Use of the hot wire method for measuring the thermal conductivity of lightweight, fiber, and powder refractory materials

Abstract

In a number of countries of the European Ceramic Union and the Council for Mutual Economic Aid the method of a linear probe of constant power, known as the hot wire method, has been adopted as the standard for determination of the thermal conductivity of refractories with % < 2 W/(m-K).At present a plan has been developed for a standard of the Council for Mutual Economic Aid including the steady method of a plate (All-Union State Standard (GOST) 12170-76) and the hot wire method (German Democratic Republic standard TC/32153-76). The higher the thermal conductivity of the sample the lower the measuring error of the first of them while the second method is the optimum for low thermal conductivity parts and materials.In the USSR the hot wire method was first used for determining the thermal conductivity of refractories by the All-Union Institute for Refractories [i]. In the Ukrainian ScientificResearch Institute for Refractories an instrument based on the hot wire method has been built for measurements of % up to 1400~ with an error of • [2]. Standardization of the instrument using certified samples of organic and quartz glasses has shown a difference from metrological data of <2%. This article considers the possibilities of the hot wire method for thermal insulation materials with different structures including lightweight, fiber, and powder refractories.* Lightweight refractories to GOST 5040-78 with low (ShL-0.4 chamotte), medium (MKRL-0.8 mullite--silica), and high (KL-I.3 corundum) thermal conductivities were used for the tests. The sample was two bricks of normal dimensions.Between the largest surfaces of the samples was placed a measuring cross of platinum--rhodium wire with a PP thermoelectric transducer welded to it. The determinations were made in a wide range of values of power (3-60 W/m) supplied to the wire.The features of the method and the equipment used are described in The absolute values and character of the temperature relationship of thermal conductivity of the investigated lightweight parts reflect features of the influence of phase composition and structure on the processes of heat transfer in dispersed systems [3]. For the most porous material, ShL-0.4, a significant increase in thermal conductivity % with an increase in temperature t, indicating a significant contribution of radiation to heat transfer at high temperatures, is characteristic.The approximation of the experimental data by a cubic parabola ,Z=0,175+0,90 9 10-m(t,+.is shown in Fig. 1 by a broken line.The thermal conductivity of ShL-0.4 samples was also measured by methods of a cylinder [4] and a plate (GOST 12170-76).In both cases two samples each were tested, one of them cut from the brick which had served as half of the sample for measurement by the hot wire method and the other from a random brick of the same lot. The reproducibility of each method for the two samples is no poorer than 5%. Both of the methods provided results agreeing with the hot wire method within the limits of instrument errors, which are • and • respectivel...

Radiation effects on transient hot-wire measurements in absorbing and emitting porous media

“…In addition, the large influence of the emissivity of the wire observed in these cases means a dependence of the measured results on the measurement arrangement bringing measured results which are not really properties of the sample. The negative deviations vanish for very large values of E. That means, we should use the hot-wire method only for measuring samples with a large enough E. The linear relationship between the measured effective thermal conductivity and the heating power per unit length observed in experiments [13] are confirmed by calculated results of this work, as shown in Fig. 7.…”