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Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling
Accepted Manuscript Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling Chuan Gang Fan, Jian Yang PII: DOI: Reference:
S0894-1777(16)30335-1 http://dx.doi.org/10.1016/j.expthermflusci.2016.11.019 ETF 8941
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
14 July 2016 1 November 2016 19 November 2016
Please cite this article as: C.G. Fan, J. Yang, Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/ 10.1016/j.expthermflusci.2016.11.019
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Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling
Chuan Gang Fan1, * Jian Yang2 1. School of Automotive and Transportation, Hefei University of Technology, Hefei, China 2. College of Civil Engineering, Fuzhou University, Fuzhou, China Corresponding Author: Chuan Gang Fan. Address: School of Automotive and Transportation Engineering, Hefei University of Technology, Hefei 230009, Anhui, China. Tel: +86-551-0551-62901960 (Office), +86-18255170802 (Mobile) Email: [email protected]
1
Abstract Model scale tests were performed with varying tunnel widths and heights in order to study the effect of tunnel cross section and ventilation velocity on the thermal smoke backlayering length in case of large heat release rate. The results showed that in tests with the same fuel and ventilation velocity, the backlayering length trends to increase with increasing tunnel cross-sectional area when the tunnel cross-sectional area reaches a certain value, under which the critical velocity is obtained. The reason should be as the tunnel height or width increases, the blockage effect of flame inside the tunnel is reduced, thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In order to establish an integral model of backlayering length considering the effect of both tunnel height and width, new dimensionless forms of backlayering length and ventilation velocity, including the hydraulic diameter of tunnel, are introduced. The proposed equation can correlate well with all the data from the model scale tests with different fuels, ventilation velocities and tunnel cross sections, and also Runehamar full scale tunnel tests. Key words: heat release rate, tunnel cross section, mechanical ventilation, critical velocity, backlayering length
Nomenclature
A
tunnel cross-sectional area (m2)
Alocal
local tunnel cross-sectional area (m2)
cp
heat capacity (kJ/kg K)
Fr
Froude number 2
g
gravity acceleration (m/s2)
H
tunnel height (m)
Hf
distance between fuel center and tunnel ceiling (m)
H
hydraulic diameter of tunnel (m)
L
length scale (m)
Lb
backlayering length (m)
m
mass flow rate (kg/s)
P
tunnel perimeter (m)
Q
heat release rate (kW)
Ta
ambient temperature or surrounding gas temperature (K)
Tf
flame temperature (K)
t
time (s)
V
longitudinal ventilation velocity at ambient conditions (m/s)
Vc
critical velocity (m/s)
Vlocal
local ventilation velocity under the ceiling (m/s) V *
velocity W
tunnel width (m)
x
distance from fuel centre (m)
Greek a
air density (kg/m3) blockage ratio
Subscript F
full scale 3
dimensionless ventilation
M
model scale
max
maximum value
1. Introduction
In recent years, more and more tunnels are being built to ease traffic congestion and segregate hazardous goods transportation in urban areas [1-7]. The number and aggregate length of road, rail and subway tunnels in the world are increasing quickly. However, tunnels bring us not only convenience but also new research needs of energy balance and thermal flow movement. In urban road tunnels, the heat emitted by heavy traffic would easily cause temperature and concentration of harmful gas to rise to intolerable levels with ineffective ventilation. Longitudinal ventilation with jet fans is the most commonly used ventilation mode in tunnels. The longitudinal airflow induced by jet fans is the key factor in the control of heat and smoke movement. For example, when the longitudinal velocity is high enough, the fire-induced smoke will be prevented from spreading upstream and then the smoke backlayering upstream of the fire is eliminated, thus the critical velocity is reached. When the longitudinal velocity is less than the critical velocity, a smoke backlayering will form, as shown in Fig. 1. Lb
V
Fig. 1 Schematic diagram of smoke backlayering. The backlayering length is a popular subject in the tunnel research and previous studies have investigated the backlayering length extensively [8-15], as the main aim of ventilation system is to 4
confine the smoke in an acceptable zone to provide a clean route for people evacuation, fire rescue and extinguishment. Thomas [16] presented a theoretical analysis of the backlayering length in a longitudinally ventilated tunnel fire and proposed a dimensionless relation: Lb gHQ H aT f c pV 3 A
(1)
In fact, as A HW , Eq. (1) can be rewritten as: Lb gQ H aT f c pV 3W
(2)
From Eq. (2), it can be seen that heat release rate (HRR), tunnel height and width, and longitudinal velocity are key parameters influencing the backlayering length. Vantelon et al. [17] carried out small-scale experiments in a 1.5 m long semicircular pipe with 0.15 m radius, and found that the ratio of backlayering length to tunnel height varied as 0.3 power of a modified Richardson number: Lb gQ Ri '0.3 ( )0.3 3 H aTa c pV H
(3)
Li et al. [18] conducted experimental tests and theoretical analyses to investigate the critical velocity together with the backlayering length in tunnel fires. Experimental data show that the relation between the ratio of ventilation velocity to critical velocity and the dimensionless backlayering length follows an exponential relation: L V exp(0.054 b ) Vc H
(4)
However, it should be noticed that as the tunnel width is not changed systematically in [16-18] and accordingly not included in Eqs. (3) and (4), whether the tunnel width influences the backlayering length is still unknown. 5
Vauquelin and Wu [19] carried out experiments on two scale models (a thermal model using a propane gas flame to simulate the fire and a densimetric model in which the fire-induced smoke was represented by a continuous release of an isothermal buoyant mixing) to study the influence of tunnel width on critical velocity (for a given tunnel height). Complementary CFD calculations were also presented in order to describe the influence of the lateral confinement on smoke plume spreading. However, in both the scale models and CFD calculations, the possible effect of tunnel width change on the HRR was ignored (gas burners with fixed heat release rates cannot reflect the radiation heat feedback, and compared with gas burner whose flow rate is manually controlled, liquid pool fires and wood crib fires are more close to practical situation), which may influence the accuracy of the results. Overall, both tunnel height and width may have a significant effect on the smoke backlayering length. Unfortunately, no studies have been performed where these parameters have been systematically varied. This study is aimed to establish an integral model of backlayering length considering the effect of both tunnel height and width in a longitudinal ventilation tunnel. The results of this study will enrich the knowledge of smoke movement characteristics in tunnels. The integral model of backlayering length will provide a theoretical support about how to control the fire-induced smoke in vehicular tunnels with different heights and widths, such as calculating the total thrust of jet fans to satisfy the need of ventilation and safe evacuation. Moreover, it should be noticed that this study is focused on the case of large heat release rate, where the flame impinges on the ceiling and the continuous flame volume (combustion zone) extends along the ceiling.
2. Experimental set-up
Froude scaling was used in the present study [20, 21]. The model scale tunnel was built in scale 1:20, which means that the size of the tunnel is scaled geometrically according to this ratio. General 6
information about scaling theories can be obtained from [20-27]. The key parameters considered in the study and how they are scaled between the model scale and the full scale are presented in Table 1. .
Table 1 A list of scaling correlations for the model tunnel. Type of unit
Scaling model
Equation number
Heat Release Rate (kW)
QF QM ( LF / LM )5/2
(5)
Velocity (m/s)
VF VM ( LF / LM )1/ 2
(6)
Time (s)
tF tM ( LF / LM )1/2
(7)
Temperature (K)
TF TM
(8)
The tunnel, as shown in Fig. 2, was 10 m long and the tunnel width and height were varied during the test series. The widths were 0.3, 0.45 and 0.6 m, and the heights were 0.25 and 0.4 m. The ceiling, floor and one of the walls were made of Promatect H boards. The other wall was comprised of windows of fireproof glass. A longitudinal ventilation system was established in the upstream end of the tunnel. The fuel, wood or heptane, was positioned at the tunnel centre, i.e. 5 m from the inlet and outlet of the tunnel. The wood crib was constructed of four layers of long sticks and three layers of short sticks. In some tests, all short wood sticks were replaced by plastic (polyethene) with the same dimensions. In heptane tests, an almost square pool (0.155 m × 0.160 m), standing directly on the Promatect H board connected to the weighing scale, was used.
7
1250
1250
1250
1250
1250
1250
1250
1250 100
Window
x3 30
x3
0.5H
Air flow Fire
load cell
1000
220mm
172mm 125mm
30mm
78mm
300mm
350mm
thermocouple pile
100
880
thermocouple
velocity
thermocouple pile
Plate thermometer
heat flux gage
gasanalysis
wall thermocouple
target
FBG
Fig. 2 Side view of the model scale tunnel with positions of measurements (Dimensions in mm). Gas temperatures were measured under the ceiling longitudinally from -3.75 m to +3.75 m, as shown in Fig. 2. Bare thermocouples (Type K, 0.25 mm in diameter) were used. Type K is suitable for continuous measurements within the temperature range from -250 °C to 1260 °C. The ambient temperature was around 20 °C in all tests. The backlayering length was determined by measurement of gas temperature under the ceiling, owing to the fact that gas temperature distribution under the ceiling has a sharp decrease at the upstream smoke front [18], as shown in Fig. 3.
800
Smoke backlayering
T (℃)
600
400
Smoke front (sharp change of temperature)
200
Fuel center 0
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
x (m)
Fig. 3 Determination of backlayering length based on temperature change. 8
A total of 26 tests in tunnels were performed, as shown in Table 2 with details on the test series and how the parameters were varied. The HRRs were determined using the method of fuel weight loss. For all tests, the HRRs are between 63 kW and 214.8 kW, and their corresponding values in the real (full scale) tunnel are between 112.7 MW and 384.2 MW. For this series of tests, some typical cases were repeated twice and the results (such as HRR and temperature) presented good repeatability with discrepancy less than 4 %. Table 2 Description of the test conditions in tunnels.
Test
V
H
W
A
QM
QF
Lb
(m/s)
(m)
(m)
(m2)
(kW)
(MW)
(m)
Fuel
1
Heptane
0.67
0.25
0.3
0.08
185.3
331.5
0.50
2
Heptane
0.67
0.25
0.45
0.11
172.2
308.0
0.50
3
Heptane
0.67
0.25
0.6
0.15
214.8
384.3
0.75
4
Heptane
0.67
0.4
0.3
0.12
155.8
278.7
1.00
5
Heptane
0.67
0.4
0.45
0.18
139.4
249.4
2.75
6
Heptane
0.5
0.4
0.6
0.24
162.4
290.5
3.75
7
Wood crib
0
0.25
0.45
0.11
76.8
137.4
3.75
8
Wood crib
0.22
0.25
0.3
0.08
63.0
112.7
3.75
9
Wood crib
0.22
0.25
0.45
0.11
67.8
121.3
3.75
10
Wood crib
0.22
0.25
0.6
0.15
71.4
127.7
3.75
11
Wood crib
0.22
0.4
0.3
0.12
82.2
147.0
3.75
12
Wood crib
0.22
0.4
0.45
0.18
79.8
142.8
3.75
13
Wood crib
0.22
0.4
0.6
0.24
65.4
117.0
3.75
9
14
Wood crib
0.45
0.25
0.45
0.11
106.8
191.1
2.50
15
Wood crib
0.67
0.25
0.3
0.08
97.2
173.9
0.50
16
Wood crib
0.67
0.25
0.45
0.11
103.8
185.7
0.50
17
Wood crib
0.67
0.25
0.6
0.15
105.6
188.9
1.25
18
Wood crib
0.67
0.4
0.3
0.12
92.4
165.3
0.75
19
Wood crib
0.67
0.4
0.45
0.18
102.6
183.5
2.25
20
Wood crib
0.5
0.4
0.6
0.24
103.2
184.6
3.75
21
Wood crib
1.12
0.25
0.45
0.11
101.4
181.4
0.25
0.67
0.25
0.3
0.08
153.9
275.3
0.75
0.67
0.25
0.45
0.11
150.6
269.4
0.75
0.67
0.25
0.6
0.15
172.2
308.0
1.00
0.67
0.4
0.45
0.18
172.2
308.0
2.25
0.5
0.4
0.6
0.24
145.6
260.5
3.75
Wood + plastic 22 crib Wood + plastic 23 crib Wood + plastic 24 crib Wood + plastic 25 crib Wood + plastic 26 crib It should be noted that in our former work [28], a detailed analysis was performed in order to study the effect of tunnel cross section (width and height) together with ventilation velocity on ceiling gas temperatures and heat fluxes. Test results show that the maximum temperature under the ceiling is a weak function of HRR and ventilation velocity for cases with HRR more than 100 MW at full scale. It clearly varies with the tunnel height and is a weak function of the tunnel width. With a lower tunnel 10
height, the ceiling is closer to the base of continuous flame zone and the temperatures become higher. Overall, the gas temperature beneath the ceiling decreases with increasing tunnel dimensions, and increases with increasing longitudinal ventilation velocity. The HRR is also an important factor that influences the decay rate of excess gas temperature, and a dimensionless HRR integrating HRR and other two key parameters, tunnel cross-sectional area and distance between fuel centre and tunnel ceiling, was introduced to account for the effect. An equation for the decay rate of excess gas temperature, considering both the tunnel dimensions and HRR, was developed. Moreover, a larger tunnel cross-sectional area will lead to a smaller heat flux.
3. Results and discussion
To realise the effect of tunnel cross section on the backlayering length clearly, Fig. 4 shows the backlayering length as a function of tunnel cross-sectional area under the same ventilation velocity (0.67 m/s). Generally speaking, in tests with the same fuel (heptane, wood crib or wood/plastic crib) and ventilation velocity, the backlayering length trends to increase with increasing tunnel cross-sectional area when the tunnel cross-sectional area is greater than 0.1125 m2, namely the second smallest tunnel cross section, under which the critical velocity is reached. The reason should be the blockage effect of flame on the ventilation airflow near the fuel. As the tunnel height or width increases, the blockage rate (flame surface to tunnel cross section) inside the tunnel is reduced, thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In fact, assuming ventilation flow in tunnels is a steady incompressible flow, the ventilation velocity will change with tunnel cross-sectional area [18, 29, 30]: VA Vlocal Alocal Vlocal ( A Afire ) Vlocal A(1 ) 11
(9)
where is the tunnel blockage rate, namely Afire / A . When the ventilation airflow passes through the fire position, the local ventilation velocity under the ceiling will be increased due to the existence of fire, and Eq. (9) can be rewritten as: Vlocal
V V Afire 1 1 A
(10)
where Vlocal V . When the cross-sectional area is very small (such as smaller than 0.1125 m2), the influence of the fire blockage will be evident, leading to a relatively strong local ventilation airflow, under which no backlayering is formed, as shown in Fig. 4. As the cross-sectional area increases, the influence of the fire blockage will be reduced, and thus the local ventilation velocity will be smaller, as demonstrated by Eq. (10). As a result, the restriction effect of the ventilation on the smoke backlayering will be reduced. Therefore, the backlayering length will increase with increasing tunnel cross-sectional area in cases when the critical velocity is not reached, as shown in Fig. 4. More details on the blockage effect can be found in [18, 29, 30].
4
Lb (m)
3
2
1
Critical velocity case
0
-1 0.00
0.05
0.10
0.15 2
A (m )
(a) heptane, V=0.67 m/s
12
0.20
0.25
4
3
Lb (m)
2
1
Critical velocity case
0
-1 0.00
0.05
0.10
0.15
0.20
0.25
2
A (m )
(b) wood crib, V=0.67 m/s
4
Lb (m)
3
2
1
Critical velocity case
0
-1 0.00
0.05
0.10
0.15
0.20
0.25
2
A (m )
(c) wood/plastic crib, V=0.67 m/s Fig. 4 Backlayering length as a function of tunnel cross-sectional area under the same ventilation velocity. Fig. 5 shows the backlayering length as a function of ventilation velocity for wood crib under the same tunnel cross-sectional area (H=0.25 m, W=0.45 m). As the most distant thermocouple upstream of the fuel is 3.75 m away from the fuel center, the recorded maximum backlayering length, 3.75 m, shown in Fig. 5 and Table 2, means “at least 3.75 m” actually. It is easy to understand that the 13
backlayering length decreases with increasing ventilation velocity, due to that a larger ventilation velocity provides a greater resistance to the smoke from spreading upstream, as also proved by other researches [18, 31].
5
Smoke
4
Lb (m)
3
2
Critical velocity case
1
0
-1 -0.5
0.0
0.5
1.0
1.5
V (m/s)
Fig. 5 Backlayering length as a function of ventilation velocity for wood crib (H=0.25 m, W=0.45 m). Earlier researches have shown that the governing parameters for the backlayering length in a longitudinally ventilated tunnel are HRR, tunnel geometry and ventilation velocity. In general, the tunnel height was used as the characteristic length. Li et al. [18] found when the dimensionless HRR is more than 0.15, namely Q
Q ＞0.15 , the backlayering length is independent of the a c pTa g1/2 H 5/2
HRR, and depends only on the dimensionless ventilation velocity, V *
V gH
. The reason should be
that when the HRR is relatively small, buoyancy force in the smoke layer dominates the backlayering, which increases with increasing HRR. However, when the HRR increases to a certain level, the continuous or intermittent flames would reach the tunnel ceiling and occupy the upper part of the tunnel. The flames have the feature of constant flow speed, therefore the buoyancy force in the smoke 14
backlayering (the backlayering length correspondingly) would be insensitive to HRR. All dimensionless HRRs in this series of tests meet the definition (the dimensionless HRR is more than 0.15) of Li et al. [18]. Ingason and Li [31] proposed a simple equation to predict the backlayering length in a large tunnel fire. The relationship of dimensionless backlayering length, L*b
Lb , and dimensionless ventilation H
velocity can be expressed as: L*b 17.3ln(0.4 / V * )
(11)
Fig. 6 shows the comparison of measured dimensionless backlayering length in this series of tests with Eq. (11), which demonstrates most scatters deviate from the equal line beyond 20%. As a whole, Eq. (11) overestimates the dimensionless backlayering length for this series of tests with different fuels, ventilation velocities and tunnel cross sections. By carefully examining Eq. (11), it is found that it lacks the effect of tunnel width on the backlayering length.
20% error bar
10
Heptane Wood Wood+plastic
Calculated Lb of all tests (m)
8
20% error bar
6
4
2
0
0
2
4
6
8
10
Measured Lb of all tests (m)
Fig. 6 Comparison of measured dimensionless backlayering length with Eq. (11). In order to establish an integral model of backlayering length considering the effect of both tunnel height and width, new dimensionless forms of backlayering length and ventilation velocity are 15
introduced： Lb H
L** b
V **
(12)
V
(13)
gH which modify the common used characteristic length, tunnel height, as hydraulic diameter of tunnel ( H 4 A / P ) to reveal the effect of tunnel cross section (actually both tunnel height and width). Fig. 7 shows the modified dimensionless backlayering length as a function of modified dimensionless ventilation velocity. When V ** 0.38 , the backlayering is eliminated, proving the critical velocity is reached. When V **＜0.38 , the proposed line in Fig. 7 can correlate well with all the data from the model scale tests with different fuels, ventilation velocities and tunnel cross sections, and also Runehamar full scale tunnel tests (data shown in Table 3) [32]. The proposed equation shown in Fig. 7 can be expressed as:
L**b 30 79V **
V **＜0.38
(14)
It should be noticed that Eq. (14) is suitable at higher HRRs (the flame impinges on the ceiling) where the backlayering length is independent of the HRR, as stated above.
18
Heptane Wood Wood+plastic Runehamar tests
15
12 **
L L b H ** b
V = 0.38 9
6
Smoke
3 ** L** b 30 79V
Lb= 0
0
-3 -0.2
0.0
0.2
0.4
V **
V gH
16
0.6
0.8
Fig. 7 Modified dimensionless backlayering length as a function of modified dimensionless ventilation velocity.
Table 3 Description of Runehamar full scale tunnel tests [32].
Test
V
H
W
A
Qmax
Lb
(m/s)
(m)
(m)
(m2)
(MW)
(m)
1.93
6
9
54
202
~100
1.99
6
9
54
157
~100
2.04
6
9
54
119
~100
2.09
6
9
54
66
~100
Fuel
Wood, PE plastic and 1 polyester tarpaulin Wood, PUR mattresses and 2 polyester tarpaulin Furniture and fixtures, 3
rubber tyres and polyester tarpaulin Corrugated paper cartons, polystyrene (PS) cups,
4 wood, and polyester tarpaulin
4. Conclusion
Model scale tests were carried out to investigate the backlayering length in longitudinally ventilated tunnels, where the flame impinges on the ceiling. The effects of tunnel height and width on the backlayering length were analyzed emphatically. The major conclusions are: 1. In tests with the same fuel and ventilation velocity, the backlayering length trends to increase 17
with the increasing tunnel cross-sectional area when the tunnel cross-sectional area is greater than a certain value, under which the critical velocity is reached (Fig. 4). As the tunnel height or width increases, the blockage rate (flame surface to tunnel cross section) inside the tunnel is reduced (Eq. (10)), thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In tests with the same tunnel cross-sectional area, the backlayering length decreases with the increasing ventilation velocity (Fig. 5), due to that a larger ventilation velocity provides a greater resistance to the smoke from spreading upstream. 2.
In order to establish an integral model of backlayering length considering the effect of both
tunnel height and width (Eq. (14)), new dimensionless forms of backlayering length and ventilation velocity were introduced (Eqs. (12) and (13)), which modify the common used characteristic length, tunnel height, as hydraulic diameter of tunnel to reveal the effect of tunnel cross section (actually both tunnel height and width). Data of backlayering length in Runehamar full scale tunnel tests was also demonstrated to validate the correlation.
Acknowledgments
This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 51608163, Fujian transportation science and technology project under Grant No. 201526, and the Fundamental Research Funds for the Central Universities.References
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1972, pp. 1021-1030. [21] J.G. Quintiere, Scaling Applications in Fire Research, Fire Safety Journal, 15 (1989) 3-29. [22] J. Ji, X. Yuan, K. Li, J. Sun, Influence of the external wind on flame shapes of n-heptane pool fires in long passage connected to a shaft, Combustion and Flame, 162 (2015) 2098-2107. [23] P. Zhu, X. Wang, C. Tao, Experiment study on the burning rates of ethanol square pool fires affected by wall insulation and oblique airflow, Experimental Thermal and Fluid Science, 61 (2015) 259-268. [24] J. Ji, M. Li, Y. Li, J. Zhu, J. Sun, Transport characteristics of thermal plume driven by turbulent mixing in stairwell, International Journal of Thermal Sciences, 89 (2015) 264-271. [25] L.J. Li, J. Ji, C.G. Fan, J.H. Sun, X.Y. Yuan, W.X. Shi, Experimental investigation on the characteristics of buoyant plume movement in a stairwell with multiple openings, Energy and Buildings, 68 (2014) 108-120. [26] W. Shi, J. Ji, J. Sun, S. Lo, L. Li, X. Yuan, Influence of fire power and window position on smoke movement mechanisms and temperature distribution in an emergency staircase, Energy and Buildings, 79 (2014) 132-142. [27] D. Yang, R. Huo, X.L. Zhang, S. Zhu, X.Y. Zhao, Comparative study on carbon monoxide stratification and thermal stratification in a horizontal channel fire, Building and Environment, 49 (2012) 1-8. [28] C.G. Fan, Y.Z. Li, H. Ingason, A. Lönnermark, Effect of tunnel cross section on gas temperatures and heat fluxes in case of large heat release rate, Applied Thermal Engineering, 93 (2016) 405-415. [29] L. Li, X. Cheng, Y. Cui, S. Li, H. Zhang, Effect of blockage ratio on critical velocity in tunnel fires, Journal of Fire Sciences, 30 (2012) 413-427. 21
[30] Y.P. Lee, K.C. Tsai, Effect of vehicular blockage on critical ventilation velocity and tunnel fire behavior in longitudinally ventilated tunnels, Fire Safety Journal, 53 (2012) 35-42. [31] H. Ingason, Y.Z. Li, Model scale tunnel fire tests with longitudinal ventilation, Fire Safety Journal, 45 (2010) 371-384. [32] H. Ingason, Y.Z. Li, A. Lönnermark, Runehamar tunnel fire tests, Fire Safety Journal, 71 (2015) 134-149.
22
Highlights Backlayering length increases with the increasing tunnel cross-sectional area. As tunnel height or width increases, blockage effect of flame inside tunnel is reduced. A smaller local ventilation velocity occurs with increasing tunnel cross-sectional area. New dimensionless forms of backlayering length and ventilation velocity are introduced. The new model can correlate well with data from both full and model scale tests.
23
S0894-1777(16)30335-1 http://dx.doi.org/10.1016/j.expthermflusci.2016.11.019 ETF 8941
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
14 July 2016 1 November 2016 19 November 2016
Please cite this article as: C.G. Fan, J. Yang, Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/ 10.1016/j.expthermflusci.2016.11.019
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Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling
Chuan Gang Fan1, * Jian Yang2 1. School of Automotive and Transportation, Hefei University of Technology, Hefei, China 2. College of Civil Engineering, Fuzhou University, Fuzhou, China Corresponding Author: Chuan Gang Fan. Address: School of Automotive and Transportation Engineering, Hefei University of Technology, Hefei 230009, Anhui, China. Tel: +86-551-0551-62901960 (Office), +86-18255170802 (Mobile) Email: [email protected]
1
Abstract Model scale tests were performed with varying tunnel widths and heights in order to study the effect of tunnel cross section and ventilation velocity on the thermal smoke backlayering length in case of large heat release rate. The results showed that in tests with the same fuel and ventilation velocity, the backlayering length trends to increase with increasing tunnel cross-sectional area when the tunnel cross-sectional area reaches a certain value, under which the critical velocity is obtained. The reason should be as the tunnel height or width increases, the blockage effect of flame inside the tunnel is reduced, thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In order to establish an integral model of backlayering length considering the effect of both tunnel height and width, new dimensionless forms of backlayering length and ventilation velocity, including the hydraulic diameter of tunnel, are introduced. The proposed equation can correlate well with all the data from the model scale tests with different fuels, ventilation velocities and tunnel cross sections, and also Runehamar full scale tunnel tests. Key words: heat release rate, tunnel cross section, mechanical ventilation, critical velocity, backlayering length
Nomenclature
A
tunnel cross-sectional area (m2)
Alocal
local tunnel cross-sectional area (m2)
cp
heat capacity (kJ/kg K)
Fr
Froude number 2
g
gravity acceleration (m/s2)
H
tunnel height (m)
Hf
distance between fuel center and tunnel ceiling (m)
H
hydraulic diameter of tunnel (m)
L
length scale (m)
Lb
backlayering length (m)
m
mass flow rate (kg/s)
P
tunnel perimeter (m)
Q
heat release rate (kW)
Ta
ambient temperature or surrounding gas temperature (K)
Tf
flame temperature (K)
t
time (s)
V
longitudinal ventilation velocity at ambient conditions (m/s)
Vc
critical velocity (m/s)
Vlocal
local ventilation velocity under the ceiling (m/s) V *
velocity W
tunnel width (m)
x
distance from fuel centre (m)
Greek a
air density (kg/m3) blockage ratio
Subscript F
full scale 3
dimensionless ventilation
M
model scale
max
maximum value
1. Introduction
In recent years, more and more tunnels are being built to ease traffic congestion and segregate hazardous goods transportation in urban areas [1-7]. The number and aggregate length of road, rail and subway tunnels in the world are increasing quickly. However, tunnels bring us not only convenience but also new research needs of energy balance and thermal flow movement. In urban road tunnels, the heat emitted by heavy traffic would easily cause temperature and concentration of harmful gas to rise to intolerable levels with ineffective ventilation. Longitudinal ventilation with jet fans is the most commonly used ventilation mode in tunnels. The longitudinal airflow induced by jet fans is the key factor in the control of heat and smoke movement. For example, when the longitudinal velocity is high enough, the fire-induced smoke will be prevented from spreading upstream and then the smoke backlayering upstream of the fire is eliminated, thus the critical velocity is reached. When the longitudinal velocity is less than the critical velocity, a smoke backlayering will form, as shown in Fig. 1. Lb
V
Fig. 1 Schematic diagram of smoke backlayering. The backlayering length is a popular subject in the tunnel research and previous studies have investigated the backlayering length extensively [8-15], as the main aim of ventilation system is to 4
confine the smoke in an acceptable zone to provide a clean route for people evacuation, fire rescue and extinguishment. Thomas [16] presented a theoretical analysis of the backlayering length in a longitudinally ventilated tunnel fire and proposed a dimensionless relation: Lb gHQ H aT f c pV 3 A
(1)
In fact, as A HW , Eq. (1) can be rewritten as: Lb gQ H aT f c pV 3W
(2)
From Eq. (2), it can be seen that heat release rate (HRR), tunnel height and width, and longitudinal velocity are key parameters influencing the backlayering length. Vantelon et al. [17] carried out small-scale experiments in a 1.5 m long semicircular pipe with 0.15 m radius, and found that the ratio of backlayering length to tunnel height varied as 0.3 power of a modified Richardson number: Lb gQ Ri '0.3 ( )0.3 3 H aTa c pV H
(3)
Li et al. [18] conducted experimental tests and theoretical analyses to investigate the critical velocity together with the backlayering length in tunnel fires. Experimental data show that the relation between the ratio of ventilation velocity to critical velocity and the dimensionless backlayering length follows an exponential relation: L V exp(0.054 b ) Vc H
(4)
However, it should be noticed that as the tunnel width is not changed systematically in [16-18] and accordingly not included in Eqs. (3) and (4), whether the tunnel width influences the backlayering length is still unknown. 5
Vauquelin and Wu [19] carried out experiments on two scale models (a thermal model using a propane gas flame to simulate the fire and a densimetric model in which the fire-induced smoke was represented by a continuous release of an isothermal buoyant mixing) to study the influence of tunnel width on critical velocity (for a given tunnel height). Complementary CFD calculations were also presented in order to describe the influence of the lateral confinement on smoke plume spreading. However, in both the scale models and CFD calculations, the possible effect of tunnel width change on the HRR was ignored (gas burners with fixed heat release rates cannot reflect the radiation heat feedback, and compared with gas burner whose flow rate is manually controlled, liquid pool fires and wood crib fires are more close to practical situation), which may influence the accuracy of the results. Overall, both tunnel height and width may have a significant effect on the smoke backlayering length. Unfortunately, no studies have been performed where these parameters have been systematically varied. This study is aimed to establish an integral model of backlayering length considering the effect of both tunnel height and width in a longitudinal ventilation tunnel. The results of this study will enrich the knowledge of smoke movement characteristics in tunnels. The integral model of backlayering length will provide a theoretical support about how to control the fire-induced smoke in vehicular tunnels with different heights and widths, such as calculating the total thrust of jet fans to satisfy the need of ventilation and safe evacuation. Moreover, it should be noticed that this study is focused on the case of large heat release rate, where the flame impinges on the ceiling and the continuous flame volume (combustion zone) extends along the ceiling.
2. Experimental set-up
Froude scaling was used in the present study [20, 21]. The model scale tunnel was built in scale 1:20, which means that the size of the tunnel is scaled geometrically according to this ratio. General 6
information about scaling theories can be obtained from [20-27]. The key parameters considered in the study and how they are scaled between the model scale and the full scale are presented in Table 1. .
Table 1 A list of scaling correlations for the model tunnel. Type of unit
Scaling model
Equation number
Heat Release Rate (kW)
QF QM ( LF / LM )5/2
(5)
Velocity (m/s)
VF VM ( LF / LM )1/ 2
(6)
Time (s)
tF tM ( LF / LM )1/2
(7)
Temperature (K)
TF TM
(8)
The tunnel, as shown in Fig. 2, was 10 m long and the tunnel width and height were varied during the test series. The widths were 0.3, 0.45 and 0.6 m, and the heights were 0.25 and 0.4 m. The ceiling, floor and one of the walls were made of Promatect H boards. The other wall was comprised of windows of fireproof glass. A longitudinal ventilation system was established in the upstream end of the tunnel. The fuel, wood or heptane, was positioned at the tunnel centre, i.e. 5 m from the inlet and outlet of the tunnel. The wood crib was constructed of four layers of long sticks and three layers of short sticks. In some tests, all short wood sticks were replaced by plastic (polyethene) with the same dimensions. In heptane tests, an almost square pool (0.155 m × 0.160 m), standing directly on the Promatect H board connected to the weighing scale, was used.
7
1250
1250
1250
1250
1250
1250
1250
1250 100
Window
x3 30
x3
0.5H
Air flow Fire
load cell
1000
220mm
172mm 125mm
30mm
78mm
300mm
350mm
thermocouple pile
100
880
thermocouple
velocity
thermocouple pile
Plate thermometer
heat flux gage
gasanalysis
wall thermocouple
target
FBG
Fig. 2 Side view of the model scale tunnel with positions of measurements (Dimensions in mm). Gas temperatures were measured under the ceiling longitudinally from -3.75 m to +3.75 m, as shown in Fig. 2. Bare thermocouples (Type K, 0.25 mm in diameter) were used. Type K is suitable for continuous measurements within the temperature range from -250 °C to 1260 °C. The ambient temperature was around 20 °C in all tests. The backlayering length was determined by measurement of gas temperature under the ceiling, owing to the fact that gas temperature distribution under the ceiling has a sharp decrease at the upstream smoke front [18], as shown in Fig. 3.
800
Smoke backlayering
T (℃)
600
400
Smoke front (sharp change of temperature)
200
Fuel center 0
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
x (m)
Fig. 3 Determination of backlayering length based on temperature change. 8
A total of 26 tests in tunnels were performed, as shown in Table 2 with details on the test series and how the parameters were varied. The HRRs were determined using the method of fuel weight loss. For all tests, the HRRs are between 63 kW and 214.8 kW, and their corresponding values in the real (full scale) tunnel are between 112.7 MW and 384.2 MW. For this series of tests, some typical cases were repeated twice and the results (such as HRR and temperature) presented good repeatability with discrepancy less than 4 %. Table 2 Description of the test conditions in tunnels.
Test
V
H
W
A
QM
QF
Lb
(m/s)
(m)
(m)
(m2)
(kW)
(MW)
(m)
Fuel
1
Heptane
0.67
0.25
0.3
0.08
185.3
331.5
0.50
2
Heptane
0.67
0.25
0.45
0.11
172.2
308.0
0.50
3
Heptane
0.67
0.25
0.6
0.15
214.8
384.3
0.75
4
Heptane
0.67
0.4
0.3
0.12
155.8
278.7
1.00
5
Heptane
0.67
0.4
0.45
0.18
139.4
249.4
2.75
6
Heptane
0.5
0.4
0.6
0.24
162.4
290.5
3.75
7
Wood crib
0
0.25
0.45
0.11
76.8
137.4
3.75
8
Wood crib
0.22
0.25
0.3
0.08
63.0
112.7
3.75
9
Wood crib
0.22
0.25
0.45
0.11
67.8
121.3
3.75
10
Wood crib
0.22
0.25
0.6
0.15
71.4
127.7
3.75
11
Wood crib
0.22
0.4
0.3
0.12
82.2
147.0
3.75
12
Wood crib
0.22
0.4
0.45
0.18
79.8
142.8
3.75
13
Wood crib
0.22
0.4
0.6
0.24
65.4
117.0
3.75
9
14
Wood crib
0.45
0.25
0.45
0.11
106.8
191.1
2.50
15
Wood crib
0.67
0.25
0.3
0.08
97.2
173.9
0.50
16
Wood crib
0.67
0.25
0.45
0.11
103.8
185.7
0.50
17
Wood crib
0.67
0.25
0.6
0.15
105.6
188.9
1.25
18
Wood crib
0.67
0.4
0.3
0.12
92.4
165.3
0.75
19
Wood crib
0.67
0.4
0.45
0.18
102.6
183.5
2.25
20
Wood crib
0.5
0.4
0.6
0.24
103.2
184.6
3.75
21
Wood crib
1.12
0.25
0.45
0.11
101.4
181.4
0.25
0.67
0.25
0.3
0.08
153.9
275.3
0.75
0.67
0.25
0.45
0.11
150.6
269.4
0.75
0.67
0.25
0.6
0.15
172.2
308.0
1.00
0.67
0.4
0.45
0.18
172.2
308.0
2.25
0.5
0.4
0.6
0.24
145.6
260.5
3.75
Wood + plastic 22 crib Wood + plastic 23 crib Wood + plastic 24 crib Wood + plastic 25 crib Wood + plastic 26 crib It should be noted that in our former work [28], a detailed analysis was performed in order to study the effect of tunnel cross section (width and height) together with ventilation velocity on ceiling gas temperatures and heat fluxes. Test results show that the maximum temperature under the ceiling is a weak function of HRR and ventilation velocity for cases with HRR more than 100 MW at full scale. It clearly varies with the tunnel height and is a weak function of the tunnel width. With a lower tunnel 10
height, the ceiling is closer to the base of continuous flame zone and the temperatures become higher. Overall, the gas temperature beneath the ceiling decreases with increasing tunnel dimensions, and increases with increasing longitudinal ventilation velocity. The HRR is also an important factor that influences the decay rate of excess gas temperature, and a dimensionless HRR integrating HRR and other two key parameters, tunnel cross-sectional area and distance between fuel centre and tunnel ceiling, was introduced to account for the effect. An equation for the decay rate of excess gas temperature, considering both the tunnel dimensions and HRR, was developed. Moreover, a larger tunnel cross-sectional area will lead to a smaller heat flux.
3. Results and discussion
To realise the effect of tunnel cross section on the backlayering length clearly, Fig. 4 shows the backlayering length as a function of tunnel cross-sectional area under the same ventilation velocity (0.67 m/s). Generally speaking, in tests with the same fuel (heptane, wood crib or wood/plastic crib) and ventilation velocity, the backlayering length trends to increase with increasing tunnel cross-sectional area when the tunnel cross-sectional area is greater than 0.1125 m2, namely the second smallest tunnel cross section, under which the critical velocity is reached. The reason should be the blockage effect of flame on the ventilation airflow near the fuel. As the tunnel height or width increases, the blockage rate (flame surface to tunnel cross section) inside the tunnel is reduced, thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In fact, assuming ventilation flow in tunnels is a steady incompressible flow, the ventilation velocity will change with tunnel cross-sectional area [18, 29, 30]: VA Vlocal Alocal Vlocal ( A Afire ) Vlocal A(1 ) 11
(9)
where is the tunnel blockage rate, namely Afire / A . When the ventilation airflow passes through the fire position, the local ventilation velocity under the ceiling will be increased due to the existence of fire, and Eq. (9) can be rewritten as: Vlocal
V V Afire 1 1 A
(10)
where Vlocal V . When the cross-sectional area is very small (such as smaller than 0.1125 m2), the influence of the fire blockage will be evident, leading to a relatively strong local ventilation airflow, under which no backlayering is formed, as shown in Fig. 4. As the cross-sectional area increases, the influence of the fire blockage will be reduced, and thus the local ventilation velocity will be smaller, as demonstrated by Eq. (10). As a result, the restriction effect of the ventilation on the smoke backlayering will be reduced. Therefore, the backlayering length will increase with increasing tunnel cross-sectional area in cases when the critical velocity is not reached, as shown in Fig. 4. More details on the blockage effect can be found in [18, 29, 30].
4
Lb (m)
3
2
1
Critical velocity case
0
-1 0.00
0.05
0.10
0.15 2
A (m )
(a) heptane, V=0.67 m/s
12
0.20
0.25
4
3
Lb (m)
2
1
Critical velocity case
0
-1 0.00
0.05
0.10
0.15
0.20
0.25
2
A (m )
(b) wood crib, V=0.67 m/s
4
Lb (m)
3
2
1
Critical velocity case
0
-1 0.00
0.05
0.10
0.15
0.20
0.25
2
A (m )
(c) wood/plastic crib, V=0.67 m/s Fig. 4 Backlayering length as a function of tunnel cross-sectional area under the same ventilation velocity. Fig. 5 shows the backlayering length as a function of ventilation velocity for wood crib under the same tunnel cross-sectional area (H=0.25 m, W=0.45 m). As the most distant thermocouple upstream of the fuel is 3.75 m away from the fuel center, the recorded maximum backlayering length, 3.75 m, shown in Fig. 5 and Table 2, means “at least 3.75 m” actually. It is easy to understand that the 13
backlayering length decreases with increasing ventilation velocity, due to that a larger ventilation velocity provides a greater resistance to the smoke from spreading upstream, as also proved by other researches [18, 31].
5
Smoke
4
Lb (m)
3
2
Critical velocity case
1
0
-1 -0.5
0.0
0.5
1.0
1.5
V (m/s)
Fig. 5 Backlayering length as a function of ventilation velocity for wood crib (H=0.25 m, W=0.45 m). Earlier researches have shown that the governing parameters for the backlayering length in a longitudinally ventilated tunnel are HRR, tunnel geometry and ventilation velocity. In general, the tunnel height was used as the characteristic length. Li et al. [18] found when the dimensionless HRR is more than 0.15, namely Q
Q ＞0.15 , the backlayering length is independent of the a c pTa g1/2 H 5/2
HRR, and depends only on the dimensionless ventilation velocity, V *
V gH
. The reason should be
that when the HRR is relatively small, buoyancy force in the smoke layer dominates the backlayering, which increases with increasing HRR. However, when the HRR increases to a certain level, the continuous or intermittent flames would reach the tunnel ceiling and occupy the upper part of the tunnel. The flames have the feature of constant flow speed, therefore the buoyancy force in the smoke 14
backlayering (the backlayering length correspondingly) would be insensitive to HRR. All dimensionless HRRs in this series of tests meet the definition (the dimensionless HRR is more than 0.15) of Li et al. [18]. Ingason and Li [31] proposed a simple equation to predict the backlayering length in a large tunnel fire. The relationship of dimensionless backlayering length, L*b
Lb , and dimensionless ventilation H
velocity can be expressed as: L*b 17.3ln(0.4 / V * )
(11)
Fig. 6 shows the comparison of measured dimensionless backlayering length in this series of tests with Eq. (11), which demonstrates most scatters deviate from the equal line beyond 20%. As a whole, Eq. (11) overestimates the dimensionless backlayering length for this series of tests with different fuels, ventilation velocities and tunnel cross sections. By carefully examining Eq. (11), it is found that it lacks the effect of tunnel width on the backlayering length.
20% error bar
10
Heptane Wood Wood+plastic
Calculated Lb of all tests (m)
8
20% error bar
6
4
2
0
0
2
4
6
8
10
Measured Lb of all tests (m)
Fig. 6 Comparison of measured dimensionless backlayering length with Eq. (11). In order to establish an integral model of backlayering length considering the effect of both tunnel height and width, new dimensionless forms of backlayering length and ventilation velocity are 15
introduced： Lb H
L** b
V **
(12)
V
(13)
gH which modify the common used characteristic length, tunnel height, as hydraulic diameter of tunnel ( H 4 A / P ) to reveal the effect of tunnel cross section (actually both tunnel height and width). Fig. 7 shows the modified dimensionless backlayering length as a function of modified dimensionless ventilation velocity. When V ** 0.38 , the backlayering is eliminated, proving the critical velocity is reached. When V **＜0.38 , the proposed line in Fig. 7 can correlate well with all the data from the model scale tests with different fuels, ventilation velocities and tunnel cross sections, and also Runehamar full scale tunnel tests (data shown in Table 3) [32]. The proposed equation shown in Fig. 7 can be expressed as:
L**b 30 79V **
V **＜0.38
(14)
It should be noticed that Eq. (14) is suitable at higher HRRs (the flame impinges on the ceiling) where the backlayering length is independent of the HRR, as stated above.
18
Heptane Wood Wood+plastic Runehamar tests
15
12 **
L L b H ** b
V = 0.38 9
6
Smoke
3 ** L** b 30 79V
Lb= 0
0
-3 -0.2
0.0
0.2
0.4
V **
V gH
16
0.6
0.8
Fig. 7 Modified dimensionless backlayering length as a function of modified dimensionless ventilation velocity.
Table 3 Description of Runehamar full scale tunnel tests [32].
Test
V
H
W
A
Qmax
Lb
(m/s)
(m)
(m)
(m2)
(MW)
(m)
1.93
6
9
54
202
~100
1.99
6
9
54
157
~100
2.04
6
9
54
119
~100
2.09
6
9
54
66
~100
Fuel
Wood, PE plastic and 1 polyester tarpaulin Wood, PUR mattresses and 2 polyester tarpaulin Furniture and fixtures, 3
rubber tyres and polyester tarpaulin Corrugated paper cartons, polystyrene (PS) cups,
4 wood, and polyester tarpaulin
4. Conclusion
Model scale tests were carried out to investigate the backlayering length in longitudinally ventilated tunnels, where the flame impinges on the ceiling. The effects of tunnel height and width on the backlayering length were analyzed emphatically. The major conclusions are: 1. In tests with the same fuel and ventilation velocity, the backlayering length trends to increase 17
with the increasing tunnel cross-sectional area when the tunnel cross-sectional area is greater than a certain value, under which the critical velocity is reached (Fig. 4). As the tunnel height or width increases, the blockage rate (flame surface to tunnel cross section) inside the tunnel is reduced (Eq. (10)), thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In tests with the same tunnel cross-sectional area, the backlayering length decreases with the increasing ventilation velocity (Fig. 5), due to that a larger ventilation velocity provides a greater resistance to the smoke from spreading upstream. 2.
In order to establish an integral model of backlayering length considering the effect of both
tunnel height and width (Eq. (14)), new dimensionless forms of backlayering length and ventilation velocity were introduced (Eqs. (12) and (13)), which modify the common used characteristic length, tunnel height, as hydraulic diameter of tunnel to reveal the effect of tunnel cross section (actually both tunnel height and width). Data of backlayering length in Runehamar full scale tunnel tests was also demonstrated to validate the correlation.
Acknowledgments
This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 51608163, Fujian transportation science and technology project under Grant No. 201526, and the Fundamental Research Funds for the Central Universities.References
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Highlights Backlayering length increases with the increasing tunnel cross-sectional area. As tunnel height or width increases, blockage effect of flame inside tunnel is reduced. A smaller local ventilation velocity occurs with increasing tunnel cross-sectional area. New dimensionless forms of backlayering length and ventilation velocity are introduced. The new model can correlate well with data from both full and model scale tests.
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