Device for measuring temperature of outer wall of direct-current heating metal tube

Abstract

The invention relates to the field of physical electrical temperature measurement, and discloses a device for measuring the outer wall temperature of a direct-current heating metal tube, which is mainly applied to measurement of the outer wall temperature of the tube in an experiment for researching the convective heat transfer characteristic of fluid in the tube, and comprises a direct-current power supply, a current direction change-over switch, a copper electrode, a metal tube, a thermocouple and a data acquisition system, according to the invention, the extra potential difference brought to the thermocouple welded on the pipe wall when the direct-current power supply is used for heating can be effectively eliminated, so that the outer wall temperature of the metal pipe under the direct-current heating condition can be accurately measured.

Description

Technical Field

[0001] The invention relates to the field of physical electrical temperature measurement, and in particular to a device for measuring the outer wall temperature of a tube used in an experiment to study the convective heat transfer characteristics of a fluid in a tube, and in particular to a device for measuring the outer wall temperature of a metal tube used for direct current heating. Background Art

[0002] In the convection heat transfer experiment in the tube, the experimental metal tube is usually heated by electric heating to simulate the boundary conditions of equal heat flow heating. Electric heating uses Joule's law to convert electrical energy on the conductive element into thermal energy, including direct heating and indirect heating. Direct heating uses the resistance of the heated object itself to obtain heat; while indirect heating is that the heat is generated by the heating element and transferred to the heated object through thermal radiation, heat conduction and other methods. The object to be directly heated must be a conductor with a high resistivity. Since the heat is generated by the heated object itself, it can be regarded as internal heat source heating, so the thermal efficiency of direct heating is higher; in indirect heating, since the heated object and the heating element are divided into two parts, the type of heated object is generally not restricted, but its heating thermal efficiency is much lower than that of direct heating.

[0003] In scientific experiments and industrial applications, two different electric heating methods, alternating current and direct current, are mainly used. For AC heating, electromagnetic effects are generated, resulting in reactive power during the heating process. When the same apparent power is applied, the temperature measured by AC heating is always lower than the measured value for DC heating. When using higher-frequency AC heating, the Joule effect generated by the current harmonics will cause uneven local heating of the metal tube, thereby affecting the local heat transfer characteristics. In addition, AC heating will also produce a skin effect, resulting in uneven current density distribution of the heated metal, which further affects the distribution of heat in the conductor. For DC heating, there is no skin effect because the magnitude and direction of its current and voltage are stable. Although it will also produce a weak electromagnetic effect, its additional power loss is much smaller than that of AC heating, and can be ignored in practical applications.

[0004] The methods for measuring wall temperature are mainly divided into two categories: non-contact measurement and contact measurement. The most commonly used non-contact measurement method is infrared thermal imager temperature measurement. The principle of infrared thermal imager temperature measurement is to sense the infrared radiation emitted by the object, convert it into an electrical signal, and then map the temperature distribution to the image according to different gray levels. It will not destroy the thermal equilibrium state of the object being measured, and can measure the temperature field distribution of the object. However, when using infrared thermal imagers to measure temperature, it is easily affected by environmental radiation, has strict requirements on the environment during temperature measurement, and has a low response frequency. The contact measurement method usually uses thermocouples for measurement. The principle of thermocouple temperature measurement is to use the Seebeck effect to generate a potential difference signal, and then the digital acquisition system converts the measured electrical signal into a temperature value. The most commonly used thermocouple is the armored thermocouple. Because it is wrapped with a layer of armor sleeve, it is more stable during use, but this also leads to a slow response time and the measured value is slightly lower than the true value of the actual temperature. In order to avoid the above shortcomings, the thermocouple wire can be directly welded to the metal pipe wall. This temperature measurement method can not only directly and accurately measure the pipe wall temperature, but also has a high temperature measurement frequency response.

[0005] However, when using DC power supply to heat and weld thermocouples to measure the outer wall temperature of metal pipes, the output signal of the thermocouple will be interfered by the DC power supply. Due to the thin wire of the thermocouple and manual operation, a linear gap will inevitably appear between the two wires during welding, and the thermoelectric signal is generally in the millivolt level. Therefore, as long as there is a small gap between the two wires of the thermocouple spot welded on the pipe wall, it will cause great interference to the output signal of the thermocouple. Summary of the invention

[0006] The object of the present invention is to provide a device for measuring the outer wall temperature of a metal tube heated by direct current, so as to solve the problems raised in the above background. The device comprises a direct current power supply (1), a current direction conversion switch (2), a copper electrode (3), a metal tube (4), a thermocouple (5), and a data acquisition system (6). The direct current power supply (1) is connected to the input end of the current direction conversion switch (2); the output end of the current direction conversion switch (2) is connected to the copper electrode (3); the copper electrode (3) is installed at both ends of the metal tube (4); the thermocouple (5) is directly welded to the outer wall surface of the metal tube (4) by spot welding; and the data acquisition system (6) is connected to the thermocouple (5).

[0007] The DC power supply (1) uses a low-voltage, high-current DC power supply to simulate the boundary conditions of equal heat flow heating; the current direction conversion switch (2) uses an LW26 type 2-section 8-contact universal conversion switch and is connected in series in the heating circuit to control the direction of the heating current.

[0008] The thermocouple (5) wire is directly welded on the pipe wall by a spot welding machine and is used to measure the temperature distribution of the outer wall of the metal pipe (4). The thermocouple model is a commonly used T-type or K-type thermocouple, the wire diameter is 0.1 mm, and the measurement accuracy is less than 0.5%. The data acquisition system (6) is connected to the thermocouple (5). The data acquisition system (6) is composed of a thermocouple signal acquisition module and a computer, wherein the thermocouple signal acquisition module first converts the potential difference signal output by the thermocouple into a temperature value, and then inputs it into the computer for data calculation, recording and storage.

[0009] The copper electrodes (3) are mounted at both ends of the metal tube (4) and connected to a DC power source (1); the copper electrodes (3) are made of red copper, which has excellent electrical conductivity and ductility; the metal tube (4) is made of 304 stainless steel, which has a relatively large resistivity, good corrosion resistance, good processing performance, and is easy to cut and bend.

[0010] Compared with the traditional measurement method, the present invention has the following advantages.

[0011] 1. This device adopts a combination of DC power supply heating with higher heating efficiency and wall welding thermocouple temperature measurement with higher temperature measurement frequency response.

[0012] 2. This device is equipped with a current direction conversion switch in the heating circuit, which eliminates the additional potential difference caused by DC power heating to the wall welding thermocouple. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of a device for measuring the outer wall temperature of a DC-heated metal pipe provided by the present invention

[0014] Figure 2 Schematic diagram of an additional potential difference generated by a pipe wall welding thermocouple in a device for measuring the outer wall temperature of a direct current heated metal pipe provided by the present invention

[0015] Figure 3 A schematic diagram of the wiring of a current direction conversion switch circuit in a device for measuring the outer wall temperature of a DC-heated metal pipe provided by the present invention

[0016] Figure 4 The results of measuring the outer wall temperature distribution of a metal pipe using a device for measuring the outer wall temperature of a DC-heated metal pipe provided by the present invention are shown in FIG.

[0017] In the figure: DC power supply 1, current direction conversion switch 2, copper electrode 3, metal tube 4, thermocouple 5, data acquisition system 6. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] See also Figure 1-Figure 4 The present invention provides a technical solution: a device for measuring the outer wall temperature of a metal pipe heated by direct current, the device comprising a direct current power supply (1), a current direction conversion switch (2), a copper electrode (3), a metal pipe (4), a thermocouple (5), and a data acquisition system (6), such as Figure 1 As shown; the device applies direct current to the metal tube (4) to simulate the boundary condition of equal heat flow heating, and eliminates the additional potential difference caused by the direct current voltage heating to the wall welding thermocouple (5) through the current direction conversion switch (2), and finally obtains the actual outer wall temperature of the metal tube (4) through the data acquisition system (6). The specific process is as follows:

[0020] A plurality of thermocouples (5) are spot welded in an orderly manner along the axial direction to the outer wall surface of a metal tube (4) to measure the outer wall temperature. The thermocouples (5) are connected to a data acquisition system (6). Two copper electrodes (3) are respectively installed at both ends of the experimental tube and connected to the output end of a current direction conversion switch (2). The input end of the current direction conversion switch (2) is connected to a direct current power supply (1). Different heat flux densities of the tube wall are obtained by adjusting the heating power of the direct current power supply.

[0021] Turn on the DC power supply, the heating power is,

[0022] Q=I 2 R(1)

[0023] The heat flux density is obtained as follows:

[0024]

[0025] In the formula, Q is the heating power of the metal tube, I is the output current of the DC power supply, R is the resistance of the metal tube, q is the heat flux density of the metal tube, and A is the surface area of ​​the metal tube.

[0026] Due to the thinness of the thermocouple wires and the manual operation, a gap ΔL is inevitably formed between the two thermocouple wires during welding. When a DC power supply is used to heat the experimental pipeline, the gap between the thermocouple wires will generate an additional potential difference ΔU, such as Figure 2 As shown, it will be added to the potential difference ΔU, generated by the thermocouple itself due to the thermoelectric effect. Therefore, the potential difference actually output by the wall-welded thermocouple is ΔU+ΔU,.

[0027] In order to solve this problem, when measuring the wall temperature, the device uses a current direction conversion switch (2), whose handle can be rotated clockwise or counterclockwise, with a conversion angle of 60°. Figure 3 As shown, when the handle of the current direction conversion switch (2) is at the scale 0 position, it indicates that the switch is disconnected. At this time, the circuit is in an open circuit state, and no current flows through the experimental metal tube (4). When the handle of the current direction conversion switch (2) is rotated 60° counterclockwise to scale 1, the metal tube (4) will be heated under the positive heating current (current direction 1). After the wall temperature stabilizes, the wall temperature value at this time is recorded. Then, the handle of the current direction conversion switch (2) is quickly rotated 120° clockwise to scale 2, so that the metal tube (4) is heated under the same voltage reverse current (current direction 2). After the wall temperature stabilizes, the temperature value is recorded again. Finally, the two recorded wall temperature values ​​are averaged by the computer in the data acquisition system (6) to obtain the actual wall temperature value under the heating heat flux density, as shown in FIG. Figure 4 shown. Figure 4 The vertical coordinate T w is the wall temperature value, and the horizontal axis n is the value of the number of thermocouples welded along the axial direction of the metal tube (4). In this embodiment, n is 20, that is, 20 thermocouples are welded equidistantly along the axial direction.

[0028] The device adopts a combination of a DC power supply with higher heating efficiency and a wall-surface welding thermocouple with higher temperature measurement frequency response to measure the wall temperature of the metal pipe in the electric heating experiment. The current direction conversion switch (2) can effectively eliminate the additional potential difference caused by the DC power supply heating to the wall-surface welding thermocouple, thereby measuring the true value of the DC electric heating pipe wall temperature.

[0029] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A device for measuring the outer wall temperature of a DC-heated metal tube, characterized in that: The device comprises a direct current power supply (1), a current direction conversion switch (2), a copper electrode (3), a metal tube (4), a thermocouple (5), and a data acquisition system (6); the direct current power supply (1) is connected to the input end of the current direction conversion switch (2); the output end of the current direction conversion switch (2) is connected to the copper electrode (3); the copper electrode (3) is installed at both ends of the metal tube (4); the thermocouple (5) is directly welded to the outer wall surface of the metal tube (4) by spot welding; and the data acquisition system (6) is connected to the thermocouple (5).

2. The device according to claim 1, characterized in that: The DC power supply (1) is a low-voltage, high-current DC power supply; the current direction conversion switch (2) is an LW26 type 2-section, 8-contact universal conversion switch, and is connected in series in the heating circuit.

3. The device according to claim 1, characterized in that: The thermocouple (5) wire is directly welded to the pipe wall by a spot welder. The thermocouple model is a commonly used T-type or K-type thermocouple. The wire diameter is 0.1 mm and the measurement accuracy is less than 0.5%. The data acquisition system (6) is connected to the thermocouple (5). The data acquisition system (6) is composed of a thermocouple signal acquisition module and a computer.

4. The device according to claim 1, characterized in that: The copper electrodes (3) are installed at both ends of the metal tube (4) and connected to the DC power supply (1). The material of the copper electrodes (3) is copper, and the material of the metal tube (4) is 304 stainless steel.

Images