Factors on the Measurement of Effective Thermal Diffusivity of Molten Slag Using Double Hot Thermocouple Technique.

Abstract

The molten slags that are used not only in the continuous caster but also in every metal industry play an important role and affect the quality of products. The authors initially developed the double hot thermocouple technique (DHTT) for in situ observation of mold slag crystallization.In this study, the DHTT was further developed to allow the measurement of the overall thermal diffusivity of molten slag applying the principle of the laser flash method. The affecting factors (finite pulse time, shape of pulse and heat loss from sample surface) on the measurement of thermal diffusivity using the DHTT were discussed theoretically using both the analytical and the numerical methods. New relationship between the thermal diffusivity a and the time at half-maximum temperature t 0.5 was obtained as follows:a(ϫ10 4 m 2 /s)ϭ0.000707(t 0.5 /t p ) Ϫ1.8946 The thermal diffusivity obtained from the experimental half-maximum time t 0.5 /t p (t p is the time of peak on the heat pulse) was in good agreement with the one from literature.KEY WORDS: thermal diffusivity; thermal conductivity; slag melt; laser flash; hot thermocouple; DHTT.DHTT and the laser flash method. The laser flash method is an excellent method and can predict a relatively suitable value of thermal diffusivity using Parker's method 4) (Eq. (3)). However, it will need a special alignment for the measurement of liquid and/or transparent material. Furthermore, a direct observation during measurement is almost impossible. On the other hand, the DHTT can observe the molten sample in situ. As shown in the previous study, 1,2) the solidification of mold slag is complicate phenomena including the bubble formation/breaking and the crystallization. It would be very important to know the sample image during measurement. The thermal diffusivity obtained from DHTT, however, is not yet established and it is not reported what kinds of factors affected on the measurement until now.For the preliminary approach to the application of the DHTT to the measurement of thermal diffusivity, the affecting factors on the measurement were elucidated from both the theoretical and the experimental method. In the case of laser flash method, an instantaneous laser pulse was used as a heat source and the temperature increase at the rear surface was measured by an infrared pyrometer (IR detector). In the case of DHTT, the temperature of one side thermocouple (CH-1, in Fig. 1) increased in a pulse shape and the other side of thermocouple (CH-2) was used as a detector. The typical temperature profiles of both CH-1 and CH-2 are shown in Fig. 2. The time t 0.5 at the half-maximum temperature (1/2 DT max ) was defined as shown in Fig. 2 and used for the calculation of thermal diffusivity according to the laser flash method (the details will be shown in below). The large differences between the laser flash method and DHTT are as follows: (1) Heat source is the order of several seconds (millisecond order in laser flash method). (2) Shape of pulse is not square. This factor might be decisive in bo...

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