Hydrogen diffusion flame, temperature

There has been interest in the low radiative background, low quenching argon-hydrogen diffusion flame. The temperature of this flame is too low to prevent severe chemical interferences and therefore the argon-separated air-acetylene flame has been most widely used. The hot nitrous oxide-acetylene flame (argon separated) has been used where atomization requirements make it essential. In all cases, circular flames, sometimes with mirrors around them, offer the preferred geometry. 

Case 3 Both temperature and species variation - In this case, additional information is required. This could be obtained from another diagnostic or a mathematical model. Smith (10) used an extensive mathematical model of a laminar hydrogen diffusion flame to predict the species distribution throughout the flame having this, the temperature could be inferred from the Rayleigh scattering intensity. 

Smith, J. R., "Rayleigh Temperature Profiles in a Hydrogen Diffusion Flame," Proceedings of SPIE Vol 158 Laser Spectroscopy (1978) p. 84-90. 

Figure 11-20. Comparison of preheating temperatures of the eounterflow hydrogen diffusion flames using preheater versus the preheater and gliding are aetivation of the air stream.

A typical chemiluminescence detector consists of a series-coupled thermal decomposition and ozone reaction chambers. The selective detection of nitrosamines is based on their facile low-temperature (275-300°C) catalytic pyrolysis to release nitric oxide. Thermal decomposition in the presence of oxygen at about 1000°C affords a mechanism for conversion of nitrogen-containing compounds to nitric oxide (catalytic oxidation at lower temperatures is also possible). Decomposition in a hydrogen-diffusion flame or thermal oxidation in a ceramic furnace is used to produce sulfur monoxide from sulfur-containing compounds. 

The ability of the hydrogen diffusion flame to act as a temperature and concentration gradient across its body can be shown with a simple experiment. If tin(II) bromide (SnBr2) is aspirated into the flame, then three colored regions appear (Figure 1). In region 1, the concentration of atomic hydrogen is low and the excited particles (marked with an asterisk) are formed by the reaction... 

Direct aspiration of the sample into the hydrogen diffusion flame is always associated with certain problems. The low temperature of the flame may lead to formation of solid particles, which reduce the production of emitting species and decrease sensitivity. The solvent and other components of the analyte solution may alter the temperature and disturb the radical distribution and concentration within the flame body. The distribution of the sample vapors all over the flame may be responsible for generation of more than one emitting species, such as with tin(II) bromide. Production of excited species will also be affected by the difference of temperature at various points of the flame. Moreover, the emitting species will spread over a wide region and the intensity per unit area of flame facing the detector will be low. Finally, the residence time of the analyte within the flame is short and cannot be increased since it is mainly controlled by the flow rate of the support gas. 

Figures 11.1 and 11.2 illustrate the concentration and temperature profiles in a hydrogen diffusion flame and the typical S-curve denoting temperature dependencies on the Damkohler number that describe the simplified diffusion flame. Two characteristic points are seen on the S-curve. The lower characteristic point Ti is a firing point, the upper one Tad is the point of extinction. The curve itself is often called a fundamental curve of extinction because it is used for predicting the extinction level of a diffusion flame.

Let us reconsider the critical flame temperature criterion for extinction. Williams [25], in a review of flame extinction, reports the theoretical adiabatic flame temperatures for different fuels in counter-flow diffusion flame experiments. These temperatures decreased with the strain rate (ua0/x), and ranged from 1700 to 2300 K. However, experimental measured temperatures in the literature tended to be much lower (e.g. Williams [25] reports 1650 K for methane, 1880 K for iso-octane and 1500 K for methylmethracrylate and heptane). He concludes that 1500 50 K can represent an approximate extinction temperature for many carbon-hydrogen-oxygen fuels burning in oxygen-nitrogen mixtures without chemical inhibitors . 

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. 

In lean hydrogen mixtures the flame temperature in diffusive combustion is higher than in normal propagation and so, in a wide region of concentrations (from 4 to 9% H2 in a mixture of hydrogen with air), normal propagation is impossible, only diffusive combustion is possible. Our point of view is in accord with the observed properties of flame propagation in this concentration interval. 

All of what we have said above relates equally to the surface of a flame on which a rapid and full homogeneous chemical reaction takes place. In a lean hydrogen mixture on the surface of a flame at rest with respect to the gas to which hydrogen and oxygen are supplied by diffusion, a temperature significantly exceeding the theoretical temperature of combustion is established. 

E. Gutheil and F.A. Williams, A Numerical and Asymptotic Investigation of Structures of Hydrogen-Air Diffusion Flames at Pressures and Temperatures of High Speed Combustion, 23th Symp. (Int.) Comb. (1990) pp. 513-521. 

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