Diffusion ignition

Spontaneous Generation of Diffusion Flames (Diffusion Ignition)... 

Fig. 11.26 Hypothetical diagram of diffusion ignition of a compressed hydrogen jet directly into the air...

Fig. 11.27 Diagram of the diffusion ignition of a compressed hydrogen jet into the air through a system of intermediate tubes 1 - hydrogen in a gate valve tube 2 - monometer 3 - main cutoff valve 4 - pressure transducer 5 - feeder 6 - diaphragm 7 - variable-length extender [50]...

Fig. 11.30 Diagram of the facility for diffusion ignition investigation containing intermediate volumes and adjoining post-membrane ducts open at one end [49, 50] 1 - hydrogen vessel 2 - reduction gear, 3 - gate valve 4 - drainage 5 - needle valve 6 - pressure transducer 7 - control volume 8 - membrane unit 9 - set of lengthening ducts [47]...

Fig. 11. 37 Lower boundary of diffusion ignition, explosion modes are not specihed [48]...

Due to the lower methane diffusion constant and to the longer period of selfignition delay time, spontaneous methane ignition has not been achieved at the same conditions as for hydrogen. Detailed investigation of methane diffusion ignition under decompression requires special experiment for better analysis of mine accidents [3]. 

Figure 11.34b specifies the case of methane + air mixture compression by the reflected wave at the methane decompression level P4 = 10 MPa (point + ). Gas mixture ignition is anticipated in areas of potential pressure wave focusing on curved surfaces. In [47], while discussing hydrogen diffusion ignition problems, it was correctly reasoned that an approach considering a flat contact surface is bounded. The authors in [47] assumed, and it has been proved by long-term experience in shock tubes operation [43], that the disk shape is not flat before the rupture (Fig. 11.35a). For this reason, the contact surface is curved (Fig. 11.35b), and it noticeably affects the ready-to ignite gas mixture.

The lower diffusion ignition boundary is plotted in [48], but the potential combustion modes are not specified (Fig. 11.37). Some decrease of the experimentally obtained boundary was pointed out when circular ducts are replaced by rectangular ones. 

P. Wolanski, S. Wojcicki, Investigation into mechanism of the diffusion ignition of a combustible gas flowing into oxidizing atmosphere. Proc. Combust. Inst. 14, 1217-1223 (1973)... 

P. Wolanski, Forty years of investigation of diffusion ignition. Paper at 7th international symposium on hazards prevention and mitigation of industrial explosions, St. Petersburg, 2008... 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Combustors All gas turbine combustors perform the same function They increase the temperature of the high-pressure gas at constant pressure. The gas turbine combustor uses veiy little of its air (10 percent) in the combustion process. The rest of the air is used for cooling and mixing. The air from the compressor must be diffused before it enters the combustor. The velocity leaving the compressor is about 400-500 ft/sec (130-164 m/sec), and the velocity in the combustor must be maintained at about 10-30 ft/sec (3-10 iTi/sec). Even at these low velocities, care must be taken to avoid the flame to be carried downstream. To ensure this, a baffle creates an eddy region that stabi-hzes the flame and produces continuous ignition. The loss of pressure in a combustor is a major problem, since it affecls both the fuel consumption and power output. Total pressure loss is in the range of 2-8 percent this loss is the same as the decrease in compressor efficiency. 

The flame behavior of a fire is important in determining tlie causes and effects of fires. There are several classificiitions of flames orifice flames, pool flames, fireballs. Jet fimnes, and flash fires. Orifice or pipe flames are characterized as eitlier prenii. ed flame or diffusion flmiies. Pool flames are flames on ground pools and flames on tanks. Fireballs radiate intense heat, wliich can cause fatal bums and can quickly ignite otlier materials. Jet flame or flares also radiate intense heat. 

It must be appreciated that at high temperatures platinum permits the flame gases to diffuse through it, and this may cause the reduction of some substances not otherwise affected. Hence if a covered crucible is heated by a gas flame there is a reducing atmosphere in the crucible in an open crucible diffusion into the air is so rapid that this effect is not appreciable. Thus if iron(III) oxide is heated in a covered crucible, it is partly reduced to metallic iron, which alloys with the platinum sodium sulphate is similarly partly reduced to the sulphide. It is, advisable, therefore, in the ignition of iron compounds or sulphates to place the crucible in a slanting position with free access of air. 

Parametric studies showed that mass diffusion in the gas phase could be neglected under most conditions. The calculations also show that the selection of the hypergolic combination (i.e., the gaseous oxidizer and the propellant system) fixes all of the parameters except the initial temperature and the oxidizer concentration. A general solution of the model shows that the ignition-delay time is approximately rated to the gaseous oxidizer concentration by the relation... 

Gas density Propellant density Boltzmann constant A factor to account for temperature oscillations ignition delay time Diffusion time... 

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