CN104965010B - A kind of low temperature condenser type void fraction measuring device - Google Patents

Abstract

The invention discloses a kind of measurement by capacitance low temperature two-phase-flow void fraction devices, including：Test pipeline, curved surface electrode, electro-magnetic screen layer, vacuum sleeve and data collecting system.Experiment pipeline is non-metallic insulation cry-fluid line, including upper and lower pipe interface.Data collecting system includes conversion circuit (CDC), I2C buses and computer；Electro-magnetic screen layer provides electromagnetic shielding for curved surface electrode.The principle that this method change using cryogen gas-liquid difference in dielectric constant and total dielectric constant with void fraction measures low temperature two-phase-flow void fraction indirectly, will not stream field generate and interfere, while have it is easy to operate, it is at low cost, respond the advantages that fast.

Description

A low-temperature capacitive cavitation rate measuring device

technical field

The invention relates to the technical field of low-temperature refrigeration engineering, in particular to a low-temperature capacitive void ratio measuring device.

Background technique

Low-temperature gas-liquid two-phase flow widely exists in energy, air separation, aerospace and other fields. As an important parameter in two-phase flow, the cavitation rate is closely related to the manifold and local gas-liquid velocity; Monitoring has an important impact on the normal operation and process control of the two-phase flow system.

The method of measuring the void rate in two-phase flow based on the dielectric constant has the characteristics of fast response, low cost, and no interference to the flow field, and is widely used in real-time measurement of two-phase flow, such as oil-water-gas phase content measurement, Refrigerant flow status monitoring, etc. The measurement principle is that the electrodes arranged on both sides of the flow channel form a capacitor, and the capacitance value is a function of the dielectric constant of the two phases and the phase containment rate, and the phase containment rate is calculated by measuring the capacitance value. The relative permittivity of most gases is very close to 1, and compared with normal temperature fluid water, refrigerant, etc., the dielectric constant of cryogenic fluid is less different from the corresponding gas, as shown in Table 1.

At the same time, due to the low temperature environment, the dielectric constant of the organic material will change, which will also affect the measurement process. Therefore, at present, only the Russian Institute of Nuclear Engineering has applied the capacitance method to measure the two-phase flow in a low temperature environment, but the radio frequency method used to measure the capacitance is slow and difficult to meet the measurement fluctuation characteristics.

There are also literature reports on a measuring device that can be used in a deep low temperature environment using a similar method. Chinese patent CN102759492 discloses a device that uses the dielectric constant method to measure the density of low temperature fluids, but this invention focuses on the measurement of low temperature two phases in a static and stable state. The density of the mixture cannot satisfy the measurement of the cryogenic fluid in the flowing state. The application number is 201410146048.3, a gas-liquid two-phase flow phase holdup real-time capacitance measurement system and its measurement method, which require the liquid phase dielectric constant to be much larger than the gas phase dielectric constant, which is not suitable for small differences in the gas-liquid phase dielectric constant. cryogenic fluids. U.S. Patent US5291791 discloses a multi-electrode device for measuring multiple flows. Its main content is to measure the average void rate by changing the electric field. layer flow.

Contents of the invention

The invention provides a low-temperature capacitive cavitation rate measuring device with simple structure, convenient testing and accurate test results. The device can accurately measure the phase content of the two-phase flow in the low-temperature pipeline, thereby facilitating the monitoring of the working conditions in the pipeline.

Technical scheme of the present invention is as follows:

A capacitive device for measuring the void ratio of low-temperature two-phase flow includes an experimental pipeline, a curved surface electrode, an electromagnetic shielding layer, a vacuum jacket, and a data acquisition system, and the curved surface electrode and the electromagnetic shielding layer are attached to the pipe wall of the experimental pipeline As for the vacuum jacket, the data acquisition system is composed of a capacitance-to-digital conversion circuit, an I2C bus and a computer. The vacuum data line connector is connected to the I2C bus to communicate with the computer.

The experimental pipeline is a low-temperature-resistant thin-walled insulating pipeline. The two ends of the experimental pipeline are provided with inlet and outlet interfaces, and the whole is placed in a vacuum jacket, and a vacuum data line connector is provided on the vacuum jacket.

The curved surface electrodes are two metal sheets with the same curvature as the experimental pipeline, which are arranged symmetrically on both sides of the outer wall of the test pipeline. Tape wrapped.

The electromagnetic shielding layer is a long metal sheet with a width greater than that of the curved surface electrode, and completely covers the curved surface electrode. The inner side is two curved surface electrodes wrapped with insulating tape, and the two are separated by insulating tape. The electromagnetic shielding layer is welded with wires , connected to ground in the capacitance-to-digital conversion circuit.

The capacitance-to-digital conversion circuit is composed of a capacitance-to-digital conversion chip and peripheral circuits, and completes capacitance data acquisition and conversion from analog to digital. It requires an effective resolution of 4aF and an update rate of more than 20Hz to meet real-time performance. measurement requirements.

The inner diameter of the vacuum jacket is larger than that of the experimental pipeline, and has a vacuum data line connector.

The invention is a low-temperature capacitive cavitation rate measuring device, which adopts a non-invasive design, has a simple structure, is convenient for testing, and has no influence on the original flow field; at the same time, the measurement speed is fast, and it can have a real-time dynamic response to the flow field.

Description of drawings

Fig. 1 is a structural sectional view of a low-temperature capacitive cavitation rate measuring device of the present invention;

Fig. 2 is a schematic structural diagram of a low-temperature capacitive void ratio measuring device of the present invention;

Figure 3 shows the changes in capacitance and flow for multiple stable flows.

Figure 4 is the verification of capacitance and liquid level changes.

In the figure: 1. Experimental pipeline, 2. Curved surface electrode, 3. Electromagnetic shielding layer, 4. Vacuum jacket, 5. Insulating tape, 6. Experimental pipeline interface, 7. Capacitance-to-digital converter, 8. Vacuum data interface.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings, but the embodiments of the present invention are not limited thereto.

As shown in Figures 1 and 2, a capacitive device for measuring the void ratio of low-temperature two-phase flow includes an experimental pipeline 1, a curved surface electrode 2, an electromagnetic shielding layer 3, a vacuum jacket 4, and a data acquisition system, and the curved surface electrode 2 and the electromagnetic shielding layer 3 are pasted on the pipe wall of the experimental pipeline 1, and together in the vacuum jacket 4, the data acquisition system is composed of a capacitance-to-digital conversion circuit 7, an I2C bus and a computer, wherein the capacitance-to-digital conversion circuit and The polar plates on both sides of the experimental pipeline 1 are placed in the vacuum layer 4 together, and the digital signal is connected to the I2C bus through the vacuum data line connector 8 to communicate with the computer.

In one embodiment of the present invention, the experimental pipeline 1 is a low-temperature-resistant thin-walled insulating pipeline, and the two ends of the experimental pipeline 1 are provided with inlet and outlet interfaces 6, which are placed in the vacuum jacket 4 as a whole, and the vacuum jacket has Vacuum data line connector 8.

In one embodiment of the present invention, the curved electrode 2 is two metal sheets with the same curvature as the experimental pipeline, which are arranged symmetrically close to the two sides of the outer wall of the experimental pipeline 1, and wires are welded on the outside of the curved surface electrode and connected to the Capacitance-to-digital conversion circuit 7, and the outside of the curved surface electrode 2 is wrapped with insulating tape 5 at the same time.

In one embodiment of the present invention, the electromagnetic shielding layer 3 is a long metal sheet with a width greater than that of the curved surface electrode 2, and completely covers the curved surface electrode 2, and the inner side is two pieces of curved surface electrodes 2 wrapped with insulating adhesive tape 5, two They are separated by insulating tape 5 , and the electromagnetic shielding layer 3 is welded with wires and connected to the ground in the capacitance-to-digital conversion circuit 7 .

In one embodiment of the present invention, the capacitance-to-digital conversion circuit 7 is composed of a capacitance-to-digital conversion chip and peripheral circuits, and completes capacitance data acquisition and conversion from analog to digital. The effective resolution is required to reach 4aF, and at the same time The update rate reaches over 20Hz to meet the real-time measurement requirements.

In one embodiment of the present invention, the inner diameter of the vacuum jacket 4 is larger than that of the experimental pipeline, and has a vacuum data line connector 8 .

In one embodiment of the present invention, the test bench is mainly composed of three parts: a liquid nitrogen supply system, an adiabatic test tube and a measurement system. Saturated nitrogen under pressure presses saturated liquid nitrogen out of the Dewar, controls the flow by changing the pressure, passes through the cryogenic turbine flowmeter (model: HOFFOR 1/4X1/4-.35-3.5, accuracy: ±0.5%), and enters Experimental pipeline 1. The experimental pipeline is a quartz glass material tube with an inner diameter of 34mm and a length of 800mm. Capacitive sensors are arranged on the experimental pipeline, and the outer vacuum layer is used to realize the heat insulation of the experimental pipeline.

The hyperbolic panel electrode structure with the highest sensitivity is selected, and it is placed close to both sides of the insulating pipe and placed symmetrically. The thickness of the copper sheet of the capacitor electrode group is 0.5mm, the width of the copper sheet is 20mm, and the coverage angle α is 170°. It also includes a shielding layer that provides electromagnetic shielding for the measuring electrodes.

The data acquisition system is mainly composed of a capacitance-to-digital conversion circuit CDC7, an I2C bus, and a computer. The capacitance-to-digital conversion circuit 7 and the plates on both sides of the experimental pipeline 1 are placed in the heat insulation layer to minimize the transmission distance of the analog signal. At the same time, pay attention to the shielding layer around the electrodes to reduce interference signals; the digital signal is connected to the I2C bus through the vacuum connector to communicate with the computer. Among them, the capacitance-to-digital conversion circuit 7 is mainly composed of the AD7746 chip of ADI Company and peripheral circuits, and mainly completes capacitance data acquisition and conversion from analog to digital. The AD7746 chip has an effective resolution of 21-bit, about 4aF, and its factory calibration accuracy is 4fF; at the same time, the update rate of the chip can reach up to 90Hz to meet the real-time measurement requirements.

First pre-cool the pipeline with a small amount of liquid nitrogen, wait until the pipeline is pre-cooled to the temperature of liquid nitrogen, and then carry out the test measurement. During the experiment, the curved surface electrodes on both sides constitute a capacitor. When the flow rate of liquid nitrogen is changed, the content of the two-phase flow in the experimental pipeline changes accordingly, that is, the electrolyte between the curved surface electrodes changes, and the capacitance value of the capacitor is measured through the data acquisition system. , the change of phase content in the low temperature two-phase flow can be obtained.

The experimental results are shown in Figure 3, the dotted line is the flow data Q, and the solid line is the collected capacitance value C. During the experiment, the flow rate of liquid nitrogen was changed many times, and it can be seen that the signal of the capacitive sensor and the signal of the flowmeter have corresponding changes. Except for the wave peak formed by a large amount of gas passing through the flowmeter at the beginning, every time the stable flow is maintained for a period of time, the capacitance The value also fluctuates within a certain range. It can be seen that the two signals are highly correlated. After closing the liquid nitrogen valve, the flow rate returns to zero quickly, and the capacitive sensor also drops to the lowest point. After turning on the flowmeter again, it appeared again, until finally the liquid nitrogen was basically used up, a large amount of gas appeared in the flow channel, and the capacitance sensor gradually decreased to the initial value.

In order to further verify the accuracy of the present invention's measurement, read out all the continuous and steady flow states in Fig. 3, calculate the average flow rate at this stage, and apply the laminar flow assumption to calculate the height under the corresponding flow rate. The experimental points where the capacitance increment varies with height are obtained as shown in Figure 4. At the same time, the theoretical curve calculated based on the finite element software is shown in the figure, and the two are in good agreement in the experimental interval.

Claims (2)

1. a kind of measurement by capacitance low temperature two-phase-flow void fraction device, which is characterized in that it includes experiment pipeline (1), curved surface electricity Pole (2), electro-magnetic screen layer (3), vacuum jacket (4) and data collecting system, the curved surface electrode (2) and electro-magnetic screen layer (3) it pastes and on the tube wall of experiment pipeline (1), jointly as in vacuum jacket (4), the data collecting system is by capacitance number Conversion circuit (7), I2C buses and computer composition, capacitance digital conversion circuit (7) therein and experiment pipeline (1) both sides Pole plate is collectively disposed in vacuum layer (4), and digital signal is connected to I2C buses after vacuum data wire terminal (8), with computer It is communicated；

The experiment pipeline (1) is low temperature resistant thinwalled insulation pipeline, and experimental channel (1) is provided at both ends with inlet and outlet connectors (6), Entirety is placed in vacuum jacket (4), has vacuum data wire terminal (8) on vacuum jacket (4)；

The curved surface electrode (2) is two panels sheet metal identical with experiment pipeline (1) curvature, is close to test pipeline (1) outside wall surface Both sides are simultaneously arranged symmetrically, and welding lead on the outside of curved surface electrode (2) is connected to capacitance digital conversion circuit (7), curved surface electrode (2) It is wrapped up simultaneously with insulating tape (5) in outside；

The copper thickness of curved surface electrode (2) group is 0.5mm, and copper sheet width is 20mm, and two pieces of capacitor plate cone of coverage α are 170 °；

The electro-magnetic screen layer (3), the long sheet metal of curved surface electrode (2) is more than for width, and curved surface electrode (2) is covered completely Lid, inside is the two panels curved surface electrode (2) for being surrounded by insulating tape (5), is separated by insulating tape (5) therebetween, electromagnetic shielding Layer (3) welding lead is connected to ground connection in capacitance digital conversion circuit (7)；

The capacitance digital conversion circuit (7), is made of capacitance number conversion chip and peripheral circuit, completes capacitance data acquisition With the conversion work of analog quantity to digital quantity, it is desirable that effective resolution reaches 4aF, while renewal rate being wanted to reach 20Hz or more, To meet the measurement request of real-time.

2. measurement by capacitance low temperature two-phase-flow void fraction device as described in claim 1, it is characterized in that the vacuum jacket (4), internal diameter is more than experiment pipeline (1), has vacuum data wire terminal (8).