Numerical modelling of vented lean hydrogen deflagations in an ISO container

Hydrogen process equipment are often housed in 20-foot or 40-foot container either be at refueling stations or at the portable standalone power generation units. Shipping Container provide an easy to install, cost effective, all weather protective containment. Hydrogen has unique physical properties, it can quickly form an ignitable cloud for any accidental release or leakages in air, due to its wide flammability limits. Identifying the hazards associated with these kind of container applications are very crucial for design and safe operation of the container hydrogen installations. Recently both numerical studies and experiment have been performed to ascertain the level of hazards and its possible mitigation methods for hydrogen applications. This paper presents the numerical modelling and the simulations performed using the HyFOAM CFD solver for vented deflagrations processes. HyFOAM solver is developed in-house using the opensource CFD toolkit OpenFOAM libraries. The turbulent flame deflagrations are modelled using the flame wrinkling combustion model. This combustion model is further improved to account for flame instabilities dominant role in vented lean hydrogen-air mixtures deflagrations. The 20-foot ISO containers of dimensions 20′ × 8′ × 8′.6″ filled with homogeneous mixture of hydrogen-air at different concentration, with and without model obstacles are considered for numerical simulations. The numerical predictions are first validated against the recent experiments carried out by Gexcon as part of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU) under the Horizon 2020 Framework Programme for Research and Innovation. The effects of congestion within the containers on the generated overpressures are investigated. The preliminary CFD predictions indicated that the container walls deflections are having considerable effect on the trends of generated overpressures, especially the peak negative pressure generated within the container is overestimated. Hence to account for the container wall deflections, the fluid structure interactions (FSI) are also included in the numerical modelling. The final numerical predictions are presented with and without the FSI. The FSI modelling considerably improved the numerical prediction and resulted in better match of overpressure trends with the experimental results

Numerical modelling of vented lean hydrogen–air deflagrations using HyFOAM

Hydrogen is being considered as a sustainable future energy carrier with least environmental impact in terms of combustion by-products. It has unique physical properties of very wide flammability range, between 4% to 75% by volume and high flame speeds, which are challenging factors in designing safe hydrogen installations. An accidental release in enclosures can easily result in the formation of flammable mixtures, which may upon ignition lead to fast turbulent deflagrations or even transition to detonation. Explosion venting is frequently used to mitigate explosions in industry, but it is not straightforward to design vent systems that will reduce the explosion pressure sufficiently to prevent collapse of structures and formation of projectiles. Validated predictive techniques will be of assistance to quantified analysis of possible accidental scenarios and designing effective mitigation measures such as vents. While explosion venting has been previous studied experimentally and numerically, relatively little information has been gathered about the configurations used in hydrogen energy applications and in the presence of obstacles; a viable predictive technique for such scenario is still lacking. The use of standard 20 feet ISO shipping containers for self-contained portable hydrogen fuel cell power units is being widely considered. Fresh experiments for this configuration have been carried out by GexCon AS as part of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU) under the Horizon 2020 Framework Program for Research and Innovation. In the present study, numerical modelling and simulations have been conducted to aid our understanding of the vented gas explosion in these self-contained portable power units using HyFOAM, an in-house modified version of the open source Computational Fluid Dynamics (CFD) code OpenFOAM for vented hydrogen explosions. The convective and diffusive terms are discretised using Gaussian-Gamma bounded and Gaussian linear corrected numerical schemes with in OpenFOAM. The temporal terms are discretised using Euler implicit scheme making the solver second order accurate both in spatial and time coordinates

VENTED HYDROGEN DEFLAGATIONS IN AN ISO CONTAINER

The commercial deployment of hydrogen will often involve housing portable hydrogen fuel cell power units in 20-foot or 40-foot shipping containers. Due to the unique properties of hydrogen, hazards identification and consequence analysis is essential to safe guard the installations and design measures to mitigate potential hazards. In the present study, the explosion of a premixed hydrogen-air cloud enclosed in a 20-foot container of 20’ x 8’ x 8’.6” is investigated in detail numerically. Numerical simulations have been performed using HyFOAM, a dedicated solver for vented hydrogen explosions developed in-house within the frame of the open source computational fluid dynamics (CFD) code OpenFOAM toolbox. The flame wrinkling combustion model is used for modelling turbulent deflagrations. Additional sub-models have been added to account for lean combustion properties of hydrogen-air mixtures. The predictions are validated against the recent experiments carried out by Gexcon as part of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU) under the Horizon 2020 Framework Programme for Research and Innovation. The effects of congestion within the containers on the generated overpressures are also investigated

Vented hydrogen deflagations in an ISO container

The commercial deployment of hydrogen will often involve housing portable hydrogen fuel cell power units in 20-foot or 40-foot shipping containers. Due to the unique properties of hydrogen, hazards identification and consequence analysis is essential to safe guard the installations and design measures to mitigate potential hazards. In the present study, the explosion of a premixed hydrogen-air cloud enclosed in a 20-foot container of 20’ x 8’ x 8’.6” is investigated in detail numerically. Numerical simulations have been performed using HyFOAM, a dedicated solver for vented hydrogen explosions developed in-house within the frame of the open source computational fluid dynamics (CFD) code OpenFOAM toolbox. The flame wrinkling combustion model is used for modelling turbulent deflagrations. Additional sub-models have been added to account for lean combustion properties of hydrogen-air mixtures. The predictions are validated against the recent experiments carried out by Gexcon as part of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU) under the Horizon 2020 Framework Programme for Research and Innovation. The effects of congestion within the containers on the generated overpressures are also investigated

Performance evaluation of empirical models for vented lean hydrogen explosions

This paper aims to provide a comprehensive review of available empirical models for overpressures predictions of vented lean hydrogen explosions. Empirical models and standards are described briefly, with discussion on salient features of each model. Model predictions are then compared with the available experimental results on vented hydrogen explosions. First comparison is made for standards tests, with empty container and quiescent starting conditions. Comparisons are then made for realistic cases with obstacles and initial turbulent mixture. Recently, a large number of experiments are carried out with standard 20-foot container for the HySEA project. Results from these tests are also used for model comparison. Comments on accuracy of model predictions, their applicability and limitations are discussed. A new model for vented hydrogen explosion is proposed. This model is based on external cloud formation, and explosion. Available experimental measurements of flame speed and vortex ring formation are used in formulation of this model. All assumptions and modelling procedure are explained in detail. The main advantage of this model is that it does not have any tuning parameter and the same set of equations is used for all conditions. Predictions using this model show a reasonably good match with experimental results

Evaluation of engineering models for vented lean hydrogen deflagrations

Hydrogen gas produced from the renewable energy source can be the prefect future energy carrier. It will not only reduce the demands for depleting hydrocarbons fuels but will also help in reducing the greenhouse gas emissions. The 20-ft ISO standard containers are widely considered for building self-contained portable fuel cell based power generation units. Safety analysis of these installations is essential to prevent any future catastrophic accidents. The present paper evaluates existing engineering models to predict vented explosion peak overpressures in case of an accident release of hydrogen in these container. Such predictions are required in the design of venting panels, which are commonly used to prevent damage to enclosures by reducing overpressure of combusting gases. Although various engineering models and empirical correlations have been developed, a number of which have been included in engineering standards and guidelines [4-7]. These correlations, however, often have conflicting recommendations [3]. None of the engineering models in the public domain have been validated with vented hydrogen tests data in realistic configurations, such as ISO shipping containers, used in hydrogen energy applications. Evaluating/improving these engineering models with the aid of full scale experimental data and computational fluid dynamics (CFD) based numerical modelling is a main objective of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU) under the Horizon 2020 Framework Program for Research and Innovation. The present study aims to assess capabilities of existing engineering models for vented deflagrations of lean hydrogen-air mixtures. As hydrogen has much higher flame speeds than hydrocarbon fuels like methane and propane, it is not possible to use models derived for hydrocarbons directly with hydrogen flames. The leaner flames of hydrogen are also susceptible to instabilities like Darius-Landau instability, Rayleigh-Taylor instability, which are often overlooked in the derivation of engineering models

Numerical implementation and validation of turbulent premixed combustion model for lean mixtures

The present paper discusses the numerical investigation of turbulent premixed flames under lean conditions. Lean premixed combustion, a low NOx emission technique but are prone to instabilities, extinction and blow out. Such flames are influenced by preferential diffusion due to different mass diffusivities of reactants and difference between heat and mass diffusivities in the reaction zone. In this numerical study, we estimate non-reacting flow characteristics with implementation of an Algebraic Flame Surface Wrinkling Model (AFSW) in the open source CFD code OpenFOAM. In these flows, the mean velocity fields and recirculation zones were captured reasonably well by the RANS standard k-epsilon turbulence model. The simulated turbulent velocity is in good agreement with experiments in the shear-generated turbulence layer. The reacting flow study was done at three equivalence ratios of 0.43, 0.5 and 0.56 to gauge the ability of numerical model to predict combustion quantities. At equivalence ratios 0.5 and 0.56 the simulations showed numerical oscillations and non-convergence of the turbulent quantities. This leads to a detailed parametric variation study where, the pre-constant of AFSW model is varied with values 0.3, 0.35 and 0.4. However the study revealed the weak dependence of pre-constant value on the equivalence ratio. Hence the pre-constant value is fit for specific equivalence ratio based on the parametric variation study. The tuned AFSW model with fitted pre-constant specific to given equivalence ratio predicted are compared with experiments and discussed. The tuned AFSW model produced turbulent flame speed values which are good agreement with experiments

Performance evalulation of empirical models for vented lean hydrogen explosions

Explosion venting is a method commonly used to prevent or minimize damage to an enclosure caused by an accidental explosion. An estimate of the maximum overpressure generated though explosion is an important parameter in the design of the vents. Various engineering models (Bauwens et al., 2012, Molkov and Bragin, 2015) and European (EN 14994 ) and USA standards (NFPA 68) are available to predict such overpressure. In this study, their performance is evaluated using a number of published experiments. Comparison of pressure predictions from various models have also been carried out for the recent experiments conducted by GexCon using a 20 feet ISO container. The results show that the model of Bauwens et al. (2012) predicts well for hydrogen concentration between 16% and 21% and in the presence of obstacles. The model of Molkov et al. (2015) is found to work well for hydrogen concentrations between 10% and 30% without obstacles. In the presence of obstacles, as no guidelines are given to set the coefficient for obstacles in the model, it was necessary to tune the coefficient to match the experimental data. The predictions of the formulas in NFPA 68 show a large scatter across different tests. The current version of both EN 14994 and NFPA 68 are found to have very limited range of applicability and can hardly be used for vent sizing of hydrogen-air deflagrations. Overall, the accuracy of all the engineering models was found to be limited. Some recommendations concerning their applicability will be given for vented lean-hydrogen explosion concentrations of interest to practical applications