Elsevier

Fuel

Volume 237, 1 February 2019, Pages 486-493
Fuel

Full Length Article
The evolution of flame height and air flow for double rectangular pool fires

https://doi.org/10.1016/j.fuel.2018.10.027Get rights and content

Highlights

  • Flame length and air entrainment rate of double rectangular pool fires are studied.

  • The flame height increases with the reduction of distance between these two pools.

  • The velocity and temperature increases as the two pools move close to each other.

  • A correlation of flame height is developed considering aspect ratio and distance.

Abstract

In this study, a series of experiments were performed to investigate the flame height of double rectangular pool fires with different aspect ratios and distances in open space. Two identical rectangular gas burners with the same heat release rate were used as the fire sources. Four gas burners with the same surface area but with different aspect ratios were used. Different distances between two gas burners were considered. The experimental results indicated that the flame height decreased sharply with the increase of distance and subsequently exhibited a slight change with further increase of distance. The buoyancy force in the region between these two pools is the main factor to influence the flame height. A characteristic length D, which depends on the gas burner geometrical shape and distance, were proposed, and the experimental data indicate that the value of coefficient C decreases with the increase of D. A correlation has been achieved which can estimate the flame height of the rectangular pool fires with different aspect ratios in open space.

Introduction

Multiple fires are common in daily life, and variability exists in the geometrical shapes of fire sources. The flame-flame interaction between the double pool fires is still an important issue which has not been paid enough attention until now. For example, parallel parked overcharged electric cars and electric bicycles accidentally setup fires, wooden structures stand on both sides of the streets. These can be treated as double pool fires (like line source) in the event of a fire. And the same phenomenon can be occured for the trees on both sides of the streets. The flame bodies would be merged if their distance is small which brings a large flame height. It should be note that the air entrainment rate would be very different for the rectangular pool fires with different aspect ratios. Air entrainment rate is restricted significantly when the distance between the double pool fires is sufficiently small, and it impacts the combustion characteristic obviously. Flame height is an extremely important parameter of combustion characteristics that has been examined by extant studies over several decades [1], [2], [3], [4], [5], [6], [7], [8]. Flame height is a function of the dimensionless heat release rate that is related to the burner geometry and heat release rate of a fire source. Extant studies proposed several correlations between the flame height and dimensionless heat release rate. Heskestad proposed that the non-dimensional flame height is a power function of dimensionless heat release rate with a linear relationship [9], [10]. Hasemi considered the influence of fire geometry on flame height, examined the flame height of the line fire source, and indicated that it is proportional to 2/3rd the power of the dimensionless heat release rate [11]. Quintiere et al. investigated the flame height of rectangular fire source and proposed a more common correlation for a fire plume [12]. The aforementioned studies assumed that sufficient air is entrained into the flame region, and this indicates that air entrainment rate remains as a constant. However, the flame height changes significantly if the air entrainment is restricted physically. For example, the flame height increases if the fire is adjacent to the wall or at the corner when compared with that in open space [13], [14], [15], [16], and it is also a function of ambient pressure [17], [18], [19], [20]. Hu et al [16] studied the flame height of double jet flame with different rectangular nozzles, they found that the flame height decreases with the increase of distance firstly, then it increases slightly since the flame body tilts with a little angle. The flame height remains as a constant when the distance is large enough finally. The air entrainment for the double pool fires differs from that for single pool fires. The central temperature between the double pool fires increases sharply and the density of the air decreases. Subsequently, the flame body is deflected to the central line. Both the pool fires merge together if the distance is sufficiently small [21]. The ambient air entrainment is limited by the neighbor flame body and subsequently the flame height increases. Thus, double pool fires lead to a greater hazard when compared with single pool fires [22].

Previous studies also examined the flame height of multiple pool fires. Putnam and Spich initially investigated the interaction of multiple fires [23]. Subsequently, Thomas et al. [24] investigated the flame height and merging behavior of two rectangular horizontal fuel beds. Furthermore, a correlation to estimate the merging flame height was proposed, and this is expressed as follows:Lf/B=9S3/BA21/2where Lf denotes the flame height, A and B denote the lengths of the long side and short side of the fire source, respectively, and S denotes the characteristic distance between both the pools. Sugawa et al. experimentally examined the merged or inclined flame height for the flames of two rectangular fire sources [25]. A model to estimate the flame height was obtained, as given in Eq. (2):Lf=4.2(fs)2/3Q̇2/3Bfs=(2AB+S2)/2(AB+S2)

Weng et al. [26] indicated that the flame height of the merged flame is related to the number, distances, mass flow rate, and geometrical shape of fire sources. As widely-known, the flame height is a power function of heat release rate, and the power value is affected by the number of fire sources. They also investigated the flame height of propane and wood crib burner configurations and various separation distances, a dimensionless heat release rate has been developed to estimate the total flame height [27]. Baldwin deduced a theoretical scaling law in relation to the flame height for the conditions under which the flames are merged [21]. Wan et al. analyzed the relationship between air entrainment and flame height from two square gas burners. An explicit model was developed to predict the flame height of double fire in open space [28]. Furthermore, the merging behavior of double turbulent diffusion flames in the tunnel was also examined [29]. An extant study by Liu et al revealed that flame height of multiple flames is influenced by air entrainment as well as the heat feedback [30]. The fuel surface receives heat feedback from its own flame and the adjacent flame, and subsequently the mass burning rate and flame height increases with the heat feedback. Lu et al. [31] experimentally examined the merging behavior of facade flames ejected from two windows. A coefficient termed as additional entrainment area to characterize the change of entrainment was proposed. Their experimental results indicate a high correlation between the facade flame height and ratio of additional air entrainment surface area divided by the total surface area.

In the study, a series of experiments were performed to investigate the flame height of double rectangular pool fires with different aspect ratios. Two identical rectangular gas burners with identical heat release rates were used in each test. A characteristic length was proposed to characterize the gas burner shape and distance. A non-dimensional correlation was proposed to predict the total flame height.

Section snippets

Experimental setup and conditions

The experimental facility is shown in Fig. 1-a, and it consists of a fuel supply system, two rectangular gas burners, and a CCD camera. Propane was used as the fuel, and two gas flow meters of ±2% monitor control the fuel supply rate. We assume that the combustion efficiency in the experiment is 90% [32], [33], and both the gas burners are assumed as identical. In the experiments, two rectangular gas burners (A × B) with dimensions of 10 × 10, 14.2 × 7.1, 20 × 5, and 28.3 × 3.53 cm were

Experimental results

The mean flame height is recorded by a CCD camera. Approximately 1500 frames for each condition are achieved, and subsequently each frame is transferred into a gray scale image. Then, the images are converted into the binary image by the Otsu method subsequently [34]. The image is finally achieved based on the average value in each pixel position, and this shows the intermittency. The mean flame height corresponds to intermittency at 0.5 [35] as shown in Fig. 2. With the variation of the fuel

Conclusions

This paper investigates the flame height of double rectangular pool fires with different aspect ratios and distances in the open space. Major findings include:

  • (1)

    The air flow speed in the additional region and the temperature in the central line increase as the two pools move close to each other, as shown in Fig. 5, Fig. 6. The air flow speed from the ambient region to the additional region and flame region are almost not influenced by the variations of the heat release rates and the distance.

  • (2)

    A

Acknowledgments

The work in this study was financially supported by the National Key R&D Program of China (No. 2017YFC0803300), National Natural Science Foundation of China (Grant No. 51408181), and the Fundamental Research Funds for the Central Universities (No. JZ2017HGTB0208).

References (36)

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