Thermal inhomogeneities

The CTM is capable of reducing nerve for problem compounds such as those based on natural rubber, by ironing out thermal inhomogeneities and overcoming elastic memory. However, it is not self-cleaning, it generates some additional heat, it does not have forward conveying ability and it can also create back pressure. 

FED (3)] and viscous dissipation in the molten regions. As melting progresses the latter mechanism becomes dominant. Mixing disperses the newly formed melt into the mass [creating a solids-rich suspension] the melt that comes in intimate contact with solid particles cools down and at the same time heats up the surface layer of the particles the particulate solid charge is eventually converted into a richer, thermally inhomogeneous suspension and ultimately into a... 

On scaling-up the equipment, the surface-to-volume ratio decreases, which limits the heat removal in production machines. Moreover, in production-size machines thermal inhomogeneities may occur that do not exist in laboratory equipment This requires a careful design of the experiments on laboratory scale in order to assure a reliable scale-up procedure. 

A special complication is formed by thermal inhomogeneities. The polymer follows a circular path in the cross-channel direction, which implies that only a certain amount of polymer passes the thermal boundary layer where it is cooled directly. Therefore, for deep cut channels the heat of reaction in the middle of the channel is difficult to remove. Janeschitz Kriegl [18] compared the thickness of the thermal boundary layer with the channel depth, and identified a criterion for thermal homogeneity in a single-screw extruder. This can be adapted to twin-screw extruders with m thread starts per screw. It may be concluded that thermal inhomogeneities become important if ... 

This increases the risk of loss of product quality due to a decrease in the molar mass, thermal inhomogeneities and melting problems. The extruder and screw concepts need to be constantly optimized and adapted to avoid quality loss. 

At a depth of 500 m (and lower), the seasonal differences in the climatic temperature and salinity fields are statistically indistinguishable therefore, in Fig. lie, only the mean annual salinity field is presented. At this level, the temperature field is especially homogeneous (and, therefore, it is not shown) because of the existence of the intermediate isothermal layer in this depth range (see Sect. 2), in which vertical water motions produce no thermal inhomogeneities. 

Nonsteady-state (nonstationary) systems with regular fluxes homogeneous, thermally inhomogeneous ... 

It should be noted that low levels of birefringenee ean be induced by mechanical stresses in materials that are not birefringent in their perfect crystalline state/ Such stresses are typically created if there are large thermal inhomogeneities in the furnaces in which the crystal of fused silica are grown/ ... 

A polydispersity of about 2 is typical of high molecular weight condensation polymers. A polydispersity of 1.5-2.0 is typical of the instantaneous molecular weight distribution of a free-radical polymer, but the composite distribution from a moderate- to high-conversion reactor will often be broader than this due to thermal inhomogeneities. Coordination catalysis produces very broad distributions, while some low-temperature, ionic polymerizations can give nearly monodisperse polymers (see Peebles [1] for a comprehensive treatment of molecular weight distributions). 

Rotational Speed (rpm) Above Which Thermal Inhomogeneities Can... 

In large extruders thermal inhomogeneities can occur, where the temperature in the center of the channel can be much higher than near the barrel walls. This effect increases with an increasing scale and can be detrimental for scaling up. Therefore, reactive extrusion often requires a special experiment design as will be dealt with in Chapter 12. 

It can be seen that thermal inhomogeneities increase strongly for larger-scale extruders, where the channels are deeper cut. This is especially the case for... 

For process development, it is often important to scale the prospected process down to lab experiments. A dimensionless number can be defined that indicates whether large thermal inhomogeneities may be expected. Especially in reactive extrusion, neglecting the importance of this number by using too small extruders will inevitably lead to erroneous results and problems in the large-scale equipment. 

To measure the spectra of cold, biomolecular ions, one must first produce them in the gas phase and cool them to low temperatures to eliminate thermal inhomogeneous broadening. Because of their net charge, one can use electric or magnetic fields to trap the ions in space before spectroscopic interrogation. To obtain a spectrum one has to detect the absorption of light, and because the density of ions is low, it is extremely difficult to do so directly. One typically uses some type of action spectroscopy in which the consequences of light absorption are detected... 

As was estimated in Ref. [25] only very small isotherm deviations from hnearity of 1-2 K are enough to reach the critical resolved shear stress (CRSS) for dislocation multiplication in GaAs crystals at high temperatures ( 0.5 MPa). In the production of compound semiconductor crystals of diameters 100 and 150 mm it has been not been possible to reduce the thermal inhomogeneities and the related thermal-induced stress to a sufficiently low level at aU process phases in... 

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