SU823999A1 - Device for measuring thermal conductivity of thin-walled cylinders - Google Patents

Description

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The invention relates to thermophysical measurements, and more specifically to devices that serve to determine the thermal conductivity of thin-walled cylinders.

 A device for determining the coefficient of thermal conductivity of tubular samples is known, which contains a heater installed inside the sample and a sinker, which is a water-cooled system that is in close contact with the external surface of the sample.

However, in the device due to the difference

thermal expansion coefficients of the heaters heaters materials. and the sample under test forms a gap between the test sample on one side and the heater and the heat remover on the other. This leads to an unreadable error in determining the thermal conductivity due to the additional thermal resistance in the zone of contact of the test sample with heat. salmon and heater.

The closest in technical essence to the present invention is a device for measuring the thermal conductivity of tubular samples containing a heater in contact with the external surface of the sample and a heat sink inserted into the internal cavity of the sample, while temperature sensors are installed directly on the heater and in the heat remover 2.

However, the use of this device is associated with the appearance of a large uncontrolled contact resistance, since the classical methods of reducing this effect in this case are unacceptable. With a sliding fit between the contacting surfaces, the average gap is about 50 microns, which, even in the case of using a highly heat-conducting inert gas helium, with

& W

heat flux of about 0.5-10

m

leads to a parasitic temperature difference of about. At heatplug, the water content of the cage is about 3 -

  . MK -

The specific error in determining the thermal conductivity reaches 50%. In addition, due to the small thickness of the samples, there is a restriction on the minimum allowable heat flux.

The purpose of the invention is to improve the accuracy of thermal conductivity measurements.

The goal is achieved by the fact that between the heater and the heat remover of the coaxialio in relation to them there are two cylindrical split sleeves made of highly thermally conductive material, made so that, in the presence of a radial radiant, the gaps change due to thermal expansion, the temperature sensors are fixed in sleeves, while the sample is located between the sleeves. The bushings are made with an axial cut. The cut of the sleeves provides a free and at the same time tight fit of the cylinder surface to the sleeves.

The drawing shows schematically the measuring node, a section.

The assembly contains sample 1, for example, made of laminated material, inserted into the gap formed by cylindrical cuts with 1 sleeves 2, which are mounted coaxially with respect to heater 3 and water-tight heat shooting 4, and temperature sensors 5 are located in axial grooves made on the sleeves. During the measurements, as the heat flux increases, the temperature difference between the copper bushings increases and they tightly fit the test case. The use of high-conductive material, such as copper, is aimed at reducing the contact resistance and at averaging temperatures and heat fluxes over the surface of the measuring unit. The selection of the heat flow at which close contact is ensured is carried out during assembly attempts in vacuum, argon, helium, which allow to estimate the magnitude of the contact resistance. Temperature sensors 5 are dispensed directly in the sleeves and can be calibrated with sufficient accuracy measure radial temperature differences and

Calculate thermal conductivity using known formulas.

Two split sleeves reduce uncontrolled thermal resistance, while providing free and at the same time tight fit of the sample.

The number of grooves on the outer forming sleeves is determined by the appropriateness of the accuracy of temperature measurements. In the proposed device, there are four of them, although with a well-established methodology, there can be only one of them:

The size of the cut is determined from the characteristics of thermal expansion of materials, so that at the maximum heat flux the ends of the cut are connected.

The size of the cut of the internal ytula located at the heater is determined by the ratio

.oL, lT, -% V2n8j., oC2lT2-Tol,

lip V

Where

average diameters of the sleeve and the sample, respectively; about („(X, 2 coefficients of linear

expansion sleeve image; /. .. -. , “Td; T ,; Tj is the initial temperature and

sample sleeve temperature; ,

V value technological G9 gap between the sleeve and the sample; ,

Claims (2)

P is the coefficient FU “1. The size of the cut of the outer sleeve, located at the refrigerator, is determined by the ratio S2p4pg 2lT2-%) ,, “s.Lp average diameters of the sleeve and the sample, respectively; OS 5, the linear expansion coefficient of the sample sleeve; T is the initial temperature and the temperature of the sleeve and the sample. The proposed device has been worked out when measuring the thermal conductivity of cylinders from pyrographite PGV with a thickness of up to 0.6 mm and zirconium carbide with a thickness of up to 0.8 mm. The device was tested when measuring the thermal conductivity of a cylinder, 5 mm thick, made of graphite of the brand ARB. The device works as follows; The carbide cylinder under investigation, Qi horses, with a diameter of 60 mm, slid over the fit, is inserted into two copper split sleeves, the cut width of the inner sleeve is 2.7 mm, and the outer 0.3 m. In them, the graduation thermocouple is caulked HA. With an increase in the level of the heat flux to 0.3 - 10 W / / m, the thermal resistance of the gap between the copper bushings and the surface of the sample did not exceed 20%. A further increase in heat flux has virtually no effect on the magnitude of the thermal resistance. The thermal resistance effect of the gaps is estimated when changing gels to argon and when testing in vacuum. The obtained values of the thermal conductivity of the cylinder from the APB are well (within 20%) consistent with the literature data, which makes it possible to conclude that the maximum error in determining the effective thermal conductivity is no more. thirty%. The destruction of the clips is not npoHCXOjEpiT. The accuracy of measurements was carried out using the described device for determining the thermal conductivity of thin-walled cylinders by conducting methodological experiments in vacuum and in a medium of gases with different thermal density, during which the contact resistance value was increased, which allows to increase the accuracy of thermal conductivity measurements to 15-20 % Claim 1. Device for determining the thermal conductivity of thin-walled cylinders including a coaxially arranged heater, cooler and temperature sensor, characterized in that, in order to improve measurement accuracy at high temperatures, the device further comprises two cylindrical split sleeves, on the outer core of which grooves are made for the installation of temperature sensors, these bushings are made of the height of one material and are installed with a heater and a refrigeration, and between volts lkami by sliding sediment located sample. 2. The device according to claim 1, wherein the cut size of the inner sleeve located at the heater is determined by the relation. , | T - T;, у-2Т1Бз-ПсЗ (.р () is the average diameter of the sleeve and the sample, respectively. But;. /.: 0 (linear expansion coefficients of the sleeve and the sample; initial temperature and temperature of the sleeve and sample; value technical gap between the sleeve and the sample. 3. The device according to paragraphs I and 2, from the fact that the cut size of the outer sleeve located at the refrigerator, is determined by the ratio of L2-P cfr "71 7-P)) - 2 Pb-Pasr .eos IT (-TQ) ,. where d. gpch average bushing diameter. and sample, respectively; OiijOfj is the linear expansion coefficient of the sleeve and the sample; . fo; 4; the initial temperature and temperatures of the sleeve and sample; Sj, is the size of the technological gap between the sample and the sleeve. Sources of information received during the B1 schmanie prng examination. USSR Author's Certificate No. 156323, cl. 6 About N 25/18, 1961. 2. US patent 3592060, cl. 73-15, 1971 (prototype).