SU1059494A1 - Device for determination of local convective heat transfer between phase separation surface anv moving medium - Google Patents

Abstract

1. DEVICE FOR DETERMINING COEFFICIENTS OF LOCAL HEAT TRANSFER BETWEEN PHASE AND SURFACE SECTION moving medium containing heat meters, made with identical contact surfaces of the movable and fixed on the medium temperaturovyravnivayuschey plate, characterized in that, in order to expand the field of application due to the flow expansion investigated class , a thermal collector is introduced into the device, and the temperature-equalizing plate is made with a notch, forming-. A flat cavity filled with a medium with a known thermal conductivity, the lateral surfaces of the cavity are heat insulated. 2. The device according to claim 1, wherein the cavity is that the cavity between the calorimeter and the temperature-leveling plate is partitioned by a foil located parallel to the plate of the temperature-equalizing plate, providing respecting the ratio .1000, where G, the Großof test; Pf, - Prandtl criterion.

Description

The invention relates to thermal measurements, namely the study of the parameters of convective heat transfer between the interface and the moving medium. A device for determining local heat transfer coefficients is known, comprising a battery thermoelectric heat meter for measuring the heat flux density and a differential thermocouple for measuring the temperature difference between the heat meter surface and the moving medium, and the heat meter is set flush with the heat exchange surface. This device allows the determination of heat transfer coefficients in any direction of the heat flux (both from the object under study to the moving medium, and vice versa) in the study of the heat exchange of bodies of various shapes. To calculate the heat transfer coefficient, use the known; Newton-Richman C ID Dependency The disadvantage of this device is low accuracy of determining heat transfer due to errors arising from measuring the temperature difference due to the difficulty of determining the location of the temperature of the moving medium. The device for determining local temperature is the closest to the proposed method. heat transfer coefficients between the interface and the moving medium with a medium containing heat meters, made with identical contact surfaces with the driving with the medium attached to the temperature-equalizing plate, the basis of which is the determination of the heat transfer coefficient with respect to the increment of the heat flux density (Hl I temperature difference DT - .T-D surfaces of heat meters 2J. The temperature difference between heat meter surfaces results from the use of heat meters with different thermal resistances And and RI The determination of a small temperature difference (T; –T2) in this device is carried out according to the readings of heat meters, which measure the heat flux densities tj, n and c, through them, according to the thermal resistance values of R. and provided that the heat meters are mounted on an iso thermal surface, the temperature of the plate is aligned with the temperature TO, T, .T2 (VTJ - {% To,) ("V,) - 1 The desired heat transfer coefficient is found in Formula 1 v, L in the described construction of the device, heat-exchanging thermoelectric heat flux transducers are used as heat radiators, for which the heat flux density v penetrating the heat meter is proportional to the thermal emulator generated by the thermopile i . (3) where K is the conversion factor, Substituting relation (3) into formula (2), we obtain the calculated dependence for the device G /. R. oi d () .W "2 where K is the device constant, if K K2, then K 1. It follows from equation (4) that heat meters are of the same type and their coefficients have the same temperature dependence, then the device constant K does not depend on temperature When designing a device, one of the nodal moments is the choice of values of thermal resistances, and 2 When choosing values of R, go to provide the required measurement range for the ratio q ,,. the condition must be met at the maximum possible value of the heat transfer coefficient in the process under study. At the same time, the temperature difference between the surfaces of heat meters (must be such that the heat transfer coefficients for the surfaces of both heat meters are the same, and the difference between heat flows (must be two orders of magnitude greater than the non-synchronization fluctuations of q, and v v. Since the minimum value R is determined by the technological conditions for the manufacture of heat meters rf by the magnitude of the contact thermal resistances in the plane of attachment of the heat meter with a temperature-equalizing plate, the main value that can be To vary to meet the above conditions, is the difference between the thermal resistances of heat meters and the correctness of the measurements depends on the correct choice. Therefore, devices made to study the heat transfer of gases are often incorrectly used to study the heat transfer processes of the surface washed by liquids and vice versa. application is the main disadvantage of the known device, which can be eliminated if it is possible to vary the values ydy depending on the type of coolant and limits the heat transfer coefficient changes. The purpose of the invention is to expand the scope of application by determining local heat transfer coefficients in various environments. The goal is achieved by the fact that the device for determining local heat transfer coefficients between the interface and the moving medium, containing heat measures, made with identical contact surfaces with the moving medium and fixed on the temperature-equalizing plate, additionally contains a thermal collector and the temperature-equalizing plate is made with a notch forming with the heat meter surface, on which the heat collector is mounted, a flat cavity filled with a medium with known heat conductivity, with the side surfaces of the cavity insulated. In addition, the cavity between the calorimeter and the temperature-equalizing plate is partitioned by a foil located parallel to the temperature-equalizing plate at a distance of the scientific research institute between adjacent surfaces, which ensures ratios; 1000. FIG. 1 shows a first modification of a device for determining local heat transfer coefficients; in fig. 2 - the same, the second modification; in fig. 3 - indicator of heat conductivity of the moving medium; in fig. 4 - the third modification of the device for the study of convective-radiation heat exchange; in fig. 5 shows the layout of the foil in flat CAVITY; in fig. b - nomogram for selecting the required values of the value of LK f na fig. 7 - calibration characteristic for the case of filling a flat cavity with various substances. This device can be made in several versions, First & the device modification (Fig. 1) contains heat meters 1 and 2, as well as a temperature sensor 3 (for example, a thermocouple) mounted on the surface of the heat meter 1, facing the moving medium. Heat meters are made in one heat meter unit; connected to the temperature-equalization plate 4, in which the notch is milled, located under the heat meter 2 and forming with the heat collector 5 installed on its surface facing the thermal plate-leveling plate 4 the flat cavity whose side surfaces are thermally insulated with a profiled liner 6. fittings 7 and 8 are mounted with the exit of their channels into a flat cavity filled with a medium with a known thermal conductivity. In order to simplify the manufacturing technology and improve the accuracy of determining the value (Vi / V 15, heat meters 1 and 2 are usually identical in terms of thermophysical values for thermophysical characteristics and sensitivity. To protect heat meters 1 and 2 and temperature sensor 3 from the effects of a moving medium, as well as to ensure that the characteristics of their surfaces are identical, washed by the moving medium (roughness, coefficients of thermal radiation and absorption), these surfaces are covered with a protective film or foil, sectioned It is for reducing heat transfer along it.To ensure simultaneous temperature field heating meters 1 and 2 are surrounded by protection zones with thermal characteristics equal to those of heat meters. Aligning temperature of high heat conducting material 4 ensures that the known heat resistance of heat meters 1 and 2 and the additional thermal resistance of the medium of the medium filling the cavity and the measured densities of heat fluxes "and q, 2 There was a difference in temperature (T: - T) between the surfaces of heat meters 1 and 2, with moving media. The heat collector 5 ensures the constancy of the heat fluxes passing through the thermoelectrodes of the thermometer battery of heat meter 2 and through the filler between them, regardless of the thermal conductivity of the medium in the flat cavity, and also ensures that the surface of heat meter 2 isothermal with the temperature equalization plate 4. convection in a flat cavity, its thickness, i.e. The gap between the thermal collector 5 and the temperature-equalizing plate 4 is selected when designing the device so that during the measurement process at maximum values of the temperature difference in the layer of the medium filling the cavity, the Kraussold condition (, 1000) is observed and the lateral surfaces of the cavity are thermally insulated. In some cases, as the medium filling the cavity, it is possible to use a working medium (liquid or gas), the heat exchange with which is investigated. When etsil it is advisable to use the second modification of the device (Fig. 2), in which instead of one of the nozzles in the heat-metering unit an opening 9 is made, through which the cavity is filled with the working medium. If the thermal conductivity of the working medium changes during the time of the research, to monitor this change, the thermal conductivity indicator of the moving medium (Fig. 3), operating according to the principle of a thermal meter bridge, assembled from the same elements as the proposed device ( Fig. 2), and the difference between it and the fact that instead of a protective pad or foil on the surfaces of heat meters 1 and 2, an additional temperature equalizing plate 10 is mounted, the application of the heat conduction indicator (phi 3) placed in the immediate vicinity of the device, allows to obtain information on the local in space and time values of the thermal conductivity of the working medium, the heat exchange with which is studied In relation to the research problems of convective-radiative heat exchange, it is advisable to use the third modification of the device ( Fig. 4), in which, in contrast to the first and second (Fig. 1 and .2), a heat meter 11 was additionally inserted, mounted in a single heat meter unit with heaters 1 and 2 installed on a common temperature-equalizing plate 4 with them and made with the same sensitivity as thermal heat meters 1 and 2 available to thermal resistance, but differing from them in that its backhole film has different values of thermal radiation and absorption coefficients, compared with these characteristics for coatings of heat meters 1 and 2. If a predominantly gaseous medium is used to fill a flat cavity, then to reduce heat transfer by radiation and to ensure predominantly conductive heat transfer The sediment between the thermal collector 5 and the temperature-equalizing plate 4 cavity is performed with a partitioned metal foil 12 (FIG. 5), parallel to the temperature of the injec- tion plate 4 and the surface of the thermal collector, divide the strip on the target, for which the condition G, .Pf, 1000 is met. In this case, the surfaces of the thermal collector 5, the temperature-equalizing plate 4 and the foil 12 are made with mass values of the coefficients of thermal radiation and absorption. This solution provides predominant heat transfer heat conductor-. in a flat cavity. To determine the parameters of convective heat transfer using the proposed device, it is necessary to select and ensure the required value of R to install the device on the surface of the object under study; as well as measure thermoelectric power, and calculate the required values using formulas. Based on a priori information about the possible range of changes in heat transfer coefficients in the heat exchange between the surface of the object and the moving medium, about the thermophysical characteristics of the environment and about its eEc modes of its movement, choose the desired value of LK using a generalized nomogram (fig.b) satisfying the requirements similar to the above in the description of the prototype. The required value of the coefficient of thermal conductivity of the medium filling the cavity is determined from the selected value of the value of ai, using the previously obtained calibration dependences dK (A j, T) and Rp / aR f2 {Aj, T), it is necessary to fill the cavity to obtain the desired value of the magnitude of dK. The cavity is filled with the selected substance through fittings, which, after filling, are sealed. If used as a filling substance; it is necessary to dear it before using it, and when filling the cavity it is necessary to ensure that the gas bubbles are completely removed from it. Since the actual values of thermal conductivity of a substance may differ from reference data, usually before installing on the object under test, the values of R ;, / and ft and are checked, placing the devices between two isothermal surfaces with different temperatures. When determining the values, RI / AR (and DCI (1) use the values of K and Ri, determined earlier in the calibration experiments. After filling the cavity with the required substance, the device is installed; installation of the device for any reason: it can be mounted directly on the surface of the object under study, but before and after it for sure

Environments establish special wedges made of high-quality material and serve to reduce the distortion of the hydrodynamics of the incoming flow of the medium and the heat flow.

Having installed the device on the object under study, they connect current-collecting wires of heat meters 1 and 2 and temperature sensor 3 to the secondary equipment and measure the thermal emf generated by the primary converters, and determine the heat transfer coefficient.

In the study of local convective heat transfer, in addition to the heat transfer coefficient, it is necessary to determine the temperature of the medium T (.р, corresponding to the measured value of the heat transfer coefficient and the heat flux "Vx of the surface of the Object under study that is not disturbed by the presence of the device.

The device also makes it possible to determine the heat flux density on the surface of the object under study undisturbed by the presence of the device, if we additionally measure the surface temperature T of the object at a point in the same hydrodynamic conditions with the device, under the condition of the same temperature history of the incident flow .-K3t () which includes only the quantities measured directly during the experiment. Thus, the use of the proposed device in a heat engineering experiment in the study of convective heat exchange makes it quite easy to take into account the distortions of the heat-flux density of the flow introduced by the device when it is placed on the interface between the phases of the object under study.

FIG. 7 shows the calibration characteristic of one of the devices for a temperature interval of 30-40 C, the cavity of which is filled with air, transformer oil, glycerin and water, from the analysis of which it follows that depending on the process being studied, varying the medium filling the cavity, it is possible to obtain optimal values of uKf, thereby expanding the scope of the proposed device.

If the thermal conductivity of the working medium is known and satisfies in its magnitude the value of the thermal conductivity of the medium necessary to provide the required value of the AR unit, then a second device modifier can be used (Fig. 2), the use of which simplifies the preparation of the experiment, since it is eliminated the need to compensate for the change in pressure and volume expansion of the substance filling the cavity under the influence of the temperature conditions of the device and the change in the thermodynamic parameters of the moving working environment.

The method of preparation and measurement with the second modification of the device is similar to that described.

If the thermal conductivity varies over time during the study (for example, by changing the composition or pressure of the incoming medium), then it is included in the package with a specimen. An additional indicator of thermal conductivity of the moving medium is introduced into my devices (Fig. 3). In this case, it is placed in the immediate vicinity of the device on the surface of the object under study. When this fitting 8 devices (Fig. 2) and fitting 8

0 the heat conduction indicator of the moving medium (Fig. 3) is connected to a single coaxial system, with the help of which the medium in the cavities of the device (Fig. 2) is periodically updated and

5 indicator (Fig. 3).

. Measurement of the coefficient of heat. water content is carried out at steady thermal conditions in the cavity. The desired thermal conductivity coefficient

0 moving media are calculated by

5 where R

- thermal resist42

T1

of heat meters 1 and 2 (fig. 3) forming the heat meter pavement q, and “v heat fluxes that passed through heat meters 1 and 2 (fig. 3) and measured by them; Aj-p is the thermal conductivity of the medium;

h is the thickness of the gap in the cavity of the indicator (Fig. 3);

A and B are permanent instruments obtained in calibration experiments with standard samples.

The use of the indicator of thermal conductivity of the moving medium (Fig. 3), installed in close proximity to the main device

0 (Fig. 2) allows obtaining information on the local in space and time values of the thermal conductivity of the medium, the heat exchange with which is investigated, thereby improving the accuracy of determining the values of the heat transfer coefficient and processing the results in the criterion form (Ro., Pp, - G,. I, since

 oil,. ....

number includes value

The coefficient of thermal conductivity

For the study of convective-radiation heat exchange, a third modification of the device (Fig. 4) is used in which

ff

ten,

FIG. 2

k- / r / fj,

Claims (2)

1. DEVICE FOR DETERMINING LOCAL HEAT TRANSFER COEFFICIENTS BETWEEN THE SURFACE OF THE PHASE AND MOVING MEDIA, containing heat meters made with identical contact surfaces with a moving medium and fixed to a temperature-leveling plate, as a rule In order to expand the scope by expanding the class of the studied flows, a thermal collector is introduced into the device, and the temperature-leveling plate is made with a recess, forming -. adjacent to the surface of the heat meter on which the heat collector is mounted, a flat cavity filled with a medium with known thermal conductivity, the side surfaces of the cavity being insulated. 2. The device according to π. 1, the fact that the cavity between the heat meter and the temperature-leveling plate is partitioned by a foil located parallel to the temperature-leveling plate, at a distance between adjacent surfaces, ensuring compliance with the ratio GpPp <1000, where Gp is the Groshoff criterion; R _g - Prandtl criterion. FIG. 7