Radiation properties of refractory materials

Abstract

Refractory materials (refractories) are products made on the basis of mineral raw materials, and are widely used as construction linings of industrial furnaces and a whole range of other thermal-energy units. In conditions of high-temperature heating the refractory lining not only delineates the working space, but also takes a direct part in the processes of heat exchange, and above all in such processes through radiation, which as a rule are the dominant processes. Therefore, the effectiveness of the work of industrial furnaces, their reliability and economic effectiveness largely depend on the proper choice of refractory materials and a knowledge of their reflective and radiating capacities (radiation characteristics). The study of the properties of the thermal radiation of refractories is extremely important, both technologically and from the viewpoint of speeding up heating processes and the rational use of energy [1][2][3]. All this leads to the need for a detailed investigation of the radiation properties of materials.Refractory materials can be classified into several groups. The main ones are siliceous, aluminosilicate, magnesia, chrome-magnesite, high-alumina, zirconia, and oxide materials [4]. In Soviet scientific literature several reviews have appeared dealing with the measurement of the properties of thermal radiation of refractories. Noteworthy are the reviews and summaries given in the handbook under the editorship of A. E. Sheindlin [5], in the work of Landa and Glazachev [6], and in the factual data [7]. The reviews cover a wide range of experimental works and so we shall concentrate mainly on investigations which for one reason or another have not found the necessary reflection in publications [5-7]~ Of great practical importance is a study of the influence of prolonged industrial use of articles on their radiation properties. These questions are examined in [8][9][10], in which measurements were made of the spectral normal radiation capacity SAn of high-alumina and chamotte (fireclay) refractories with different services periods in metallurgical furnaces.Measurements were made by comparing the radiation of specimens and a model of an absolutely black body, which was used in the form of a tubular resistance furnace. The design of the apparatus was described in detail in [ii]. Heating was done by using flat specimens, tightly pressed to a metal plate made of nichrome or molybdenum, coated with molybdenum disilicide, through which an electric current was passed. The temperature of the refractory materials was measured with two chromel-alumel thermocouples which were placed in special channels 0.6 and 1.2 mm deep, and attached to the specimen with a refractory cement. The temperature of the working (radiating) surface was determined by extrapolation to the zero depth of the reading of the thermocouple. Investigations were made in the range 973-1273 K and a wavelength of 0.95-15 ~m in air. The experimental error is assessed at 8-10%.It is established (Fig. i) that the considerable change in the spec...