Thermal conductivity of refractories in various media

Abstract

The need in electric furnace construction for data on thermal conductivity of refractory materials in various media, including vacuum, inert, and carbon-and hydrogen-containing media with different concentrations of the components, has made it necessary to develop a calculation method for determination of thermal conductivity making it possible to reliably calculate it for any composition of medium without making laborious experimental investigations.Many different approaches to obtaining methods of calculation of the thermal conductivity of porous materials are known [i]. This article develops a semiempirical approach which earlier made it possible to obtain a satisfactory description of the thermal conductivity of free-flowing materials under arbitrary operating conditions [2] and a description of the thermal conductivity of certain solid porous materials at low temperatures [3]. Such an approach with the use of a minimum of experimental data on the thermal conductivity of a number of aluminosilicate refractory materials produced in the Czechoslovakian SSR and the USSR in hydrogen and vacuum made it possible to develop a method of calculation of their thermal conductivity under arbitrary temperature, pressure, and gaseous-medium operating condit ions.The basis of description of the thermal conductivity of any composite materials, particularly refractories, is the existence of a characteristic relationship relating the thermal conductivity of the material %m to the thermal conductivities of the solid skeleton %so and of the gas in the pores %g [4]:;~so It is assumed that relationship (la) covers description of the effective thermal conductivity of a material %ef under arbitrary operating conditions: Xso< when Xef is determined by the thermal conductivity in a pore Xpo, taking into consideration the radiation thermal conductivity X r and the temperature jump expressed by the coefficient [5,6] according to the relationshipThe accepted identities of the functional relationships for X m and Xef assume that the equations describing the coefficient of the temperature jump on the walls of the void and the radiation thermal conductivity in the void are obtained by generalization of experimental data with the use of the same characteristic relationship (la).For the function f is taken the description given in [3] and showing good agreement with the experimental characteristic relationships of a large group of porous ceramic materials:[----(l--p)Kn Lp 'with the condition that the functional K is approximated by the expression l--Klo/n IK \ l/n ]K~n--1