Volume 2, Issue 1 (6-2018)
Abstract
In human-made materials, aerogels have the least thermal conduction coefficient. The least thermal conduction, which aerogels can have, is equivalent to air conduction, about 0.021W/mK. In some applications, lowest conductions is to be needed. In this study, novolac aerogles are used as spacer, for designing and making multilayer super insulators with aluminum foil reflective layers. The performance of these insulators, are extremely depends on density, porosity and the size of pores in aerogel and number of layers in the overall thickness of insulator. In this study, for decreasing effective thermal conduction of multilayer insulations, tow parameters of density of aerogel and ratio of number of layers to thickness of insulator (layer density), are examined in 25 ˚C to 200 ˚C boundary conditions. First, by assessment of aerogel novolac density effect on thermal conduction, aerogel with density of 0.076 g/cm3 was chosen as the best spacer. In the next part, ratio of 25 layer per centimeter of thickness was chosen as the best layer density. Finally, by making multilayer super insulators, based on this results, an insulator with 5×10-4W/mK effective thermal conductivity was obtaind without evacuation of spacer.
S. Sabbagh-Yazdi, M. Jamshidi,
Volume 20, Issue 5 (5-2020)
Abstract
Galloping is a large-amplitude, low frequency, wind-induced oscillation of overhead power transmission lines with one or multi loops of standing waves per span which occurs due to wind flow. Based on the field data, numerous galloping oscillations occurs in the form of one loop oscillation which whereby high dynamic loads are imported to the support structures. In this research, the results of wind tunnel tests have been performed on a two-span distorted scale model with an ice-accreted cross-section under uniform and non-uniform aerodynamic loadings. Dead-end and suspension insulators have been applied to the support points. Then, based on identifying the most critical state of the lines oscillation, a solution has been proposed based on increasing their bending strength through the application of hardening local covering. The results showed that the most critical state of the cables oscillation in the galloping is related to the one-loop oscillation, which occurs as a result of interactions between the cables of adjacent spans under uneven aerodynamic loading and the use of suspended insulators, and the dynamic forces applied to the supports are about 20% more than the case when the cables oscillate due to the dead-end connections attached to the support structure. Also, applying the local covering with a length of 20% of cable span leads to a 27% reduction in dynamic support reaction of one-loop galloping.
