Hot spot-temperature

For busbars and busbar connections the highest temperature rise will not exceed the hot spot temperature rise as recommended in Table 14.5. 

In nonreactive materials, regions of elevated temperature, or hot spots, have an influence on material strength. In solid explosives this is also true, but the additional effeet is to start exothermal ehemieal reaetions which then lead to detonation. The hot-spot temperatures generated in typical secondary explo-... 

The fact that hot spots are required for explosive initiation can be seen by calculating for the bulk temperature, say 350 K, and the anticipated hot-spot temperature, say 700 K. We take typical values of Arrhenius constants for secondary explosives QjCp 2500 K, //c = 25,(X)0 K, and V = 10 s V Hence... 

As particle size decreases, hydrogen leakage decreases and hot spot temperature in the bed is higher. Thus the smaller particle size has greater activity (see Table VI). A kinetic system which defines the reaction in terms of CO and C02 methanation and CO shift conversion was used to determine the activity (see last column of Table VI). 

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

It is suggested that the objection can be overcome if initiation temperature is inde-. pendent of the size and duration of the hot spot and if these latter quantities are important only as they influence the hot spot temperature. In these spots, at least, the temperature in the shock front may be high enough to initiate chemical reaction in the ordinary sense. This brings both initiation and the subsequent chemical reaction into the domain of ordinary chemical kinetics (Ref 5,p 216)... 

Impact produces hot spots, the temperatures of which are (frequently) determined by melting of the solid, being effectively buffered at the melting point. Hence, the mp frequently determines the hot-spot temperature, T0 in the adiabatic-decomposition equation 8.8, listed on p 174 of Cook. If T0 is below a certain critical value, the reaction will not be adiabatic and, owing to heat loss, may not undergo reaction build-up. But above this critical value it becomes effectively adiabatic and expln then always results after a time T. The failure of grit to sensitize an expl may, however, depend simply on the ratio of the mp of the expl to that of the grit particle. 

Other methods of making hot spot calculations are reviewed in Chap 10 of Ref 18. This Ref gives generalized, non-dimensioned curves for estimating hot spot temperatures and dimensions... 

The maximum hot spot temperature is limited by the melting point of the impacted medium, but this is the melting point at the elevated pressure of the impact and not the ordinary melting point at one atmosphere... 

However, hot-spot temperatures required for ignition of various fuels determined under different conditions agree reasonably well, as shown in Table IX. 

Experimentally measured hot-spot temperatures required for surface ignition of iso-octane and benzene in an operating engine are similar in spite of known great differences in actual surface-ignition resistance as measured by other procedures. Alquist and Male (1) and Male and Eward (57) concluded that hot-spot temperatures could not be used to characterize the surface ignition resistance of fuels. 

Recently Livengood, Toong, Rona, Taylor, and Black used an externally heated hot spot to induce surface ignition in a motored engine (53). Under the conditions of these experiments, benzene required a somewhat higher hot-spot temperature for surface ignition than iso-octane, but the difference in temperature was far less than the difference between the spontaneous ignition temperatures of the two hydrocarbons as determined in the laboratory. 

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 ... 

A theoretical and experimental study of multiplicity and transient axial profiles in adiabatic and non-adiabatic fixed bed tubular reactors has been performed. A classification of possible adiabatic operation is presented and is extended to the nonadiabatic case. The catalytic oxidation of CO occurring on a Pt/alumina catalyst has been used as a model reaction. Unlike the adiabatic operation the speed of the propagating temperature wave in a nonadiabatic bed depends on its axial position. For certain inlet CO concentration multiplicity of temperature fronts have been observed. For a downstream moving wave large fluctuation of the wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions erratic behavior of temperature profiles in the reactor has been observed. 

The reaction front was ignited at the reactor outlet and moved upstream. The hot spot temperature increased toward the reactor inlet. Decreasing the inlet temperature the reaction front moves downstream and disappears in the middle part of the reactor. Experiments and numerical simulation indicated that in long nonadiabatic reactor the ignition process does not start at the reactor outlet but inside the bed [21]. 

Investigation of the propagating fronts for nonadiabatic conditions shown that the front velocity is not constant and depends on the position of the front in the reactor [15]. For a downstream propagating front, the velocity, hot spot temperature and exit conversion exhibited an oscillatory character [7]. 

The former corresponds to low rates of deactivation,while for high deactivation rates,travelling wave deactivation occurs. For the standing wave deactivation,the hot spot temperature decreases during deactivation, while for the travelling wave deactivation, constant pattern profiles exist and the hot spot temperature increases. 

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 vinyl acetate reactor we use in Chap. 11 has been designed to be insensitive to parameter variations under normal operating conditions. The hot-spot temperature is only 162°C with an exit temperature of 159°C. It is adequate to control the exit temperature instead of the hot spot. Since multiplicity, open-loop stability, and sensitivity are of no concern for this reactor, we can focus our attention on the open-loop characteristics relevant, to the control of exit temperature with jacket cooling. 

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