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Influence of sidewalls on width effects of upward flame spread
Fire Safety Journal 46 (2011) 294–304
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Influence of sidewalls on width effects of upward flame spread Kuang-Chung Tsai Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, 2 Juoyue Road, Nantzu, Kaohsiung 811, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2010 Received in revised form 28 March 2011 Accepted 30 March 2011 Available online 21 April 2011
A previous study has reported width effects for turbulent diffusion upward flame spread on thermally thick materials with sidewalls. However, sidewalls are not realistic. The present study has revisited this topic by performing experiments without sidewalls using 9 mm thick and 1000 mm tall PMMA slabs with widths of 100, 200, 300, 500 and 700 mm and by providing a hypothesis of the sidewall effects. Experimental data have revealed that the width effects still exist when sidewalls are absent. Flame heights and spread rates were higher for wider flames, although heat feedback to the fuel did not vary much with flame width. Compared to flames without sidewalls, the existence of sidewalls lengthened flame heights and generally reduced heat feedback along the central lines of the flames, resulting in higher flame spread rates for narrower flames and lower flame spread rates for wider flames. In addition, the absence of sidewalls enhanced the delivery of pyrolyzate towards the central line of the flames throughout the whole flame width. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Upward flame spread Width effect Sidewall effect
1. Introduction Previous studies have identified upward flame spread on vertical surfaces as one of the most hazardous fire scenarios due to the concurrent direction of flame propagation and air flow [1]. The prediction of upward flame spread rate is of importance, and many experimental studies and several models [1–13] have been developed owing to the potential of leading to more severe fire hazards [14,15]. Most of these studies and models have regarded upward flame spread as being one dimensional, and do not consider the effect of the width of the burning zone. However, Thomas and Webster [16] demonstrated that, for thermally thin materials, flame spread is faster for wider, width 6–100 mm, freely suspended strips of cotton fabric. Tsai and Drysdale [17] showed a weak width effect on flame height using well-controlled vertical gas panel fires with fixed burning areas and steady fuel flow. Tsai and Wan [18] studied spreading PMMA wall fires, and showed significant width effects on flame spread rates for samples narrower than 300 mm and insignificant width effects for samples with widths 300–900 mm. Additionally, Rangwala et al. [19] discussed finite-width effects, theoretically and experimentally, using 500 mm long and 25 mm thick PMMA samples with varying widths from 25 to 150 mm. They tried to prove the theory of Pagni and Shih [20] for laminar flames (width o200 mm), addressing the width effect caused primarily by a fraction of fuel diffusing to the sides and consequently changing the amount of fuel available to participate in flame spread as excess pyrolyzate. The reduction of pyrolyzate near the sides was
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relatively significant for narrower flames. In addition, a parallel, but secondary, mechanism was the heat loss by conduction at the sides, where the pyrolysis rate was reduced compared with that at the centre of the sample. Both the flame spread rate and flame height consequently decreased while the width decreased in their study. Pizzo et al. [21] also experimentally studied the width effect for PMMA slabs without sidewalls with widths from 0.025 to 0.2 m. They observed width-independent flame height and flame spread rate correlations for flames wider than 0.1 m due to a decrease in the influence of lateral diffusion for wider flames. Rangwala et al. [19] and Pizzo et al. [21] both considered laminar flames. Recently, Tsai [22] developed a hypothesis and performed experiments with sidewalls using 18 mm thick and 1000 mm tall PMMA slabs with widths of 100, 200, 300, 500 and 700 mm. This study [22] was the first one for turbulent diffusion flames on thermally thick materials. (There were well-mixed air and fuel pyrolysis products in the gas phase, particularly near the bottom of the wall. A premixed-like flame was generated, as described by Tsai and Drysdale [11].) In the hypothesis of Tsai [22], a lateral fractional force appeared, both by the sides and throughout the flame width towards the central line of a flame, while entrained air and pyrolyzate mixed and enhanced the efficiency of the diffusion of the flammable mixture. The former effect formed a curved flame surface, i.e. the flame was thicker along the central line, while the latter resulted in more efficient combustion. Additionally, flame height and heat feedback to the fuel were discussed by similarity analysis, correlating flame height (Xf) and 0 heat feedback (q_ 00 ) using the heat release rate per unit width (Q_ ) and the ratio of vertical position and flame height (X/Xf), respectively. Larger flame heights (i.e. higher heat release rate per unit width), more heat feedback to the surface (against X/Xf) and
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higher flame temperatures were consequently present for wider flames. The corresponding flame spread rate also increased with width, and a power value of 0.35 existed between the flame spread rate and the width in the study of Tsai [22]. All the experimental results clearly supported Tsai’s hypothesis [22]. In Tsai’s hypothesis and experiments [22], sidewalls were used. Sidewalls can reduce the effect of local lateral diffusion of pyrolyzate near the edges of the flame, and the effect of air entrainment throughout the whole flame width can be focused. However, a wall fire without sidewalls is more realistic. This study has applied a similar experimental rig, but without sidewalls, to investigate the width effects on upward flame spread. Additionally, the influence of sidewalls on the width effects of upward flame spread is discussed.
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along the central line of the sample. Additionally, the flame temperature and visual flame thickness (standoff distance) were recorded when the pyrolysis front reached the top of the samples via a portable K-type thermocouple and a camcorder for further radiation discussion. The rate of upward flame spread was determined by analysing infra-red video recordings of each experiment. This infra-red experimental technique with narrow band filters was proven by Arakawa et al. [23] and Qian et al. [24] to enable wall surface temperature measurements. The accompanying software allowed the pyrolysis front to be tracked as the 350 1C contour as it advanced upwards. This experimental arrangement permitted simultaneous measurement of the flame height, total heat transfer and flame spread rate.
3. Experimental results (effects of width) 2. Experimental design A series of experiments was designed to study the influence of sidewalls on the width effect of wall fires. Fig. 1 schematically illustrates the experimental set-up. The samples were 9 mm thick and 1 m tall clear PMMA, with widths of 100, 200, 300, 500 and 700 mm. The samples were held against a 3 mm thick steel plate to prevent flame spreading up on the back of the sample, distortion and slumping. A 25 mm thick and 1 m tall marinite board was setup on top of the sample to allow the flames to continue to spread as ‘‘wall’’ fires until the end of the experiments. For comparison, two 50 mm high sidewalls made of 9 mm thick marinite were used for the cases with sidewalls. A hand-held butane-fuelled blowtorch was used to ignite the bottom 50 mm of the sample and was removed following ignition. The flame height was determined visually [11]. A Gardongauge total heat flux metre was positioned at a height of 850 mm
The effects of width are discussed in this section using flame height, heat flux to unburned surface and flame spread rate. 3.1. Flame development and flame height correlation After ignition, a flame established and propagated upwardly. Figs. 2 and 3 demonstrate the shapes of visual flames and infrared images with various widths (100, 300 and 700 mm) with and without sidewalls, respectively, when pyrolysis fronts reached the position of the heat flux meter (850 mm high). For 100 mm wide flames, the shapes of visual flames and infra-red images with and without sidewalls were similar. However, for flames wider than 100 mm, the shapes with and without sidewalls were different. The flame tips were higher near the edges when sidewalls were present, and were higher along the central line when sidewalls were absent. In this study, flame height, heat flux to fuel and
Marinite board
Total heat flux meter PMMA
PMMA sidewall PMMA ignition
x
y z
Fig. 1. Schematic of experimental rig: (a) with sidewalls and (b) without sidewalls.
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Fig. 2. Pictures of flames of different widths with or without sidewalls. (a) 100 mm wide without sidewalls; (b) 100 mm wide with sidewalls; (c) 300 mm wide without sidewalls; (d) 300 mm wide with sidewalls; (e) 700 mm wide without sidewalls and (f) 700 mm wide with sidewalls.
Fig. 3. Infra-red images of different widths with or without sidewalls. (a) 100 mm wide without sidewalls; (b) 100 mm wide with sidewalls; (c) 300 mm wide without sidewalls; (d) 300 mm wide with sidewalls; (e) 700 mm wide without sidewalls and (f) 700 mm wide with sidewalls.
flame spread rate were measured along the central line of the flames. The effects of the sidewalls will be discussed in Section 4. Fig. 4 displays the flame height measurements (average of three tests) against pyrolysis height for the 100, 200, 300, 500 and 700 mm wide samples without sidewalls. The flames were clearly higher for wider flames. This observation is consistent with measurements by Tsai and Drysdale [17] in their gas panel flames, and Tsai and Wan [18] and Tsai [22] in their PMMA studies, although sidewalls were present in these studies [17,18,22]. Orloff et al. [25] identified a transition from laminar to fully turbulent flames at heights from 10 to 15 cm from their wall-fire experiments using 41 cm wide and 157 cm high vertical PMMA slabs with sidewalls. The flame behaviour (laminar or turbulent) according to corresponding controlling heat transfer feedback mechanisms was identified [25]. The flame spread of laminar flames was controlled by convective heat transfer or edge conduction, and their flame spread rate decreased with size. Intermediate-scale turbulent flames were primarily controlled by turbulent convective heat transfer, with some augmentation by radiation, resulting in a minimum burning rate that was independent of size. Fully turbulent flames were controlled by radiation, and an increased burning rate was observed. An approximately constant flame spread rate at heights from 10 to
15 cm was observed [25], and it was identified that this height was associated with a transition from laminar to turbulent flames. Additionally, Tamanini [26] reported a height of 0.2 m above which a fire plume becomes turbulent and radiative heat transfer starts to play an important role in numerical analysis. In this study, constant flame spread rates were addressed with flames of heights below approximately 20 cm (see Fig. 4). Thus, flames shorter than 20 cm may be identified as a transition from laminar to turbulent, while flames higher than 20 cm may be turbulent. 3.2. Heat flux correlation Fig. 5 shows the total heat flux distributions (average of three tests) of the spreading PMMA wall fires plotted as a function of height (X) normalised against the flame height (Xf). The total heat flux distributions were similar to Hasemi’s correlation [27] (not shown), yielding q_ 00 ¼ 12:3ðX=Xf Þ2:5 (for X/Xf 40.7). Clearly, the difference of primary total heat fluxes (X/Xf o1) for flames with different widths was not significant, although the total heat fluxes were lower for 100 mm wide flames. The lower heat feedback for 100 mm wide flames may have been caused by significant lateral diffusion of the pyrolyzate to the sides for the narrow flames described by Rangwala et al. [19]. The amount of fuel available to
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Fig. 4. Flame height correlation (against pyrolysis height) of the 100, 200, 300, 500 and 700 mm wide samples.
Fig. 5. Total heat flux distributions of the spreading PMMA wall fires without sidewalls plotted as a function of height (X) normalised against the flame height.
participate in upward flame spread as excess pyrolyzate was reduced. The heat feedback associated with the flames higher than 20 cm was primarily radiative [22] and can be related to the flame temperature and flame thickness by: Q_ rad ¼ esT 4
ð1Þ
where e denotes the emissivity, s is the Stefan–Boltzmann constant (5.67 10 8 W/m2K4) and T represents the flame
temperature (K). The emissivity can be estimated by
e ¼ 1expðKLÞ
ð2Þ
where K denotes an effective emission coefficient and L represents the visual flame thickness (or mean beam length). (The ‘‘thickness’’ is measured orthogonal to the PMMA surface.) Table 1 lists the flame temperatures and thicknesses when the pyrolysis fronts reached the position of the heat flux meter. Clearly, the flame temperatures were higher for wider flames, while the differences of the flame thicknesses were not obvious. The insignificant influence
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of width on the total heat fluxes in Fig. 5 may result from the similar flame thicknesses among the flames with different widths. Additionally, K may depend on the sample width because soot concentration changes with flame size [28]. More research is required to quantify the effects of each parameter.
3.3. Flame spread rate Fig. 6 shows the flame spread rates (average of three tests) of the wall fires with different widths. A clear width effect can be observed. Wider flames spread faster than narrower flames. The flame spread rate correlates with width in the form of Eq. (3). In this study, the values of n without and with sidewalls (Fig. 7, as discussed in Section 3.4) were 0.35, identical to the value found in Tsai’s study [22] using 18 mm thick PMMA with sidewalls. Therefore, the value of n is independent of fuel thickness and presence of sidewalls in this study. However, the dependence of fuel on the value of n requires further investigation. In addition, the higher flame spread rate with wider flames without sidewalls can be explained primarily due to the larger flame height. Vpwn
ð3Þ
Table 1 Flame temperature and thickness when pyrolysis fronts reached the position of heat flux meter. Width (mm)
100 200 300 500 700
Without sidewalls
With sidewalls
Flame temperature (1C)
Flame thickness (mm)
Flame temperature (1C)
Flame thickness (mm)
6707 20 689 7 36 666 7 24 691 7 15 735 7 33
55 73 60 72 60 73 58 74 59 74
715 7 21 655 7 33 665 7 30 676 7 28 729 7 26
62 74 62 74 62 73 63 75 65 74
where V denotes the upward flame spread rate and w is the flame width. 3.4. Comparison with Tsai’s study [22] Fig. 7 presents the flame spread rate with sidewalls for comparison with Tsai’s study [22]. The only difference between Fig. 7 in this study and the study of Tsai [22] was the thickness of the PMMA. Tsai [22] used 18 mm thick PMMA, and the present study applied 9 mm thick PMMA slabs. Clearly, flames spread faster for thicker fuel. When the fuel is thinner, heat generated on the fuel’s surface can conduct to the 3 mm thick backing steel plate faster, reducing the heat contributing to the flame spread.
4. Experimental results (effects of sidewalls) Fig. 2 demonstrates the shapes of visual flames and infra-red images of pyrolysis regions with and without sidewalls. For 100 mm wide flames, the shapes of flames with and without sidewalls were similar. However, for flames with widths of 300 and 700 mm, the shapes of flames and infra-red images with and without sidewalls were different. A sidewall effect was clearly more significant for wider flames. Wang et al. [29] showed a curved flame shape because of the absence of sidewalls in their numerical analysis, and pointed out that the principal air entrainment was from the front side of the plume. The presence of sidewalls influences the pattern of air entrainment and flow structure [30]. Fig. 8 demonstrates the flame height correlations along the central lines against pyrolysis height for each width, with and without sidewalls. Clearly, the flames were higher when sidewalls were present. Fig. 9 exhibits the heat flux along the central lines of the flames. For 100 mm wide flames, the heat fluxes were higher when sidewalls were present. However, for 200, 300, 500 and 700 mm wide flames, the heat fluxes were higher without sidewalls. The higher heat fluxes for the 100 mm
Fig. 6. Flame spread rates of the wall fires without sidewalls.
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Fig. 7. Flame spread rates of the wall fires with sidewalls.
wide flames with sidewalls may be caused by relatively less lateral heat loss. The higher heat fluxes for flames wider than 100 mm without sidewalls can be explained by higher temperatures (shown in Table 1) along the central line due to more efficient combustion. The enhancement of combustion efficiency may be caused by better mixing due to stronger lateral air entrainment towards the central lines. Fig. 10 shows the flame spread rate along the central line for each width, with and without sidewalls. Flames with widths of 100, 200 and 300 mm spread faster when sidewalls were present. The sidewall effects on flame spread rate were not significant for the 500 mm wide flames. However, the 700 mm wide flames without sidewalls propagated faster than those with sidewalls.
5. Discussion of sidewall effects According to the experimental results in this study, a hypothesis of sidewall effect on upward flame spread is provided. Sidewall effects occur along the sidewalls and throughout the whole flame width, influencing the lateral air-entrainment pattern and heat transfer, respectively. Fig. 11 shows air-entrainment patterns in upward flame spread with different widths, with and without sidewalls. Near the edges of the flames, air entrainment is limited when sidewalls exist. In addition, the pyrolyzate generated near the edges does not diffuse laterally, but moves upwards along the sidewalls. Therefore, when sidewalls are present, the flame tips are higher near the edges. Furthermore, some of the heat generated near the edges conducts to the sidewalls. The conductive heat loss to the sidewalls depends on the conductivity of the sidewall material. In this study, the lateral heat loss was only slightly due to the relatively low conductivity of marinite. Sidewall effects also occur throughout the whole flame width. Air entrainment towards the central line of the flames is enhanced throughout the whole flame width due to the absence of sidewalls. The pyrolyzate generated is subsequently delivered towards the central line of the flames, giving relatively higher flame tips and increasing the flame thickness along the central line rather than near the edges of the flames. The shapes of
the flames shown in Fig. 2 consequently form. Additionally, the enhanced air entrainment towards the central lines uplifts the mixing of air and pyrolyzate, resulting in more efficient combustion. Generally, higher temperatures (shown in Table 1) for flames without sidewalls were observed, causing higher heat flux distribution along central lines. The effects of sidewalls occur near the edges of flames and throughout the whole flame width. For 100 mm wide flames, the sidewall effects near the edges are dominant. With an increase of flame width, the sidewall effects throughout the whole flame width become important. The presence of sidewalls influences flame height, heat feedback to the fuel and the consequent flame spread rate. According to the experimental data (Fig. 8), flames (along the central lines) were higher when sidewalls were present. For the 100 mm wide flames, the sidewall effects near the edges were dominant where sidewalls formed a channel for flames to extend. For 200, 300, 500 and 700 mm wide flames, the extension of flames along sidewalls would only slightly influence the flame heights along the central lines. The shorter flames without sidewalls may be caused by more efficient combustion. When combustion is more efficient, the pyrolyzate generated is consumed quickly. The location of flame tips was theoretically defined by Delichatsios [31] as the region above which burning is complete. After the pyrolyzate is generated, it moves upwards due to buoyancy, mixes with air and is then burnt. When combustion is more efficient, the pyrolyzate generated is consumed at relatively lower regions along the route for the pyrolyzate to move upwards, consequently lowering the height of the flame tips, i.e. flame height. Eqs. (4) and (5) show the correlations of the flame height (Xf) with pyrolysis height (Xp) for turbulent wall flames ( 420 cm) without and with sidewalls for similarity analysis [32,33]. The power values of Xp were very close to 0.66, which is the theoretical value in many correlations [11,12,27,34]. The heights of flames without sidewalls were more sensitive to width than those with sidewalls according to the higher power value of the width. The delivery of pyrolyzate towards the central line of the flames throughout the whole flame width was strong when the sidewalls were absent. This observation corresponds to the stronger air entrainment for flames without sidewalls than for
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Fig. 8. Comparison of flame height correlations (against pyrolysis height) of the 100, 200, 300, 500 and 700 mm wide samples with and without sidewalls.
those with sidewalls. Xf ¼ 1:65Xp0:67 w0:2 , Xf ¼ 1:72Xp0:68 w0:14 ,
for flames without sidewalls for flames with sidewalls
ð4Þ ð5Þ
Sugawa et al. [35] investigated side-wind effects on flames ejected from an opening of a compartment, and estimated the flame inclination slope caused by the side wind. They demonstrated that the inclination slope increased with side-wind velocity [35]. In the present study, the lateral air entrainment was involved in the burning zone as a side wind. The lateral air entrainment was strong when sidewalls were absent. Figs. 2 and 3 show shapes of flames and pyrolysis zones for flames with and without sidewalls. For flames with sidewalls, higher flames and pyrolysis fronts were observed near the edges and along the central lines. For flames without sidewalls, higher flames and pyrolysis fronts were observed only along the central lines. Fig. 12(a) demonstrates schematic diagrams of the shape of the
pyrolysis zone and the inclination angle at the edges. The inclination angles were 01 throughout the fuel width initially because the bottom edges of the fuels, which were 5 cm high, were ignited. Then, the pyrolysis zones grew, as Fig. 3(a), (c) and (e) displayed, showing higher pyrolysis fronts near the central lines and lower ones near the edges. Near the central lines, the inclination angles were 01. The inclination angles decreased from y near the edges to 01 near the central lines; the inclination angle reflects the relative strength of lateral air entrainment, buoyancy and inertia. Therefore, only the development of y is discussed here. Fig. 12(b) plots the inclination angle y at the edges as flames propagated upwards. The y value increased and reached a steady state gradually as the flames grew. Clearly, y was larger for narrower flames when the flames were below 200 mm, and larger for wider flames when they were higher than 400 mm. Flames lower than 200 mm were laminar and convection controlled, while flames higher than 400 mm were turbulent and radiation controlled. At the early stages of narrow flames, strong buoyancydriven convection dominated the air flow, and achieved large
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301
Fig. 9. Comparison of total heat flux distributions of the wall fires with and without sidewalls.
values of y. When flames were higher than 400 mm, the influence of lateral air entrainment was less important, and achieved large values of y. Moreover, the air-entrainment pattern influences the heat feedback to fuel. For the 100 mm wide flames, the channel formed by the sidewalls prevented the pyrolyzate diffusing laterally and reduced lateral heat loss. A strong convective upward current moved along the channel due to the sidewall effects, particularly for narrow samples. Additionally, the flames were relatively thicker and with higher temperatures along the sidewalls (see Table 1). Thus, the heat feedback to the fuel with sidewalls was stronger. For the 200, 300, 500 and 700 mm wide flames, lateral air entrainment towards the central line and subsequent mixing of air and pyrolyzate were enhanced when the sidewalls
were absent. The better mixing enhanced combustion efficiency. Thus, the heat feedback to the fuel without sidewalls was stronger. In addition, flame spread rate is governed by flame height and heat flux to fuel. The 100 mm wide flames with sidewalls propagated faster than those without sidewalls due to longer flame height and more heat feedback to the fuel when the sidewalls were present. The 200 mm wide flames with sidewalls also propagated faster than those without sidewalls due to longer flames caused by the sidewalls, although heat feedback was less. The influence of flame height was stronger than heat flux to the fuel. However, with the increase of flame width, flames were generally thicker and with higher temperatures, so the influence of radiative heat flux to the fuel became dominant. For the
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Fig. 10. Comparison of flame spread rates of the wall fires with and without sidewalls.
fuel Sidewall or extension flame entrained air
Fig. 11. Schematic (top view) of different air-entrainment patterns of wall fires with different widths with and without sidewalls.
500 mm wide flames, the flame spread rates with and without sidewalls were almost identical due to the equivalent influence of longer flame height and less heat flux to the fuel with sidewalls.
For the 700 wide flames, the flame spread rates were primarily controlled by stronger radiative heat flux to the fuel, performing higher flame spread rates without sidewalls.
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Fig. 12. Correlation of inclination angle near edges and pyrolysis height. (a) Schematic diagrams of the shape of the pyrolysis zone and the inclination angle at the edges and (b) inclination angle y at the edges as flames propagate upwards.
6. Conclusion This study has investigated the effects of width on upward flame spread without sidewalls by performing experiments using 9 mm thick and 1000 mm tall PMMA slabs with widths of 100, 200, 300, 500 and 700 mm and conducting analyses of sidewall effects. Experimental data have revealed that width effects still exist when sidewalls were absent. Flame height and spread rate were higher for wider flames, although heat flux to the fuel did not vary much with flame width. Flame spread rate was correlated with flame width to a power of 0.35. The value of the power was independent of fuel thickness and presence of sidewalls. Additionally, sidewalls produced higher flames and generally less heat feedback in this study, resulting in higher flame spread rates with sidewalls when flames were narrower, and lower flame spread rates with sidewalls when flames were wider. The analysis of sidewall effects highlighted the influence of entrained air near the edges and throughout the whole flame width. Near the edges of flames when sidewalls were present, flames were extended and heat loss was decreased. Throughout the whole width of the flames, delivery of the pyrolyzate towards the central lines of the flames when sidewalls were absent played an important role.
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Influence of sidewalls on width effects of upward flame spread Kuang-Chung Tsai Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, 2 Juoyue Road, Nantzu, Kaohsiung 811, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2010 Received in revised form 28 March 2011 Accepted 30 March 2011 Available online 21 April 2011
A previous study has reported width effects for turbulent diffusion upward flame spread on thermally thick materials with sidewalls. However, sidewalls are not realistic. The present study has revisited this topic by performing experiments without sidewalls using 9 mm thick and 1000 mm tall PMMA slabs with widths of 100, 200, 300, 500 and 700 mm and by providing a hypothesis of the sidewall effects. Experimental data have revealed that the width effects still exist when sidewalls are absent. Flame heights and spread rates were higher for wider flames, although heat feedback to the fuel did not vary much with flame width. Compared to flames without sidewalls, the existence of sidewalls lengthened flame heights and generally reduced heat feedback along the central lines of the flames, resulting in higher flame spread rates for narrower flames and lower flame spread rates for wider flames. In addition, the absence of sidewalls enhanced the delivery of pyrolyzate towards the central line of the flames throughout the whole flame width. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Upward flame spread Width effect Sidewall effect
1. Introduction Previous studies have identified upward flame spread on vertical surfaces as one of the most hazardous fire scenarios due to the concurrent direction of flame propagation and air flow [1]. The prediction of upward flame spread rate is of importance, and many experimental studies and several models [1–13] have been developed owing to the potential of leading to more severe fire hazards [14,15]. Most of these studies and models have regarded upward flame spread as being one dimensional, and do not consider the effect of the width of the burning zone. However, Thomas and Webster [16] demonstrated that, for thermally thin materials, flame spread is faster for wider, width 6–100 mm, freely suspended strips of cotton fabric. Tsai and Drysdale [17] showed a weak width effect on flame height using well-controlled vertical gas panel fires with fixed burning areas and steady fuel flow. Tsai and Wan [18] studied spreading PMMA wall fires, and showed significant width effects on flame spread rates for samples narrower than 300 mm and insignificant width effects for samples with widths 300–900 mm. Additionally, Rangwala et al. [19] discussed finite-width effects, theoretically and experimentally, using 500 mm long and 25 mm thick PMMA samples with varying widths from 25 to 150 mm. They tried to prove the theory of Pagni and Shih [20] for laminar flames (width o200 mm), addressing the width effect caused primarily by a fraction of fuel diffusing to the sides and consequently changing the amount of fuel available to participate in flame spread as excess pyrolyzate. The reduction of pyrolyzate near the sides was
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relatively significant for narrower flames. In addition, a parallel, but secondary, mechanism was the heat loss by conduction at the sides, where the pyrolysis rate was reduced compared with that at the centre of the sample. Both the flame spread rate and flame height consequently decreased while the width decreased in their study. Pizzo et al. [21] also experimentally studied the width effect for PMMA slabs without sidewalls with widths from 0.025 to 0.2 m. They observed width-independent flame height and flame spread rate correlations for flames wider than 0.1 m due to a decrease in the influence of lateral diffusion for wider flames. Rangwala et al. [19] and Pizzo et al. [21] both considered laminar flames. Recently, Tsai [22] developed a hypothesis and performed experiments with sidewalls using 18 mm thick and 1000 mm tall PMMA slabs with widths of 100, 200, 300, 500 and 700 mm. This study [22] was the first one for turbulent diffusion flames on thermally thick materials. (There were well-mixed air and fuel pyrolysis products in the gas phase, particularly near the bottom of the wall. A premixed-like flame was generated, as described by Tsai and Drysdale [11].) In the hypothesis of Tsai [22], a lateral fractional force appeared, both by the sides and throughout the flame width towards the central line of a flame, while entrained air and pyrolyzate mixed and enhanced the efficiency of the diffusion of the flammable mixture. The former effect formed a curved flame surface, i.e. the flame was thicker along the central line, while the latter resulted in more efficient combustion. Additionally, flame height and heat feedback to the fuel were discussed by similarity analysis, correlating flame height (Xf) and 0 heat feedback (q_ 00 ) using the heat release rate per unit width (Q_ ) and the ratio of vertical position and flame height (X/Xf), respectively. Larger flame heights (i.e. higher heat release rate per unit width), more heat feedback to the surface (against X/Xf) and
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higher flame temperatures were consequently present for wider flames. The corresponding flame spread rate also increased with width, and a power value of 0.35 existed between the flame spread rate and the width in the study of Tsai [22]. All the experimental results clearly supported Tsai’s hypothesis [22]. In Tsai’s hypothesis and experiments [22], sidewalls were used. Sidewalls can reduce the effect of local lateral diffusion of pyrolyzate near the edges of the flame, and the effect of air entrainment throughout the whole flame width can be focused. However, a wall fire without sidewalls is more realistic. This study has applied a similar experimental rig, but without sidewalls, to investigate the width effects on upward flame spread. Additionally, the influence of sidewalls on the width effects of upward flame spread is discussed.
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along the central line of the sample. Additionally, the flame temperature and visual flame thickness (standoff distance) were recorded when the pyrolysis front reached the top of the samples via a portable K-type thermocouple and a camcorder for further radiation discussion. The rate of upward flame spread was determined by analysing infra-red video recordings of each experiment. This infra-red experimental technique with narrow band filters was proven by Arakawa et al. [23] and Qian et al. [24] to enable wall surface temperature measurements. The accompanying software allowed the pyrolysis front to be tracked as the 350 1C contour as it advanced upwards. This experimental arrangement permitted simultaneous measurement of the flame height, total heat transfer and flame spread rate.
3. Experimental results (effects of width) 2. Experimental design A series of experiments was designed to study the influence of sidewalls on the width effect of wall fires. Fig. 1 schematically illustrates the experimental set-up. The samples were 9 mm thick and 1 m tall clear PMMA, with widths of 100, 200, 300, 500 and 700 mm. The samples were held against a 3 mm thick steel plate to prevent flame spreading up on the back of the sample, distortion and slumping. A 25 mm thick and 1 m tall marinite board was setup on top of the sample to allow the flames to continue to spread as ‘‘wall’’ fires until the end of the experiments. For comparison, two 50 mm high sidewalls made of 9 mm thick marinite were used for the cases with sidewalls. A hand-held butane-fuelled blowtorch was used to ignite the bottom 50 mm of the sample and was removed following ignition. The flame height was determined visually [11]. A Gardongauge total heat flux metre was positioned at a height of 850 mm
The effects of width are discussed in this section using flame height, heat flux to unburned surface and flame spread rate. 3.1. Flame development and flame height correlation After ignition, a flame established and propagated upwardly. Figs. 2 and 3 demonstrate the shapes of visual flames and infrared images with various widths (100, 300 and 700 mm) with and without sidewalls, respectively, when pyrolysis fronts reached the position of the heat flux meter (850 mm high). For 100 mm wide flames, the shapes of visual flames and infra-red images with and without sidewalls were similar. However, for flames wider than 100 mm, the shapes with and without sidewalls were different. The flame tips were higher near the edges when sidewalls were present, and were higher along the central line when sidewalls were absent. In this study, flame height, heat flux to fuel and
Marinite board
Total heat flux meter PMMA
PMMA sidewall PMMA ignition
x
y z
Fig. 1. Schematic of experimental rig: (a) with sidewalls and (b) without sidewalls.
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Fig. 2. Pictures of flames of different widths with or without sidewalls. (a) 100 mm wide without sidewalls; (b) 100 mm wide with sidewalls; (c) 300 mm wide without sidewalls; (d) 300 mm wide with sidewalls; (e) 700 mm wide without sidewalls and (f) 700 mm wide with sidewalls.
Fig. 3. Infra-red images of different widths with or without sidewalls. (a) 100 mm wide without sidewalls; (b) 100 mm wide with sidewalls; (c) 300 mm wide without sidewalls; (d) 300 mm wide with sidewalls; (e) 700 mm wide without sidewalls and (f) 700 mm wide with sidewalls.
flame spread rate were measured along the central line of the flames. The effects of the sidewalls will be discussed in Section 4. Fig. 4 displays the flame height measurements (average of three tests) against pyrolysis height for the 100, 200, 300, 500 and 700 mm wide samples without sidewalls. The flames were clearly higher for wider flames. This observation is consistent with measurements by Tsai and Drysdale [17] in their gas panel flames, and Tsai and Wan [18] and Tsai [22] in their PMMA studies, although sidewalls were present in these studies [17,18,22]. Orloff et al. [25] identified a transition from laminar to fully turbulent flames at heights from 10 to 15 cm from their wall-fire experiments using 41 cm wide and 157 cm high vertical PMMA slabs with sidewalls. The flame behaviour (laminar or turbulent) according to corresponding controlling heat transfer feedback mechanisms was identified [25]. The flame spread of laminar flames was controlled by convective heat transfer or edge conduction, and their flame spread rate decreased with size. Intermediate-scale turbulent flames were primarily controlled by turbulent convective heat transfer, with some augmentation by radiation, resulting in a minimum burning rate that was independent of size. Fully turbulent flames were controlled by radiation, and an increased burning rate was observed. An approximately constant flame spread rate at heights from 10 to
15 cm was observed [25], and it was identified that this height was associated with a transition from laminar to turbulent flames. Additionally, Tamanini [26] reported a height of 0.2 m above which a fire plume becomes turbulent and radiative heat transfer starts to play an important role in numerical analysis. In this study, constant flame spread rates were addressed with flames of heights below approximately 20 cm (see Fig. 4). Thus, flames shorter than 20 cm may be identified as a transition from laminar to turbulent, while flames higher than 20 cm may be turbulent. 3.2. Heat flux correlation Fig. 5 shows the total heat flux distributions (average of three tests) of the spreading PMMA wall fires plotted as a function of height (X) normalised against the flame height (Xf). The total heat flux distributions were similar to Hasemi’s correlation [27] (not shown), yielding q_ 00 ¼ 12:3ðX=Xf Þ2:5 (for X/Xf 40.7). Clearly, the difference of primary total heat fluxes (X/Xf o1) for flames with different widths was not significant, although the total heat fluxes were lower for 100 mm wide flames. The lower heat feedback for 100 mm wide flames may have been caused by significant lateral diffusion of the pyrolyzate to the sides for the narrow flames described by Rangwala et al. [19]. The amount of fuel available to
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Fig. 4. Flame height correlation (against pyrolysis height) of the 100, 200, 300, 500 and 700 mm wide samples.
Fig. 5. Total heat flux distributions of the spreading PMMA wall fires without sidewalls plotted as a function of height (X) normalised against the flame height.
participate in upward flame spread as excess pyrolyzate was reduced. The heat feedback associated with the flames higher than 20 cm was primarily radiative [22] and can be related to the flame temperature and flame thickness by: Q_ rad ¼ esT 4
ð1Þ
where e denotes the emissivity, s is the Stefan–Boltzmann constant (5.67 10 8 W/m2K4) and T represents the flame
temperature (K). The emissivity can be estimated by
e ¼ 1expðKLÞ
ð2Þ
where K denotes an effective emission coefficient and L represents the visual flame thickness (or mean beam length). (The ‘‘thickness’’ is measured orthogonal to the PMMA surface.) Table 1 lists the flame temperatures and thicknesses when the pyrolysis fronts reached the position of the heat flux meter. Clearly, the flame temperatures were higher for wider flames, while the differences of the flame thicknesses were not obvious. The insignificant influence
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of width on the total heat fluxes in Fig. 5 may result from the similar flame thicknesses among the flames with different widths. Additionally, K may depend on the sample width because soot concentration changes with flame size [28]. More research is required to quantify the effects of each parameter.
3.3. Flame spread rate Fig. 6 shows the flame spread rates (average of three tests) of the wall fires with different widths. A clear width effect can be observed. Wider flames spread faster than narrower flames. The flame spread rate correlates with width in the form of Eq. (3). In this study, the values of n without and with sidewalls (Fig. 7, as discussed in Section 3.4) were 0.35, identical to the value found in Tsai’s study [22] using 18 mm thick PMMA with sidewalls. Therefore, the value of n is independent of fuel thickness and presence of sidewalls in this study. However, the dependence of fuel on the value of n requires further investigation. In addition, the higher flame spread rate with wider flames without sidewalls can be explained primarily due to the larger flame height. Vpwn
ð3Þ
Table 1 Flame temperature and thickness when pyrolysis fronts reached the position of heat flux meter. Width (mm)
100 200 300 500 700
Without sidewalls
With sidewalls
Flame temperature (1C)
Flame thickness (mm)
Flame temperature (1C)
Flame thickness (mm)
6707 20 689 7 36 666 7 24 691 7 15 735 7 33
55 73 60 72 60 73 58 74 59 74
715 7 21 655 7 33 665 7 30 676 7 28 729 7 26
62 74 62 74 62 73 63 75 65 74
where V denotes the upward flame spread rate and w is the flame width. 3.4. Comparison with Tsai’s study [22] Fig. 7 presents the flame spread rate with sidewalls for comparison with Tsai’s study [22]. The only difference between Fig. 7 in this study and the study of Tsai [22] was the thickness of the PMMA. Tsai [22] used 18 mm thick PMMA, and the present study applied 9 mm thick PMMA slabs. Clearly, flames spread faster for thicker fuel. When the fuel is thinner, heat generated on the fuel’s surface can conduct to the 3 mm thick backing steel plate faster, reducing the heat contributing to the flame spread.
4. Experimental results (effects of sidewalls) Fig. 2 demonstrates the shapes of visual flames and infra-red images of pyrolysis regions with and without sidewalls. For 100 mm wide flames, the shapes of flames with and without sidewalls were similar. However, for flames with widths of 300 and 700 mm, the shapes of flames and infra-red images with and without sidewalls were different. A sidewall effect was clearly more significant for wider flames. Wang et al. [29] showed a curved flame shape because of the absence of sidewalls in their numerical analysis, and pointed out that the principal air entrainment was from the front side of the plume. The presence of sidewalls influences the pattern of air entrainment and flow structure [30]. Fig. 8 demonstrates the flame height correlations along the central lines against pyrolysis height for each width, with and without sidewalls. Clearly, the flames were higher when sidewalls were present. Fig. 9 exhibits the heat flux along the central lines of the flames. For 100 mm wide flames, the heat fluxes were higher when sidewalls were present. However, for 200, 300, 500 and 700 mm wide flames, the heat fluxes were higher without sidewalls. The higher heat fluxes for the 100 mm
Fig. 6. Flame spread rates of the wall fires without sidewalls.
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Fig. 7. Flame spread rates of the wall fires with sidewalls.
wide flames with sidewalls may be caused by relatively less lateral heat loss. The higher heat fluxes for flames wider than 100 mm without sidewalls can be explained by higher temperatures (shown in Table 1) along the central line due to more efficient combustion. The enhancement of combustion efficiency may be caused by better mixing due to stronger lateral air entrainment towards the central lines. Fig. 10 shows the flame spread rate along the central line for each width, with and without sidewalls. Flames with widths of 100, 200 and 300 mm spread faster when sidewalls were present. The sidewall effects on flame spread rate were not significant for the 500 mm wide flames. However, the 700 mm wide flames without sidewalls propagated faster than those with sidewalls.
5. Discussion of sidewall effects According to the experimental results in this study, a hypothesis of sidewall effect on upward flame spread is provided. Sidewall effects occur along the sidewalls and throughout the whole flame width, influencing the lateral air-entrainment pattern and heat transfer, respectively. Fig. 11 shows air-entrainment patterns in upward flame spread with different widths, with and without sidewalls. Near the edges of the flames, air entrainment is limited when sidewalls exist. In addition, the pyrolyzate generated near the edges does not diffuse laterally, but moves upwards along the sidewalls. Therefore, when sidewalls are present, the flame tips are higher near the edges. Furthermore, some of the heat generated near the edges conducts to the sidewalls. The conductive heat loss to the sidewalls depends on the conductivity of the sidewall material. In this study, the lateral heat loss was only slightly due to the relatively low conductivity of marinite. Sidewall effects also occur throughout the whole flame width. Air entrainment towards the central line of the flames is enhanced throughout the whole flame width due to the absence of sidewalls. The pyrolyzate generated is subsequently delivered towards the central line of the flames, giving relatively higher flame tips and increasing the flame thickness along the central line rather than near the edges of the flames. The shapes of
the flames shown in Fig. 2 consequently form. Additionally, the enhanced air entrainment towards the central lines uplifts the mixing of air and pyrolyzate, resulting in more efficient combustion. Generally, higher temperatures (shown in Table 1) for flames without sidewalls were observed, causing higher heat flux distribution along central lines. The effects of sidewalls occur near the edges of flames and throughout the whole flame width. For 100 mm wide flames, the sidewall effects near the edges are dominant. With an increase of flame width, the sidewall effects throughout the whole flame width become important. The presence of sidewalls influences flame height, heat feedback to the fuel and the consequent flame spread rate. According to the experimental data (Fig. 8), flames (along the central lines) were higher when sidewalls were present. For the 100 mm wide flames, the sidewall effects near the edges were dominant where sidewalls formed a channel for flames to extend. For 200, 300, 500 and 700 mm wide flames, the extension of flames along sidewalls would only slightly influence the flame heights along the central lines. The shorter flames without sidewalls may be caused by more efficient combustion. When combustion is more efficient, the pyrolyzate generated is consumed quickly. The location of flame tips was theoretically defined by Delichatsios [31] as the region above which burning is complete. After the pyrolyzate is generated, it moves upwards due to buoyancy, mixes with air and is then burnt. When combustion is more efficient, the pyrolyzate generated is consumed at relatively lower regions along the route for the pyrolyzate to move upwards, consequently lowering the height of the flame tips, i.e. flame height. Eqs. (4) and (5) show the correlations of the flame height (Xf) with pyrolysis height (Xp) for turbulent wall flames ( 420 cm) without and with sidewalls for similarity analysis [32,33]. The power values of Xp were very close to 0.66, which is the theoretical value in many correlations [11,12,27,34]. The heights of flames without sidewalls were more sensitive to width than those with sidewalls according to the higher power value of the width. The delivery of pyrolyzate towards the central line of the flames throughout the whole flame width was strong when the sidewalls were absent. This observation corresponds to the stronger air entrainment for flames without sidewalls than for
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Fig. 8. Comparison of flame height correlations (against pyrolysis height) of the 100, 200, 300, 500 and 700 mm wide samples with and without sidewalls.
those with sidewalls. Xf ¼ 1:65Xp0:67 w0:2 , Xf ¼ 1:72Xp0:68 w0:14 ,
for flames without sidewalls for flames with sidewalls
ð4Þ ð5Þ
Sugawa et al. [35] investigated side-wind effects on flames ejected from an opening of a compartment, and estimated the flame inclination slope caused by the side wind. They demonstrated that the inclination slope increased with side-wind velocity [35]. In the present study, the lateral air entrainment was involved in the burning zone as a side wind. The lateral air entrainment was strong when sidewalls were absent. Figs. 2 and 3 show shapes of flames and pyrolysis zones for flames with and without sidewalls. For flames with sidewalls, higher flames and pyrolysis fronts were observed near the edges and along the central lines. For flames without sidewalls, higher flames and pyrolysis fronts were observed only along the central lines. Fig. 12(a) demonstrates schematic diagrams of the shape of the
pyrolysis zone and the inclination angle at the edges. The inclination angles were 01 throughout the fuel width initially because the bottom edges of the fuels, which were 5 cm high, were ignited. Then, the pyrolysis zones grew, as Fig. 3(a), (c) and (e) displayed, showing higher pyrolysis fronts near the central lines and lower ones near the edges. Near the central lines, the inclination angles were 01. The inclination angles decreased from y near the edges to 01 near the central lines; the inclination angle reflects the relative strength of lateral air entrainment, buoyancy and inertia. Therefore, only the development of y is discussed here. Fig. 12(b) plots the inclination angle y at the edges as flames propagated upwards. The y value increased and reached a steady state gradually as the flames grew. Clearly, y was larger for narrower flames when the flames were below 200 mm, and larger for wider flames when they were higher than 400 mm. Flames lower than 200 mm were laminar and convection controlled, while flames higher than 400 mm were turbulent and radiation controlled. At the early stages of narrow flames, strong buoyancydriven convection dominated the air flow, and achieved large
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Fig. 9. Comparison of total heat flux distributions of the wall fires with and without sidewalls.
values of y. When flames were higher than 400 mm, the influence of lateral air entrainment was less important, and achieved large values of y. Moreover, the air-entrainment pattern influences the heat feedback to fuel. For the 100 mm wide flames, the channel formed by the sidewalls prevented the pyrolyzate diffusing laterally and reduced lateral heat loss. A strong convective upward current moved along the channel due to the sidewall effects, particularly for narrow samples. Additionally, the flames were relatively thicker and with higher temperatures along the sidewalls (see Table 1). Thus, the heat feedback to the fuel with sidewalls was stronger. For the 200, 300, 500 and 700 mm wide flames, lateral air entrainment towards the central line and subsequent mixing of air and pyrolyzate were enhanced when the sidewalls
were absent. The better mixing enhanced combustion efficiency. Thus, the heat feedback to the fuel without sidewalls was stronger. In addition, flame spread rate is governed by flame height and heat flux to fuel. The 100 mm wide flames with sidewalls propagated faster than those without sidewalls due to longer flame height and more heat feedback to the fuel when the sidewalls were present. The 200 mm wide flames with sidewalls also propagated faster than those without sidewalls due to longer flames caused by the sidewalls, although heat feedback was less. The influence of flame height was stronger than heat flux to the fuel. However, with the increase of flame width, flames were generally thicker and with higher temperatures, so the influence of radiative heat flux to the fuel became dominant. For the
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Fig. 10. Comparison of flame spread rates of the wall fires with and without sidewalls.
fuel Sidewall or extension flame entrained air
Fig. 11. Schematic (top view) of different air-entrainment patterns of wall fires with different widths with and without sidewalls.
500 mm wide flames, the flame spread rates with and without sidewalls were almost identical due to the equivalent influence of longer flame height and less heat flux to the fuel with sidewalls.
For the 700 wide flames, the flame spread rates were primarily controlled by stronger radiative heat flux to the fuel, performing higher flame spread rates without sidewalls.
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Fig. 12. Correlation of inclination angle near edges and pyrolysis height. (a) Schematic diagrams of the shape of the pyrolysis zone and the inclination angle at the edges and (b) inclination angle y at the edges as flames propagate upwards.
6. Conclusion This study has investigated the effects of width on upward flame spread without sidewalls by performing experiments using 9 mm thick and 1000 mm tall PMMA slabs with widths of 100, 200, 300, 500 and 700 mm and conducting analyses of sidewall effects. Experimental data have revealed that width effects still exist when sidewalls were absent. Flame height and spread rate were higher for wider flames, although heat flux to the fuel did not vary much with flame width. Flame spread rate was correlated with flame width to a power of 0.35. The value of the power was independent of fuel thickness and presence of sidewalls. Additionally, sidewalls produced higher flames and generally less heat feedback in this study, resulting in higher flame spread rates with sidewalls when flames were narrower, and lower flame spread rates with sidewalls when flames were wider. The analysis of sidewall effects highlighted the influence of entrained air near the edges and throughout the whole flame width. Near the edges of flames when sidewalls were present, flames were extended and heat loss was decreased. Throughout the whole width of the flames, delivery of the pyrolyzate towards the central lines of the flames when sidewalls were absent played an important role.
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