Hydrogen flame speed

Hydrogen has high flame speed at stoichiometric ratios. Under these conditions, the hydrogen flame speed is nearly an... 

The results in Tables 4. la and 4. lb demonstrate that in the absence of obstacles, the highest flame speed observed was 84 m/s, and it was accompanied by an overpressure of 60 mbar for hydrogen-air in a 10-m radius balloon (Schneider and Pfortner 1981). For all other fuels, flame speeds were below 40 m/s and corresponding overpressures were below 35 mbar. Hence, weak ignition of an unconfined... 

Ishizuka, S., Koumura, K., and Hasegawa, R., Enhancement of flame speed in vortex rings of rich hydrogen/air mixtures in the air, Proceedings of the Combustion Institute, 28,1949-1956, 2000. 

Subsequently, the problem was investigated by Karpov and Severin [6]. They used closed vessels with a diameter of 10cm and 10, 5, and 2.5cm width, initially at atmospheric pressure. The vessels were filled with different lean hydrogen and methane/air mixtures and rotational speeds in the range of 130-4201/s were employed. They also included data from the study of Babkin et al. [3] in their analysis. Unfortunately, they did not observe the flame itself and measured only the pressure rise in the vessel, which was compared with pressure development in the vessel without rotahon, to draw a conclusion with respect to flame speeds and quenching. 

Very early, from the analysis of ignition, flame speed, and detonation velocity data, investigators realized that small concentrations of hydrogen-containing materials would appreciably catalyze the kinetics of CO—02. The H20-catalyzed reaction essentially proceeds in the following manner ... 

The variation of flame speed with equivalence ratio follows the variation with temperature. Since flame temperatures for hydrocarbon-air systems peak slightly on the fuel-rich side of stoichiometric (as discussed in Chapter 1), so do the flame speeds. In the case of hydrogen-air systems, the maximum SL falls well on the fuel-rich side of stoichiometric, since excess hydrogen increases the thermal diffusivity substantially. Hydrogen gas with a maximum value of 325 cm/s has the highest flame speed in air of any other fuel. 

There is, of course, a chemical effect in carbon monoxide flames. This point was mentioned in the discussion of carbon monoxide explosion limits. Studies have shown that CO flame velocities increase appreciably when small amounts of hydrogen, hydrogen-containing fuels, or water are added. For 45% CO in air, the flame velocity passes through a maximum after approximately 5% by volume of water has been added. At this point, the flame velocity is 2.1 times the value with 0.7% H20 added. After the 5% maximum is attained a dilution effect begins to cause a decrease in flame speed. The effect and the maximum arise because a sufficient steady-state concentration of OH radicals must be established for the most effective explosive condition. 

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization... 

On what side of stoichiometric would you expect the maximum flame speed of hydrogen-air mixtures Why ... 

The most important of these are the Wobbe index [or Wobbe number = calorific value/(specific gravity)] and the flame speed, usually expressed as a factor or an arbitrary scale on which that of hydrogen is 100. This factor can be calculated from the gas analysis. In fact, calorific value and specific gravity can be calculated from compositional analysis (ASTM D3588). 

The H + O2 competition is responsible for several important aspects of combustion phenomena. For example, the second explosion limit for hydrogen-oxygen mixtures is explained by the competition between H + O2 branching and termination (Section 13.2.6). The observed reduction in hydrocarbon-air flame speeds with increased pressure between 1 and 10 atm is caused by the branching-termination competition. For a given temperature, as the pressure increases, the concentration of [M] increases, which favors the termination reaction. Thus the chain branching competes less favorably for a greater portion of the flame, which diminishes the flame speed [427]. 

Explosion suppression is used for the protection of extremely hazardous systems in industry. Explosions that develop very high radial flame speeds (such as hydrogen-oxygen) are too fast for existing equipment. Many detonations (ultrasonic) also develop from an initial deflagration. It is possible... 

The flame-speeds of combustible mixtures of hydrogen and air are less easy to determine since the flame travels more rapidly and in some... 

Fig. 27.—Flame speeds of carbon monoxide, hydrogen, and methane.

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