CFD Modeling of Industrial-Scale Roll-Paper Fires

Transcription

1 CFD Modeling of Industrial-Scale Roll-Paper Fires Ren N.*, Zeng D., Karl V. M., Marcos C., Wang Y., Sergey B. D. FM Global, Research Division, 1151 Boston-Providence Turnpike, Norwood, MA, 02062, USA *Corresponding author ABSTRACT The purpose of this study is to develop and validate a pyrolysis model for modeling industrial-scale rollpaper fire growth. The rapid growth of a roll-paper fire is attributed to paper delamination caused by thermally-induced stress and paper peeling at the pyrolysis front. The paper delamination effect has been modeled in a previous study. This study presents a paper peeling model using a two-zone approach. In the vicinity of the pyrolysis front, the surface paper sheet is treated using a thermally-thin heat transfer model; while the rest of the paper sheets conduct heat in the in-depth direction. This study includes a small-scale configuration for model calibration and four large-scale arrays for model validation. The large-scale arrays use two array arrangements (standard array and open array) and three array heights (2-, 3- and 4-rolls high). The modeled flame heights and chemical heat release rates show good agreement with the measurements. Specifically, the open array shows a faster fire growth rate and a larger fire size than that of the standard array. The fire growth rate and fire size increase significantly with the array height. The maximum fire growth rate is between 1-4 MW/s, which is almost one order of magnitude higher than the standard Class 2 rack storage fires of similar height. KEYWORDS: CFD, FireFOAM, fire growth modeling, roll paper, pyrolysis. INTRODUCTION A roll-paper fire, due to its rapid fire growth rate, is one of the most severe hazards in the forest products industry [1]. There are many factors affecting the fire hazard of an array of stacked roll paper, such as paper weight (measured as area density, kg/m 2 ), paper composition, array configuration, array height and ceiling clearance, etc [2]. In most roll-paper storage warehouses, paper rolls are stored onend and stacked vertically with two typical configurations: standard array and open array [1], examples of which are shown in Fig. 1. The most hazardous arrangement is the open array, where fire can spread along all horizontal directions. The array height has a strong effect on the fire growth rate; as the array height increases, the fire growth rate increases dramatically. Driven by material handling and economic issues, roll-paper is increasingly being stored in taller, open arrays along with higher ceilings. Thus, providing adequate fire protection becomes considerably more challenging. Protection guidelines for high-storage, high-ceiling and open-array configurations are currently under development. However, the full-scale tests are prohibitively expensive and limited by the scale of the test laboratory. To gain a better understanding of the fire suppression physics and to facilitate optimal testing, CFD modeling study for roll-paper fire is carried out. The objective of the current study is to model the initial fire growth (free-burn) of a selected paper (Kraft paper, area density is kg/m 2 ). In this study, the tested and modeled configurations include one small-scale parallel-roll configuration (2 rolls, 2.13 m high), one standard array (2 3, 6.39 m high) and three open arrays (2 3) of 4.26 m, 6.39 m, and 8.52 m high. Fire growth modeling of lightweight papers and higher array will be conducted in the follow-up studies. Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8), pp Edited by Chao J., Liu N. A., Molkov V., Sunderland P., Tamanini F. and Torero J. Published by USTC Press ISBN: DOI: /c.sklfs.8thISFEH

2 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) Figure 1. Roll-paper array configurations. APPROACH Gas-phase model The numerical modeling tool used in the present study is FireFOAM [3, 4], which was developed recently based on the OpenFOAM [5] platform. In the gas-phase, FireFOAM solves the filtered transport equations for mass, species and sensible enthalpy together with the fully compressible Navier-Stokes equations [3]. The k-equation LES turbulence model [6] is used to solve the sub-grid kinetic energy and viscosity. Combustion is modeled by the eddy dissipation model (EDC) [7] with a single-step, infinitely-fast, non-reversible chemical reaction. The radiation transfer equation is modeled using the finite-volume discrete ordinate method. The emission source is assumed to originate solely from the reaction zone with a fixed radiant fraction of the chemical heat release rate (HRR). A 22% of radiant fraction is used assuming roll-paper pyrolysate has the same composition as the corrugated-cardboard pyrolysate [8]. The absorption of the participating media is neglected considering the short mean beam length of the configuration in this study. An empirical model, following the work of Wang et al. [9], is used to correct the convective heat flux on the wall. A detailed modeling description can be found in Ref. [9]. Solid-phase model Compared to the typical charring materials, such as corrugated cardboard and wood, roll-paper pyrolysis has three unique behaviors. The first is thermally-induced paper delamination, which has been modeled in the bench-scale fire propagation apparatus (FPA) study [2] and implemented into FireFOAM [10]. This form of delamination occurs when an intact sheet of paper ruptures upon charring. The second behavior is the paper-peeling phenomenon, occurs once initial delamination has happened. Along the edge of the pyrolysis front as shown in Fig. 2, the paper tends to peel away from the surface underneath. Once the paper peels, it becomes thermally-thin and is able to burn quickly. Consequently, the flame spread rate is significantly promoted. A third behavior, which consists of large-scale exfoliation of the paper sheets as they become separated and fall off the rolls, is not accounted for in this model. This behavior is observed much later in the tests and is not relavant to the initial fire growth phase. 454

3 Part II Fire Figure 2. Description and model of paper peeling in the vicinity of the pyrolysis front. In this study, a two-zone model is proposed and implemented into FireFOAM to model the paper sheet peeling. A major assumption for this two-zone model is that the width of the peeling zone, δ p, is a constant value, as illustrated in Fig. 2. Within the peeling zone, the paper is thermally-thin and its temperature is modeld by dt t q net = ρ C d A p, (1) where q net is the net heat flux to thermally-thin paper; ρ A is the paper area density (kg/m 2 ); C is the p specific heat. Outside the peeling zone, the paper is thermally-thick. The temperature profile in the in-depth direction is modeled by ( ρct p ) t ( k T) =, (2) where ρ is the bulk density; k is the thermal conductivity. The thermally-thick paper sheet becomes thermally-thin either (1) when paper sheet temperature reaches the critical delamination temperature T, or (2) by peeling at the pyrolysis front. d When a thermally-thin paper sheet burns out, the next paper sheet is exposed to fire and becomes the new solid surface. In the model, when paper delamination or peeling occurs, the stacked paper sheets are moved toward the solid surface by a paper-sheet thickness. Detailed description of this approach is available in Ref. [10]. The local burning rate is determined by both the formation rate of thermally-thin paper (by delamination or peeling) and the flame propogation rate along the thermally-thin paper sheet. The burning of a thermally-thin paper is simply modeled as 455

4 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) q m = net, (3) loss H p where m loss is the mass loss rate of a thermally-thin paper; ΔH p is the effective heat of pyrolysis. The paper material properties are listed in Table 2. Paper density, emissivity and heat of combustion, are directly measured. The peeling distance is obtained by calibrating the model against the smallscale parallel-panel test. The heat of pyrolysis is estimated based on the input heat flux and paper sheet burning rate in the FPA tests. Conductivity, heat capacity and critical delamination temperature are effective properties optimized from the bench-scale FPA study [2]. Property Table 2. Effective material properties [2]. Value Conductivity (W/(m K)) 0.2 Density (kg/m 3 ) 689 Heat Capacity (J/(kg K)) 1396 Emissivity a Heat of Combustion, ΔH c (J/kg) Critical Delamination Temperature (K) 608 Peeling Distance b, δ p (cm) 5 Heat of Pyrolysis, ΔH p (J/kg) a Value shown assuming flame temperature of 1400 K. Actual value varies. b Calibrated from a small-scale parallel-roll test. EXPERIMENTAL AND NUMERICAL CONFIGURATION The array arrangement has been illustrated in Fig. 1. The paper rolls are stacked vertically. For the standard array, the paper rolls are placed against each other in one horizontal direction and have a 15 cm flue space in the other horizontal direction. For the open array, there is a 15 cm flue space in both horizontal directions. In the test, the paper is ignited in the center of the array at the bottom of the paper rolls using two igniters (7.6 cm diameter and 7.6 cm long cellucotton cylinder soaked with 120 ml of gasoline). The peak chemical heat release rate (HRR) of one igniter is about 10 kw. The chemical HRRs were measured using a 20 MW Fire Products Collector (FPC) using oxygen consumption and carbon dioxide generation calorimetry. Fig. 3 shows an example of the mesh setup for the 2 3, 6.39 m high open array. The computational domain is m long (x-direction), m wide (y-direction), m high (z-direction). Four grid refinement zones are used to reduce the computational cost. The finest grid resolution is 2.54 cm. This finest mesh covers the whole roll-paper array. In the solid region, a non-uniform mesh is applied, with the smallest grid on the paper surface (equal to paper thickness, 1.84 mm) and a stretch ratio of 10%. The bottom surface of the domain is the floor and uses a wall boundary. The side and top surfaces use open boundaries for entrainment flow. In the test, the top of the array does not have delamination or peeling, and it was not ignited during the test. Therefore, the top of the array is set as an inert wall boundary in the simulations. The simulations are conducted on the EOS cluster provided by the Oak Ridge National Laboratory. For the 2 3, 6.39 m high open-array case, there are 3.36 M cells in the 456

5 Part II Fire gas phase and 4.11 M cells in the solid phase. The simulations were carried out using 256 CPU cores (Intel Xeon E5-2670, 2.60 GHz). Figure 3. Description of computational mesh for the 2 3, 6.39 m high open array. MODEL IMPLEMENTAITON A critical component of the current pyrolysis model is to identify which surface computational cells are in the thermally-thin zone. The judgement is based on the distance of a cell to its closest pyrolysis front (defined as d p). Fig. 4 presents a snapshot of the distance to the pyrolysis front and the corresponding thermally-thin zone. In Fig. 4, the front rolls of the array are removed to allow better visibility. The bottom of the center column is the ignition location. At this moment, three paper sheets on the center column have been ignited forming three distinct pyrolysis fronts. The paper peeling at the pyrolysis front is attributed to the discontinuity of paper sheet. For a given computational cell, the pyrolysis front chosen for calculating d p is the one that is located on the same paper sheet. An example is shown on the left of Fig. 4. For the point b, d p is defined as the distance of b to c, not b to a, even though the distance of b to a is shorter. Figure 4. Illustration of the distance to the pyrolysis front and the thermally thin zone at the pyrolysis front at t = 30 s, 2 3, 6.39 m high standard array. 457

6 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) For the standard array, the paper rolls touch with each other in one horizontal direction. However, the paper sheets are not physically connected. Therefore, the pyrolysis front on one column does not cause paper peeling on the neighboring column. In the simulation, a one-cell wide inert region is used to separate the rolls. In the model, the initiation of the pyrolysis front on a new paper sheet originates only from thermally-induced paper delamination (i.e., when the paper sheet temperature reaches the critical delamination temperature). RESULTS We first present the model calibration using the small-scale parallel-roll configuration. This configuration has two rolls with a separation distance of 15 cm. The detailed description of this configuration is available in Ref. [10]. Fig. 5 shows the modeled HRR with three peeling distances. If paper peeling is disabled, the modeled HRR grows much slower than that of the test. As the peeling distance increases, the modeled HRR growth rate becomes faster. For this paper type, a 5 cm peeling distance is found to provide an optimal HRR compared to the test. This peeling distance is then used in the simulations of four large-scale arrays. Figure 5. Study of parallel-roll configuration: chemical heat release rate with different peeling distance. Based on observations from large-scale fire tests, the fire grows in two initial stages. In the first stage the fire size is so small that it has negligible influence to the ceiling sprinkler activation (flame height < 2 m). The duration of this initial stage has large variability from test to test and is sensitive to factors such as lab condition and ignitor location relative to paper rolls. In the second stage, as the fire size becomes sufficiently large, the fire growth rate significantly increases, as in a typical vertical firespread scenario. In this study, we focus on the rapid fire growth period, as sprinklers are usually activated when the fire size is sufficiently large, where the flame height is comparable to the ceiling height. To enable direct comparison among tests as well as between test and model, the presented data have been time shifted based on the flame height time history curves. An example is shown in Fig. 6 for the 6.39 m high standard array and the 8.52 high open array, in which the flame height curves are shifted to roughly align at the 2 m high location. The flame height of the tests is obtained visually. In the simulation, it is obtained at the highest location of stoichiometric mixture fraction. The modeled flame heights shows good agreement with the tests. The standard array shows the same flame height as the open array. When flame height is lower than the array height (t < 40 s), the side 458

7 Part II Fire rolls have not been ignited. The fire mainly grows along the center rolls. Therefore, no difference is observed between standard and open arrays. Figure 6. Flame height comparison; experimental curves are shifted to align flame height at 2 m. The modeled and measured chemical heat release rate (HRR) are shown in Fig. 7. We first discuss the HRR of the 2 3, 6.39 m high standard array shown in Fig. 7(a). Before 35 s, the HRR is below 2 MW and the fire size grows slowly. At about 35 s, when the flame tip reaches the top of the array, the fire size starts growing rapidly from 2 MW to 15 MW in about 15 s. At 50 s, the flame has already spread to the end of the array and the HRR reaches a quasi-steady state, during which fire spread slowly to the outer surface of the array. The modeled HRR shows a reasonable agreement with the measurement. In the rapid fire growth stage, the predicted fire size shows a faster growth rate than the measurement. The fire size in the quasi-steady state is about 15% higher than the measurement. The HRRs for the 3 open arrays are shown in Fig. 7(b). Similar to that of the standard array, the openarray HRRs show the same trend initially. As the fire can spread in all horizontal directions, the open array has a longer rapid fire growth period and consequently a larger fire size. In the later stage during the tests, paper sheets in the outer layer of the array exfoliate, as shown in Fig. 8. The exfoliated paper sheets are quickly ignited and causes rapid fire growth. Compared to the standard array, paper exfoliation occurs earlier in the open-array configuration. A possible reason is that in the standard-array configuration, as the paper rolls are placed against each other in one direction, the paper sheet is held between the rolls; while in the open-array configuration, paper sheets are free on both sides. Therefore, the open-array paper sheets exfoliate more easily. Upon sprinkler activation (which is largely determined by the first rapid fire-growth stage) if the delivered water is sufficient, the fire can be controlled before paper exfoliation occurs. Therefore, the latter stage is less important for the fire suppression study. In the scope of this numerical study, the large-scale paper exfoliation is not modeled and the modeled HRR in this stage will be smaller than the measurements (e.g., at 80 s of the 8.52 m high open array). For the open arrays, the modeled HRR shows a good agreement with the measurements before paper exfoliation occurs. There is a clear turning point in the modeled HRR at 45 s. The sudden increase in the modeled HRR is attributed to the ignition of the side rolls of the array. Compared to the standard 459

8 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) array, the modeled ignition of the open-array side rolls is delayed for about 5 s, which is due to the increased distance between the side columns to the ignition location. However, this delay is not clearly observed in the fire tests. (a) (b) Figure 7. Chemical heat release rates; (a) standard array; (b) open array. Figure 8. Illustration of paper exfoliation in large-scale tests; left: 2 3, 6.39 m high standard array at 105 s; right: 2 3, 8.52 m high open array at 70 s. Flame contour snapshots are shown in Fig. 9 for the 2 3, 6.39 m high open array. At 35 s, the flame height reaches the top of the array. The side rolls have not been ignited. At 45 s, the paper sheets of the side columns are ignited at the second and third roll locations. At 60 s, the fire has spread to the outside surface of the array. The modeled flame contour agrees with the test well. Fig. 10 shows the maximum fire growth rate, which is obtained by taking a time derivative of the HRR using a 5 second time interval. Generally, the modeled maximum fire growth rate is larger than 460

9 Part II Fire the measurement for both standard and open arrays. For the open array, the discrepancies becomes larger as the array height increases, which may be attributed to the fact that the model prediction of ignition time of the side rolls is earlier than that in the test. The discrepancies will be further investigated in a future study. Figure 9. Illustration of flame contour, 2 3, 6.39 m high open array; top: test; bottom: model (front rolls are removed at 35 and 45 s to illustrate the flame contour inside the flue space). Figure 10. Maximum fire growth rates. 461

10 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) CONCLUSION This study presented a newly developed pyrolysis model for modeling industrial-scale roll-paper fires. The new model addressed the paper peeling phenomena at the pyrolysis front. The implementation of the paper peeling model uses a two-zone approach, where the paper sheet in the peeling zone is treated using a thermally-thin heat transfer model. The peeling distance was calibrated using a small-scale parallel-roll configuration and then the model was validated against four large-scale configurations: one standard array and three open arrays. The modeled flame height and HRR shows good agreement with the measurements in the intial fire growth stage. When paper exfoliation occurs, the mesasured HRR significantly increases and the predicted HRR is lower than the measurement. The modeled maximum fire growth rate is larger than the measurement. The model obtained the same trend with the test as the array height increases. Pyrolysis model improvements, such as fire growth modeling of higher open arrays and light-weight paper, as well as fire suppression modeling will be continued in follow-on studies. ACKNOWLEDGEMENT The work presented in this paper was funded by FM Global and performed within the framework of the FM Global Strategic Research Program on Fire Modeling. The authors thank the FM Global Research Campus staff for performing the full-scale measurements described herein. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR The authors gratefully acknowledge Ms. Ashley Barker and Ms. Suzy Tichenor at the Oak Ridge National Lab for their assistance. REFERENCES 1. Xin, Y. B., and Troup, J. M. A. Advancements in Fire Hazard Classification of Roll Paper, Proceedings of the 5th International Seminar on Fire and Explosion Hazards, Edinburgh, , Zeng, D., Chaos, M., Wang, Y., and Dorofeev, S. B. Experimental and Pyrolysis Modeling Study of Delaminating Materials, Proceedings of the 14th International Conference on Fire and Materials, San Francisco, , Wang, Y., Chatterjee, P., and De Ris, J. L. Large Eddy Simulation of Fire Plumes, Proceedings of the Combustion Institute, 33(2): , FireFOAM, available at 5. OpenFOAM, available at 6. Yoshizawa, A., and Horiuti, K. A Statistically-Derived Subgrid-Scale Kinetic Energy Model for the Large- Eddy Simulation of Turbulent Flows, Journal of the Physical Society of Japan, 54(8): , Magnussen, B. F., and Hjertager, B. H. On Mathematical Modeling of Turbulent Combustion with Special Emphasis on Soot Formation and Combustion, Proceedings of the Combustion Institute, 16(1): , Zeng, D., Chaos, M., Khan, M. M., and Dorofeev, S. B. Radiation Characteristics of Corrugated Cardboard Flames, In: Nilsson, D., Van Hees, P., and Jansson, R. (Ed.), Fire Safety Science Proceedings of the Eleventh International Symposium, 11: , Wang, Y., Meredith, K. V., Zhou, X., Chatterjee, P., Xin, Y., Chaos, M., Ren, N., and Dorofeev, S. B. Numerical Simulation of Sprinkler Suppression of Rack Storage Fires, In: Nilsson, D., Van Hees, P., and Jansson, R. (Eds.), Fire Safety Science Proceedings of the Eleventh International Symposium, 11: , Ren, N., Zeng, D., Meredith, K., Chaos, M., and Wang, Y. CFD Modeling of Fire Growth between Vertical Paper Rolls, 9th U.S. National Combustion Meeting, Cincinnati, Ohio, 17-20,