The installation for measuring thermophysical characteristics of the solid materials using a flat probe method

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Institute of Physical and Technical Problems of the North

Siberian Branch of the Russian Academy of Sciences

The installation for measuring thermophysical characteristics of the solid materials using a flat probe method

Timofeev A.M., Doctor of technical sciences, chief researcher

Malyshev A.V., Candidate of technical sciences, researcher

Stepanov A.A., Candidate of technical sciences

Kirillin A.R. Leading engineer

ABSTRACT

The paper considers the method of a round flat probe of the constant power used to determine the thermal characteristics of the solids. The formulae for the calculation of thermal conductivity and thermal diffusivity are given. The measuring equipment is described and the results of the experiment are presented.

Keywords: thermal conductivity, thermal diffusivity, non-stationary method, flat probe method.

Nowadays, methods for measuring thermophysical characteristics of various solid materials, which do not require complicated technical equipment, are widely used. When using such methods it is sufficient to perform temperature measurements at certain points of the studied body and to register the power of the heater. To measure the thermophysical characteristics of solid materials, it is possible to apply the method of a round flat constant-power probe in various versions presented in the works [1-4].

This method belongs to non-stationary relative methods of monitoring thermophysical characteristics. The backbone of the method is a constant heat flux which is supplied through a round flat heater to the surfaces of two semi-bounded in the thermal sense bodies, one of which is the researched one. This heat flow consists of two flows directed to each of the bodies, while

Here the registered parameters are the excess average integral temperature of the contact spot of the heater surface depending on the time and power of the heater. According to the data obtained, the dependence of the excess average integral temperature of the contact spot on the reverse square root of the current heating time of the researched sample is plotted.

The measuring installation that implements the constant power flat probe method is shown in Fig. 1.

Fig.1. Measuring equipment.

The equipment includes a heating element - 1, two temperature sensors - 2, body with known thermal properties - 3, loading cover - 4, a test sample - 5, a connector - 6 and a computer-measuring system - 7. The heating element consists of two round plates made of copper sheets with a thickness of 0.15 mm. The man- ganin wire with a diameter of 0.1 mm is used as a heating wire. The length of the wire with a copper plate diameter of 0.03 m is 1.7 m. The wire is folded in half and glued to one of the plates, with a pre-applied thin layer of the glue. The wire is layed in a spiral of Archimedes starting from the center of the round plate. A temperature sensor is mounted on one of the surfaces of the heating element, and the other sensor is installed at a considerable distance from it, inside the body of the material with known thermal properties. Differential copper-constantan thermocouple or a pair of miniature thermistors can be used as temperature sensors. The only requirement for a thermistor is low thermal inertia. The choice of a thermistor as a temperature sensor is also justified by the fact that it is much more accurate than a thermocouple. The instrumental error of temperature measurement by platinum thermistors is ± 0.01 °С without taking into account the error of the measuring system. In our case, we used a Pt100 thin film thermometer with Wioo=1,385. The contact area of the thermistor is 5 *10^-6 m². The thermistor was mounted on the heater surface by applying a thin layer of the glue on a pre-degreased surface. The connector is used to communicate with the computer-measuring system (CMS). The AK-6.25 was used as a CMS. We have compiled the measurement algorithm and made a program for it using the Turbo Pascal v7.0 language for MS DOS v.6.22.

Results

thermophysical solid flat probe

The results of measuring the thermal characteristics of solid materials were obtained using the reference optical glass. The experimental data were processed in

MS Excel 2016, the experiment showed the average heater on the reverse root of the heating time of the ob value of the electric current of 54.6-10-3 А passing served sample.

Fig.2. The reference optical glass heating thermogram.

Placing the known values of the thermal characteristics of the second body, which is extruded polystyrene foam, in the formulae (2) and (3), the value of the heat flux density, the heater radius, and the і9_ст and points determined from the thermogram, we can find the desired thermal characteristics of the investigated optical glass. The values of the thermal conductivity and thermal diffusivity of the reference glass calculated using the experimental thermogram were 1,22 W/ (m-°C) and 8,8-10-7 m²/s, respectively.

Comparison of the obtained thermal characteristics of the reference glass with the data taken from [5] shows that the relative errors in the determination of the thermal diffusivity and thermal conductivity using this experimental equipment are 14% and 3%, respectively.

Conclusion

This paper considers measuring equipment that implements the flat probe method, its automation is carried out using the AK-6.25 computer measuring system. A measurement algorithm was developed and a computer program was written. The relative error in the determination of the thermal conductivity using this equipment falls within the range of ± 5%, and the thermal diffusivity in ± 15%, which is satisfactory for most measurements used in practice. This installation allows to determine the thermal characteristics of solid materials with X>0.2 W/(m-°Q such as optical glasses, natural hard rocks, soils in a frozen state, constructional materials. And as for heat insulation materials, which have X<0.15 W/(m-°Q as noted in [4], it is necessary to take into account the heat capacity of the heater itself, especially for the materials with X<0.05 W/(m-°G).

References

1. Gavrilyev R.I. Metod ploskogo zonda dlya opredeleniya teplofizicheskikh svoystv massiva mate- rialov [The flat probe method for determining the thermal properties of an array of materials] // Journal of Engineering Physics and Thermophysics. -1993. - Vol.64. - № 2. - p.221-227 (in Russian).

2. Sviridenko V.I., Ushakov S.I. Issledovaniye teplofizicheskikh kharakteristik kvartsevogo stekla nestatsionarnym metodom [Investigation of the thermophysical characteristics of quartz glass by non-sta- tionary method] // Metrological assurance of thermal measurements at low temperatures. - Khabarovsk, 1988. - p. 36-37 (in Russian).

3. Serykh G.M., Gergesov B.A. Novyy skorostnoy metod issledovaniya teplofizicheskikh svoystv sypuchikh pishchevykh produktov [New high-speed method for studying the thermophysical properties of bulk food products] // Food Technology. - 1976. - № 2. - p.162-164 (in Russian).

4. Shashkov A.G., Kozlov V.L., Stankevich A.V. Korrektsiya pogreshnosti opredeleniya teplofizi- cheskikh kharakteristik, obuslovlennoy teployem- kost'yu nagrevatelya [Correction of the error due to the heat capacity of the heater in determining the thermophysical characteristics] // Journal of Engineering Physics and Thermophysics. - 1986. - vol.50. - № 6. - p. 1007-1013 (in Russian).

5. Golubev M.P., Pavlov A.A., Pavlov Al.A., Shiplyuk A.N. Opticheskiy metod registratsii teplovykh potokov [Optical method of heat flux recording]// PMTF - Novosibirsk: Publishing House of the Siberian Branch of the Russian Academy of Sciences, 2003. - vol.44. - № 4. - p. 174-184 (in Russian).

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