Hot-spot phenomenon

Cavity design is an important factor in the control, or the utilization of this hot spots phenomenon. 

In parallel with the development of high-activity catalysts, researchers are studying other types of reactors that would prevent the hot-spot phenomenon associated with the current fixed-bed reactor and/or increase the single-pass conversion. These include fluidized-bed, recirculating fluidized-bed, slurry, trickle-bed, gas-solid-solid trickle-flow, and liquid-phase reactors. Complete single-pass conversion has been demonstrated using continuous methanol removal by Kquid or solid absorbents [18,19]. 

In addition to these bulk temperature effects, which can be readily predicted, local hot spots can develop at the point of reaction. This phenomenon is aggravated in this system because the recycle make-up... 

Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

W.A. Gey M.A. Cook (Ref 4). Their experiments in propagation of deton thru steel glass plates showed that thin plates of inert material invariably interrupt the deton wave completely, requiring the deton to re-form if it continues to propagate beyond the interrupter. A remarkable "new phenomenon, called flash-across, was observed when a bluish-white hot spot on one frame and another hot spot that developed between adjacent frames on the opposite SPHF plate had both flashed across the chge and met at the collision interface... 

In order to accurately determine the speed of the flash-across phenomenon, the experiment was repeated and recorded by streak camera with color film. Also thinner SPHF plates were used. In the streak camera trace, 8.5 (isec after each initial wave entered the NM, a hot spot appeared at the surface of each plate and flashed to the center of the chge each at the phenomenal speed of 35 mm/fisec. Cook (Ref 3) considers the flash-across phenomenon to be the heat pulse predicted by M.A. Cook, R. Keyes A.S. Filler (Ref 1)... 

Hot Spots. Combustion-Chamber Deposits) Surface ignition is the initiation of a flame front by any hot surface other than the spark discharge prior to the arrival of the normal flame front. The flame front or fronts so established propagate at normal velocities. This phenomenon can be further subdivided into preignition and postignition. 

Apart from oxidation of the lubricant and the metal surfaces, there can be complex tribo-chemical reactions. Chemical reactions at the surfaces can be stimulated by different factors. One factor is heating due to friction. This can either be a global effect (elevated mean temperature of surfaces and lubricant) or a localized phenomenon. Especially in situations where mixed or boundary lubrication exists, the direct contact of surface asperities can lead to high flash temperatures. At these hot spots temperatures in excess of 1000°C promote chemical reactions and surface melting. Other factors promoting chemical reactions are ... 

The self-sustaining decomposition is a phenomenon whereby the decomposition is initiated by a hot spot, and then propagates through to the solid with a velocity of some millimeters to centimeters per second. The decomposition does not require oxygen, so it cannot be avoided by using an inert atmosphere. 

The phenomenon of increasing combustion catalyst activity was frequently observed in literature before [64, 65]. At both levels of Pt loading, the coolant temperature at the exit was higher than the temperature of the products which was explained by hot-spot formation. 

The radial activity profile is a simple parabola-like function with a minimum in the center of the tube. As a result, for a 2.54 cm tube, the deactivation process can be simulated very accurately from the one-dimensional approximation. We can also notice that both the one-and two-dimensional models predict correctly the growing transient hot spot temperature. This effect was predicted by Blaum (3) for extreme reaction conditions and was experimentally observed by Mikus et al. (7). in a quasiadiabatic laboratory reaction. Evidently,this phenomenon can be observed also for a rather mild condition in a deactivating bed of full size. After 25 hours of deactivation,the hot spot moved from z=0.75m to z=1.65m and the temperature increased by 15°C. 

The phenomenon of impact or mechanical initiation is really thermal in nature. This was first proposed by Bowden and Yoffe (Refs. 2 and 3), who postulated that any of several mechanical mechanisms could produce heat at tiny local areas and thus raise the local temperature to the ignition point of an explosive. They referred to these tiny locally heated areas as hot spots. The mechanisms that they proposed were ... 

Intrinsic approaches focus on the detection of trace of chemicals and molecules with unique Raman spectra that are small enough to bind at SERS hot spots such as tips and crevices of metal nanoparticles. Direct measurement of small chemicals has a long history in SERS. The SERS phenomenon itself was discovered by the measurement of pyridine molecules adsorbed to a silver electrode that had been electro-chemically roughened to increase the surface area and therefore the signal [16]. 

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