flow is dominated by laminar, but this critical value will also change with spacing D .
Rec = f (D), and Rec
increases with D
q̇ V ρ̃ ΔHg + ρcp (Tig − T∞)
(6)
Comparing Fig. 4 and the following 11, the variation trend is similar as Eq. (6) shows. When curtain wall spacing D is small, the curtain wall and gypsum board are heated by the flame at the same time, the heat received at unburned area is relatively large, leading to the faster flame spreading rate. With the increasing D , the internal boundary layer disturbance intensifies and the combustion instability increases, which may weaken both the temperature and the continuous flame zone. At the same time, due to the decreasing heat from the flame by reflection of the opposite curtain wall, this additional thermal feedback effect would reduce rapidly. Meanwhile, as illustrated above, as spacing D become larger, the combustion becomes more sufficient, and the flame spreading velocity tend to maintain a stable level.
(5)
It should be noted that, the larger D is, the smaller the turbulence disturbance of the bottom entrainment to the flame will be. This is further confirmed by the results of the FPU pulsation frequency change of the flame shown in Fig. 8. Therefore, considering the coupling effect of upward entrainment and turbulence intensity (the stronger the turbulence in the flame area, the weaker the stretching effect of updraft), the overall flame height shown in Fig. 6 is not surprising. 3.4. Temperature field and flame spreading velocity
4. Conclusions
Flame and hot plume temperature field is key factor for heat transfer [20–23], mass loss rate, flame spread rate, etc. The temperature by two sequences of thermocouples located at the central and lateral side under curtain wall spacing D = 6 cm conditions are depicted in Fig. 9. The Tf − max (the peak temperature measured by each thermocouple location) were analyzed here. It is shown that the temperature at lateral was higher than central condition generally. With the curtain wall spacing D variation, the temperature field of the middle and both sides will be affected. It is mainly subjected to the changed air entrainment manner. For small spacing with limited upward air supplied, the air ventilation from lateral side was strengthened and controlled the combustion sufficiency. This phenomenon was also verified by An [24] which simulated the vortex field by using CFD method. In An's study, when the curtain wall spacing become narrower, the external large-scale vortex structure was pulled and splited due to lateral air entrainment force, small inverted vortex occurred near the centerline, and finally lead to the temperature along the centerline derceased and higher temperature area appears at both sides that consistent with our experiment results. Further, for enlarged spacing D with sufficient air supply, the flame mainly developed along the vertical direction and heat accumulates between the two plates, so the middle temperature could be increased slightly. When the spacing D is large enough, two plates constained structure become to a open veneer structure, and with the invalid curtain wall effect, the temperature along centerline and lateral side would become similar. Fig. 9 shows a typical result of hot gas temperature field development near the plate surface, that is, the temperature on the outside is higher than the center slightly. According to our measurements, the
The influence of parallel curtain wall on the characteristics of downward flame spread over FPU foam board was investigated in this work by series of laboratory-scale experiments. Further, the flame leading edge shape, mass loss rate, flame height, temperature field, radiation heat flux and flame spread rate were obtained and analyzed based on heat transfer mechanism and fire dynamics. The conclusions are as following:
Fig. 7. The state of interplate flow changes with the increase of D . 6
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Fig. 11. Average flame spreading rate at different curtain wall spacing.
Fig. 8. Average flame plusation frequency with increasing spacing.
effects by curtain wall configuration. Also, phenomenology explaination was proposed based on the analysis of heat feedback mechanisms and turbulent flow characteristics. (3) The average flame height was found to first drop and then increase with the enlarged spacing by a critical spacing, i.e. Dc . The variation of the average flame height is significant as D ⩽ Dc while indistinctive change is observed for D > Dc due to the combined chimney and restriction effects induced by changes in the curtain wall spacing. (4) The temperature at lateral was higher than that of at central condition in general, which is related to the air entrainment supply and the sufficient degree in burning. Also, the development of radiation in interlayer zone was disccussed. The results of this work should be helpful with respect to the fire hazard assessment and safe design of adjacent building facades. Declaration of Competing Interest Fig. 9. Temperatures field at the central and lateral side under the condition of curtain wall spacing D = 6 cm .
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was supported by the National Key Research and Development Program of China (No. 2017YFC0803300), National Natural Science Foundation of China (No. 51776060 & No. 51606215 & No. 51606002), Key Research and Development Plan of Anhui Province, Granted (201904a07020070, 201904a07020072), Anhui Provincial Natural Science Foundation of China (Grant No. 1908085QE250), Opening Fund of State Key Laboratory of Fire Science (Grant No. HZ2019-KF13). The authors gratefully acknowledge all these supports. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114694.
Fig. 10. Radiation heat flux of curtain wall spacing D = 4.5 cm, 8.5 cm, ∞.
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(1) The constraint effect of curtain wall would made the lateral side air entrainment weaken, and delay the phenomenon of gradient inverted “V” shape of flame leading edge. Furthermore, the meltingdripping phenomenon was more distinct due to the heat accumulation under narrow spacing condition which could complicate fire rescue. (2) The mass loss showed non-monotonous relationship with curtain wall spacing attributed to the competition between the multiple
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