Flammable Region Research Articles

Quantitative prediction of the flammability limits of filter paper in microgravity conditions

Under JAXA's FLARE project, ongoing since May 2022, long-duration microgravity combustion experiments have been conducted using filter paper. These experiments utilize the Solid Combustion Experiment Module (SCEM) in the ISS/Kibo module. The dimensions of the filter paper used were 130 mm in length and either 40 mm or 20 mm in width, with the experiments conducted in an environment of 298 K and 100 kPa. This paper highlights the results of 19 flame spread experiments performed in opposed-flow environments and compares the predetermined flammability limiting curve with the flammability map obtained in orbital experiments. One of the objectives of the FLARE project is to predict the flammability limiting curve in microgravity environments based on the results of tests that can be conducted on the ground. These experiments produced intriguing findings, demonstrating the effects of both radiation loss and blow-off during flame spread, as the opposed flow velocity was incrementally increased from 10 cm/s to 18 cm/s in 2 cm/s steps, within an environment containing 15.5 % oxygen concentration. When comparing with the combustible map obtained from orbital experiments, it was discovered that the original limit curve accurately replicated the blow-off branch but overestimated the flammable region in areas with opposed-flow velocity below 10 cm/s. This discrepancy was attributed to the inadequate consideration of the effect of opposed-flow velocity on the radiation quenching branch. Therefore, by considering the velocity distribution measurement results obtained in orbit and modeling the impact of the effective flow velocity opposing the flame front on both the radiation quenching branch and the blow-off branch, we successfully quantitatively replicated the results of the orbital experiments. These findings indicate that the flammability limits of thin flat plate samples can be accurately predicted by properly assessing the effect of the boundary layer on the sample surface.

Temporal and spatial evolution of hydrogen leakage and diffusion from tube fittings on fuel cell vehicles under the effect of ambient wind

Hydrogen has become a competitive energy carrier owing to its high fuel value, cleanliness, and renewability. Fuel cell vehicles (FCVs) are crucial in achieving global carbon neutrality in the transportation sector. This study used Flame Acceleration Simulator (FLACS) software to model hydrogen leaks and diffusion from tube fittings of hydrogen systems with the numerical model validated by experimental data. The effect of wind direction on hydrogen leakage and diffusion in the chassis was investigated. Hydrogen leakage rates from the tube fittings under various pressures, tightening angles, and torques were determined; a release rate of 0.6 NL/min is considered a typical value for hydrogen leakage from the tube fittings of a 1.2-MPa hydrogen supply line. The results demonstrate that in windless conditions, the high concentration region with 1–2% vol. was located around the group of hydrogen storage tanks and that there was no flammable region in the entire chassis area. Wind accelerated hydrogen diffusion and reduced the extension range of the hydrogen envelope by 1–2% vol. Compared with longitudinal wind conditions, a crosswind can more effectively reduce the hydrogen concentration. Cutting off the hydrogen supply resulted in rapid contraction of the hydrogen envelope of 1–2% vol. within 5 s. These results have implications for installation of hydrogen sensors in the chassis of FCVs and the location of the blower for forced ventilation by first responders in the event of hydrogen leakage.

An investigation on the thermal emission of hydrogen enrichment fuel in a gas turbine combustor

In this study, numerical methods were used to assess the effects of adding hydrogen and varying the equivalence ratios on the combustion and emission properties of a non-premixed combustor. The results revealed that the additions of H2 and varying Φs significantly affected the flame configuration and emission characteristics. An elevation in the H2 vol. Promotes fuel combustion, yielding a higher flame temperature and a wider flammability region. Lower Φs and increased H2 vol. Significantly decrease CO and CO2 emissions by 87.48% and 96.63% respectively. Conversely, NOx emissions increase with higher H2 vol. and Φs due to the catalytic effect of elevated flame temperature. At Φ = 0.2 and 50 vol% of H2, outlet temperature and pattern factor were enhanced by 61.66% and 30.16% respectively. Under these conditions, NOx emission was calculated by a factor of 74.14. Investigation emphasizes hydrogen-enriched fuels’ potential for enhancing combustion and emissions control through vol. and equivalence ratios.

Investigation of the leakage and diffusion characteristics of hydrogen-addition natural gas from indoor pipelines

The hydrogen-addition natural gas pipeline is a promising technology for large-scale civilian use, revealing the indoor diffusion behaviors of flammable gas in a leakage accident is of great importance for formulating a safety warning. A three-dimensional unsteady model is established, the distribution characteristics of CH4 and H2 are studied and the dynamic flammable regions are investigated. Case studies are performed to evaluate the effects of hydrogen-addition ratios and floor levels on average concentrations and flammable volumes of CH4 and H2. The results show that the high-concentration region of H2 is always near the leakage hole. The increase rate of flammable gas mixture volume is larger than that of the volume of high-concentration CH4. Under a higher hydrogen-addition ratio, the high-concentration region of CH4 and the flammable region of gas mixture both float up while the high-concentration region of H2 keeps horizontal, the flammable volume of gas mixture increases faster but the final volume is smaller. When the room is ventilated, a bigger wind speed leads to a smaller flammable volume of gas mixture. The flammable volume can be constrained at a quite low value from the beginning as the wind speed surpasses 1.5 m/s. When the leakage of hydrogen-addition natural gas occurs, floor 1 takes the most serious ignition risk with the largest flammable volume of 9.87 m3, occupying 44% of the room space. By contrast, the upper floors have quite smaller flammable volumes of gas mixture (under 0.05 m3) and the flammable region is near the leakage hole. The high-concentration region of H2 is affected slightly by the floor height.

Effects of water vapor addition on downstream interaction in CO/O2 counterflow premixed flames

The effects of H2O addition on downstream interaction in counterflow premixed CO/O2 flames are investigated by varying the global strain rate (ag) and CO mole fractions (XCO,L, XCO,U) in the lower and upper nozzles, respectively. For interacting premixed CO/O2 flames, the flammable region is very narrow such that the flames cannot be sustained for ag > 11.75 s−1. When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low strain rate, the lean-lean and rich-rich extinction boundaries show strong and weak interactions similar to those observed previously in hydrocarbon fuels. The flammable lean-lean and rich-rich regions gradually shrink with the increase of ag. When XCO,U is small for asymmetric lean double flames at low strain rate, the extinction boundary exhibits weak interaction behavior, where the weaker flame is parasitic to the stronger flame by XCO,L. The stronger flame experiences heat loss to the weaker flame. As the strain increases, the reaction cannot be completed due to the reduction in the flow time. The thermal energy loss by incomplete reaction leads to the flame extinction. This effect changes the qualitative nature of extinction boundary as strain rate increases, resulting in the extinction boundary having only the strong interaction mode having near constant (XCO,L + XCO,U) and bending toward larger XCO,L, and eventually forming an island shape at higher strain rate. The local equilibrium temperature (LET) concept is introduced to explain these flame extinction mechanisms. Local temperature behaviors are well explained by investigating major reaction contributions to heat release rate. In all cases, LET decreases by the effect of preferential diffusion because of the Lewis number of deficient reactant being larger than unity. For asymmetric double flames, conductive heat transfer (CHT) from the stronger to weaker flame reduces the LET of the stronger flame. Flame extinction mechanism can be explained by introducing a loss ratio. For CO/O2 flames, the effects of incomplete reaction as well as preferential diffusion and CHT lead to flame extinction. For (CO/O2 + 1.0% H2O) flames with the increase of strain rate, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.

Influence of jet parameters of fluidic oscillator-type fuel injector on the mixing performance in a supersonic flow field

Numerical analyses are performed to clarify the influence of jet parameters, i.e., the sweeping jet angle and oscillation frequency, of a fluidic oscillator on the mixing performance. Specifically, hydrogen is injected into the supersonic freestream through a fluidic oscillator, and the corresponding mixing performance is evaluated. The mixing performance evaluated is penetration length, area of a flammable region, and mixing efficiency. In all cases, the fluidic oscillator has better mixing performance than a steady transverse jet. The results show that the influence of the sweeping jet angle on the mixing performance is more notable than that of the oscillating frequency. At 58 and 580 Hz frequency, 24° has the highest penetration length ratio and largest area of the flammable region. The tendency of the mixing efficiency ratio depending on the sweeping angle at low frequencies is unclear. At high frequencies, 24° has the biggest mixing efficiency and largest area of the flammable region, whereas 18° has the highest penetration length ratio. The results showed that the appropriate combination of the frequency and sweeping jet angle to maximize the mixing performance.

Reacting flow characteristics based on the axis-switching phenomenon in a baffled micro combustor with rotated noncircular holes for micro-thermophotovoltaic system

• The geometric features related to the axis-switching phenomenon are numerically investigated for a multihole baffled micro combustor. • In the combustor of noncircular holes, secondary flows are more developed. • The flow mixing and combustion efficiency are improved by noncircular air holes. • The best thermal field for a combustion-based TPV system is obtained for baffles with 30°-rotated square holes and 15°-rotated triangular holes. • The emitter efficiency for TPV applications is better than the swirling flow combustor. For a thermophotovoltaic (TPV) generator, a combustor design for strengthening the axis-switching phenomenon is explored. Various simulations based on a Reynolds stress model and detailed chemical mechanism of GRI 3.0 are performed for methane (CH 4 )–air flames in a multihole baffled combustor. The reacting flow fields are investigated with the baffle plates of rotated square and triangular holes. The spreading and merging of multiple air jets past the baffle plate are significantly affected by rotating noncircular air holes. Considering the crossover location changes of the jet half-width, the air jets have a flow structure that quickly generates the axis-switching phenomenon. Compared with the baffled combustor of circular air holes, the center recirculation zone in the combustor of noncircular holes is enlarged by approximately 200% with a hole rotation. As the hole rotation increases, secondary flows become more complicated and the merging of air jets is quickly finished. Based on these flow structures, the region with the stoichiometric condition is observed in various forms of hexagonal, star, circular, and flower shapes. As a result, the flammable region and combustion efficiency are considerably increased. The flame lengths are roughly evaluated as three times the mixing lengths. From the viewpoint of a heat emitter for the TPV application, the heat transfer on the combustion chamber wall is discussed. Compared with the combustor with circular air holes, the combustor with noncircular air holes shows a 5% increase in flammable region and a 5.9% improvement in combustion efficiency. For a combustion-based TPV system, the best uniformity of wall temperature is observed for baffles with 30°-rotated square holes and 15°-rotated triangular holes and the highest mean wall temperatures are obtained when the flame length is approximately 1/3 of the combustor length.

Ethylene combustion performance with varying the N2 content in a porous burner

Direct emissions of volatile organic compounds (VOCs) not only pollute the environment, but also waste a lot of resources. To overcome above problem, this paper creatively proposed the solution by porous media combustion technology. In this study, ethylene with different N2 content levels of 10%, 30%, and 50% (used to simulate VOCs) was experimentally burned in a self-designed porous medium burner stacked with 9 mm alumina balls. The effects of equivalence ratio, premixed gas flow velocity and dilution ratio on flammable region, maximum combustion temperature, combustion wave velocity, pollutant emissions and combustion treatment efficiency were investigated. The maximum combustion temperature increased with the equivalence ratio and the premixed gas flow velocity. Consequently, the corresponding combustion treatment efficiency increased and CO emissions decreased. With increasing the dilution ratio, the flammable region, maximum combustion temperature, and combustion treatment efficiency became smaller but CO emissions and the combustion wave velocity became larger, except the case with 10% N2 content in which combustion was better than that with pure ethylene. In general, with relatively higher φ (φ ≥ 0.50), medium Vfv (Vfv ≥ 0.50), and appropriate α (α = 10%), the premixed gas could burn well with high combustion treatment efficiency.

Modeling the diffusion of flammable hydrogen cloud under different liquid hydrogen leakage conditions in a hydrogen refueling station

Constructing hydrogen refueling stations will be popular for hydrogen energy use in the future, and investigating the diffusion characteristics of hydrogen in a leakage incident is quite significant. The instantaneous evolution of flammable hydrogen clouds arising from liquid hydrogen leakage in a hydrogen refueling station is predicted using Ansys Fluent, and parametric analyses are conducted to reveal the effects of storage pressure, source height, and leakage direction on the distributions of the flammable regions. In addition, the feasibilities of heating the ceiling or the ground of the station after the leakage of liquid hydrogen to accelerate the hydrogen dilution are examined. The results show that the flammable region is stabilized at 90 s, the corresponding flammable hydrogen cloud volume is about 333 m3, and the extensions of downwind and vertical directions reach 10 m and 9.3 m. Storage pressure has a finite effect on the downwind diffusion distance of the flammable cloud. A lower source height tends to format the high-concentration hydrogen cloud near the ground while a higher source height helps separate the flammable clouds from the ground. The upward leakage direction leads to the maximum downwind diffusion distance of about 10.2 m while the downward leakage direction makes the high hydrogen concentration region confined below the ceiling. Just maintaining the ceiling at the initial temperature of 300 K is effective for accelerating the hydrogen dilution in the upward leakage. The maximum hydrogen concentration and the flammable volume can be reduced at rates of 0.35 vol % and 8% for every 50 K increase in heating temperature. For the downward leakage, keeping the ground at the initial temperature just works for the first 40 s in reducing the maximum hydrogen concentration, while increasing the heating temperature receives a gradually declined effect on reducing the flammable volume.

Hydrogen gas dispersion studies for hydrogen fuel cell vessels II: Fuel cell room releases and the influence of ventilation

Results are presented for computational fluid dynamics (CFD) modeling for varying hydrogen leaks within a hydrogen vessel's Fuel Cell Rack inside a Fuel Cell Room. In the limiting case of no room ventilation, modeling shows that the flammable region produced by the hydrogen leak is initially limited by self-induced entrainment and recirculation of air caused by the buoyant rising of hydrogen. Locally and at shorter times (minutes), this effect can be even more influential in limiting the size of the flammable envelope than Fuel Cell Room ventilation. Interestingly, the more diffuse detectable (but sub-flammable) region is not self-limited. This indicates the recirculation pattern required for the self-limiting effect requires a sufficient concentration of hydrogen to establish and differentiate the rising hydrogen mass from the surrounding air, thereby establishing the recirculation pattern that self-limits the flammable region at short times. Modeling results with the Fuel Cell Room ventilation activated shows that several seconds after a hydrogen leak is initiated, the flammable region reaches a steady state, with only minor fluctuations due to the air currents created by ventilation. The expected trends with ventilation rate are found: for a given leak size, a decreasing flammable envelope is found as ventilation is increased and for a given level of ventilation, an increasing hydrogen leak rate produces a larger flammable region. For the cases and ventilation rates examined, flammable H2/air mixtures greater than 4% clear the Fuel Cell Room within 1.5 s after the hydrogen leak is turned off. The CFD modeling results for the detectable level of hydrogen that would trigger an alarm showed that higher ventilation rates might have the unintended consequence of making a hydrogen leak harder to detect, depending on the location of the gas detector in the Fuel Cell Room For the hydrogen leak rates considered in this study, we find that a ventilation rate of 15 ACH provides timely hydrogen evacuation while allowing the leak to be detected by the ceiling-mounted hydrogen monitor (for most monitor locations).

Sensitivity Analysis and Multi‐Objective Optimization of Oxidative Dehydrogenation of Propane in a Fixed‐bed Reactor over Vanadium/Graphene for Propylene Production

AbstractOxidative dehydrogenation of light alkanes is an attractive route for the production of olefins. This method does not need much energy, while common processes such as propane dehydrogenation and cracking require external energy resources and cause rapid catalyst deactivation. In this research, an isothermal one‐dimensional model of a fixed‐bed reactor was considered for oxidative dehydrogenation of propane over a V2O5/graphene catalyst. Operational parameter sensitivity analysis showed that high temperature (450–500 °C), high pressure (7–8 atm), a moderate molar propane to air feed ratio (about 0.7), and a low feed flow rate enhanced the propane conversion and propylene productivity. However, for the production of propylene as desirable product, low temperature, low pressure, a high molar ratio, and a high feed flow rate are preferred. Thus, to determine the exact interval of operational parameter values, a multi‐objective optimization was performed for the yields of propylene and COx, revealing a maximum propylene yield of ∼40 %. For safety reasons, a feed molar ratio of propane to air of > 0.6 was preferred, which is not in the flammable region.

Numerical Description of Jet and Duct Ventilation in Underground Garage after LPG Dispersion

Contamination of toxic and odorous gases emitted from stacks in buildings located in an urban environment are potential health hazards to citizens. A simulation using the computational fluid dynamic technique may provide detailed data on the flammable region and spatial dispersion of released gases. Concentrations or emissions associated with garage sources and garage-to-house migration rates are needed to estimate potential exposures and risk levels. Therefore, the aim of the study was to use an original mathematical model to predict the most accurate locations for LPG sensors in an underground garage for vehicles powered with LPG. First, the three-dimensional geometry of an underground garage under a multi-family building was reconstructed. Next, two types of ventilation, jet and duct, were considered, and different sources of LPG leakage were assumed. Then, the Ansys Fluent software was applied as a solver, and the same initial value of released LPG (5 kg) was assumed. As a simplification, and to avoid the simulation of choked outflow, the emission from a large area was adopted. The results showed stagnation areas for duct ventilation in which gas remained for both the jet and duct ventilation. Moreover, it was observed that the analyzed gas would gather in the depressions of the ground in the underground garage, for example in drain grates, which may create a hazardous zone for the users of the facility. Additionally, it was observed that for jet ventilation, turbulence appearance sometimes generated differentiated gas in an undesirable direction. The simulation also showed that for blowing ventilation around the garage, and for higher LPG leakage, a higher cloud of gas that increased probability of ignition and LPG explosion was formed. Meanwhile, for jet ventilation, a very low concentration of LPG in the garage was noticed. After 35 s, LPG concentration was lower than the upper explosive limit. Therefore, during the LPG leakage in an underground garage, jet ventilation was more efficient in decreasing LPG gas to the non-explosive values.

Flame structures and ignition thresholds of hydrogen jets containing sodium mist under various gas concentrations

ABSTRACT Non-premixed combustion of hydrogen jets containing sodium vapor and mist reduces threats to reactor containment integrity in sodium-cooled fast reactors (SFRs) because it gradually consumes hydrogen gas generated mainly by a reaction between sodium and concrete. Previous studies have been limited to experimentally determining ignition thresholds on the jet temperature and the sodium concentration under specific gas concentrations. In this study, ignition experiments on hydrogen jets containing sodium mist were carried out at a specific jet temperature and sodium concentration under various gas concentration conditions (1–15 vol% hydrogen and 3–21 vol% oxygen). As a result, a stable sodium flame was observed in the jet and then formed a lifted hydrogen flame from a fuel nozzle outlet. An attached hydrogen flame on the outlet was also formed under high hydrogen concentration conditions. These flame structures seemed to be attributed to hydrogen flame propagation, which depends on the hydrogen concentration, jet temperature, and jet velocity. Additionally, the experimental results revealed ignition thresholds on the gas concentration and indicated a flammable region where the hydrogen-sodium jet combustion was more advantageous than an explosive premixed hydrogen combustion. Our study will enable the advancement of safety assessment technology in SRFs.

Tussocks Enduring or Shrubs Greening: Alternate Responses to Changing Fire Regimes in the Noatak River Valley, Alaska

AbstractAs the Arctic warms, tundra wildfires are expected to become more frequent and severe. Assessing how the most flammable regions of the tundra respond to burning can inform us about how the rest of the Arctic may be affected by climate change. Here we describe ecosystem responses to tundra fires in the Noatak River watershed of northwestern Alaska using shrub dendrochronology, active‐layer depth monitoring, and remotely sensed vegetation productivity. Results show that relatively productive tundra is more likely to experience fires and to burn more severely, suggesting that fuel loads currently limit tundra fire distribution in the Noatak Valley. Within three years of burning, most alder shrubs sampled had either germinated or resprouted, and vegetation productivity inside 60 burn perimeters had recovered to prefire values. Tundra fires resulted in two phases of increased primary productivity as manifested by increased landscape greening. Phase one occurred in most burned areas 3–10 years after fires, and phase two occurred 16–44 years after fire at sites where tundra fires triggered near‐surface permafrost thaw resulting in shrub proliferation. A fire‐shrub‐greening positive feedback is currently operating in the Noatak Valley and this feedback could expand northward as air temperatures, fire frequencies, and permafrost degradation increase. This feedback will not occur at all locations. In the Noatak Valley, the fire‐shrub‐greening process is relatively limited in tussock tundra communities, where low‐severity fires and shallow active layers exclude shrub proliferation. Climate warming and enhanced fire occurrence will likely shift fire‐poor landscapes into either the tussock tundra or erect‐shrub‐tundra ecological attractor states that now dominate the fire‐rich Noatak Valley.

상호작용하는 SNG-Air 희박 예혼합 비대칭 화염의 소화 특성

Experimental and numerical studies were conducted to clarify the extinction mechanism in mutually interacting SNG (synthetic natural gas) - air premixed asymmetric counter-flow flames. The detailed kinetic mechanism of UC San Diego with which the priority of predicting measured extinction boundaries was validated was adopted to analyze various aspects via up and downstream interactions on extinction boundaries in the flame stability map. The flame stability map was presented with a functional dependency on methane mole fractions in the cold stream ejecting from upper and lower nozzles by varying the global strain rate. Increasing global strain rate lead gradually slanted and configuring of island flammable region and finally only one flammable condition at 740 s<SUP>-1</SUP> through the shrinkage of flammable region. The interacting lean-lean asymmetric flames of extinction boundaries have flame speed of positive (negative) depending on the deviation of methane mole fraction for two reactants. The extinction mechanism of those flames was explained and discussed by emphasizing important role of downstream chemical interaction (via H and CO) and upstream thermal interaction (via conductive heat loss from stronger flame to unburned mixture).

Effect of opening area on the suppression of self-ignition of high-pressure hydrogen gas leaking in the air by an extension tube

The most influential factor for self-ignition of high-pressure hydrogen is known to be the strength of the shock. Thus, the self-ignition can be suppressed by weakening the shock strength, which is possible by reducing the area where the hydrogen is ejected in this study. To confirm the possibility of this method, experiments were done by controlling the burst pressure of up to 302 bar and the ratio of the opening area. The experimental results showed that the minimum burst pressure of self-ignition is increased exponentially as the opening area is reduced. This confirmed that reducing the opening area under the same burst pressure conditions has an effect on the suppression of self-ignition. However, it was also found that the minimum shock speed that causes self-ignition gradually decreases as the opening area becomes smaller, which results from an increasing in mixing. The CFD simulation results showed that the volume of the flammable region in the tube was increased and the hydrogen-air mixing efficiency also increased when the opening area became smaller. The results suggest that reduction of the opening area can suppress a self-ignition by weakening the shock strength, but it should be noted that an increase in mixing effect also occurs.

Mutually interacting SNG-air premixed flames

Experimental and numerical studies were conducted to grasp the flame characteristics for mutually interacting SNG-air premixed flames with a counterflow configuration. Five different reaction mechanisms (GRI 3.0, USC 2.0, Aramco 2.0, UCSD, and CRECK) were tested to give the priority in tracing measured lean extinction boundaries. UCSD mechanism best traced the measured data. The flame stability map was presented with a functional dependency on methane mole fractions in the reactant streams issuing from the lower and upper nozzles, by varying the global strain rate.With increasing global strain rate, lean and rich extinction boundaries were gradually slanted, forming an island of flammable region and finally only one flammable condition at 740 s−1 via a continuous shrinkage of flammable region. For asymmetric lean-lean and rich-rich flames, the weaker flame was survived via thermally and chemically symbiotic relation of it to the stronger flame. For lean-rich flames, triple flame is changed into bi-brachial flames via a merging of diffusion flame lean flame at high strain rates. This behavior could be explained well by introducing a specific heat release rate. Extinction mechanism could be reasonably explained by the relation of specific heat release rate (as a reactivity of flame) to conductive heat loss from the strong flame to unburned mixture.

An investigation of flammability limits of gaseous mixtures at reduced pressures

An experimental investigation of flammability limits of hydrogen, methane and propane in air and oxygen at reduced pressures was carried out. A slow influence of sizes of an experimental vessel of a diameter higher than 125 mm on the flammability limits was revealed, but an influence of a type of an oxidizer (air or oxygen) and an ignition energy is significant. Critical values of an initial pressure for a possibility of a flame propagation were determined. The limiting values of the ignition energy were determined, for which an elevation of this parameter does not influence the critical pressure and the flammability region. A qualitative interpretation of obtained experimental results is given, which is based on a peculiarities of a flame initiation.

Revisiting the dispersion safety factor (DSF) for vapor clouds of liquefied flammable gases (LNG and propane)

The concept of the Dispersion Safety Factor (DSF) was introduced by Vílchez et al. (2013, 2014) and has been revisited in this work. The DSF is defined as the ratio between the flammable region of the vapor cloud (set at a concentration equal to the low flammability level (LFL)) and the corresponding visible boundary of the cloud. We have used a computational fluid dynamics model (FLACS v.10.4) to simulate the dispersion of two liquefied flammable fuels (LNG and propane). DSF results have been analyzed using main effects and interaction plots, and a complementary metric (DSF50) has been introduced in order to establish more conservative threat areas in flammable vapor cloud scenarios. We have observed an interaction between relative humidity and wind velocity for DSF in the low-to-medium range of RH and wind velocity values. Four regression models have been proposed for the computation of DSF and DSF50 for LNG and propane dependent on ambient wind velocity and relative humidity. Contour plots have been prepared to be used as a practical tool, because through the reading of these plots the DSF (and DSF50) can be obtained immediately given wind velocity and relative humidity data.

Effect of initial pressure on the lower explosion limit of nicotinic acid/acetone mixture

In the work presented in this paper, the effect of initial pressure on the lower explosion limit (LEL) of the hybrid nicotinic acid/acetone mixture was investigated through standard explosion tests carried out in the 20 L sphere. From experimental results, the flammability diagram was built in the plane (concentration/minimum explosive concentration) of nicotinic acid versus (concentration/LEL) of acetone. Interestingly, it has been found that, in going from low pressures (P < 1 atm) to high pressures (P > 1 atm), the extension of the flammability region increases. This behavior has been attributed to the fact that the turbulence kinetic energy (and thus the energy dissipation) decreases with increasing initial pressure. Bartknecht's correlation for LEL of hybrid mixtures was modified to take into account the effect of pressure, and two correlations were obtained able to give satisfactory predictions of experimental data at both low pressures and high pressures.