SU1712849A1 - Method for determination of thermophysical characteristics of materials - Google Patents

Abstract

The invention relates to experimental physics and can be used in the complex determination of the thermophysical characteristics of materials. The purpose of the invention is to improve accuracy. The surface of the studied material is exposed to a heat pulse from a linear heat source and the temperature is recorded on the line of heat and at a given distance from this line at a fixed point in time, and the moment corresponding to the equality of the excess temperature at a point at a given distance is chosen as a fixed point in time. from the heating line and the difference between the excess temperatures on the heating line and at a given distance from this line. 3 il.

Description

The invention relates to technical physics, in particular to thermophysical measurements.

The known method for determining the thermophysical characteristics of materials by the method of instantaneous heat source. I conclude a 1.C14is in local or total heating of the surface of the test material with a short thermal pulse, recording the time interval corresponding to the extremum of the temperature curve after exposure to a thermal impulse x material at any point of the test material remote from the place of heating production.

The disadvantage of this method is that. that it does not provide high accuracy in determining the thermal diffusivity of materials. This is due to the fact that the determination of the extremum of the temperature curve is accompanied by significant errors due to the diffuse nature of this extremum, which causes the subjective errors of the experimenter and the reduced accuracy of the electronic differentiating devices.

The closest in technical essence and the achieved result to the proposed is a method for determining the thermal characteristics of materials, including a pulsed thermal effect of a linear source of heat on the surface of the material under study, recording the temperature on the line of action of the heat source and at a given distance from it at a fixed time the subsequent calculation of the desired characteristics.

The disadvantage of this method is its low accuracy, due to the need to measure two coordinates of points on the surface of the material under study from the line of action of heat, in which the temperature is recorded, as well as the relative complexity of the calculation formulas to find the desired characteristics. The purpose of the invention is to improve accuracy. FIG. 1 shows an embodiment of the method; in fig. 2 shows the connection circuit of the temperature measuring converters; in fig. 3 - graphs of excess temperatures on the line of action of the heat source T {0. d), at a point at a given distance from the heating line T (xi, i and the difference AT (i :) T (O, D) T (XI, d). On the thermally insulated surface of the test sample 1 is placed at distance XI a linear heater 2 and the temperature sensor 3 and effect the heat pulse. The temperature field T (x, under the action of a linear pulsed heat source on the surface of the semi thermally limited test sample {Z 0) is described by the dependence T () 2G (-)) where Q is heat pulse per unit length, J / m; X - coordinate, measured from the axis of the source, m; I - coefficient of thermal conductivity, W / m; TO; a - coefficient of thermal diffusivity, g-time, s. The temperature on the line of action of the heat source is determined by the ratio (a linear source can be structurally combined with a thermal sensor) P0.d) To determine the thermophysical characteristics of materials, a time instant n is recorded that corresponds to the equality Between excess temperature T (xc n) at a point at a given distance X1 from the heating line and the difference between the excess temperatures AT (ri) on the line of heat T (0, ri) m at a given distance from the heating line T (xi, TI). Consequently, T (xi, Ti) AT (ri) T (0, ri) -T (xi, ri), D ((1-exp (-41t) b () Then expression (3) can be written Q / x PTTT 4 а п / 2ггАгЛ (-Р (-4) -5 From here d 0.36 X) Substituting the expression (4) into the formula (4), the thermal conductivity coefficient g - 0.08 Q D1 Ctr, Ti Formula (7) and (8) allows To obtain an expression for the volumetric heat capacity of 0.22 Q and AT (Tf) x Thus, with a known value of Q const after applying a heat pulse, I register a time TI corresponding to the equality between the excess temperature T (xi, n) at a point at a given distance xi from the heating line and the difference AT (TI) excess Thermal characteristics of the materials can be determined using the formulas (7), (8) and (9}) for the heat source lines T (0, ri) and at a given distance from it T (xi, ri). A relative method for determining the thermophysical characteristics of the materials should be recommended. For this purpose, a linear heater and a sensor (and temperature) are placed on the heat insulator, which in general are a measuring probe. In the calibration mode, the specified measuring probe is brought into thermal contact with a material with known thermophysical characteristics and is determined by a, X C / o by formulas (7), (8) and (9). After that, the thermophysical characteristics of the tested materials are measured with regard to the data obtained during the calibration of the measuring probe. Such an approach eliminates substantially systematic measurement errors and does not make direct and indirect measurements of Q values. As an example of the technical implementation of the proposed method, a linear electric heater can be combined with a thermocouple, butt-welded, for measuring the temperature T (0. On a given distance xi ot of the linear heater, the temperatures T (x1, g) 8 are measured at two points, while these points can be located on one side of the heat source (heater ) or symmetrically: Т {х1, i) and Т (-Х1,. After applying a thermal impulse by a switch, the electrodes - the thermopairs are connected according to the following scheme (Fig. 3). a TPp thermocouple according to the differential scheme. At the moment n zero, the indicator registers the difference Т (xi, ri) -ТТ (ri) 0. Е Я АТ (1 - thermoEMF, which is a function of the difference between the excess temperatures on the line of heat source and on the set distance from it. The method of mathematical and machine modeling on an EC 1840 computer is used to calculate excess temperatures at, and -16 m, .5 J / m ..1W / m K, a. The calculation results are shown in B1 in the form of graphs in FIG. 3. On the basis of half-thermal thermograms, the time instant ri 4.81 s was determined, which corresponds to the equality T (x1, ri) AT (ri) 1 ,. According to formulas (7) and (8), the coefficients of heat and thermal diffusivity are calculated for arsch.2,994-U m / s; Arasch 0.1004W / m -K. Then the relative errors of determining the coefficients of thermal and thermal diffusivity will be, respectively, 2%, 4%. Thus, the use of the proposed method for determining the thermophysical characteristics of materials makes it possible to increase the accuracy of determining the required coefficients in comparison with the prototype. Formula and method of determining the thermophysical characteristics of materials, including a pulsed thermal effect of a linear heat source on the surface of the material under study, recording the temperature at two points on the surface at a fixed point in time, followed by calculating the desired characteristics, characterized in that, in order to improve accuracy, as a fixed point in time choose a moment of time corresponding to the equality of the excess temperature at a point at a given distance from the load line Eva and the difference of excess temperatures on the heating line and at a given distance from this line, and the desired characteristics are determined by the formulas 0.36 x D GST1} -T1 DT (t1) -x where a. A, C, p-, respectively, thermal diffusivity, thermal conductivity, specific heat capacity and density of the material under study; Q is the energy of the heat pulse per unit length; XI is the distance between the linear heat source and the control point on the line parallel to the heating line; AT (r1) is the difference of excess temperatures on the line of action of the heat source and. at a given distance from it at time t when this difference is equal to the excess temperature at the control point.

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Claims (1)

Claim A method for determining the thermophysical characteristics of materials, including a pulsed thermal effect by a linear heat source on the surface of the material under study, recording the temperature at two points on the surface at a fixed point in time, followed by calculating the desired characteristics, characterized in that, in order to increase accuracy, they choose as a fixed point in time point in time corresponding to the equality of the excess temperature at a point at a given distance from the heating line and the difference x temperatures on heating line and at a predetermined distance from this line, and the desired characteristics determined by the formulas _ 0,36x3 ^and "inf ....... ^{_:, _ 0,08 Q Ζ"} ΔΙ (τι) · τί ' 0.22 Q ^CP AT (p) -xG where a, A, C, p are respectively the coefficient of thermal diffusivity, thermal conductivity, specific heat and density of the material under study; Q is the energy of the heat pulse per unit length; xi is the distance between the linear heat source and the control point on a line parallel to the heating line; Δ Т (η) is the difference of excess temperatures on the line of action of the heat source and. at a given distance from it at time τι, when this difference is equal to the excess temperature at the control point. see th