Thermal conductivity determination

Conduction is heat transfer through a solid nonporous barrier when a temperature difference exists across the barrier. The thermal transfer capability of the specific barrier or wall material, known as thermal conductivity, determines the temperature gradient that will exist through the material. 

Using the experimental values for the width of the traveling wave front (portion be, Fig. 8), let us estimate the propagation velocity for the case of a thermal mechanism based on the Arrhenius law of heat evolution from the known relationship U = a/d, where a 10"2 cm2/s is the thermal conductivity determined by the conventional technique. We obtain 5 x 10"2 and 3 x 10-2cm/s for 77 and 4.2 K, respectively, which are below the experimental values by about 1.5-2 orders of magnitude. This result is further definite evidence for the nonthermal nature of the propagation mechanism of a low-temperature reaction initiated by brittle fracture of the irradiated reactant sample. 

APPARATUS FOR THE THERMAL CONDUCTIVITY DETERMINATION OF SOME ORGANIC COOLANTS USED IN NUCLEAR REACTORS. 

EXPERIMENTAL THERMAL CONDUCTIVITY DETERMINATION OF NORMAL SATURATED HYDROCARBON VAPORS UP TO 450 DEGREES. 

Figure 23.16. Solid thermal conductivity determined for a set of carbon aerogels treated at different temperatures. The values were determined for a temperature of 300°C under vacuum [43].

Vacuum glazing performance (in terms of equivalent thermal conductivity) determined from potrai-tial U-values on the order of 0.5-0.3 W m and a thickness of the evacuated gap d—0.3... 1.0 mm) without taking into account the thickness of the glass panes. 

A laser flash technique has been used to determine the diffusivity of pyroelectric polymers such as polyvinylidene fluoride [83], whereas hot-wire techniques have been used to determine the thermal diffusivity of high-density polyethylene, low-density polyethylene propylene, and polystyrene [83], Dos Santos and coworkers [84] utilized the laser flash technique to study the effect of recycling on the thermal properties of selected polymers. Thermal diffusivity expresses how fast heat propagates across a bulk material, and thermal conductivity determines the woiking temperature levels of a material. Hence, it is possible to assert that those properties are important if a polymer is used as an insulator, and also if it is used in applications in which heat transfer is desirable. Five sets of virgin and recycled commercial polymers widely used in many applications (including food wrapping) were selected for this study. 

The economics of processing depend on the thermal characteristics of the material. The thermal properties of PP are compared with some other plastics in Table 23. The heat transfer requirement for cooling from the melt temperature to mould temperature in the case of PP is much higher than those in the case of amorphous polymers such as ABS and PS. Hence, the processing of PP is costly. In addition, thermal conductivity determines the cooling time of the material in the mould. It can be seen that the thermal conductivity of PP is less than HOPE. It would require more cooling time, and hence, a slower production rate. 

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