Errors in dynamic methods of measuring thermal conduction of gases

Autor: A. V. Krymasov, Yu. A. Gorshkov, A. S. Umanskii
Rok vydání: 1980
Předmět:
Zdroj: Measurement Techniques. 23:435-439
ISSN: 1573-8906
0543-1972
DOI: 10.1007/bf00824537
Popis: In the case when the cell contains stationary temperature-field distortions, even with a value exceeding the amplitude of the field variable component, yet sufficiently small to consider the thermophysical properties to be independent of temperature, the solution of the thermal conduction problem can be represented as the sum of the solution of the stationary problem within whose boundaries the stationary distortions are considered [i] and that of the transient problem in which the external-wall temperature is assumed to be constant. Thus, in the case of dynamic methods, small stationary distortions in the working cell do not affect the measured value of X, thus making it possible to lower the stringency of requirements for the thermostatic control of the cell. These methods can be classified with respect to the thermal disturbance in the following manner: linear-source method (MLI), whose source's power remains constant [2]; periodic heating method (MPN) [3]; and filament impulse method (instantaneous source of heat [4]). Differences in the nature of heat disturbances produce differences in the theory of methods, technical means of measurement, and techniques for processing results and they obviously affect the influence exerted by the systematic error sources. However, insufficient attention is paid to analyzing these sources, often making the error evaluations cited in literature unreliable. Below we examine the effect of the basic systematic-error sources on the measurements of thermal conduction in gases by means of dynamic methods. Since the filament impulse method is dealt with in a separate article, we shall only consider the first two methods. Let us first of all examine the basic limitations which exist in their practical implementation. The MLI measuring cell is originally in thermal equilibrium with the thermostat in which it is located. Later, under the effect of the constant heat source, the filament temperature begins to rise monotonically. In the course of heating, filament temperature variations are produced and recorded on the basis of the filament resistance changes. Typical thermograms are shown in Fig. i, where curve 1 corresponds to a constant-power source qz which is uniformly distributed in the cylinder with the radius RI (filament radius) and the same thermophysical properties (X1, Cpi, 01) as the tested gas (Kcp = I); curves 2, 4, and 5 correspond to a constant power source ql with radius RI and an infinite thermal conduction (XI = ~) whose volumetric thermal capacity differs from that of the tested gas (for curves 2, 4, and 5 the value of Kco is equal to 2, i0, and 50, respectively): curve 3 corresponds to .a linear heat source (i)). The filament thermal inertia which distorts the thermogram is reduced with time and the filament excess temperature characteristic t(R:, x) degenerates into a straight line which can be represented by a simple relationship [2]
Databáze: OpenAIRE
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