CFD based design for blast walls to mitigate explosion

20e Congrès de maîtrise des risques et de sûreté de fonctionnement - Saint-Malo 11-13 octobre 2016

CFD BASED DESIGN FOR BLAST WALLS TO MITIGATE EXPLOSION CONSEQUENCES: VALIDATION AND BEST PRACTICES

DIMENSIONNEMENT DE MURS DE PROTECTION CONTRE LES EFFETS DES

EXPLOSIONS A L’AIDE DE LA MECANIQUE DES FLUIDES NUMERIQUES : VALIDATION DE LA METHODE ET BONNES PRATIQUES

Elena Vyazmina, Simon JALLAIS Sophie Trélat Air Liquide – Centre de Recherche Paris-Saclay IRSN 1 chemin de la Porte des Loges 31 Avenue de la Division Leclerc, 78354 LES LOGES-EN-JOSAS 92260 Fontenay-aux-Roses Résumé Les murs de protection sont un moyen reconnu et efficace pour atténuer dans la zone post-mur les effets de surpression dus à des explosions de gaz accidentelles ou malveillantes. L'objectif de cet article est de présenter une nouvelle méthodologie pour la conception de ces murs pour un objectif de protection de biens et des personnes dans la zone post-mur. Cette nouvelle méthodologie utilise un calcul de dynamique des fluides numérique et se base sur l'analogie entre un éclatement du réservoir et une explosion de gaz. Des bonnes pratiques de conception basée sur cette approche sont proposées. Summary Protective wall is a well-known efficient way to protect people and infrastructures of overpressure effects from accidental or malicious explosions. The objective of this paper is to present a new methodology for an accurate design of protective blast walls. This new methodology is based on the analogy between vessel burst and vapor cloud explosion (VCE) and it is consequently used by a computational fluid dynamics code (CFD). Guidelines based on this approach are suggested to simplify the design of the protective barriers on an industrial site and to assess increase their efficiency.

Introduction

Nowadays, blast walls are an efficient way to significantly reduce the overpressure effects produced by an accidental explosion or a malicious explosion. These are passive mitigation barriers used to protect people and infrastructures from overpressure effects on industrial sites. However, there are no clear guidelines for the design of blast wall, which are adapted to a real accidental scenario. The prediction of blast effects behind the blast walls requires a detailed understanding of the phenomena taking into account the interaction of blast waves with a barrier and with the ground. This interaction strongly depends on the wall dimensions and on geometrical characteristics: height, width and thickness of the wall, angle of inclination of the front and the back side, distance between the centre of the explosive charge and the blast wall. Due to the complexity of the phenomena associated with the interaction between blast waves appearing from fast deflagration and the wall, computational fluid dynamics (CFD) based method is developed. The aim of this method is to reproduce the decay of blast waves (generated by a fast deflagration or detonation) in a far field upstream and downstream of the barrier, without modeling combustion process. This approach is validated versus experimental data for a simple configuration (free field) and for geometry with a blast wall.

Experimental setup The setup of Trélat, 2006 and Trélat et al., 2007 is a small-scale gaseous detonation experiments. The gas is a stoichiometric propane-oxygen mixture confined in a soap bubble (radius = 6 cm) on a large plane surface (length 1.8 m, width 1.2 m). Experiments were performed for a free field geometry and geometry with structures. Only results in free field from Trélat are used in the current paper. The detonation is ignited by an exploding wire (energy 200 mJ) at the center of the hemispherical bubble. The pressure gauges (Kistler 603B) are distributed on the plane surface in front of the soap bubble. S. Eveillard (2013) in the frame of an ANR project BARPPRO performed the same detonation experiments in the presence of a protection barrier in a geometry form of a merlon. The merlon is regular (angle 45°) with base length of 80 cm (cross-stream direction). High frequency pressure gauges are used to monitor overpressure history before, on and behind the blast wall. The experimental sensors are shown in the diagram below by green circles. The merlon located 14 cm downstream of the detonation hemisphere. Only the sensor located at 35 cm from the explosive charge on the top of the merlon did not give reliable results. No deformation of the merlon was observed. S. Eveillard, 2013 emphasizes that the measurements uncertainty varied from +/- 14% (maximum measured uncertainty) to +/- 10%. Error of 10% is recommended to apply to overpressure measurements.

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Figure 1. Propane/oxygen detonation experiment with a merlon sketch adapted from S. Eveillard, 2013.

Modeling approaches Pressure waves generated after a detonation are equivalent to the ones generated by a vessel burst problem (burst of a high-temperature and high-pressure reservoir) provided that the energy contained in the high-temperature and pressure reservoir is equivalent to the detonation energy released in the form of blast. In current simulations the temperature inside this reservoir is equal to the corresponding Chapman Jouguet (CJ) temperature (3843 K) and the pressure is CJ pressure (34.1barg). The radius Reqv of this high-pressure and high-temperature equivalent hemispherical vessel is calculated as follows: 23𝜋𝜋𝑅𝑅𝑒𝑒𝑒𝑒𝑒𝑒3 Δ𝑃𝑃

𝛾𝛾−1= α𝐸𝐸 (1)

Here Reqv is the radius of the equivalent vessel burst, ΔP is the overpressure inside the vessel (34.1 bars), 𝐸𝐸 is the energy released by the combustion (6875 J considering CO2 and H2O as products) and α is the energy equivalency ratio (1 for complete combustion giving final products CO2 and H2O but less than 1 in case of non-complete combustion, this will be discussed later on). For modeling of these experiments two commercial CFD codes are used: code FLACS v10.4 (Flame Acceleration Simulator) from GEXCON and the compressible solver of LS-DYNA from LSTC. There are different versions of FLACS dedicated to the modeling of different phenomena (see FLACS overview). FLACS-Blast is a special version of FLACS for simulating propagation of blast waves from detonation of condensed explosives. FLACS-Blast solves the Euler equations with a flux-corrected transport scheme of Boris et al., 1973. This version was recently validated for 3 different charges of high explosive in Davis et al., 2014. FLACS standard version is dedicated to explosions of gases and dusts. Since blast propagation from gas detonation is considered here, the standard version of FLACS is considered in the study. In current simulations FLACS version 10.4 is used. FLACS v 10.4 solves the compressible Navier-Stokes equations on a 3-D Cartesian grid using a finite volume method. FLACS 10.4 solves the RANS (Reynolds-Averaged Navier-Stokes) k-eps model equations for turbulence. The gravity is activated and is parallel to the vertical Z axis. A very short time step is used (CFLC=0.1, CFLV=0.1, also the option “keep low” is activated to keep very low time step). For FLACS simulations in the case of free field geometry the computational domain is 1.2 m in both horizontal directions [–0.6m; 0.6m] and 0.6m in the vertical direction [0m, 0.6m]]; two computational grids of 1cm and 0.5cm are used, with total number of cells of ~0.5M and ~1.5M correspondingly. For the geometry with a protection barrier simulation the computation domain is 2.3 m in the downstream direction [–0.6m; 1.6m], 2.4 in cross-stream direction [–1.2m; 0.2m] and 0.6m in the vertical direction [0m, 0.6m]]; 1cm and 0.5 cm grids are used with total number of cells of ~2.5M and ~15.2M correspondingly. The compressible fluid solver in LS-DYNA (named CESE) solves the compressible Navier-Stokes equations based upon the space-time conservation element and solution element method with a second order explicit scheme. A very short time step is used (0.2 µs). For LS-DYNA simulations in the case of free field geometry, the computational domain is 1.5 m horizontal direction [0 m; 1.5 m] and 0.8 m in the vertical direction [0m, 0.8 m]; a computational grid between 1 and 5 mm (stretching method), with total number of cells of ~120 000. For the geometry with a protection barrier the computation domain is the same as in the free field with total number of cells of ~450 000 (finer cells are used close to the barrier in order to catch properly the reflection waves in this area).

Free field Figure 2 shows the comparison of simulation results of FLACSv10.4 with experimental data for an energy equivalency ratio of 1. Simulations clearly overestimate experimental pressure at each position. In the case of the complete combustion of the stoichiometric mixture C3H8/O2 the combustion products are CO2 and H2O. In this case the combustion is isobar at temperature Chapman-Jouguet, This will give the molar enthalpy ∆H = -2.044 MJ/mole and the released energy of 15.2 MJ/m3 correspondingly.



Figure 2. Propane/oxygen detonation: comparison of FLACS with experiment: full combustion energy contributes to blast However these calculations seem to be too conservative (see figure 2). Detonation is a combustion process where the pressure increases violently, and the combustion is not necessarily completed especially with pure O2 as oxidant. In the case of the stoichiometric mixture C3H8+5O2 the combustion products are CO, H2, CO2, H2O and radicals. In this case, calculation performed with the detailed thermo-chemistry code GASEQ (considering dissociation) gives a molar enthalpy ∆H = - 1.005 MJ/mole, given the released energy of 7.48 MJ/m3. Hence the energy released by a detonation of stoichiometric mixture of propane/oxygen in a 6 cm radius hemisphere should be 3400J. Furthermore, parametric study shows that the best fit with experimental data is obtained for energy of 4100J (α = 60%), figure 2; the difference can be due to the part of energy induced by the explosive wire which cannot be neglected (cf. Trélat, 2006). Finally, it can be seen on figure 3 that computed energy and results from the parametric study are in quite good agreement for α = 60%.

Figure 3. Propane/oxygen detonation: estimation of energy contribution to blast waves

In the same way, simulations using LS-DYNA/CESE code (α = 60%) are in close agreement with experimental data too, as presented on figure 4. So the energy of 4100 J contained in the vessel will be used in the study. According to S. Eveillard, 2013 for stoechiometric mixture of propane/oxygen, Chapman-Jouguet velocity is 2357m/s, thus it will take the detonation front to reach the limits of the charge more than ~0.02ms. Thus the simulation results must be shifted in time on 0.02 ms for the comparison with experimental pressure signals.



Figure 4. Propane/oxygen detonation: comparison between experiments and LS-DYNA/CESE results

Figure 5 shows the pressure signal in time at 10cm, 20cm and 30cm from the charge. CFD (FLACS and LS-DYNA/CESE) simulation results match well experimental data in terms of the maximum overpressure and corresponding impulse.

Figure 5. Pressure evaluation in time for propane/oxygen detonation in a free field: FLACS results, vs. LS-DYNA/CESE vs.

experiment Trélat (2006).

Geometry with a protective barrier For a geometry with a protective barrier (merlon) α is taken to be 60% in equation (1). Figure 6 shows the comparison of experimental overpressure (in red color) with CFD results of LS-DYNA/CESE (in blue) and FLACS v10.4 (in green and black). Simulation results are in close agreement with experimental data. Since LS-DYNA/CESE is a non-viscous code the pressure rise is much steeper than for k-epsilon model (FLACS simulation). However FLACS gives good results as well, the maximum error between FLACS simulations and experiment is smaller than the uncertainty of the experimental measurements. In the case of monitor position 2 (located on the ground, 13.7 cm from the center of the explosive charge), experimentally and numerically (LS-DYNA/CESE) two peak-shape signal is observed. The first peak is the incident peak (from detonation) and the



second one is due to the pressure reflection from the merlon. In the case of FLACS simulations a quite coarse grid is used,which does not allow the separation of these peaks. The finest grid used by FLACS is of 0.5cm, where the distance from monitor point to the merlon is only 0.3cm. Hence to be able to resolve this double-peak structure the simulation grid should be atleast twice smaller than the distance between the monitor point and the merlon.



Figure 6. Pressure evaluation in time for propane/oxygen detonation for the geometry with a merlon: the monitor point position (on the left); FLACS results, vs. LS-DYNA/CESE vs. experiment (on the right)

The comparison of pressure signals behind the merlon (in the “wall shade”), shows good agreement between simulations and experimental data. Figure 7 demonstrates the maximum overpressure decay in the wall shade as a function of distance from the charge. The error between simulations and experiment is smaller than the experimental uncertainty. Figure 7 also represent the mitigation effect of the merlon. Just behind the merlon (at distance equal to a half-height of the merlon) the overpressure is twice lower than in the free field, at larger distances (at 4 heights of merlon) the overpressure is still 1.5 times lower than in the free field. This behavior represents the mitigation effects of the merlon on blast attenuation.



Figure 7. Maximum overpressure in the wall shade: simulations vs. experiments

Guidelines for design of barriers An extensive parametric study using previously validated method is performed. The first investigations focused on the length L of the barrier, which can strongly affect its mitigation properties. For the comparison with the infinite barrier, three finite lengths of wall are considered. It is found that for walls with aspect ratio higher than 6 (the ratio of wall length to its height L/h>6), the mitigation is approximately the same as for the infinite wall. For shorter walls, for instance L/h~3, the mitigation effect is strongly reduced. The overpressure for short walls is much higher than for the infinite wall, due to the pressure wave lateral overturning The effect of the amplitude of the incoming pressure on the mitigation of a barrier is an important parameter for the design of a protective wall. To understand this effect, the wall location is varied in order to obtain different levels of the incoming overpressure.

Incoming overpressure (mbarg)

Reflected overpressure (mbarg)

Max overpressure behind the wall (mbarg)

410 1 017 55 300 634 46 150 321 32

Table 1. Free field and reflected overpressure and maximum overpressure behind the wall for various upcoming overpressure levels (infinite wall)

The maximum overpressure value in the shade of the barrier depends on the magnitude of the incoming overpressure wave. Mitigation effect is more visible for higher incoming overpressure levels; however the reflected pressure is much higher as well.

Conclusion An equivalence vessel burst approach for a detonation can be used for simulations of blast propagation in a far field. The comparisons of simulation results and experimental measurement for the overpressure show a good agreement with and without a merlon in all the investigated configurations. This good agreement validates the presented modeling approach. The most important advantage of the use of CFD code is that it takes into account obstacles and protective walls, which can help to significantly reduce the associated distances. To avoid conservative results, it is needed to correctly estimate combustion energy contributing to blast. This can be done by a correct estimation of the combustion products (in current case the presence of radicals). The approach of the complete combustion for detonation seems to be too conservative. Parametrical studies performed by FLACS gives several guidelines for design of protective barriers:

1) it is recommended to build walls with aspect ratio L/h>6 to avoid overturning. 2) to obtain overpressure less than 50mbarg behind the wall the incoming pressure must be less than 300mbarg,

however the wall must be designed for 650mbarg to resist the reflected overpressure.



Acknowledgments The authors thank their companies for sponsoring this study and for permission to publish this paper.

References S. Trélat, Impact de fortes explosions sur les bâtiments représentatifs d’une installation industrielle, Thèse de doctorat, Université d’Orléans, 2006. S. Trélat, I. Sochet, B. Autrusson, O. Loiseau, K. Cheval, Strong explosion near a parallelepiped structure, Shock Waves 16 pp. 349–357, 2007. S. Eveillard, Propagation d’une onde de choc en présence d’une barrière de protection, Thèse de doctorat, Université d’Orléans, 2013. E. Vyazmina, S. Jallais, A. Beccantini, S. Trelat, “CFD design of protective walls against the effects of vapor cloud fast deflagration of hydrogen”, ICHS 6th, Yokohama, Japan, 2015. S. Trélat, E.Vyazmina, A. Beccantini, J. Daubech, S. Jallais, “Mitigation efficiency of a protective barrier against the effects of a Vapor Cloud Explosion”, ISSW30, Tel-Aviv, Israel, 2015. J.P. Boris & D.L. Book, “Flux-corrected transport, I SHASTA - a fluid transport algorithm that works”, Journal of Computational Physics, 11: 38–69, 1973. S.G. Davis & P. Hinze, “Simulating explosive pressure in test geometries with FLACSBlast”, Tenth International Symposium on Hazards, Prevention, and Mitigation of Industrial Explosions (X ISHPMIE), Bergen, Norway, 10-14 June 2014. FLACS overview: http://gexconus.com/FLACS_overview


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CFD based design for blast walls to mitigate explosion