Flammability Limits Of Mixtures Research Articles

Research on flammability of R290/R134a, R600a/R134a and R600a/R290 refrigerant mixtures

As a natural substance, HCs (hydrocarbons) refrigerant have no destructive effect on ozone and has little impact on greenhouse effect. It is an ideal new refrigerant. R290 and R600a have been used to a certain extent, but hydrocarbon refrigerant has a high risk of flammability, which hinders its engineering application. This paper focuses on using hydrocarbon refrigerants as alternative refrigerants, taking R290 and R600a as the main research objects, adding refrigerant R134a to hydrocarbon refrigerants to study the flammability change of mixtures. The flammability limits of R290/R134a, R600a/R134a and R600a/R290 mixture refrigerants were measured under different components. In comparison, R134a had more obvious flammability inhibition effect on R290 than R600a. The influencing factors of the flammability limits of HCs and the estimation method of the flammability limits of mixtures with the average MAPEs ranged from 0.62% to 2.94% was discussed. In addition, the refrigeration cycle of HCs/R134a mixtures with various concentrations was simulated. The COP, volume refrigeration capacity per unit and condensation pressure of the mixtures had local maximum near the azeotropic point, respectively.

Experimental and theoretical study of flammability limits of hydrocarbon–CO2 mixture

The hydrocarbon–CO2 mixture is identified as the most advantageous working medium in the medium-high temperature Organic Rankine cycle (ORC). However, the flammable zones of hydrocarbon–CO2 mixtures at atmospheric condition are rare. In order to measure the flammability limits of mixture, an experimental setup based on the ASTM E681-09 has been built completely. In the design of the experiment, an innovative tactic about gas distribution is proposed to improve the precision. The flammable zones of five mixtures were measured carefully at 30 °C in this paper. But in reality, ORC system operates within the temperature range from 87 to 300 °C. Meanwhile the flammability zones of mixtures at higher temperature are not available in the existing literatures. To solve this problem, a modified critical flame temperature model is modified to estimate the flammable zones of mixtures at different temperatures. The modified model presents a higher prediction precision with average relative difference of 1.24% at LFL. Thus, it is easier to estimate the flammable zones of mixtures at different temperatures with the modified model.

A new model based on adiabatic flame temperature for evaluation of the upper flammable limit of alkane-air-CO2 mixtures

For security issue of alkane used in Organic Rankine Cycle, a new model to evaluate the upper flammability limits for mixtures of alkanes, carbon dioxide and air has been proposed in present study. The linear relationship was found at upper flammability limits between molar fraction of diluent in alkane-CO2 mixture and calculated adiabatic flame temperature. The prediction ability of the variable calculated adiabatic flame temperature model that incorporated the linear relationship above is greatly better than the models that adopted the fixed calculated adiabatic flame temperature at upper flammability limit. The average relative differences between results predicted by the new model and observed values are less than 3.51% for upper flammability limit evaluation. In order to enhance persuasion of the new model, the observed values of n-butane-CO2 and isopentane-CO2 mixtures measured in this study were used to confirm the validity of the new model. The predicted results indicated that the new model possesses the capacity of practical application and can adequately provide safe non-flammable ranges for alkanes diluted with carbon dioxide.

Critical concentration of helium making hydrogen–oxygen with various fractions nonflammable

Abstract The objective of this study is to determine the critical concentrations of helium making hydrogen–oxygen nonflammable under various volume fraction ratios of hydrogen and oxygen. Tests were conducted for three volume fraction ratios of hydrogen and oxygen 2:1, 1:1 and 1:2, at the initial pressures of 0.1, 0.2, 0.3 and 0.4 MPa and the initial temperatures of 21, 40, 60, 75 and 90 °C. The flammability limits of mixtures with the lower volume fraction ratios between hydrogen and oxygen are much different from the lower flammability limits of hydrogen in oxygen or air. Compared with the lower flammability limits of hydrogen in oxygen or air, the critical concentrations of helium determined in this study are significant for process safety of the mixtures with various volume fraction ratios between hydrogen and oxygen. For the mixtures at 21 °C, the critical concentrations of helium making hydrogen–oxygen nonflammable increase with the volume fraction ratios of hydrogen and oxygen in the range from 0.5 to 2. The variation of the critical concentrations of helium with the initial pressure depends on the initial temperature. For the mixtures at the initial temperature of 60 °C and the initial pressure of 0.3 MPa, the critical concentration of helium reaches to its limit of 95% in the case of volume fraction ratio of hydrogen and oxygen 2:1. The limit values of the critical concentration of helium are 93% for 1:1 volume fraction ratio of hydrogen and oxygen and 89% for 1:2 volume fraction ratio of hydrogen and oxygen ratio, respectively.

Flammability limits of binary mixtures of ammonia with HFO-1234yf, HFO-1234ze, HFC-134a, and HFC-125

Flammability limits of binary mixtures of ammonia with HFO-1234yf, HFO-1234ze, HFC-134a, and HFC-125 were measured in dry air. The measurements were made in a 12L spherical glass flask using ASHRAE method. It has been found that the flammability limits of ammonia and HFO-1234yf mixtures are extremely different from the values predicted by Le Chatelier's equation. Modification of Le Chatelier's equation was made by introducing Ellipse function to analytically express the flammability limits of the mixtures of arbitrary concentrations. HFO-1234ze is non-flammable in dry air, but becomes flammable if ammonia is added more than four percent. If the content of ammonia exceeds 20 percent, the values of flammability limits become very close to the corresponding ones for HFO-1234yf and ammonia. On the other hand, the flammable regions of ammonia mixed with each of HFC-134a and HFC-125 are much narrower than the mixtures of HFO-1234ze. Extension of the ellipse modified Le Chatelier's equation introduced in the above was made to successfully explain the flammability limits of these mixtures.

Correlations for estimating flammability limits of pure fuels and fuel-inert mixtures

Flammability limits are one of the most important characteristics to evaluate fire hazards associated with fuels. Currently, numerous correlations, based on either chemistry (stoichiometric oxygen number) or energy (heat of combustion), have been proposed to estimate flammability limits. In this study, a thermal model is proposed to incorporate the dual role (quenching and heating) of each component in a mixture. The energy term plays a vital role in Excess Heating Potential (EHP) and Quenching/Heating Ratio (QHR) and hence an association between flammability limits and energy terms can be established. In addition, oxygen calorimetry bridges chemistry formula with energy release of a certain fuel, thus all existing correlations are unified under the principle of energy conservation. Since this methodology is universal, it can be applied to estimate the flammability limits of any mixtures or guide future correlations with improved precision. Advantages and limitations for this methodology are also discussed.

An Experimental Investigation of Flammability Limits and Autoignition Temperatures of Petrofuels and Biofuels in a Tubular Burner

A set of experiments to determine the flammability limits of pure and binary blends of fuels was carried out in a combustion tube operated at atmospheric pressure. The lowest and highest volumetric fraction of vapor fuels that allowed flame propagation when the air/fuel mixture was submitted to a high voltage spark with energy higher than the minimum ignition energy defined the lower and upper flammability limit, respectively. The obtained experimental results for liquefied petroleum gas (LPG), acetone, ethylic alcohol, gasoline, diesel, and bioethanol are in close agreement with results reported in the literature. Diagrams of flammability limits of mixtures of gasoline/bioethanol and diesel/biodiesel were found within the range of composition of standard commercial blends of these fuels available for market in Brazil. The autoignition temperature and the influence of temperature on flammability limits of pure ethanol and a commercial mixture with 4% of biodiesel and 96% of diesel were experimentally determined.

Experimental and numerical studies of the lower flammability limit of mixtures of C1–C5 hydrocarbons with air

The lower concentration limit of flammability of hydrocarbon-air mixtures has been studied experimentally and by numerical simulation. Simulation using a detailed mechanism of chemical reactions has shown that calculations results are in good agreement with experimental data on the effect of water vapor on the lean concentration limit of flammability of hydrocarbon mixtures with air. The presence of water vapor at low concentrations in the mixture does not affect the lower concentration limit of flammability, but, at the same time, significantly changes the flame propagation velocity. Key words: concentration limits of flammability, opposed-jet premixed flame, hydrocarbons.

Berechnung von Explosionsgrenzen ethylenoxidhaltiger Gasphasen in Sterilisatoren / Calculation of flammability limits of gas phases with ethylene oxide in sterilisers

A calculation method for flammability limits of gas phases with ethylene oxide in sterilisers was developed. Using the Software GasEq and the newly developed Makro "SterEx" for MS-Excel, flammability limits of mixtures with ethylene oxide, air and inert gases at temperatures between 20 degrees C and 100 degrees C and pressures between 0.4 bar and 1.0 bar can be calculated. This method can be used to easily determine safe operating conditions. The used semi-empirical model is based upon the assumption of constant flame temperature profiles at the flammability limits subject to the EO-concentration for different mixtures. To collect model parameters and to validate the model, several experiments with mixtures of ethylene oxide, nitrogen, carbon dioxide, water vapour and air were carried out to determine flammability limits. To simulate the structural conditions of sterilisers, the experiments were conducted in accordance to DIN EN 1839-B in a closed autoclave with temperatures and pressures relevant for sterilisation processes. The calculation of flammability limits of process gas mixtures with "SterEx" provides good agreement with flammability limits that were determined in experiments.

A study on flammability limits of fuel mixtures

Flammability limit measurements were made for various binary and ternary mixtures prepared from nine different compounds. The compounds treated are methane, propane, ethylene, propylene, methyl ether, methyl formate, 1,1-difluoroethane, ammonia, and carbon monoxide. The observed values of lower flammability limits of mixtures were found to be in good agreement to the calculated values by Le Chatelier's formula. As for the upper limits, however, some are close to the calculated values but some are not. It has been found that the deviations of the observed values of upper flammability limits from the calculated ones are mostly to lower concentrations. Modification of Le Chatelier's formula was made to better fit to the observed values of upper flammability limits. This procedure reduced the average difference between the observed and calculated values of upper flammability limits to one-third of the initial value.

Flammability limits of isobutane and its mixtures with various gases

Flammability limits of isobutane and five kinds of binary mixtures of isobutane were measured by the ASHRAE method. Propane, nitrogen, carbon dioxide, chloroform, and HFC-125 (1,1,1,2,2-pentafluoroethane) were used as the counter part gases in the mixtures. The observed data were analyzed using the equations based on Le Chatelier's formula. The flammability limits of mixtures with propane were well explained by the original Le Chatelier's formula. The flammability limits of mixtures with nitrogen and the ones with carbon dioxide were adequately analyzed by the extended Le Chatelier's formula. It was found that the extended Le Chatelier's formula is also applicable to the flammability limits of mixtures with chloroform and HFC-125.

The role of pulsating instability and global lewis number on the flammability limit of lean heptane/air flames

The unsteady propagation of lean heptane/air planar flames in the doubly infinite domain was computationally simulated with detailed chemistry and transport and with and without radiative heat loss. For the adiabatic situation, thermal-diffusive pulsating instability, promoted for large values of the Lewis and Zeldovich numbers, was observed to develop when the mixture became sufficiently lean. With subsequent progressive reduction in the equivalence ratio ( φ ), the mode of pulsation changes from that of monochromatic, to period doubling, and to hibernation characterized by bursts of high burning intensity separated by long periods of dormancy. The flame nevertheless does not extinguish. This behavior is similar to that of the rich hydrogen/air flame studied previously. However, when heat loss is considered, the onset of pulsation is facilitated, although the regime in φ for sustained pulsating propagation is extremely narrow, and extinction occurs readily when the amplitude of the flame temperature oscillation becomes large enough to extinguish the flame during the negative phase of the excursion. The abruptness of transition to extinction is quite different from that of the rich hydrogen/air flame. The pulsating extinction occurs at a large φ (=0.5063) than steady extinction ( φ =0.4762), again implying that the flame extinguishes in the pulsating instead of the steadily propagating mode, and that the flammability limit is accordingly narrowed. It is further demonstrated that the states for the onset of adiabatic and non-adiabatic pulsation can be accurately estimated by using the criteria of Sivashinsky and of Joulin and Clavin, respectively, provided the Lewis number used is the global one, extracted from the response of stretched flames, demonstrating that it is a property of the flame instead of the unburned mixture alone. It is also suggested that the lean flammability limits of mixtures of large hydrocarbons and air can be estimated by using the criterion of Joulin and Clavin.

Effect of Chemistry and Transport Properties on Near-Limit Flames at Microgravity

Abstract The role of chemical kinetics in the behavior of near-limit flames at rnicrogravity (μg) was investigated experimentally by comparison of the properties of these flames in mixtures having varying chemistry but similar diffusive transport properties. Conversely, the importance of unequal rates of diffusion of thermal energy and mass was evaluated by comparison of flames in mixtures having similar chemistry but varying Lewis numbers, It was found that the characteristics of near-limit flames at μg were affected much more by Lewis number effects than chemistry. It is concluded that there are at least three regimes of near-limit behavior at μg, depending upon the Lewis number (Le): (1) cellular flames, which are highly resistant to extinguishment, for mixtures with Le≪l, (2) self-extinguishing flames and well-defined flammability limits for mixtures with Le≃ 1, and (3) ignition energy limited flame propagation for Le ≪ 1, Gradual rather than abrupt transitions between regimes were observed.

Gaseous detonations: their nature, effects and control

1 Introduction.- 1.1 General remarks.- 1.2 Definitions of commonly-used terms.- 1.3 Unidimensional models of detonations.- 1.4 Structure of a detonation wave.- 1.5 Philosophy of presentation.- 2 Unidimensional models.- 2.1 Introductory remarks.- 2.2 Properties of unidimensional shock waves.- 2.3 Properties of unidimensional shock waves with energy addition.- 2.4 Properties of the Chapman-Jouguet state.- 2.5 Comparison of C-J predictions and experiment.- 2.6 The Zel'dovich, von Neumann, Doring model.- 2.7 Comparison of the ZND model and experiment.- 2.8 The Taylor expansion wave.- 2.9 Concluding remarks.- 3 Structure of detonation fronts.- 3.1 General remarks.- 3.2 'Spinning' detonation fronts.- 3.3 'Galloping' fronts.- 3.4 Experimental studies of multi-headed fronts.- 3.5 Theoretical treatments of multi-headed fronts.- 3.6 Concluding remarks.- 4 Detonable media.- 4.1 General remarks.- 4.2 Confined and unconfined detonations.- 4.3 Gases and vapours which are detonable in the absence of an oxidant.- 4.4 Comparison of detonation limits for confined and unconfined detonations with flammability limits for mixtures of hydrocarbons with oxygen and air.- 4.5 Homology hypothesis for predicting detonation limits.- 4.6 Detonations with oxidants other than oxygen.- 4.7 Influence of initial pressure and temperature on detonability.- 4.8 Influence of additives on detonability.- 4.9 Detonations in suspensions of dusts and droplet mists in oxidizing atmospheres.- 5 Initiation of a detonation wave.- 5.1 General remarks.- 5.2 Initiation of confined detonations by shock waves.- 5.3 Initiation by blast waves from electrical and laser sparks and charges of conventional explosives.- 5.4 Detonations in large unconfined clouds of vapour.- 5.5 Minimum ignition energies.- 5.6 Laminar burning velocities.- 5.7 Expansion ratios.- 5.8 Detonations arising from accelerating flames.- 5.9 Influence of initial temperature and pressure of the medium on run-up distances.- 5.10 Influence of diameter of pipeline on run-up distances.- 5.11 Effect of additives on pre-detonation distances.- 5.12 Effects of surface roughness and obstacles on the acceleration of confined flames.- 5.13 Pressure piling (cascading).- 5.14 Concluding remarks.- 6 Interaction of a detonation with confinement.- 6.1 Introductory remarks.- 6.2 Diffraction at an isolated wall, ?w < 0 .- 6.3 Diffraction at an isolated wall, 0 < ?w < ?crit.- 6.4 Diffraction at an isolated wall, ?crit < ?w < 90 standard two- and three-shock theory for non-reactive media and the effects of reaction.- 6.5 Normal reflection of a detonation wave.- 6.6 Transmission of a planar detonation wave through an abrupt expansion in area.- 6.7 Propagation of detonations through bends and junctions.- 6.8 Interaction of a detonation with an inert surrounding gas.- 6.9 Refraction of detonations in mixtures of different composition.- 6.10 Concluding remarks.- 7 Damage caused by detonations.- 7.1 Introduction.- 7.2 Early experiments on effective pressures generated by detonations.- 7.3 Damage produced by detonations in chemical plant.- 7.4 Experimental studies of failure resulting from detonations.- 7.5 Concluding remarks.- 8 Prevention and mitigation of detonations.- 8.1 Introductory remarks.- 8.2 Inhibition of flames of normal burning velocity.- 8.3 Venting in the early stages of an explosion.- 8.4 Quenching of flame-shock complexes.- 8.5 Suppression of detonations.- 8.6 Mitigating the effects of detonations.- 8.7 Concluding remarks.- 9 Concluding recommendations.- 9.1 Introductory remarks.- 9.2 Stress waves in confining walls.- 9.3 Planned deformations as safety measures.- 9.4 Designing to minimize the effects of local peaks in pressure.- 9.5 Suggestions for further studies of detonations.- References.- Author index.

Experimental studies of lower flammability limits of gases and mixtures of gases at elevated temperatures

Experimental studies of lower flammability limits for methane, butane, hydrogen, carbon monoxide, in addition to mixtures of these gases are performed at temperatures up to 450°C and at atmospheric pressure. The experimental tests are mostly carried out for upward propagation. The flammability limits for each gas are found to decrease linearly with increasing temperature in the temperature range tested. A new mixing rule for calculating lower flammability limits for mixtures at elevated temperatures in suggested. The mixing rule is a simplification of that of Le Chatelier's and is based on the established equations for each gas component in the mixture.

Some considerations of the lean flammability limits of mixtures involving hydrogen

Consistent data regarding the flammability limits of homogeneous mixtures of hydrogen gas in air at atmospheric pressure are presented for various temperatures extending down to −130 °C. The retardation of the combustion of lean homogeneous mixtures of hydrogen and air due to the homogeneous mixing of inerts such as CO 2, N 2 and He is also considered. Moreover, the enhancement of the flammability limits of fuel mixtures involving CO, CH 4, C 3H 8 and C 2H 4 in air due to the presence of some hydrogen is also presented. Guidelines are then suggested for predicting the role of the presence of hydrogen in the fuel-air mixtures over a wide range of concentrations and temperatures.