Early ignition

Many furnace residues (fine powders and salts) deposited in the upper parts of furnaces used for thorium melting operations, are highly pyrophoric and often ignite as the furnace is opened. Such residues may be rendered safe by storage under water for 60-90 days. If the water is drained off early, ignition may occur. 

An increase in droplet size with axial position is observed for all three gases. However, the relative trend of smallest droplet mean size with steam and largest with normal (unheated) air remains unchanged. As an example, at 50 mm downstream from the nozzle exit at r = 0, droplet mean size for steam, preheated air, and normal air were found to be 69, 86, and 107 pm, respectively see Fig 16.3. The droplet size with steam is also significantly smaller than air at all radial positions see Fig. 16.3. The droplet size with preheated air is somewhat smaller than normal air due to the decreased effect of preheated air at this location and increased effect of combustion. Early ignition of the mixture with preheated air (see Fig. 16.1) must provide a longer droplet residence time which results in a smaller droplet size. In addition, the increased flame radiation with preheated air increased droplet vaporization at greater distances downstream from the nozzle exit. Indeed, the results indicate that the measured droplet sizes with preheated atomization air are smaller than normal air in the center... 

A low-pitched thudding noise different from knock and accompanied by engine roughness. One of the causes probably is the high rate of pressure rise associated with very early ignition or multiple surface ignition. 

For this case, unlike run 4, the temperature profile for steady state 1 is drastically dilFerent to that of the early ignited steady states 2, 3. However, steady states 2, 3 are still very close to each other. 

In the examples given above we have tried to describe some of the phenomena which arise as a result of chemical kinetic-fluid dynamic coupling. First, we described studies of the isolated effects of chemical-acoustic coupling, emphasizing the effects on the chemical kinetics. The major conclusion is that sound waves and entropy perturbations can alter chemical timescales, and that this effect can be quantified. We then described a system in which sound waves and entropy perturbations behind a shock wave caused early ignition at unpredictable locations and at reduced ignition times. A series of reaction centers formed and one of these close to the shock front eventually ignited. 

---