An experimental study of flammability limits of LPG/air mixtures

The liquefied petroleum gas (LPG) is generally considered to be eco-friendly viable fuel not only in domestic sector but also for transport sector. The inhibition of LPG-air premixed flames is a very important practical problem that has received relatively little attention. This paper is concerned with experimental determination of the flammability limits of LPG-air mixture. The standard procedure suggested by US Bureau of mines has been adopted for the present studies for determining the flammability limit of LPG-air mixture. The lower flammability limit (LFL) is found to be 1.81% and upper flammability limit (UFL) is 8.86% of LPG for upward propagation of flame. Whereas, for downward propagation of flame, the LFL and UFL are 1.87 and 7.69% of LPG, respectively. The nitrogen dilution effects on the flammability limits have been explored, which is presented on a flammability limit plot. It is believed that these data will be very useful for developing fire extinguishers and other combustion devices.     

Upper flammability limits of some organosilicon compounds

This paper reports the upper flammability limits of 26 organosilicon compounds commonly used in the silicone industry and investigates the correlation between the upper flammability limit and the net heat of combustion. An empirical equation has been developed to predict the upper flammability limit of organosilicon compounds using the net heat of combustion. Copyright © 2004 John Wiley & Sons, Ltd.     

Specific Features of the Mechanism of Flame Propagation in Rich Hydrogen–Propane–Air Mixtures

Propagation of a planar laminar flame in rich homogeneous propane–air and hydrogen–propane–air mixtures is numerically studied. It is shown that the maximum burning temperature is higher than the thermodynamically equilibrium value and is reached when chemical and physical processes have not been yet completed. The degree of superadiabaticity depends on the ratio of hydrogen and propane concentrations. Superadiabaticity in rich hydrogen–propane–air mixtures, in addition to the inhibiting action of propane, determines the deviation from Le Chatelier's principle for flammability limits in these mixtures.     

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.     

Flame retardation of poly(ethylene terephthalate) containing poly(4-bromo styrene), poly(vinyl bromide), and poly(vinylidene bromide)

The pyrolysis and gaseous combustion of poly(ethylene terephthalate) (PET) incorporating poly(4-bromostyrene), poly(vinyl bromide), and poly(vinylidene bromide) has been studied using thermogravimetry, flammability limit evaluation, and hydrogen bromide (HBr) evolution techniques. The data obtained have been compared with limiting oxygen index (LOI) flammability data to elucidate flame retardation mechanisms. All the organo bromides studied (applied either via topical treatment or radiation grafting) released HBr on pyrolysis which is capable of inhibiting the gas phase combustion reactions. Condensed phase interactions were also detected which were capable of altering the gaseous pyrolysates. Thermal stability considerations suggest that, although the aliphatic bromides are excellent sources of HBr, they are not ideal flame retardants for PET.     

THE EFFECT OF ANTIMISTING ADDITIVES ON FLAMMABIL1TY OF JET FUELS

High molecular weight (MW) polyisobutylenes were effective in suppressing flammability of Jet-A fuel sprays when present at concentrations of 100 ppm or less in the fuel. Six polyisobutylene solutes, with viscosity-average MW ranging from 0.7 to 12 million, were tested in wind-shear created fuel sprays against both spark and flame ignition sources. Air velocities up to 140 m/s and fuel/air mass ratios up to 10 were used to examine effects of spray conditions on solute concentrations necessary to prevent ignition. The effectiveness of the additives increased directly with their MW for both spark and flame ignition, with the higher-MW polymers providing flammability suppression at concentrations that caused less than 20 percent increases in fuel viscosity.     

Characteristic modes and flow structure of non-premixed flame in humid-air combustion

An experimental study has been performed in order to determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame. Two-dimensional Particle Image Velocimetry (PIV) measurements were made to quantify the velocity field, with and without steam injected. The results indicate the addition of steam decreases the recirculation flow and reduces the distance between the forward and aft stagnation points. The detailed stabilization regimes show that the critical fuel-to-air velocity ratios of the central fuel penetration in the humid air case are 16% to 22% lower, and the partially quenching limits are at least 25% lower. The decreased penetration limit is due to a reduction in momentum of the humid air. An analysis of flamelet concepts reveals that increased chemical reaction time leads to lower partially quenching limits in the humid air combustion.     

Laminar burning velocities and Markstein numbers of syngas-air mixtures

In the currently reported work, three typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas-air flames. The maximum value of syngas-air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas-air flames are generally unstable. Karlovitz numbers indicates that typical syngas-air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas-air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H₂/95% CO and proved to be similar to methane, although somewhat slower than propane.     

Experimental and numerical investigation of vortex‐induced flame propagation in a biomass furnace with tangential over fire registers

The tangentially fired furnaces have evolved because of rapid contacting of the fuel and air flame impingement, and the increased particulate residence time due to vortex motion. Tangentially fired units have a good record in being able to meet emission regulation on NOx as a result of their flexibility and the ability to control the heat release rate. Yet, the flow inside the tangentially fired furnaces is known to have its own peculiar aerodynamics; it is quite complicated in such a way that it is not easy to reach a satisfactory model to describe it. The drawbacks with the traditional tangentially fired furnaces are burner velocities. Low velocities are not suitable for fuels having high volatile contents, as ignition occurs in or near the burner causing slugging and distortion problems. Very high velocities on the other hand are undesirable as fuel particles can centrifuge out of the main combustion zone as unburnt carbon. The test boiler used in this work has tangential over fire registers located in the side walls, which are directed to form an imaginary circle at the centre to aid the suspension burning. The vortex formed by these jets, is then induced by the under grate air in reaching the higher levels in the furnace. In the test furnace, the fuel is not coming along with the tangential over fire air, but enters the furnace through the bagasse spreaders and carried by the distributor air. The combustion of bagasse and the propagation of the flame within the furnace are influenced by the tangential over fire air and the under grate air. In the present work, the furnace is simulated and analyzed for the propagation of flame and the patterns at various heights of the furnace supported by the measurements.     

Dust/gas mixtures explosion regimes

The explosion features of nicotinic acid dust in atmosphere of methane and air at different concentrations of either dust or gaseous fuel are studied. Experimental measurements of the pressure history, deflagration index and flammability limits are performed by the standard 20 l Siwek bomb though adapted for such hybrid mixtures.Data show non linear effect of explosion severity and the synergistic effects when hybrid mixtures explode. Results allow the definition of five different regimes of the gas/dust/air mixture explosion in the plane dust concentration vs. fuel concentration.     

Determining flammability limits by analyzing diffusive-thermal flame instability. methane–air–diluent mixture

This paper describes the method for determining flammability limits with the use of equations for diffusive-thermal stability boundaries. The key parameter responsible for the existence of flammability limits is the thermal effect produced by the combustion of gas mixtures. The thermal effect and the equation for diffusive-thermal stability boundaries are used to determine a minimum flame temperature below which combustion is impossible. Flammability limits are significantly affected by the heat capacity of components of the mixture if it is strongly dependent on temperature. For upper and lower flammability limits, a minimum flame temperature is generally different and dependent on the relative concentration and properties of the diluent. The theoretical methods for calculating flammability limits are tested according to the experimental data on the combustion of a methane–air–diluent mixture.     

On the Flammability Limit

The upper flammability limit of ethane–air and ethane–oxygen mixtures was determined experimentally for various initial temperatures and pressures. In the experiments, the upper flammability limit increased with increasing initial temperature and pressure, which is consistent with literature data. The limit was determined in a closed vessel with central ignition. The obtained limit corresponds to the quenching of the flame propagating downward after its rise under the action of the Archimedes force.     

Effect of trimethylphosphate additives on the flammability concentration limits of premixed methane-air mixtures

The effect of small additives of trimethylphosphate (TMP) on the lean and rich flammability concentration limits of CH₄/air gas mixtures were studied using an opposed-flow burner and numerical modeling based on detailed kinetic mechanisms. TMP was found to narrow the flammability concentration limits of premixed CH₄/air mixtures. Modeling using a previously developed model for flame inhibition by phosphorus compounds showed that the model provides a satisfactory fit to experimental results on the effect of TMP additives on the lean concentration limit. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 12–21, January–February, 2008.     

Flammability limits of the polymer pyrolysates generated from flame retarded poly (ethylene terephthalate)

A device constructed and used to determine the flammability limits of pyrolysate–air mixtures as a function of percentage weight loss on pyrolysis has been employed to investigate the pyrolysate gases generated from poly (ethylene terephthalate) both in the presence and absence of chemical flame retardants. The chemicals tripropyl phosphate (TPP), dibromopropanol (DBP) and tris (2, 3 dibromopropyl) phosphate (TRIS) all influenced both the lower and upper flammability limits, with the largest effects being obtained when both phosphorus and bromine were present, followed by bromine only and then phosphorus only. The results indicate TPP has negligible condensed phase activity with only small gas phase action. DBP has no condensed phase activity but is a very active gas phase inhibitor. In contrast, TRIS has a detrimental effect upon the condensed phase reactions in that it is responsible for the formation of a more flammable pyrolysate gas mixture. Fortunately, its gas phase inhibition reaction is capable of reducing the overall flammability.     

Percolation Phase Transition in Combustion of Heterogeneous Mixtures

Combustion of heterogeneous mixtures with a stepwise dependence of the reaction rate on temperature is considered. A twodimensional model of the process is proposed, which takes into account, in addition to other factors, the random distribution of fuel particles in the mixture and nonisothermality of the latter. The flammability limits are compared by methods of numerical simulation for two types of heterogeneous systems with an identical mean density of the fuel: with a uniform distribution of the fuel for all particles of the mixture and with its distribution based on random sampling of particles. It is shown that, as the degree of heterogeneity increases (dimensionless coefficient of heat transfer between the particles, Bi, decreases), the flammability limit for systems of both types becomes significantly higher than its thermodynamic value, and the upper boundary of the limit is twice as high. Differences in flammability limits and burning rates for heterogeneous systems of these types are found. A relationship between the flammability limit of a random system and a percolation phase transition, which occurs in a heterogeneous condensed mixture with scarce inclusions of the fuel, is demonstrated. An analytical approach for evaluation of the threshold concentration of the fuel in such systems, based on the problem of percolation on random nodes, is proposed.     

Laminar flame in fine-particle dusts

A simple conductive model of the laminar flame in fine-particle dusts, with particles burning in the diffusion mode, is presented. The model is based on the assumption about wide zones of combustion in dusts; therefore, the main contribution to the formation of the heat flux toward the pre-flame zone is made by the heat released on burning particles near the interface between the burning and pre-flame zones. The normal flame velocity is demonstrated to increase with decreasing particle size and with increasing fuel and oxidizer concentrations. The maximum velocities are reached under the condition of identical volume heat capacities of the solid and gas phases, which requires the fuel concentration to be several times higher than the stoichiometric value. The calculated results are validated by experimental data on the normal flame velocity as a function of the fuel concentration for gas suspensions of magnesium, aluminum, zirconium, iron, and boron particles.     

Ermittlung sicherheitstechnischer Kenngrößen explosionsfähiger Gasgemische

Determination of safety characteristics for explosive gas mixtures. This paper presents equipment for determining safety characteristics such as explosion limits, flame temperatures, and laminar flame velocities. Contactless methods of measurement such as infrared thermography, interferometry, and Laser-Doppler anemometry are of particular importance. By means of selected gas mixtures it is shown that the composition of the gas mixture, the initial temperature and, as far as explosion limits are concerned, also the type of ignition source are important parameters. Finally, the influence of additives is reported which even in relatively small quantities strongly modify the whole combustion process and are therefore also relevant in safety problems.     

空气雾化技术在燃油锅炉上的应用

研究设计内混式高效空气雾化轻柴油燃烧器。针对石油、石化、化工行业中锅炉在开工阶段没有燃料气和蒸汽供应,燃烧器的燃气枪和雾化蒸汽油枪无法投用,因此,锅炉装置不能运行。探讨在原燃烧器的基础上,按尽量不改变原燃烧器结构、燃烧方式和尽量少投入的原则,将原蒸汽雾化燃油枪改用空气雾化方式,开工用燃料油采用0#柴油,其它都不作改动。针对冷态试验结果,讨论了气压和油压对火炬形状的影响,油枪燃烧能量和雾化剂耗量,燃烧火盆与雾化角的关系。使用表明油枪喷嘴喷孔夹角、喷孔数、孔径、燃烧火盆、助燃风设计合理,雾化粒度细,燃烧完全,火焰刚直明亮、集中,喷嘴不结焦,操作性能良好。     

Structure of lean premixed hydrogen-air flames in an annular microcombustor

Self-sustainable combustion and narrow stability limits are the most critical issues in microcombustors (length scale 1 mm) as increased heat losses can lead to thermal quenching of the flame. Hydrogen is a potential fuel for microcombustion due to its high specific energy and wide flammability limits. This work focuses on the lean premixed hydrogen-air flames stabilized in a newly developed annular microcombustor. Detailed axisymmetric numerical calculations involving multistep kinetics, multicomponent mass-heat transport, conjugate heat transfer, and thermal radiation in gas and solid media are performed. It has been shown that flame stabilization occurs by preheating even though wall temperatures are higher than the autoignition temperature. Results unravel the importance of H radical reactions in the kinetics of microflames in low- and high-temperature regions of the microcombustor.