A comparison study into low leak rate buoyant gas dispersion in a small fuel cell enclosure using plain and louvre vent passive ventilation schemes

Hydrogen, producing electricity in fuel cells, is a versatile energy source, but with risks associated with flammability. Fuel cells use enclosures for protection which need ventilating to remove hydrogen emitted during normal operation or from supply system leaks. Passive ventilation, using buoyancy driven flow is preferred to mechanical systems. Performance depends upon vent design, size, shape, position and number. Vents are usually plain rectangular openings, but environmentally situated enclosures use louvres for protection. The effect of louvres on passive ventilation is not clear and has therefore been examined in this paper. Comparison ‘same opening area’ louvre and plain vent tests were undertaken using a 0.144 m3 enclosure with opposing upper and lower vents and helium leaking from a 4 mm nozzle on the base at rates from 1 to 10 lpm, simulating a hydrogen leak. Louvres increased stratified level helium concentrations by typically in excess of 15 %. The empirical data obtained was also used in a validation exercise with a SolidWorks: Flow Simulation CFD model, which provided a good qualitative representation of flow behaviour and close empirical data correlations

Energy losses during drop weight mechanical impacts with special reference to ignition of flammable atmospheres in nuclear decommissioning: theory and determination of experimental coefficients for impact analysis and prediction

The major purpose of this study is to provide a framework for determination of energy losses resulting from mechanical impacts of the kind that could occur during nuclear decommissioning of waste material. Measurements have been made of final translational and rotational velocities for impacts between projectiles of different length and a massive barrier. This enabled determination of experimental values of the impact coefficients and energy losses. It was found that the total energy losses could be accurately accounted for by the sum of those pertaining to the normal and tangential processes, thus indicating that these include any losses due to vibration. The results obtained clearly support an Amontons–Coulomb friction model and the previously held contention that there is a limiting value for the impulse ratio at low angles of barrier inclination. Although sliding surfaces are likely to be modified during impact, it is shown that any original contamination on the contacting surfaces results in a very large decrease in impulse ratio or friction coefficient. This represents an important finding in the context of mechanical ignition testing indicating that the state of the impact surfaces and their handling need to be taken into account. The difficulties in establishing appropriate values for the impact coefficients and dealing with the effect of mechanical vibrations on the energy losses are discussed and equations derived for determining the tangential and normal energy losses from known initial velocities

Application of Bayesian methods and networks to ignition hazard event prediction in nuclear waste decommissioning operations

The major purpose of the study is to examine how Bayesian networks can be used to represent and understand potential ignition scenarios in nuclear waste decommissioning. This is illustrated using a network to represent a situation with stacked storage boxes containing pyrophoric material removed from waste storage silos. Corrosion of this material during storage produces hydrogen which is released through a filter medium into the gap between the boxes. The probabilistic relationships used to indicate dependence between network nodes are expressed by conditional probability tables or C++ coded equations that relate to UK nuclear industry corrosion and storage data. The study focuses on optimal prediction of the likelihood of a flammable hydrogen atmosphere arising in the gap between stacked boxes and the conditions necessary to exceed the lower flammable limit. It is concluded that the approach offers a useful means of easily determining the manner in which varying the controlling parameters affects the possibility of an ignition event. The effect of data variation can be examined at first hand using the supplementary Bayesian Network that accompanies the article

Chemochromic Pd-V2O5 Sensors for Passive Hydrogen Detection in Nuclear Containments

The ability to detect and monitor hydrogen gas efficiently in process and storage facilities, handling nuclear material, is crucial to ensuring their safety. The accumulation of hydrogen gas, above the lower flammable limit (LFL), in a nuclear waste containment is a concern since it creates the potential for a hydrogen-air explosion to occur, which could lead to a loss of containment and result in the uncontrolled release of radioactive material into the surrounding environment. The events that took place at Fukushima Daiichi Nuclear Power Plant highlighted the vulnerability of conventional hydrogen detection to extreme events, where power may be lost. In the present work, chemochromic hydrogen sensors have been fabricated, using transition metal oxide thin films, to provide eye-readable detection systems that would be resilient to plant power failure. Vanadium oxide (V2O5) films were prepared on quartz glass substrates by sol-gel deposition and sensitized with a palladium (Pd) catalyst, deposited by electron beam evaporation. When exposed to hydrogen, the Pd catalyst dissociates H2 to H atoms, which diffuse into the V(V)2O5 forming a hydrogen-vanadium metal bronze, H2V(III)2O5, resulting in a noticeable colour change from orange to dark green. To assess their viability for nuclear safety applications, these sensors have been irradiated to total doses between 5 and 250 kGy using a Co-60 gamma isotope irradiator. The results suggest that gamma irradiation, at the levels examined, has an effect on the initial colour of the V2O5 and Pd-V2O5 thin films with decreased transmittance above 540 nm. The orange starting colour darkened and developed a green tone, with the degree of colour change depending on the applied total dose. Changes in surface morphology and characteristics have been examined by using Scanning Electron Microscopy (SEM) and Raman spectroscopy. High level (250 kGy) gamma radiation exposure begins to produce surface degradation on V2O5 thin films; however this behaviour is not observed for films that are also coated with palladium. Chemochromic properties of both un-irradiated and irradiated Pd-V2O5 thin films were determined by examining their optical transmittance, using UV-vis spectroscopy, under exposure to a 4% H2-N2 gas mixture. Exposure to gamma radiation has been found to have negligible effect upon colour change behaviour after 30 minutes exposure to hydrogen gas. The results suggest that the thin film V2O5 sensitised with Pd is a plausible technique for application in the monitoring of hydrogen gas in low-level gamma radiation environments

Characterising the performance of hydrogen sensitive coatings for nuclear safety applications

The detection of hydrogen gas is essential in ensuring the safety of nuclear plants. However, events at Fukushima Daiichi NPP highlighted the vulnerability of conventional detection systems to extreme events, where power may be lost. Herein, chemochromic hydrogen sensors have been fabricated using transition metal oxide thin films, sensitised with a palladium catalyst, to provide passive hydrogen detection systems that would be resilient to any plant power failures. To assess their viability for nuclear safety applications, these sensors have been gamma-irradiated to four total doses (0, 5, 20, 50 kGy) using a Co-60 gamma radioisotope. Optical properties of both un-irradiated and irradiated samples were investigated to compare the effect of increased radiation dose on the sensors resultant colour change. The results suggest that gamma irradiation, at the levels examined (>5 kGy), has a significant effect on the initial colour of the thin films and has a negative effect on the hydrogen sensing abilities

Cross-sectional measures and modelled estimates of blood alcohol levels in UK nightlife and their relationships with drinking behaviours and observed signs of inebriation

<p>Abstract</p> <p>Background</p> <p>Management of nightlife in UK cities focuses on creating safe places for individuals to drink. Little is known about intoxication levels as measuring total alcohol consumption on nights out is complicated by early evening interviews missing subsequent consumption and later interviews risking individuals being too drunk to recall consumption or participate at all. Here we assess mixed survey and modelling techniques as a methodological approach to examining these issues.</p> <p>Methods</p> <p>Interviews with a cross sectional sample of nightlife patrons (n = 214) recruited at different locations in three cities established alcohol consumption patterns up to the point of interview, self-assessed drunkenness and intended drinking patterns throughout the remaining night out. Researchers observed individuals' behaviours to independently assess drunkenness. Breath alcohol tests and general linear modelling were used to model blood alcohol levels at participants' expected time of leaving nightlife settings.</p> <p>Results</p> <p>At interview 49.53% of individuals regarded themselves as drunk and 79.43% intended to consume more alcohol before returning home, with around one in ten individuals (15.38% males; 4.35% females) intending to consume >40 units (equal to 400 mls of pure alcohol). Self-assessed drunkenness, researcher observed measures of sobriety and blood alcohol levels all correlated well. Modelled estimates for blood alcohol at time of going home suggested that 71.68% of males would be over 0.15%BAC (gms alcohol/100 mls blood). Higher blood alcohol levels were related to drinking later into the night.</p> <p>Conclusions</p> <p>UK nightlife has used substantive health and judicial resources with the aim of creating safer and later drinking environments. Survey and modelling techniques together can help characterise the condition of drinkers when using and leaving these settings. Here such methods identified patrons as routinely getting drunk, with risks of drunkenness increasing over later nights. Without preventing drunkenness and sales to intoxicated individuals, extended drinking hours can simply act as havens for drunks. A public health approach to nightlife is needed to better understand and take into account the chronic effects of drunkenness, the damages arising after drunk individuals leave city centres and the costs of people avoiding drunken city centres at night.</p

Surface temperature generation during drop weight mechanical impact and the usefulness of dynamic thermocouple measurements for predicting impact ignition of flammable gases

The ignition of flammable atmospheres from hot surfaces arising from mechanical interactions has been a significant cause of many industrial and mining explosions. An investigation of the surface temperature generation resulting from sliding friction during short duration mechanical impacts has been carried out and the nature and usefulness of dynamic thermocouple measurement examined in the context of predicting mechanical ignition. The experimental results reveal that there is only a limited relationship between the measured maximum temperatures and the tangential energy loss during an impact. This appears to be mostly due to variation of the extent to which the tangential energy loss represents frictional loss (associated with tip sliding) rather than material deformation. Whilst an increase in impact energy tends to raise the measured surface temperature, there is significant random variation under nominally similar conditions. It is considered that this is associated with the randomness and changing nature of the contacting areas. During the small time-period of a mechanical impact, there is insufficient time for any equalisation of temperature between neighbouring contact zones to take place. With reference to the ignition of flammable gases brought about by mechanical impact, surface temperatures measured by dynamic thermocouple appear to offer only limited predictive usefulness since they could be associated with contact areas of insufficient size to transfer enough energy into the gas mixture to cause ignition. Finger-marking impact surfaces has the effect of greatly reducing the frictional energy loss but this is not fully reflected in the measured maximum surface temperature. It is concluded that ignition prediction should still be based on tests conducted with mechanical impacts taking place in an ambient flammable atmosphere

An experimental and CFD study into the dispersion of buoyant gas using passive venting in a small fuel cell enclosure

© 2016 IChemE. With the emergence of a 'Hydrogen Economy', fuel cell (FC) deployment in small enclosures will become common place. However, hydrogen's wide flammable range (4-74%) poses a significant safety concern. Without adequate ventilation, a hydrogen gas leak from a FC could create flammable mixtures in the enclosure, and hence the potential for an explosion. Traditionally, a mechanical ventilation system would be employed in an enclosure to ensure that the hydrogen gas is removed and prevent a flammable concentration forming. However, in many applications (e.g. low power and remote installations) mechanical ventilation is undesirable, since it would drain the FC output and its operation would be vulnerable to any power failures that may occur. In such situations, it is therefore desirable to be able to employ a passive ventilation system to remove the hydrogen gas from the FC enclosure. Passive ventilation relies upon buoyancy driven flow, with the size, shape and position of ventilation openings critical for producing predictable flows and maintaining low gas concentrations. Determining the relationship between gas leak rate, ventilation configuration and internal concentration of buoyant gas will help to inform and optimise FC enclosure safety design. An experimental and Computational Fluid Dynamics (CFD) study was therefore carried out to investigate helium gas dispersion (employed as a safe analogue for hydrogen gas) in a 0.191 m 3 ventilated enclosure. The helium gas was released from a centrally positioned, vertical, 4mm diameter nozzle at low flow rates (1-5 L/min), to simulate a hydrogen leak from a fuel cell (FC) in a small enclosure. A single narrow horizontal vent was created at the top of one vertical face. The helium leak rate was varied and observations of dispersal behaviour and gas concentration made. Similarly positioned vents were introduced on the remaining vertical faces and further observations made. Ventilation flow rates were found to increase as the number of vents increased, and became more effective at keeping helium concentrations below 4% v/v, across the range of leak rates investigated. A cross-flow passive ventilation scheme using opposing lower and upper matched vents provided comparative data. The cross-flow arrangement provided effective displacement ventilation and performed best. The more challenging, high-level vent arrangements provided mixing/exchange ventilation, which became more effective with increasing number of vents. The CFD model was found to be able to replicate the experimental flow behaviour observed, but with variance in concentration levels produced

Domestic Refrigerator Design -Safety Issues and Opportunities

There are around 300 house fires in Great Britain each year involving a fridge or freezer. This paper describes the examination of evidence of domestic fires in London between 2005 and 2015 where fridges or freezers have been cited as a fire source. It describes the generic reasons for the cause and spread of the fire and potential ways of constructing and using fridges and freezers to significantly reduce risk and spread

Fire suppression systems in aircraft: Their past, present & future

Fire suppressants are used as part of our everyday lives due to their ability to safeguard life and property. Until recently halons and other ozone depleting substances were commonly used, but for many ground based applications these have now been replaced. However, in aeronautics the challenge to identify suitable replacements for some applications has been difficult, particularly in relation to cargo hold protection. The following article will discuss the history of halon use and withdrawal, and the challenges we are facing to develop new aeronautical fire suppression systems