Bilateral estimates of thermal conductivity characteristics of anisotropic heat-shielding materials


Аuthors

Zarubin V. S.*, Zimin V. N.**, Leonov V. V.***, Zarubin V. S.

Bauman Moscow State Technical University, MSTU, 5, bldg. 1, 2-nd Baumanskaya str., Moscow, 105005, Russia

*e-mail: zarubin@bmstu.ru
**e-mail: zimin@bmstu.ru
***e-mail: lv-05@mail.ru

Abstract

The re-entry spacecraft heat-shielding coating surface is subjected to the uneven heating. For the temperature reduction on the surface areas, experiencing the most intense heating, it is advisable to employ an anisotropic heat-protective coating material. Such material layer has significantly larger thermal conductivity coefficient in the tangential direction compared to the normal direction to the layer surface. This allows removing a part of thermal energy, received by the most thermally stressed surface areas, in tangential direction. This possibility is up-to-date in the first place for the re-entry spacecraft while passing through the dense atmosphere.

The presence of blunting made from thermal protective material is characteristic for the front part of the re-entering spacecraft. The external blunting surface is bounded, as usual, by the fragment of a spherical surface. The highest density of the heat flux delivered to this surface in the laminar flow mode corresponds to the front critical point. It is located at the intersection of the surface by the flight direction line passing through the center of the spherical surface. With the turbulent flow-around mode, the region of the increased heat flux density shifts to the socalled sonic point. Within the vicinity of this point outside the boundary layer, the flow velocity reaches the value of the local speed of sound.

Employing the anisotropic material of a heat-shielding coating with higher thermal conductivity in each tangential direction compared to the normal direction to the surface allows easing the intensive local thermal flow delivery impact on the coating operability. This effect is associated with redistribution in tangential direction of the considerable portion of the thermal energy delivered to the coating surface. With a high degree of anisotropy of the heat-shielding material, characterized by the ratio of its thermal conductivity coefficients in the tangential and normal directions to the surface, it is possible to prevent the destruction process of such material or (at least) to reduce this process intensity.

The majority of the materials being employed for the heat protective coatings of modern air-craft are isotropic relative to the thermal conductivity. Implementation of the highly thermal conductive inclusions of various shape, oriented in such materials in a certain manner relative to the normal to the coating surface, may ensure sufficiently high degree of thermal conductivity anisotropy of the resulting A composite. Carbon fibers, pyrographite fragments, nanotubes, or graphene fragments can be used as such inclusions. The specific inclusion type selection and its voluminous concentration in the thermal protective material should rely on preliminary estimations of the effect being expected. Such estimates reliability depends upon the fidelity of the initial information on the heat conductivity of the basic material, hosting the composite matrix and inclusions possible for implementation, as well as correctness of the thermal interaction model of the matrix and inclusions used as a basis of the estimation technique being employed.

This article presents a dual variation model of the thermal conductivity process in anisotropic composite material consisting of an isotropic matrix and anisotropic high thermal conductivity inclusions. This model was used for developing the two-side (from the top and bottom) esteems of the effective thermal conductivity characteristics of such materials. The article presents the example of quantitative esteems obtaining of the dependence of thermal conductivity anisotropy degree of the transversal isotropic composite with inclusions of pyrographite into isotropic matrix from glassy carbon on the temperature.

Keywords:

anisotropic cooling material, isotropic composite matrix, high heat-conducting inclusions, effective thermal conductivity coefficients, transversally isotropic composite

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