Ignition time moments

Generally, at any moment of time the concentration of components within a vapor cloud is highly nonhomogeneous and fluctuates considerably. The degree of homogeneity of a fuel-air mixture largely determines whether the fuel-air mixture is able to maintain a detonative combustion process. This factor is a primary determinant of possible blast effects produced by a vapor cloud explosion upon ignition. It is, therefore, important to understand the basic mechanism of turbulent dispersion. 

A much more effective mixture was the one invented in 668 AD by Kallinikos and called Greek Fire (see below) or Sea Fire. The exact formula of the original composition is not known because the Greeks kept it a secret. However, it seems that it contained, besides the combustible materials such as naphtha, pitch and sulfur, some oxidizer, which could have been saltpeter. This mixture was liquid that was discharged against the enemy either from pots, tubes or siphons, installed in the bows of ships. The moment the liquid came in contact with the water it was ignited. This device was so effective that it caused several defeats of the Arabic fleets in the 7th and 8th centuries, of the Russian fleet in 941 and 1043 and saved Constantinople several times from invaders. Greek... 

The gases disengaged in the combustion of this powder, would. comprise, at 32°, a volume three hundred and twenty-nine times as groat as that occupied by the powder. The force exerted hy the evolution of these gases is, however, mainly dependent upon their enormous expansion at the instant of the explosion, by the heat evolved in the action for it is calculated that one volume of powder of the above composition, yields, at the moment of ignition, at least two thousand times its volume of gas. ... 

I will now take such a flame as 1 had a moment since, and set free from it the particles of carbon. Here is some camphine, which will burn with a smoke but if I send these particles of smoke through this pipe into the hydrogen flame, you will see they will burn and become luminous, because we heat them a second time. There they are. Those are the particles of carbon re-ignited a second time. They arc those particles which you can easily see by holding a piece of paper behind them, and which, whilst they are in the flame, arc ignited by the heat produced, and, when so ignited, produce this brightness. When the particles are not separated, you get no... 

The pressure drop over the filter increases continuously with time until the moment, when a critical soot load has accumulated on the filter surface. Self-ignition of the soot then occurs, which in addition depends on the exhaust gas temperature, the filter temperature, the oxygen concentration, the type and amount of the volatiles adsorbed onto the carbonaceous soot matrix, and the packing density of the soot on the filter surface. The soot-laden filter bums free, the pressure drop decreases and the loading/regeneration cycle repeats itself again, as shown in Fig. 15.4. 

Details of this resolution can be found in any heat-transfer book [12]. Figure 3.4 presents a typical set of data that show the evolution of the surface temperature as a function of time. The peaks indicate the moment of ignition. 

The rapid increase in the rate of an oxidation reaction when the reaction mixture exceeds a certain temperature is called ignition the temperature at which this phenomenon occurs is called the ignitiontemperaturet and the time between the instant when the mixture reaches the ignition temperature and the moment of ignition is the ignition lag. The ignition temperature and lag are shown here on a representative plot of the temperature of a fuel mixture that is being heated. 

Later, as the front of in situ combustion shifts away fiom the well, the receptivity of the oilbearing bed to air may again increase or become stable. The speciHc time when the receptivity to air attains its maximum value only to drop sharply immediately thereafter signals the moment in which oil ignition in the bed occurs. 

The next important element in the scheme is the production of a preliminary plasma. As the real coefficient of volume compression in (cylindrical) geometry is 200 to 400, we can then expect a 20 to 30 times temperature increase during compression. Therefore about 5% of plasma energy (at the moment before ignition) is required for putting into preliminary plasma (foreplasma). 

The time that elapses between bringing an exothermic reactant into surroundings that put it at risk and the moment of ignition depends strongly upon the temperature. If heat-losses are small, then regarding the system as an adiabatic one overestimates the risk and gives a safer lower limit. For a first-order reaction in the terms we have introduced. 

Ignition Delay. Ignition of a flammable mixture raised to or above the temperature at which spontaneous combustion occurs is not instantaneous the time delay between the moment of exposure to high temperature and visible combustion is called the ignition delay. This time delay decreases as the ignition temperature increases. The time delay may be as little as a fraction of a second at higher temperatures, or several minutes close to the autoignition temperature. 

Figure 3.19 presents some recorded data from the photodiodes PhDl-PhD4 response to the flame propagation along the EC. The time t is counted from the moment of ignition in all experiments it is = 43 ms following the membrane burst. 

Rather complicated flow occurs near the reflector, where there is no direct detonation initiation. The initial reflected wave and the combustion front exist independently in the system, Fig. 6.27. At some moment in time, self-ignition ceases and the combustion is controlled by small-scale turbulence and diffusion. Because the deflagration-to-detonation transition takes place, in principle, it cannot be excluded, but this phenomenon has not been discussed yet. 

Ignition delay time (IDT) Time delay since the instant of the temperature jump to the moment of ignition. IDT depends on the temperature and ranges from several minutes (not high pressure and low temperature) to microseconds (high temperature behind the shock waves). 

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