Cornstarch explosion experiments and modeling in vessels ranged by height/diameter ratios

Size: px

Start display at page:

Download "Cornstarch explosion experiments and modeling in vessels ranged by height/diameter ratios"

Teresa Peters
5 years ago
Views:

1 Journal of Loss Prevention in the Process Industries 14 (2001) Cornstarch explosion experiments and modeling in vessels ranged by height/diameter ratios S. Radandt a,*, Jianye Shi a, A. Vogl a, X.F. Deng b, S.J. Zhong b a Forschungsgesellschaft für angewandte Systemsicherheit und Arbeitsmedizin (FSA), Mannheim, Germany b Industrial Explosion Protection Institute (IEPI), Northeastern University, Shenyang, PR China Abstract Height to diameter (H/D) ratio is one of the important parameters affecting premixed particle air combustion characteristics. This paper focuses on the behavior of cornstarch combustion in closed vessels with changed H/D ratios and fixed volumes; and a combustion model is employed to simulate the experiments. An Eulerian Lagrangian approach for two-phase flows was used in the model and conservation equations of unsteady turbulent two-phase reacting flows were solved in two-dimensional domains. Heat loss to the vessel walls was taken into consideration in the model. The simulation results have a good agreement with those of experiments. Further simulations were carried out for higher H/D ratios from 8 to 15. These results show that H/D=8 is a changing point. When H/D 8, the maximum pressure and the rate of maximum pressure rise decrease with increasing H/D ratios. While H/D 8, the both have an increasing tendency with increasing H/D ratios Published by Elsevier Science Ltd. Keywords: Cornstarch; Dust explosion experiment; Closed vessel; Modeling; CFD 1. Introduction Numerous experiments in elongated vessels show that the height to diameter H/D ratio has an effect on the behavior of dust explosions (Bartknecht, 1993; Radandt and Shi, 1997). NFPA 68 (NFPA68, 1988) also suggests that explosion venting of vessels of H/D 5 should be considered specially. Pu (1988) had made laboratory scaled tests with different H/D ratios and volumes. Bartknecht (1998) had done large-scale explosion tests in vessels of which volumes and H/D ratios were changed but surface to volume ratio was fixed. Radandt (1983) had done venting experiments in silos with both horizontal and vertical vessels (V=20 m 3, H/D=6.25). Vogl (1996) investigated flame propagation in pipelines with different lengths (40, 48, and 60 m) and diameters (0.10, 0.15 and 0.20 m). Gardner, Winter, and Moore (1986) had studied explosion development and the transition from deflagration to detonation in coal dust/air suspensions in a long duct (H/D=70, D=1.2 m). Tests in vessels with changed H/D ratios and fixed volumes are left unknown in the former researches and * Corresponding author. address: fsa@radandt.de (S. Radandt). experiments. Our projects aimed to get more knowledge on dust explosion behaviors and phenomena on this aspect, and to lay a basis for dust combustion modeling. By using the combustion modeling, once the initial conditions are given, the explosion behaviors can be predicted. This paper focuses on the maize starch combustion in closed vessels with changed H/D ratios and fixed volumes. That is, the volume is kept to be 1 m 3 and H/D ratios are changed from 1, 2, 4 and 6, respectively. Generally, dust combustion models considering local flow variables can be cataloged into two classes according to multi-phase sub-models. One model named Eurlerian Eulerian (E E) considers particle phases as some special kind of fluid. Particles are grouped according to the particle size and each group is considered as a kind of fluid species. Another model is the Eulerian Lagrangian (E L) model. In this model particles are treated as non-continuous phases. A lot of work had been done by former researchers in the field of dust flame propagation and combustion modeling. Clark and Smoot (1985) had modeled the coal dust flame acceleration using three-zone model. Smirnov and Nikitin (1996) had modeled dust explosion in a 1.25 m 3 confined vessel. In their papers E L multi-phase model was used, and volatilization and combustion of char were taken into consideration. Korobeinikov, Mar /01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S (01)

2 496 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) Nomenclature A interface area of heat convection, m 2 A s pre-exponential factor, unit is mechanism dependent B s temperature exponential factor c dust concentration, kg/m 3 C p specific heat capacity, J/(kg K) C d drag force coefficient c s Rosseland mean absorption coefficient D diameter of the test vessel, m D s diffusion coefficient of specie s, m 2 /s d p diameter of particle, m E specific mixture energy (including chemical energy), J/kg E p energy flux from particulate phase, J/(m 3 s) f d drag force, N G gravity acceleration constant, m/s 2 G s mass flux of a single particle combustion, kg/s H height of the vessel, m h heat transfer coefficient, W/(m 2 K) I unit tensor of the second order I s mass flux of diffusion in gas phase, kg/(m 2 s) I q heat flux of thermal conduction in gas phase, J/(m 2 s) I s heat flux of thermal radiation, J/(m 2 s) k turbulent kinetic energy, J/kg M p momentum flux from particulate phase, kg/(m 2 s 2 ) m p mass of a particle, kg Nu Nusselt number P, p pressure, 10 5 Pa Pr Prandt number, 0.7 Q rate of heat transfer, J/(m 3 s) q specific heat, J/kg Re Reynolds number RMS Root Mean Square velocity, m/s S p net mass flux from particle phase, kg/(m 3 s) T temperature, K t time, s u velocity vector of gas phase, m/s u p velocity vector of a particle, m/s V volume, m 3 W molecular weight, kg/mol ẇ s net reaction rate of species sth in gas phase, kg/(m 3 s) x position vector of a particle, m mass fraction of sth species in gas phase Y s Greek a volume fraction of gas phase a s mass fraction of specie s in S p r density of gas, kg/m 3

3 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) l thermal conductivity coefficient, J/(s m K) t stress tensor of second order, N/m 2 s s Schmit number, 1.0 s B Stefan Boltzmann Constant, J/(m 2 k 4 ) n kinetic viscosity, m 2 /s e radiation emissivity; decay rate of turbulent kinetic energy, J/(kg s) Superscripts g l t w gas laminar properties turbulent properties wall kov, Klemer, Klemens, and Wolanski (1996) had used an E E method to model dust explosions and detonations and radiative heat transfer was taken into consideration. A simple dust flame propagation model on turbulent fluctuation velocity (RMS velocity) and laminar burning velocity was integrated in a gas explosion code FLACS (FLame ACceleration Simulation). Explosions in a 20 m 3 chamber were simulated with this modified computational fluid dynamics (CFD) code by Van Wingerden (1996). Krause and Kasch (1998) had modeled the flame velocity and laminar burning velocities using code FLU- ENT 4. Of these methods E E one is relative simple because it only needs minor modification of the gas combustion code to incorporate the function to deal with dust combustion. This method, however, is time consuming when particle size groups increase, and there are unnecessary diffusions for the particle phases. E L method adopts an Eulerian approach for the gas phase and a Lagrangian approach for the particle phase. The trajectories, mass changes, motions and combustion of particles in the calculation domain are traced in Lagrangian coordinates. Because of the large amount of the particles, some particles (typically several thousands) are represented as one computational particle (model particle). The E L model is adopted in present modeling. In this model, dust combustion process is considered as a transient two-phase turbulent reacting flow. The flow and combustion phenomena are governed by a set of conservation equations. The following sub-models are employed: Multi-phase model; Turbulence model; Combustion model of gas phase and of particle phase; Model of heat transfer to the vessel wall. 2. Experimental configuration A test method with a ring-pipe-nozzle to disperse dust is recommended by ISO6184 guideline (1985). When the volume of the vessels are small and kept to be constant, however, and H/D ratios are changed bigger the shapes of the vessels will be changed from cubic one to pipe. In this case, it is difficult to get homogeneous dust air mixture by using only one ISO recommended ring nozzle, and more ring-nozzles in a vessel will influence on the pre-ignition turbulence. To reduce those shortcomings, a new mushroom nozzle system was designed for these specific tests. Four mushroom nozzles are mounted on the surface of the wall of a vessel. The turbulence induced by mushroom nozzles is reduced to a lower level, and more homogeneous dust air mixture can be generated. Fig. 1(a) shows the experimental layout of ignition and dispersion system in a H/D=1 vessel. Four small dust chambers connected with a air reservoir are linked to the nozzles and the compressed air can be discharged at the same time by electromagnetic valves. Ignition is triggered after a pre-defined time (ignition delay time) of the dispersion. One setup of the test vessels (H/D=2) is shown as Fig. 1(b). The pre-existing turbulence is a function of ignition delay time. In the current experiment setup, when the ignition delay time is 0.38 s, the RMS turbulent velocity is similar to that of 0.60 s in a standard 1 m 3 vessel test chamber. The sample dust is cornstarch and some of the characteristics are listed in Table Physical model 3.1. Gas phase equations (ary s ) (ary t s u ) I s a s S p ẇ s (1)

498 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) 495 502 Table 1 Characteristics of cornstarch Medium diameter (µm) 15 Moisture (weight) 3.

4 498 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) Table 1 Characteristics of cornstarch Medium diameter (µm) 15 Moisture (weight) 3.5 (dried) Minimum ignition energy (J) Minimum ignition temperature (K) 673 Lower explosible limit (g/m 3 ) ~40 Maximum pressure (10 5 Pa) 9.4 Dust explosion index K st (10 5 Pa m/s) 217 (t u ) E p where I s ar D I n t s s Y (4) t ar(v I v t ) u ( u ) T 2 3 u 23 I arki (5) I q l I arc pn t Pr T (6) I s 16 s B T 3 c 3 T. (7) s The kinetic turbulent viscosity is got by: n t c m k 2 /e (8) and k, e are determined by k e model Particle phase equations m p d u p dt f d m p G (9) d x dt u p (10) dt p m p C p dt Q surface reaction Q convective Q rediation (11) where Fig. 1. Experiment layout of H/D=1 and H/D=2 vessels, V=1 m 3. (a) H/D=1, (b) H/D=2. f d 1 8 C darpd 2 p( u u u p) u u u p. (12) (ar u ) (ar u u ) (ap) t M t p (2) (are) (ar u E) (apu ) (I t q I s ) (3) 3.3. Combustion mechanism The combustion mechanism had been described in detail by Zhong, Deng, and Li (1998). The volatilization

5 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) of particles, combustion in gas phase and heterogeneous combustion of fix carbon were taken into consideration Heat loss to the wall of a vessel It is assumed that before the fireball reach the wall of a vessel, the radiation from the fireball to the wall can be ignored. In the calculation domain, only the cells adjacent to the wall are considered. The heat transfer to the wall includes heat convection and heat radiation. The convection transfer from a cell to the wall can be expressed as: q c ha(t g T w ) (13) where the convection heat transfer coefficient is obtained by: h Nul (14) D where Nu the Nusselt number, l the thermal conductivity coefficient and D the diameter of the vessel. Nu can be obtained by Chen (1990): Nu Re 0.8 Pr 0.33 (15) where Re u D (16) n I Pr is assumed to be 0.7. The radiation from a cell to the wall is: q r Aes B (T 4 g T 4 w) (17) where s B, Stefan Boltzmann Constant s B = W/(m 2 k 4 ); e, the absorption factor of a wall, ranged from 0.37 to 0.9 according to the lubricity and bright degree of the wall surface. The wall temperature is unknown, but it can be obtained by solving the heat conduction equation inside the vessel wall, or taken as a constant for simplification. 4. Comparison of the experiment and modeling results Typical temperature profiles of the simulation are shown as Fig. 2(a) and (b) Explosion pressure and maximum rate of pressure rise as the function of dust concentration The maximum pressures and maximum rates of pressure rise of different dust concentrations for H/D=1 are listed in Table 2. Fig. 3 demonstrates that both P max and (dp/dt) max of Fig. 2. Typical temperature fields in the H/D=1 vessel. (a) At 25 ms after ignition, (b) at 65 ms after ignition. Table 2 P max and (dp/dt) max under dust concentration in the H/D=1 vessel Dust Experiment Simulation Experiment Simulation concentration (g/m 3 ) P max (10 5 Pa) (dp/dt) max (10 5 Pa/s) experiment and simulation increase at first with increasing dust concentration ( g/m 3 ), keep nearly constant in the middle ( g/m 3 ) and then fall down ( 750 g/m 3 ). From Table 2 and Fig. 3, it can be seen

6 500 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) Fig. 3. P max and (dp/dt) max vs. dust concentrations in the H/D=1 vessel. (a) P max vs. concentration, (b) (dp/dt) max vs. concentration. Fig. 4. P max and (dp/dt) max in different vessels ranged by H/D ratios. (a) P max to different H/D ratios, (b) (dp/dt) max to different H/D ratios. that the simulation results have a good agreement with experiment ones The effect of H/D ratio It can be found in Table 3 and Fig. 4 that simulation results and experiment ones have a good agreement when H/D ratio is changed from 1 to 6. P max and (dp/dt) max decrease with increasing H/D ratio. The reason of this phenomenon is that the heat loss to the wall in vessels with smaller H/D is smaller, because the inner surface areas of vessels are smaller. For instance, the surface area is 3.69 m 2 for a H/D=1 vessel, but 6.71 m 2 Table 3 P max and (dp/dt) max in vessels ranged by H/D ratios H/D Experiment Simulation Experiment Simulation P max (10 5 Pa) (dp/dt) max (10 5 Pa/s) N/A 9.2 N/A N/A 10.7 N/A N/A 17.5 N/A 1070 for a H/D=6 one. Higher heat loss results in lower pressure. The compressing effect, on the other hand, plays an increasingly role with increasing H/D ratio. This effect can be described as follows: In a long duct, compared with a cubic enclosure with the same volume, there is narrow space for the expansion of hot combustion gases. The gases can only expand in two directions (sometimes one direction), unlike those in the cubic vessel they can expand in every direction. The unburned mixtures in the duct will be more pre-compressed than in the cubic vessel. Pre-compressing will make higher temperature and velocity. Higher velocity will then generate higher turbulence, and therefore the deflagration becomes stronger and stronger. When H/D 8, both P max and (dp/dt) max increase with increasing H/D ratio. That means when H/D 8 the compression effect plays dominant role when compared the role of heat loss (when H/D 8). In a H/D=15 vessel, P max can achieve Pa and (dp/dt) max up to Pa/s, which is a very sharp increase when compared with the results of the H/D=8 vessel (P max = Pa and (dp/dt) max = Pa/s). This result is reasonable when referring to Gardner s experiments (1986). It had been found in his experiments that in a 42 m long duct, P max could be (30 80) 10 5 Pa, and detonation had been observed. Pressure time curves in dif-

7 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) ferent positions (distance from the center) from the H/D=1 vessel to H/D=15 one are shown from Figs The pressure time histories at different positions in vessels of H/D=1 6 are nearly the same. That means a uniform distribution of pressure in the cubic and cubiclike vessels. On the contrary, in vessels of H/D=10 15, different positions have different pressure time histories. The pressure and rate of pressure rise at the end position, which are far away from the ignition source, are much higher than those at the center. After about s, the pressures at different positions in the vessel will become the same. 5. Conclusions and discussion Experiments in vessels ranged by H/D ratio show that when H/D increases from 1 to 6, the maximum pressure P max decreases a little, and the maximum rate of pressure rise decreases apparently; Simulation and experiment results have a good agreement in vessels when H/D is between 1 6; Simulation shows that when the H/D ratio increases from 8 to 15, P max and (dp/dt) max have different tendency than those when H/D ratio increases from 1 to 6. Both P max and (dp/dt) max increase with increasing H/D ratios when H/D are changed from 8 to 15. According to the simulation H/D=8 is the changing Fig. 6. Explosion pressure time histories in H/D=4 and 6 vessels. (a) H/D=4 vessels, (b) H/D=6 vessels. Fig. 5. Explosion pressure time histories in H/D=1 and 2 vessels. (a) H/D=1 vessel, (b) H/D=2 vessels. Fig. 7. Explosion pressure time histories in H/D=8 and 10 vessels. (a) H/D=8 vessels, (b) H/D=10 vessels.

8 502 S. Radandt et al. / Journal of Loss Prevention in the Process Industries 14 (2001) Fig. 8. Explosion pressure time history in a H/D=15 vessel. point of the decreasing tendency (H/D=1 8) and increasing one (H/D=8 15); The heat loss has the most important effect in H/D 6 vessels. While the compressing effect may play a dominant role in H/D 8 vessels. Acknowledgements This work of modeling part was supported by the National Natural Science Foundation of China (Project No ) and the experiment part by Forschungsgesellschaft für angewandte Systemsicherheit und Arbeitsmedizin (Project No. F ). The authors acknowledge Dr. F. Hauert for providing turbulence measurement data. References Bartknecht, W. (1993). Explosionsschutz Grundlagen und Anwendung. Berlin: Springer pp Bartknecht, W. (1998). Einfluß des Höhen/Durchmesser Verhältnisses Duckentlasteter Behälter auf den Flächenbedarf. Basel: Ciba-Geigy Bericht D 15/88. Clark, D. P., & Smoot, L. D. (1985). Model of accelerating coal dust flames. Combustion and Flame, 62, Chen, S. (1990). Heat transfer. Beijing: High Education Press of China. Gardner, B. R., Winter, R. J., & Moore, M. J. (1986). Explosion development and deflagration to detonation transition in coal dust/air suspensions. In: The 21st Symposium (International) on Combustion (pp ). The Combustion Institute. ISO 6184/1 Explosion Protection System Part 1 (1985). Determination of Explosion Indexes of Combustible Dusts in Air. Korobeinikov, V. P., Markov, V. V., Klemer, J., Klemens, R., & Wolanski, P. (1996). The unsteady flows of dusty gases with chemical reactions. In: Proceedings of International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions and 7th ICDE (pp ). Bergen, Norway. Krause, U., & Kasch, T. (1998). The influence of flow and turbulence on flame propagation through dust air mixtures. In: International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions and The 8th International Colloquium on Dust Explosions, IL. NFPA 68 Guide for Venting of Deflagrations, Nation Fire Protection Association (1988). Pu, Y. K. (1988). Fundamental characteristic of laminar and turbulent flames in cornstarch dust/air mixtures. Ph.D. thesis, Department of Mechanic Engineering, McGill University, Canada. Radandt, S. (1983). Staubexplosionen in Silos. Symposium Heft 12 (pp ), Berufsgenossenschaft Nahrungsmittel und Gaststätten. Radandt, S., & Shi, J. Y. (1997). Dust explosion in different vessels ranged by height to diameter ratios. FSA/BGN Internal Report. Smirnov, N. N., & Nikitin, V. F. (1996). Dust air mixtures evolution and combustion in confined and turbulent flows. In: Proceedings of International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions and 7th ICDE (pp ), Bergen, Norway. Vogl, A. (1996). Flame propagation in tubes of pneumatic conveying systems and exhaust equipment. In: The 29th Loss Prevention Symposium, New Orleans. Van Wingerden, K. (1996). Simulation of dust explosion using a CFD code. In: Proceedings of International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions and 7th ICDE (pp ), Bergen, Norway. Zhong, S. J., Deng, X. F., & Li, G. (1998). Modeling maize starch explosions in the 12 m 3 silo. In: Proceedings of the International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions, Schaumburg, IL.

Simulation of dust explosions in spray dryers

Simulation of dust explosions in spray dryers Dr.-Ing. K. van Wingerden, Dipl. Ing. T. Skjold, Dipl. Ing. O. R. Hansen, GexCon AS, Bergen (N) and Dipl. Ing. R. Siwek, FireEx Consultant GmbH, Giebenach