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The reaction of a fire plume to a droplet spray
ARTICLE IN PRESS
Fire Safety Journal 41 (2006) 390–398 www.elsevier.com/locate/firesaf
The reaction of a fire plume to a droplet spray John A. Schwille, Richard M. Lueptow Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208-3111, USA Received 18 April 2005; received in revised form 16 January 2006; accepted 4 February 2006 Available online 17 April 2006
Abstract The reaction of a fire plume to an applied water spray is crucial to fire suppression. While considerable research has been devoted toward fire plumes without suppression, and some computer simulations of fire suppression have provided insight, little experimental data exist detailing how the structure of a fire plume changes during suppression. Experiments were performed in which 5, 15 and 50 kW gas burner fires were exposed to a spray from one of three spray sources. Flow rates from the nozzles or fire sprinkler ranged from 6 to 106 L/min. Contours of infrared (IR) intensity of the fire plume show that the plume decreases in height and increases in width with the increasing strength of the applied spray. Based on the height of the maximum fluctuations of IR intensity, the thermal plume height decreases with increasing spray strength, but the overall projected area of the plume changes very little. The plume height depends on the ratio of the drag of the droplets on the air to the momentum of the plume, allowing the results to be generalized to typical fire suppression applications. r 2006 Elsevier Ltd. All rights reserved. Keywords: Plume; Interaction; Thermography; Spray
1. Introduction A key element in fire suppression with water is the reaction of the fire plume to the applied droplet spray. This reaction is not well understood, largely because fire plumes have rarely been studied in conjunction with suppression sprays. In fact, widely used correlations for fire plumes [1] are based on relations developed 50 years ago to describe the width, velocity and density difference in a buoyant plume [2]. While these standard plume equations are effective in modeling the fire hazard before suppression systems activate, the plume correlations do not account for a fire suppression spray. In an attempt to understand the effect of the fire plume on the droplet spray, actual delivered density (ADD) tests [3,4] are used to measure the water delivered to a fire by a sprinkler by collecting the water in pans placed beneath a fire, which is designed to replicate fires found in actual storage applications. However, these tests provide little information regarding the reaction of a fire to an applied Corresponding author.
E-mail address: [email protected] (R.M. Lueptow). 0379-7112/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2006.02.005
droplet spray. Some work in detailing how the heat release rate of a fire is changed due to a spray has been performed. Previous work in large-scale experiments has shown that the heat release rate of a rack storage fire decreases exponentially with the water application rate [5]. On the other hand, studies of the suppression of pool fires with a droplet spray indicate that the heat release rate of pool fires increases under direct suppression by water droplets [6]. One explanation for the increased heat release rate is that the change in structure of the fire allows for more fuel and air mixing. Thus, it is clear that the dynamics of the interaction between the suppression spray and the fire have an impact on the heat release rate, fire spread, and suppression of the fire. Current knowledge of this interaction is mainly limited to insight gained through CFD simulations of the fire field. Previous simulations [7–9] have shown that the droplet spray pushes the fire plume downward. Under certain conditions, the simulations show an interaction boundary between the downward flow of air entrained in the spray and the upward fire plume [8] suggesting that the downward momentum of the spray balances the upward momentum of the plume.
ARTICLE IN PRESS J.A. Schwille, R.M. Lueptow / Fire Safety Journal 41 (2006) 390–398
The research described here presents the first direct measurements of the general structure of a fire plume as measured using infrared intensity as it interacts with a suppression spray. The work is aimed at directly measuring the effect of suppression on the fire plume and determining how the interaction boundary depends on the fire and the spray. In addition, although recent work has provided laws for scaling up laboratory experiments related to droplets and flames [10], generalizing the interaction between sprinkler sprays and fire plumes may assist sprinkler manufacturers and fire protection system designers in making initial assessments of suppression effectiveness. Furthermore, these direct measurements of the reaction of a fire plume to a spray provide experimental data against which computer simulations can be compared. Finally, the results of these experiments provide insight into the physics of fire suppression. 2. Methodology A variety of fire sizes and spray strengths were used to investigate the reaction of the fire plume to the applied droplet spray. To provide a controlled fire, methane flowed at a specified rate into a burner that consisted of a cylindrical can filled with pea gravel to uniformly distribute the fuel and constrain the initial fire diameter. The methane fuel entered into the lower portion of the can via tubing and exited the tube by way of vertically oriented holes at the center of the can. The gas diffused through several inches of the pea gravel before igniting at the top of the can. Fire sizes were 5, 15 and 50 kW, which required three different sized burners of diameters 10, 18 and 25 cm to maintain a relatively small fuel velocity, corresponding to 1oFro2, consistent with buoyancy dominated fires. The Froude number, Fr ¼ u2 =gDf , depends on the velocity of the fuel from the surface, u, the acceleration due to gravity, g, and the diameter of the fire, Df , as constrained by the burner size. Temperature distributions of the plume from these burners have been shown to follow standard correlations for fire plumes [11]. To provide a situation representative of a typical fire suppression application, the spray source was located 1.6 m above the center of the top of the burner, which served as the origin of a cylindrical coordinate system. The reaction of the fire plume to the droplet spray was examined using an infrared camera in the arrangement shown in Fig. 1. The ThermaCAM PM390 infrared camera, which is sensitive to wavelengths ranging from 3.4 to 5 mm, was positioned about 7 m from the fire. The camera uses a cryogenically cooled platinum silicide detector to convert infrared radiation to a 256 256 pixel image, which corresponds to a spatial resolution of 8.5 mm/pixel. Typically, infrared technology is used to measure surface temperatures of opaque objects. However, other researchers have used IR thermography to map the structure of pool fires by considering statistical quantities of the IR radiation emitted by the CO2 [12], and to measure the
391
IR thermography measurement region
Spray source
Infrared Camera
1.6 m
7m
Fire z r Burner
Fig. 1. Experimental setup of the fire, spray source and infrared camera.
emissivity of flames for hydrocarbon pool fires [13]. With a flame, temperature measurement is complicated by the dependence of flame emissivity on several variables, including the path length and the type of fuel being used [14]. One method to overcome this limitation is to calibrate the IR signal against thermocouple temperatures. This has been used successfully in a few other studies related to fire [11,15]. For instance, the IR signature of the fire was compared to thermocouple measurements for a fire with no spray to determine the correlation between the measured IR radiance and approximate temperature [11]. In the current work, thermocouple measurements were made along the centerline of the plume and compared to the values in the IR images for the corresponding pixels for three different size fires. Thermocouples can measure a wide range of temperatures, but the IR radiation (from the CO2 of the methane flame) could not be calibrated for the entire range of thermocouple measurements. Nevertheless, IR intensities could be matched to the excess temperatures, DT, over a range of excess temperatures typical of the plume for all three fire sizes. In any case, the intent of the experiments described here is not to measure temperature, but to characterize the impact of a water spray on the thermal plume. The IR intensity provides a measure of the height and width of the plume. Dense sprays can reduce the transmitted radiation from fires [16]. Since the IR thermography measurements described here were made in the presence of a water spray, it was necessary to quantify the attenuation of the fire’s IR radiation by the spray and the uncertainty it introduces in the results. To do this, a hot plate was placed behind droplet sprays of varying strengths. The hot plate was held at approximately 200 C, typical of the temperature in the plume that was measured in the experiments, and droplet sprays were introduced between the IR camera and the hot plate. Although attenuation increased slightly with spray strength, the effect on the measured IR temperature was less than 10 C for all spray conditions used in this study. Thus, the attenuation effect of the spray on the IR signature of the suppressed fires was small enough so it could be neglected.
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Table 1 Spray source parameters
Wide angle nozzle Narrow angle nozzle F980 sprinkler
Manufacturer
Orifice size (mm)
Nominal K factor (L/min kPa1=2 )
Spray angle
Dv50 at 137.9 kPa (mm)
Spraying systems Spraying systems Central
3.97 3.57 12.2
0.76 0.73 8.07
120 50 NA
721 708 833
Two spray nozzles and a standard spray fire sprinkler were used to generate the sprays. The mass flow of water through the nozzle or fire sprinkler was controlled by establishing and monitoring the pressure differential across the orifice of the nozzle or sprinkler. The nozzles were chosen for their general uniformity of spray characteristics such as velocity and drop size, while the fire sprinkler was included in the study to ensure the applicability of the results to practical fire suppression sprays. The relevant spray characteristics of the spray sources are presented in Table 1. The height of the spray source results in only a portion of the spray interacting with the fire. Details of the spray field in the presence of the fire are presented in Ref. [17]. The volumetric median droplet diameter, Dv50 , was measured using phase doppler interferometry at distance of 0.15 m from the spray orifice along the centerline of the spray. Experiments began by igniting the methane burner and establishing the desired fire size. Water was then allowed to flow to the nozzle or fire sprinkler to create a droplet spray. After allowing the flame and spray to stabilize, 25 IR images were taken at approximately 1 s intervals. A total of 46 combinations of fire size and spray strength were investigated. 3. Results 3.1. Plume structure One way to characterize the reaction of the flame and fire plume to the droplet spray is to consider the situation as a competition between the downward momentum of the spray and the upward momentum of the fire plume. Computer simulations have shown that if the downward spray is strong enough to balance or overpower the upward momentum of the fire plume, the structure of the fire plume changes [9] . Fig. 2 provides a qualitative measure of the effect of increasing spray strengths on the 15 kW fire for the wide angle nozzle. Each individual subfigure shows 25 contours, one from each of the 25 images, in a vertical plane, where r is the radius from the centerline of the fire and z is the height above the burner. The nozzle is located beyond the bounds of the figure, at r ¼ 0 m and z ¼ 1:6 m. Each contour represents a temperature difference between the ambient temperature and the elevated plume temperature of DT ¼ 190 10 C. This temperature was chosen
because it was in the middle of the calibration range for the IR camera, clearly represents a plume contour that is distant from the luminous region of the flame, and is a contour that readily depicts differences in the plumes of the fire sizes that were considered. For the case with no spray, shown in Fig. 2a, the contours generally lie on top of one another, particularly in the lower half of the plume, although the fire plume is clearly unsteady. This is consistent with results for the unsteadiness of flame height, as measured in terms of intermittency in which the flame height is defined as the elevation above which the luminous region is present 50% of the time [18]. Once the spray is introduced at a flow rate of 7.57 L/min, the situation changes. The overall height of the contours decreases as the downward momentum of the spray is introduced. In addition, the contours overlap less in Fig. 2b than in Fig. 2a, demonstrating a strongly timedependent response of the fire plume to the droplet spray. This is even more evident when comparing individual contours. The individual contours in Fig. 2a are quite similar to one another, where those in Fig. 2b vary substantially from one another, indicating a higher degree of unsteadiness and larger deviation from the average contour. This unsteady response was noted in early research on fire extinction [19,20], but not clearly demonstrated. The maximum lateral extent of the plume is also slightly increased due to the applied droplet spray. For the flow rates of 9.65 L/min (Fig. 2c) and 11.17 L/min (Fig. 2d), these effects are more pronounced. With increasing flow rate, the maximum vertical extent of the plume decreases. Along with that decrease, the height of the zone where the majority of the contours cross the centerline ðr ¼ 0 mÞ also decreases. For instance, when the flow rate through the nozzle is 7.57 L/min (Fig. 2b) the zone 0:4ozo0:8 contains most of the contours along the centerline at r ¼ 0. As the spray is increased this zone moves to 0:3ozo0:6 for 9.65 L/min (Fig. 2c) and then to 0:05ozo0:4 for 11.17 L/ min (Fig. 2d). The width of the plume also increases as the spray strength increases. For the largest flow rate used, 11.17 L/min, the lateral extent of the fire plume has increased by nearly a factor of 2 compared to no spray. This suggests that lateral flame spread may occur under heavy suppression conditions. This lateral spread is more clearly evident if the 25 contours are ensemble averaged. Fig. 3 shows a single contour representing the ensemble averaged excess
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0.8
0.8
0.6
0.6
z (m)
z (m)
1
393
0.4
0.4
0.2
0.2
0 -0.5
-0.25
0
0.25
r (m)
(a)
0 -0.5
0.5
-0.25
0
0.25
0.5
r (m)
(b) 1
0.8
0.8
0.6
0.6
z (m)
z (m)
1
0.4
0.4
0.2
0.2
0 -0.5
-0.25
0
0.25
r (m)
(c)
0 -0.5
0.5
(d)
-0.25
0
0.25
0.5
r (m)
Fig. 2. Contours of IR intensity from a 15 kW fire corresponding to DT ¼ 190 10 C at 25 time intervals ðDt ¼ 1 sÞ for wide angle nozzle flow rates of (a) 0 L/min, (b) 7.57 L/min, (c) 9.65 L/min and (d) 11.17 L/min.
1 0.9 0.8 0.7
z (m)
0.6 0.5 0.4 0.3 0.2 0.1 0 -0.5
-0.25
0
0.25
0.5
r (m) Fig. 3. Suppression of a 15 kW fire (0.18 m diameter burner) with the wide angle nozzle based on ensemble averages of IR contours corresponding to DT ¼ 190 10 C. (Heavy solid contour: 0 L/min, dashed contour: 7.57 L/ min, dashed-dotted contour: 9.65 L/min, solid contour: 11.17 L/min.)
temperature of DT ¼ 190 10 C for wide angle nozzle flow rates of 0 L/min (no spray), 7.57, 9.65, and 11.17 L/ min. As the flow rate increases, the width of average fire plume increases substantially, from about 0.3 m with no spray to nearly 0.5 m when the spray flow rate is 11.17 L/ min. Note that since the base of the flame is constrained by the gravel burner’s area, this increase in width is a result of the dynamics of the flame and not due to an increase in the burning surface. However, if the fire occurred on a flammable surface, this could easily result in lateral flame spread. The distance from the base of the fire to the location of this maximum width also decreases as the strength of the spray increases. The ensemble averaged contours also clearly show the vertical suppression of the fire with increasing spray strength. As the spray increases in strength, the height of the plume decreases. This effect is fairly sensitive to flow rate, with the height of the thermal plume for a spray flow rate of 11.17 L/min being about half that of the height of the thermal plume for a spray flow rate of 7.57 L/min. The increase in fire plume radius and the decrease in the fire plume height is consistent with previous
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measurements for pool fires [6] and with computer simulations [9]. In addition to the dynamics of the fire plume under suppression and the effective height and width of the plume, the individual IR images provide information on the total size of the plume under suppression. Although the excess temperature contour of DT ¼ 190 10 C does not define the flaming region or the full extent of the thermal plume, a change in the projected area represented by this contour is indicative of a change in the volume of the flaming region and the thermal plume. Fig. 4 shows the projected area, Ap , contained by the ensemble averaged contour representing DT ¼ 190 10 C for the wide angle nozzle as a function of flow rate. As expected, Ap increases with increasing fire size. The increase in Ap is much greater between 15 and 50 kW fires than between 5 and 15 kW fires. As the spray is introduced, there is a slight decrease in the area with increasing flow through the nozzle. Ap for a 50 kW fire decreases somewhat more than it does for the other fire sizes. This may occur because for the 50 kW fire, the contour corresponding to DT ¼ 190 10 C reaches further from the base of the fire, and the plume velocity is lower at this point according to the plume correlations [1]. However, the projected area, Ap , decreases only about 10% as the suppression flow rate increases from zero to the maximum. For smaller fires, the projected area is nearly constant with the spray flow rate. Thus, there is only a very minor overall change in the projected area of the fire plume with the applied spray, clearly indicating that the decrease in the height of the plume during suppression is offset by a broadening of the plume width.
3
3.2. The plume–spray interaction region It is quite difficult to quantify the height at which the thermal plume and the spray interact, because the height depends on the particular temperature contour that is used and the position of that contour varies in time. However, it would be expected that the IR intensity would have its greatest fluctuations where the plume and spray interact. Below the interaction region the plume dominates, and above this region the spray dominates, resulting in relatively small temperature fluctuations in either case than at the interaction boundary. Thus, the standard deviation of the IR images in time was analyzed. Fig. 5 shows the standard deviation, sI , of the IR intensity for the 25 IR images at points along the centerline r ¼ 0, normalized with the maximum possible IR intensity value, I sat ¼ 255, for a 15 kW at three different spray flow rates. A small value of the standard deviation of the IR intensity indicates that the temperature varies very little (as would be expected near the base of the fire or above the plume), while a large standard deviation indicates that the IR intensity varies substantially (as would be expected at the location of the interaction of the plume with the spray). The curve for a single combination of spray and fire size can be separated into three zones. For example, the curve shown for 7.57 L/ min has very little deviation in the IR intensity in the region above z ¼ 0:9 m. In this region, the spray dominates the flow field, and there is no fire or thermal plume to emit IR radiation. In the region below z ¼ 0:15 m, the fire is fairly steady, such that even though the IR intensity is strong, the standard deviation of the intensity is nearly zero. In between these two zones, the fire has the largest fluctuations. From Figs. 2a–d, it is clear that the height at which these fluctuations occur decreases as the spray increases. Thus, we can use the position of maximum fluctuations as a measure of the location of the interaction between the fire
2.5 0.45
Ap (m2)
2
0.4
1.5 0.35 0.3
s I /Isat
1 0.5 0
0
0.05
0.1
0.15
0.2
Q w (L/s) Fig. 4. Plume area measured as the projected area enclosed within the ensemble-averaged contour corresponding to DT ¼ 190 10 C for the wide angle nozzle. Error bars indicate the standard deviation of the area contained by the DT ¼ 190 10 C contour for the 25 image set. For 5 and 15 kW, the error bars are smaller than the symbols. Open symbols represent the plume with no spray present (E: 50 kW, ’: 15 kW, .: 5 kW).
0.25 0.2 0.15 0.1 0.05 0
0
0.25
0.5
0.75 z (m)
1
1.25
1.5
Fig. 5. Standard deviation of IR intensity at r ¼ 0 for a 15 kW fire (’: 0 L/min, E: 7.57 L/min, .: 11.17 L/min).
ARTICLE IN PRESS J.A. Schwille, R.M. Lueptow / Fire Safety Journal 41 (2006) 390–398
1.2
z b (m)
1 0.8 0.6 0.4 0.2 0
0
0.05
0.1 QW (L/s)
0.15
0.2
Fig. 6. The location of the interaction boundary, zb , as a function of the wide angle nozzle flow rate. Open symbols represent the plume with no spray present (E: 50 kW, ’: 15 kW, .: 5 kW).
0.8 0.7 0.6 0.5
z b (m)
and the spray. For a flow rate of 7.57 L/min, sI is a maximum near z ¼ 0:5. We define the height of the maximum of sI as the ’’interaction boundary,’’ zb , which represents a boundary between the downward spray momentum and the upward plume momentum. The concept of an interaction region between the suppression spray and the fire plume comes about from computational simulations of fires that show the downward flow of air entrained with the spray and the upward buoyant plume forming a stagnation point where the two flows meet [7,8]. As is evident in Fig. 5, the interaction boundary moves towards the fire base as the spray flow rate increases. Of course, for the case with no spray (0 L/min), there is no interaction location. However, the position of maximum fluctuations still serves as a characteristic height of the thermal plume when there is no suppression spray. By defining the location of the interaction boundary based on the standard deviation of the IR intensity, the effect of the relevant variables on the location of the boundary can be investigated. Fig. 6 shows the height of the interaction boundary, zb , as a function of the wide angle nozzle flow rate for three fire sizes. As the flow rate increases, the momentum of the downward spray increases and pushes the location of the interaction closer to the base of the fire for all three fire sizes. Given that the downward momentum of the spray opposes the upward momentum of the fire, it is no surprise that at any given flow rate for the spray, increasing the fire size pushes the interaction region higher above the base of the fire. For example, when the nozzle is flowing approximately 0.11 L/s, the distance from the base of the fire to the interaction region increases by a factor of more than three when the fire size increases from 5 to 50 kW. In addition to changing the flow rate and heat release rate (5, 15 or 50 kW), the spray characteristics were changed by selecting either one of two spray nozzles or
395
0.4 0.3 0.2 0.1 0
0
0.5
1
1.5
Q W (L/s) Fig. 7. The location of the interaction boundary, zb , for a 15 kW fire. Open symbols represent the plume with no spray present (K: fire sprinkler, ’: wide angle nozzle, .: narrow angle nozzle).
the fire sprinkler. Fig. 7 shows the height of the interaction region for each spray source over a range of flow rates. The nature of the spray clearly has a large effect on the location of the interaction boundary. The wide angle nozzle has an interaction boundary that is higher than that of the narrow angle nozzle at an equivalent flow rate, indicating the wide angle nozzle is not as effective as the narrow angle nozzle at pushing the fire downward. This is a consequence of the more focused spray of the narrow angle nozzle. For the fire sprinkler, the wide distribution of the spray results in the need for higher flow rates, ranging from 1 to 1.5 L/s, to achieve the same interaction boundary height for zb than are necessary for the nozzles having almost an order of magnitude less flow rate. While these results indicate the nozzles push the fire plume downward more effectively than the fire sprinkler, they do not indicate that nozzles are more effective in suppression applications. Instead, the results show that spray characteristics such as droplet velocity and droplet concentration play an important role in determining the structure of the fire plume under suppression conditions. The interaction between a sprinkler and fire has often been cast in terms of the competition between the momentum of the downward spray and the momentum of the upward fire plume [7,8,21]. While this has been helpful, a slightly modified approach is used here to better understand the data. The momentum of the fire plume, M p , is still used to characterize the fire size. However, to characterize the strength of the spray, the drag of the spray, Ds , is used since this is the physical mechanism of the interaction between the droplets and the gas of the plume. Thus, a spray that has a very large effect on a fire plume does so by creating a large drag on the fire plume. The ratio of the drag of the spray to the momentum of the plume, Ds =M p , is a nondimensional parameter that characterizes the effect of the spray on the dynamics of the fire.
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To calculate this parameter, the drag of the spray, Ds , can be expressed as Ds ¼ 12 nrp C D Ad ð~ v ~ vd j, v ~ vd Þj~
(1)
where n is the total number of drops in the spray, rp is the density of the plume, C D is the drag coefficient of the droplets, Ad is the cross-sectional area of the droplets, ~ v is the velocity in the plume and ~ vd is the velocity of the droplets. For this simple analysis, the drag coefficient of the droplets is assumed to be close to that of solid spherical particles. This assumption ignores secondary effects such as droplet deformation, internal flow and evaporation, but serves as a good estimate. It is quite difficult to know the plume velocity, ~ v, but it can be approximated based on the plume relations for a fire with no spray [1]. Although the droplets will clearly affect the plume velocity based on the spray characteristics, this approximation suffices to develop a scaling parameter. The plume momentum can be calculated from the plume velocity profile and width as Z ~ 2 dA, Mp ¼ rvðrÞ (2) A
where A is the area of the plume at a given height. As in the spray drag calculation, the plume velocity and area can be calculated for correlations developed for fire plumes in isolation. A Gaussian profile based on plume correlations [1] was used to calculate the integral term in Eq. (2). The area of integration covers the region 0oro2 m. This area contains almost all of the plume momentum considering that at a radius of r ¼ 2 m the plume velocity has decreased to much less than 0.01% of its value at r ¼ 0, even for the 50 kW fire. Because the plume velocity changes with height, the plume momentum and spray drag were both calculated at a height of 1 m above the fire. Although somewhat arbitrary, this height is in the region where the droplets and plume interact, so it serves in defining a parameter to characterize the interaction. The ratio of the spray drag to plume momentum can then be expressed as 1 nC D Ad rp ð~ v ~ vd Þj~ v ~ vd j Ds ¼2 . R 2 Mp ~ dA r vðrÞ
0.8 0.7 0.6
(3) 0.5
p
The droplet velocity was measured using particle image velocimetry [11]. The droplet area was calculated from the volumetric median diameter measured at a limited number of flow rates using Phase Doppler Interferometry. Volumetric median diameters at other flow rates were interpolated from those measurements. Of course, another average diameter could have been used to calculate Ad , such as the Sauter mean diameter. While another method of averaging the diameters in a polydisperse spray would change the magnitude of the ratio Ds =M p , the volumetric median diameter serves to characterize the droplet size in a particular spray for relative comparison between different sprays and is often reported as a spray parameter [22]. Phase Doppler Interferometry also provided a droplet
zb /zb0
A
number density, which was used along with an estimated spray volume to provide a first order estimate for n. The droplet area and velocity measured near the spray source (approximately 0.15 m below the spray source on the centerline) are used to characterize the spray, since this represents the initial condition of the spray before it is affected by the plume. When the interaction boundary (normalized by the height of the maximum fluctuations for no spray) for 46 experiments performed with the three nozzles covering an order of magnitude variation in flow rate from 5.7 to 105.6 L/min and an order of magnitude variation in fire sizes from 5 to 50 kW are plotted as a function of Ds =M p , the height of the normalized interaction boundary decreases as the ratio of spray drag to plume momentum increases, as shown in Fig. 8. Although the collapse of the data is not perfect, the results show that once the spray drag is about 10% of the plume momentum, the plume is pushed down nearly to the base of the fire. At first glance it is surprising that the ratio Ds =M p is so small for high levels of suppression. However, some droplets in a spray may be outside the fire plume and thus not contributing their drag to suppression. The collapse of the data is much better for each individual spray source. The nonuniformities in the sprays and different spray areas may make it difficult to perfectly collapse these results. Of course, the nondimensional parameter Ds =M p is not intended to serve as an absolute fire sprinkler design parameter. Instead, it shows that the drag of the spray is critical to suppressing the structure of the fire. A recent theoretical study [23] has analyzed a fire plume under suppression by adding a droplet momentum term to Morton’s original plume analysis [2], which is the basis for generally accepted plume correlations used in fire analysis [1]. This model predicts a particular height above the fire
0.4 0.3 0.2 0.1 0
0
0.05
0.1
0.15
0.2
0.25
Ds/Mp Fig. 8. The location of the normalized interaction boundary, zb =zb0 , as a function of the drag on the air due to the droplet spray, Ds , to the momentum of the fire plume, M p (K: fire sprinkler, ’: wide angle nozzle, .: narrow angle nozzle).
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origin where the plume widens rapidly and the plume velocity goes to zero. This height represents the boundary between the fire plume and the spray, much like the current measurements of zb represent the height of the interaction between the fire plume and the spray. This suggests that a comparison between the theoretical model and the measurements of the height of the interaction boundary would be useful. Fig. 9 shows the experimental data from all 46 experiments over a wide range of spray flow rates, fire sizes, and types of spray along with the model’s prediction of the interaction boundary for the spray and fire conditions of the corresponding experiment. The inputs to the model, which assumes a homogenous droplet spray counteracting a standard fire plume, are the droplet number density, the droplet velocity, the droplet diameter, the fire size and the initial diameter of the fire. The droplet parameters were those used in Eq. (3), and the fire parameters are directly known from the experiments. The results shown in Fig. 9 are encouraging. The trend of the data matches the trend of the model predictions with higher ratios of spray drag to plume momentum resulting in interaction regions closer to the base of the fire. The experimental data matches the model fairly well except for four data points (between zb ¼ 0:75 and 1.1 m, all related to 50 kW fires using the narrow angle nozzle or the fire sprinkler for suppression. For the fire sprinkler data, this is probably because the model assumes a uniform spray, which is not a good approximation for the fire sprinkler. In spite of the four data points that do not match the model, demonstrating that the IR measurements, which track the dynamics of the structure of the thermal plume, are similar to the predictions from a model based on the momentums of the fire plume and the spray indicates that momentum plays a key role in the interaction between the fire and the spray.
397
3.3. Droplet velocity within the plume The IR measurements of the fire and thermal plume presented in this paper can be compared with measurements of droplet velocity in the same experimental configuration [17] to provide a better understanding of droplet penetration through fires. Figs. 10a–c show contours of both IR intensity and droplet velocity for the wide angle nozzle flowing 8.4 L/min for three fire sizes. The IR contours are from ensemble averages of the 25 IR intensity images, while the droplet velocities were measured with PIV [17]. The IR contour represents DT ¼ 190 10 C and the gray areas are regions where droplets are moving upward with a positive velocity, the boundary of which can be considered the limit of droplet penetration. For the 5 kW case (Fig. 10a), there is no gray region within the spray pattern of the wide angle nozzle, so it is clear that the droplets fall through the fire plume, which, although small, is still present near the burner. For the case of the 15 kW fire (Fig. 10b), the droplet velocity data indicate that the droplets have difficulty penetrating the plume to be exposed to temperatures over DT ¼ 190 10 C. This is evident from the maximum height of the gray region corresponding to upward-moving droplets, which is somewhat higher than the height of the contour for DT ¼ 190 10 C. For the largest fire (Fig. 10c), the region for positive droplet velocity is almost coincident with the contour for DT ¼ 190 10 C in the upper regions of the plume. Although the 15 kW fire shows a region of positive droplet velocity above the DT ¼ 190 C 1.6
1.6
1.6
1.4
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1.4
1.2
1.2
1.2
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1
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1.8 z (m)
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z b (m)
1.2 1
(a)
0.4 0.2 0
0 0
0
0.05
0.1
0.15
0.2
0.25
Ds /M p Fig. 9. Comparison of measured zb to the interaction region found using a momentum-based model [23]. Open symbols are model predictions, filled symbols are IR-based measurements (K: fire sprinkler, ’: wide angle nozzle, .: narrow angle nozzle).
0.25
r (m)
0.5
0 0
(b)
0.25
r (m)
0.5
0
(c)
0.25
0.5
r (m)
Fig. 10. Plume area contour for the wide angle nozzle compared to droplet velocity measurements [17] for fire sizes of (a) 5 kW, (b) 15 kW, and (c) 50 kW. (Heavy solid curves: IR contour representing DT ¼ 190 10 C, light solid curve: contours of zero droplet velocity as measured with PIV. Gray regions have an upward velocity.) The center of the base of the fire is at r ¼ 0, z ¼ 0; the orifice of the wide angle nozzle is at r ¼ 0, z ¼ 1:6 m. The gray region in the upper right corner for the 50 kW fire represents droplets blown above the cone angle of the spray. The lower bound of the gray regions in the upper right corner for the 5 and 15 kW fires indicate the cone angle of the spray.
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contour while the 50 kW shows the reverse, the DT ¼ 190 C contour generally coincides with positive droplet velocity. Overall, for the wide angle nozzle most droplets are slowed to zero velocity before they have penetrated a significant portion of the high-temperature environment of the fire plume unless the spray is strong enough to push the fire plume close to the base of the fire as for the 5 kW fire.
4. Conclusions Results showing the structure and location of a fire plume for a variety of fire sizes and spray strengths have been presented. The IR images of the fire show that fire suppression is a very dynamic process, with the fire plume moving back and forth in response to the applied spray. The effective width of the fire is increased with increasing spray strength. However, this increase in effective width is not a result of an increase in projected area of the fire. In fact, the projected area of the fire remains relatively unchanged from no spray to fires being suppressed by wide range of spray flow rates. The location of the maximum deviation of the IR intensity of the fire can be used to define the interaction region between the fire plume and the suppression spray. The height from the base of the fire to this interaction region decreases as spray strength increases for all the sprays used in this study. Increasing the fire size increases the distance from the base of the fire to this interaction region. Finally, it is clear that the IR measurements define a portion of the plume into which droplets have difficulty penetrating. Previous researchers have often characterized the interaction between a spray and a fire plume by comparing the momentum of the spray to the momentum of the plume [24,25]. While this has been effective to generalize the penetration of water through a fire plume, the current study shows that the decrease in the thermal plume height due to spray application is governed by the ratio of the drag due to the spray to the momentum of the fire plume. Thus, while larger drops may penetrate through a fire plume, a spray at the same mass flow rate that consists of a greater number of smaller drops may serve better at pushing the thermal plume downward because of its higher drag.
Acknowledgments J.A.S. was supported by a National Science Foundation Graduate Research Fellowship and Northwestern University. Underwriter’s Laboratories (UL) provided laboratory facilities for this research. We gratefully acknowledge the support and encouragement of Dr. Pravin Gandhi (UL).
References [1] Heskestad G. Fire plumes, In: Beyler CL, Custer RLP, Walton WD, Watts JM, Drysdale D, Hall JR, DiNenno PJ, editors. SPFE Handbook of Fire Protection Engineering. 2nd ed. National Fire Protection Association; 1995 [chapter 2], p. 2-1–2-19. [2] Morton BR, Taylor GI, Turner JS. Turbulent gravitational convection from maintained and instantaneous sources. Proc R Soc London 1956;234:1–23. [3] Yao C. The development of the ESFR sprinkler system. Fire Saf J 1988;14:65–73. [4] Chan TS, Kung HC, Yu HZ, Brown WR. Experimental study of actual delivered density for rack-storage fires. In: Fire Safety Science—Proceedings of the Fourth International Symposium, Ontario, Canada; 1994. p. 913–24. [5] Yu HZ, Lee JL, Kung HC. Suppression of rack-storage fires by water. In: Fire Safety Science—Proceedings of the Fourth International Symposium, Ontario, Canada; 1994, p. 901–12. [6] Kim MB, Jang YJ, Kim JK. Burning rate of a pool fire with downward-directed sprays. Fire Saf J 1996;27:37–48. [7] Nam S. Numerical simulation of actual delivered density of sprinkler spray through fires. Atomization Sprays 1994;4:385–404. [8] Nam S. Development of a computational model simulating the interaction between a fire plume and a sprinkler spray. Fire Saf J 1996;26:1–33. [9] Alpert RL. Numerical modeling of the interaction between automatic sprinkler sprays and fire plumes. Fire Saf J 1985;9:157–63. [10] Heskestad G. Scaling the interaction of water sprays and flames. Fire Saf J 2002;37:535–48. [11] Schwille JA, Lueptow RM. The flow field between a fire and a fire sprinkler spray. Fire Technol, submitted for publication. [12] Hatasaka H. A study on pool flame structure using thermography. In: Fire Safety Science—Proceedings of the Fourth International Symposium, Ontario, Canada; 1994, p. 125–36. [13] Planas-Cucni E, Chatris JM, Lopez C, Arnoldos J. Determination of flame emissivity in hydrocarbon pool fires using infrared thermography. Fire Technol 2003;39:261–73. [14] Ris J. Fire radiation—a review, Technical Report 0A0E6.BU-2, Factory Mutual Research Corporation; 1978. [15] Brahmi L, Vietoris T, Torero JL, Joulain P. Estimation of boundary layer diffusion flame temperatures by means of an infrared camera under microgravity conditions. Meas Sci Technol 1999;10:859–65. [16] Dembele S, Wen JX, Sacadura JF. Experimental study of water sprays for the attenuation of fire thermal radiation. J Heat Transfer 2001;123:534–43. [17] Schwille JA, Lueptow RM. The effect of a fire on suppression spray droplet motion. Atomization Sprays, to appear. [18] Zukoski EE, Kubota T, Cetegen B. Entrainment in fire plumes. Fire Saf J 1980;3:107–21. [19] Rasbash DJ, Rogowski ZW. Extinction of fires in liquids by cooling with water sprays. Combust Flame 1957;1:453–66. [20] Rasbash DJ, Rogowski ZW, Stark GWV. Mechanisms of extinction of liquid fires with water sprays. Combust Flame 1960;4:223–34. [21] Yao C. Overview of sprinkler technology research. In: Fire Safety Science—Proceedings of the Fifth International Symposium, Melbourne, Australia; 1997, p. 93–110. [22] Chan T. Measurements of water density and drop size distributions of selected ESFR sprinklers. J Fire Prot Eng 1994;6:79–87. [23] Schwille JA, Lueptow RM. Simple model of the interaction between a fire plume and a droplet spray. J Fire Prot Eng, to appear. [24] Alpert RL. Modeling of sprinkler spray suppression of fires. In: Proceedings of the First International Conference on Fire Suppression Research, Stockholm, Sweden; 1992. [25] Nam S. Numerical simulation of the penetration capability of sprinkler sprays. Fire Saf J 1999;32:307–29.
Fire Safety Journal 41 (2006) 390–398 www.elsevier.com/locate/firesaf
The reaction of a fire plume to a droplet spray John A. Schwille, Richard M. Lueptow Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208-3111, USA Received 18 April 2005; received in revised form 16 January 2006; accepted 4 February 2006 Available online 17 April 2006
Abstract The reaction of a fire plume to an applied water spray is crucial to fire suppression. While considerable research has been devoted toward fire plumes without suppression, and some computer simulations of fire suppression have provided insight, little experimental data exist detailing how the structure of a fire plume changes during suppression. Experiments were performed in which 5, 15 and 50 kW gas burner fires were exposed to a spray from one of three spray sources. Flow rates from the nozzles or fire sprinkler ranged from 6 to 106 L/min. Contours of infrared (IR) intensity of the fire plume show that the plume decreases in height and increases in width with the increasing strength of the applied spray. Based on the height of the maximum fluctuations of IR intensity, the thermal plume height decreases with increasing spray strength, but the overall projected area of the plume changes very little. The plume height depends on the ratio of the drag of the droplets on the air to the momentum of the plume, allowing the results to be generalized to typical fire suppression applications. r 2006 Elsevier Ltd. All rights reserved. Keywords: Plume; Interaction; Thermography; Spray
1. Introduction A key element in fire suppression with water is the reaction of the fire plume to the applied droplet spray. This reaction is not well understood, largely because fire plumes have rarely been studied in conjunction with suppression sprays. In fact, widely used correlations for fire plumes [1] are based on relations developed 50 years ago to describe the width, velocity and density difference in a buoyant plume [2]. While these standard plume equations are effective in modeling the fire hazard before suppression systems activate, the plume correlations do not account for a fire suppression spray. In an attempt to understand the effect of the fire plume on the droplet spray, actual delivered density (ADD) tests [3,4] are used to measure the water delivered to a fire by a sprinkler by collecting the water in pans placed beneath a fire, which is designed to replicate fires found in actual storage applications. However, these tests provide little information regarding the reaction of a fire to an applied Corresponding author.
E-mail address: [email protected] (R.M. Lueptow). 0379-7112/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2006.02.005
droplet spray. Some work in detailing how the heat release rate of a fire is changed due to a spray has been performed. Previous work in large-scale experiments has shown that the heat release rate of a rack storage fire decreases exponentially with the water application rate [5]. On the other hand, studies of the suppression of pool fires with a droplet spray indicate that the heat release rate of pool fires increases under direct suppression by water droplets [6]. One explanation for the increased heat release rate is that the change in structure of the fire allows for more fuel and air mixing. Thus, it is clear that the dynamics of the interaction between the suppression spray and the fire have an impact on the heat release rate, fire spread, and suppression of the fire. Current knowledge of this interaction is mainly limited to insight gained through CFD simulations of the fire field. Previous simulations [7–9] have shown that the droplet spray pushes the fire plume downward. Under certain conditions, the simulations show an interaction boundary between the downward flow of air entrained in the spray and the upward fire plume [8] suggesting that the downward momentum of the spray balances the upward momentum of the plume.
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The research described here presents the first direct measurements of the general structure of a fire plume as measured using infrared intensity as it interacts with a suppression spray. The work is aimed at directly measuring the effect of suppression on the fire plume and determining how the interaction boundary depends on the fire and the spray. In addition, although recent work has provided laws for scaling up laboratory experiments related to droplets and flames [10], generalizing the interaction between sprinkler sprays and fire plumes may assist sprinkler manufacturers and fire protection system designers in making initial assessments of suppression effectiveness. Furthermore, these direct measurements of the reaction of a fire plume to a spray provide experimental data against which computer simulations can be compared. Finally, the results of these experiments provide insight into the physics of fire suppression. 2. Methodology A variety of fire sizes and spray strengths were used to investigate the reaction of the fire plume to the applied droplet spray. To provide a controlled fire, methane flowed at a specified rate into a burner that consisted of a cylindrical can filled with pea gravel to uniformly distribute the fuel and constrain the initial fire diameter. The methane fuel entered into the lower portion of the can via tubing and exited the tube by way of vertically oriented holes at the center of the can. The gas diffused through several inches of the pea gravel before igniting at the top of the can. Fire sizes were 5, 15 and 50 kW, which required three different sized burners of diameters 10, 18 and 25 cm to maintain a relatively small fuel velocity, corresponding to 1oFro2, consistent with buoyancy dominated fires. The Froude number, Fr ¼ u2 =gDf , depends on the velocity of the fuel from the surface, u, the acceleration due to gravity, g, and the diameter of the fire, Df , as constrained by the burner size. Temperature distributions of the plume from these burners have been shown to follow standard correlations for fire plumes [11]. To provide a situation representative of a typical fire suppression application, the spray source was located 1.6 m above the center of the top of the burner, which served as the origin of a cylindrical coordinate system. The reaction of the fire plume to the droplet spray was examined using an infrared camera in the arrangement shown in Fig. 1. The ThermaCAM PM390 infrared camera, which is sensitive to wavelengths ranging from 3.4 to 5 mm, was positioned about 7 m from the fire. The camera uses a cryogenically cooled platinum silicide detector to convert infrared radiation to a 256 256 pixel image, which corresponds to a spatial resolution of 8.5 mm/pixel. Typically, infrared technology is used to measure surface temperatures of opaque objects. However, other researchers have used IR thermography to map the structure of pool fires by considering statistical quantities of the IR radiation emitted by the CO2 [12], and to measure the
391
IR thermography measurement region
Spray source
Infrared Camera
1.6 m
7m
Fire z r Burner
Fig. 1. Experimental setup of the fire, spray source and infrared camera.
emissivity of flames for hydrocarbon pool fires [13]. With a flame, temperature measurement is complicated by the dependence of flame emissivity on several variables, including the path length and the type of fuel being used [14]. One method to overcome this limitation is to calibrate the IR signal against thermocouple temperatures. This has been used successfully in a few other studies related to fire [11,15]. For instance, the IR signature of the fire was compared to thermocouple measurements for a fire with no spray to determine the correlation between the measured IR radiance and approximate temperature [11]. In the current work, thermocouple measurements were made along the centerline of the plume and compared to the values in the IR images for the corresponding pixels for three different size fires. Thermocouples can measure a wide range of temperatures, but the IR radiation (from the CO2 of the methane flame) could not be calibrated for the entire range of thermocouple measurements. Nevertheless, IR intensities could be matched to the excess temperatures, DT, over a range of excess temperatures typical of the plume for all three fire sizes. In any case, the intent of the experiments described here is not to measure temperature, but to characterize the impact of a water spray on the thermal plume. The IR intensity provides a measure of the height and width of the plume. Dense sprays can reduce the transmitted radiation from fires [16]. Since the IR thermography measurements described here were made in the presence of a water spray, it was necessary to quantify the attenuation of the fire’s IR radiation by the spray and the uncertainty it introduces in the results. To do this, a hot plate was placed behind droplet sprays of varying strengths. The hot plate was held at approximately 200 C, typical of the temperature in the plume that was measured in the experiments, and droplet sprays were introduced between the IR camera and the hot plate. Although attenuation increased slightly with spray strength, the effect on the measured IR temperature was less than 10 C for all spray conditions used in this study. Thus, the attenuation effect of the spray on the IR signature of the suppressed fires was small enough so it could be neglected.
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Table 1 Spray source parameters
Wide angle nozzle Narrow angle nozzle F980 sprinkler
Manufacturer
Orifice size (mm)
Nominal K factor (L/min kPa1=2 )
Spray angle
Dv50 at 137.9 kPa (mm)
Spraying systems Spraying systems Central
3.97 3.57 12.2
0.76 0.73 8.07
120 50 NA
721 708 833
Two spray nozzles and a standard spray fire sprinkler were used to generate the sprays. The mass flow of water through the nozzle or fire sprinkler was controlled by establishing and monitoring the pressure differential across the orifice of the nozzle or sprinkler. The nozzles were chosen for their general uniformity of spray characteristics such as velocity and drop size, while the fire sprinkler was included in the study to ensure the applicability of the results to practical fire suppression sprays. The relevant spray characteristics of the spray sources are presented in Table 1. The height of the spray source results in only a portion of the spray interacting with the fire. Details of the spray field in the presence of the fire are presented in Ref. [17]. The volumetric median droplet diameter, Dv50 , was measured using phase doppler interferometry at distance of 0.15 m from the spray orifice along the centerline of the spray. Experiments began by igniting the methane burner and establishing the desired fire size. Water was then allowed to flow to the nozzle or fire sprinkler to create a droplet spray. After allowing the flame and spray to stabilize, 25 IR images were taken at approximately 1 s intervals. A total of 46 combinations of fire size and spray strength were investigated. 3. Results 3.1. Plume structure One way to characterize the reaction of the flame and fire plume to the droplet spray is to consider the situation as a competition between the downward momentum of the spray and the upward momentum of the fire plume. Computer simulations have shown that if the downward spray is strong enough to balance or overpower the upward momentum of the fire plume, the structure of the fire plume changes [9] . Fig. 2 provides a qualitative measure of the effect of increasing spray strengths on the 15 kW fire for the wide angle nozzle. Each individual subfigure shows 25 contours, one from each of the 25 images, in a vertical plane, where r is the radius from the centerline of the fire and z is the height above the burner. The nozzle is located beyond the bounds of the figure, at r ¼ 0 m and z ¼ 1:6 m. Each contour represents a temperature difference between the ambient temperature and the elevated plume temperature of DT ¼ 190 10 C. This temperature was chosen
because it was in the middle of the calibration range for the IR camera, clearly represents a plume contour that is distant from the luminous region of the flame, and is a contour that readily depicts differences in the plumes of the fire sizes that were considered. For the case with no spray, shown in Fig. 2a, the contours generally lie on top of one another, particularly in the lower half of the plume, although the fire plume is clearly unsteady. This is consistent with results for the unsteadiness of flame height, as measured in terms of intermittency in which the flame height is defined as the elevation above which the luminous region is present 50% of the time [18]. Once the spray is introduced at a flow rate of 7.57 L/min, the situation changes. The overall height of the contours decreases as the downward momentum of the spray is introduced. In addition, the contours overlap less in Fig. 2b than in Fig. 2a, demonstrating a strongly timedependent response of the fire plume to the droplet spray. This is even more evident when comparing individual contours. The individual contours in Fig. 2a are quite similar to one another, where those in Fig. 2b vary substantially from one another, indicating a higher degree of unsteadiness and larger deviation from the average contour. This unsteady response was noted in early research on fire extinction [19,20], but not clearly demonstrated. The maximum lateral extent of the plume is also slightly increased due to the applied droplet spray. For the flow rates of 9.65 L/min (Fig. 2c) and 11.17 L/min (Fig. 2d), these effects are more pronounced. With increasing flow rate, the maximum vertical extent of the plume decreases. Along with that decrease, the height of the zone where the majority of the contours cross the centerline ðr ¼ 0 mÞ also decreases. For instance, when the flow rate through the nozzle is 7.57 L/min (Fig. 2b) the zone 0:4ozo0:8 contains most of the contours along the centerline at r ¼ 0. As the spray is increased this zone moves to 0:3ozo0:6 for 9.65 L/min (Fig. 2c) and then to 0:05ozo0:4 for 11.17 L/ min (Fig. 2d). The width of the plume also increases as the spray strength increases. For the largest flow rate used, 11.17 L/min, the lateral extent of the fire plume has increased by nearly a factor of 2 compared to no spray. This suggests that lateral flame spread may occur under heavy suppression conditions. This lateral spread is more clearly evident if the 25 contours are ensemble averaged. Fig. 3 shows a single contour representing the ensemble averaged excess
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z (m)
1
393
0.4
0.4
0.2
0.2
0 -0.5
-0.25
0
0.25
r (m)
(a)
0 -0.5
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-0.25
0
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r (m)
(b) 1
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z (m)
1
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0
0.25
r (m)
(c)
0 -0.5
0.5
(d)
-0.25
0
0.25
0.5
r (m)
Fig. 2. Contours of IR intensity from a 15 kW fire corresponding to DT ¼ 190 10 C at 25 time intervals ðDt ¼ 1 sÞ for wide angle nozzle flow rates of (a) 0 L/min, (b) 7.57 L/min, (c) 9.65 L/min and (d) 11.17 L/min.
1 0.9 0.8 0.7
z (m)
0.6 0.5 0.4 0.3 0.2 0.1 0 -0.5
-0.25
0
0.25
0.5
r (m) Fig. 3. Suppression of a 15 kW fire (0.18 m diameter burner) with the wide angle nozzle based on ensemble averages of IR contours corresponding to DT ¼ 190 10 C. (Heavy solid contour: 0 L/min, dashed contour: 7.57 L/ min, dashed-dotted contour: 9.65 L/min, solid contour: 11.17 L/min.)
temperature of DT ¼ 190 10 C for wide angle nozzle flow rates of 0 L/min (no spray), 7.57, 9.65, and 11.17 L/ min. As the flow rate increases, the width of average fire plume increases substantially, from about 0.3 m with no spray to nearly 0.5 m when the spray flow rate is 11.17 L/ min. Note that since the base of the flame is constrained by the gravel burner’s area, this increase in width is a result of the dynamics of the flame and not due to an increase in the burning surface. However, if the fire occurred on a flammable surface, this could easily result in lateral flame spread. The distance from the base of the fire to the location of this maximum width also decreases as the strength of the spray increases. The ensemble averaged contours also clearly show the vertical suppression of the fire with increasing spray strength. As the spray increases in strength, the height of the plume decreases. This effect is fairly sensitive to flow rate, with the height of the thermal plume for a spray flow rate of 11.17 L/min being about half that of the height of the thermal plume for a spray flow rate of 7.57 L/min. The increase in fire plume radius and the decrease in the fire plume height is consistent with previous
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measurements for pool fires [6] and with computer simulations [9]. In addition to the dynamics of the fire plume under suppression and the effective height and width of the plume, the individual IR images provide information on the total size of the plume under suppression. Although the excess temperature contour of DT ¼ 190 10 C does not define the flaming region or the full extent of the thermal plume, a change in the projected area represented by this contour is indicative of a change in the volume of the flaming region and the thermal plume. Fig. 4 shows the projected area, Ap , contained by the ensemble averaged contour representing DT ¼ 190 10 C for the wide angle nozzle as a function of flow rate. As expected, Ap increases with increasing fire size. The increase in Ap is much greater between 15 and 50 kW fires than between 5 and 15 kW fires. As the spray is introduced, there is a slight decrease in the area with increasing flow through the nozzle. Ap for a 50 kW fire decreases somewhat more than it does for the other fire sizes. This may occur because for the 50 kW fire, the contour corresponding to DT ¼ 190 10 C reaches further from the base of the fire, and the plume velocity is lower at this point according to the plume correlations [1]. However, the projected area, Ap , decreases only about 10% as the suppression flow rate increases from zero to the maximum. For smaller fires, the projected area is nearly constant with the spray flow rate. Thus, there is only a very minor overall change in the projected area of the fire plume with the applied spray, clearly indicating that the decrease in the height of the plume during suppression is offset by a broadening of the plume width.
3
3.2. The plume–spray interaction region It is quite difficult to quantify the height at which the thermal plume and the spray interact, because the height depends on the particular temperature contour that is used and the position of that contour varies in time. However, it would be expected that the IR intensity would have its greatest fluctuations where the plume and spray interact. Below the interaction region the plume dominates, and above this region the spray dominates, resulting in relatively small temperature fluctuations in either case than at the interaction boundary. Thus, the standard deviation of the IR images in time was analyzed. Fig. 5 shows the standard deviation, sI , of the IR intensity for the 25 IR images at points along the centerline r ¼ 0, normalized with the maximum possible IR intensity value, I sat ¼ 255, for a 15 kW at three different spray flow rates. A small value of the standard deviation of the IR intensity indicates that the temperature varies very little (as would be expected near the base of the fire or above the plume), while a large standard deviation indicates that the IR intensity varies substantially (as would be expected at the location of the interaction of the plume with the spray). The curve for a single combination of spray and fire size can be separated into three zones. For example, the curve shown for 7.57 L/ min has very little deviation in the IR intensity in the region above z ¼ 0:9 m. In this region, the spray dominates the flow field, and there is no fire or thermal plume to emit IR radiation. In the region below z ¼ 0:15 m, the fire is fairly steady, such that even though the IR intensity is strong, the standard deviation of the intensity is nearly zero. In between these two zones, the fire has the largest fluctuations. From Figs. 2a–d, it is clear that the height at which these fluctuations occur decreases as the spray increases. Thus, we can use the position of maximum fluctuations as a measure of the location of the interaction between the fire
2.5 0.45
Ap (m2)
2
0.4
1.5 0.35 0.3
s I /Isat
1 0.5 0
0
0.05
0.1
0.15
0.2
Q w (L/s) Fig. 4. Plume area measured as the projected area enclosed within the ensemble-averaged contour corresponding to DT ¼ 190 10 C for the wide angle nozzle. Error bars indicate the standard deviation of the area contained by the DT ¼ 190 10 C contour for the 25 image set. For 5 and 15 kW, the error bars are smaller than the symbols. Open symbols represent the plume with no spray present (E: 50 kW, ’: 15 kW, .: 5 kW).
0.25 0.2 0.15 0.1 0.05 0
0
0.25
0.5
0.75 z (m)
1
1.25
1.5
Fig. 5. Standard deviation of IR intensity at r ¼ 0 for a 15 kW fire (’: 0 L/min, E: 7.57 L/min, .: 11.17 L/min).
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1.2
z b (m)
1 0.8 0.6 0.4 0.2 0
0
0.05
0.1 QW (L/s)
0.15
0.2
Fig. 6. The location of the interaction boundary, zb , as a function of the wide angle nozzle flow rate. Open symbols represent the plume with no spray present (E: 50 kW, ’: 15 kW, .: 5 kW).
0.8 0.7 0.6 0.5
z b (m)
and the spray. For a flow rate of 7.57 L/min, sI is a maximum near z ¼ 0:5. We define the height of the maximum of sI as the ’’interaction boundary,’’ zb , which represents a boundary between the downward spray momentum and the upward plume momentum. The concept of an interaction region between the suppression spray and the fire plume comes about from computational simulations of fires that show the downward flow of air entrained with the spray and the upward buoyant plume forming a stagnation point where the two flows meet [7,8]. As is evident in Fig. 5, the interaction boundary moves towards the fire base as the spray flow rate increases. Of course, for the case with no spray (0 L/min), there is no interaction location. However, the position of maximum fluctuations still serves as a characteristic height of the thermal plume when there is no suppression spray. By defining the location of the interaction boundary based on the standard deviation of the IR intensity, the effect of the relevant variables on the location of the boundary can be investigated. Fig. 6 shows the height of the interaction boundary, zb , as a function of the wide angle nozzle flow rate for three fire sizes. As the flow rate increases, the momentum of the downward spray increases and pushes the location of the interaction closer to the base of the fire for all three fire sizes. Given that the downward momentum of the spray opposes the upward momentum of the fire, it is no surprise that at any given flow rate for the spray, increasing the fire size pushes the interaction region higher above the base of the fire. For example, when the nozzle is flowing approximately 0.11 L/s, the distance from the base of the fire to the interaction region increases by a factor of more than three when the fire size increases from 5 to 50 kW. In addition to changing the flow rate and heat release rate (5, 15 or 50 kW), the spray characteristics were changed by selecting either one of two spray nozzles or
395
0.4 0.3 0.2 0.1 0
0
0.5
1
1.5
Q W (L/s) Fig. 7. The location of the interaction boundary, zb , for a 15 kW fire. Open symbols represent the plume with no spray present (K: fire sprinkler, ’: wide angle nozzle, .: narrow angle nozzle).
the fire sprinkler. Fig. 7 shows the height of the interaction region for each spray source over a range of flow rates. The nature of the spray clearly has a large effect on the location of the interaction boundary. The wide angle nozzle has an interaction boundary that is higher than that of the narrow angle nozzle at an equivalent flow rate, indicating the wide angle nozzle is not as effective as the narrow angle nozzle at pushing the fire downward. This is a consequence of the more focused spray of the narrow angle nozzle. For the fire sprinkler, the wide distribution of the spray results in the need for higher flow rates, ranging from 1 to 1.5 L/s, to achieve the same interaction boundary height for zb than are necessary for the nozzles having almost an order of magnitude less flow rate. While these results indicate the nozzles push the fire plume downward more effectively than the fire sprinkler, they do not indicate that nozzles are more effective in suppression applications. Instead, the results show that spray characteristics such as droplet velocity and droplet concentration play an important role in determining the structure of the fire plume under suppression conditions. The interaction between a sprinkler and fire has often been cast in terms of the competition between the momentum of the downward spray and the momentum of the upward fire plume [7,8,21]. While this has been helpful, a slightly modified approach is used here to better understand the data. The momentum of the fire plume, M p , is still used to characterize the fire size. However, to characterize the strength of the spray, the drag of the spray, Ds , is used since this is the physical mechanism of the interaction between the droplets and the gas of the plume. Thus, a spray that has a very large effect on a fire plume does so by creating a large drag on the fire plume. The ratio of the drag of the spray to the momentum of the plume, Ds =M p , is a nondimensional parameter that characterizes the effect of the spray on the dynamics of the fire.
ARTICLE IN PRESS J.A. Schwille, R.M. Lueptow / Fire Safety Journal 41 (2006) 390–398
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To calculate this parameter, the drag of the spray, Ds , can be expressed as Ds ¼ 12 nrp C D Ad ð~ v ~ vd j, v ~ vd Þj~
(1)
where n is the total number of drops in the spray, rp is the density of the plume, C D is the drag coefficient of the droplets, Ad is the cross-sectional area of the droplets, ~ v is the velocity in the plume and ~ vd is the velocity of the droplets. For this simple analysis, the drag coefficient of the droplets is assumed to be close to that of solid spherical particles. This assumption ignores secondary effects such as droplet deformation, internal flow and evaporation, but serves as a good estimate. It is quite difficult to know the plume velocity, ~ v, but it can be approximated based on the plume relations for a fire with no spray [1]. Although the droplets will clearly affect the plume velocity based on the spray characteristics, this approximation suffices to develop a scaling parameter. The plume momentum can be calculated from the plume velocity profile and width as Z ~ 2 dA, Mp ¼ rvðrÞ (2) A
where A is the area of the plume at a given height. As in the spray drag calculation, the plume velocity and area can be calculated for correlations developed for fire plumes in isolation. A Gaussian profile based on plume correlations [1] was used to calculate the integral term in Eq. (2). The area of integration covers the region 0oro2 m. This area contains almost all of the plume momentum considering that at a radius of r ¼ 2 m the plume velocity has decreased to much less than 0.01% of its value at r ¼ 0, even for the 50 kW fire. Because the plume velocity changes with height, the plume momentum and spray drag were both calculated at a height of 1 m above the fire. Although somewhat arbitrary, this height is in the region where the droplets and plume interact, so it serves in defining a parameter to characterize the interaction. The ratio of the spray drag to plume momentum can then be expressed as 1 nC D Ad rp ð~ v ~ vd Þj~ v ~ vd j Ds ¼2 . R 2 Mp ~ dA r vðrÞ
0.8 0.7 0.6
(3) 0.5
p
The droplet velocity was measured using particle image velocimetry [11]. The droplet area was calculated from the volumetric median diameter measured at a limited number of flow rates using Phase Doppler Interferometry. Volumetric median diameters at other flow rates were interpolated from those measurements. Of course, another average diameter could have been used to calculate Ad , such as the Sauter mean diameter. While another method of averaging the diameters in a polydisperse spray would change the magnitude of the ratio Ds =M p , the volumetric median diameter serves to characterize the droplet size in a particular spray for relative comparison between different sprays and is often reported as a spray parameter [22]. Phase Doppler Interferometry also provided a droplet
zb /zb0
A
number density, which was used along with an estimated spray volume to provide a first order estimate for n. The droplet area and velocity measured near the spray source (approximately 0.15 m below the spray source on the centerline) are used to characterize the spray, since this represents the initial condition of the spray before it is affected by the plume. When the interaction boundary (normalized by the height of the maximum fluctuations for no spray) for 46 experiments performed with the three nozzles covering an order of magnitude variation in flow rate from 5.7 to 105.6 L/min and an order of magnitude variation in fire sizes from 5 to 50 kW are plotted as a function of Ds =M p , the height of the normalized interaction boundary decreases as the ratio of spray drag to plume momentum increases, as shown in Fig. 8. Although the collapse of the data is not perfect, the results show that once the spray drag is about 10% of the plume momentum, the plume is pushed down nearly to the base of the fire. At first glance it is surprising that the ratio Ds =M p is so small for high levels of suppression. However, some droplets in a spray may be outside the fire plume and thus not contributing their drag to suppression. The collapse of the data is much better for each individual spray source. The nonuniformities in the sprays and different spray areas may make it difficult to perfectly collapse these results. Of course, the nondimensional parameter Ds =M p is not intended to serve as an absolute fire sprinkler design parameter. Instead, it shows that the drag of the spray is critical to suppressing the structure of the fire. A recent theoretical study [23] has analyzed a fire plume under suppression by adding a droplet momentum term to Morton’s original plume analysis [2], which is the basis for generally accepted plume correlations used in fire analysis [1]. This model predicts a particular height above the fire
0.4 0.3 0.2 0.1 0
0
0.05
0.1
0.15
0.2
0.25
Ds/Mp Fig. 8. The location of the normalized interaction boundary, zb =zb0 , as a function of the drag on the air due to the droplet spray, Ds , to the momentum of the fire plume, M p (K: fire sprinkler, ’: wide angle nozzle, .: narrow angle nozzle).
ARTICLE IN PRESS J.A. Schwille, R.M. Lueptow / Fire Safety Journal 41 (2006) 390–398
origin where the plume widens rapidly and the plume velocity goes to zero. This height represents the boundary between the fire plume and the spray, much like the current measurements of zb represent the height of the interaction between the fire plume and the spray. This suggests that a comparison between the theoretical model and the measurements of the height of the interaction boundary would be useful. Fig. 9 shows the experimental data from all 46 experiments over a wide range of spray flow rates, fire sizes, and types of spray along with the model’s prediction of the interaction boundary for the spray and fire conditions of the corresponding experiment. The inputs to the model, which assumes a homogenous droplet spray counteracting a standard fire plume, are the droplet number density, the droplet velocity, the droplet diameter, the fire size and the initial diameter of the fire. The droplet parameters were those used in Eq. (3), and the fire parameters are directly known from the experiments. The results shown in Fig. 9 are encouraging. The trend of the data matches the trend of the model predictions with higher ratios of spray drag to plume momentum resulting in interaction regions closer to the base of the fire. The experimental data matches the model fairly well except for four data points (between zb ¼ 0:75 and 1.1 m, all related to 50 kW fires using the narrow angle nozzle or the fire sprinkler for suppression. For the fire sprinkler data, this is probably because the model assumes a uniform spray, which is not a good approximation for the fire sprinkler. In spite of the four data points that do not match the model, demonstrating that the IR measurements, which track the dynamics of the structure of the thermal plume, are similar to the predictions from a model based on the momentums of the fire plume and the spray indicates that momentum plays a key role in the interaction between the fire and the spray.
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3.3. Droplet velocity within the plume The IR measurements of the fire and thermal plume presented in this paper can be compared with measurements of droplet velocity in the same experimental configuration [17] to provide a better understanding of droplet penetration through fires. Figs. 10a–c show contours of both IR intensity and droplet velocity for the wide angle nozzle flowing 8.4 L/min for three fire sizes. The IR contours are from ensemble averages of the 25 IR intensity images, while the droplet velocities were measured with PIV [17]. The IR contour represents DT ¼ 190 10 C and the gray areas are regions where droplets are moving upward with a positive velocity, the boundary of which can be considered the limit of droplet penetration. For the 5 kW case (Fig. 10a), there is no gray region within the spray pattern of the wide angle nozzle, so it is clear that the droplets fall through the fire plume, which, although small, is still present near the burner. For the case of the 15 kW fire (Fig. 10b), the droplet velocity data indicate that the droplets have difficulty penetrating the plume to be exposed to temperatures over DT ¼ 190 10 C. This is evident from the maximum height of the gray region corresponding to upward-moving droplets, which is somewhat higher than the height of the contour for DT ¼ 190 10 C. For the largest fire (Fig. 10c), the region for positive droplet velocity is almost coincident with the contour for DT ¼ 190 10 C in the upper regions of the plume. Although the 15 kW fire shows a region of positive droplet velocity above the DT ¼ 190 C 1.6
1.6
1.6
1.4
1.4
1.4
1.2
1.2
1.2
1
1
1
0.8
0.8
0.8
0.6
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0
1.8 z (m)
1.6 1.4
z b (m)
1.2 1
(a)
0.4 0.2 0
0 0
0
0.05
0.1
0.15
0.2
0.25
Ds /M p Fig. 9. Comparison of measured zb to the interaction region found using a momentum-based model [23]. Open symbols are model predictions, filled symbols are IR-based measurements (K: fire sprinkler, ’: wide angle nozzle, .: narrow angle nozzle).
0.25
r (m)
0.5
0 0
(b)
0.25
r (m)
0.5
0
(c)
0.25
0.5
r (m)
Fig. 10. Plume area contour for the wide angle nozzle compared to droplet velocity measurements [17] for fire sizes of (a) 5 kW, (b) 15 kW, and (c) 50 kW. (Heavy solid curves: IR contour representing DT ¼ 190 10 C, light solid curve: contours of zero droplet velocity as measured with PIV. Gray regions have an upward velocity.) The center of the base of the fire is at r ¼ 0, z ¼ 0; the orifice of the wide angle nozzle is at r ¼ 0, z ¼ 1:6 m. The gray region in the upper right corner for the 50 kW fire represents droplets blown above the cone angle of the spray. The lower bound of the gray regions in the upper right corner for the 5 and 15 kW fires indicate the cone angle of the spray.
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contour while the 50 kW shows the reverse, the DT ¼ 190 C contour generally coincides with positive droplet velocity. Overall, for the wide angle nozzle most droplets are slowed to zero velocity before they have penetrated a significant portion of the high-temperature environment of the fire plume unless the spray is strong enough to push the fire plume close to the base of the fire as for the 5 kW fire.
4. Conclusions Results showing the structure and location of a fire plume for a variety of fire sizes and spray strengths have been presented. The IR images of the fire show that fire suppression is a very dynamic process, with the fire plume moving back and forth in response to the applied spray. The effective width of the fire is increased with increasing spray strength. However, this increase in effective width is not a result of an increase in projected area of the fire. In fact, the projected area of the fire remains relatively unchanged from no spray to fires being suppressed by wide range of spray flow rates. The location of the maximum deviation of the IR intensity of the fire can be used to define the interaction region between the fire plume and the suppression spray. The height from the base of the fire to this interaction region decreases as spray strength increases for all the sprays used in this study. Increasing the fire size increases the distance from the base of the fire to this interaction region. Finally, it is clear that the IR measurements define a portion of the plume into which droplets have difficulty penetrating. Previous researchers have often characterized the interaction between a spray and a fire plume by comparing the momentum of the spray to the momentum of the plume [24,25]. While this has been effective to generalize the penetration of water through a fire plume, the current study shows that the decrease in the thermal plume height due to spray application is governed by the ratio of the drag due to the spray to the momentum of the fire plume. Thus, while larger drops may penetrate through a fire plume, a spray at the same mass flow rate that consists of a greater number of smaller drops may serve better at pushing the thermal plume downward because of its higher drag.
Acknowledgments J.A.S. was supported by a National Science Foundation Graduate Research Fellowship and Northwestern University. Underwriter’s Laboratories (UL) provided laboratory facilities for this research. We gratefully acknowledge the support and encouragement of Dr. Pravin Gandhi (UL).
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