Hot-zone model

The main respects in which the hot-zone model differs from the other models previously described will be considered next. 

The hot-zone model differs from the random-fragmentation model in an important respect. The random-fragmentation model is based upon free radical recombinations. The authors model suggests that in view of the relatively low concentration of free radicals present in the hot zone, chemical reactions with the parent species may also play an important part. The picture may be further complicated by ion-molecule reactions (71). 

G. Harbottle and N. Sutin USA Hot zone model in solid systems... 

Azone model calculatesthe fire environment by dividing each compartment in the model into two homogeneous zones. One zone is an upper hot smoke zone that contains the fire products. The other zone is a lower, relatively smoke-free zone that is cooler than the hot zone. The vertical relationship between the zones changes as the fire develops, usually via expansion of the upper zone. The zone approach evolved from observations of such layers in full-scale fire experiments. While these experiments show some variation in conditions within the zones, the variations are most often small compared to the difference between the zones themselves. 

Of a more complete approach are the zone models [3], which consider two (or more) distinct horizontal layers filling the compartment, each of which is assumed to be spatially uniform in temperature, pressure, and species concentrations, as determined by simplified transient conservation equations for mass, species, and energy. The hot gases tend to form an upper layer and the ambient air stays in the lower layers. A fire in the enclosure is treated as a pump of mass and energy from the lower layer to the upper layer. As energy and mass are pumped into the upper layer, its volume increases, causing the interface between the layers to move toward the floor. Mass transfer between the compartments can also occur by means of vents such as doorways and windows. Heat transfer in the model occurs due to conduction to the various surfaces in the room. In addition, heat transfer can be included by radiative exchange between the upper and lower layers, and between the layers and the surfaces of the room. 

The concept of chemical reactions during the lifetime of the hot zone, which is an important feature of the authors model, is similar to the epithermal-reaction concept proposed for liquid systems by Miller and Dodson. However, the latter authors did not attempt to estimate such parameters as the size, temperature, and lifetime of the hot zone. 

Summarizing the observations on the ionic crystals it may be said that there is abundant evidence that reactions in the hot zone play an important part. Thus, any theory, such as the elastic-collision model which neglects specific chemical effects, e.g., reduction by NH4+ and H2O, or oxidation by C104, under the influence of the high local temperature," cannot ve a complete explanation of the data. In none of the studies of the hot-atom chemistry of a series of oxyanion salts, e.g., the permanganates, have correlations been established between the retention and... 

The feeder and injector produced a thin pencil-like p.c. stream which passed down through the hot zone. The total combustion air supplied was approximately 3 liters/min for the bituminous coals, giving between 10 and 25 percent excess air for p.c. feed rates of 0.24 to 0.28 g/min. The flow and heat transfer conditions were modeled using the methods described by Pigford (16) for conditions of superimposed natural and forced convection at very low mass flow rates. Particle residence times were calculated by summing the centerline gas velocity and terminal velocity using Stokes s law (17). The error introduced using this method should never have exceeded 10 percent, even when pyrite was tested and particle Reynold s numbers approached one. The residence times thus calculated were found to be between one and two seconds. 

Computations based on the extended 3D model of the hot zone have shown, in particular, that the distribution of the temperature and of the heat at the melt/cmcible and the crystal/melt boundaries are essentially nonstationary and greatly deviate from the axisymmetric ones [37, 38]. Note that strong spatial and temporal nonuniformity of the heat flux at the crystal/melt interface (and, hence, of the growth rate) is probably a cause of striations in the resistivity and the oxygen level in the grown crystals. 

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