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Hydrogen is garnering attention as a future energy source due to its environmentally friendly characteristics and high energy density. However, it is crucial to recognize the risks associated with hydrogen, stemming from its extensive flammable range and rapid flame speed. Establishing safety distance criteria and conducting hazard analysis require the collection of data on the behavior of leaked liquid hydrogen under various weather conditions. In this study, a simulated atmospheric chamber is designed to mimic atmospheric conditions, and reliable data is being acquired for trend analysis in various leakage scenarios. Ultimately, the flammable area is most influenced by wind speed, and utilizing hydrogen in environments with temperatures above a certain threshold can minimize the likelihood of accidents resulting from leaks. • Confirming the flammable region resulting from liquid hydrogen spillage. • Presenting the changes in the flammable region according to atmospheric conditions. • Providing an analysis of the spillage behavior of liquid hydrogen in different environments. [ABSTRACT FROM AUTHOR]

This study investigates cavitation in a rectangular-profile Venturi tube using numerical simulations and four turbulence models. The unsteady Reynolds-averaged Navier–Stokes technique is employed to simulate vapor cloud formation and compared against experimental data. κ-ε realizable, κ-ε RNG, κ-ω SST, and κ-ω GEKO models are evaluated. The simulation results are analyzed for pressure, turbulence, and vapor cloud formation. Discrepancies in cavitation cloud formation among turbulence models are attributed to turbulence and vapor cloud interactions. RNG and SST models exhibit closer alignment with the experimental data, with RNG showing a superior performance. Key findings include significant vapor cloud shape differences across turbulence models. The RNG model best predicts velocity at the throat exit with an error of 4.145%. Static pressure predictions include an error of 4.47%. The vapor cloud length predictions show variation among models, with the RNG model having a 0.386% error for the minimum length and 4.9845% for the maximum length. The SST model exhibits 4.907% and 13.33% errors for minimum and maximum lengths, respectively. Analysis of the cavitation number reveals agreement with the experimental data and sensitivity to cavitation onset. Different turbulence models yield diverse cloud shapes and detachment points. Weber number contours illustrate the variation in the cavitation cloud behavior under different turbulence models. [ABSTRACT FROM AUTHOR]

The flame spread behavior over a filter paper in opposed flow was investigated on the ISS/Kibo, and the results were compared with the flammability map predicted in advance by scale analysis for a two-dimensional flame. The on-orbit experiments were conducted in the solid combustion experimental module (SCEM) as the first combustion test of the FLARE project by JAXA. A filter paper with a length of 130 mm, a width of 40 mm or 20 mm, and a thickness of 0.12 mm was used as the specimen. The opposed flow velocity and the oxygen concentration were varied from 0 cm/s to 25 cm/s, and from 13.5% to 34%, respectively. The ambient pressure is 101.3 kPa. The predicted limiting curve reasonably agreed with the minimum limiting oxygen concentration observed. On the other hand, in extremely slow flows, very robust flames were observed beyond the limiting curve. When the condition reached the limit of two-dimensional flame, the flame front changed from a linear shape to a spherical shape (flamelet) to shorten the preheat zone length, reducing the radiative heat loss to survive. In these cases, the flamelet was sustained for a long time (> 100 s), and a large amount of decomposed gas was released at the periphery of the pyrolysis zone generating a combustible vapor cloud around the spherical flame font. The presence of a long-life robust flamelet and the accumulation of such combustible mist can be fire hazards in microgravity. [ABSTRACT FROM AUTHOR]

Characterization of the dispersion behaviors of spilled liquid hydrogen is necessary from the safety prospective. In this study, the two-phase flow of liquid hydrogen spill is predicted with the mixture multiphase model, Lee model and Realizable k - ε model, and the dispersion of hydrogen vapor cloud with the atmospheric inversion layer is numerically analyzed. The inversion layer restrains the upward movement of the cloud and the mixing of the cloud with air, increasing its ground-level hydrogen concentration. The heights of the flammable cloud can be reduced by 5.71%, 10.49% and 12.86%, respectively, with the temperature lapse rates of 0.03, 0.06 and 0.10 K/m in our investigated scenarios. Besides, the restraining effect is strengthened by increasing the ground air temperature if the temperature lapse rate remains unchanged. Image 1 • Dispersion of hydrogen vapor cloud in presence of atmospheric inversion layer was analyzed. • The inversion layer restrains the upward movement of the hydrogen vapor cloud. • Increasing the temperature lapse rate strengthens the restraining effect. • A higher ground air temperature is also adverse for the cloud dilution. [ABSTRACT FROM AUTHOR]

• Soybean oil droplet shows four-staged evaporation characteristics at high temperatures. • The micro-explosion strength increases with the increase of ambient temperature. • Evaporation induced vapor cloud was first observed at 973 K. • Homogenous and heterogeneous nucleation were observed at 873 K, 923 K and 973 K. Micro-explosion characteristics of soybean oil droplet were investigated using high speed backlight imaging technique at ambient temperature 873 K, 923 K and 973 K. Experimental results indicate that soybean oil droplet underwent transient heating phase, micro-explosion evaporation phase, equilibrium evaporation phase, and residue evaporation phase at all the studied temperatures. The evaporation rate of equilibrium evaporation phase is increased when the temperature increased. However, a decrease for the evaporation rate of residue evaporation phase. Homogeneous and heterogeneous nucleation are observed at all temperatures, homogeneous nucleation lead to strong micro-explosion and heterogeneous nucleation lead to weak micro-explosion. Moreover, micro-explosion strength increased when the ambient increased. It was found that the micro-explosion occurred at the end of droplet evaporation, this is one of the reasons for the poor atomization characteristics of bio-oil in the engine. More importantly, the evaporation induced vapor cloud is observed at 973 K, this is mainly due to the non-isothermal condensation caused by Stefan outflow. [ABSTRACT FROM AUTHOR]

• High expansion foam used to mitigate the vapor risk due to LNG. • Adding nanoparticles to improve foam stability. • Studying effects of heat transfer mechanisms on nanoparticle stabilized foam. • Understanding the mechanism of improved stability. Natural gas consumption has increased as it is a cleaner energy source when compared to coal or petroleum. It can be stored in its liquid form as liquefied natural gas (LNG) because it has a much lower liquid volume. This has allowed ease of storage and transportation enabling it to be used as a global commodity. In case of an incident resulting in leaked LNG, the National Fire Protection Agency (NFPA) and American Gas Association (AGA) recommend high expansion foam use to mitigate the risk due to a cryogenic vapor cloud. This work investigates the role of exfoliated zirconium phosphate (ZrP) nanoplates in stabilizing high expansion foam. Experiments performed under wind induced forced convection and thermal radiation illustrate that exfoliated ZrP nanoplates can stabilize high expansion foam. Up to 40% reduction in the foam breakage rates was observed when using exfoliated ZrP nanoplate stabilized foam. Use of this foam can increase the duration over which it needs to be replenished, while allowing rising LNG vapors to become lighter by exchanging heat with the foam and ensuring effective vapor dispersion. [ABSTRACT FROM AUTHOR]

Abstract Safety is one of the most important concerns in handling liquid hydrogen. Mixture model and Realizable k-ε model are adopted to simulate the spills of liquid hydrogen in the present work. Effects of a dike around the liquid hydrogen source on vapor cloud dispersion are numerically investigated. The dike increases the hydrogen concentration near the source, and promotes the upward movement of flammable cloud thanks to the enhanced mixing with air, reducing the cloud detachment distance. The time needed for the hydrogen vapor cloud to be diluted out of hazards is largely influenced by dike size, and introducing the dike does not necessarily increase the overall hazard duration. Increasing the height to length ratio of the dike makes the near-source region exposed to high hydrogen concentration for longer hazard duration, while detachment distance reduces with the increment in dike height. Highlights • Effects of dike around liquid hydrogen source on cloud dispersion are simulated and analyzed. • Dike promotes the upward movement of flammable cloud due to the enhanced mixing with the air inside dike. • Introducing the dike does not necessarily increase the overall hazard duration. • Increasing the height to length ratio of dike worsens the situation near the source. [ABSTRACT FROM AUTHOR]

Abstract A three-dimensional CFD model for large-scale liquid hydrogen spills is developed and validated by the experiments carried out by NASA. The effect of humidity on the development of hydrogen vapor cloud is emphasized, with the modified expressions of Lee model accounting for the phase changes of water and hydrogen. The results show that the numerical prediction is more consistent with the experiment considering the presence of air humidity. The condensation of water in the atmosphere increases the buoyancy of the vapor cloud, and promotes the diffusion of the cloud in vertical direction. The dimension of the cloud in streamwise direction changes little under different humidity, due to the balance between the height-dependent wind speed and the induced buoyancy. The scope of visible cloud indicated by the condensed water vapor expands with the increasing air humidity, and still lies within the flammable domain when the relative humidity approaching to 75%. Water vapor condensation induces the cloud temperature rise under the same concentration, and the leeward part is more influenced compared with the upwind part. Highlights • CFD model for large-scale liquid hydrogen spills considering air humidity is built. • The effect of humidity on the development of hydrogen vapor cloud is analyzed. • The streamwise dimension of hydrogen vapor cloud changes little with different humidity. • The scope of visible cloud expands with the increasing air humidity. • Water vapor condensation increases the cloud temperature under same concentration. [ABSTRACT FROM AUTHOR]

Dilution of hydrogen vapor cloud formed by the liquid hydrogen spills under different spill amounts, spill rates and liquid mass fractions at the exit were numerically investigated. Three dimensional CFD simulations were performed with ANSYS FLUENT using Realizable k - ε turbulence model. Time variation of the maximum hydrogen concentration ( t = 0 s at the end of the spill) is most sensitive to the liquid mass fraction, followed by the spill rate, and finally the spill amount. The dilution process is nearly unaffected by the spill amount. With the increment of spill rates, the dilution speed first decreases and then remains approximately unchanged, which can be attributed to the combined effects of the gaseous hydrogen generated per unit time and the source disturbances generated by the spill and evaporation process. In addition, the dilution accelerates with the increment of liquid mass fraction at the exit. The turbulence induced by the evaporation of liquid hydrogen promotes the mixing and dilution processes, while the temperature drop in the ground and the ambient air due to liquid evaporation has little influence on the overall dilution of the vapor cloud. The durations of detonation and flammability when the liquid mass fraction is 0% are 1.65 and 1.88 times of those when liquid mass fraction is 100%, respectively. [ABSTRACT FROM AUTHOR]

When hot vapor condenses the nucleation particles initially formed coalesce rapidly and exothermically. Most of the heat of coalescence is released in less than a millisecond while droplets are less than 100 nm diameter. Consequently, the heat is released close to the source of the vapor and has no effect on dispersion. Coagulation rate is governed by an inverse square law with respect to number density; calculations showed that a vapor cloud initially within its flammable range should produce droplets smaller than 10 µm after a minute of coalescence. Such micronized droplets completely vaporize in flame preheat zones and behave as premixed vapors rather than as individual droplets; this characteristic is not due to the increased surface energy accompanying subdivision into micronized droplets but to short evaporation times in the thick preheat zones of limit flames. Droplet growth time is a nonlinear function of cloud mass concentration and if the latter exceeds about 10 times the minimum explosible concentration (MEC), droplets quickly grow to above 20–40 µm. These larger droplets tend to burn individually and the estimation of MEC becomes complicated, although rain‐out of large droplets might hasten dispersal to below the MEC. With the assumption that micronized crystalline organic materials behave similarly to liquid droplets, we derive “intrinsic” MEC values for both hydrocarbons and a series of CHOx fuels having increasing O:C ratios. In each case, the MEC attains a constant “terminal value” as carbon number is increased. Terminal MEC values are 40 g m−3 for CH (hydrocarbons), 50 g m−3 for CHO (aldehydes, ethers, etc.), 60 g m−3 for CHOO (esters), 67 g m−3 (polyethylene glycols), and 103 g m−3 (carbohydrates). MEC values for CHO and CHOO decrease rapidly with increased carbon number since additional hydrocarbon units dilute the functional oxygen group. For polyethylene glycols and carbohydrates, the MEC is less dependent on carbon number since oxygen is contained in a repeating group. Although scission and/or decomposition replace simple evaporation as carbon number increases, flash pyrolysis data for polyethylene in air show that the products of decomposition can have little effect on MEC. Nevertheless, owing to experimental deficiencies published MEC values for polyethylene are as low as 10–30 g m−3. Of the many factors affecting MEC tests, we focus on the effects of gravity and flame radiation. Radiation not only affects MEC but also the burning rates of metal dusts such as titanium; such tests should be carried out on an adequately large scale. © 2017 American Institute of Chemical Engineers Process Process Saf Prog 37:4–17, 2018 [ABSTRACT FROM AUTHOR]

Formation of lithium vapor cloud occurs above lithium surface as temperature goes up. Experiments devoted to investigating lithium vapor cloud interacting with expanding argon plasma under applied electric field have been carried out in a linear plasma device. Lithium spectrum has been characterized by utilizing optical emission spectroscopy (OES) and many emission lines of lithium atoms have been observed, including Li_670.78 nm (1.9 eV), Li_610.36 nm (3.87 eV), Li_460.30 nm (4.54 eV) and Li_323.26 nm (3.83 eV). Enhanced erosion of lithium surface caused by surface adatoms was found when bias is high, its effective adatoms sublimation energy was calculated to be 1.72 eV at the high temperature through Arrhenius plot in enhanced region. The surface temperature of liquid lithium has been monitored by a thermocouple. Since incident argon ion energy has been elevated due to the applied electric field acceleration, the temperature of lithium surface will have different behaviors with different biases, and it turns out that the temperature has a linear relation with bias, about an increase of 16–17 °C per 5 V bias voltage. By calculating evaporation power, excitation power and ionization power, indicating that at temperatures above 700–800 °C, heat flux dissipated in the lithium vapor can have a huge meaningful decrease. [ABSTRACT FROM AUTHOR]

In this study, the authors used a specially designed experimental device to detect the amount of diesel that evaporated at different diesel and environmental temperatures and quantify the distribution of diesel at different heights within the subsequent vapor cloud. They also examined the relationship between the total diesel concentration of the vapor cloud and the amount of diesel evaporated, the relationship between the explosive threshold of the diesel vapor cloud and the environmental temperature, and the formation of the vapor cloud. Finally, the authors investigated the conditions relevant to the formation of an explosive vapor cloud after diesel spillage. [ABSTRACT FROM AUTHOR]

The evaporation of volatile heavy hydrocarbon liquid from coaxial well configuration is studied to understand the vapor phase transport, vapor cloud interaction and its effect on the local evaporation rate of two evaporating coaxial cavities of microliter volume. The gap between the two coaxial cavities is varied without varying the dimensions (width and depth) of the coaxial cavities for studying the vapor cloud interaction. Digital holographic Interferometry (DHI) is used to decipher the vapor mole fraction field above the coaxial well. Gravimetric measurement has been carried out for measuring the evaporation rate. Diffusion-limited simulation of evaporation process has been carried out using COMSOL Multiphysics software to understand the role of convective motion. IR thermography measurement of liquid interface temperature has been carried out to correlate the vapor cloud interaction with local evaporation rate and evaporation induced cooling. A flat-disk shaped vapor cloud surrounds the coaxial cavities. Total evaporation rate measured from DHI shows good agreement with the gravimetric measurement having maximum deviation of 3%. Good match between the evaporation rate measured from gravimetry and holography with diffusion limited model is observed at low Grashof number. The mismatch in evaporation rate with the diffusion limited model is observed at high Grashof number due to higher radius of the coaxial cavity indicating the dominance of convective motion. The evaporation rate from individual well decreases when liquid simultaneously evaporates from the inner and outer coaxial cavities. This reduction in evaporation rate increases with decrease in gap between the two coaxial cavities due to increase in interaction between the vapor cloud. The decrease in local evaporation rate from individual cavity due to presence of a neighboring evaporating cavity correlates with the evaporative cooling effect using temperature measurement of the interface from IR thermography. Vapor cloud interactions of microliter volume coaxial cavities can influence the evaporation rate of individual coaxial cavity and the convection inside the liquid phase. The present study demonstrates the capability to precisely control the evaporation process by appropriate design of coaxial well configuration. The results from the present study can be useful for controlling the evaporation rate in several applications i.e. protein crystal growth, DNA/RNA sequencing and microfabrication etc. by suitable design of coaxial well. [Display omitted] [ABSTRACT FROM AUTHOR]

• The flame evolution behavior of vapor cloud explosion ignited at different positions were analyzed. • Experiments of chained explosions for vertically distributed vapor clouds were conducted. • The effect of vapor cloud diameter and separation distance on the secondary explosion was discussed. • The regime of the enhanced secondary explosion was revealed. Chained explosions happened occasionally, leading to heavy casualties and huge property losses. However, the development regime of the chained explosions is unclear. Experiments of both single and two vertically distributed vapor clouds with different sizes, volume concentrations and ignition conditions were conducted. The experimental results show that the maximum overpressure generated by the primary methane/air mixture explosion increases with vapor diameter, but hardly changes with the ignition energy. The effect of ignition position on chained explosions is significant since the development trajectory of the primary explosion varies with the ignition position. The secondary explosion is more likely to be triggered in the case of ignition at the center of the primary vapor cloud. Because of the enhanced ignition and turbulence induced by the primary explosion, the maximum overpressure and rate of pressure rise of the secondary explosion are larger than that of the primary explosion and increase with a decrease in the separation distance between the two vapor clouds. In addition, the development of the explosion flame for the secondary explosion induced by a primary explosion is asymmetric due to the uncertain ignition point. [ABSTRACT FROM AUTHOR]