Experimental Research on Gas Fire Backdraft Phenomenon

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Environmental Sciences ProcediaProcedia Environmental Environmental Sciences 0011 (2011)(2011) 000–000 1542 – 1549 www.elsevier.com/locate/procedia

Experimental research on gas fire backdraft phenomenon

Jinxiang Wu*, Yingting Zhang, Xiang Gou, Meijuan Yan, Enyu Wang, Liansheng Liu

School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China *Corresponding author e-mail: [email protected]

Abstract

The fire triggered by the leak of combustible gas is a serious threat to the safety of the public and firefighters. Backdraft is a special fire phenomenon in a limited-ventilation apartment. It can develop from fires of combustibles that become oxygen-starved and continue to generate a fuel-rich environment. If abundant fresh air is abruptly supplied by opening a door or a window, the hot gas in this vitiated apartment will rapidly combust and a fire ball and a blast wave will take place. This paper presents laws of gas fire backdraft phenomenon based on series of experimental tests in a reduced-scale compartment (length×width×height with 1.08m×0.36m×0.72m), fitted with an opening which could be changed in six cases quickly. The effects of varying ventilation conditions, ignition locations, and mass fluxes of gas leakage are discussed. In addition, the effect of the gas fire backdraft phenomenon on the temperature of the compartment is analyzed. The experimental results and the analysis show that the smaller the opening is, the further the ignition position is away from the center of the compartment and the larger the mass flux of gas leakage is, the more easily the gas fire backdraft is produced. Furthermore, it is found that the ventilation condition is the key parameter determining the occurrence of gas fire backdraft, followed by the ignition position, and the mass flux of gas leakage through the analysis of orthogonal experiments. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Intelligent © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer] Information Technology Application Research Association. Keywords:Gas fire; backdraft phenomenon; experimental research；orthogonal experiment

1. Introduction

It is important to understand and predict the behavior of compartment fires in order to predict the impact on structural elements and the dynamics of fire growth [1]. Although much work has been done, generalities concerning fuel properties, configuration and ventilation are yet to be fully correlated [2]. The fire triggered by the leak of combustible gas, especially the backdraft phenomenon is a serious threat to the public and firefighters[3, 4]. Backdraft is a special fire phenomenon in a limited-ventilation apartment. In the case of poor ventilation, the fire does not normally die out but becomes ventilation-controlled [5] and its behaviour is very dependent on the geometry of the opening. If the fresh air is allowed to flow into the vitiated space, such as by opening a door or breaking a window, a gravity current of colder air will flow into the space while the hot fuel-rich gases flow out through the top of the opening. The air and fuel-rich gases will mix along the interface of the two flow streams [6]. Backdraft could increase the interior temperature and pressure dramatically and strengthen the fire. C. M. Fleischmann, et al [7-9] performed a two-dimensional simulation and a series of salt water experiments in order to clarify the mixing that occurs in a gravity current which precedes a backdraft. Daniel T. Gottuk [10] found the development and mitigation laws of backdrafts through a real-scale shipboard experimental test series. Weicheng Fan, et al [11] undertook plenty of research on the mechanism and critical condition of backdraft phenomenon. In addition, some systematic study on indoor fuel about the spread of the dynamic process and development, the changes of the combustible region and the danger of fire taking place [12-14]. In this paper, laws of backdraft phenomenon are based on the results of a reduced-scale experiment in a reduced-scale compartment (length×width×height with 1.08m×0.36m×0.72m), fitted with three different geometries for the opening on

1878-0296 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Intelligent Information Technology Application Research Association. doi:10.1016/j.proenv.2011.12.232 Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 1543 Author name / Procedia Environmental Sciences 00 (2011) 000–000 one end-wall, which could be changed in six cases quickly. A series of experiments were designed involving different ventilation conditions, ignition locations and gas velocities in the inlet. In addition, the temperature of the compartment was measured to quantify the effect of backdraft .

2. Experimental Design and Procedure

Considering the explosive nature of backdraft phenomenon, the compartment was totally made by 2.5-mm-thick stainless steel, welded together to ensure strength. An observation window (quartz glass, 0.72m high×1.08m wide) was installed in one of the long walls shown in Fig.1. Bolt systems were built so that the window could open with different geometries, as shown in Fig. 2, could be easily modified by replacing three face-plates bolted to the compartment, and there were 6 six cases in all. These openings were opened after the fire had been burning into limited-ventilation situation. Every effort was made to seal all the construction holes to control leakage. A LPG leaking point was placed against the wall opposite to that with the openings shown in Fig. 1. In order to ensure the similarity of the flow field and the reliability of ignition, a candle was provided for an ignition source. In this paper, the rectangular compartment vertical axis symmetric is known as the observation surface (centro- symmetric surface). There are three ignition positions on this surface, corresponding to the 6th, the 5th and the 4th thermocouples. In order to facilitate the description below, they are called ignition position A, ignition position B and ignition location C respectively, as shown in Fig.1.

Figure 1. Model of experimental compartment

Figure 2. Sketch of window structure

Nine thermocouples are placed on the centro-symmetric surface shown in Fig. 1. Data from the thermocouple are recorded by using a HP 34970A with a 40-channel highspeed scanning A/D converter. The flow rate of LPG was measured by flowmeter with an effective range of 600–6000 L/h. Video data were captured using Samsung S600 digital camera, which had 30 frames per second. The experiment system is shown in Fig.3.

1—Computer, 2—Data Acquisition, 3—Compartment, 4—Thermo-couple, 5—Stilling Chamber, 6—flowmeter, 7—Regulating Valve, 8—Pressure Reducing Valve, 9—Liquified Petroleum Gas Container 1544 Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 Author name / Procedia Environmental Sciences 00 (2011) 000–000 Figure 3. Experiment system

3. Experimental Results and Discussion

In order to facilitate the description below, the four ventilations involved are described as 0.05, 0.25, 0.5 and 0.75 respectively.

3.1. Influence of Different Openings on the Backdraft Phenomenon

As shown in Fig.4 and Fig.5, the window openings are in different conditions: the window size changes from 0.05 to 0.25, from 0.25 to 0.5, as well as from 0.5 to 0.75. Other conditions are kept constant: ignition location C, and the gas flow rate of 0.85 m3 / h. In Fig.4, the time is marked from the ignition moment, the actual moment of ignition from the leakage moment are 78.63s, 70.24s and 31.16s respectively. The white bright spot in the pictures is the reflection of the video indicator light. In Fig.5, the time is marked from the leakage moment, and 1-9 curves represent the temperatures from thermocouple 1-9 respectively.

Figure 4. Influences of different openings on backdraft

Fig.4 and Fig.5 show that when the window size changes from 0.05 to 0.25 or from 0.25 to 0.50, the backdraft phenomenon is obvious, however, when it changes from 0.50 to 0.75, no backdraft takes place. The reason probably is that in the initial condition of large window opening, the primary combustion is sufficient, and there is no backdraft occurrence Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 1545 Author name / Procedia Environmental Sciences 00 (2011) 000–000 condition. In addition, when the window changes from 0.05 to 0.25 or changes from 0.25 to 0.50, the primary combustion produces lots of incomplete combustion products. While fresh air enters, the second combustion takes place and then the second temperature rise can be seen in Fig.5.(a) such as curve 2, 3, 5. Therefore, it is found that it is easier for backdraft to take place in a compartment with a small opening change than with a large opening change. From the previous experimental analysis, it can be seen that when the window opening changes from 0.05 to 0.25, backdraft phenomenon is most obvious. Therefore, the following experimental work is carried out on this basis.

1000 3 900 thermocouple 1 5 thermocouple 2 2 800 thermocouple 3 thermocouple 4 700 thermocouple 5 600 thermocouple 6 thermocouple 7 500 thermocouple 8

Temperature/K thermocouple 9 400

300

200 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time/s (a) from 0.05 to 0.25 1000

900 thermocouple 1 thermocouple 2 800 thermocouple 3 thermocouple 4 700 thermocouple 5 600 thermocouple 6 thermocouple 7 500 thermocouple 8 thermocouple 9 Temperature/K 400

300

200 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time/s (b) from 0.25 to 0.50 1000

900 thermocouple 1 800 thermocouple 2 700 thermocouple 3 thermocouple 4 600 thermocouple 5 thermocouple 6 500 thermocouple 7

Temperature/K thermocouple 8 400 thermocouple 9 300

200 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time/s (c) from 0.50 to 0.75

Figure 5. influences of the change of window size on temperature

3.2. Influence of Different Ignition Location on the Backdraft Phenomenon

As shown in Fig.6, the ignition locations are in different conditions: ignition position A, ignition position B and ignition location C. Other conditions are kept constant: the window size changes from 0.05 to 0.25, and the flow rate is 0.85 m3 / h. In Fig.6, the time is marked from the ignition moment, the actual moment of ignition from the leakage moment are 77.84s, 75.15s and 78.63s respectively. Fig.6 illustrates that both ignition position A and ignition location C are easier to bring out backdraft phenomenon than ignition location B, and ignition location C is the easiest among the three locations. 1546 Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 Author name / Procedia Environmental Sciences 00 (2011) 000–000 The reason probably is that when the fire is caught on position C, the flame front spreads fast to the window, and the bottom part under the window becomes a “combustion dead corner” which leads to the accumulation of lots of incomplete combustion products. When a gravity flow of fresh oxygen-enriched air enters the compartment suddenly, the secondary combustion takes place, which is the backdraft. Therefore, it is found that it is easier for a backdraft to take place in a compartment with an ignition position which is further away from the center.

Figure 6: Influences of different ignition location on backdraft

3.3. Influence of Different Gas Velocities in the Inlet on the Backdraft Phenomenon

As shown in Fig.7, the flow rates are in different conditions: the flow rates are 0.85 m3/h, 1.4 m3/h and 2.0 m3/h (corresponding to the gas velocities of 3 m/s, 5 m/s and 7 m/s) respectively. Other conditions are kept constant: the window size changes from 0.05 to 0.25, and ignition location is at point A. In Fig.7, the time is marked from the ignition moment. Fig.7 shows that with the increase of flow rate in the inlet, the combustion becomes more intense and the backdraft phenomenon becomes more and more obvious. The reason probably is that the higher the gas velocities in the inlet, the greater the speed of the gas jet, the more inadequate the gas mixed in the compartment, the much more the incomplete combustion products, and the easier the appearance of backdraft phenomenon.

Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 1547 Author name / Procedia Environmental Sciences 00 (2011) 000–000

Figure 7. Influences of different flow rates in the inlet on backdraft

3.4. Orthogonal Experiment on the Backdraft Phenomenon

In order to determine the influence degree of these three factors on backdraft phenomenon, a series of orthogonal experiments were designed, as shown in Table 1 and Table 2. Because the backdraft experiment indicator is a qualitative indicator, in order to change it into quantitative one, a quantitative description is defined as follows: 90% represents very obvious backdraft, 70% stands for obvious backdraft, 50 % shows less obvious backdraft, 30% means poor obvious, 10% represents no backdraft. It is found that the ventilation condition is the key parameter determining the occurrence of backdraft, followed by the ignition position, and the leakage rate through the analysis of orthogonal experiments.

TABLE I. FACTORS LEVEL OF MEDIUM-SIZED BOX’S BACKDRAFT TEST

Factors ⅠLeakage Ⅱ Ignition Ⅲ Ventilation Project Velocities Location Condition （m/s） 1548 Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 Author name / Procedia Environmental Sciences 00 (2011) 000–000 1 0.7 A 0.05-0.25 Level 2 1.4 B 0.05-0.50 3 2.1 C 0.25-0.50

TABLE II. RESULTS AND ANALYSIS OF MEDIUM-SIZED BOX’S BACKDRAFT TEST

Factors Ⅰ Ⅱ Ⅲ Experimental Experimental Leakage Ignition Ventilation Indicator Number Velocities Location Condition （%）（m/s） 1 0.7 (1) A (1) 0.05-0.25 (1) 90 2 0.7 (1) B (2) 0.05-0.50 (2) 70 3 0.7 (1) C (3) 0.25-0.50 (3) 10

4 1.4 (2) A (1) 0.05-0.50 (2) 70 5 1.4 (2) B (2) 0.25-0.50 (3) 10 6 1.4 (2) C (3) 0.05-0.25 (1) 50

7 2.1 (3) A (1) 0.25-0.50 (3) 50 8 2.1 (3) B (2) 0.05-0.25 (1) 30 9 2.1 (3) C (3) 0.05-0.50 (2) 70

K1 170 210 170 K2 130 110 210 K3 150 130 70

k 1 57 70 57 k 2 43 37 70 k 3 50 43 23

R 14 33 47

Ⅰ1 Ⅰ2 Ⅰ3 Ⅱ1 Ⅱ2 Ⅱ3 Ⅲ1 Ⅲ2 Ⅲ3 Leakage Velocities Ignition Location Ventilation Condition Figure 8: Relationship between factors and indicators of backdraft phenomenon Fig.8 shows the relationship between factors and indicators of backdraft phenomenon. From the above analysis, it is found that among these nine experiments the best combination of factors isⅠ1Ⅱ1Ⅲ1, while through calculation and analysis the best combination of factors is Ⅰ1Ⅱ1Ⅲ2. We made some confirmatory experiments, as shown in Table 3, to found that Ⅰ1Ⅱ1Ⅲ1 is the better one.

TABLE III. CONFIRMATORY EXPERIMENT

Experimental Experimental Experimental Average Number Condition Indicator（%） Value（%）

10, 11 Ⅰ1Ⅱ1Ⅲ1 90, 90 90

12, 13 Ⅰ1Ⅱ1Ⅲ2 70, 90 80

4. Conclusion

In this paper, the results of some reduced-scale experiment series in a reduced-scale compartment were shown. The experimental variables included the different ventilation conditions, the ignition locations, and the gas velocities in the inlet. In addition, the temperature of the compartment was measured to quantify the effect of backdraft. The experimental Jinxiang Wu et al. / Procedia Environmental Sciences 11 (2011) 1542 – 1549 1549 Author name / Procedia Environmental Sciences 00 (2011) 000–000 results and the analysis show that the smaller the opening, the ignition position is further away from the center of the compartment and the larger the gas leak speed, the more easily backdraft is produced. Furthermore, it is found that the ventilation condition is the key parameter determining the occurrence of backdraft, followed by the ignition position, and the leakage rate through the analysis of orthogonal experiments.

5. Acknowledgment

This work was supported by the National Natural Science Foundation of China No. 50676028.

References

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