numerical simulation of hydrogen release in a private garage

Proceedings of Hypothesis IV, 9-14 September 2001, Strahlsund, Germany, p. 368 NUMERICAL SIMULATION OF HYDROGEN RELEASE IN A PRIVATE GARAGEW. Breitung, G. Necker*, B. Kaup, A. Veser*Research Center Karlsruhe, Institute for Nuclear and Energy Technologies, P.O. Box 3640,76021 Karlsruhe, Germany, breitung@iket.fzk.de*ProScience GmbH, Parkstr. 9, 76275 Ettlingen, necker@pro-science.deKey words: Numerical simulation, hydrogen distribution, garage, computational fluid dynamics, risk assessment, safetyAbstract:This paper investigates the formation of combustible clouds from pulsed release of hy-drogen in a garage. The analysis consists of two steps: (1) 3d calculation of the time and space dependent H2-concentration field, and (2) evaluation of the resulting combustion hazard, using newly developed criteria for flame acceleration and detonation potential. In case of a low release rate (0.34 g/s over 100s) the possibility for a slow or fast deflagration exists essentially only during the release period and involves only few g H2. Detonations are excluded. The mixing processes have sufficient time for H2 dilution. In case of the fast release rate (3.4 g/s over 10s) the balance between H2-source and mixing processes is dominated by the source leading to large inhomogeneities, a long term potential for slow deflagration, a short term (20 s) potential for fast turbulent deflagration and detonation. For control of such situations effective mitigation measures appear necessary and feasible.1. Introduction and objectivesWith the introduction of hydrogen fueled passenger cars into the market, the inadvertent release of hydrogen in a private garage will become an important safety issue. The risk of such an event is re-lated to the fact that combustible H2-air clouds will develop in the garage, even when on a homoge-neously mixed basis the average gas composition is far below the flammability limit. The time de-pendent cloud size and its composition result from a complicated interaction between properties of the hydrogen source, the active transport and mixing processes, and the available hydrogen sinks.In this paper we investigate the evolution of combustible H2-air mixtures during H2-release in a garage and evaluate the resulting hazard potential with newly developed criteria. The objective is to quantify the risk for slow and fast combustion modes by calculating associated hazard parameters like size, composition and time of existence of reactive clouds.2. MethodologyA systematic step-by-step procedure for the deterministic analysis of hydrogen behaviour and miti-gation was developed previously in the frame of nuclear safety research programs at FZK [1].The procedure can be subdivided into four main parts (Fig.1):(1) modelling of the H2-air mixture generation within the enclosure,(2) prediction of the hazard potential of this mixture with respect to flammability, flame accel-eration and detonation on-set, using newly developed criteria,(3) numerical simulation of the appropriate combustion regime with verified 3D-computationalfluid dynamics (CFD) codes, and(4) consequence analysis based on the calculated pressure and temperature loads.In this paper we restrict the analysis to the first two parts. Each part consists of several steps. The information needed for each step, and the assumptions made for this analysis are now summarized.2.1 Combustible mixture generationStarting point of any analysis is the geometrical modeling of the building in an appropriate compu-tational grid. The building walls and the inner surfaces define the mathematical boundary conditions for the solution of the 3d fluid-flow equations solved in the CFD model.Fig.1 Step-wise procedure for deterministic analysis of hydrogen behaviour in risk studies.In the present analysis a simple rectangular garage of 2.4 m height, 4.5 m width and 6.5 m length containing a parked vehicle, is modeled. The garage door has two vent openings in the upper re-gion. Otherwise the garage is closed to the outside. Two different meshes were applied in the analy-sis, one consisting of 10´10´10 cm3 cubic mesh cells (» 70 000 cells), the other with a refined mesh of 10´10´3.3 cm3 in the upper 70 cm of the garage volume (» 111 000 cells).For a defined geometry, the next step of the hydrogen analysis is definition of the mitigation meas-ures which are to be included in the modeling. In the present case one could e.g. investigate the ef-fects from additional vent openings, from forced ventilation or from a catalytic garage ceiling. The GASFLOW code includes verified models which allow to predict the effects of these mitigation ap-proaches on the hydrogen distribution process. Mitigation measures are not addressed in this paper due to space limitations.For a given geometry and mitigation system, the next question is what events should be covered in the analysis as representative accident scenarios. For the present study an intermittend release of cold hydrogen gas, which could e.g. represent the boil-off from a cryogenic tank is investigated.After definition of the relevant scenarios, the corresponding hydrogen sources must be specified. For consistent CFD calculations the mass, momentum and energy of the released hydrogen are needed for the whole duration of the simulation. In addition, the location and direction of the H2-source flow must be selected. For estimation of the release rate due to boil-off, a thermal energy deposition into the cryogenic tank of 1 W is assumed. The vaporized liquid (170 g/day) is released in 5 pulses per day (=34 g/pulse) over a period of 10 or 100 seconds, respectively (3.4 and 0.34 g H2/s). The gaseous hydrogen is released at a temperature of 22.3 K underneath the trunk in down-ward direction, which represents conservative assumptions with respect to hydrogen mixing with air. If homogeneously mixed with the available air in the garage, the released H2-mass per pulse (34g) would lead to a concentration far below the flammability limit (0.6% H2). However, due to rate limitations in the physical mixing processes even for such apparently sub-critical H2-sources, heterogeneities and combustible clouds must be expected. The question is where do they develop, how much hydrogen is involved, and what combustion regime could result for what time period. With given characteristics of the hydrogen source, the distribution and mixing with air can be calcu-lated using the verified 3d CFD code GASFLOW [2,3]. The outcome from this step of the analysis are temperature, pressure and composition of the H2-air mixture as function of time and location. In the present case the following physical phenomena must be modeled or considered: single-phase, multicomponent gas flow under the influence of gravity, pressure gradients, viscous stresses and drag from inner surfaces and flow restrictions, molecular transport including hydrogen diffusion, heat transfer from hydrogen (22 K) to air (300 K), and effect of turbulence on mass, energy and momentum transport.2.2 Hazard potentialThe second part of the risk assessment evaluates the hazard potential that the calculated hydrogen-air distribution may represent to structures and persons if an ignition should occur (second column in Fig.1). Within the FZK hydrogen research program three criteria were developed to classify the possible combustion regimes and their consequences. These empirical criteria were derived from available literature data and many own tests with H2-air-mixtures. The criteria capture the observed combustion regimes in a condensed form and allow predictions with respect to the risk of a given theoretically calculated H2-air distribution.We distinguish three hazard levels. The first one is flammability of the mixture. In the present case of pure hydrogen-air mixtures at ambient conditions we assume that flammability exists if either the hydrogen concentration reaches 4 vol % (lower explosion limit) or the oxygen concentration reaches 5 vol% (upper explosion limit, = 75 vol % H2 in air) [4]. To quantify and compare the haz-ard with respect to flammability the burnable volume and hydrogen mass within the range of 4 to 75 vol% H2 was computed for the investigated release scenarios.A second hazard level is reached if the computed mixture can support flame acceleration under conservative conditions, i.e. with high degree of confinement and turbulence generation by flow ob-stacles. The systematic analysis of all experimental data with different scaling methodologies showed that the expansion ratio s (= ratio of specific volumes, m3kg-1, of burned to unburned gas at constant pressure) is the dominating mixture parameter which determines the flame propagation re-gime [5]. The critical s-value above which flame acceleration was observed in H2-air mixtures at ambient initial temperature iss crit = 3.75 ± 0.25. (1)This expansion ratio corresponds to 10.5 vol% H 2 on the lean side, and 75 vol% H 2 on the rich side. For the present analysis the potential for flame acceleration is quantified by computing from the known 3d hydrogen distribution as function of time the cloud volume for which the expansion ratio exceeds the critical value.The third and highest hazard level is reached if the computed H 2-air mixture can support detonation on-set under conservative conditions, i.e. high confinement and turbulence generation. A large number of joint experiments were performed in various facilities of the Kurchatov Institute Moscow and Research Center Karlsruhe [6]. In addition the available literature data on detonation onset con-ditions were collected and critically evaluated [7]. The final result is that a minimum scale require-ment for onset of deflagration-to-detonation-transition (DDT) exists, which can be expressed as: D ³7l (2) where D is the characteristic size of the reactive gas cloud taken as third root of the cloud volume (D=V 1/3) and l is the detonation cell width of the average gas mixture in the reactive cloud. In the present study we have evaluated the ratio D(t)/7l (t) for the flame acceleration cloud (³ 10.5% H 2) during and after the hydrogen release to quantify the potential risk for DDT in the different release scenarios.3. Calculations performed3.1 ScenariosHydrogen is released in a simple rectangular garage with two different rates into initially stagnant air at ambient temperature. Table 1 summarizes the assumed parameters.Table 1 Investigated garage geometry and hydrogen release scenariosGEOMETRY HYDROGEN SOURCE CASE Volume (m 3) Vent Openings H 2-Rate (g/s) Duration (s) Total Mass (g) Release Temp. (K) Release Location Nr. 70.2 Two times 10´20 cm 23.40 10 34 22.3 under- neath 10.34 100 34 22.3 trunk 23.2 GASFLOW sensitivity studyTo investigate the effects from numerical methods on the computed hydrogen distribution in the garage the following modeling variations were made:- first and second order numerical solver (Donor, vanLeer),- equidistant grid (D x=D y=D z=10 cm), and vertically refined grid near the ceiling (D z=3.3 cm), - diffusion terms on and off, k-e turbulence model on and off.The highest correlation was observed between the size of the computed combustible cloud (>4 % H 2) and the used grid resolution. The edge of the cloud is sensitive to numerical diffusion because of its very low H 2-concentration value compared to the source. The inner, more H 2 enriched regions of the combustible cloud, which are evaluated in the s - and l -criterion resp., showed little depend-ence on any of the investigated model variations. The results described in the next section refer to the calculation with second order solver, refined grid, no turbulence and no molecular diffusion. Weconsider this our current best estimate calculation because the neglection of of these processes re-sults in a conservative H2-distribution, and because the isotropic k-e turbulence model is not fully applicable to this special case.4. Results4.1 Hydrogen distributionFig.2 shows typical results for the computed hydrogen distribution in the garage for case 1 (3.4 g H2/s). Initially two separate plumes rise along two sides of the trunk. After reaching the ceil-ing a mushroom shaped cloud forms. About 20 seconds after begin of the release the combustible cloud has obtained its largest extension. The velocities in the rising plume reach about 1 m/s. The momentum in the flow is redirected at the ceiling causing an initially circular expansion of the burnable cloud and then reflections at the nearby walls (Fig.2c). These reflections lead to a prefer-ential flow along the ceiling towards the front of the car (Fig. 2e). Air is entrained by these convec-tive gas motions, leading to decrease of the combustible volume. The calculation finally predicts a stable stratification with most of the released hydrogen in a shallow layer underneath the ceiling.In case of the slow release (0.34 g H2/s) a different distributions process is predicted (Fig.3). Only one thin, tube-like H2-air plume extends from the source below the trunk to the ceiling. The plume pulsates slightly in flow rate and diameter, but is otherwise quasi-stationary. As in case 1, a stable stratified hydrogen distribution develops in the garage, however this time with less gas motion along the ceiling. The stratification existing after the end of the release in the garage is demon-strated in Figures 3d and e. Enough time was available to dilute the released hydrogen practically completely to inert mixtures with less than 4% H2.4.2 Hazard evaluationFor a risk assessment the question must be addressed what the calculated hydrogen distributions in the garage mean in terms of hazard. The three criteria described in Section 3 were therefore evalu-ated from the computed time dependent H2-air distributions. The results for these three hazard lev-els are shown in Fig. 4 for cases 1 and 2.Case 1: The high release rate of 3.4 g H2/s leads to a maximum characteristic size of the combusti-ble cloud near 1.6 m, which is equivalent to 4.1 m3. This maximum value is reached at about 20s. After the end of the release (10s) the combustible cloud still grows due to dilution of its enriched kernel by further entrainment of air. After reaching its maximum the cloud size decreases due to convective mixing by the momentum induced gas flows. Thereafter the volume of the combustible mixture approaches a constant value.The cloud which could support flame acceleration begins to grow directly with the start of the hy-drogen release and reaches a maximum volume of about 0.7 m3 shortly after termination of the re-lease. The corresponding H2 inventory is 20 g. Thereafter the cloud disappears in about 10 seconds. This dynamic growing and shrinking of the sensitive cloud results from the balance between the hydrogen source strength and the mixing processes. In the first phase (< 12s) the release dominates and in the second phase (> 12s) the mixing dominates. Comparison of the time scale shows that the decay of the sensitive cloud (³ 10.5%) feeds the growth of the combustible cloud (³ 4%).The evaluation of the DDT potential shows even higher dynamics. The cloud reaches detonable composition and size about 2s after begin of the hydrogen release. The most sensitive situationa) 7 seconds after begin ofH2 release underneathtrunkb) 10 secondsc) 15 secondsd) 25 secondse) 35 secondsFig. 2: Computed hydrogen concentration fields in the garage, case number 1, isosurface in space for 4 vol% hydrogen, release of 3.4 g H2/s for 10 seconds.a) 20 seconds after beginof release, 4 vol% H2isosurfaceb) 50 seconds, 4 vol% H2c) 80 seconds, 4 vol% H2d) 120 seconds, 2 vol% H2e) 120 seconds, 1 vol% H2Fig. 3: Computed hydrogen concentration fields in the garage, case number 2, isosurfaces in space, release of 0.34 H2/s for 100 seconds.CASE 1CASE 20204060801001201401601800102030405060d (cm)cc d (cm)cc 010203040506070809005010015020025000.10.20.30.40.50.60.70.80102030405060V (m³)fa V (m³)fa 00.020.040.060.080.100.120501001502002500.511.522.530102030DDT no DDT405060D7lno DDT00.20.40.60.81.01.21.4050100150200250D 7lTime (s) Time (s)Fig.4:Computed hazard parameters for Left: Case 1, hydrogen release rate 3.4 g H 2/s for 10 s. Right: Case 2, hydrogen release rate 0.34 g H 2/s for 100 s. Top: Characteristic dimension of the combustible cloud, d cc , with H 2concentrations between 4 and 75 vol% H 2.Middle: Volume of the cloud which could support spontaneous flame acceleration,V fa , volume with H 2-concentrations between 10.5 and 75%.Bottom: DDT index D/7λ of cloud with H 2-concentrations between 10.5 and 75 vol%.exists a few seconds later during which hydrogen accumulates underneath the car. DDT possibility is predicted for a total duration of about 10s (D/7l >1). Analog to above, the decay of the detonable cloud supports the simultaneous growth of the acceleration cloud.Case 2: A different picture evolves for the slow release rate. Fig.4 shows that a much smaller com-bustible cloud is formed in case 2 (d cc £ 0.8 m, V cc £ 0.5 m 3) and that 20s after begin of the H 2 re-lease a quasi-equilibrium is established between the source and the mixing processes, leading to a nearly constant cloud volume (Fig 3b and c). After termination of the source the combustible cloud dissolves within 20 seconds, because the plume is cut off and regions with 4% H 2 or more could not sustain under the ceiling during the long release period.The cloud capable to support flame acceleration is also much smaller in case 2 than in case 1 (about 0.1 m 3 vs. 0.7 m 3). Again a quasi-equilibrium exists between sources and sinks. The cloud disap-pears within 10 s after termination of the hydrogen source. The maximum hydrogen inventory of the flame acceleration cloud was only 2.7 g. The evaluation of the DDT criterion D/7l shows that the size and the hydrogen concentration of the enriched cloud are insufficient for a transition to detonation for practically the whole duration of the hydrogen release.Table 2: Summary of computed hazard parametersCASE FLAMMABLE CLOUD FLAME ACCELERATION CLOUDDETONABLE CLOUDMax.Size (m 3) Time of Existance (s) Max.Size (m 3) Max Mass (gH 2) Time of Exist (s) Max. Size (m 3) Max. Mass(g/H 2)Time ofExist. (s) Case 1 (3.4 gH 2/s) 4.1 complete analysis time (50 s) 0.7 20 release time +15s= 25 s0.7 20 10 Case 2 (0.34g H 2/s) 0.5 release time+15 s= 115 s0.1 2.7 release time +15s = 115 s - - -Table 2 summarizes the computed hazard parameters, which show a strong dependence on the hy-drogen release rate. In case of the low rate only a relatively small flammable cloud develops(<0.5 m 3). Combustible mixtures from during the release period only in the rising plume. Shortly after termination of the source no combustible mixtures are left in the garage and no combustion hazard remains.(Another risk which is not addressed here, would arise if released hydrogen could penetrate into highly confined compartments of the car.)For the large release rate up to about 4 m 3 of combustible mixture develop in the garage. As long as the source is active, an inner kernel of enriched mixtures exists (up to 0.7 m 3) which has the poten-tial for sustaining supersonic combustion modes. These enriched regions dilute however rapidly to more insensitive mixtures after termination of the source. This process of the other hand supports the persistance of the deflagrative cloud (³ 4% H 2), which at the end of the calculation shows a sta-ble stratified distribution near the ceiling of the garage. Accidental ignition of such a mixture would lead to substantial damage of the car and the garage. To control H 2 release rates in the range of grams per second active or passive countermeasures seem necessary, which however need careful design calculations to prove the required efficiency.5. Summary and conclusionsThe risk from pulsed release of hydrogen in a garage was investigated. The analysis concentrated on two examples for “subcritical” hydrogen sources, which when homogeneously distributed in the ga-rage would result in inert mixtures far below the flammability limit (0.6 vol% H2). The presented 3d computations have identified the size, shape, composition and evolution of the reactive H2-air clouds which would develop during and after the pulsed H2-release period. The applied criteria al-lowed to quantify several risk parameters like size, hydrogen mass, and times of existance of the mixtures which under inadvertent conditions could sustain a slow deflagration, a fast turbulent de-flagration or even a detonation (Table 2).The dynamics of the reactive cloud evolution are governed by the balance between the H2 source and the physical mixing mechanisms, which tend to dilute the pure hydrogen released at the H2 exit. In case of the high release rate the source dominated over the mixing processes for the whole re-lease period (10 s), leading to a large and continously growing cloud. In case of the low release rate source and mixing mechanisms reached a quasi-equilibrium, resulting in a small but practically constant cloud size during the release period.After termination of the hydrogen release the sensitive clouds which could support spontaneous flame acceleration (Eq.1) or detonation on-set (Eq.2) disappear for both release rates within 10-15 seconds due to convective mixing. The combustible cloud (³4% H2) behaves differently however. In case of the high release rate a long-term potential for slow deflagration of much of the injected hydrogen exists in form of a stable stratified layer under the ceiling. In case of the low release rate the mixing processes have had sufficient time to prevent the formation of a combustible gas, once the source is terminated.Hydrogen distribution and the associated formation of reactive clouds is a complex process which depends on many parameters. Modern 3d CFD simulations can capture the essential phenomena and make predictions about effective countermeasures for risk control.References[1]W. Breitung, P. Royl, Nuclear Engineering and Design 202 (2000), 249-268[2]J.R. Travis, P. Royl, R. Redlinger, G.A. Necker, J.W. Spore, K.L. Lam, T.L. Wilson,B.D. Nichols,C. Müller, 1998. GASFLOW-II: Theory and Computational Model, Vol 1,Reports FZKA-5994, LA-13357-MS.[3]J.R. Travis, P. Royl, R. Redlinger, G.A. Necker, J.W. Spore, K.L. Lam, T.L. Wilson,B.D. Nichols,C. Müller, 1998. GASFLOW-II: Users Manual, Vol 2, Reports FZKA-5994,LA-13357-MS.[4]U. Bielert, W. Breitung, Proceedings Annual Meeting on Nuclear Technology ‚99, ISSN 0720-9207, Karlsruhe, 18.-20. May 1999, p. 167[5]S.B. Dorofeev, M. Kuznetsov, M. Alekseev, A. Efimenko, A. Bezmelnitsyn, A. Yankin,W. Breitung, Reports IAE-612713, Kurchatov Institute Moscow, Russia, and FZKA6268,Forschungszentrum Karlsruhe, Germany (1999).[6]W. Breitung, S.B. Dorofeev, Proceedings Annual Meeting on Nuclear Technology ’99, ISSN0720-9207, Karlsruhe, Germany 18-20 May 1999, p.185[7]S.B: Dorofeev, Chapter 3 in W. Breitung, C. Chan, S. B. Dorofeev, A. Eder, B. Gelfand, M.Heitsch, R. Klein, A. Malliokos, E. Shepherd, E. Studer, P. Thibault, ReportNEA/CSNI/2000/7 (August 2000), OECD Paris, see also ftp://inrh2b.FZK.de/pub/SOAR.。