CONDUCTIVE-CONVECTIVE HEAT TRANSFER IN THIN-FILM THERMAL INSULATION

The relevance of the study is explained by the fact that thermal protection of equipment and pipelines is important when implementing energy-saving measures at energy facilities for various purposes. The increase in heat or cold losses during transportation of energy carriers is the reason for development of new approaches to energy-saving measures when performing thermal insulation actions. The main method of reducing heat energy losses during its transportation and storage is the use of highly efficient thermal insulation materials. This material is thin-film thermal insulation. The unique thermal characteristics of thin-film thermal insulation coatings are the reason for their use in various energy systems and equipment. Despite this, the technology of using thin-film insulating coatings has not been developed. This is due to the following reasons: lack of knowledge about the physical properties and mechanisms of heat and mass transfer in thin-film thermal insulation coatings. The main aim of the research is to study the conductive-convective heat transfer in the layer of thin-film thermal insulation taking into account the heterogeneity of the properties of the microspheres and binders. The object of the research is a cylindrical layer of thin-film insulating coating. The temperature is constant on the inner and outer surfaces of the thermal insulation coating. The geometry of the thin-film thermal insulation coating was a binder and hollow microspheres. Scientific research was carried out for a layer of insulation thickness of 0,33 mm. The temperatures on the inner and outer surfaces of the insulation were taken in accordance with the experimental data. The layer of thin-film thermal insulation for 62 % consists of microspheres with a diameter of 50 microns and for 38 % from binder. Two types of hollow microspheres with wall thicknesses (5 and 2 mu m) were considered. Methods. The solution of the problems posed is obtained by the finite element method. Galerkin approximation, non-uniform finite element mesh was used. The parameters of the grid elements were chosen from the conditions of convergence of the solution. An increase in the number of elements of the computational grid was carried out using the Delaunay method. Results. The author has investigated the effect of the type of binder and the characteristics of the microspheres, the wall thickness of the microsphere and the gas phase contained in the cavity of the microsphere on heat loss. For the case under consideration, the deviation from the experimental data was up to 90 %, depending on the composition of the thin-film thermal insulation coating. Comparison of the results of numerical simulation of heat transfer in a layer of thin-film thermal insulation, performed using a conductive-convective heat transfer model with the results for a conductive model showed a discrepancy between them does not exceed 3 %. This is due to the errors in numerical calculations. For this reason, a simpler conductive model of heat transfer can be used in practical calculations.