Car Park Fire Tests and Smoke Movements

Transcription

1 Car Park Fire Tests and Smoke Movements István Horváth 1, Jeroen van Beeck 2, Bart Merci 3 1, 2: Environmental and Applied Fluid Dynamics Department, Von Karman Institute, Rhode-St- Genèse, Belgium 3: Ghent University, Dept. of Flow, Heat and Combustion Mechanics, Ghent, Belgium 1: horvath@vki.ac.be Keywords: Car Park Fire, Smoke Extraction, Smoke Back-layering Distance Abstract In the frame of the Flamand IWT-SBO Project (#81) full- and small-scale measurements along with numerical simulations were carried out in order to increase fire safety in car parks. Because the primary design objective for smoke ventilation in car parks is typically not the evacuation of people but to allow fire services to approach the fire within a certain distance, the smoke back-layering (SBL) was investigated. SBL is the distance covered by the smoke upstream of the ventilation flow with respect to the fire source. It was measured in both full- and small-scales and the results are compared to the literature. Because there is extensive literature on road tunnels whilst car parks lack for it, the investigated car park (full- and smallscale) was simplified in order to be comparable to road tunnel results. Ceiling and walls were plain and the final layout is similar to a very wide road tunnel. Horizontal and vertical beams supporting the ceiling are negligible. Therefore the results obtained are only valid for large closed parks with a flat ceiling and uni-directional smoke and ventilation patterns within the investigated heat release rate range. Such study can serve double purposes: Empirical formula can be obtained and the best model for SBL can be chosen from the wide variety for road tunnel fires, which can be applied to car parks with the above described simplified geometries. If the results of the small-scale car park model are in accordance with the full-scale ones, the smallscale model can be used for more complex car park layouts, which is a very favorable solution in terms of both time and costs. The aim of this study is to present the measurement technique and results of full-scale car park and to compare the results to those obtained in the small-scale model and by models of road tunnel related literature. The description of the small-scale measurements is beyond the scope of this study and it can be found in [1]. Pool fire in the middle of car park was applied during the full-scale measurements to simulate car fires, which was substituted by an isothermal fire model in case of the small-scale measurements. Beyond of the SBL investigation, unexpected flow phenomena at the entrance and extractors, respectively, could be explained also by small-scale trials. i

2 List of Abbreviations and Symbols Abbreviation Meaning A Cross Section Area (m 2 ) c p Heat Capacity (J/kgK) D Diameter (m) f Frequency (Hz) g Gravitational Acceleration (m/s 2 ) H Height (m) HRR Heat Release Rate (kw) l* Dimensionless Smoke Back-layering Distance P Perimeter (m) PIV Particle Image Velocimetry Re D Reynolds Number based on diameter SBL Smoke Back-layering T Temperature (K) TI Turbulence Intensity (-) U Horizontal Velocity (m/s) U** Dimensionless Confinement Velocity U cr Critical Velocity (m/s) U fs Free-stream Velocity (m/s) Vel Velocity (m/s) VKI Von Karman Institute for Fluid Dynamics W Width (m) WIDIM WIndow Displacement Iterative Multigrid η, κ Constants are Depending on the Three Regions of Flame Defined by McCaffrey ρ Density (kg/m 3 ) z Height in Flame Midline 1 Introduction With the increasing number of road tunnels and car parks, the possibility of hazardous fire scenarios has increased as well. Whereas there is extensive literature on road tunnels, car parks lack for it. In the frame of the Flamand IWT-SBO Project (#81) researchers were given the opportunity to synthesize and to extend their knowledge on car park fires and to come up with solutions and advice that can increase fire safety. The full- (3 m x 28.6 m x 2.6 m) and small-scale (1.2 m x 1.2 m x.15 m) measurements were carried out on the premises of WarringtonFireGent NV, and at the Von Karman Institute, respectively, in Belgium. Simultaneously, numerical simulations were carried out in the Gent University [2]. The latter study was conducted separately and therefore its results are only cited. A detailed description of the small-scale measurements is given in [1]. Because the primary design objective for smoke ventilation in car parks is not evacuation of people but to allow fire services to approach the fire source within a certain distance [3], SBL was investigated in detail. SBL is the distance covered by the smoke upstream of the ventilation flow with respect to the fire source. Because it is measured at ceiling height, it can be negative as well if smoke reaches the ceiling downstream from the upstream edge of the fire source. In this study a simplified full-scale car park was investigated. The SBL results are compared to those of models developed for road tunnels ([4][5][6]) in order to choose the best one and an empirical formula is derived. The small-scale results are compared to the full-scale ones in order to verify the small-scale model, if it can be applied for more complex car park layouts in the future. Unexpected flow phenomena are also explained by small-scale trials. 1

3 Different heat release rates (2 kw - 4 kw) were investigated with different inlet velocities (.5 m/s 3 m/s) to characterize the SBL. The inlet velocity was set by the fans of the extraction system mounted in the ceiling at the rear end of the car park. The car park was empty during the experiments because this is the basic assumption in current standards for smoke and heat control [7], [8]. 2 Car Park Lay-out and Flow Phenomena In this chapter the lay-out and the flow phenomena are first introduced for the full-scale car park model prior to the small-scale measurement results. 2.1 Full-Scale Car Park Model Figure 1 shows the top view, with the fire (in the middle) under investigation and with the extractors. The entrance ( Inlet Opening ) can cover the entire front side, or only part of it. In the actual figure only the central 2% is open. The full-scale car park model has the dimensions of 28.6 m x 3 m x 2.6 m. The reader is referred to [2] and to [1] for more detailed description of the full-scale fire measurements. The fire source is a hexane pool burner, placed in the middle of the car park. The fuel floats on water in a 3 m x 1.4 m tray which is only partly covered by the carburant depending on the needed heat release rate (HRR) under investigation. In the present study, fire heat release rates of 2, 5, 1, 2 and 4 kw are discussed. Figure 1. Full-Scale Car Park Top View & 3D Sketch without Ceiling Figure 2 gives visual impressions of the different fire sizes. A picture of a real car experiment is also included (the HRR at the time of the picture is app. 15 kw). The pool and the real car fires were ignited with a torch on a long bar (Figure 1/a), and with some combustible material put in the rear seat, respectively. It can be seen that the flame size clearly increases with the HRR. The images in Figure 2 have been taken at the beginning of the fire cases, for visualization and for safety reasons. The smoke is still accumulating in the compartment and therefore the amount shown is less than in the final steady states. As will be discussed, one of the major problems with PIV measurements in fire is indeed the presence of smoke which is shown in Figure 3 and in Figure 4 at the extractors and at the entrance, respectively. At higher HRRs the smoke can fill the car park in time and exclude visualization, therefore images like in Figure 2/e & f cannot be acquired after a certain time. Moreover, without the sight of fire source, fire-fighting is blocked. Consequently, the aim of 2

4 ventilation in car parks is to allow fire services to see and approach the fire within a certain distance (typically: ~1 m). a) 2kW b) 5kW c) 1kW d) 2kW e) 4kW f) Car Fire (~ 15kW at time of picture) Figure 2. Investigated Heat Release Rates of Pool Fires & Real Car on Fire (note: the driver s window was open and the rear window of the car was removed prior to the test, which explains the flame plume at the back side of the car). Figure 3. Smoke from Extractors at 4kW Figure 4. Smoke at the Entrance at 4kW 3

5 Ceiling Temperature (ºC) Car Park Midline 16 m from Entrance Figure 5 shows the time evolution of the temperature as measured by means of a thermocouple under the roof in the symmetry plane of the car park, at 16 m from the car park entrance (position: Figure 1/X), for a Volvo 46 real car fire, with maximum extraction rate (V in = 195 m 3 /h). After reaching the maximum temperature, the fully-developed fire conditions last for about 1 minutes of which about 5 minutes are labeled as MaxFire. This is the highest HRR and therefore the most critical period of the car fire from the fire safety point of view Volvo 46 MaxFire 16:3:5 16:8:5 16:13:5 16:18:5 Time (hh:mm:ss) on 211/9/27 Figure 5. Evolution of Real Car Fire Ceiling Temperature in Car Park symmetry plane at 16m from the car park entrance (maximum smoke extraction flow rate). Note that the rear window of the car was removed prior to the test, so that air could enter the fire in its initial stages. When the ventilation openings are restricted, the fire development would be much slower (if it does not extinguish). While investigating the results of time resolved LS-PIV on the pool fire presented in [1], it was observed that the flame manifests a downstream-wise wave-like motion when the extraction is in working order. This can be the reason of the oscillation in the temperature plot of Figure 5. Although, recent studies indicate an increase in car fire HRR values [9], based on in situ real car fire measurements (Figure 6), 4kW was used as an average HRR of passenger car fire. The values shown in Figure 6 are obtained by averaging the results over the period of maximum temperature occurrence ( MaxFire in Figure 5; sampling at.1 Hz over the time period of 16:7:45 16:1:45 (hh:mm:ss)). The verification is indirect, since averaged temperature values along the midline are used, so that possible differences in radiation between the hexane fuel and the car are not fully accounted for. Yet, it is clear that the shapes of the car fire and the 4kW pool fire cases agree well, even though the 4 kw pool fire manifests higher temperature values at the inlet where locally the plot of 2kW pool fire gives better agreement. This discrepancy might be due to the fact that in the case of the 4kW pool fire the SBL reaches the entrance even at the highest frequency of the extractors. It is therefore advisory to use this higher HRR value in order to stay on the safe side and in order to count for the possible better burning passenger cars. 4

6 Ceiling Temperature (ºC) Volvo 46 4kW 2kW Distance from the Entrance of Car Park (m) Figure 6. Comparison of Max. Car Fire and 4kW Pool Fire Avg. Temp. Distribution in the Car Park Midline In order to characterize the smoke and heat control (SHC) system, the four frequency-controlled exhaust fans have been investigated in terms of extraction flow rates at different frequencies. The fans (V1, V2, V3, V4) in use were mounted in the ceiling according to Figure 1. The velocity of the flow at the extractors was obtained by a vane type anemometer. The volume flow rate is linearly dependent on the overall frequencies of the fans as follows in the fan law: Equation 1. Fan Law [1] Where the and 1 suffixes are for the reference and for the new state, respectively. The inlet velocity can be computed from the extraction flow rate, but the temperature difference between the inlet and the extractors has to be taken into account because the exhausted hot air has higher specific volume than the intake. This temperature difference is small because the temperature at the fans without fire and with maximum fire (4 MW) varied only between 25 C and 5 C (i.e vs. 1.9 kg/m 3 in terms of density) due to the immense amount of entrained fresh air. The inlet velocity, as well as in case of road tunnels, is defined as the volume flow rate of the intake divided by the area of the cross section filled by the flow. This can be done because the studied car park geometry is a quasi 1 D simplified geometry leading to nearly flat velocity profiles upstream of the fire. These profiles can be found in [1]. Since the frequencies of the fans are continuously controlled, and the temperatures at the inlet and at the extractors are monitored, they were used to set the inlet velocity. This can only be done by taking into account the former described density difference of the flow between the entrance and the extractors as follows: Equation 2. Velocity Ratio Between the Entrance and Extractors of the Car Park That is, the inlet velocity obtained by the fluid density at the extractors has to be multiplied by the Vel ratio in order to get the actual inlet velocity. This ratio is computed for each HRR case and it is inferred -along with Equation 14- by the multiplication described in In order to provide explanations for phenomena observed in full-scale, detailed reduced-scale measurements have been simultaneously carried out at the Von Karman Institute. The complete description of the reduced-scale measurements, which can be found in [1], is beyond the scope of this study, but some results for comparison and for elucidatory purposes are presented. 5

7 The relevant small-scales are the following: geometrical scale: 1:25; velocity scale: 1:5. The fire was modeled by an isothermal model where the buoyancy was reproduced by a mixture of Helium and Air in order to meet the requirements of the densimetric Froude number. Full-scale pool fire heat release rates of 2 kw, 5 kw, 1kW, 2kW and 4kW were scaled down. The extractors were situated in the scaled down positions of the real car park fans. The ventilation was realized by 4 extraction pipes, which meet in 1 common line driven by an axial fan. Therefore the flow-rates in the extraction pipes are the same and can only be changed together. 2.2 Flow Phenomena Interpreted by Small-Scale Measurement Results In order to understand the unexpected flow phenomena observed in the full-scale car park, PIV measurements were carried out on the small-scale model, which provides 2D velocity fields discussed below. Small-scale experiments revealed the reason of the smoke accumulation due to recirculation bubble at the entrance and the surprisingly low temperatures due to the fresh air indraught at the extractors. The thick smoke layer at the entrance of the car park (Figure 4) can be explained by Figure 7. This figure presents streamlines. It reveals that there is a recirculation bubble underneath the ceiling at the entrance. The turbulence intensity, defined in Equation 3, is at its maximum on the edge of this recirculation bubble. Equation 3. Turbulence Intensity The third fluctuating velocity component is assumed to be negligible with respect to the presented ones. Figure 7. Small-Scale Stream Lines at the Entrance on Turbulence Intensity Color-map (; at Mid.Entrance) Figure 8. Small-Scale Stream Lines at the Extractors on Turbulence Intensity Color-map (; at Mid.Extractor) The reason of small temperature difference between the entrance and the extractors is shown in Figure 8. It is only a small portion of the extracted volume flow rate, which is drawn from the hot upper layer, while the vast majority of the extracted volume flow rate originates from the cooler layers below. This explains why the temperature, measured in the center of the extractors, remains low (below 5 C), even in the case of 4 kw maximum heat release rate. 6

8 U** 3 Comparison of Full- and Small-Scale SBL Results of the Present Study A comparison is made in based on the pure critical velocity values obtained by different methods and by different scales. In this section the former results will be compared in the dimensionless way of Li et al. [11]. Figure 9 shows the dimensionless interpretation of the smoke back-layering distance in function of the dimensionless confinement velocity. It can be seen that both of the full-scale methods (45ºC / STD) are in agreement with the small-scale ones. Their trendlines practically coincide even if they differ below l* <. However these exponential trendlines are different from that of Li et al. in their slopes. It is probably due to the different geometries of road tunnels and car parks. Due to this difference there is not yet an ultimate dimensionless number or method by means of which all the different scales and geometries can be compared without discrepancies. This is dismissed hereafter for U cr with respect to HRR. Li et al. Full-Scale STD Expon. (Full-Scale 45C) Expon. (Small-Scale) 1,4 1,2 1,8 Full-Scale 45C Small-Scale Expon. (Full-Scale STD) y = e -,54x,6 y =,9254e -,153x,4,2 y =,9557e -,159x y =,9853e -,179x Figure 9. Full and Small-Scale Dimensionless Confinement Velocity (U**) vs. Dimensionless Back-layering Length (l*) Comparison of the Results with Li et al. [11] l* Equation 4. Dimensionless Confinement Velocity Equation 5. Dimensionless Smoke Back-layering Length 4 Comparison of Results - Present Study vs. Literature In the literature, many correlations exist of critical velocities for road tunnel fires. Concise descriptions are given e.g. in [12] and in [13]. After the investigation of the applicability of road tunnel models to car parks it was concluded that in the case of car parks the hydraulic diameter is applicable instead of the height and width dimensions of the room. Consequently the obtained data will be handled with the correlations of Wu and Bakar [5]. This correlation was derived in a study of on different aspect ratios (W/H =.5 4), concluding that neither the tunnel height nor the tunnel width is suitable itself for use as the characteristic length in the analysis of the critical velocity if the aspect ratio significantly changes. The hydraulic diameter is defined as follows: Equation 6. Hydraulic Diameter 7

9 The conclusion of Wu and Bakar is in agreement with numerical [3] and experimental [14] studies where the effect of different widths on the SBL distance proved to be either negligible or changed sign with its increasing value. This latter phenomenon was also observed with the increase in height. Hereafter the definitions of the different models are presented with the experimental data of the present study: Thomas (1968),[4] assumed that at critical velocity the Richardson number and therefore its reciprocal, the densimetric Froude number must be close to unity where the inertial and buoyant forces balance each other. This assumption resulted in the following equation: Equation 7. Thomas Critical Velocity Applying Equation 9 and Equation 1 on Equation 7 and using the hydraulic diameter instead of the height and width as described above, the dimensionless equations of Thomas forms as: Equation 8. Thomas Dimensionless Critical Velocity Wu and Bakar (2) [5] carried out a series of experimental tests in five model tunnels having the same height but different cross-sectional geometry (H =.25m; H/W =.5, 1, 2, 4). The study was a response to the uncertainties of the former semi-empirical equations obtained from the Froude number preservation combining with some experimental data. They use hydraulic diameter to describe the geometry of the cross section. The normalized critical ventilation velocity: Equation 9. Normalization of Critical Velocity - Applied to Car Parks (W/H > 3) The normalized heat release rate: Equation 1. Normalization of Heat Release Rate - Applied to Car Parks (W/H > 3) Finally the Wu-model can be computed as follows: Equation 11. Wu Dimensionless Critical Velocity Kunsch (22) [6], [15] yields an analytic formula for estimating the critical ventilation velocity, which is also given in dimensionless form. It is applicable for ventilation rates and heat release rates occurring in real road tunnel fires. Again, applying the hydraulic diameter instead of the height and width dimensions alike in the case of the model of Thomas, the definition forms as: 8

10 Dimensionless Critical Velocity Equation 12. Kunsch Dimensionless Critical Velocity Small-Scale & Small-Scale Eq. (211) are the small-scale results of the present study and their proposed empirical fit, respectively. Equation 13. Small-Scale Eq. Dimensionless Critical Velocity The corresponding velocity and heat release rate results were normalized by Equation 9 and Equation 1, respectively. Full-Scale (211) refers to the results of the presented Full-Scale measurement of the present paper. The critical velocity measurements are based on the standard deviation of temperature. The corresponding velocity and heat release rate results were normalized by Equation 9 and Equation 1, respectively.,45,4,35,3,25,2,15,1,5 Thomas - Model Kunsch - Model Horvath - Small-Scale Car Park Meas. Wu - Model Horvath - Emp.Eq. Horvath - Full-Scale Car Park Meas.,1,2,3,4,5,6,7,8,9 Dimensionless Heat Release Rate Figure 1. Dimensionless Critical Velocities in Function of the Dimensionless Heat Release Rate Literature vs. Model and Full-Scale Measurements It can be observed in Figure 1 that the small-scale and full-scale experiments of the present study are in agreement and they can be represented by the small-scale empirical equation (Equation 13). The discrepancy at the highest HRR*, which is related to the 2 kw case in full-scale, can be explained by the high uncertainty due to the coarse measurement points, described in Wu and Bakar significantly underestimate the critical velocity of the present study. The obtained experimental data are in agreement with the simplified equation of Kunsch and are close to the model of Thomas below HRR* =.5. The model of Kunsch can therefore be applied also for car park cases with flat ceiling and uni-directional smoke and ventilation pattern within the investigated heat release rate range. Even in the smaller HRR* values where the discrepancy is more significant the model predicts slightly higher critical velocity values, thus the designer considering them stays on the safe side. Apart from the measurement point of the smallest HRR*, qualitatively speaking, a difference of less than 1% is observed between the model of Kunsch and the present measurement results. The 9

11 critical velocity is more sensitive to the heat release rate at lower values; moreover these lower HRR values have higher uncertainty during the measurements. This reasoning, beyond the possible influence of the different geometries, can count for the discrepancy at the lowest HRR*, even though the small- and full-scale results show nice agreement. It is important to emphasize that the model of Thomas and the small-scale empirical equation are not able to represent the asymptotic behavior of the critical velocity for higher values of heat release rates and therefore the extension of the equations is limited and needs special attention to be paid on. The other models on the other hand take into account the so called super-critical wind velocity above which the critical velocity does not increase anymore with the power of the fire. 5 Conclusion In the frame of the Flamand IWT-SBO Project (#81) researchers were given the opportunity to synthesize and to extend their knowledge on car park fires and to come up with solutions and advice that can increase fire safety. The present study focused on the smoke back layering phenomenon. Whereas there is extensive literature on road tunnels, car parks lack for it, therefore the investigated car park (full- and small-scale) was simplified in order to be comparable to road tunnel results. In this way the simplified present test case can be considered as a high aspect ratio (W/H) road tunnel. Because the primary design objective for smoke ventilation in car parks with respect to fire safety is not evacuation of people but to allow fire services to approach the fire source within a certain distance [3], smoke back-layering (SBL) was investigated in detail. Different heat release rates (2 kw - 4 kw) were investigated with different inlet velocities (.2 m/s 3 m/s) to characterize it. An important output of SBL investigation is the critical velocity, at which no upstream smoking occurs in the ventilation flow with respect to the fire source. By comparing the former values to those of the literature obtained by different models for road tunnels the best model for car parks can be chosen. It is the model of Kunsch [15] simplified with the hydraulic diameter, instead of the use of the height and width dimensions. It is very close to the presented experiments, quantitatively a difference of less than 1% is observed. Based on the experimental results the follwoing equation can be introduced: The new empirical formula is only valid for large closed car parks with a flat ceiling and unidirectional smoke and ventilation pattern within the investigated heat release rate range (2KW - 4 kw). The simultaneous small-scale measurements, which were carried out at the Von Karman Institute, supported the full-scale measurements. They revealed the reason of the smoke accumulation due to recirculation bubble at the entrance and the surprisingly low temperatures due to the fresh air indraught at the extractors. Moreover the corresponding results of the small- and full-scale models in terms of dimensionless critical velocity prove that the isothermal fire modeling described in [1] can be applied with confidence for fires in case of more complex car park layouts, which cannot be handled by existing analytical models. By comparing the theoretical and empirical models, experiments and CFD simulations for road tunnels and for car parks it is obvious that there is no adimensionalization, which is capable to put all the data in the literature on one coherent curve from the critical velocity point of view. Moreover the effect of height and width dimensions on the SBL is ambiguous (see: [3], [14], [5]). The reason can be due to the fact that present models were verified only by small-scale model experiments or by rough full-scale experimental data. It is mainly because of the current lack of accurate full-scale experiments 1

12 on critical velocity. Therefore a future study with wide range of aspect ratios from the characteristics of road tunnels to those of car parks with wide range of heat release rates would be beneficial to better synthesize the existing models, amongst which there is significant difference in anticipation the critical velocity for the moment. 11

13 6 Appendices 6.1 Assessment of Smoke Back Layering Distance (SBL) SBL is the distance covered by the smoke upstream of the ventilation flow with respect to the fire source. It was measured in both full- and small-scales and the results are compared here. The beams on the ceiling in Figure 1 does not play important role. It has been investigated again in the smallsmall-scale model [1]. The reason is that the beams are shorter than the thickness of the smoke under the roof; moreover the isolation protecting the roof in the smoke area makes them even smaller in height Full-Scale Temperature as Smoke Tracer Parameter & SBL Indicator In the full-scale experiments, thermocouple rows have been installed underneath the car park ceiling (see: Figure 1) to monitor the temperature distribution. Table 1 shows the measurement grid of the actual thermocouples with the examples of averaged temperature values for the 1 kw heat release rate at the inlet velocity of 1.5 m/s. A temperature rise implies the presence of smoke. The temperature values in each measurement case were sampled at.1 Hz for a time interval of approximately 2s. Investigating the midline resolution, it can be seen that upstream, close to the fire source (14 m from opening) the intervals decrease to.3 m in order to capture the SBL with higher accuracy. Further from the source these intervals are increasing and by the entrance and by the rear end of the car park they reach 1-2 m. This means that the resolution of the SBL is higher close to the fire (SBL ) than far from it. This is reasonable because the aim is to obtain the critical velocity, which corresponds to zero SBL. Table 1. Measurement Grid of Thermocouples and Avg. Temperature Examples (1kW / 1.5m/s) Principle and Results of the Methods As shown in Figure 1, only the middle part of the car park (6 m) was open at the front side for the measurements presented here. The velocity upstream of the fire can be estimated from the accumulated frequencies of the extraction fans assuming that the inlet flow propagates with a linear increase in the width of cross section from the 6 m wide opening to the 16.2 m wide distance between the outer parts of extraction fans (see diverging dashed arrows in Figure 1). Note that Equation 2 has to be applied. The ratio between the velocity just upstream the fire (U fire ) -that is 14 m from the entrance- and the entrance velocity (U ent ) can be expressed by the area ratios (continuity equation) of the linearly developing flow as follows: Equation 14. Velocity Ratio at Different Positions: Pool Fire / Entrance 12

14 Temperature (ºC) Standard Deviation of Temperature This assumption gains confidence when comparing the results to those obtained in the case of smallscale and those of mathematical models (see: Figure 1). For the comparison the computed velocity at the fire was handled as the inlet velocity for a completely open car park. From the experiments carried out in the frame of this study it was proved that the temperature can be used as a tracer parameter for the smoke by means of two different methods. Either by simply applying the 45 C threshold value (see: 6.2) or by investigating the standard deviation of the temperature values over the elapsed time and finding the break points (transient) on the plots (see: 6.3). To apply the simple temperature threshold is evident and it can be seen in the cited appendix section. In the present study, at the height of the thermocouples in the midline of the car park, if the average temperature value over the time of investigation rises above 45 C, it is considered to be smoke, otherwise fresh air. 4m 5,5 7,3m ,2m 1m 12m :13: 16:13:44 16:14:27 16:15:1 Time (hh:mm:ss) Figure 11. Time Histories of Temperatures Measured by Thermocouples in the Midline of the Car Park at Various Distances from Fire Source HRR= 1kW;Uin = 1,8m/s , Upstream Distance from Fire Source (m) Figure 12. Standard Deviation of Temperature Histories of Figure 11 The use of standard deviation of temperature as a smoke tracer is more complicated thus Figure 11 and Figure 12 are shown for better comparison. From the figures it can be understood that the standard deviation of temperature manifests itself in distinct change in the edge of the smoke and thus can be applied to measure the smoke back-layering distance. The critical velocity is when the SBL distance is zero. It is an important measure of fire safety because it implies that the smoke is trapped downstream of the fire source and therefore this source can be found and approached by fire services. In the following figures SBL distances are plotted versus the inlet velocities, that is the plots cross the abscissa at the wanted critical velocity. 13

15 Critical Velocity (m/s) Critical Velocity (m/s) Smoke Back Layering (m) by Temperatue (45ºC) Smoke Back Layering (m) by STD of Temperature ,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5, Inlet Velocity (m/s) 2kW 5kW 1kW 2kW Linear (2kW) Linear (5kW) Linear (1kW) Linear (2kW) Figure 13. Smoke Back-layering in the case of Different Velocities and Different Heat Release Rates by means of the 45C Temperature Threshold ,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5, Inlet Velocity (m/s) 2kW 5kW 1kW 2kW Linear (2kW) Linear (5kW) Linear (1kW) Linear (2kW) Figure 14. Smoke Back-layering in the case of Different Velocities and Different Heat Release Rates by means of the Standard Deviation of Temperature Heat Release Rate (kw) Small-Scale Critical Velocity (m/s) Small-Scale Full-Scale by Model STD Break Original Full-Scale Critical Velocity (m/s) STD Break Multiplied 45C Node Original 45C Node Multiplied Table 2. Measured Full-Scale Smoke Back-layering Distances by Temperature Figure 15 and Figure 16 visualize the data of Table 2 about the critical velocities -obtained by either the 45 C threshold value or by the STD break point- in function of the heat release rates. Complimentary the results of the small-scale measurements are also shown for comparison. The three full-scale values reported are derived from: scaling up the small-scale model results ( Full-Scale by Model ), the actual critical velocity measurements without the correction of Equation 2 and Equation 14 ( Original ) the actual critical velocity measurements corrected by both Equation 2 and Equation 14 ( Multiplied ) Full Scale by Model 45C Node Original 45C Node Multiplied Heat Release Rate (kw) Figure 15. Critical Velocity Obtained by STD of Ceiling Temperature vs. HRR Full Scale by Model STD Break Original STD Break Multiplied Heat Release Rate (kw) Figure 16. Critical Velocity Obtained by Threshold Temperature of 45 C vs. HRR Clearly, agreement of the Multiplied velocities with those derived from small-scale measurements is excellent. Thus, the reduced scale model approach is favorable to carry out easy-to-handle and cost 14

16 effective measurements for car park fires with more complex layouts where analytical models cannot be applied. It can be observed that the number of samples for 2 kw is only 2, and that the 4kW case is completely missing. The reason is that the measurements had to be finished before accomplishing the prescribed cases. Because of this, the uncertainty on the 2 kw case is high. Using a threshold value of 45 C (see: 6.2) is faster, while using the break point (transient) of STD of temperature (see: 6.3) is a rather meticulous, but more objective way to obtain the smoke backlayering distance. Applying the simple temperature threshold value is faster because it needs only the average midline temperature values to be plotted along the length of the car park. The method of using the break point (transient) of STD of temperature histories is meticulous because it needs the time histories of temperatures at the different thermocouple locations, the STDs of temperatures have to be computed and finding the breakpoints cannot be automized. In Figure 1 the results of the STD method will be used for comparison purposes. It is also important to mention that the linear interpolation used at the full-scale measurements in order to obtain the critical velocities (see: Figure 13 & Figure 14) is just a simplification and it is only due to the coarse full-scale data. With the refined small-scale data the SBL distances manifested themselves in a polynomial curve in function of the inlet velocity as shown in Figure 19. It can be seen from the former plot that the linear simplification is reasonable if the data set contains points around zero Smoke Back-Layering Distance (SBL) at Small-Scale Model SBL values have been obtained by investigating the smoke concentration with the Mie scattering concentration measurement technique [16], as presented in Figure 17 (corresponding to a full-scale fire of 1kW). SBL is the distance covered by the smoke upstream of the ventilation flow with respect to the fire source (see: Figure 17/a/SBL). Because the SBL is measured at ceiling height, it can be negative as well if the smoke reaches the ceiling downstream from the upstream edge of the fire source (see: Figure 17/e,f). a) U in =.95 (.19) m/s d) U in = 1.85 (.37) m/s b) U in = 1.1 (.22) m/s e) U in = 2.1 (.42) m/s c) U in = 1.2 (.24) m/s f) U in = 2.2 (.44) m/s Figure 17. Small-Scale SBL by means of Mie Scattering Concentration Measurement Technique for 1kW Fullscale Case with different Inlet Velocities Small-Scale Values in Brackets 15

17 Critical Velocity (U cr ) [m/s] {O} Dimensionless Critical Velocity (U cr *) {X} Smoke Back-layering (mm) Smoke Back-layering (mm) The modeled pool fires for different real-scale heat release rates (2 kw, 5kW, 1 kw, 2 kw, 4kW) were investigated for different upstream velocities in the small-scale car park. The smoke concentration cut-off threshold was set at 5% with respect to the concentration of the source. The results of small-scale experiments are presented along with the full-scale and literature data in Figure ,5,15,25,35,45,55-1 Inlet Velocity (m/s) 2kW 5kW 1kW 2kW 4kW Intersect Poly. (2kW) Poly. (5kW) Poly. (1kW) Poly. (2kW) Poly. (4kW) Figure 18. Small-Scale Smoke Back-layering Distance (SBL) vs. Inlet Velocities (U in ) kW 5kW 1kW 2kW 4kW All(Ucrit-Uin) Poly. (All(Ucrit-Uin)) y = 516,5x ,9x ,13x R² =,9656 -,5,5,15,25-1 Critical Velocity - Inlet Velocity (m/s) Figure 19. Small-Scale Smoke Back-layering Distance (SBL) vs. Difference of Critical and Inlet Velocities (U cr - U in ) The critical velocity results for all of the HRR cases are shown in Figure 18 & in Figure 19. The critical velocities are summed up and empirical equations are proposed in Figure 2 and in Equation 15, respectively. 3 2,5 2 1,5 1,5 Dimensionless Heat Release Rate (HRR*),2,4,6,4,3,2, Heat Release Rate (HRR) [kw] Figure 2. Small-Scale Critical Velocities vs. HRR with Empirical Equations: Figure 2/Dashed Line Figure 2/Dotted Line Figure 2/Solid Line 16 Equation 15. Small-Scale Possible Empirical Equations for Dimensionless Critical Velocity It can be concluded that the 1/3 power of HRR*, widely used in the literature [17], [5], does not give the best fitting. This might be due to the fact that the cited formulas are based on road tunnel measurements and not on car parks as the present study. The 1/4 power gives better approximation in agreement with the tendency of the full-scale simulations of Tilley [3], who was also working on car parks. Nevertheless, the best trendline is provided by the natural logarithmic equation, which will therefore be used in Figure 1 of the next section, where further conclusions are drawn.

18 Ceiling Temperature (ºC) Ceiling Temperature (ºC) Ceiling Temperature (ºC) Ceiling Temperature (ºC) 6.2 Smoke Back-layering by the Average Midline Ceiling Temperature (Threshold: 45 C) 135 9,93m/s 1,86m/s 1,49m/s Node Upstream Distance from Real Scale Fire Source (m) Figure 21. Node Points of Midline Ceiling Temp. with 45 C in the case of Different Inlet Vels 2kW Pool Fire ,93m/s 1,86m/s 3,36m/s Node 1,49m/s 2,8m/s 3,73m/s Upstream Distance from Real Scale Fire Source (m) Figure 22. Node Points of Midline Ceiling Temp. with 45 C in the case of Different Inlet Vels 5kW Pool Fire ,93m/s 1,49m/s 1,86m/s 2,8m/s 18 3,36m/s 3,73m/s C Node Upstream Distance from Real Scale Fire Source (m) Figure 23. Node Points of Midline Ceiling Temp. with 45 C in the case of Different Inlet Vels 1kW Pool Fire ,8m/s 18 3,73m/s 135 Node Upstream Distance from Real Scale Fire Source (m) Figure 24. Node Points of Midline Ceiling Temp. with 45 C in the case of Different Inlet Vels 2kW Pool Fire 17

19 STD of Temperature (ºC) STD of Temperature (ºC) STD of Temperature (ºC) STD of Temperature (ºC) 6.3 Smoke Back-layering by the Standard Deviation of Midline Ceiling Temperature ,93m/s 1,86m/s 1,49m/s Break Upstream Distance from Real Scale Fire Source (m) Figure 25. Break Points of STD of Midline Ceiling Temp. in the case of Different Inlet Velocities 2kW Pool Fire ,49m/s 1,86m/s 2,8m/s 3,36m/s 3,73m/s Break Upstream Distance from Real Scale Fire Source (m) Figure 26. Break Points of STD of Midline Ceiling Temp. in the case of Different Inlet Velocities 5kW Pool Fire 35 1,49m/s 1,86m/s 3 2,8m/s 3,36m/s 3,73m/s Break Upstream Distance from Real Scale Fire Source (m) Figure 27. Break Points of STD of Midline Ceiling Temp. in the case of Different Inlet Velocities 1kW Pool Fire ,8m/s 3,73m/s Break Upstream Distance from Real Scale Fire Source (m) Figure 28. Break Points of STD of Midline Ceiling Temp. in the case of Different Inlet Velocities 2kW Pool Fire 18

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