Limiting Oxygen Concentration Research Articles

Evaluation of an oxygen reduction system (ORS) in large-scale fire tests

This work evaluates the oxygen reduction system (ORS) approach to fire prevention in large-scale fire tests. A two-tier fuel array of standard commodities is used in a rack-storage configuration within an enclosure. A constant nitrogen/air mixture flow is supplied to the enclosure at a desired oxygen concentration. The oxygen concentration varies from 9% to 17%. A premixed propane ignitor is used as ignition source. The tested materials include five standard commodities of Class 3, CUP, CEP, UUP and UEP. The results of fire propagation success are obtained for the five standard commodities under different oxygen concentrations with a sustained igniter (hard limits) and without a sustained igniter (soft limits). The resulting limiting oxygen concentration (LOC) values are shown to be generally lower than the oxygen design concentrations recommended by existing standards including VdS 3527 and EN 16750 due to different test conditions. The hard limits are close to the fundamental LOC values for gases and vapors and do not depend significantly on the ignition duration and array size, while the soft limits vary significantly with the size and configuration of the fuel array and ignition duration. It is concluded that the hard limits are more suitable for ORS design purposes.

Enhanced Predictive Modelling of Steel Corrosion in Concrete in Submerged Zone Based on a Dynamic Activation Approach

A numerical model for enhanced service life prediction of concrete infrastructure is presented which includes transient analysis of processes during corrosion initiation as well as propagation stage. The temporal and spatial transition of Steel–Concrete Interface during depassivation events is described by a randomly varying chloride threshold function. As such random activation events can be accounted for, rather than having to pre-describe the anode size and location as in many existing models. The aim of the study is to investigate random spatial activation events in concrete structures in submerged zones based on dynamically changing boundary conditions on the rebar surface to control transition from passive to active state. Investigations are carried out to realize the sustainability of corrosion processes in limiting oxygen concentrations in dissolved seawater. The model showcases the numerical architecture, the associated concept of randomly varying chloride threshold and predicts that among other factors, the rate of oxygen strongly influences corrosion rate in submerged locations.

Experimental investigation of limiting oxygen concentration of hybrid mixtures

An investigation into the limiting oxygen concentration (LOC) of fifteen combustible dusts and methane, ethanol and isopropanol hybrid mixtures in the standard 20 L explosion chamber was performed. Three ignition energies (10 J, 2 kJ and 10 kJ) were used. The results show that a 10 J electrical spark ignition leads to significantly higher limiting oxygen concentration values than either 2 kJ or 10 kJ pyrotechnic igniters. This could be due to the “overdriving” effect of the chemical igniters, which produce a hot flame that virtually covers the entire explosion chamber during combustion. With respect to hybrid mixture investigation, the 20 L sphere was modified to allow the input of methane gas and flammable solvents. The limiting oxygen concentrations of the hybrid mixtures were found to be considerably lower than those of dust air mixtures when the relatively weaker spark igniter was used. There was no significant change in limiting oxygen concentration when the higher energy chemical igniters were used.

An experimental study on self-extinction of tunnel fire under natural ventilation condition

Fire tests with both methanol and propane fires ranging from 5.6 to 16.8 kW were conducted in two 1:20 reduced-scale tunnels with identical cross-section size of 0.45 m wide × 0.23 m high but different lengths, i.e., 20.8 m and 10 m long respectively. Natural smoke control was adopted for both tunnels. Fires were located at the center of the tunnels. Temperatures and gas concentrations (i.e., O2, CO and CO2) were measured in the experiments. The experimental results showed that both methanol fires and propane fires self-extinguished in the 20.8 m-long tunnel within around 600 s, except the 5.6 kW methanol fire. The larger the heat release rates were, the quicker the self-extinction of fires were. The oxygen concentrations in the 20.8 m-long tunnel declined monotonously until they approached around 12% except for the 5.6 kW methanol fire, which kept well above the limiting oxygen concentration. For fires close to the self-extinction, the flames lifted from the burner surface and fluctuated strongly. Self-extinction was not observed with both methanol fires and propane fires of the same heat release rates in the 10 m-long tunnel. The oxygen concentrations in the 10 m-long tunnel decreased temporarily to 17% and rose up to around 20%. In the long tunnel with large fire sizes, the smoke layer descended to ground level at a certain distance away from the fire as the temperature of smoke layer dropped to ambient temperature. The descended smoke layer stopped the supply of fresh air reaching the fire seat. The backflow of air toward the fire was highly vitiated due to the mixing of combustion products with fresh air. The fires self-extinguished when the concentration of oxygen close to fire seat approached the limit of flammability. The self-extinguishment of fires in tunnel provides us new insight on the strategy of firefighting in tunnel fires.

Evaluation of Oxygen Reduction System (ORS) in Large-scale Fire Tests

An oxygen reduction system (ORS) is a fire prevention system that uses a low-oxygen environment to reduce, if not eliminate, the potential for ignition and fire propagation in a protected space. The key parameter for ORS design is the limiting oxygen concentration (LOC), defined as the lowest O2 concentration that can support combustion for a given fuel. The present work examines LOCs of solid fuels at large scale in a configuration representative of a rack-storage and discusses the results in relation to the design concentration for ORS.To simulate ORS applications in engineering practice, a two-tier fuel array of standard commodities in rack storage configuration was set up in an enclosure. A constant N2/Air mixture flow was supplied to the enclosure at a desired oxygen concentration. The oxygen concentration was varied from 9% up to 17%. A premixed flame with a constant heat release rate was used as the ignition source to maintain repeatable test conditions. This premixed flame ignitor represents potential heat sources such as electric arc and hot work that are not sensitive to oxygen level. The tested materials included five standard commodities: Class 3, Cartoned Unexpanded Plastic (CUP), Cartoned Expanded Plastic (CEP), Uncartoned Unexpanded Plastic (UUP) and Uncartoned Expanded Plastic (UEP). This paper reports detailed test results at different oxygen levels only for Class 3. The impact of the test conditions on fire propagation was examined and the results showed that the oxygen concentration was the major parameter to control fire propagation. When successful flame spread is initiated by the igniter, the fire size tends to be larger as the igniter is sustained for a longer time. The results of fire propagation success were obtained for the five standard commodities under different oxygen concentrations with a sustained igniter (hard limits) and without a sustained igniter (soft limits). The LOC was defined as an oxygen concentration at 0.05 probability of flame spread by using statistical analysis. The resulting LOC values measured for different commodities in a two-tier rack storage are:• Cartoned (Class 3, CUP and CEP) – hard limit 11.1%,• Uncartoned (UUP and UEP) – hard limit 13.0%,• Cartoned (Class 3, CUP and CEP) – soft limit 13.8%,• Uncartoned (UUP and UEP) – soft limit 14.7%.These LOCs are generally lower than the oxygen design concentrations recommended by existing standards including VdS 3527 and EN 16750 due to different test conditions. The hard limits resemble fundamental LOC for gases and vapors and do not depend significantly on the ignition duration and array size, while the soft limits vary significantly with the size and configuration of the fuel array and ignition duration. It is concluded that the hard limits are more suitable for ORS design purposes.

Gold nanoparticles enhance cisplatin delivery and potentiate chemotherapy by decompressing colorectal cancer vessels.

BackgroundTumor vessels were persistently compressed by solid stress from tumor interstitial matrix, resulting in limited vessel perfusion and oxygen concentrations. Collagen within matrix participated in transmitting the solid stress to tumor vessels and limiting drug delivery.PurposeThe objective of this study was to identify whether gold nanoparticles (AuNPs) were able to decompress colorectal cancer vessels and enhance vessel perfusion as well as drug delivery in colorectal cancer.MethodsColorectal cancer xenograft mice were treated with AuNPs or normal saline for 14 days. The cancer stromal collagen I level, cancer vessel perfusion, hypoxia of tumor were tested by histological examination. We also test the solid stress in the two groups. Furtherly, the effect and the drug delivery of combined using AuNPs and cisplatin were tested. The effect and the underlying mechanism of AuNPs on SW620 cells were tested by CCK8, flow cytometry, Western-blot and atomic force microscope.ResultsAuNPs were able to decrease the density of colorectal cancer associated fibroblasts (CAFs), to reduce the production of tumor stromal collagen I, and to diminish the expression of profibrotic signals, including CTGF, TGF-β1 as well as VEGF in vivo and vitro via Akt signaling pathway. Consequently, AuNPs could alleviate solid stress in tumors, subsequently leading to enhanced vessel perfusion. Therefore, cisplatin as well as oxygen delivery to tumors were improved by AuNPs, which reduced hypoxia while sensitizing therapy of cisplatin in colorectal cancer model.ConclusionAuNPs were effective agents in enhancing cisplatin delivery and potentiating inhibiting tumor growth by decompressing colorectal cancer vessels.

The effect of ultra-low temperature on the flammability limits of a methane/air/diluent mixtures.

Natural gas represents an attractive fuel for industrialized and developing countries seeking an alternative to petroleum. Due to economic and safety considerations, liquefied natural gas (LNG) at cryogenic conditions is preferred for storage and transportation. The main drawback is the poor understanding of the physical and chemical phenomena that occur at the storage conditions of liquid methane, i.e. at ultra-low temperatures around 110 K and, if released, at temperatures below ambient. In this work, a procedure to evaluate the laminar burning velocity, the flammability limit (FL) and the limiting oxygen concentration (LOC) of methane-air-diluent mixtures based on detailed kinetic mechanism at ultra-low temperatures is proposed. The estimation of the FL was obtained with the limiting burning velocity theory. The effects of inert content (extinguishing) and agent (N2, H2O and CO2) on FL were evaluated and compared with data retrieved from the literature. The agreement between experimental observation and model results from 200 K-300 K incentivizes the adoption of the new procedure for further studies of fuel reactivity and safety parameters. Moreover, the proposed procedure may be suitable for the estimation of the safety parameters of complex fuel mixtures whose composition is closer to the actual values of LNG.

Characterization of thermal decomposition and combustion for commercial flame-retardant rubber floor cloth in TG–FTIR and FPA

The commercial flame-retardant rubber samples were investigated in a TG–FTIR with different heat rates and fire propagation apparatus under different external heat fluxes to study thermal and fire characteristics, respectively. The thermal decomposition, ignition time, heat release rate, limiting oxygen concentration, mass loss rate and CO/CO2 yield qualitatively and quantitatively are analyzed, respectively. It was found that the second main reaction of sample was the main decomposition process. The sample was recognized as thermal thin material. The first peak heat release rate of commercial flame-retardant rubber was lower than the second one due to the internally formed charred layer, and the sample would break into two pieces when the external heat flux was less than 40 kW m−2. The limiting oxygen concentration of sample was gradually reducing with external heat flux increasing. The mass loss rate was effected by radiation heat blockage, thus the increase rate of mass loss rate would reduce under higher external heat flux. CO yield increased first and then decreased with the heat flux increasing. These data and models could be used to evaluate the fire risk of flame-retardant rubber for high-speed train department.

Flammability limit of thin flame retardant materials in microgravity environments

The flammability limits of thin flame retardant materials in an opposed flow under microgravity condition have been discussed by scale analysis. The predicted trends of flammability were verified by parabolic flight experiments. As the samples, meta- and para-aramid fabrics (NOMEX and Kevlar), polyimide film (Kapton) and polycarbonate (PC) were investigated. In order to predict the limiting curve accurately, blow-off tests in high forced flow were conducted and the results were used to obtain proper pre-exponential factor, A, and activation energy, E, for blow-off phenomena. The blow-off tests revealed that the material with high pyrolysis temperature, such as Kevlar and Kapton, had its minimum limiting oxygen concentration (MLOC) at higher opposed velocity region. For such materials, it was predicted that the limiting oxygen concentration in microgravity environments was higher than that in normal gravity, and the prediction was consistent with the result of parabolic flight experiment. For NOMEX and PC, the limiting oxygen concentration increased monotonically with increase of opposed flow velocity. The developed model predicted that such materials could have their MLOC in microgravity environments. Actually, the MLOCs of NOMEX and PC were observed under microgravity condition and they were 2% and 0.5% lower than their limiting oxygen concentrations in normal gravity (LOC1g), respectively. The developed model with blow-off test data could predict the difference between the MLOC and LOC1g quantitatively, and it was found that the model was a good tool to discuss the flammability of the flame retardant materials in microgravity environments.

Concurrent flame spread over externally heated Nomex under mixed convection flow

Fire-resistant materials are used in multiple applications where protection from fire is needed. Their fire-resistant capacity is often tested under specific conditions that might not represent the situation in an actual fire. Particularly relevant for this work is the application for astronaut spacesuit, since a spacecraft environment may be different than earth atmospheres. There, a material is exposed to low velocity flows, microgravity, reduced pressure, and enriched oxygen concentration. Under these conditions, material flammability can be altered. In addition, flammability tests are based primarily on the exposure of the material to an external radiant flux to simulate an adjacent fire, but not a real flame. In this work, an experimental study was performed to investigate the effect of ambient pressure and oxygen concentration on the concurrent/upward flame spread over a fire-resistant fabric (Nomex HT90-40) exposed to two different external heat sources. One is the radiation from infrared lamps, and the other is the flame from a burning polymethyl methacrylate (PMMA) sheet placed below the fabric. The experimental results show that an external heat flux extends the limiting oxygen concentration (24% LOC) of Nomex. This effect is more pronounced when the PMMA flame provides the heat flux (17% LOC). For oxygen concentrations larger than the Nomex LOC, the flame spread rate decreases as the ambient pressure is decreased, indicating that reducing buoyancy reduces the flame spread rate. A simple analysis of concurrent flame spread that incorporates mixed flow heat transfer correlates well with the experimental data. This suggest that flame spread in microgravity can be predicted in terms of a mixed flow velocity that includes the Reynolds and Grashof numbers. The results of this work provide further information about the effect of the type of external heating on material flammability. They may also guide future fire safety design in space exploration.

The competitive reaction mechanism between oxidation and pyrolysis consumption during low‐rank coal combustion at lean‐oxygen conditions: A quantitative calculation based on thermogravimetric analyses

AbstractThe evolution of the oxidation‐pyrolysis competitive mechanism during low‐rank coal combustion at lean‐oxygen conditions determines the development of the coalfield fires and the burning loss rate of the coal resource. In the present work, the mass increments of oxygen‐chemisorption were measured and calculated firstly. An abstract shrinking core model was built for distinguishing the oxidation and pyrolysis pathways. On this basis, the consumption rates of oxidation and pyrolysis were respectively calculated with the TGA data obtained under the lean‐oxygen gradient of 21–0 %. The results demonstrate that the oxidation path gradually delays in temperature while the pyrolysis exhibits great temperature stability as the oxygen concentration decreases. Besides, the significant transition of the competitive consumption mechanism exists below 9 % for the oxidation phase before ignition and 5 % for the combustion phase after ignition, respectively. Two temperature indicators of TC1 and TC2 for guiding the optimal fire extinguishing schemes based on the actual oxygen concentrations and temperatures are proposed. Furthermore, 1 % is identified as the limiting oxygen concentration for the development of the coalfield fires where the ignition mechanisms of both the HST and WD coal transform into the homogeneous ignition. The WD lignite, which always maintains the coke state as the burnout mechanism, holds higher reactivity but wider dispersion degrees of combustion compared with the HST sub‐bituminous coal.

Potential explosion hazard of polyester resin dust formed from a granulation process: Limiting oxygen concentration with different pressures

When polyester resin (PR) is manufactured on an industrial scale, the spreading of the dust cloud is a potential explosion hazard. The maximum explosion pressure and maximum rate of pressure rise were in the range 3.6 bar < Pmax(500) < 9.8 bar and 32 < (dP dt–1)max(500) < 356 bar s–1, respectively, which is more than that of sugar dust explosion. The explosion index Kst = V1/3(dP dt–1)max was in the range 8.86 bar m s–1 <Kst(500) < 96.63 bar m s–1, which belongs to dust explosion class St-1. In addition, the results indicated that the explosion pressure can be significantly lessened with lower LOC. However, the inhibiting effect is not obvious at higher initial pressure. Therefore, the oxygen concentration must be considered as low as feasibly possible at a higher pressure to lessen the explosion hazards in production as well as handling process. Moreover, according to rough set analysis, the most significant factors for explosion intensity (Pmax) are dust concentration and initial pressure. The two parameters should be prudently controlled during the storage, transportation, and manufacturing of PR dust.

Prediction of Limiting Oxygen Concentration of Thin Materials in Microgravity

Prediction of Limiting Oxygen Concentration of Thin Materials in Microgravity

Limiting Oxygen Concentration Trend of ETFE-Insulated Wires under Microgravity

Limiting Oxygen Concentration Trend of ETFE-Insulated Wires under Microgravity

The impact of operational strategies on the performance of a photo-EBPR system

A novel Phototrophic - Enhanced Biological Phosphorus Removal (Photo-EBPR) system, consisting of a consortium of photosynthetic organisms and polyphosphate accumulating organisms (PAOs), was studied in this work. A sequencing batch reactor was fed with a mixture of acetate and propionate (75%–25%) and subjected to dark/light cycles in order to select a photo-EBPR system containing PAOs and photosynthetic organisms, the latter likely providers of oxygen to the system. The results from the selection period (stage 1) showed that the photo-EBPR culture was capable of performing P release in the dark and P uptake in the presence of light, under limited oxygen concentrations. During the optimization period, the aeration period, which was initially provided at the end of the light phase, was gradually reduced until a non-aerated system was achieved, while the light intensity was increased. After optimization of the operational conditions, the selected consortium of photosynthetic organisms/PAOs showed high capacity of P removal in the light phase in the absence of air or other electron acceptor. A net P removal of 34 ± 3 mg-P/L was achieved, with a volumetric P removal rate of 15 ± 2 mg-P/L.h, and 79 ± 8% of P removal from the system. Also, in limiting oxygen conditions, the P uptake rate was independent of the PHA consumption, which demonstrates that the organisms obtained energy for P removal from light. These results indicated that a photo-EBPR system can be a potential solution for P removal with low COD/P ratios and in the absence of air, prospecting the use of natural sunlight as illumination, which would reduce the costs of EBPR operation regarding aeration.

Experimental investigations on ignition sensitivity of hybrid mixtures of oil shale dust and syngas

Hybrid mixtures of a combustible dust and flammable gas are usually encountered in many process industries. Ignition sensitivity parameters of such mixtures are critical to assess the possibility for accident to happen in industry. The work of this paper investigates the ignition sensitivity of the main four kinds of oil shale dusts of China and related dust/syngas hybrid mixtures. The experiments were carried out on the standardized 20-L spherical vessel, Hartmann apparatus and 1-m3 explosion vessel. Comparing the results with that of bituminous coal, it was found that the ignition sensitivities of oil shale dusts are generally weaker than that of selected bituminous coal. Results show that the presence of syngas would significantly increase the ignition sensitivity of the hybrid mixtures. With the increasing of syngas contents (below its lower explosion limit) in the mixtures, the minimum ignition energy (MIE) and the minimum explosion concentration (MEC) decrease sharply. For example, the MIE of oil shale from Fushun and Huadian was observed to decrease from 16J, 10J to 1.25mJ and 0.98mJ, and the MEC dropped from 280g/m3, 250g/m3 to 50g/m3, 50g/m3. When the syngas content is increased to 3% in volume, the influence of oil shale volatile content on MIE and MEC can be ignored for the four shale samples. The limiting oxygen concentration (LOC) has been investigated systemically in a 1-m3 explosion vessel. It has been found that the LOC of oil shale dust from Longkou is 17% in volume, while the hybrid of 2% syngas and 350g/m3 of dust revealed the lowest LOC of 16% when nitrogen was used as the inert.

Igniter-induced hybrids in the 20-l sphere

Dust explosibility is traditionally described by two parameters, namely the maximum explosion pressure, Pmax, and the deflagration index, KSt, usually determined through testing in a closed, pressure-resistant spherical vessel, either 20 L or 1 m3 in volume. These parameters constitute key variables in the design of explosion protection systems, such as venting, suppression or isolation systems.The potential for overdriving dust combustion with pyrotechnical igniters in the 20-l sphere has been recognized, discussed and analyzed for many years, notably in the determination of the minimum explosible and limiting oxygen concentrations, which has led to specific guidelines regarding the ignition source strength in ASTM standards.The current paper presents new experimental evidence that the energy provided by pyrotechnical igniters may, in some instances, physically alter the dust being tested in the 20-l sphere. KSt values can be several times greater in the small vessel compared to those measured in the 1-m3 chamber. Further visual evidence is provided to show that high energy ignition can produce a turbulent flame region, possibly consisting of a hybrid mixture of flammable gas (or vapor) and dust, which can propagate faster than the corresponding pure dust. The experiments suggest that KSt values measured in the 20-l sphere may no longer be representative of a dust deflagration in a real process environment. We recommend additional tests in a 1-m3 chamber when a dust exhibits a low flash point, or when it's KSt is above 300 bar m/s in the 20-l sphere.

Flammability of gases in focus of European and US standards

The presentation will discuss the difference between EU and US standards for the determination of explosion (flammability) limits and limiting oxygen concentration. Small differences observed in measured values can be traced back to the different test apparatuses and criteria. The discrepancies can be much greater in the case of limiting oxygen concentration because of the high amount of inert gases and the corresponding low laminar burning velocities. The paper describes some examples and the influence of the chosen criteria on the results. The European and US standards use the criteria of flame propagation in open test vessels and of pressure rise in closed ones. The examples discussed show that flame propagation is still possible at very small pressure rise values, as observed much below the pressure rise criterion of usual standards. However, flame propagation in a process plant can cause an accident or explosion and must be avoided. Therefore, the flame propagation criterion is recommended to be used in chemical safety engineering. The European safety database CHEMSAFE contains expert-evaluated safety data for cases where the determination method and criteria are known. Flammability characteristics based on the pressure rise criterion may suffice in certain cases, e.g. for explosion protection in closed vessels without any connecting pipes.

Flame spread limits (LOC) of fire resistant fabrics

Selecting fabrics based on their fire resistance is important for professions with substantial fire risk such as firefighters, race car drivers, and astronauts suits. Generally, fire resistant materials are tested under standard atmospheric conditions. However, their flammability properties can change when the ambient conditions deviate from standard atmospheric conditions. Particularly in high altitude locations, aircraft, and spacecraft, the pressure and oxygen concentrations are different than in a standard atmosphere. Also, the presence of external radiation (i.e. overheating component or nearby fire) can reduced the fire resistance of a material. In this work, an experimental study was conducted to analyze the influence of environmental variables such as oxygen concentration, ambient pressure, and external radiant heat flux on the flame spread limits of two different fire resistant fabrics: Nomex HT90-40 and a blend made of Cotton/Nylon/Nomex. Ambient pressure was varied between 40 and 100kPa and ambient oxygen concentrations were decreased until the Limiting Oxygen Concentration (LOC), limiting conditions which would permit flame propagation, were found. Experiments were conducted using no external radiant flux or a radiant flux of 5kW/m2 to examine the influence of the presence of a nearby heat source. Among the results, it was found that as ambient pressure is reduced the oxygen concentration required for the flame to propagate must be increased. The external radiant heat flux acts as an additional source of heat and allows propagation of the flame at lower oxygen concentrations. An analysis of the propagation limits in terms of the partial pressure of oxygen suggest that the LOC of a material is not only determined by heat transfer mechanisms but also by chemical kinetic mechanisms. The information provided in this work helps characterize increased flammability risk of materials when in environments different from the standard atmospheric conditions at which they are typically tested.

Formate simultaneously reduces oxidase activity and enhances respiration in Campylobacter jejuni

The foodborne microaerophilic pathogen, Campylobacter jejuni, possesses a periplasmic formate dehydrogenase and two terminal oxidases, which serve to metabolize formate and facilitate the use of oxygen as a terminal electron acceptor, respectively. Formate, a primary energy source for C. jejuni, inhibits oxidase activity in other bacteria. Here, we hypothesized that formate might affect both energy metabolism and microaerobic survival in C. jejuni. Subsequently, we showed that C. jejuni 81–176 (wildtype) exhibited enhanced chemoattraction to and respiration of formate in comparison to other organic acids. Formate also significantly increased C. jejuni’s growth, motility, and biofilm formation under microaerobic (5% O2) conditions. However, formate reduced oxidase activity under microaerobic conditions as well as aerotolerance and biofilm formation under ambient oxygen conditions. The expression of genes encoding the ribonucleotide reductase (RNR) and proteins that facilitate the use of alternative electron acceptors generally increased in the presence of formate. Taken together, formate might play a role in optimizing C. jejuni’s adaptation to the oxygen-limited gastrointestinal tract of the host. By affecting oxidase activity, formate possibly facilitates shuttling electrons to alternative acceptors, while likely conserving limited oxygen concentrations for other essential functions such as DNA synthesis via RNR which is required for C. jejuni’s growth.