Prediction of structural deformations in a research stand for the study of hydrogen explosions

. Along with the development of facilities that employ hydrogen in many aspects such as hydrogen production, storage systems, fueling stations, and so on, it is critical to understand the features of this gas, particularly those affecting the explosive qualities. Researchers can create advanced modeling approaches, risk assessment procedures, and safety standards to lessen the potential impact of accidents by researching the behavior of hydrogen explosions. The present paper deals with the design of a stand intended for hydrogen explosion experimentation, anticipating the possible values of the explosion overpressures to which the stand structure is subjected. To approximate these values, a computer simulation of the hydrogen explosion was carried out in the virtual environment, in two stages. The explosive process was first simulated in a strictly fluid environment, with overpressure values at the domain's limits being recorded. The values of the explosion overpressures from the first stage were taken in the second stage and applied to the surfaces where the fluid and solid environments came into contact, ultimately yielding the potential deformations of the stand construction. The map of the resulting deformations becomes a point of reference in the design of the stand considering, first of all, its safe use.


Introduction
In addition to being the most common chemical element in the universe, hydrogen may also be extracted from water, fossil fuels, and renewable energy sources.Hydrogen has potential as a fuel because of its many benefits, including the fact that it can generate electricity without releasing any carbon dioxide.However, advancements in technology and infrastructure are needed before hydrogen can be used on a broad scale as a fuel source.
Hydrogen has the potential to address a number of pressing energy issues, including the need to better balance supply and demand from intermittent renewable sources like solar photovoltaics and wind power.It provides options for decarbonizing industries including long-distance transportation, chemicals, and iron and steel, where significant emission reductions have thus far proven elusive.It also has the potential to boost energy security and better the air we breathe [1].
Hydrogen can be produced from a wide range of fuels, including renewable sources, nuclear sources, natural gas, coal, and oil.Like LNG, hydrogen can be delivered either as a gas through pipelines or as a liquid on ships.It can be used to generate energy and methane for heating and cooking, transportation fuels for automobiles, trucks, ships, and airplanes, and to provide food for livestock.
The necessity for a reference for risk assessment and explosion evacuation system design can be satisfied by taking hydrogen explosion characteristics into account in engineering design and application [2].When it comes to maintaining worker safety in fields as diverse as power generation and aerospace, knowing the science behind hydrogen explosions and how it interacts with risk mitigation mechanisms in confined facilities is essential.Nowadays, the production of hydrogen is still pre-dominated by fossil-based techniques, which is considered undesirable due to low con -version efficiency and release of greenhouse gases.It is necessary to find green and sustainable hydrogen production routes with low energy consumption and cost [3].As various scientists are stressing the gravity of climate change effects that are reaching the physical environment, ecosystems, and humanity in general, concern for the future is becoming the main global topic.Consequently, governments are implementing new sustainable policies that promote RES as a substitute for fossil fuels.Increasing progress in hydrogen technology instigated nations worldwide to incorporate hydrogen in their energy legislations and national development plans, which resulted in numerous national hydrogen strategies [4].
This study discusses the design of an infrastructure for hydrogen explosion experimentation, taking into account the range of probable explosive overpressures.These numbers were calculated via a two-part, computer-generated simulation of the hydrogen explosion in a virtual environment.Overpressure data at the domain's boundaries were recorded after first simulating the explosive process in a purely fluid environment.Potential stand deformations were calculated by applying values of first-stage explosion overpressures to contact surfaces between fluid and solid environments in the second stage.The generated deformation map is then used to ensure the stand is safe for its intended use.

Simulation of the air-hydrogen mixture explosion 2.1 Geometry and meshing
The stand's geometry resembles a rectangular spiral with a spiral explosion path for studying how the explosive process behaves when the propagation direction is changed.The future experimental stand must meet a number of operational safety requirements and must ensure that explosion characteristics and video content may be recorded.
The stand will be constructed for this purpose out of a metal spiral with polycarbonate covers.Ten spaces will be created from the explosion route (which has a free section of 30x50mm) using membranes fixed inside the stand using rectangular supports.The intended route for the gas explosions goes through the spiral from its centre outwards.In the example at hand, the membranes were eliminated, leaving only their supports and a free surface with geometry-conforming dimensions of 20x40mm.The stand's fluid internal volumes and the solid pieces that define them were both constructed in a virtual environment.

Fig. 1. Stand virtual geometry
The virtual geometry was divided or discretized into 687562 calculation elements (finite volumes) for use in the first stage of the simulation, respectively the simulation of the explosive process in a strictly fluid environment, in order to obtain the overpressure field generated by the explosion process of the air-hydrogen mixture.Overpressures caused by explosions were calculated by isolating the surfaces where fluid and solid media made contact.The free surface was also determined to be the spiral's exit hole.

The virtual explosion of the mixture
Several crucial parameters are initially adjusted to determine the conditions under which the explosion will occur in the fluid environment.The cells, hydrogen concentration, temperature, pressure, and position inside the spiral are all variables that may be adjusted.
The initial settings for the explosion in the fluid medium involve several key parameters that determine the conditions under which the explosion will occur.These settings include the cells involved, the concentration of hydrogen in the air, the temperature, pressure, and the starting point within the spiral.Cells 1, 2, 3, and 4 are the first to be set with an explosive atmosphere, with the explosion beginning at the spiral's centre.29,53% volume of hydrogen is the specified concentration in the air.Because hydrogen is so highly flammable, it can produce a highly explosive atmosphere when there is enough of it in the air.A large amount of hydrogen is present at a 29,53% concentration, raising the possibility of a strong explosion.20 degrees Celsius is the fixed temperature.The rate and intensity of chemical reactions are greatly influenced by temperature.Higher temperatures typically result in quicker reactions, which could speed up the explosion's spread.The pressure is set at 101325 Pa, which is equivalent to atmospheric pressure.The starting point of the explosion is cell 1, located at the centre of the spiral.The choice of the starting point can impact the propagation and direction of the explosion.
The explosion of the air-hydrogen mixture was carried out in the Fluent module of Ansys platform with the following settings: -Model transient: pressure-based; -Turbulence chemistry interaction: Finite rate; -Species model: mixture material hydrogen-air (hydrogen mass-fraction of 0,028 equivalent of 29,53% volumetric hydrogen in air); The recording of the explosion overpressure values was carried out on the boundary surfaces of the fluid environment.
Initially, the fluid medium was set to the values of normal atmosphere (101325Pa, 293K, 9.81m/sec 2 gravitational acceleration) the walls having the same temperature of 293K.After initiation, rooms 1-4 were patched to the values of the stoichiometric reaction of hydrogen with oxygen (29.53% H2).

Stage 1 results:
A series of images with explosion pressures and temperatures were obtained in the process, as following:

Fig. 3 Image sequences with explosion pressures
A series of color-contoured images showing the development of pressure levels is provided here.
In Figure 3, we can observe the progression of pressure changes over time.Through a series of images depicted by color contours of temperatures, the behavior of the flame front may be observed in Figure 4.
Since the purpose of this work is to anticipate the maximum deformations of solid structures, in the first stage of the simulation, the maximum values of the explosion overpressures obtained in the volumes of the stand/spiral rooms were taken into account.
The maximum pressure obtained was 352660Pa.The Ansys platform offers the ability to combine Multiphysics modules with the retrieval of data resulting from a simulation from a certain domain (e.g.fluid environment) and the transposition of these data as input data to obtain the effects of force pressures etc. on another domain (e.g.solid environment).In general, in CFD, this operation is called Fluid Structure Interaction (FSI) [5].
After running a virtual simulation of an air and hydrogen combination with the explosion process in mind, the fluid volume of the stand was recorded together with the pressure values on the boundary walls and the maximum overpressure values in the cells designed to explode.Cellular membranes have been discarded, exposing the underlying structures of the cells (open surfaces).

Solid environment discretization
In the Transient Structural module, the fluid bodies used in the first stage of the simulation in the Fluent module were suppressed [6].The geometry of the solid objects, respectively of the metal spiral and of the upper and lower covers was discretized according to the requirements of this simulation module in 23309 calculation elements/finite volumes.

Anticipation of stand structural deformations
Only the stand's solid parts-including their contact surfaces with the fluid environmentwere taken into account for the simulation's second stage of implementation.These surfaces become the target of the transposition of the first-stage explosion overpressures (the results obtained in Fluent overpressure field).
However, only the pressures on the lids were addressed in order to shorten the calculation procedure (associated time), as the metal component was thought to be sufficiently strong to withstand the applied overstresses.The lids are constructed of polycarbonate and have a 10mm thickness.In order to simulate four vises for attaching the covers to the metal spiral (from the case of the actual experiment), the corners of the upper and lower covers were set up as fixed supports.

Stage 2 results:
As a result of this stage, the total deformation of the lids highlighted in Figure 10 was obtained.This concludes that the lids used were not strong enough to withstand the overpressures of the air-hydrogen explosion.The highest degree of deformation, 33.2mm, was roughly found in the middle of the plates.
A different scenario was therefore generated because this result cannot be accepted in the context of an actual experimental stand.In this scenario, the metal spiral's sealing lids were firmly attached to it (to the spiral), replicating the mounting of screws on the metal spiral's top and bottom outlines.
According to the picture below, the lid deformations were significantly reduced compared to the prior scenario when the same pressure field acquired in the first phase of the simulation was applied in the Fluent module, reaching a maximum value of 0.136mm.The goal of this simulation was to foresee the deformations of the lids under the effect of explosion overpressures, which was useful for the design of an experimental stand for the study of hydrogen explosions and its use in safe conditions.However, the simulation was not validated by comparison with a physical experiment [7].

Conclusions
-This virtual simulation served as a platform for detailed investigation and identification of the precise elements, materials, and safety precautions required to successfully carry out experiments with air-hydrogen mixtures [8].
-Using CFD simulations can significantly enhance safety and reduce costs.By conducting simulations, potential hazards and risks associated with the explosion can be identified and addressed before any physical tests or experiments are conducted.This reduces the chances of accidents, injuries, or damage to expensive equipment, thereby minimizing the costs associated with experimental failures.
-The use of simulation tools allows for easy exploration of various parameters and scenarios, providing a deeper understanding of the explosion's behavior.By altering parameters such as mixture composition, initial conditions, geometry, or ventilation, like in this present case, modifying the lids structure, it is possible to evaluate different scenarios and optimize designs for safety and efficiency.This can be time-consuming and costly in experimental tests, but simulations enable quick iterations and analysis.

Fig. 2
Fig. 2 Discretization of the virtual geometry in the first stage

Fig. 4
Fig. 4 Image sequence with explosion temperatures

Fig. 6
Fig. 6 Workbench -Combining Multiphysics modules on the Ansys work platform

Fig. 9
Fig. 9 The imported pressures from Fluent module

Fig. 10
Fig. 10 Total deformation of the structure

Fig. 11
Fig. 11 Deformation of the structure after replicating the mounting of screws on the metal spiral's top and bottom outlines