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Experimental and numerical study on attenuation of thermal radiation from
large-scale pool fires by water mist curtain
Article in Journal of Fire Sciences · July 2015
DOI: 10.1177/0734904115585796
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Original Article
Experimental and numerical
study on attenuation of
thermal radiation from
large-scale pool fires by
water mist curtain
Journal of Fire Sciences
2015, Vol. 33(4) 269–289
Ó The Author(s) 2015
Reprints and permissions:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/0734904115585796
jfs.sagepub.com
Pei Zhu1, Xishi Wang1,2, Zhigang Wang1, Haiyong Cong1
and Xiaomin Ni1
Date received: 13 February 2015; accepted: 15 April 2015
Abstract
Full-scale experiments and numerical simulations were conducted to study the thermal radiation
attenuation from large fires by water mist curtain with low and intermediate pressures. Fire
dynamics simulator (version 6) was used for numerical simulations. A novel multi-injector nozzle
was designed to generate a homogeneous water mist curtain with low water consumption.
Water mist characteristics of one of the injectors were measured by Shadowgraphy technology.
A 1 3 1 m diesel pool fire was considered as the fire source. The experimental results show that
the water mist curtain has high thermal radiation attenuation efficiency, for example, about 82.7%
radiant heat flux could be attenuated by the water mist curtain with 2 MPa working pressure and
flow rate of 13.3 L/min. Comparisons with the experimental results show that the calculated radiant heat flux and temperature are slightly underestimated.
Keywords
Water mist curtain, thermal radiation attenuation, large pool fire, computational fluid dynamics
Introduction
Thermal radiation as a hazardous factor in fire scenarios, especially for large-scale fires, has
been received more attention not only on its thermal hazards to personnel and equipment
1
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, China
Collaborative Innovation Center for Urban Public Safety, Hefei, China
2
Corresponding author:
Xishi Wang, State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China.
Email: wxs@ustc.edu.cn
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Figure 1. Schematic diagram of thermal radiation attenuation by water mist curtain.
but also on its potential igniting of nearby combustible materials. Traditional methods on
fire compartmentation in buildings known as fire resisting shutter, fire door, and fire wall
being considered have limitations on personnel evacuation. Water mist as a clean and efficient fire extinguishing agent has been widely used in practical fire suppression applications.
The fire suppression mechanisms of water mist include removal of heat from the gases, the
displacement of oxygen, fuel surface cooling, attenuation of thermal radiation, and so on.1–4
The knowledge of thermal radiation attenuation by water mist can be utilized as thermal
radiation protection technology, as shown in Figure 1. So, water mist curtain would be a
better choice for fire compartmentation via attenuating flame thermal radiation and screening fire smoke; it also has the advantage of low water consumption. Therefore, the objective
of this article is to develop and evaluate a novel water mist curtain system, which is a more
realistic fire compartmentation technique for attenuating strong thermal radiation of a large
fire.
Many previous studies on the interaction between water droplets and thermal radiation
have been conducted. The study5 on radiation blockage by water curtain showed that the
radiation attenuation factor could reach 69.7% by drencher nozzle with working pressure of
0.6 MPa and water flow rate of 1.30 L/s, but the water consumption was too large. The
numerical and experimental studies6–15 have been conducted on the radiant heat transfer in
water mist curtain, which showed that the water volumetric fraction, the droplet size, the
residence time of the droplets, the width of curtain, and so on were the key parameters for
thermal radiation attenuation by water spray curtain. However, most of the previous experimental studies6,7 used a hot plate or radiation spectrum from a laser as a radiative source to
simplify the problem with laboratory scale tests, which were not validated in practical application. The previous numerical studies9–15 have the features that limit their applicability on
practical fire simulation. When considering the realistic fire scenario, the flow, radiation,
and temperature field in the space would become more complicated for simulations, while
the large-scale experimental data are still lacking. In addition, the traditional water mist curtain was mostly generated with multiple injectors along a line on the pipe,8,9,15 which would
generate inhomogeneous mist curtain and be easily affected by the numbers of the injector
in practical application.
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271
In this study, a novel multi-injector nozzle was designed, which could generate a homogeneous water mist curtain. The large-scale experiments were conducted to investigate the
attenuation of thermal radiation from a large realistic fire by this water mist curtain, and
the test results were then used to evaluate the properties of the novel water mist curtain nozzle. However, the targets of the full-scale experiment were limited, and the repeatability was
also poor due to the unstable nature of the fire. Numerical simulation analyses often serve
to explore complicated full-scale fires. Fire dynamics simulator (FDS)16,17 based on large
eddy simulation (LES) model was often used to solve Navier–Stokes (N-S) equations of the
fire plume with low Mach number. The model has been subjected to numerous validations
and studies18–20 on the simulation of flame radiation or fire suppression by water spray. The
recent studies21,22 on the simulation of water mist systems and large water-based fire suppression systems indicated that the ability of the new version of FDS on predicting the interaction of fire with water mist has been significantly improved. Thus, a series of numerical
simulations on water mist characteristics and their effects on thermal radiation attenuation
were conducted by FDS in this work. Comparisons were also performed with the full-scale
experiments to evaluate its applicability and precision. So, it is expected that the conclusions
of this study may have certain guidance for practical design and application of water mist
curtain system in fire safety protection.
Experimental descriptions
Measurements of water mist characteristics
Figure 2 shows the schematic diagram of water mist characterization by Particle-Master
Shadow system, which is based on high-resolution imaging with pulsed backlight illumination. Due to the difficulties caused by large field of water mist on optical measurement, only
one injector (with one orifice) of the water mist curtain nozzle was used during the characterization measurements. The single injector was placed 0.5 m above the measurement point
along the central axis. The diameter of the injector orifice was about 1.0 mm. The experiments were conducted with operating pressure of water mist system from 0.5 to 3.0 MPa.
Figure 3(a) shows the appearance of the water mist curtain nozzle, which consists of nine
injectors and distributed equally around two semicircle arcs, where the first arc has five
Figure 2. Schematic diagram of water mist characterization by Particle-Master Shadow system.
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(a)
(b)
(c)
Figure 3. Water mist curtain nozzle pattern: (a) shape and construction, (b) water mist curtain pattern,
and (c) design drawing.
injectors and the second one has four injectors. Figure 3(b) indicates that the relatively uniform and dense water mist curtain would be generated by this novel nozzle. Figure 3(c) presents the elaborate design drawing of the water mist curtain nozzle.
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273
Figure 4. Schematic of the experimental apparatus for radiation attenuation by water mist curtain.
Table 1. Cases of the full-scale experimental tests.
Test cases
Operating pressure (MPa)
Initial fuel mass (L)
Fuel pan size (m2)
Case 1
Case 2
Case 3
Case 4
0
1
2
3
4
4
4
4
1.0
1.0
1.0
1.0
3
3
3
3
1.0
1.0
1.0
1.0
On full-scale experiments
Full-scale experiments on flame radiation attenuation were performed in a large space as
schematically shown in Figure 4. The red frames in Figure 4 represent the open area and the
other surfaces were all walls. Two water mist curtain nozzles were installed at the top of the
open area between the two rooms to generate a water mist curtain which could cover the
whole open area. The width of the opening was 6 m and the two nozzles were evenly distributed with 2 m distance from the two ends, respectively. The fuel pan was placed at the middle
position in Room 1, and a square diesel oil pool fire with dimension of 1.0 3 1.0 m was used
as fire source, which was ignited with gasoline. The measurement system including one set of
K-type thermocouple trees and a radiometer (TS-30) was located in Room 2 about 4.5 m
away from the pool fire center. The total radiative heat flux was measured at 1 m above the
floor. Seven thermocouples were vertically installed from 0.5 to 2.6 m with 0.3 m interval.
The experimental test cases are specified in Table 1. Each test case was carried out at least
two times. In each test, the data acquisition system was activated first, after about 30 s, the
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Figure 5. Typical pattern of the full-scale experimental test: (a) water mist curtain without fire, (b) pool
fire without water mist curtain, and (c) pool fire with water mist curtain.
water mist curtain was activated and the fuel was ignited at the same time. Figure 5(a) shows
the water mist curtain without fire. Figure 5(b) shows the fire after ignition without water
mist curtain. Figure 5(c) shows the case of fire smoke shielding with water mist curtain.
Numerical simulations
Model descriptions
Physical phenomena related to thermal radiation attenuation by water mist curtain include
the thermal hydraulic transfer, turbulence mixing, chemical combustion, spray dynamics,
and radiative heat transfer. The basic governing equations and solution methods for gas
phase were described in FDS Technical Reference manual.16 LES with deardorff turbulence
model for turbulence and mixture fraction combustion model for fire were used in FDS
code. Only the water spray model and thermal radiation model are described briefly in the
following.
Water spray model. Water spray was modeled as an Eulerian–Lagrangian system, where the
gas phase was solved using an Eulerian method and the liquid phase was tracked as numerous Lagrangian particles with mass, momentum, and temperature. Ignoring buoyancy, lift,
and forces arising from the fluid acceleration, the motion of a single spherical droplet can be
governed by21
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Zhu et al.
275
dmp vp
= mp g + rg CD prp2 (vp vg )vp vg dt
ð1Þ
where mp is the mass of the droplet, vp is the velocity of the droplet, g is the gravitational
acceleration, rg is the density of the surrounding gas, vg is the velocity of the surrounding
gas, rp is the radius of the droplet, and CD is the drag coefficient.
The mass and energy transfer between the gas and the liquid droplets can be considered as
follows
dmp
= Ap hm rg (Ya, l Ya, g )
dt
dTp
dmp
1
=
hv
Ap h(Tg Tp ) + q_ r +
mp cp
dt
dt
ð2Þ
ð3Þ
where mp is the mass of the liquid droplet, Ap is the surface area of the liquid droplet, hm is
the mass transfer coefficient, rg is the gas density, cp is the liquid specific heat, h is the heat
transfer coefficient between the droplet and the gas, q_ r is the rate of radiative heating of the
droplet, and hv is the latent heat of vaporization of the liquid. The vapor mass fraction of the
gas, Ya, g , can be obtained from the gas phase mass transport equations, and the liquid equilibrium vapor mass fraction, Ya, l , can be obtained from the Clausius–Clapeyron equation.16
In addition, since there is no condensation in FDS code, the simulations would over-estimate
the evaporation of the small droplets.
Thermal radiation models. The radiative transport equation (RTE)16 for an absorbing, emitting, and scattering medium is
s rIl (x, s) =
k(x, l)Il (x, s)
|fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl}
ss (x, l)Il (x, s) +
|fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl}
Energy loss by absorption Energy loss by scattering
B(x, l)
|fflfflffl{zfflfflffl}
Emission source term
+
ð
ss (x, l)
F(s0 , s)Il (x, s0 )ds0
4p
4p
|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
ð4Þ
Inscattering term
where Il (x, s) is the radiation intensity at wavelength, l; s is the direction vector of the intensity; k(x, l) and ss (x, l) are the local absorption and scattering coefficients, respectively.
F(s0 , s) is the scattering phase function that gives the scattered intensity fraction from direction s0 to s; B(x, l) is the emission source term, describing how much heat is emitted by the
local mixture of gas, soot, and droplets. The emission source term for radiation band n can
be expressed as
Bn (x) = kn (x)Ib, n (x)
ð5Þ
In practical simulations, the spectral dependence of the RTE cannot be resolved accurately. The RTE for a non-scattering gas can be adapted by dividing the radiation spectrum
into several bands (N). Even with a reasonably small number of bands, solving multiple
RTEs is very time consuming. Fortunately, in most large-scale fire scenarios, soot dominates
the overall radiative properties of the fire and the hot smoke,23 so gray model was used for
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the radiation solver in this study. The spectral dependence is then lumped into one absorption coefficient (N = 1). Therefore, the source term can be expressed by the radiation intensity of blackbody
Ib (x) =
sT (x)4
p
ð6Þ
where s is the Stefan–Boltzmann constant and T (x) is the blackbody temperature. In largescale fire calculation, the temperature near the flame surface cannot be relied upon when
computing the source term in the radiation transport equation for large grid size. So, the
parameter of radiative fraction, xr , which was the fraction of energy released from the fire,
was used to amend the Emission Source Term, which has been described in detail in FDS
Technical Reference Guide.16
When considering the attenuation of thermal radiation by liquid droplets, the radiation–
spray interaction must be solved for both the accurate prediction of the radiation and the
energy balance of the droplets. So, the local absorption and scattering coefficients in equation (4) would include the absorption and scattering of water mist droplets. The detail solution for absorption and scattering coefficients of droplets based on Mie theory and the
numerical method for the RTE can be seen in the FDS Technical Reference Guide.16
Numerical simulations on water mist characteristics
Figure 6(a) shows the computational domain with dimension of 0.5 3 0.5 3 2.0 m, whose
sides were all open. The single injector was placed at the center and 1.5 m high above the
floor, and the samples were taken at 0.5 and 1.0 m under the injector. The initial conditions
were set as same as the experimental conditions.
The initial droplet velocities were calculated with
sffiffiffiffiffiffi
2P
v0 = C
ð7Þ
rl
where P is the operating pressure of the nozzle, rl is the density of the liquid, and C is taken
to be 0.95 to account for the friction losses in the nozzle.21 A user-defined cumulative number
fraction (CNF) based on experimental measurement data was used to describe the droplet
size distribution,16 which will be further described in the following parts. The other simulation parameters including mesh size (Dx), droplets per second (DPS) inserted into simulation,
and offset parameter (R) were determined through a series of grid sensitivity tests, as suggested by Sikanen et al.21 The multi-injector nozzle was modeled by positioning several single
injectors with different orientations at one point in the computational domain according to
the real water mist curtain nozzle (as shown in Figure 3(a) and (c)). Figure 6(b) gives the
simulated spray flow pattern of the water mist curtain, which corresponds to the real spray
pattern (as shown in Figure 3(b)).
Numerical simulations on full-scale fire experiment
The geometry of the openings and layout of the fire source were identical to the full-scale
experimental test. The diesel fuel was used in the experiments, and details of its properties
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Zhu et al.
277
Figure 6. Simulation on water mist curtain characteristics: (a) the computational domain and grid and (b)
the spray flow pattern of water mist curtain.
Table 2. The properties of diesel oil.
Fuel
Specific heat at
constant pressure
Flash point
Heat of combustion
Density
Boil point ( °C)
Diesel oil
2.1 kJ/kg °C
38 °C
44,400 kJ/kg
0.86 g/cm3
170–390 °C
can be seen in Table 2. Due to the limitations of the current experimental facilities, the burning rate of the diesel pool fire was predicted based on the empirical relationship (pool diameter, D . 0.2 m) proposed by Burgess et al.24
m_ emp = m_ ‘ (1 expkuD )
ð8Þ
where m_ ‘ is the burning rate for a pool fire with infinite diameter, k is the radiative emission
coefficient, and u is the mean beam-length corrector. The constant of heat release rate
(HRR), Q_ emp , can be obtained based on the empirical relationship as
Q_ emp = m_ emp Af DHC, eff
ð9Þ
where Af is the pool surface area and DHC, eff is the effective heat of combustion.
For diesel oil, following values as given in SFPE handbook were used in the calculations25
m_ ‘ = 0:045 kg=m2 s, DHC, eff = 44400 kJ=kg, ku = 2:1 m1
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2000
䊡
HRR curve in FDS
HRR(kw)
1500
1000
䊠
䊢
500
0
0
50
100
150
200
250
Time(s)
Figure 7. The HRR curve used in the simulation.
The total HRR was calculated to be 1.81 MW with equations (8) and (9), which was in
the steady burning state. The prescribed HRR was used in the simulation, and the growth of
HRR was estimated based on the fuel burning time and variation trend of measured radiative heat flux without water mist curtain (as shown in Figure 7), which includes three phases,
such as the growing phase (I), the steady phase (II; from 55 to 110 s), and the decay phase
(III).
One of the most significant factors influencing the solution accuracy and computing time
is the size of the computational grid specified by the user. The grid size near the fire source
was initially determined by the non-dimensional expression D*/dx,16 where dx is the nominal
size of a grid cell and D* is a characteristic fire diameter and it can be expressed as
2=5
Q_
D =
pffiffiffi
r‘ cp T‘ g
ð10Þ
where Q_ is the total HRR of the fire, r‘ is the ambient air density, cp is the specific heat at
constant pressure, T‘ is the ambient temperature, and g is the gravitational acceleration.
When the grid size was taken as 0.1 D*, the average axle center velocity and temperature in
the LES model will meet Baum and McCaffrey26 experimental curve fitting equation. For
fire size Q_ = 1.81 MW, D* is computed to be 1.216 m, and then, 0.1 D* is approximately
0.1216 m, which can be taken as a reasonable grid size. The simulations without water mist
curtain were conducted in sensitivity studies with grid size from 0.083 to 0.25 m. Figure 8(a)
presents the predicted radiative heat flux variation with various mesh sizes for the cases without water mist curtain. The results of mesh size with 0.125, 0.1, and 0.083 m get slight differences, and there is no significant improvement but more time consuming when the mesh size
is smaller than 0.1 m. So, the final grid cell size of 0.1 m was selected in the simulations. In
addition, the solid angle number is also important for the accuracy of radiative transfer solver. Figure 8(b) shows the predicted radiative heat flux variation with various solid angles
for grid size of 0.1 m without water mist curtain. Similarly, the solid angle number 500 was
used in the simulations. Figure 8(c) shows the comparison of the predicted radiative heat flux
with the experimental data for fire test without water mist curtain.
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3.5
Without water mist
3.0
Grid size Dx
0.083m
0.10m
0.125m
0.167m
0.20m
0.25m
2.5
2.0
1.5
1.0
0.5
0.0
0
50
100
150
Time(s)
200
250
Radiation heat flux(kw/m2)
Radiative heat flux(kw/m2)
Zhu et al.
3.0
Solid angles
100
300
500
800
1000
2.5
2.0
1.5
1.0
0.5
0.0
0
50
Radiative heat flux(kw/m2)
(a)
100 150
Time(s)
200
250
(b)
Steady stage
3.0
FDS
Exp
2.5
2.0
1.5
1.0
0.5
0.0
0
50
100 150
Time(s)
200
250
(c)
Figure 8. The simulations without water mist curtain: (a) predicted radiative heat flux variation with
different mesh sizes, (b) predicted radiative heat flux variation with different solid angles for grid size 0.1 m,
and (c) comparison of the simulated radiative heat flux with experimental data.
The error in the simulations is quantified as
e=
Expi Simi
Expi
ð11Þ
where Expi and Simi are the experimental measurements and simulation results, respectively.
So, the maximum error does not exceed 626.2% at the steady stage for the cases without
water mist curtain. So, it can be said that FDS code is acceptable in predicting the basic fire
characteristics in this study.
Results and discussions
Water mist characteristics
Single injector of water mist curtain nozzle. The measured results of single-injector nozzle are
shown in Table 3. It can be seen that all of the cumulative volume diameter of Dv90, Dv50,
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Table 3. Characteristics of the single injector of the water mist curtain nozzle.
Test case
Operating
pressure (MPa)
*
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
0.5
1.0
1.5
2.0
2.5
3.0
165
125
116
90
80
79
Dv90 (mm)
*
D32 (mm)
Spray
Angle (u)
Flow rate
Q (L/min)
114
87
77
60
59
58
88
72
65
54
53
53
22
22
22
21
20
20
0.74
1.01
1.20
1.44
1.58
1.75
Dv50 (mm)
*
Dvf represents the droplet diameter such that the cumulative volume, from zero diameter to this respective diameter, is
the fraction, f, of the corresponding sum of the total distribution.
and the Sauter mean diameter (D32) decrease with increase in the operating pressure. It
should be noted that the reduction of D32 is not obvious when the operating pressure exceeds
2.0 MPa. This means that to this kind of nozzle, it is not the better way to generate even
more fine water mist through continuing to increase operating pressure. In addition, the
changes of the spray angle are relatively small as the operating pressure is increased.
Another key factor of the nozzle is the flow coefficient, which is calculated by the following equation
Q
K = pffiffiffiffiffiffiffiffi
10P
ð12Þ
where K is the flow coefficient of nozzle (L/min/MPa0.5), Q is the flow rate of water (L/min),
and P is the water pressure (MPa) measured at the spray head. So, the average K factor of
the single injector was calculated as 0.32 L/min/MPa0.5. This small K factor indicates that
the water consumption of this nozzle is quite small.
Figure 9(a) and (b) shows some typical measured and simulated results of the droplets’
size distribution. Figure 9(c) gives the results of the droplets’ average velocity both of measured and simulated. The maximum differences between the measured and the simulated
results do not exceed 22%. So, it can be said that FDS can predict water mist characteristics
well.
Water mist curtain nozzle. The measurement results of general water mist curtain characteristics are shown in Table 4. From Table 4 and equation (5), the average K factor was obtained
as 3.0. It means that the flow rate of this novel water mist curtain nozzle is very small compared with the traditional water curtain nozzle.5,8
Results of the full-scale experiments and numerical simulations
Comparison of radiative heat flux. The large fire experiments were conducted in a large space (as
shown in Figures 4 and 5). The total radiative heat flux meter was located 1.0 m above the
floor which is about 40% of the flame height, where the thermal radiation from the flame
was supposed to be the largest one in vertical direction. When considering the interaction
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281
100
100
Experiment 2.5MPa
FDS 2.5MPa
80
% of Max
% of MAX
80
60
40
40
20
20
0
60
20
40
60
80
Diameter(μm)
100
0
120
0
20
40 60 80 100 120
Diameter(μm)
1.0
2.5MPa
1.5MPa
CNF
0.8
0.6
0.5MPa
FDS
Expleriment
0.4
0.2
Droplet velocity(m/s)
(a)
0.0
0 20 40 60 80 100120140160180
Diameter(μm)
9
8
7
6
5
4
3
2
Experiment
FDS
0.5
1.0
1.5
2.0
2.5
Operating pressure(MPa)
(b)
(c)
Figure 9. Typical results: experiment and simulation. (a) The histograms of droplets’ size distribution with
operating pressure of 2.5 MPa, (b) typical cumulative number distribution, and (c) centerline droplets’
average velocity.
Table 4. The general characteristics of water mist curtain.
Operating pressure (MPa)
Flow rate (L/min)
Injection length (m)
Mist curtain thickness (m)
1.0
1.5
2.0
2.5
3.0
10.0
12.5
13.3
14.2
15.5
3.0
3.6
3.8
4.3
4.5
0.35
0.38
0.40
0.50
0.50
between water mist and the thermal radiation, the parameter of DPS is also an important
factor for the solver of RTE.
Figure 10 presents the results of the predicted radiative heat flux with different DPS for
the cases with water mist curtain. The large DPS would lead to very time consuming simulations, so the DPS of 50,000 is relatively appropriate for all of the cases. In addition, it also
indicates that the radiative heat fluxes are underestimated slightly by simulations, especially
for the cases with high working pressure. Under high working pressure, the droplets would
be relatively small and the thermal radiation attenuation ability of small droplets is relatively
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1.6
Experiment
Simulation
DPS=5000
DPS=10000
DPS=30000
DPS=40000
DPS=50000
1MPa
1.2
0.8
0.4
0.0
0
50
100
Time(s)
150
200
250
Radiative heat flux(kw/m2)
Journal of Fire Sciences 33(4)
Radiative heat flux(kw/m2)
282
1.0
Experiment
Simulation
DPS=5000
DPS=10000
DPS=30000
DPS=40000
DPS=50000
2MPa
0.8
0.6
0.4
0.2
0.0
0
50
100
150
Time(s)
Radiative heat flux(kw/m2)
(a)
200
250
(b)
0.8
3MPa
Experiment
Simulation
DPS=5000
DPS=10000
DPS=30000
DPS=40000
DPS=50000
0.6
0.4
0.2
0.0
0
50
100
150
Time(s)
200
250
(c)
Figure 10. The comparisons between the predicted radiative heat flux and experimental measurements
with different DPS at different operating pressures: (a) 1MPa, (b) 2MPa, and(c) 3MPa.
large. Therefore, FDS would over-predict the radiative absorption and scattering by water
mist droplets, especially for small droplets.
In order to indicate the effects of the water mist curtain on the radiative heat flux measurement, the case with water mist curtain activation but without fire was conducted, as
shown in Figure 11(a) (from 220 to 0 s). It can be seen that the effects of water mist curtain
on the radiative heat flux measurement were small.
As shown in Figure 11(a), the measurement value of radiative heat flux is fluctuating to
both cases of experiments and FDS simulations, even though in a steady burning state. This
may be caused not only by the fluctuations of the fire but also by the fluctuations of the
spray dynamics. It can be also seen that the radiative heat flux value decreases with the
increase in operating pressure, but tends to the limit at about 2.0 MPa. Figure 11(b) presents
the time-average value of radiative heat flux in steady stage for experiments and simulations.
The error bars show the fluctuation of the measurement results. According to equation (11),
the errors between simulations and experiments in steady stage are shown in Figure 12,
which means that most errors are in the range from 230% to + 40%.
Comparison of thermal radiative attenuation efficiency. Generally, the total attenuation factor, At,
can be defined by
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283
3.0
2.5
2.0
Starting time for normal Experiment
experiments
No mist
1MPa
2MPa
3MPa
1.5
Simulation
No mist
1MPa
2MPa
3MPa
Steady stage
1.0
0.5
Without
fire
0.0
-50
0
50
100 150 200 250
Time(s)
3.0
Radiative heat flux(kw/m2)
Radiative heat flux(kw/m2)
Zhu et al.
Experiment
Simulation
2.5
2.0
1.5
1.0
0.5
0.0
0
1
2
Operating pressure(MPa)
(a)
3
(b)
Figure 11. Comparisons of the measured radiative heat flux with the simulated results: (a) temporal
evolution value and (b) time-average value.
No mist
1MPa
2MPa
3MPa
Eorror(%)
100
80
60
40
20
0
-20
-40
-60
-80
-100
50
60
70
80
90
100
110
Time(s)
Figure 12. The results of relative error between simulations and experiments.
At = 1 Iwith
Iwithout
3100%
ð13Þ
where Iwith is the radiative heat flux measurement value with water mist curtain and Iwithout is
the radiative heat flux measurement value without water mist curtain. Figure 13(a) shows the
temporal evolution of the attenuation factor for all cases calculated by equation (13). For
the initial stage, the predicted value is larger than the measured one, which may be caused by
the slightly fast growth of the fire in simulation. In the steady burning state, the predicted
value is in relatively good agreement with the measured one. In the decay stage, the predicted
value is smaller due to the small radiation and the unsteady flow field of the flame. The timeaverage attenuation factor in the steady stage was used to evaluate the attenuation efficiency
of the water mist curtain. Figure 13(b) presents the time-average attenuation factors for all
cases. It can be seen that the attenuation factor increases with the increase in operating
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Journal of Fire Sciences 33(4)
Attenuation facor (%)
120
Steady stage
100
80
60
40
Experiment
1MPa
2MPa
3MPa
Simulation
1MPa
2MPa
3MPa
20
0
-20
0
Attenuation factor(%)
284
140
120
Experiment
Simulation
100
80
60
40
20
0
Time
1
2
3
Operating pressure(MPa)
(a)
(b)
30 60 90 120 150 180 210 240 270
Figure 13. The attenuation factors between simulations and experiments: (a) temporal evolution value
and (b) time-average value.
pressure, which is mainly due to the decrease in droplets’ diameter and increase in mist curtain’s thickness and flow rate. For this novel water mist curtain, the attenuation factor
increases from 59% to 82.7% for operating pressure from 1.0 to 2.0 MPa and flow rate from
10 to 13.3 L/min. Continuing to increase the operating pressure to 3.0 MPa, the attenuation
factor only increases about 6%. So in the practical applications, the attenuation efficiency
has been enough high for the operating pressure of 2.0 MPa, which may be selected preferentially. Although the results of average attenuation factor shows very agreeable, the fluctuation of simulation results is relatively large and the instantaneous results between them are
also relatively large. Therefore, results of simulation would under-predict thermal radiation
attenuation slightly, but the general trend agrees well with the experimental results.
Comparison of temperature. Figure 14 shows the results of temperature variation measured by
the thermocouple trees and calculated with FDS code for different cases. The temperature
measurement points were located 0.5 to 2.6 m high above the floor. For the cases without
water mist curtain (as shown in Figure 14(a)), the temperature first increases and then
decreases gradually with time, but the temperature rise is relatively small, that is, less than
10 °C. The increase in the temperature is mainly due to the radiant heating. When the water
mist curtain is activated, the measured temperature would be affected by the mist droplets
and the smoke due to smoke decline. From Figure 14(b), it can be seen that the temperature
is disturbed in different heights for 1.0 MPa case and increases slightly at the upper part due
to smoke decline but decreases at the lower part due to droplets’ cooling. With continuous
increase in operating pressure, the mean droplets’ diameter would decrease but the droplets’
velocities increase, so the effects of droplets and smoke become more obvious due to spray
dynamic. As shown in Figure 14(c) and (d), the temperature rises slightly again, but the difference of temperature becomes small, which is mainly caused by the more smoke going
downward due to high mist momentum.
However, the maximum temperature rise for the cases with water mist curtain does not
exceed 5 °C compared with the case without water mist curtain. The comparisons between
the experiments and the simulations indicate that the simulated temperature variation trend
agrees well with the measured one, but the temperature value is slightly underestimated.
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Zhu et al.
285
30
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
No mist for experiment
28
24
24
22
20
20
18
18
0
50
100
150
Time(s)
200
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
26
22
16
No mist for FDS
28
T(䉝㻕
T(䉝㻕
26
30
16
250
0
50
100
150
Time(s)
200
250
(a)
32 1MPa for experiment
32
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
24
28
T(䉝㻕
T(䉝㻕
28
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
1MPa for FDS
20
24
20
16
16
0
50
100
150
200
250
0
50
Time(s)
100
150
200
250
Time(s)
(b)
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
T(䉝㻕
30
25
30
20
15
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
2 MPa for FDS
35
T(䉝㻕
2MPa for experiment
35
25
20
0
50
100
150
Time(s)
200
15
250
0
50
100 150
Time(s)
200
250
(c)
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
T(䉝㻕
30
25
30
25
20
20
15
0.8m
1.1m
1.4m
1.7m
2.0m
2.3m
2.6m
35 3 MPa for FDS
T(䉝㻕
3 MPa for experiment
35
0
50
100
150
200
15
250
Time(s)
0
50
100 150
Time(s)
200
250
(d)
Figure 14. Comparison of temperature variations between measured and simulated results: (a) without
water mist curtain, (b) with water mist curtain generated with 1.0 MPa pressure, (c) with water mist
curtain generated with 2.0 MPa pressure, and (d) with water mist curtain generated with 3.0 MPa pressure.
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286
Journal of Fire Sciences 33(4)
There is no condensation in FDS code, and droplets’ diameter of water mist in this study is
relative small. So, the relatively low temperature in simulation may be due to the overestimation of the droplets’ evaporation.
Conclusion
Thermal radiation attenuation by water mist curtain has been studied through full-scale
experiments and numerical simulations. The following conclusions can be drawn:
1.
2.
3.
A novel water mist curtain nozzle was developed to generate more uniform water mist
curtain, and its characteristics were predicted by FDS and validated with experimental measurement of a single injector.
About 83% thermal radiation of the fire flame can be attenuated by the water mist
curtain with operating pressure of 2.0 MPa, flow rate of 13.3 L/min, mean droplet diameter of 56 mm, and mist curtain thickness of 0.4 m, but the increase in attenuation
factor was not obvious when there is a continuous increase in the operating pressure.
The simulated results of radiative heat flux and temperature were underestimated
comparing with the experimental data, but they have the same variation trend qualitatively. The errors of radiative heat flux value between simulations and experiment
were kept in the range from 230% to + 40%. Although the results of the average
attenuation factor agree well between the simulation and experiment, the fluctuation
of the simulated results was relatively large and the instantaneous results between
them were also relatively large. So, an improved numerical model is still needed to be
established for better predicting.
Future work will focus on the topic related to the optimized characteristics of water mist
curtain system, such as the working pressure, flow rate, and curtain width.
Declaration of conflicting interests
The authors declare that there is no conflict of interest.
Funding
This study was supported by the Natural Science Foundation of China (grant no. 51323010), the Anhui
Provincial Natural Science Foundation (grant no. 1408085MKL95), and the Fundamental Research
Funds for the Central Universities (grant no. WK2320000033).
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Appendix 1
Notation
A
Af
At
B
cp
C
CD
D
Expi
g
h
hm
hv
DHC, eff
Ib
Iwith
Iwithout
Il
m
m_ emp
m_ ‘
P
q_ r
Q_ emp
r
s, s0
Simi
F(s0 , s)
t
T
v
v0
x
Ya
surface area (m2)
fuel surface (m2)
thermal radiation attenuation factor
emission source term
specific heat (kJ/kg/K)
friction losses constant of nozzle
drag coefficient
_ ‘ cp T‘ pffiffiffi
g)2=5
characteristic diameter (m) D = (Q=r
results of experiments
gravitational acceleration (m/s2)
heat transfer coefficient between the droplet and the gas (W/m2 K)
mass transfer coefficient (kN/m2)
latent heat of vaporization of the liquid (kJ/kg)
effective heat of combustion of diesel oil
radiation blackbody intensity
radiative heat flux with water mist curtain (kW/m2)
radiative heat flux without water mist curtain (kW/m2)
radiation intensity at wavelength, l
mass (kg)
burning rate predicted based on the empirical relationship
burning rate of pool fire with infinite diameter
operating pressure near the nozzle exit (MPa)
rate of radiative heating of the droplet (kW)
predicted heat release rate
radius of droplet (m)
unit vector in direction of radiation intensity
results of simulations
scattering phase function
time (s)
temperature (K)
velocity (m/s)
initial droplet velocities at the exit of nozzle
coordinate (m)
vapor mass fraction of species a
a
e
u
k
l
r
s
ss
gas species
error between simulations and experiments
mean beam-length corrector
absorption coefficient
wavelength (m)
density (kg/m3)
Stefan–Boltzmann constant
scattering coefficient
Subscript
f
fuel
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Zhu et al.
g
l
n
289
gas phase
liquid phase
number of radiation band
Author biographies
Pei Zhu is a PhD student of the State Key Laboratory of Fire Science, University of Science and
Technology of China from September 2012.
Xishi Wang is an associate professor of the State Key Laboratory of Fire Science, University of
Science and Technology of China. He received his PhD from the University of Science and
Technology of China in 2002. His research focuses on fire suppression mechanisms and technologies,
two/multiphase flows, laser-based diagnostic techniques, and so on.
Zhigang Wang is a master student of the State Key Laboratory of Fire Science, University of Science
and Technology of China from September 2013.
Haiyong Cong is a master student of the State Key Laboratory of Fire Science, University of Science
and Technology of China from September 2013.
Xiaomin Ni is a lecturer of the State Key Laboratory of Fire Science, University of Science and
Technology of China. She received her PhD from the University of Science and Technology of China
in 2006.
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