Gas Flame Structure and Optical Assessment of the Flame Speed and Combustion Efficiency

We perform the analysis of a prepared propane-butane flame structure, by using the computer processing of the radiation from the chemical reaction zone. We mark out the stoichiometric reaction along with the zones of the external oxidant inflow into the flame for different burner diameters. We suggest a method of determining the normal flame speed based on catching the moment of the complete fuel combustion in the upper part of a flame. We show a role of the external oxidant inflow in the kinetic processes within the burning zone. The absolute value of the normal component of the flame speed and its dependence on the burner diameter and on the excess oxidant ratio for a prepared propane-butane flame are determined experimentally.

On the extinction limit and flammability limit of non-adiabatic stretched methane–air premixed flames

Extinction limits and the lean flammability limit of non-adiabatic stretched premixed methane–air flames are investigated numerically with detailed chemistry and two different Planck mean absorption coefficient models. Attention is paid to the combined effect of radiative heat loss and stretch at low stretch rate. It is found that for a mixture at an equivalence ratio lower than the standard lean flammability limit, a moderate stretch can strengthen the combustion and allow burning. The flame is extinguished at a high stretch rate due to stretch and is quenched at a low stretch rate due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with two distinct extinction limits, a radiation extinction limit and a stretch extinction limit respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing the flammability limit of the stretched methane–air flame is obtained by plotting these two extinction limits in the mixture strength coordinate. A good agreement is shown on comparing the predicted results with the experimental data. For equivalence ratio larger than a critical value, it is found that the O-shaped temperature curve opens up in the middle of the stable branch, so that the stable branch divides into two stable flame branches; a weak flame branch and a normal flame branch. The weak flame can survive between the radiation extinction limit and the opening point (jump limit) while the normal flame branch can survive from its stretch extinction limit to zero stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits of stretched methane–air flames is presented. It is found that the critical equivalence ratio for opening up corresponds to the standard flammability limit measured in microgravity. Furthermore, the results show that the flammability limit (inferior limit) of the stretched methane–air flame is lower than the standard flammability limit because flames are strengthened by a moderate stretch at Lewis number less than unity.

Evaluation of a Glass Frit Nebulizer for Use in Atomic Absorption Spectrophotometry

A glass frit nebulizer has been designed and constructed specifically for use in flame atomic absorption spectroscopy. Its performance has been compared with that of conventional nebulizers using normal flame conditions. The performance level depends strongly on the pore size of the glass frit used. A fine frit is capable of operation at efficiencies close to 100% for low solution introduction rates. The noise level of the absorption signal when the fine glass frit was used was considerably less than that measured for the conventional nebulizer. The use of glass frits with larger pore sizes does not produce efficiency or noise data quite as good as those measured for the fine frit, but does offer extended operating ranges.