Dependence Of Burning Velocity Research Articles

Effects of temperature and pressure on quenching distances of difluoromethane (R32) and ammonia (R717)

Temperature and pressure effects on quenching distance have been studied for difluoromethane (R32) and ammonia (R717), two mildly flammable refrigerant gases. By studying ignition and quenching behavior in conditions beyond standard test conditions, the authors aimed at understanding whether combustion takes place in practical situations. First, the quenching distance of R32 and R717 in the temperature range of 298 to 333 K and pressure range of 0.79 to 1.12 atm was measured. The quenching distance decreased slowly with increasing temperature and decreased steeply with increasing pressure. Second, the authors examined the relationship between quenching distance and burning velocity at various temperature and pressure. It was found that quenching distance is essentially related to (unburned gas density × burning velocity)−1, and changes in quenching distance observed in the present experiment can be explained by the temperature and pressure dependences of burning velocity and unburned gas density. It was also found that the quenching distance and burning velocity for R32 were not affected much by the humidity of air in the practical condition.

Dependence of burning velocity on the sample size in the nonactivated and mechanically activated Ni + Al systems

The burning velocities of Ni + Al films 270–360 μm thick are greater than those of the samples 8–12 mm in diameter and 15 mm in height pressed from Ni + Al powder by a factor of 4–20. The data are explained in terms of a convective–conductive model of combustion wave propagation.

CELLULAR STRUCTURE OF EXPLOSION FLAMES: MODELING AND LARGE-EDDY SIMULATION

Simulations of deflagration propagation through initially quiescent stoichiometric hydrogen/air mixture inside a large-scale 2.3-m-diameter spherical vessel are performed. A large-eddy simulation (LES) model of premixed combustion is suggested, which is based on a subgrid-scale turbulence model of renormalization group theory, a gradient method for combustion modeling and the dependence of burning velocity on transient pressure and temperature. An unstructured grid with one level of refinement/de-refinement around the flame front area is used. Wrinkled by hydrodynamic instability, the flame front structure has been resolved for the first time in LES of large-scale explosions. The growth of cell sizes with flame radius is reproduced numerically in agreement with theoretical and experimental results. The minimum resolved size of the flame cells is of the order of the simulated flame front thickness, which is about three control volume edges. The fractal dimension of the wrinkled flame front surface grows and reaches a value of 2.15, close to observed in experiments.

Study of Measuring Burning Velocity of Fuel-Air Mixture under Compression by Using Double Kernel Method.

This study aimed at revealing the characteristics of burning velocity of fuel-air mixtures under the ambient temperature and pressure changing condition which was made by using rapid compression machine. The burning velocity was measured by a double kernel method applied to the schlieren photographs. At first, the applicability of the double kernel method was examined and verified by utilizing a proper technique to analyze the schlieren photographs. At the next stage, under a standard condition (298 K, 0.10 MPa), the dependence of burning velocity on the fuel type was obtained and compared with those of the other studies. Finally, the dependence of burning velocity on the ambient temperature and pressure was obtained and discussed under the condition that the ambient state was varying with time according to the flame propagation.

Burning velocities and flammability limits of gaseous mixtures at elevated temperatures and pressures

Burning velocities and flammability limits of gaseous mixtures of combustible gas (hydrogen and methane), oxidizer (oxygen and air), and diluent (nitrogen, argon, helium, carbon dioxide, steam, water aerosol formed by evaporation of superheated water) have been measured at elevated temperatures (up to 250°C) and pressures (up to 4.0 MPa). It was found that with increasing temperature, the flammability region is widened for all the mixtures studied. With increasing pressure, the flammable region for mixtures of hydrogen-oxygen-diluent is narrowed, except where steam or water aerosol is the diluent. A more complicated dependence of flammability limit on pressure is found for mixtures of hydrogen-oxygen-steam. The influence of water aerosol (formed by the rapid evaporation of superheated water having an initial temperature of 150°C) on the flammability limits of methane in air was also investigated. It was found that the inerting action of such an aerosol is mainly due to the steam available within it; water droplets which are present in the mixture do not have a significant influence on the flammability limits. The dependences of burning velocity on temperature and pressure were investigated for stoichiometric hydrogen-air mixtures diluted by nitrogen and steam. It has been shown that, over the ranges of temperature and pressure considered, the temperature index of burning velocity is positive (that is, burning velocity increases with temperature), while the pressure index changes sign from positive to negative on dilution by inert agents. The observed differences in the influences of steam and nitrogen on burning velocity cannot be explained on the basis of a thermal mechanism alone and is mainly due to the more active role of water molecules in enhancing termolecular recombination reactions in the flame front. The influence of small quantities of methane on the burning velocity of hydrogen-air mixtures also was investigated.

Premixed silaneoxygennitrogen flames

The burning velocities of lean premixed silaneoxygennitrogen flames were measured in the silane and oxygen concentration ranges from 1.6% to 2.9% and from 4% to 24%, respectively. Combustion product analyses and flame temperature measurements were also carried out. The burning velocity of a silaneair flame is around 55 cm/s at a silane concentration of 2%. For lean mixtures, when the oxygen concentration is reduced, dependence of burning velocity upon silane concentration decreases but does not significantly affect the flame temperature. For extremely lean flames, the degree of hydrogen production increases with decreasing silane, although silane is consumed almost completely. On the other hand, if the silane concentration exceeds stoichiometric, the burning velocity increases gradually with increasing silane concentration. In that case, silane as well as oxygen are consumed completely and, at the same time, hydrogen rather than water production becomes dominant. The mechanism of silane combustion is discussed, based on numerical calculations, where the mechanism used in the calculation is assembled by analogy of silane to methane combustion.

Flame structure and flame reaction kinetics. VI. Structure, mechanism and properties of rich hydrogen+nitrogen+oxygen flames

Burning velocities of a number of slow-burning, fuel-rich hydrogen + nitrogen + oxygen flames have been measured and compared with values computed on the basis of the chemical mechanism proposed by Dixon-Lewis (1970 a ). This mechanism is shown to be incomplete, and in order to obtain agreement between theory and experiment over the composition range studied, it is necessary to invoke some further chain breaking steps involving hydroxyl radicals and oxygen atoms. The steps which have been considered lead to the extended mechanism OH + H 2 ⇌ H 2 O + H, (i) H + O 2 = OH + O, (ii) O + H 2 ⇌ OH + H, (iii) H + O 2 + M = HO 2 + M, (iv) H + HO 2 = OH + OH, (vii) H + HO 2 = O + H 2 O, (vii a ) H + HO 2 = H 2 + O 2 , (xii) OH + HO 2 = H 2 O + O 2 , (xiii) O = HO 2 = OH +O 2 , (xiv) H + H + M = H 2 + M, (xv) H + OH + M = H 2 O + M, (xvi) H + O + M = OH + M, (xvii) With the use of steady-state approximations for the small concentrations of OH and O present in the flames, the important new parameters in the analysis are k 16 , k 17 , and the ratios k 7 / k 12 , k 7a / k 12 , k 13 / k 12 and k 14 / k 12 . Although the number of these precludes an unambiguous assignment of values to them, numerical experimentation on reasonable assumed values of k 15 , k 16 and k 17 has led to the conclusions that: ( a ) The ratio ( k 7 + k 7a )/ k 12 lies in the range 6.5 ± 1.0, assumed independent of temperature in the flame reaction zone. ( b ) The ratio k 7 / k 7a is unlikely to be much greater than 0.1, and it may be much less than this. ( c ) Assuming k 7a = k 14 = 0, a maximum possible value of k 137 / k 12 = 5.5 may be deduced from the dependence of burning velocity on composition. A further important feature shown by the computation is that for the imposed condition of satisfactory prediction of measured burning velocities, the hydrogen-atom concentration profiles in specific flames are not appreciably affected by the particular combination selected from the six adjustable parameters given above. In this context the more important parameters, apart from k 15 (Dixon-Lewis 1970 a ), are shown to be the ratios 2 k 2 / k 4 . Also in connexion with the atom and radical concentrations, a comparison of measured profiles of the relative intensity of chemiluminescence when traces of sodium salts are added to three of the flames studied, with profiles derived from the computed H and OH radical concentrations, leads to the conclusion that reaction (xix) is some 25 times faster than reaction (xviii) H + H + Na = H 2 + Na*, (xviii) H + OH + Na = H 2 O + Na*. (xix) Lastly, the satisfactory prediction of the burning velocity of a much faster and hotter flame than those concerned in the bulk of the investigation provides further very powerful support for the reaction mechanism involving participation of hydroperoxyl. Effects of thermal diffusion in this and the other flames are discussed.

The acetylene decomposition flame and the deduction of reaction mechanism from ‘global’ flame kinetics

A method of stabilizing a pure acetylene decomposition flame on a burner, over a range of temperatures and pressures, has been developed. It depends for its feasibility and safety on a novel flow and compressor system, details of which are described. By its use, the variation of burning velocity with temperature and pressure has been determined not only for acetylene but also for a series of hydrocarbon + air mixtures, in order to test both apparatus and method of interpretation. The latter involves an attempt to deduce rate-controlling reaction steps from effective activation energies and orders derived from the pressure and temperature dependence of burning velocity. It is shown that, while these ‘global’ parameters are in good agreement with values determined for the hydrocarbon + air mixtures, for instance, by ‘well-stirred reactor’ methods, a logical modification in interpretation brings them into accord with the conclusions, concerning detailed mechanism, of recent flame-structure sampling studies. A similar analysis of the acetylene decomposition results yields effective orders and activation energies which differ from those of both low-temperature pyrolysis and shock-tube studies. A proposed reaction mechanism, which is likely to be dominant only in the case of flames and in the deliberately contrived absence of soot deposits, accounts for these differences as well as for the numerical values obtained. In the presence of soot, an alternative heterogeneous surface-reaction is likely to become important and experimental support for this concept is provided by the behaviour of acetylene during its interaction with hot-ware igniters above the burner.

Laminar Burning Velocities of Methane-Oxygen-Diluent Gas Mixtures

The burning velocities of methane--oxygen--diluent mixtures were determined at atmospheric pressure by use of bunsen flames under laminar flow conditions. The fuel-lean region was investigated at theoretical flame temperatures of 2000, 2250 and 2500/sup 0/K employing nitrogen, helium and argon as diluents. By maintaining constant calculated flame temperatures for the different mixtures, evaluation of the effects of the properties of the diluent and of O/sub 2//CH/sub 4/ ratio was greatly simplified. The dependence of burning velocity on O/sub 2//CH/sub 4/ ratio was not in accord with the approximate theories of combustion. Therefore, an empirical expression was developed to describe the effect of diluent, O/sub 2//CH/sub 4/ ratio, and flame temperature on burning velocity. The evaluation of this expression is discussed in terms of effective transport properties and concentrations.