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Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel
Applied Energy xxx (2016) xxx–xxx
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Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel Huaxian Wan a, Zihe Gao a, Jie Ji a,b,⇑, Kaiyuan Li c, Jinhua Sun a, Yongming Zhang a a
State Key Laboratory of Fire Science, University of Science and Technology of China, JinZhai Road 96, Hefei, Anhui 230026, China Institute of Advanced Technology, University of Science and Technology of China, Hefei, Anhui 230088, China c Department of Civil Engineering, School of Engineering, Aalto University, 02150 Espoo, Finland b
h i g h l i g h t s Experiments were conducted for two energy sources burning in a model tunnel. Energy release rate and burner spacing were varied. Flame merging criteria in tunnel were proposed. Correlations for estimating the ceiling gas temperature profiles were developed. The interacting flame lengths in tunnel were compared with the open space.
a r t i c l e
i n f o
Article history: Received 26 August 2016 Received in revised form 24 October 2016 Accepted 31 October 2016 Available online xxxx Keywords: Tunnel Multiple energy sources Flame merging Temperature decay profile Flame length
a b s t r a c t Multiple energy sources in a tunnel might lead to merge of flames with small enough spacings, releasing more heat and pollutant emissions than a single energy release source in tunnel and thus posing a great threat to tunnel structure, facilities and trapped people. As the heat detection, controlling and cooling systems are originally designed for the single energy release source, while the spacing between energy sources in tunnel is changeable and unpredictable. Then it is important and helpful to research on the much different characteristics of multiple energy sources with interacting ceiling flames for effective control the high risk scenarios. This paper aims to study the ceiling gas temperature profile and flame properties induced by two interacting energy sources in tunnel so as to improve the understanding of the arrangement of heat detectors and water sprinklers in tunnel. Two identical propane burners were used as energy sources located in a longitudinal array in tunnel. The total energy release rate and burner spacing were varied. The flame merging probability, ceiling gas temperature, vertical flame height and longitudinal flame extension were measured. The criteria of beginning merging and fully merging of flames are respectively proposed for two energy sources in tunnel. Results showed that the area of ceiling flame region increases with higher energy release rate. Models for predicting the ceiling gas temperature profiles induced by two energy sources in tunnel are established respectively for weak and strong plumes impinging on the ceiling. A modified model for predicting the combined vertical and longitudinal flame lengths from two burners in tunnel is proposed involving the normalized energy release rate, burner size and spacing. Finally, the comparison between models proposed for ceiling gas temperatures and flame lengths in tunnel and other configurations identifies the high risk of multiple energy sources in tunnel. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Two or more flames burning interactively is termed as multiple flames. In the past decades, most studies on multiple flames were conducted in open space. The key parameter involved in multiple ⇑ Corresponding author at: State Key Laboratory of Fire Science, University of Science and Technology of China, JinZhai Road 96, Hefei, Anhui 230026, China. E-mail address: [email protected] (J. Ji).
flames is spacing [1] as the flames might lean to each other and merge if sufficiently close [2]. Flame merging is believed as more destructive and uncontrollable [3]. Finney and McAllister [4] reviewed the empirical merging criteria for multiple flames derived in open space. As a typical confined space, tunnel might involve multiple vehicles burning simultaneously caused by collisions or subsequent accidents [5]. Generally, there are many mechanical and electrical facilities in tunnels, such as heat detectors and sprinklers. When the uncontrollable energy along with
http://dx.doi.org/10.1016/j.apenergy.2016.10.131 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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Nomenclature b cp D g Hef LB Lef Lf N P Pm Q_ Q_ c Q_
D
Q_ 0 g
plume radius at ceiling level, where the velocity is onehalf of the centerline value (m) specific heat of air (kJ/(kgK)) energy source diameter/length (m) gravitational acceleration (m/s2) ceiling height above the burner surface (m) impingement point induced by burner B to the center of burner B (m) effective flame length (m) flame height (m) flame number of N N array perimeter of air entrainment (m) flame merging probability energy release rate of a single flame (kW) convective energy release rate (kW) dimensionless energy release rate for each individual burner dimensionless energy release rate for group flames
enormous pollutant emissions and high temperatures induced by flame and smoke are released to a tunnel, the facilities within the tunnel should be activated simultaneously to achieve quick energy detection, controlling and cooling. Optimizing installation spacing of heat detectors and sprinklers could improve the effectiveness of energy controlling at the early burning stage. As a matter of fact, in many tunnel accidents, two or more crashed vehicles were burnt simultaneously [6], the resultant merging flames will release more heat and pollutant emissions than a single vehicle accident in tunnel. And as the spacing between the crashed vehicles in tunnel is changeable and unpredictable, it will dramatically increase the difficulty of quick heat detection and cooling. However, little effort has been paid attention to multiple flames in tunnels and so far no merging criterion is proposed for multiple flames in tunnel. This paper attempts to bridge this knowledge gap. Gaseous and liquid fuels were commonly used as energy sources in free and confined spaces [7]. For multiple liquid flames in open space, Huffman et al. [8] and Liu et al. [9] found that the heat release rate (HRR) developed a non-monotonous trend with decreasing the burner spacing due to the competition between heat feedback enhancement and air entrainment restriction. Since the HRR of liquid sources is strongly affected by the radiation from flame and smoke, it is difficult to quantify the effect of spacing. On the other hand, gas burners are affected only by the fuel flow rate while the impact of heat feedback is insignificant [10]. Therefore, the gas burners were used as energy sources in former studies [3,10–14]. When flame height is smaller than the ceiling height, the ceiling jet is a weak buoyant smoke flow. Fukuda et al. [3] concluded that when the HRR of each burner is low, the flames in a square array are hardly merging and the free flame height is little affected by the spacing. Gao et al. [14] found that if the flame tip did not hit the ceiling the flame height from a single energy source in tunnel is almost the same as the one in free space. When the free flame height is equal to or exceeds the ceiling height above the fuel surface, the resultant ceiling jet is driven by a strong plume. It was found that the pollutants emission from an impinging flame is a crucial behavior of the flame and the emission characteristics of impinging flames might differ from that of the same energy source in open space [15–17]. As strong plume hit the ceiling with flames spread radially along the ceiling, the gas temperature and the resultant heat flux received by the ceiling will increase dramatically and the risk of flame propagation enhances [18]. Estimations of the temperature field and flame length can facilitate determina-
Q_ total r rB rf S T T1 DT DTmax
total energy release rate of two burners (kW) radial distance from the flame axis and/or impingement point (m) horizontal distance from the center of burner B (m) distance from the impingement point to the flame tip in the downstream of burner B (m) burner spacing (m) gas temperature (K) ambient temperature (K) temperature rise (K) maximum temperature rise at impingement zone (K)
Greek symbols a power value of Q_ DHef in Eq. (4) q1 ambient density (kg/m3)
tions on the heat transfer from energy sources to receiving mediums such as the ceiling, nearby objects and secondary fuels [19]. In addition, the study of highest temperatures under a ceiling is significant since it has a great impact on the arrangement and activation of sprinkler systems in tunnels [20]. Alpert [21] and Heskestad and Hamada [22] proposed a set of correlations, which have been widely used, for predicting the maximum temperature decay profiles for both weak and strong plumes induced by a single source under unconfined ceilings. However, whether these correlations are suitable for multiple flames in tunnel is unclear. In spite of numerous studies focused on the free flame height of multiple energy sources [2,3,12,13,23,24], limited physical model is developed to predict the interacting flame length in tunnel. The objective of this work is to understand the ceiling gas temperature and flame behaviors from two propane burners located along the longitudinal centerline of tunnel. The total energy release rate and burner spacing were varied to quantitatively study the flame merging behaviors. Attempts have also been made to develop empirical models to predict not only the longitudinal ceiling gas temperature profile but also the flame length. The current study will provide beneficial scientific supports to the optimal arrangements of heat detectors and sprinklers, resulting in high efficiency of energy detecting and cooling as well as achieving optimum energy controlling.
2. Experiments Fig. 1 shows the experimental setup. The experiments were conducted in a 1/6-scale model tunnel which is 6 m long, 2 m wide and 0.86 m high. Fireproofing boards of 20 mm thick were used to make the top, bottom and one sidewall of the tunnel while the other sidewall was made of 10 mm thick fire-resistant glass for observation. The left and right ends of the tunnel were opened for natural ventilation. Two square gas burners denoted as A and B with a side length (D) of 15 cm were located at the longitudinal centerline with 22 cm higher than the tunnel floor. Propane was used as the fuel with different HRRs adjusted using a flow meter. The heat of combustion of propane was 46.3 MJ/kg and the combustion efficiency was assumed to be unity [25]. In the experiments, 6 HRRs (Q_ total ) were used including two burners, which are 21.6, 43.2, 64.8, 86.4, 108.0 and 129.6 kW, while the respective HRR (Q_ ¼ Q_ total /2) of burners A and B can be
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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Fig. 1. Experimental setup.
regarded as identical. To express the data conveniently, a normalized HRR Q_ ¼ Q_ /(q1cpT1g1/2D5/2) for each burner is used in the D
later pages, where q1, cp, T1 refer to the properties of ambient air. The calculated Q_ corresponding to various Q_ are 1.12, 2.24, D
3.36, 4.48, 5.60, 6.71. During experiments, the position of burner A was fixed at 2.25 m away from the left end of tunnel. The burner edge spacing (S) was set by moving burner B, including S/D = 0, 1/3, 2/3, 1, 4/3, 2, 3. Additionally, the cases with only burner A burning on were also conducted for comparison. The arrangement of the measuring points is also shown in Fig. 1. A total of 96 thermocouples were installed 10 mm beneath the ceiling (in order to obtain the vertical maximum temperature [26]) in a rectangular array with longitudinal intervals of either 0.05 m or 0.1 m and transverse interval of 0.2 m. The average values recorded by thermocouples during the steady burning stage are regarded as the maximum temperatures, which will be studied in the latter sections. Note that the K-type thermocouples with 1 mm diameter and response time less than 1 s were used in this work. Considering the uncertainty of experiments and combined effects of burner spacing and energy release rate on ceiling gas temperature and flame length, typical tests were repeated. The repeatability error of measurements was found to be less than 5%. On the left side of burner A, flush to the burner surface, four water-cooled heat flux gauges of type SBG01 with range up to 20 kW/m2 were placed to measure the total heat flux. Flame shapes were monitored from the front view and side view using two digital cameras (DV) with a frequency of 50 frames per second. In the experiments, the ambient temperature was about 30 °C. 3. Results and discussion 3.1. Flame merging behaviors Fig. 2 shows the typical snapshots of flame shape in longitudinal and transverse directions with increasing spacing during the steady burning stage. It is observed that the flames cannot impinge on the ceiling when Q_ total ¼ 21:6 kW (Fig. 2a) and the induced
flame height is weakly affected by spacing. For the rest Q_ total , the flame extensions under the ceiling can be found. As the spacing increases, the interaction between flames undergoes the following stages (see Fig. 2b–d). Firstly, both the vertical and ceiling flames merge at small spacings. As the spacing increases, the vertical flames separates while the ceiling flames still merge. Further increasing the spacing, the ceiling flames transit from intermittent merging to completely separated. For transverse flames (Fig. 2e), the achieved images can present from one side where it looks like burning a single flame due to the overlapping of the two flames. Increasing the spacing would lead to shorter transverse flame extensions.
The image processing method developed by Yan et al. [27] was adopted to judge whether the flames merged or not. In each test, the midpoint between the centerlines of two burners was specified as the center, the image processing area is 1 mm wide and has the same height from the burner surface to the ceiling. The flame merging probability (Pm) which is adopted to characterize the intensity of merging [13], was determined using the number of frames with flame lengths longer than 0 divided by total 3000 frames within 60 s after the flames were stable. Fig. 3 shows Pm against normalized spacing S/D. It can be seen in Fig. 3 that at a given Q_ , flames always merge if D
S/D is small enough, thus Pm = 1. Then Pm decays with increasing the S/D while Pm > 0, indicating that flames merge intermittently. When S/D is large enough, flames are separated, resulting in Pm = 0. Dimensional analysis method is adopted to determine the correlation of the flame merging probability. From the discussion in Fig. 3, it can be concluded that the burner properties (Q_ , D), ambi-
ent parameters (q1, cp, T1, g) and the burner spacing (S) are governing parameters for determining the Pm. Besides, the height of ceiling above the burner surface (Hef) is added due to that it directly determines whether the ceiling flames are formed or not. Therefore, the flame merging probability can be expressed as
Pm ¼ f Q_ ; q1 ; cp ; T 1 ; g; D; Hef ; S
ð1Þ
Based on dimensional analysis, Eq. (1) can be derived as
! cp T 1 Hef S Pm ¼ f ; ; ; q1 g 3=2 D7=2 gD D D Q_
ð2Þ
Incorporating the first two terms at the right side gives _ Q_ D ¼ q c T Qg1=2 D5=2 . To account for the confinement of tunnel struc1
q 1
ture, the term Hef is incorporated into Q_ D , then a widely used Q_ dimensionless HRR Q_ DHef ¼ 3=2 for tunnel [14,28,29] is 1=2 q1 cq T 1 g
DH
ef
introduced. Hence, Eq. (2) can be rewritten as
Pm ¼ f Q_ DHef ; S=D
ð3Þ
We firstly attempt to determine the relationship between Pm and S/D. From Fig. 3, it is reasonable to assume that at a given Q_ , Pm decreases linearly with S/D when 0 < Pm < 1. Since Pm D
increases with Q_ DHef at a given S/D (see Fig. 3), Eq. (3) can be rewritten as
0
1 S=D A Pm ¼ f @ Q_ a
ð4Þ
DHef
where a is the power value of Q_ DHef . It is easily deduced that a is a positive constant. Based on numerous former studies, five widely
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
S/D=0
S/D=1/3
S/D=2/3
S/D =1
S/D=4/3
S/D=2
S/D=3
S/D=2
S/D=3
(a) Qtotal =21.6 kW (front view)
(b) Qtotal =43.2 kW (front view)
(c) Qtotal =86.4 kW (front view)
(d) Qtotal =129.6 kW (front view)
S/D=0
S/D=1/3
S/D=2/3
S/D=1
S/D=4/3
(e) Qtotal =129.6 kW (side view) Fig. 2. Snapshots of flame shape in (a) – (d) longitudinal and (e) transverse directions with increasing spacing.
1.2 Normalized HRR Q 1.12 2.24 3.36 4.48 5.60 6.71 * D
1.2 1.0 0.8 0.6 0.4 0.2 0.0
-0.33 0.00 0.33 0.67 1.00 1.33 1.67 2.00 2.33 2.67 3.00 3.33
Normalized spacing S/D Fig. 3. Flame merging probability Pm against normalized spacing S/D.
used empirical values of a are employed, namely, a = 1/2, 3/4, 1, 5/4, and 3/2. The comparison shows that the data points with a = 1 are more converged than the other a values. Therefore a is finally determined as 1. Fig. 4 shows the flame merging probability Pm against the normalized term ðS=DÞ=Q_ . It is observed that Pm can be DHef
divided into three stages with increasing ðS=DÞ=Q_ DHef . Fitting the data points using piecewise linear function, the final expression of Pm is established as
Flame merging probability Pm
Flame merging probability Pm
1.4
Merging stages Fully merging Intermittent merging Non-merging
1.0 0.8
S/D=0 S/D=2/3 S/D=4/3 S/D=3
0.6 0.4
S/D=1/3 S/D=1 S/D=2 Fitting line
0.2 0.0 -0.1 -2.5
1.0 7.67 0
5
10
15
20
25
Normalized HRR and spacing ( S / D ) / Q
* DH ef
Fig. 4. Flame merging probability Pm against normalized HRR and spacing ðS=DÞ=Q_ DHef .
8 > 1; 0 6 ðS=DÞ=Q_ DHef < 1:0 > > < Pm ¼ 1:15 0:15ðS=DÞ=Q_ DHef ; 1:0 6 ðS=DÞ=Q_ DHef < 7:67 > > > : 0; 7:67 6 ðS=DÞ=Q_ DHef < 23:6
ð5Þ Eq. (5) shows that flames merge completely when ðS=DÞ=Q_ DHef is smaller than 1.0 and flames begin to merge when ðS=DÞ=Q_ DHef is smaller than 7.67.
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
5
y (cm)
90 90 90 S/D=0 S/D=1/3 S/D=2/3 60 60 60 30 30 30 530 0 0 0 -30 -30 -30 -60 -60 -60 -90 -90 -90 -90-60-30 0 30 60 90 -90-60 -30 0 30 60 90 -90-60-30 0 30 60 90
y (cm)
90 90 90 S/D=4/3 S/D=1 S/D=2 60 60 60 30 30 30 0 0 0 -30 -30 -30 -60 -60 -60 -90 -90 -90 -90-60-30 0 30 60 90 -90-60-30 0 30 60 90 -90-60-30 0 30 60 90
y (cm)
H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
90 S/D=3 60 30 0 -30 -60 -90 -90-60-30 0 30 60 90
x (cm)
x (cm)
Top view
x (cm)
x (cm)
x (cm)
Transverse
x (cm)
(
)
870 810 740 670 600 530 460 390 320 250 180
x (cm)
burner A
y O
burner B x
Centerline S/2 Longitudinal O: the middle point between the two burners x: longitudinal distance from the point O y: transverse distance from the point O
Fig. 5. Ceiling gas temperature profiles with various S/D at Q_ D ¼ 4:48.
3.2. Gas temperature properties under the tunnel ceiling 3.2.1. Ceiling gas temperature decay profile for a single flame When a weak plume impinges on an unconfined ceiling, Alpert [21] developed the following widely used correlation to quantify the maximum gas temperature rise (DT = TT1), assuming an axisymmetric plume flow under a flat, horizontal ceiling without sidewall effect.
DT ¼ 16:9
DT ¼ 5:38
Q_ 2=3 5=3
Hef
;
for r=Hef 6 0:18
Q_ 2=3 =H5=3 ef ðr=Hef Þ2=3
;
ð6aÞ
for r=Hef > 0:18
ð6bÞ
where r is the radial distance from the flame axis. For a strong buoyant plume impinging on the ceiling, Alpert’s correlation is no longer applicable. Replacing the ceiling height (Hef) used for a weak plume with the ceiling plume radius, b, as a length scale, Heskestad and Hamada [22] proposed a correlation of excess temperature for strong plume.
r 1 h DT r i ; ¼ 1:92 exp 1:61 1 b b DT max
for 1 6
r 6 40 b
ð7aÞ
3.2.2. Gas temperature profile under the ceiling During the steady burning stage, the gas temperatures show basically unchanged. As a result, the average temperatures are used in the following contents. Fan et al. [30] found that the maximum smoke temperatures induced by both methanol and heptane pools were weakly affected by the longitudinal distance between the flame center and the tunnel end. Hence, the impact of the longitudinal distance between burners to tunnel ends is regarded as ignorable in this work. As the two burners with identical configurations and energy release rates are placed at the same level along the longitudinal centerline of tunnel, it is reasonable to assume that the symmetrical temperature profiles under the ceiling are formed by the two burners. Taking the midpoint between the centerlines of two burners as the center (Fig. 5), the 1/4 total ceiling gas temperature profiles induced by the two burners can be obtained based on the measurements of thermocouple arrays located 10 mm below the ceiling (Fig. 1). Through longitudinal and transverse symmetries, the total ceiling gas temperature profiles can be determined. Note that other than the cases of Q_ ¼ 1:12, ceiling flames are formed for the rest HRRs. Taking D
where DTmax is the maximum temperature rise at the impingement zone, b is the radius where the velocity of the impinging plume is one-half the centerline value. The expression for b is given by
h i1=2 T 1=2 Q_ 2=5 2=5 max c b ¼ 0:42 ðcp q1 Þ4=5 T 3=5 1 g DT 3=5 max
flame axis. Therefore, it is worthwhile examining the feasibility of Eqs. (6) and (7) to the ceiling gas temperature decay profile in cases with interacting flames in tunnel.
ð7bÞ
where Q_ c is the convective energy release rate, which can be calculated as Q_ c ¼ 0:8Q_ [22]. It should be noted that Eqs. (6) and (7) are proposed for single flame cases without flame tilting. However, flames can tilt due to external wind, slope, or the interaction with other flames [4], resulting in the impinging position deviated from the vertical
the cases of Q_ D ¼ 4:48 as examples, ceiling gas temperature profiles with various S/D are plotted in Fig. 5, where the point O in the middle of two burners is regarded as the origin of coordinates, the longitudinal and transverse directions are respective x-axis and y-axis. It can be seen that when the spacing is small enough (S/ D 6 1/3), the fully merging flames make the fire plume spread out radially with a circular shape. As the spacing increases (2/3 6 S/D 6 4/3), the inner core zones are separated, while in the slightly far region the temperature contours become combined and spread out radially with an elliptical shape. Further increasing the spacing (S/D P 2), temperature contours varies from an elliptical shape with the longer x-axis to a peanut-like shape.
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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Position of the impingement point, LB (cm)
6
20
S/D=0 S/D=1/3 S/D=2/3 S/D=1 S/D=4/3 S/D=2 S/D=3
15 10 5
upstream of burner B. As Q_ D increases, both pressure drop and horizontal inertial force increase, the competition effect between the two forces results in the increasing of LB. When Q_ is large enough,
Front view upstream downstream LB<0 o LB>0
D
burner B
burner A
0 -5 -10 -15 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
Normalized HRR QD*
the hot current flow is dominated by the weaken impact of horizontal inertial force, resulting in LB > 0. Fig. 6 also shows that the values of LB in all tests are varied within 10 cm 5 cm, suggesting the weak extent of flame tilting under the ceiling. As the temperatures within the confined area between two burners are quite high and difficult to quantify, the ceiling gas temperatures located in the downstream of impingement point induced by burner B is the focus in this work. In the following, the radial distance r in Eq. (6) is redefined as the horizontal distance from the impingement point to the point in the downstream of burner B and along the longitudinal centerline of tunnel. For the cases of Q_ ¼ 1:12, weak plumes impinge on the ceiling. D
Fig. 6. Position of the impingement point from the center of burner B (LB) against Q_ D .
To apply this model for scenarios with different scales, Eq. (6) should be dealt with dimensionless method by which the ceiling excess temperature is normalized by the ambient one and the energy release rate is normalized by the commonly used Q_ Hef
3.2.3. Longitudinal maximum gas temperature decay profile in the downstream of impingement point induced by burner B It should be noted that the cases with merging vertical flames will lead to only one impingement point, while the other cases will induce two impingement points (see Fig. 5). Based on the symmetrical assumption, the impingement point induced by burner B to the center of burner B is denoted as LB, as shown in Fig. 6. Due to the intervals of thermocouples, Oka et al. [33,34] proposed a suitable method to estimate the impingement point and the resultant maximum temperature rise (DTmax). Using Oka’s method to our work, the values of LB and DTmax can be determined based on a quadratic fit to the measured data from three points involving the measured maximum temperature rise and the temperatures of two adjoining points. Fig. 6 shows LB against Q_ at various norD
malized spacings, where the positive value of LB indicates that the impingement point is located at the downstream of burner B (towards the right end of tunnel), and vice versa. It can be seen that at a given S/D, LB increases with Q_ and its D
value varies from negative to positive. This is because the gas flow under the ceiling is controlled by two governing forces. One is the horizontal inertial force generated by buoyancy force, which will weaken the interaction of flames. The other is the pressure drop between flames, which will reinforce the interaction of flames. Under small Q_ and S/D, the buoyance force is weak, the reinforced D
pressure drop between flames plays a dominant role in the gas flow, leading to flame/smoke impinging on the ceiling at the 1
For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
[28,35,36]. Using the ambient temperature of 303 K in our experiments, Eq. (6) can be non-dimensionalized as follows.
8 _ 2=3 > < 5:96Q Hef ;
r=Hef 6 0:18 DT 2=3 ¼ _ QH > T1 : 1:90 ðr=H efÞ2=3 ; r=Hef > 0:18
ð8aÞ
ef
where,
Q_ Hef ¼
Q_
ð8bÞ
q1 cp T 1 g 1=2 H5=2 ef
Due to that Eqs. (6) and (8) are previously applied for a single energy release source under an unconfined ceiling. To validate the applicability of Alpert’s correlations for two energy sources in confined tunnel scenarios, similar forms shown in Eq. (8) were used for reference. The normalized temperature rise D_ T2=3 against T1 Q H
ef
normalized horizontal distance r/Hef is plotted in Fig. 7. The single flame cases in tunnel and Alpert’s model are also presented for comparison. It can be seen that the data points from various spacings are quiet concentrated, indicating that the gas temperature rise in the downstream of burner B is weakly affected by the spacing. For a single flame in tunnel, Eq. (8) can predict well in the far field from the impingement point, while it underestimates the temperatures in the near region. Besides, ceiling gas temperatures from two burners in tunnel calculated by Eq. (8) are lower than the
20
Normalized temperature rise
In former studies, the flame tips can be determined using temperature measurement. If DT = 500 °C [31,32] was used to represent the temperature of the flame tip, the flame shape under the ceiling can be determined by the contour of T = 530 °C, as shown the red1 lines marked in Fig. 5. Assuming that the region enclosed by the red line is flame while the external of the region is smoke plume. It can be observed that as the spacing increases, the flame shape of two burners experience five stages: a whole circular shape, a whole elliptical shape, a whole peanut shape, two separate elliptical shapes and two separate circular shapes with large enough spacings. In addition, the area of flame region increases with decreasing the spacing, which indicates the higher risk of interacting flames. Similar trends of temperature profiles and flame shapes can be found for the other HRRs. And the area of ceiling flame region increases with the energy release rate at a given burner spacing.
6.6, r / H ef ≤ 0.18 ΔT = −1/2 T∞ QH*2/3 2.8 ( r / H ef ) , r / H ef > 0.18 ef
10
ΔT T∞QH*2/3 ef
S/D=0 S/D=2/3 S/D=4/3 S/D=3 Eq. (8) [21]
1 0.8 0.015
S/D=1/3 S/D=1 S/D=2 Single fire This work 0.18 0.1
1
3
Normalized longitudinal distance r/Hef Fig. 7. Normalized temperature rise D_ T2=3 against normalized longitudinal distance T1 Q H ef r/Hef for weak plumes with Q_ ¼ 1:12. D
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
experimental values. This is because Eq. (8) is developed for a single flame under the unconfined ceiling without any smoke accumulation. While smoke can accumulate under the ceiling with the restriction of tunnel sidewalls. The smaller temperature decay for two burners than that for a single flame in tunnel is mainly due to the existence of a nearby flame. Fitting the data points in cases of two burners using the form of Eq. (8) with optimized coefficients 2=3 and shifting the term Q_ from the left to the right side gives Hef
8 _ 2=3 > < 6:6Q Hef ;
r=Hef 6 0:18 DT 2=3 ¼ _ QH > T1 : 2:8 ðr=H efÞ1=2 ; r=Hef > 0:18
ð9Þ
ef
Eq. (9) shows that when r/Hef 6 0.18, the normalized temperature rise DT/T1 is independent of the horizontal distance. While r/Hef > 0.18, DT/T1 is proportional to 1/2 power of r/Hef. Note that Eq. (9) is developed for S/D 6 3, it is not able to apply for very large spacings since it cannot predict well the case of a single flame (see Fig. 7). For the cases of Q_ 6 2:24, strong plumes impinge on the ceilD
ing, Eq. (7) proposed by Heskestad and Hamada are applied to these cases. Fig. 8 shows the normalized temperature rise DT/ DTmax against the horizontal distance normalized by characteristic plume radius r/b. The single flame cases in tunnel and Heskestad and Hamada’s correlation are also presented for comparison. It can be observed in Fig. 8 that the data points from two burners are converged, suggesting that the spacing has a little effect on DT/DTmax by using b as the characteristic length. The possible reason is that as the spacing decreases, the strong interaction between ceiling flames along the longitudinal direction makes it possible for unburned fuels spreading along the transverse direction to be fully combusted. In addition, Eq. (7) can well collapse the data of single flame cases in tunnel. It also fits well with the data in the near field of the impingement point in cases of two burners while the temperatures in the far field is underestimated. As shown in Fig. 8, the data from two burners can be matched closely by the following expression,
DT ¼ DT max
(
1; r=b 6 0:67 r 1:34 0:88 þ 0:18 b ; 0:67 < br 6 14
ð10Þ
r ΔT = 0.88 + 0.18 b ΔTmax
Normalized temperature rise
D
D
Q_ D 6 4:48, as shown in Fig. 9, there is no trend for Lef maintaining a constant. Fukuda et al. [3] experimentally studied multiple flames in open space with N N (N = 2–5) square arrays using square propane burners with each length of D = 15 cm. Afterwards, Delichatsios [23] proposed theoretical correlations for the free flame height (Lf) from multiple energy sources to fit the Fukuda’s experimental data by assuming that, (a) the air entrainment along the flame height is proportional to the stoichiometric requirements for fuel burning and (b) the volume of air entrainment is equal to pffiffiffiffiffiffiffi PLf gLf . The derived process by Delichatsios is as follows. The perimeter of air entrainment:
P ¼ 4ðND þ ðN 1ÞSÞ
ð11Þ
The volume of air entrainment:
qffiffiffiffiffiffiffi PLf gLf /
N2 Q_ q1 c p T 1
ð12Þ
Combining Eqs. (11) and (12) gives
2=3 L f / Q_ g ðN1ÞS ND 1 þ ND Q_ g ¼
ð13aÞ
Q_ D 5=2 N1=2 1 þ ðN1ÞS ND
−1.34
120
1
0.06 0.02
D
Lef decreases first and then maintains unchanged. The critical spacing to reach the unchanged Lef increases with Q_ . For the cases of
Effective flame length Lef (cm)
ΔT =1 ΔTmax
0.1
Since the energy release rate of gas burner would not be affected by the thermal feedback and the effective ceiling height (Hef), while the decrease of Hef would increase the ceiling flame extension at a given HRR [29]. Therefore, the sum of vertical flame height and longitudinal flame extension is defined as the effective flame length (Lef), as widely used in former studies [14,29,36,37]. When flames cannot impinge on the ceiling, Lef is equal to the vertical flame height. When flame height is higher than the ceiling, Lef = Hef + rf = Hef + rB LB, where rf is the longitudinal distance from the impingement point to the flame tip in the downstream of burner B and rB is the longitudinal distance from the center of burner B, as shown in Fig. 9. The mean flame height/extension induced by burner B (rB) is also determined using the image processing method. Fig. 9 shows the effective flame length Lef against spacing S. It can be observed that Lef is weakly affected by the spacing when Q_ ¼ 1:12. For strong plumes with Q_ 6 2:24, as the S increases,
ð13bÞ
where Q_ 0 g is the normalized energy release rate for group flames.
3
T/
Tmax
Eq. (10) shows that when r/b > 0.67, the normalized temperature rise DT/DTmax begins to be affected by the longitudinal distance. When 1 6 r/b 6 2.35, Heskestad and Hamada’s correlation can well collapse the temperature decay profiles induced by two burners in tunnel.
3.3. Flame length of two flames
S/D=0 S/D=1/3 S/D=2/3 S/D=1 S/D=4/3 S/D=2 r ΔT = 1.92 S/D=3 b ΔTmax Single fire Eq. (7) [22] 0.67 This work
0.1
−1
− exp 1.61 1 −
r b
2.35 1
10
30
Normalized longitudinal distance r/b Fig. 8. Normalized temperature rise DT/DTmax against normalized longitudinal distance r/b for strong plumes.
Definition of Lef (front view) impingement point rB rf = rB-LB 2.24 o 4.48 H flame tip ef 6.71 Lef=Hef+rf A B
QD* 1.12 3.36 5.60
110 100 90 80 70 60 0
10
20
30
40
50
Spacing S (cm) Fig. 9. Effective flame length Lef against spacing S.
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
We attempt to develop an alternative model for predicting the effective flame length in tunnel based on Eq. (13). Due to the longitudinal ceiling flames in the downstream of burner B can be regarded as entraining air freely. A reasonable assumption which firstly proposed by Babrauskas [38] will be adopted. Namely, at a given HRR, the air entrainment rate along the ceiling jet is the same as that in the free plume. As a result, the method of Delichatsios can be used for the current study. The effective perimeter of air entrainment for two interacting square burners is
P ¼ 6D þ 2S
ð14Þ 2
Substituting Eq. (14) into Eq. (12) and specifying N = 2, the normalized effective flame length can be expressed as
!2=3
2Q_ D
Lef / 6D þ 2S
ð15Þ
ð6 þ 2S=DÞ5=2
Defining the modified non-dimensional HRR as
Q_ 0g ¼
2Q_ D ð6 þ 2S=DÞ
ð16Þ
5=2
Lef =ð6D þ 2SÞ / Q_ 02=3 / Q_ 0g g
ð17Þ
For two interacting flames, Vasanth et al. [24] numerically studied the mass loss rate and flame length from two circular heptane pools in open space. The pool diameters (D) were 4.8 cm, 6.8 cm and 8.3 cm and S/D = 0.25–1.08. Due to the relatively small spacings, the effective perimeter of the two circular pools can be approximated as P 2pD + S 6D + 2S, indicating that Eq. (15) can also be used for small circular pools. Besides, Wan et al. [13] experimentally studied the free flame height for two square burners with D = 15 cm and S/D = 0–4. The relationships between normalized effective flame length Lef /(6D + 2S) and modified normalized HRR Q_ 0 in both open space and tunnel are shown in g
Fig. 10 in a double logarithmic coordinates. It can be seen that the data points from open space [13,24] and tunnel are individually concentrated. As the flame heights in cases of Q_ ¼ 1:12 are lower than the ceiling of tunnel, these data points D
are closed to the points induced by flames in open space. The exponential functions with optimized coefficients are used to fit the data (see Fig. 10). The fitting equations are
Normalized effective flame length Lef / (6D+2S)
(
4:29Q_ g 0:46 ; for open space 0 2:50Q_ g 0:42 ; for tunnel 0
5
ð18Þ
6 D + 2S
=
4.29Qg*'0.46 , for open space
It is found that Eq. (19) is also suitable for multiple flames in open space [12,23]. Note that Q_ varies from 1.1 to 6.7 in this work
g
D
g
should be equal to 2/5. Replacing the powers of Q_ 0 g in Eq. (18) with 2/5 and combining Eq. (16) gives
Lef / Q_ 2=5 D / Q_ D
ð20Þ
Eq. (20) suggests that the effective flame length from multiple energy sources is only related to HRR and not affected by the pool length/diameter (D) and spacing (S). This is not consistent with the real situation shown in Fig. 9 that Lef decreases with spacing for large HRRs. As a result, the 2/5 power of Q_ 0 is not adopted to fit g
the data. However, the power values of 0.46 and 0.42 in Eq. (18) are close to 2/5, showing that the impact of spacing on flame length is weaker than the HRR and the spacing has more significant effects on the flame length in open space than in tunnel. The possible reason is that as the spacing decreases, the unburned fuel spreads along transverse direction to compensate the strong restriction in the longitudinal area between the two burners under the ceiling, leading to a relatively small variation of the longitudinal flame extension. 4. Conclusions
DHef
QD* In open space: In Tunnel Wan et al. [13] Vasanth et al. [24] 0.01
and Ref. [13], while Q_ D is in between 1.5 and 2 in Ref. [24]. From Eq. 0 (19) and Q_ / Q_ in Eq. (16), it is derived that the power of Q_ 0
and merging of flame initiates when ðS=DÞ=Q_ DHef < 7:67. As the
2.50Qg*'0.42 , for tunnel
1
0.1 0.002
ð19Þ
In this paper, a set of experiments was conducted to study the temperature and flame behaviors under the tunnel ceiling for interacting flames. Two identical propane burners with the same HRR were used with varying HRRs and spacings. It is shown in this work that the flames merge completely when ðS=DÞ=Q_ < 1:0
Fitting equations:
Lef
D
8 _ 2 > Q D ; 0:01 6 Q_ D 6 0:1 Lef < _ 2=3 / Q D ; 0:1 6 Q_ D 6 1:0 > D : 2=5 Q_ D ; 1:0 6 Q_ D 6 100
D
Eq. (15) can be rewritten as
Lef ¼ 6D þ 2S
Eq. (18) indicates that the normalized effective flame length is proportional to 0.46 and 0.42 powers of the modified normalized HRR in open space and tunnel, respectively. The coefficients 4.29 and 2.50 suggests that the flame length from two flames in open space is greater than these in tunnel at given HRR and spacing. One of the reasons is that the cone-shaped vertical flame will be re-shaped into a disk when a rising flame impinges on the ceiling [39]. The other reason is that the restriction of longitudinal air entrainment in the area between the two burners leads to more combustible volatiles spread to the transverse direction to be fully combusted. For a single flame in open space, the relationship between Lef /D and Q_ can be expressed as [40]
0.1
1.12 2.24 3.36 4.48 5.60 6.71 0.5
Modified normalized HRR Qg
*'
Fig. 10. Normalized effective flame length Lef/(6D + 2S) against modified normal0 ized HRR Q_ g .
area of ceiling flame region increases with the energy release rate, which increases the heat and pollutants emission. Hence, the safety distance between vehicles with larger energy loads should be longer so as to prevent flame merging that will dramatically threaten the tunnel structure, facilities and trapped people. Models for predicting the ceiling gas temperature profile and flame length induced by two energy sources in tunnel are successively established, which helps to determine the heat transfer from energy sources to potential objects. The comparison between the current and former models reveals that when the flame height cannot reach the ceiling, the model proposed for a single energy source under an unconfined ceiling will underestimate the temperatures of two energy sources in tunnel. When the flame height is higher than the ceiling, the model proposed for a single energy source
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
under an unconfined ceiling can well predict the temperatures near the flame region induced by two energy sources in tunnel, while it will underestimate the temperatures in the region far away from the energy sources. The normalized effective flame lengths Lef/(6D + 2S) is proportional to 0.46 and 0.42 powers of the modified normalized HRR Q_ 0 in open space and tunnel, respecg
tively. The impact of spacing on flame length is weaker than HRR. The burner spacing has more significant effect on flame length in open space than in tunnel. Due to much higher temperature in the flame region than the smoke temperature, multiple peak ceiling gas temperatures exist right above the multiple energy sources. To improve the efficiency of heat detection, controlling and cooling, an adapting interval of heat detectors and sprinklers should be rearranged instead of the equally spaced arrangement of single energy source. As the heat feedback from flame, smoke and tunnel walls will affect the energy release rate of liquid and solid sources, which will then influence the ceiling characteristic parameters. Thus the current results of gas burners with fixed energy release rates might be unsuitable for liquid and solid energy sources. In addition, the two burners are located at the longitudinal centerline of tunnel without taking the spacing between burners and sidewalls of tunnel into account. Hence, the experiments using various fuel types as energy sources with decreasing distance to the tunnel sidewall will be conducted in future to further account for the more complicated scenarios. And the work will also be extended to include the interactions of more than two energy sources. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 51376173, the National Key Research and Development Plan under Grant No. 2016YFC0800100 and the Fundamental Research Funds for the Central Universities. Jie Ji was supported by the National TopNotch Young Talents Program. References [1] Hosain ML, Bel Fdhila R, Daneryd A. Heat transfer by liquid jets impinging on a hot flat surface. Appl Energy 2016;164:934–43. [2] Baldwin R. Flame merging in multiple fires. Combust Flame 1968;12 (4):318–24. [3] Fukuda Y, Kamikawa D, Hasemi Y, Kagiya K. Flame characteristics of group fires. Fire Sci Technol 2004;23(2):164–9. [4] Finney MA, McAllister SS. A review of fire interactions and mass fires. J Combust 2011;2011:1–14. [5] Gu ZH, Cheng YP, Zhou SN. Study on critical safe distance between vehicles in traffic tunnel fire. J China Univ Min Technol 2004;04:62–5. [6] Lönnermark A, Ingason H. Fire spread and flame length in large-scale tunnel fires. Fire Technol 2006;42(4):283–302. [7] Barrow H, Pope CW. Flow and heat-transfer in an internally-heated, naturallyventilated space. Appl Energy 2005;80(4):427–34. [8] Huffman K, Welker J, Sliepcevich C. Interaction effects of multiple pool fires. Fire Technol 1969;5(3):225–32. [9] Liu N, Liu Q, Lozano JS, Shu L, Zhang L, Zhu J, Deng Z, Satoh K. Global burning rate of square fire arrays: experimental correlation and interpretation. Proc Combust Inst 2009;32(2):2519–26. [10] Putnam AA, Speich CF. A model study of the interaction of multiple turbulent diffusion flames. Symp (Int) Combust 1963;9(1):867–77. [11] Lee WJ, Shin HD. Visual characteristics, including lift-off, of the jet flames in a cross-flow high-temperature burner. Appl Energy 2003;76(1–3):257–66.
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[12] Sugawa O, Takahashi W. Flame height behavior from multi-fire sources. Fire Mater 1993;17(3):111–7. [13] Wan HX, Ji J, Li KY, Huang XY, Sun JH, Zhang YM. Effect of air entrainment on the height of buoyant turbulent diffusion flames for two fires in open space. Proc Combust Inst 2017. doi: http://dx.doi.org/10.1016/j.proci.2016.07.078. [14] Gao Z, Ji J, Wan H, Li K, Sun J. An investigation of the detailed flame shape and flame length under the ceiling of a channel. Proc Combust Inst 2015;35 (3):2657–64. [15] Huang XQ, Leung CW, Chan CK, Probert SD. Thermal characteristics of a premixed impinging circular laminar-flame jet with induced swirl. Appl Energy 2006;83(4):401–11. [16] Zhen HS, Choy YS, Leung CW, Cheung CS. Effects of nozzle length on flame and emission behaviors of multi-fuel-jet inverse diffusion flame burner. Appl Energy 2011;88(9):2917–24. [17] Choy YS, Zhen HS, Leung CW, Li HB. Pollutant emission and noise radiation from open and impinging inverse diffusion flames. Appl Energy 2012;91 (1):82–9. [18] You HZ, Faeth GM. Ceiling heat transfer during fire plume and fire impingement. Fire Mater 1979;3(3):140–7. [19] Qi JA, Leung CW, Wong WO, Probert SD. Temperature-field measurements of a premixed butane/air circular impinging-flame using reference-beam interferometry. Appl Energy 2006;83(12):1307–16. [20] Gao ZH, Liu ZX, Wan HX, Zhu JP. Experimental study on longitudinal and transverse temperature distribution of sidewall confined ceiling jet plume. Appl Therm Eng 2016;107:583–90. [21] Alpert RL. Calculation of response time of ceiling-mounted fire detectors. Fire Technol 1972;8(3):181–95. [22] Heskestad G, Hamada T. Ceiling jets of strong fire plumes. Fire Safety J 1993;21 (1):69–82. [23] Delichatsios MA. A correlation for the flame height in‘‘ group” fires. Fire Sci Technol 2007;26(1):1–8. [24] Vasanth S, Tauseef S, Abbasi T, Rangwala A, Abbasi S. Assessment of the effect of pool size on burning rates of multiple pool fires using CFD. J Loss Prevent Proc Ind 2014;30:86–94. [25] Cetegen BM, Ahmed TA. Experiments on the periodic instability of buoyant plumes and pool fires. Combust Flame 1993;93(1):157–84. [26] Ji J, Fan CG, Zhong W, Shen XB, Sun JH. Experimental investigation on influence of different transverse fire locations on maximum smoke temperature under the tunnel ceiling. Int J Heat Mass Transfer 2012;55(17–18):4817–26. [27] Yan WG, Wang CJ, Guo J. One extended OTSU flame image recognition method using RGBL and stripe segmentation. Appl Mech Mater 2012;121–126:2141–5. [28] Ding H, Quintiere JG. An integral model for turbulent flame radial lengths under a ceiling. Fire Safety J 2012;52:25–33. [29] Ji J, Fu Y, Li K, Sun J, Fan C, Shi W. Experimental study on behavior of sidewall fires at varying height in a corridor-like structure. Proc Combust Inst 2015;35 (3):2639–46. [30] Fan C, Ji J, Sun J. Influence of longitudinal fire location on smoke characteristics under the tunnel ceiling. Fire Mater 2015;39(1):72–84. [31] Heskestad G. Fire plumes, flame height, and air entrainment. In: SFPE handbook of fire protection engineering. National Fire Protection Association; 2002. [32] Ingason H. Model scale tunnel fire tests with longitudinal ventilation. SP Report; 2005. [33] Oka Y, Imazeki O. Temperature and velocity distributions of a ceiling jet along an inclined ceiling – Part 1: Approximation with exponential function. Fire Safety J 2014;65:41–52. [34] Oka Y, Oka H, Imazeki O. Ceiling-jet thickness and vertical distribution along flat-ceilinged horizontal tunnel with natural ventilation. Tunn Undergr Sp Tech 2016;53:68–77. [35] Gao ZH, Ji J, Fan CG, Sun JH, Zhu JP. Influence of sidewall restriction on the maximum ceiling gas temperature of buoyancy-driven thermal flow. Energy Buildings 2014;84:13–20. [36] Lattimer BY, Mealy C, Beitel J. Heat fluxes and flame lengths from fires under ceilings. Fire Technol 2013;49(2):269–91. [37] Fan CG, Ji J, Li YZ, Ingason H, Sun JH. Experimental study of sidewall effect on flame characteristics of heptane pool fires with different aspect ratios and orientations in a channel. Proc Combust Inst 2017. doi: http://dx.doi.org/ 10.1016/j.proci.2016.06.196. [38] Babrauskas V. Flame lengths under ceilings. Fire Mater 1980;4(3):119–26. [39] Gross D. Measurement of flame lengths under ceilings. Fire Safety J 1989;15:31–44. [40] Quintiere JG, Grove BS. A unified analysis for fire plumes. Symp (Int) Combust 1998;27(2):2757–66.
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel Huaxian Wan a, Zihe Gao a, Jie Ji a,b,⇑, Kaiyuan Li c, Jinhua Sun a, Yongming Zhang a a
State Key Laboratory of Fire Science, University of Science and Technology of China, JinZhai Road 96, Hefei, Anhui 230026, China Institute of Advanced Technology, University of Science and Technology of China, Hefei, Anhui 230088, China c Department of Civil Engineering, School of Engineering, Aalto University, 02150 Espoo, Finland b
h i g h l i g h t s Experiments were conducted for two energy sources burning in a model tunnel. Energy release rate and burner spacing were varied. Flame merging criteria in tunnel were proposed. Correlations for estimating the ceiling gas temperature profiles were developed. The interacting flame lengths in tunnel were compared with the open space.
a r t i c l e
i n f o
Article history: Received 26 August 2016 Received in revised form 24 October 2016 Accepted 31 October 2016 Available online xxxx Keywords: Tunnel Multiple energy sources Flame merging Temperature decay profile Flame length
a b s t r a c t Multiple energy sources in a tunnel might lead to merge of flames with small enough spacings, releasing more heat and pollutant emissions than a single energy release source in tunnel and thus posing a great threat to tunnel structure, facilities and trapped people. As the heat detection, controlling and cooling systems are originally designed for the single energy release source, while the spacing between energy sources in tunnel is changeable and unpredictable. Then it is important and helpful to research on the much different characteristics of multiple energy sources with interacting ceiling flames for effective control the high risk scenarios. This paper aims to study the ceiling gas temperature profile and flame properties induced by two interacting energy sources in tunnel so as to improve the understanding of the arrangement of heat detectors and water sprinklers in tunnel. Two identical propane burners were used as energy sources located in a longitudinal array in tunnel. The total energy release rate and burner spacing were varied. The flame merging probability, ceiling gas temperature, vertical flame height and longitudinal flame extension were measured. The criteria of beginning merging and fully merging of flames are respectively proposed for two energy sources in tunnel. Results showed that the area of ceiling flame region increases with higher energy release rate. Models for predicting the ceiling gas temperature profiles induced by two energy sources in tunnel are established respectively for weak and strong plumes impinging on the ceiling. A modified model for predicting the combined vertical and longitudinal flame lengths from two burners in tunnel is proposed involving the normalized energy release rate, burner size and spacing. Finally, the comparison between models proposed for ceiling gas temperatures and flame lengths in tunnel and other configurations identifies the high risk of multiple energy sources in tunnel. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Two or more flames burning interactively is termed as multiple flames. In the past decades, most studies on multiple flames were conducted in open space. The key parameter involved in multiple ⇑ Corresponding author at: State Key Laboratory of Fire Science, University of Science and Technology of China, JinZhai Road 96, Hefei, Anhui 230026, China. E-mail address: [email protected] (J. Ji).
flames is spacing [1] as the flames might lean to each other and merge if sufficiently close [2]. Flame merging is believed as more destructive and uncontrollable [3]. Finney and McAllister [4] reviewed the empirical merging criteria for multiple flames derived in open space. As a typical confined space, tunnel might involve multiple vehicles burning simultaneously caused by collisions or subsequent accidents [5]. Generally, there are many mechanical and electrical facilities in tunnels, such as heat detectors and sprinklers. When the uncontrollable energy along with
http://dx.doi.org/10.1016/j.apenergy.2016.10.131 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
Nomenclature b cp D g Hef LB Lef Lf N P Pm Q_ Q_ c Q_
D
Q_ 0 g
plume radius at ceiling level, where the velocity is onehalf of the centerline value (m) specific heat of air (kJ/(kgK)) energy source diameter/length (m) gravitational acceleration (m/s2) ceiling height above the burner surface (m) impingement point induced by burner B to the center of burner B (m) effective flame length (m) flame height (m) flame number of N N array perimeter of air entrainment (m) flame merging probability energy release rate of a single flame (kW) convective energy release rate (kW) dimensionless energy release rate for each individual burner dimensionless energy release rate for group flames
enormous pollutant emissions and high temperatures induced by flame and smoke are released to a tunnel, the facilities within the tunnel should be activated simultaneously to achieve quick energy detection, controlling and cooling. Optimizing installation spacing of heat detectors and sprinklers could improve the effectiveness of energy controlling at the early burning stage. As a matter of fact, in many tunnel accidents, two or more crashed vehicles were burnt simultaneously [6], the resultant merging flames will release more heat and pollutant emissions than a single vehicle accident in tunnel. And as the spacing between the crashed vehicles in tunnel is changeable and unpredictable, it will dramatically increase the difficulty of quick heat detection and cooling. However, little effort has been paid attention to multiple flames in tunnels and so far no merging criterion is proposed for multiple flames in tunnel. This paper attempts to bridge this knowledge gap. Gaseous and liquid fuels were commonly used as energy sources in free and confined spaces [7]. For multiple liquid flames in open space, Huffman et al. [8] and Liu et al. [9] found that the heat release rate (HRR) developed a non-monotonous trend with decreasing the burner spacing due to the competition between heat feedback enhancement and air entrainment restriction. Since the HRR of liquid sources is strongly affected by the radiation from flame and smoke, it is difficult to quantify the effect of spacing. On the other hand, gas burners are affected only by the fuel flow rate while the impact of heat feedback is insignificant [10]. Therefore, the gas burners were used as energy sources in former studies [3,10–14]. When flame height is smaller than the ceiling height, the ceiling jet is a weak buoyant smoke flow. Fukuda et al. [3] concluded that when the HRR of each burner is low, the flames in a square array are hardly merging and the free flame height is little affected by the spacing. Gao et al. [14] found that if the flame tip did not hit the ceiling the flame height from a single energy source in tunnel is almost the same as the one in free space. When the free flame height is equal to or exceeds the ceiling height above the fuel surface, the resultant ceiling jet is driven by a strong plume. It was found that the pollutants emission from an impinging flame is a crucial behavior of the flame and the emission characteristics of impinging flames might differ from that of the same energy source in open space [15–17]. As strong plume hit the ceiling with flames spread radially along the ceiling, the gas temperature and the resultant heat flux received by the ceiling will increase dramatically and the risk of flame propagation enhances [18]. Estimations of the temperature field and flame length can facilitate determina-
Q_ total r rB rf S T T1 DT DTmax
total energy release rate of two burners (kW) radial distance from the flame axis and/or impingement point (m) horizontal distance from the center of burner B (m) distance from the impingement point to the flame tip in the downstream of burner B (m) burner spacing (m) gas temperature (K) ambient temperature (K) temperature rise (K) maximum temperature rise at impingement zone (K)
Greek symbols a power value of Q_ DHef in Eq. (4) q1 ambient density (kg/m3)
tions on the heat transfer from energy sources to receiving mediums such as the ceiling, nearby objects and secondary fuels [19]. In addition, the study of highest temperatures under a ceiling is significant since it has a great impact on the arrangement and activation of sprinkler systems in tunnels [20]. Alpert [21] and Heskestad and Hamada [22] proposed a set of correlations, which have been widely used, for predicting the maximum temperature decay profiles for both weak and strong plumes induced by a single source under unconfined ceilings. However, whether these correlations are suitable for multiple flames in tunnel is unclear. In spite of numerous studies focused on the free flame height of multiple energy sources [2,3,12,13,23,24], limited physical model is developed to predict the interacting flame length in tunnel. The objective of this work is to understand the ceiling gas temperature and flame behaviors from two propane burners located along the longitudinal centerline of tunnel. The total energy release rate and burner spacing were varied to quantitatively study the flame merging behaviors. Attempts have also been made to develop empirical models to predict not only the longitudinal ceiling gas temperature profile but also the flame length. The current study will provide beneficial scientific supports to the optimal arrangements of heat detectors and sprinklers, resulting in high efficiency of energy detecting and cooling as well as achieving optimum energy controlling.
2. Experiments Fig. 1 shows the experimental setup. The experiments were conducted in a 1/6-scale model tunnel which is 6 m long, 2 m wide and 0.86 m high. Fireproofing boards of 20 mm thick were used to make the top, bottom and one sidewall of the tunnel while the other sidewall was made of 10 mm thick fire-resistant glass for observation. The left and right ends of the tunnel were opened for natural ventilation. Two square gas burners denoted as A and B with a side length (D) of 15 cm were located at the longitudinal centerline with 22 cm higher than the tunnel floor. Propane was used as the fuel with different HRRs adjusted using a flow meter. The heat of combustion of propane was 46.3 MJ/kg and the combustion efficiency was assumed to be unity [25]. In the experiments, 6 HRRs (Q_ total ) were used including two burners, which are 21.6, 43.2, 64.8, 86.4, 108.0 and 129.6 kW, while the respective HRR (Q_ ¼ Q_ total /2) of burners A and B can be
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
Fig. 1. Experimental setup.
regarded as identical. To express the data conveniently, a normalized HRR Q_ ¼ Q_ /(q1cpT1g1/2D5/2) for each burner is used in the D
later pages, where q1, cp, T1 refer to the properties of ambient air. The calculated Q_ corresponding to various Q_ are 1.12, 2.24, D
3.36, 4.48, 5.60, 6.71. During experiments, the position of burner A was fixed at 2.25 m away from the left end of tunnel. The burner edge spacing (S) was set by moving burner B, including S/D = 0, 1/3, 2/3, 1, 4/3, 2, 3. Additionally, the cases with only burner A burning on were also conducted for comparison. The arrangement of the measuring points is also shown in Fig. 1. A total of 96 thermocouples were installed 10 mm beneath the ceiling (in order to obtain the vertical maximum temperature [26]) in a rectangular array with longitudinal intervals of either 0.05 m or 0.1 m and transverse interval of 0.2 m. The average values recorded by thermocouples during the steady burning stage are regarded as the maximum temperatures, which will be studied in the latter sections. Note that the K-type thermocouples with 1 mm diameter and response time less than 1 s were used in this work. Considering the uncertainty of experiments and combined effects of burner spacing and energy release rate on ceiling gas temperature and flame length, typical tests were repeated. The repeatability error of measurements was found to be less than 5%. On the left side of burner A, flush to the burner surface, four water-cooled heat flux gauges of type SBG01 with range up to 20 kW/m2 were placed to measure the total heat flux. Flame shapes were monitored from the front view and side view using two digital cameras (DV) with a frequency of 50 frames per second. In the experiments, the ambient temperature was about 30 °C. 3. Results and discussion 3.1. Flame merging behaviors Fig. 2 shows the typical snapshots of flame shape in longitudinal and transverse directions with increasing spacing during the steady burning stage. It is observed that the flames cannot impinge on the ceiling when Q_ total ¼ 21:6 kW (Fig. 2a) and the induced
flame height is weakly affected by spacing. For the rest Q_ total , the flame extensions under the ceiling can be found. As the spacing increases, the interaction between flames undergoes the following stages (see Fig. 2b–d). Firstly, both the vertical and ceiling flames merge at small spacings. As the spacing increases, the vertical flames separates while the ceiling flames still merge. Further increasing the spacing, the ceiling flames transit from intermittent merging to completely separated. For transverse flames (Fig. 2e), the achieved images can present from one side where it looks like burning a single flame due to the overlapping of the two flames. Increasing the spacing would lead to shorter transverse flame extensions.
The image processing method developed by Yan et al. [27] was adopted to judge whether the flames merged or not. In each test, the midpoint between the centerlines of two burners was specified as the center, the image processing area is 1 mm wide and has the same height from the burner surface to the ceiling. The flame merging probability (Pm) which is adopted to characterize the intensity of merging [13], was determined using the number of frames with flame lengths longer than 0 divided by total 3000 frames within 60 s after the flames were stable. Fig. 3 shows Pm against normalized spacing S/D. It can be seen in Fig. 3 that at a given Q_ , flames always merge if D
S/D is small enough, thus Pm = 1. Then Pm decays with increasing the S/D while Pm > 0, indicating that flames merge intermittently. When S/D is large enough, flames are separated, resulting in Pm = 0. Dimensional analysis method is adopted to determine the correlation of the flame merging probability. From the discussion in Fig. 3, it can be concluded that the burner properties (Q_ , D), ambi-
ent parameters (q1, cp, T1, g) and the burner spacing (S) are governing parameters for determining the Pm. Besides, the height of ceiling above the burner surface (Hef) is added due to that it directly determines whether the ceiling flames are formed or not. Therefore, the flame merging probability can be expressed as
Pm ¼ f Q_ ; q1 ; cp ; T 1 ; g; D; Hef ; S
ð1Þ
Based on dimensional analysis, Eq. (1) can be derived as
! cp T 1 Hef S Pm ¼ f ; ; ; q1 g 3=2 D7=2 gD D D Q_
ð2Þ
Incorporating the first two terms at the right side gives _ Q_ D ¼ q c T Qg1=2 D5=2 . To account for the confinement of tunnel struc1
q 1
ture, the term Hef is incorporated into Q_ D , then a widely used Q_ dimensionless HRR Q_ DHef ¼ 3=2 for tunnel [14,28,29] is 1=2 q1 cq T 1 g
DH
ef
introduced. Hence, Eq. (2) can be rewritten as
Pm ¼ f Q_ DHef ; S=D
ð3Þ
We firstly attempt to determine the relationship between Pm and S/D. From Fig. 3, it is reasonable to assume that at a given Q_ , Pm decreases linearly with S/D when 0 < Pm < 1. Since Pm D
increases with Q_ DHef at a given S/D (see Fig. 3), Eq. (3) can be rewritten as
0
1 S=D A Pm ¼ f @ Q_ a
ð4Þ
DHef
where a is the power value of Q_ DHef . It is easily deduced that a is a positive constant. Based on numerous former studies, five widely
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4
H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
S/D=0
S/D=1/3
S/D=2/3
S/D =1
S/D=4/3
S/D=2
S/D=3
S/D=2
S/D=3
(a) Qtotal =21.6 kW (front view)
(b) Qtotal =43.2 kW (front view)
(c) Qtotal =86.4 kW (front view)
(d) Qtotal =129.6 kW (front view)
S/D=0
S/D=1/3
S/D=2/3
S/D=1
S/D=4/3
(e) Qtotal =129.6 kW (side view) Fig. 2. Snapshots of flame shape in (a) – (d) longitudinal and (e) transverse directions with increasing spacing.
1.2 Normalized HRR Q 1.12 2.24 3.36 4.48 5.60 6.71 * D
1.2 1.0 0.8 0.6 0.4 0.2 0.0
-0.33 0.00 0.33 0.67 1.00 1.33 1.67 2.00 2.33 2.67 3.00 3.33
Normalized spacing S/D Fig. 3. Flame merging probability Pm against normalized spacing S/D.
used empirical values of a are employed, namely, a = 1/2, 3/4, 1, 5/4, and 3/2. The comparison shows that the data points with a = 1 are more converged than the other a values. Therefore a is finally determined as 1. Fig. 4 shows the flame merging probability Pm against the normalized term ðS=DÞ=Q_ . It is observed that Pm can be DHef
divided into three stages with increasing ðS=DÞ=Q_ DHef . Fitting the data points using piecewise linear function, the final expression of Pm is established as
Flame merging probability Pm
Flame merging probability Pm
1.4
Merging stages Fully merging Intermittent merging Non-merging
1.0 0.8
S/D=0 S/D=2/3 S/D=4/3 S/D=3
0.6 0.4
S/D=1/3 S/D=1 S/D=2 Fitting line
0.2 0.0 -0.1 -2.5
1.0 7.67 0
5
10
15
20
25
Normalized HRR and spacing ( S / D ) / Q
* DH ef
Fig. 4. Flame merging probability Pm against normalized HRR and spacing ðS=DÞ=Q_ DHef .
8 > 1; 0 6 ðS=DÞ=Q_ DHef < 1:0 > > < Pm ¼ 1:15 0:15ðS=DÞ=Q_ DHef ; 1:0 6 ðS=DÞ=Q_ DHef < 7:67 > > > : 0; 7:67 6 ðS=DÞ=Q_ DHef < 23:6
ð5Þ Eq. (5) shows that flames merge completely when ðS=DÞ=Q_ DHef is smaller than 1.0 and flames begin to merge when ðS=DÞ=Q_ DHef is smaller than 7.67.
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5
y (cm)
90 90 90 S/D=0 S/D=1/3 S/D=2/3 60 60 60 30 30 30 530 0 0 0 -30 -30 -30 -60 -60 -60 -90 -90 -90 -90-60-30 0 30 60 90 -90-60 -30 0 30 60 90 -90-60-30 0 30 60 90
y (cm)
90 90 90 S/D=4/3 S/D=1 S/D=2 60 60 60 30 30 30 0 0 0 -30 -30 -30 -60 -60 -60 -90 -90 -90 -90-60-30 0 30 60 90 -90-60-30 0 30 60 90 -90-60-30 0 30 60 90
y (cm)
H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
90 S/D=3 60 30 0 -30 -60 -90 -90-60-30 0 30 60 90
x (cm)
x (cm)
Top view
x (cm)
x (cm)
x (cm)
Transverse
x (cm)
(
)
870 810 740 670 600 530 460 390 320 250 180
x (cm)
burner A
y O
burner B x
Centerline S/2 Longitudinal O: the middle point between the two burners x: longitudinal distance from the point O y: transverse distance from the point O
Fig. 5. Ceiling gas temperature profiles with various S/D at Q_ D ¼ 4:48.
3.2. Gas temperature properties under the tunnel ceiling 3.2.1. Ceiling gas temperature decay profile for a single flame When a weak plume impinges on an unconfined ceiling, Alpert [21] developed the following widely used correlation to quantify the maximum gas temperature rise (DT = TT1), assuming an axisymmetric plume flow under a flat, horizontal ceiling without sidewall effect.
DT ¼ 16:9
DT ¼ 5:38
Q_ 2=3 5=3
Hef
;
for r=Hef 6 0:18
Q_ 2=3 =H5=3 ef ðr=Hef Þ2=3
;
ð6aÞ
for r=Hef > 0:18
ð6bÞ
where r is the radial distance from the flame axis. For a strong buoyant plume impinging on the ceiling, Alpert’s correlation is no longer applicable. Replacing the ceiling height (Hef) used for a weak plume with the ceiling plume radius, b, as a length scale, Heskestad and Hamada [22] proposed a correlation of excess temperature for strong plume.
r 1 h DT r i ; ¼ 1:92 exp 1:61 1 b b DT max
for 1 6
r 6 40 b
ð7aÞ
3.2.2. Gas temperature profile under the ceiling During the steady burning stage, the gas temperatures show basically unchanged. As a result, the average temperatures are used in the following contents. Fan et al. [30] found that the maximum smoke temperatures induced by both methanol and heptane pools were weakly affected by the longitudinal distance between the flame center and the tunnel end. Hence, the impact of the longitudinal distance between burners to tunnel ends is regarded as ignorable in this work. As the two burners with identical configurations and energy release rates are placed at the same level along the longitudinal centerline of tunnel, it is reasonable to assume that the symmetrical temperature profiles under the ceiling are formed by the two burners. Taking the midpoint between the centerlines of two burners as the center (Fig. 5), the 1/4 total ceiling gas temperature profiles induced by the two burners can be obtained based on the measurements of thermocouple arrays located 10 mm below the ceiling (Fig. 1). Through longitudinal and transverse symmetries, the total ceiling gas temperature profiles can be determined. Note that other than the cases of Q_ ¼ 1:12, ceiling flames are formed for the rest HRRs. Taking D
where DTmax is the maximum temperature rise at the impingement zone, b is the radius where the velocity of the impinging plume is one-half the centerline value. The expression for b is given by
h i1=2 T 1=2 Q_ 2=5 2=5 max c b ¼ 0:42 ðcp q1 Þ4=5 T 3=5 1 g DT 3=5 max
flame axis. Therefore, it is worthwhile examining the feasibility of Eqs. (6) and (7) to the ceiling gas temperature decay profile in cases with interacting flames in tunnel.
ð7bÞ
where Q_ c is the convective energy release rate, which can be calculated as Q_ c ¼ 0:8Q_ [22]. It should be noted that Eqs. (6) and (7) are proposed for single flame cases without flame tilting. However, flames can tilt due to external wind, slope, or the interaction with other flames [4], resulting in the impinging position deviated from the vertical
the cases of Q_ D ¼ 4:48 as examples, ceiling gas temperature profiles with various S/D are plotted in Fig. 5, where the point O in the middle of two burners is regarded as the origin of coordinates, the longitudinal and transverse directions are respective x-axis and y-axis. It can be seen that when the spacing is small enough (S/ D 6 1/3), the fully merging flames make the fire plume spread out radially with a circular shape. As the spacing increases (2/3 6 S/D 6 4/3), the inner core zones are separated, while in the slightly far region the temperature contours become combined and spread out radially with an elliptical shape. Further increasing the spacing (S/D P 2), temperature contours varies from an elliptical shape with the longer x-axis to a peanut-like shape.
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
Position of the impingement point, LB (cm)
6
20
S/D=0 S/D=1/3 S/D=2/3 S/D=1 S/D=4/3 S/D=2 S/D=3
15 10 5
upstream of burner B. As Q_ D increases, both pressure drop and horizontal inertial force increase, the competition effect between the two forces results in the increasing of LB. When Q_ is large enough,
Front view upstream downstream LB<0 o LB>0
D
burner B
burner A
0 -5 -10 -15 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
Normalized HRR QD*
the hot current flow is dominated by the weaken impact of horizontal inertial force, resulting in LB > 0. Fig. 6 also shows that the values of LB in all tests are varied within 10 cm 5 cm, suggesting the weak extent of flame tilting under the ceiling. As the temperatures within the confined area between two burners are quite high and difficult to quantify, the ceiling gas temperatures located in the downstream of impingement point induced by burner B is the focus in this work. In the following, the radial distance r in Eq. (6) is redefined as the horizontal distance from the impingement point to the point in the downstream of burner B and along the longitudinal centerline of tunnel. For the cases of Q_ ¼ 1:12, weak plumes impinge on the ceiling. D
Fig. 6. Position of the impingement point from the center of burner B (LB) against Q_ D .
To apply this model for scenarios with different scales, Eq. (6) should be dealt with dimensionless method by which the ceiling excess temperature is normalized by the ambient one and the energy release rate is normalized by the commonly used Q_ Hef
3.2.3. Longitudinal maximum gas temperature decay profile in the downstream of impingement point induced by burner B It should be noted that the cases with merging vertical flames will lead to only one impingement point, while the other cases will induce two impingement points (see Fig. 5). Based on the symmetrical assumption, the impingement point induced by burner B to the center of burner B is denoted as LB, as shown in Fig. 6. Due to the intervals of thermocouples, Oka et al. [33,34] proposed a suitable method to estimate the impingement point and the resultant maximum temperature rise (DTmax). Using Oka’s method to our work, the values of LB and DTmax can be determined based on a quadratic fit to the measured data from three points involving the measured maximum temperature rise and the temperatures of two adjoining points. Fig. 6 shows LB against Q_ at various norD
malized spacings, where the positive value of LB indicates that the impingement point is located at the downstream of burner B (towards the right end of tunnel), and vice versa. It can be seen that at a given S/D, LB increases with Q_ and its D
value varies from negative to positive. This is because the gas flow under the ceiling is controlled by two governing forces. One is the horizontal inertial force generated by buoyancy force, which will weaken the interaction of flames. The other is the pressure drop between flames, which will reinforce the interaction of flames. Under small Q_ and S/D, the buoyance force is weak, the reinforced D
pressure drop between flames plays a dominant role in the gas flow, leading to flame/smoke impinging on the ceiling at the 1
For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
[28,35,36]. Using the ambient temperature of 303 K in our experiments, Eq. (6) can be non-dimensionalized as follows.
8 _ 2=3 > < 5:96Q Hef ;
r=Hef 6 0:18 DT 2=3 ¼ _ QH > T1 : 1:90 ðr=H efÞ2=3 ; r=Hef > 0:18
ð8aÞ
ef
where,
Q_ Hef ¼
Q_
ð8bÞ
q1 cp T 1 g 1=2 H5=2 ef
Due to that Eqs. (6) and (8) are previously applied for a single energy release source under an unconfined ceiling. To validate the applicability of Alpert’s correlations for two energy sources in confined tunnel scenarios, similar forms shown in Eq. (8) were used for reference. The normalized temperature rise D_ T2=3 against T1 Q H
ef
normalized horizontal distance r/Hef is plotted in Fig. 7. The single flame cases in tunnel and Alpert’s model are also presented for comparison. It can be seen that the data points from various spacings are quiet concentrated, indicating that the gas temperature rise in the downstream of burner B is weakly affected by the spacing. For a single flame in tunnel, Eq. (8) can predict well in the far field from the impingement point, while it underestimates the temperatures in the near region. Besides, ceiling gas temperatures from two burners in tunnel calculated by Eq. (8) are lower than the
20
Normalized temperature rise
In former studies, the flame tips can be determined using temperature measurement. If DT = 500 °C [31,32] was used to represent the temperature of the flame tip, the flame shape under the ceiling can be determined by the contour of T = 530 °C, as shown the red1 lines marked in Fig. 5. Assuming that the region enclosed by the red line is flame while the external of the region is smoke plume. It can be observed that as the spacing increases, the flame shape of two burners experience five stages: a whole circular shape, a whole elliptical shape, a whole peanut shape, two separate elliptical shapes and two separate circular shapes with large enough spacings. In addition, the area of flame region increases with decreasing the spacing, which indicates the higher risk of interacting flames. Similar trends of temperature profiles and flame shapes can be found for the other HRRs. And the area of ceiling flame region increases with the energy release rate at a given burner spacing.
6.6, r / H ef ≤ 0.18 ΔT = −1/2 T∞ QH*2/3 2.8 ( r / H ef ) , r / H ef > 0.18 ef
10
ΔT T∞QH*2/3 ef
S/D=0 S/D=2/3 S/D=4/3 S/D=3 Eq. (8) [21]
1 0.8 0.015
S/D=1/3 S/D=1 S/D=2 Single fire This work 0.18 0.1
1
3
Normalized longitudinal distance r/Hef Fig. 7. Normalized temperature rise D_ T2=3 against normalized longitudinal distance T1 Q H ef r/Hef for weak plumes with Q_ ¼ 1:12. D
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
experimental values. This is because Eq. (8) is developed for a single flame under the unconfined ceiling without any smoke accumulation. While smoke can accumulate under the ceiling with the restriction of tunnel sidewalls. The smaller temperature decay for two burners than that for a single flame in tunnel is mainly due to the existence of a nearby flame. Fitting the data points in cases of two burners using the form of Eq. (8) with optimized coefficients 2=3 and shifting the term Q_ from the left to the right side gives Hef
8 _ 2=3 > < 6:6Q Hef ;
r=Hef 6 0:18 DT 2=3 ¼ _ QH > T1 : 2:8 ðr=H efÞ1=2 ; r=Hef > 0:18
ð9Þ
ef
Eq. (9) shows that when r/Hef 6 0.18, the normalized temperature rise DT/T1 is independent of the horizontal distance. While r/Hef > 0.18, DT/T1 is proportional to 1/2 power of r/Hef. Note that Eq. (9) is developed for S/D 6 3, it is not able to apply for very large spacings since it cannot predict well the case of a single flame (see Fig. 7). For the cases of Q_ 6 2:24, strong plumes impinge on the ceilD
ing, Eq. (7) proposed by Heskestad and Hamada are applied to these cases. Fig. 8 shows the normalized temperature rise DT/ DTmax against the horizontal distance normalized by characteristic plume radius r/b. The single flame cases in tunnel and Heskestad and Hamada’s correlation are also presented for comparison. It can be observed in Fig. 8 that the data points from two burners are converged, suggesting that the spacing has a little effect on DT/DTmax by using b as the characteristic length. The possible reason is that as the spacing decreases, the strong interaction between ceiling flames along the longitudinal direction makes it possible for unburned fuels spreading along the transverse direction to be fully combusted. In addition, Eq. (7) can well collapse the data of single flame cases in tunnel. It also fits well with the data in the near field of the impingement point in cases of two burners while the temperatures in the far field is underestimated. As shown in Fig. 8, the data from two burners can be matched closely by the following expression,
DT ¼ DT max
(
1; r=b 6 0:67 r 1:34 0:88 þ 0:18 b ; 0:67 < br 6 14
ð10Þ
r ΔT = 0.88 + 0.18 b ΔTmax
Normalized temperature rise
D
D
Q_ D 6 4:48, as shown in Fig. 9, there is no trend for Lef maintaining a constant. Fukuda et al. [3] experimentally studied multiple flames in open space with N N (N = 2–5) square arrays using square propane burners with each length of D = 15 cm. Afterwards, Delichatsios [23] proposed theoretical correlations for the free flame height (Lf) from multiple energy sources to fit the Fukuda’s experimental data by assuming that, (a) the air entrainment along the flame height is proportional to the stoichiometric requirements for fuel burning and (b) the volume of air entrainment is equal to pffiffiffiffiffiffiffi PLf gLf . The derived process by Delichatsios is as follows. The perimeter of air entrainment:
P ¼ 4ðND þ ðN 1ÞSÞ
ð11Þ
The volume of air entrainment:
qffiffiffiffiffiffiffi PLf gLf /
N2 Q_ q1 c p T 1
ð12Þ
Combining Eqs. (11) and (12) gives
2=3 L f / Q_ g ðN1ÞS ND 1 þ ND Q_ g ¼
ð13aÞ
Q_ D 5=2 N1=2 1 þ ðN1ÞS ND
−1.34
120
1
0.06 0.02
D
Lef decreases first and then maintains unchanged. The critical spacing to reach the unchanged Lef increases with Q_ . For the cases of
Effective flame length Lef (cm)
ΔT =1 ΔTmax
0.1
Since the energy release rate of gas burner would not be affected by the thermal feedback and the effective ceiling height (Hef), while the decrease of Hef would increase the ceiling flame extension at a given HRR [29]. Therefore, the sum of vertical flame height and longitudinal flame extension is defined as the effective flame length (Lef), as widely used in former studies [14,29,36,37]. When flames cannot impinge on the ceiling, Lef is equal to the vertical flame height. When flame height is higher than the ceiling, Lef = Hef + rf = Hef + rB LB, where rf is the longitudinal distance from the impingement point to the flame tip in the downstream of burner B and rB is the longitudinal distance from the center of burner B, as shown in Fig. 9. The mean flame height/extension induced by burner B (rB) is also determined using the image processing method. Fig. 9 shows the effective flame length Lef against spacing S. It can be observed that Lef is weakly affected by the spacing when Q_ ¼ 1:12. For strong plumes with Q_ 6 2:24, as the S increases,
ð13bÞ
where Q_ 0 g is the normalized energy release rate for group flames.
3
T/
Tmax
Eq. (10) shows that when r/b > 0.67, the normalized temperature rise DT/DTmax begins to be affected by the longitudinal distance. When 1 6 r/b 6 2.35, Heskestad and Hamada’s correlation can well collapse the temperature decay profiles induced by two burners in tunnel.
3.3. Flame length of two flames
S/D=0 S/D=1/3 S/D=2/3 S/D=1 S/D=4/3 S/D=2 r ΔT = 1.92 S/D=3 b ΔTmax Single fire Eq. (7) [22] 0.67 This work
0.1
−1
− exp 1.61 1 −
r b
2.35 1
10
30
Normalized longitudinal distance r/b Fig. 8. Normalized temperature rise DT/DTmax against normalized longitudinal distance r/b for strong plumes.
Definition of Lef (front view) impingement point rB rf = rB-LB 2.24 o 4.48 H flame tip ef 6.71 Lef=Hef+rf A B
QD* 1.12 3.36 5.60
110 100 90 80 70 60 0
10
20
30
40
50
Spacing S (cm) Fig. 9. Effective flame length Lef against spacing S.
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H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
We attempt to develop an alternative model for predicting the effective flame length in tunnel based on Eq. (13). Due to the longitudinal ceiling flames in the downstream of burner B can be regarded as entraining air freely. A reasonable assumption which firstly proposed by Babrauskas [38] will be adopted. Namely, at a given HRR, the air entrainment rate along the ceiling jet is the same as that in the free plume. As a result, the method of Delichatsios can be used for the current study. The effective perimeter of air entrainment for two interacting square burners is
P ¼ 6D þ 2S
ð14Þ 2
Substituting Eq. (14) into Eq. (12) and specifying N = 2, the normalized effective flame length can be expressed as
!2=3
2Q_ D
Lef / 6D þ 2S
ð15Þ
ð6 þ 2S=DÞ5=2
Defining the modified non-dimensional HRR as
Q_ 0g ¼
2Q_ D ð6 þ 2S=DÞ
ð16Þ
5=2
Lef =ð6D þ 2SÞ / Q_ 02=3 / Q_ 0g g
ð17Þ
For two interacting flames, Vasanth et al. [24] numerically studied the mass loss rate and flame length from two circular heptane pools in open space. The pool diameters (D) were 4.8 cm, 6.8 cm and 8.3 cm and S/D = 0.25–1.08. Due to the relatively small spacings, the effective perimeter of the two circular pools can be approximated as P 2pD + S 6D + 2S, indicating that Eq. (15) can also be used for small circular pools. Besides, Wan et al. [13] experimentally studied the free flame height for two square burners with D = 15 cm and S/D = 0–4. The relationships between normalized effective flame length Lef /(6D + 2S) and modified normalized HRR Q_ 0 in both open space and tunnel are shown in g
Fig. 10 in a double logarithmic coordinates. It can be seen that the data points from open space [13,24] and tunnel are individually concentrated. As the flame heights in cases of Q_ ¼ 1:12 are lower than the ceiling of tunnel, these data points D
are closed to the points induced by flames in open space. The exponential functions with optimized coefficients are used to fit the data (see Fig. 10). The fitting equations are
Normalized effective flame length Lef / (6D+2S)
(
4:29Q_ g 0:46 ; for open space 0 2:50Q_ g 0:42 ; for tunnel 0
5
ð18Þ
6 D + 2S
=
4.29Qg*'0.46 , for open space
It is found that Eq. (19) is also suitable for multiple flames in open space [12,23]. Note that Q_ varies from 1.1 to 6.7 in this work
g
D
g
should be equal to 2/5. Replacing the powers of Q_ 0 g in Eq. (18) with 2/5 and combining Eq. (16) gives
Lef / Q_ 2=5 D / Q_ D
ð20Þ
Eq. (20) suggests that the effective flame length from multiple energy sources is only related to HRR and not affected by the pool length/diameter (D) and spacing (S). This is not consistent with the real situation shown in Fig. 9 that Lef decreases with spacing for large HRRs. As a result, the 2/5 power of Q_ 0 is not adopted to fit g
the data. However, the power values of 0.46 and 0.42 in Eq. (18) are close to 2/5, showing that the impact of spacing on flame length is weaker than the HRR and the spacing has more significant effects on the flame length in open space than in tunnel. The possible reason is that as the spacing decreases, the unburned fuel spreads along transverse direction to compensate the strong restriction in the longitudinal area between the two burners under the ceiling, leading to a relatively small variation of the longitudinal flame extension. 4. Conclusions
DHef
QD* In open space: In Tunnel Wan et al. [13] Vasanth et al. [24] 0.01
and Ref. [13], while Q_ D is in between 1.5 and 2 in Ref. [24]. From Eq. 0 (19) and Q_ / Q_ in Eq. (16), it is derived that the power of Q_ 0
and merging of flame initiates when ðS=DÞ=Q_ DHef < 7:67. As the
2.50Qg*'0.42 , for tunnel
1
0.1 0.002
ð19Þ
In this paper, a set of experiments was conducted to study the temperature and flame behaviors under the tunnel ceiling for interacting flames. Two identical propane burners with the same HRR were used with varying HRRs and spacings. It is shown in this work that the flames merge completely when ðS=DÞ=Q_ < 1:0
Fitting equations:
Lef
D
8 _ 2 > Q D ; 0:01 6 Q_ D 6 0:1 Lef < _ 2=3 / Q D ; 0:1 6 Q_ D 6 1:0 > D : 2=5 Q_ D ; 1:0 6 Q_ D 6 100
D
Eq. (15) can be rewritten as
Lef ¼ 6D þ 2S
Eq. (18) indicates that the normalized effective flame length is proportional to 0.46 and 0.42 powers of the modified normalized HRR in open space and tunnel, respectively. The coefficients 4.29 and 2.50 suggests that the flame length from two flames in open space is greater than these in tunnel at given HRR and spacing. One of the reasons is that the cone-shaped vertical flame will be re-shaped into a disk when a rising flame impinges on the ceiling [39]. The other reason is that the restriction of longitudinal air entrainment in the area between the two burners leads to more combustible volatiles spread to the transverse direction to be fully combusted. For a single flame in open space, the relationship between Lef /D and Q_ can be expressed as [40]
0.1
1.12 2.24 3.36 4.48 5.60 6.71 0.5
Modified normalized HRR Qg
*'
Fig. 10. Normalized effective flame length Lef/(6D + 2S) against modified normal0 ized HRR Q_ g .
area of ceiling flame region increases with the energy release rate, which increases the heat and pollutants emission. Hence, the safety distance between vehicles with larger energy loads should be longer so as to prevent flame merging that will dramatically threaten the tunnel structure, facilities and trapped people. Models for predicting the ceiling gas temperature profile and flame length induced by two energy sources in tunnel are successively established, which helps to determine the heat transfer from energy sources to potential objects. The comparison between the current and former models reveals that when the flame height cannot reach the ceiling, the model proposed for a single energy source under an unconfined ceiling will underestimate the temperatures of two energy sources in tunnel. When the flame height is higher than the ceiling, the model proposed for a single energy source
Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131
H. Wan et al. / Applied Energy xxx (2016) xxx–xxx
under an unconfined ceiling can well predict the temperatures near the flame region induced by two energy sources in tunnel, while it will underestimate the temperatures in the region far away from the energy sources. The normalized effective flame lengths Lef/(6D + 2S) is proportional to 0.46 and 0.42 powers of the modified normalized HRR Q_ 0 in open space and tunnel, respecg
tively. The impact of spacing on flame length is weaker than HRR. The burner spacing has more significant effect on flame length in open space than in tunnel. Due to much higher temperature in the flame region than the smoke temperature, multiple peak ceiling gas temperatures exist right above the multiple energy sources. To improve the efficiency of heat detection, controlling and cooling, an adapting interval of heat detectors and sprinklers should be rearranged instead of the equally spaced arrangement of single energy source. As the heat feedback from flame, smoke and tunnel walls will affect the energy release rate of liquid and solid sources, which will then influence the ceiling characteristic parameters. Thus the current results of gas burners with fixed energy release rates might be unsuitable for liquid and solid energy sources. In addition, the two burners are located at the longitudinal centerline of tunnel without taking the spacing between burners and sidewalls of tunnel into account. Hence, the experiments using various fuel types as energy sources with decreasing distance to the tunnel sidewall will be conducted in future to further account for the more complicated scenarios. And the work will also be extended to include the interactions of more than two energy sources. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 51376173, the National Key Research and Development Plan under Grant No. 2016YFC0800100 and the Fundamental Research Funds for the Central Universities. Jie Ji was supported by the National TopNotch Young Talents Program. References [1] Hosain ML, Bel Fdhila R, Daneryd A. Heat transfer by liquid jets impinging on a hot flat surface. Appl Energy 2016;164:934–43. [2] Baldwin R. Flame merging in multiple fires. Combust Flame 1968;12 (4):318–24. [3] Fukuda Y, Kamikawa D, Hasemi Y, Kagiya K. Flame characteristics of group fires. Fire Sci Technol 2004;23(2):164–9. [4] Finney MA, McAllister SS. A review of fire interactions and mass fires. J Combust 2011;2011:1–14. [5] Gu ZH, Cheng YP, Zhou SN. Study on critical safe distance between vehicles in traffic tunnel fire. J China Univ Min Technol 2004;04:62–5. [6] Lönnermark A, Ingason H. Fire spread and flame length in large-scale tunnel fires. Fire Technol 2006;42(4):283–302. [7] Barrow H, Pope CW. Flow and heat-transfer in an internally-heated, naturallyventilated space. Appl Energy 2005;80(4):427–34. [8] Huffman K, Welker J, Sliepcevich C. Interaction effects of multiple pool fires. Fire Technol 1969;5(3):225–32. [9] Liu N, Liu Q, Lozano JS, Shu L, Zhang L, Zhu J, Deng Z, Satoh K. Global burning rate of square fire arrays: experimental correlation and interpretation. Proc Combust Inst 2009;32(2):2519–26. [10] Putnam AA, Speich CF. A model study of the interaction of multiple turbulent diffusion flames. Symp (Int) Combust 1963;9(1):867–77. [11] Lee WJ, Shin HD. Visual characteristics, including lift-off, of the jet flames in a cross-flow high-temperature burner. Appl Energy 2003;76(1–3):257–66.
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Please cite this article in press as: Wan H et al. Experimental study on ceiling gas temperature and flame performances of two buoyancy-controlled propane burners located in a tunnel. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.10.131