SU754976A1 - Method of solid body heat capacity determination - Google Patents

Description

The invention relates to the field of technical physics and can be used to determine the heat capacity of solids in difficult conditions, for example in the channel of a nuclear reactor.

Known non-stationary methods for determining the heat capacity, which include methods of the initial stage of the process of thermal conductivity and methods of the regular mode [1]. They consist in registering the temperature equalization process in the sample under study after pulsed heating of one of its sides. When using the methods of the regular mode, the parameters of the thermal process in the sample are determined while a change in temperature occurs on its surface (or a flow is set), changing according to a linear harmonic law.

These methods have a number of significant drawbacks, namely the difficulty of providing the required test mode, which limits their applicability in difficult conditions. Another disadvantage is the need to measure the heat fluxes acting on the sample.

The closest technical solution to the present invention is a method for determining the heat capacity of solids, which consists in the fact that the sample is continuous

but heat is supplied, ultrasonic oscillations are simultaneously excited at two frequencies in the vicinity of the sample resonant frequency, and op5 limits the heat capacity according to the rate of temperature change [2].

This method is unsuitable in the case of determining the heat capacity of materials with a large attenuation coefficient of ultrasonic vibrations, due to the large 10 measurement error arising in this

case.

The aim of the invention is to improve the accuracy of measuring the heat capacity of materials with a large 15 attenuation coefficient of ultrasonic

fluctuations.

This goal is achieved by the fact that the sample under study is mechanically connected with the reference sample with known

20 properties and a small attenuation coefficient of ultrasonic oscillations, excite ultrasound oscillations in the resulting system, consistently heat the system from the side of the test and reference samples with a heat flux of constant power and determine the rate of change of the resonant frequency of the reference sample. Next, calculate the heat capacity of the investigated sample by the formula

3

754976

/ 'isl_ l'etuet

H about ·

"Isl

where c '^t; ρ _3Τ ; / g _FL

heat capacity at constant pressure, density and thickness of the reference sample, respectively; the same for the sample under study;

the rate of change of the resonant frequency of the reference sample in the case when the system is heated by heat flux falling on the reference sample;

the rate of change of the resonant frequency of the reference sample in the case when the system is heated by heat flow falling on the sample under study, the time coefficient.

ten

15

20

25

B = 1 +2

L | yach \

^t )

/ I ²

where τ - ₅ " ^ssl is the time constant of the test sample ^π ;

and _Inst - thermal test sample;

/ _pr - time interval between the moments of reaching the maximum amplitude of the sample • oscillations at two fixed frequencies differing from each other by the value (Δ / ρ) ¹ ;

( _{| 1ah} - the time interval between the moments corresponding to the inclusion of the heat flux and the achievement of the maximum amplitude of oscillations of the sample corresponding to the first fixed frequency.

FIG. 1 shows a system for implementing the proposed method (two options); in fig. 2 - measuring circuit.

In the system consisting of the test sample 1 and the reference sample 2, ultrasonic vibrations are excited through the sound ducts 3. The test sample 1 and the reference sample 2 are connected by means of a spring element 4. The system is heated in series from the side of the test region

35

40

45

50

55

60

(one)

the sample (option I), and then from the side of the reference sample (option II) with a heat flux of constant power d.

To test the method were selected samples of fluoroplast-4. A sample of duralumin D16T was used as a reference sample.

The standard ultrasound measurement equipment was used to record the signal.

The high frequency generators 5 and 6, the frequency of the electrical signal of which is recorded by the frequency meter 7, excites the piezotransducer 9 through the switch 8. the acoustic circuit 3 is transmitted to the piezoelectric transducer 10, where it is converted into an electrical signal. Further, this signal is amplified by amplifier 11, filtered by filter 12 and recorded on the screen.

(2)

Oscilloscope 13. With the help of the detector 14, a low-frequency component is distinguished, which characterizes the rate of change of the signal amplitude during heating, which is recorded by a recorder 15. The system is heated by an incandescent lamp 11 connected to the network using a switch 17.

The results of measurements of heat capacity calculated by the formula (1) on three samples of fluoroplast-4, having a disk shape of different thickness, are in satisfactory agreement with the data taken from the literature, which confirms the efficiency of the method.

Thus, the proposed method allows to increase the accuracy of measuring the heat capacity of materials with a large attenuation coefficient of ultrasonic vibrations. One of the main advantages of the method compared to the previously known ones is the absence of the need to measure the heat flux, which greatly simplifies and speeds up the measurements. The method also allows for measurements on small samples of simple form in a short time. This allows it to be used in difficult experimental conditions (for example, in a nuclear reactor channel). Measurements using this method do not impose special requirements on the measuring device, in particular to its tep754976

isolation, which is also promising in terms of its use in difficult conditions.

Claims (1)

Claim The method of determining the heat capacity of solids, which consists in exciting mechanical oscillations of the sample, applying heat to it, determining the rate of change of the resonant frequency of the sample under heat, and calculating the specific heat from this speed in order to improve the measurement accuracy on samples with a large damping of oscillations, the sample under study is mechanically connected to a reference sample with a small damping of oscillations, excite oscillations of the resulting system, determine the rates of changes the resonant frequency of the system with sequential thermal effects on the test and reference samples and the ratio of these 5 speeds calculate the required heat capacity.