Flames detached

If air and fuel are premixed upstream of a burner nozzle, mixing (and therefore combustion) may occur more rapidly than with nozzle mixing, and surely more thoroughly than with delayed mixing (perhaps with a detached flame) out in the furnace. Presumably, faster mixing and combustion will require less furnace volume, but the aerodynamics and the directions of the velocity vectors can influence flame shape to the point where flame volume may be less dependent on air or fuel momentum. 

The methyl alcohol is poured into a round flask (250 c.c.) connected with an upright condenser. The sodium, cut into small pieces, is then added, the flask being detached from the condenser for a moment and replaced. When the sodium has dissolved, the phenol and methyl iodide are added. The mixture IS heated on the water-bath until the solution has no longer an alkaline reaction (two to three hours). As much as possible of the methyl alcohol is distilled off on the water-bath and water added to the amber-coloured residue. A colourless oil separates out, which is e.xtracted with ether. The ethereal solution is dehydrated over calcium chloride and distilled, first on the water-bath until the ether has been driven off, and then over the flame. Almost the whole of the residue distils at 150—155. Yield, nearly theoretical. 

Flame Cleaning Now little used as a preparatory method, flame cleaning is a process whereby an intensely hot oxyacetylene flame is played on the surface of the steel. In theory, differential expansion causes millscale to detach. In practice, there is evidence that the treatment may not remove thin, tightly adhering millscale. Also, steel less them 5 mm thick can buckle. Finally, the process can burn in chemicals deposited on the surface, causing premature paint failure. 

Collisional Detachment. Reactions of negative ions in flames not containing hydrocarbons have not been widely studied, although OH -ion formation is important in flames containing high electron concentrations. The rate constant k l of the reaction... 

After detachment of the flame from the wall and reduction of its width, three zones develop in the vessel parallel to each other a flame and burned gas zone and two zones (adjacent to the sidewalls), where no flame is present and where the gas temperatures are lower than behind the flame. All this happens in a field of very high centrifugal acceleration, which induces a free convection movement of the flame and the product zone behind it toward the... 

It is shown that the Ekman layers behind the flame front, generated by the rotational speed changes of the gas owing to expansion, cause flame detachment from the walls and reduction in flame width in the rotating vessels, ultimately causing flame quenching. 

After detachment of the flame from the walls, the narrow ever-diminishing hot product zone behind the flame moves owing to the free convection in the centrifugal acceleration field toward the axis of rotation, with a speed scaling with circumferential velocity at the flame location, which reduces the observed flame speed to very low values, and in some cases negative ones. 

In Chapter 6.4, J. Chomiak and J. Jarosinski discuss the mechanism of flame propagation and quenching in a rofating cylindrical vessel. They explain the observed phenomenon of quenching in ferms of the formation of fhe so-called Ekman layers, which are responsible for the detachment of flames from the walls and the reduction of fheir width. Reduction of the flame speed with increasing angular velocity of rofation is explained in terms of free convection effects driven by centrifugal acceleration. 

The burning mechanism of composite propellants differs from that described above. There is no exothermic reaction which can lead to a self-sustaining fizz zone. Instead, the first process appears to be the softening and breakdown of the organic binder/fuel which surrounds the ammonium perchlorate particles. Particles of propellant become detached and enter the flame. The binder is pyrolysed and the ammonium perchlorate broken down, initially to ammonia and perchloric acid. The main chemical reaction is thus in the gas phase, between the initial dissociation products. 

Figure 19.3 Influence of equivalence ratio on antinodal RMS pressure fluctuation annular flow arrangement, bulk mean velocity in main flow, Um = 7-5 m/s bulk mean velocity in pilot stream. Up = 8 m/s Re j = UmD/v = 40,000, axial separation between annular ring and step, A = 0.513. 1 — 4 m = 0.62 2 — 0.70 3 — 0.76 dashed line corresponds to flame detachment...

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

The sulphuric acid may be added to the solution before evaporation and in this case the evaporation on the water-bath should be continued until a semi-solid brown mass with a channelled surface is obtained. This is heated on the sand-bath or over a flame until the charcoal becomes detached from the walls of the dish and is then incinerated as above. 

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