Design studies on the reuseable surface insulation (RSI) systems for the Space Shuttle require accurate data on the thermal conductivity of ceramic-fiber insulations. Well-established laboratory techniques, such as the guarded hot plate experiment, are available for such measurements; however, caution must be exercised in applying laboratory data to widely different re-entry thermal environments. The heat-transfer associated with infrared radiation transmitted through the fibrous materials must be considered. Various analytical models have been developed to account for this radiant heat transfer, but their verification has often been incomplete. A new approach has now been applied in which infrared transmission measurements were made with an optical system that collected all the scattered radiation transmitted through the back surface of fibrous samples. Experimental data were analyzed using a “two-flux” model for the radiation scattered and absorbed in the materials and excellent correlation was obtained between theory and experiment. To account for the wavelength dependence of the scattering cross-sections of many materials, a “four-flux” model was developed. This model was used to compute the contribution of the radiation transmission to the total thermal conductivity of samples measured on the guarded hot plate equipment. The contribution was shown to be as high as 75 percent of the total conductivity at temperatures above 1000 K.
The significance of this result is that data obtained from such instruments should not be used in heat-transfer design computations without regard to the radiation transmission. Such regard will be particularly important in the thermostructural analyses of RSI for the Space Shuttle and other re-entry vehicles.
When an alternating magnetic field is applied to a type I1 superconducting material the response of the sample depends on the amplitude of the field H When Hmax is less than H is essentially diamagnetic but small surface losses are often observed (1). A s field penetration into the bulk takes place in the max' the lower critical field of the material, the sample cl' is increased above H Hmax cl' form of flwoids, for at least part of the cycle, and the movement of these in the material gives rise to dissipation (2).In view of the abrupt change in the mode of flux penetration which occurs at it is surprising that no inflexion has been observed in the ac dissipation-field H curves. Easson (3) has suggested that the transition at H surface losses, but while these undoubtedly contribute we believe bulk properties to be equally important.cl is masked by large cl Ac loss measurements have been determined from the voltage induced in a pickup coil wound directly on the cylindrical samples. A copper solenoid, cooled with liquid nitrogen, was used to apply large alternating fields parallel to the sample surface. The time average of the product of the pickup coil voltage and a field dependent signal, was obtained with a small analogue computer, thus providing a measure of the dissipation (4). In most of our measurements the dissipation per cycle, Ws (Jcm ), was found to be independent of frequency and only these values were used in our results.
-2In Fig. 1 results are presented for two niobium samples, one from heavily worked, polycrystalline rod and the other from an electron-beam zone-refined single crystal, grown from similar rod. The surfaces of both specimens were polished, the single crystal by an electrolytic method and the polycrystal mechanically t o a l pmfinish.The two outstanding features of the curves to be noted are:
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