Compound Dynamic Instability

Compound dynamic instabilities as secondary phenomena. Pressure-drop oscillations are triggered by a static instability phenomenon. They occur in systems that have a compressible volume upsteam of, or within, the heated section. Maul-betsch and Griffith (1965, 1967), in their study of instabilities in subcooled boiling water, found that the instability was associated with operation on the negative-sloping portion of the pressure drop-versus-flow curve. Pressure drop oscillations were predicted by an analysis (discussed in the next section), but because of the... 

Computer codes Because of the computer s ability to handle the complicated mathematics, most of the compounded and feedback effects are built into computer codes for analyzing dynamic instabilities. Most of these codes can analyze one or more of the following instabilities density wave instability, compound dynamic instabilities such as BWR instability and parallel-channel instability, and pressure drop oscillations. 

PDOs are categorized as compound dynamic instability and occur as secondary phenomenon triggered by stahc instability. PDO occurs in a system having compressible volume upstream or within the heated section and when the system operates in the negative slope region of the pressure drop versus flow rate curve. 

Compound dynamic instabilities Thermal oscillations Interaction of variable heat transfer coefficient Occurs in film boiling... 

Compound dynamic instability as Pressure drop Flow excursion initiates dynamic interaction Very-low—frequency periodic... 

As already mentioned, also for the other oxygenated Cl compounds, i.e. formaldehyde [118, 138-147] and methanol [148-154], as well as for larger organic molecules, dynamic instabilities are reported. Many of them are compiled in Ref. [154], for formaldehyde oxidation on Rh and Pt [147] and methanol oxidation on Pt [155] the oscillations could be clearly identified as HN-NDR type oscillations. However, in view of the number of reaction steps involved in these oxidation reactions and of the possible complexity of the interaction of the supporting electrolyte with the dynamics even in the much simpler formic acid oxidation, it is not astonishing that any quantitative considerations should still be missing. There are some attempts to qualitatively explain the observed phenomena with reaction mechanisms that go beyond the simple dual-path model described above. However, at the time being, they are quite speculative. Therefore I shall not discuss them in more detail in this article. A summary of these works can be found in [156],... 

Several limitations and shortcomings are associated with the use of micelles as the pseudosta-tionary phase. Besides the irreversible incorporation of very hydrophobic compounds within the micelles, other analytes snch as proteins may strongly interact with the free molecules of the surfactant in solntion. Moreover, the significant influence of operational parameters such as temperature, pH, and composition of the BGE on the dynamic aggregation of the surfactant molecules may result in instable micelles and consequent irreproducibility. 

Although an unsuccessful optical resolution21 of ( )-cyclodecene suggested its optical instability, Robert s dynamic NMR studies 22) of the racemization process in deuterated ( )-cycloalkenes 13 succeeded in providing a t1/2 value of 104 sec (room temperature). This optical instability, found in the parent compound, could explain Westen s fruitless attempt to prepare the chiral ( )-cyclodecenone 16 from the (+)-methanesulfonate 15 23). 

Another reason that isothermal heating methods are used in the initial screen is to identify materials that have time dependent thermal stability. These materials have a thermal decomposition that does not follow a simple Arrhenius relationship in which the reaction rate increases exponentially with an increase in temperature. Instead an extended induction period is required before the decomposition becomes detectable. An example of this behavior is shown in Figure 2. The DTA isothermal test recorder traces of methane sulfonic acid, 3,7-dimethyloctyl ester at different test temperatures are shown. The induction time varies from less than 1 hr. at 180 C to 46 hr. at 130 C. As with this compound, it is not unusual that once decomposition is detected it proceeds very rapidly, releasing all of the heat in a short period of time. Dynamic heating methods do not indicate if this type of thermal instability is present if it is, the initial detection temperature from dynamic tests will be grossly misleading as to the thermal stability of the material. 

Because of the isomorphous structures of the four compounds and their phase instabilities, they are an interesting set of compounds for detailed lattice-dynamic calculations. However, despite their relative simplicity with respect to other metal azides, their structure, with eight atoms per primitive unit cell, presents a formidable calculational problem. With compounds of this complexity it is imperative that dispersion-curve data be available to test lattice-dynamic models, and, thus far, this has been possible only for KN3. 

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