Thermal conductivity of vacuum-screen heat insulation

Abstract

Experimental data for the thermal conductivity parallel to the layers of vacuum-screen insulation at various temperatures and vacua of 760-5.10 -7 mm Hg are given. The anisotropy of the thermal conductivity is determined.Most investigations of the thermal conductivity of vacuum-screen (multilayer) heat insulation (VSHI) have been concerned with the thermal conductivity normal to the layers [1][2][3][4][5]. In most cases the investigators have determined some effective thermal conductivity, the value of which gives an idea of the mechanism of heat transfer in the laminated VSHI system. The engineering use of VSHI in various installations and the calculation of heat fluxes, however, necessitate a fuller understanding of the mechanism of heat transfer in this system and a determination of specific values of the thermal conductivity in a wide range of vacua. We believe it worthwhile to consider the mechanism of heat transfer by thermal conduction in VSHI normal to (k• and along (parallel to) the layers {klt ).According to the hypotheses of various authors, the thermal conductivity along the layers in VSHI may differ from the thermal conductivity normal to the layers by a factor of hundreds. This indicates considerable anisotropy of the thermal conductivity of a system such as VSHI.At present we have no data on thermal conductivity along VSHI layers. In the best case we have information about the thermal conductivity along a single layer of insulation, determined by the square method without inclusion of the radiative component due to reemission along the layers [1].An attempt was made in [7] to evaluate the thermal conductivity parallel to insulating layers from the thermal conductivity along layers of a cylindrical specimen of such insulation. For these tests the authors adapted a low-temperature thermal conductivity apparatus and obtained data for the thermal conductivity of Mylar superinsulation at a pressure of 10 -7 torr.In the present paper we describe an apparatus which can be used to determine the thermal conductivity along the layers (hll) of vacuum-screen insulation in the pressure range from 10 -7 to 10 -1 mm Hg for several boundary temperatures. In the light of the obtained results for the thermal conductivity along the layers (hll) and data for the thermal conductivity normal to the layers (~• [8], we discuss the mechanism of heat transfer in the laminated VSHI system. EXPERIMENTAL METHODThe effective thermal conductivity along the layers (}'ID of multilayer insulation was determined from the thermal conductivity of a cylindrical specimen formed by rolling up a piece of the investigated multilayer insulation. We used the steady-state method for a infinite plane layer with the known expression for the thermal conductivity ~,=__gQ .~ w/m.a~g.(1) t 1 --t 2 F