Publications

Passive auto-catalytic recombiners (PARs) are used as safety devices in the containments of nuclear power plants (NPPs) for the removal of hydrogen that may be generated during specific reactor accident scenarios. In the presented study it was investigated whether a PAR designed for hydrogen removal inside a NPP containment would perform principally inside a typical surrounding of hydrogen or fuel cell applications. For this purpose a hydrogen release scenario inside a garage – based on experiments performed by CEA in the GARAGE facility (France) – has been simulated with and without PAR installation. For modelling the operational behaviour of the PAR the in-house code REKO-DIREKT was implemented in the CFD code ANSYS-CFX. The study was performed in three steps: First a helium release scenario was simulated and validated against experimental data. Second helium was replaced by hydrogen in the simulation. This step served as a reference case for the unmitigated scenario. Finally the numerical garage setup was enhanced with a commercial PAR model. The study shows that the PAR works efficiently by removing hydrogen and promoting mixing inside the garage. The hot exhaust plume promotes the formation of a thermal stratification that pushes the initial hydrogen rich gas downwards and in direction of the PAR inlet. The paper describes the code implementation and simulation results.

Conditions are examined under which mechanical stimuli produced by striking controlled blows can result in sparking and ignition of hydrogen in air mixtures. The investigation principally concerns magnesium thermite reaction as the ignition source and focuses on the conditions and thermomechanical parameters that are involved in determining the probability of ignition. It is concluded that the notion of using the kinetic energy of impact as the main criterion in determining whether an ignition event is likely or not is much less useful than considering the parameters which determine the maximum temperature produced in a mechanical stimuli event. The most influential parameter in determining ignition frequency or probability is the velocity of sliding movement during mechanical stimuli. It is also clear that the kinetic energy of a moving hammer head is of lesser importance than the normal force which is applied during contact. This explains the apparent discrepancy in previous studies between the minimum kinetic energy thought to be necessary to allow thermite sparking and gas ignition to occur with drop weight impacts and glancing blow impacts. In any analysis of the likelihood of mechanical stimuli to cause ignition the maximum surface temperature generated should be determined and considered in relation to the temperatures that would be required to initiate hot surface reactions sufficient to cause sparking and ignition.

In future pressure relief devices (PRDs) should be installed on hydrogen vehicles to prevent a hydrogen container burst in the event of a nearby fire. Weakening of the container at elevated temperature could result in such burst. In this case the role of a PRD is to release some or all of the system fluid in the event of an abnormally high pressure. The paper analyzes the possibility of hydrogen self-ignition at PRD operation and ways of its prevention.

During the early phase of the transient process following a hydrogen leak into the atmosphere a contact surface appears separating hot air from cold hydrogen. Locally the interface is approximately planar. Diffusion occurs potentially leading to ignition. This process was analyzed by Lin˜a´n and Crespo (1976) for Lewis number unity and Lin˜a´n and Williams (1993) for Lewis number less than unity. In addition  to conduction these processes are affected by expansion due to the flow which leads to a temperature drop. If chemistry is very temperature-sensitive then the reaction rate peaks close to the hot region where relatively little fuel is present. Indeed the Arrhenius rate drops rapidly as temperature drops much more so than fuel concentration. However the small fuel concentration present close to the airrich side depends crucially upon the balance between fuel diffusion and heat diffusion hence the fuel Lewis number. For Lewis number unity the fuel concentration present due to diffusion is comparable to the rate of consumption due to chemistry. If the Lewis number is less than unity fuel concentration brought in by diffusion is large compared with temperature-controlled chemistry. For a Lewis number greater than unity diffusion is not strong enough to bring in as much fuel as chemistry would be able to burn and combustion is controlled by fuel diffusion. In the former case combustion occurs faster leading to a localized ignition at a finite time determined by the analysis. As long as the temperature drop due to the expansion associated with the multidimensional nature of the jet does not lower significantly the reaction rate up to that point ignition in the jet takes place. For fuel Lewis number greater than unity first the reaction rate is much lower. Second chemistry does not lead to a defined ignition. Eventually expansion will affect the process and ignition does not take place. In summary it appears that the reason why hydrogen is the only fuel for which jet ignition has been observed is a Lewis number effect coupled with a high speed of sound hence a high initial temperature discontinuity.

Deflagration-to-Detonation Transition Ratio (DDTR) is an important parameter in measuring the hazard of hydrogen detonation at given thermodynamic conditions. It’s among the major tasks to evaluate DDTR in the study of hydrogen safety in a nuclear containment. With CFD tools detailed distribution of thermodynamic parameters at each instant can be simulated with considerable reliability. Then DDTR can be estimated using related CFD output. Forstochastic or epistemic reasons uncertainty always exists in input parameters during computations. This lack of accuracy can finally be reflected in the uncertainty of computation results e.g. DDTR in our consideration. The analysis of the influence of the input uncertainty is therefore a key step to understand the model’s response on the output and possibly to improve the accuracy. The increase of computational power makes it possible to perform statistics-based sensitivity and uncertainty (SU) analysis on CFD simulations. This paper aims at presenting some ideas on the procedure in safety analysis on hydrogen in nuclear containment. A hydrogen recombiner case is constructed and simulated with CFD method. DDTR at each instant is computed using a semi-empirical method. RBD-FAST based SU analysis is performed on the result.

As a developing country the Philippines must balance its rapid industrialization efforts with the realities and consequences of climate change on the country. A feasible option to achieve this is increasing the share of renewables in power generation coupled with energy storage technology. This paper examines the present situation and opportunities for development of hydrogen and fuel cell technology in the Philippines as promising alternatives with proven applications in niche energy demand sectors aside from renewables integration. Although the Philippines is considered a latecomer there is significant renewable resource potential available local experts and trained talents and enabling legislations in the country that provide opportunities in harnessing fuel cell technologies for the transition to energy self-sufficient and low-carbon society. Current advancement of the technology in the country is limited to an initial 5-year roadmap focused on component development from cheap and local materials. Provisions for large-scale hydrogen infrastructure have not yet been realized which is comparable to the early stages of development in other countries that are also pursuing fuel cell technology. Strong industry-academe partnerships should be pursued through a specific legislated agency to ensure future development of this technology for the country’s benefit. Lastly applications in distributed power generation poised to be a lucrative direction as demonstration and validation with other potential uses such as transportation remains a challenge.

Previous studies have demonstrated that while blowdown pressure is reproduced well by both adiabatic and isothermal analytical models the dynamics of temperature cannot be predicted well by either model. The reason for the last is heat transfer to cooling during expansion gas from the vessel wall. Moreover when exposed to an external fire the temperature inside the vessel increases i.e. when a thermally activated pressure relief device (TPRD) is still closed with subsequent pressure increase that may lead to a catastrophic rupture of the vessel. The choice of a TPRD exit orifice size and design strategy are challenges: to provide sufficient internal pressure drop in a fire when the orifice size is too small; to avoid flame blow off expected with the decrease of pressure during the blowdown; to decrease flame length of subsequent jet fire as much as possible by the decrease of the orifice size under condition of sufficient fire resistance provisions to avoid pressure peaking phenomenon etc. The adiabatic model of blowdown [1] was developed using the Abel-Nobel equation of state and the original theory of underexpanded jet [2]. According to experimental observations e.g. [3] heat transfer plays a significant role during the blowdown. Thus this study aims to modify the adiabatic blowdown model to include the heat transfer to non-ideal gas. The model accounts for a change of gas temperature inside the vessel due to two “competing” processes: the decrease of temperature due to gas expansion and the increase of temperature due to heat transfer from the surroundings e.g. ambience or fire through the vessel wall. This is taken into account in the system of equations of adiabatic blowdown model through the change of energy conservation equation that accounts for heat from outside. There is a need to know the convective heat transfer coefficient between the vessel wall and the surroundings and wall size and properties to define heat flux to the gas inside the vessel. The non-adiabatic model is validated against available experimental data. The model can be applied as a new engineering tool for the inherently safer design of hydrogen tank-TPRD system.

Existing regulations and standards for the approval of composite cylinders in hydrogen service are currently based on deterministic criteria (ISO 11119-3 UN GTR No. 13). This paper provides a systematic analysis of the load cycle properties resulting from these regulations and standards. Their characteristics are compared with the probabilistic approach of the BAM. Based on Monte-Carlo simulations the available design range of all concepts is compared. In addition the probability of acceptance for potentially unsafe design types is determined.

Type IV pressure vessels are commonly used for hydrogen on-board stationary or bulk storages. During their lifetime they can be submitted to mechanical impacts creating damage within the composite structure not necessarily correlated to what is visible from the outside. When an impact is suspected or when a cylinder is periodically inspected it is necessary to determine whether it can safely stay in service or not. The FCH JU project Hypactor aims at creating a large database of impacts characterized by various non destructive testing (NDT) methods in order to provide reliable pass-fail criteria for damaged cylinders. This paper presents some of the tests results investigating short term burst) and long term (cycling) performance of impacted cylinders and the recommendations that can be made for impact testing and NDT criteria calibration.

This paper presents results from experiments on gas explosions in inhomogeneous hydrogen-air mixtures. The experimental channel is 3 m with a cross section of 100 mm by 100 mm and a 0.25 mm ID nozzle for hydrogen release into the channel. The channel is open in one end. Spectral analysis of the pressure in the channel is used to determine dynamic load factors for SDOF structures. The explosion pressures in the channel will fluctuate with several frequencies or modes and a theoretical high DLF is seen when the pressure frequencies and eigen frequencies of the structure matches.