Self-heating phenomenon

A mathematical model is described [138] in which the self-heating of material layers under industrial conditions is simulated. The model takes into account oxygen (or gas) diffusion and consumption, reactant conversion, heat conduction in, and heat transfer to and from the layer. Scale-up experiments were performed which showed the model can be successfully applied to predict the self-heating phenomenon in the layers. 

A critical size for "spontaneous detonation thru self-heating may be a general phenomenon, even though widely varying for different explosives. The view of this as a possibility arises from the indication, in the Arrhenius equation, that there is some decomposition constantly taking place even at room temperatures. As the linear dimension d of the charge increases, the rate of heat evolution increases as d, whereas the rate of heat dissipation by conduction increases only as d. Theoretically, therefore, a size must be attainable at which the sample will eventually... 

Self-heating, due to the stoppage of the alpha particles within the solid, is a well known phenomenon and calculation shows that the energy release from one gram of polonium metal would be about 140 watts. This high energy output affords a useful and absolute method for the rapid determination of polonium in large sources by calorimetry. 

It is obvious, then, that only the H2-CI2 reaction can be exploded photochemi-cally, that is, at low temperatures. The H2-Br2 and H2-I2 systems can support only thermal (high-temperature) explosions. A thermal explosion occurs when a chemical system undergoes an exothermic reaction during which insufficient heat is removed from the system so that the reaction process becomes self-heating. Since the rate of reaction, and hence the rate of heat release, increases exponentially with temperature, the reaction rapidly runs away that is, the system explodes. This phenomenon is the same as that involved in ignition processes and is treated in detail in the chapter on thermal ignition (Chapter 7). 

An adiabatic method represents the most adequate technique for determining the relative tendencies of certain coals to heat spontaneously since it simulates most closely the real phenomenon. Conceivably, a field system would be similar to the adiabatic system but with appropriate modifications to hasten the oxidation process and increase the effluent gas concentrations within a reasonable test period. This could involve a more versatile system which would allow either the study of self-heating rates, similar to a method used by Guney (10) or which may be used for adiabatic calculations of a liability index through incorporation of a constant heat input. In the latter case, the heat might be supplied exclusively from the oxidizing air stream. 

The above phenomenon is as a matter of course a direct consequence of a fact that the effect of the concentration of a chemical of the TD type on the rate of the exothermic decomposition reaction, in the early stages of the self-heating process, of the chemical is virtually of the zeroth order. 

The tests described above were, of course, all performed with the closed cell. However, it has also been ascertained that, when charged in the open-cup cell and subjected to the isothermal storage test performed at a T, of 162.1 °C, 2 g of picric acid self-heats clearly. It is certain that this phenomenon is caused by the air oxidation of picric acid. 

As the initial temperature increases, the initial self-heat rate increases following the zero-order line. One very interesting phenomenon occurs when the maximum self-heat coincides with the initial self-heat rate. Above this temperature, the reaction decelerates, even though the temperature itself continues to increase. This temperature is designated as T and can be evaluated when Tm = T0 = T and 7 = ATab -f T ... 

The tendency of expls to self-heat is an important indication of a serious stability problem. Studies of this phenomenon are performed by maintaining progressively larger r idar shapes of the expl or the proplnt (for example, right cylinders, cubes or spheres) at elevated temps until deflagration occurs (Refs 19 82). The progress of the self-heating is followed with inserted thermocouples. The critical sizes at... 

The thermal effect of a combustible system and the heat output to the environment may also serve as a basis for the interpretation of the self-ignition phenomenon. 

If the heat production extends the heat transport in any place in the bed of activated carbon, self-heating will take place and as a result a temperature peak, a so-called hot-spot , will develop. The consequence of this phenomenon may be the auto-ignition of the bed. 

These unique linear polymers are extremely lubricious (a characteristic also associated with the high-molecular-weight PEOs) and lend a beneficial slippery feel to a formulation. They are also excellent aids for dispersing sparingly water-soluble components as well as solid materials such as titanium dioxide. One particularly interesting aspect associated with these polymers is their exothermic heat of solution, a thermodynamic phenomenon that causes aqueous solutions in which the polymers are initially dissolved to warm. This quality has found commercial utility in self-heating, hot oil treatments. 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

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