Reaction front thermally initiated

The burning of a column of delay composition takes place by the passage of a reaction front along the column. The temperature profile of this reaction front can be measured by the use of suitable thermocouples and recording instruments. By analysing the shape of the front it can be shown that the reaction is a solid/solid reaction initiated by thermal conduction of heat through the unreacted material. It follows that to obtain reproducible reaction rates there must be (1) constant amount of solid to solid contact and (2) constant thermal conductivity. 

It is interesting that there is little change in the time required to cure the polymer as the concentration of BP is increased from 0.2 to 1.0 wt.%. These results suggest that the reaction proceeds to completion even before the light penetrates deep into the sample, perhaps by the propagation of a thermal front created by the initial photopolymerization at the leading surface. In any case, these results illustrate that the addition of a thermal initiator drastically reduces the polymerization time for thick systems. 

A thermal explosion is the third type of chemical explosion. In this case, no reaction front is present, and it is therefore called a homogenous explosion. Initially, the material has a uniform temperature distribution. If the temperature in the bulk material is sufficiently high so that the rate of heat generation from the reaction exceeds the heat removal, then self-heating begins. The bulk temperature will increase at an increasing rate, and local hot spots may develop as the thermal runaway proceeds. The runaway reaction can lead to overpressurization and possible explosive rupture of the vessel. 

For the example of methane steam reforming, Eq. (8) yields an acceleration factor of 4. Accordingly, the axial displacement of the reaction zone is a multiple of the axial displacement of thermal fronts. The difference of the axial displacement between the reaction front and the thermal front determines the axial profile of heat demand during the subsequent exothermic semicycle. Efficient heat recovery requires equal heat capacities of the process streams during both semicycles. The initial state can be restored by discrete heat sources distributed at equal distances along the catalytic part of the reactor. Each point source initiates a thermal wave that covers the distance to the next heating point (Fig. 1.13, right). This concept features... 

The thermal explosion mode is entered with a small temperature gradient of 1.25 K/mm. The temporal developments of the fuel coneentration, gas velocity, pressure and temperature fields are shown in Fig. 7.21. The solutions show a uniform increase in pressure to be soon attained, with but low gas velocities, throughout the volume. The apparent speed of propagation of the reaction front away from the centre is higher than the acoustic velocity, a function solely of the initial temperature gradient away from the hot spot. 

FP systems must have certain conditions for the front to autocatalytically occur. The necessary and sufficient conditions for TFP include a monomer that will polymerize via free-radical polymerization and a thermal initiator (a photoinitiator may be used to start the reaction, but a thermal initiator is necessary to sustain the reaction) [9,10]. The necessary and sufficient conditions for IFP include a monomer that will dissolve the polymer seed, polymerize via free-radical polymerization, and exhibit the gel effect a thermal initiator and a viscous region in which the gel effect can occur (i.e., the seed dissolving) [6, 11]. Ideally, another necessary IFP condition is a monomer-polymer system that produces an optically clear product because most IFP products are used in optical applications. 

An IFP system containing no dopant consists of the polymer seed, monomer, and thermal initiator. These systems produce a finished product containing no GRIN and ideally possessing a homogeneous RI. These reactions have primarily been studied to provide experimental evidence in favor of the IFP mechanism [6, 11, 41], to verify the predicted results of changing the experimental parameters within the system [6, 11], and to illustrate that the front is truly isothermal [41]. 

The results discussed here contain a wealth of dynamical details of the detonation wave profiles under different conditions. In particular, they show both thermal initiation and shock initiation of dissociation reactions, as well as the coupling of the reactions front, the shock front, and the thermoelastic properties of the lattice, all under highly nonequilibrium conditions. It is true that our hypothetical molecular model and the simulation of the "chemistry" of dissociation are too simple and perhaps simplistic. Nevertheless, because we were able to demonstrate by separate tests [36,37] that this model system was well behaved, we believe that many of the details, especially those relating to the mechanisms and rates of energy transfer and energy sharing, should have their counterparts in reality. As we further develop our techniques of modeling chemical reactions, we should be able to apply the MD method to the study of these details which are not easily accessible by any other method. 

Heat Pulse. (Also see Detonation, Flash-Across, Heat Pulse and Hypervelocity Phenomena in Vol 4, p D348-49). A concept advanced by M.A. Cook (Refs 1 2) to provide a theoretical mechanism for the shock initiation of explosives. Cook also used the heat pulse concept in his explanation of certain unusual luminosity effects observed primarily in the detonation of liquid explosives. Briefly stated, Cook believes that detonation is initiated when as a result of rising temperature, produced by reaction in the already shocked region of an explosive, a portion of the explosive becomes thermally super-conductive and a heat-pulse flashes thru it and catches up with the shock front. Studies conducted by Kendrew Whitbread (Ref 3) tend to discount the necessity for postulating a heat-pulse in a theoretical explanation of shock initiation or the above unusual luminosity effects. More recent studies of shock initiation have also failed to produce any conclusive evidence of a heat-pulse ... 

The most favorable conditions for reactive processing of monolithic articles are created when the frontal reaction occurs at a plane thermal front. For example, a frontal process can be used for methyl methacrylate polymerization at high pressure (up to 500 MPa) in the presence of free-radical initiators. The reaction is initiated by an initial or continuous local increase in temperature of the reactive mass in a stationary mold, or in a reactor if the monomer is moving through a reactor. The main method of controlling the reaction rate and maintaining stability is by varying the temperature of the reactive mass.252... 

Using the experimental values for the width of the traveling wave front (portion be, Fig. 8), let us estimate the propagation velocity for the case of a thermal mechanism based on the Arrhenius law of heat evolution from the known relationship U = a/d, where a 10"2 cm2/s is the thermal conductivity determined by the conventional technique. We obtain 5 x 10"2 and 3 x 10-2cm/s for 77 and 4.2 K, respectively, which are below the experimental values by about 1.5-2 orders of magnitude. This result is further definite evidence for the nonthermal nature of the propagation mechanism of a low-temperature reaction initiated by brittle fracture of the irradiated reactant sample. 

Detonation is a chemical reaction given by an explosive substance in which produces a shock wave. High temperature and pressure gradients are generated in the wave front, so that the chemical reaction is initiated instantaneously. Detonation velocities lie in the approximate range of 1500 to 9000 m/s = 5000 to 30000 ft/s slower explosive reactions, which are propagated by thermal conduction and radiation, are known as -> Deflagration. 

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