CN111504172B

CN111504172B - Calibration device for thin liquid film sensor of conductive ring

Info

Publication number: CN111504172B
Application number: CN202010351732.0A
Authority: CN
Inventors: 孙宏军; 张文强
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2020-04-28
Filing date: 2020-04-28
Publication date: 2021-04-27
Anticipated expiration: 2040-04-28
Also published as: CN111504172A

Abstract

The invention relates to a calibration device of a conductive ring thin liquid film sensor, which comprises two plane strip electrodes, a stepping motor, a controller, a lead screw, an internal thread sleeve, a water tank, an excitation and acquisition module and an upper computer, wherein the length and the distance of the two plane strip electrodes are the same as the perimeter and the distance of two conductive rings of the conductive ring thin liquid film sensor to be calibrated; the excitation and acquisition module is used for applying voltage signals to the two plane strip electrodes, measuring current signals passing through the two electrodes and sending liquid film thickness information represented by the current signals to the upper computer.

Description

Calibration device for thin liquid film sensor of conductive ring

Technical Field

The invention belongs to the field of measurement of an annular fog-shaped flow liquid film, and relates to a thin liquid film calibration technology which can be used for calibrating the thickness of a thin liquid film of an electrical conductance ring sensor.

Background

The moisture is a two-phase flow pattern which takes a gas (vapor) phase as a continuous phase and a liquid phase as a discrete phase, and is represented as annular fog flow when the flow rate is high, and the moisture enters a saturated or supersaturated state along with the change of working conditions after being conveyed, so that the annular fog flow is often formed due to heat loss and temperature reduction.

The liquid phase in the annular mist flow exists in the form of discrete droplets and a liquid film on the wall surface, and the liquid film is attached to the inner wall of the pipeline in an annular form [1] [2] and is usually very thin. Usually, a conductive sensor is used for measuring the thickness of a liquid film on a wall surface on line, for example, the conductive probe, the conductive ring, the planar electrode and other electrode shapes are adopted, and the conductive ring is widely applied to the measurement of the liquid film because the structure of the conductive ring is not influenced by the circumferential non-uniformity of a pipeline. Because the liquid film in the annular fog flow is very thin and basically less than 1mm, the calibration of the conductive annular liquid film sensor is difficult, and how to accurately calibrate the conductive annular liquid film sensor is a key for judging whether the sensor can be normally applied.

Reference to the literature

[1] Li Weidong, Li Rong Xian, Wang Yun Shi, et al, theoretical model for predicting the distribution of liquid film thickness in the circumferential direction of gas-liquid annular flow in horizontal pipe [ J ] chemical science, 2001(03):18-22.

[2]Setyawan A,Indarto,Deendarlianto.Experimental investigations of the circumferential liquid film distribution of air-water annular two-phase flow in a horizontal pipe[J].Experimental Thermal&Fluid Science,2017,85(Complete):95-118.

Disclosure of Invention

The invention aims to overcome the problem of calibrating a traditional pipeline electric guide ring thin liquid film sensor, and provides an equivalent calibrating device to realize quick and accurate calibration of the liquid film sensor. The technical scheme of the invention is as follows:

a calibration device for a conductive ring thin liquid film sensor comprises two planar strip electrodes, a stepping motor, a controller, a lead screw, an internal thread sleeve, a water tank, an excitation and acquisition module and an upper computer, wherein the length and the distance between the two planar strip electrodes are the same as the perimeter and the distance between two conductive rings of the conductive ring thin liquid film sensor to be calibrated; the excitation and acquisition module is used for applying voltage signals to the two plane strip electrodes, measuring current signals passing through the two electrodes and sending liquid film thickness information represented by the current signals to the upper computer.

The invention designs a calibration device for a thin liquid film electric guide ring sensor, wherein a guide ring is equivalent to a planar strip electrode, the feasibility is verified by simulating with COMSOL, the planar electrode is driven by a stepping motor to move so as to change the thickness of a liquid film on the surface of the electrode, and the device can calibrate the liquid film of the guide ring sensor with the thickness of less than 1 mm.

Drawings

FIG. 1 is a thin liquid film sensor of a conductive ring needing calibration

FIG. 2 is a COMSOL simulation model of a conductive ring equivalent to a strip-shaped planar electrode

FIG. 3 is a comparison graph of simulation results of conductive rings and long-strip planar electrodes

FIG. 4 is a diagram showing a structure of a calibration apparatus

FIG. 5 is a block diagram of an excitation and acquisition module

In the drawings: 1-metal electric guide ring 2-insulated circular measuring pipeline 3-wall surface liquid film 4-excitation and acquisition module 5-upper computer 6-plane long electrode 7-insulated flat plate 8-PIC singlechip 9-stepping motor 10-reduction gearbox 11-lead screw 12-internal thread sleeve 13-water tank

Detailed Description

The invention designs a calibration device and a calibration method for a pipeline thin liquid film thickness measurement sensor. In order to further understand the features and technical means of the present invention and achieve specific objects and functions, the following detailed description of the embodiments of the present invention is provided with reference to the accompanying drawings.

Fig. 1 is a conductance ring thin liquid film sensor needing to carry out liquid film thickness calibration, which is composed of two metal conductance rings 1, embedded in an insulating pipeline 2, and applies voltage signals to two electrodes through an excitation and acquisition module 4, measures the passing current, and sends the measurement result to an upper computer 5 through a serial port, and the thickness of a liquid film 3 between the two electrodes is calculated and displayed by the upper computer, and the conductance ring thin liquid film sensor is defined as follows:

wherein G is the conductance of the conductor to be tested, sigma is the conductivity of the conductor to be tested, S is the cross-sectional area of the conductor, and L is the length of the conductor. Since the liquid film in the pipe is attached to the wall of the pipe, the liquid film between the two electrodes is considered to be an annular column, and when h < D, the conductance is:

wherein D is the diameter of the pipeline, D is the distance between two conductive rings, sigma_lIs the conductivity of the liquid and h is the liquid film thickness. It can thus be assumed that the thickness of the liquid film is proportional to the conductance between the two metal rings. However, since the liquid film is inside the circular pipeline and the thickness of the liquid film is below 1mm, it is difficult to precisely control the thickness variation of the liquid film, and static calibration cannot be directly performed.

It can be seen that if there are two elongate electrodes, also spaced apart by D, and of length l ═ pi D, the conductance of the liquid film between them is:

therefore, when the thin liquid film is measured and h & lt D is met, the annular electrode can be equivalent to the planar strip electrode to perform a thickness measurement calibration experiment, and the calibration result of the planar strip electrode is applied to measurement of the electrical conductive ring.

The electrodes with two shapes are simulated by utilizing COMSOL software, a simulation model diagram is shown in figure 2, the left side is a conductive ring structure, the right side is a plane long-strip electrode structure, the length of the long-strip electrode is equal to the perimeter of the conductive ring, the distance between the two long-strip electrodes and the distance between the two conductive rings are both 5mm, and the width of the electrode is both 1 mm. The metal electrode material attribute is set as copper, the insulating material is plastic, and the liquid film is tap water. A sinusoidal signal of 5V500kHz is applied between the two electrodes respectively, and the current passing through the electrodes is detected along with the thickness of the liquid film. Fig. 3 is a simulation result, it can be seen that when the liquid film thickness is low, especially when the liquid film thickness is below 1mm, the measuring curves of the two shapes of electrodes are basically overlapped, when the liquid film thickness is higher, the measuring result of the electric conduction ring is smaller due to the circular nonlinear structure, so that when the liquid film thickness is below 1mm, the liquid film thickness can be calibrated by using the planar long electrode instead of the electric conduction ring.

Fig. 4 is a structure diagram of a calibration device, a PIC single-chip microcomputer 8 as a controller is used for controlling a stepping motor 9, the stepping motor drives a screw rod 11 to rotate after passing through a reduction box 10, the screw rod drives an internal thread sleeve 12 to move up and down, calibrated planar strip electrodes (6 and 7) are installed at the bottom of the sleeve and penetrate into a water tank 13 filled with water, and the distance between the electrodes and the bottom of the water tank is changed through moving up and down, so that the thickness of a liquid film on the surface of the electrodes is changed.

The screw pitches of the screw rod and the threaded sleeve are both 0.5mm, the step angle of the stepping motor is 1.8 degrees, the gear ratio of the gearbox is 1:5, 200 pulses are needed when the motor rotates for one circle, the reduction gearbox is connected in sequence at 1:5, the motor rotates for 5 circles, the screw rod rotates for one circle, the electrode moves for 0.5mm, if 0.1mm stepping is realized, the motor just rotates for one circle, namely 200 pulses control the screw rod to move for 0.1mm, the single chip microcomputer gives a control signal according to 200 pulses, the 0.1mm stepping can be realized, and the thickness of the surface of an electrode liquid film can be controlled to be adjusted from 0-2 mm to 0.1mm stepping.

Under the condition of known liquid film thickness, the electrodes are excited and collected by using FPGA, as shown in figure 5, firstly, a 5V-shaped and 500kHz sine signal is output by using DDS IP core through high-speed DA, and the sine signal is applied between the two electrodes after passing through a low-pass filtering and amplitude modulation circuit. Then measuring a current signal passing between the two electrodes, amplifying the current signal into a voltage signal through a current amplifying circuit, converting an alternating current signal into a direct current signal through a peak value detection circuit, collecting the direct current signal through an AD module, enabling the direct current signal to enter an FPGA (field programmable gate array), converting the voltage signal into liquid film thickness information by the FPGA, sending the liquid film thickness information to an upper computer through a serial port, displaying the measured liquid film thickness on the upper computer, and storing data, thereby realizing the online measurement of the liquid film thickness.

By utilizing the device and the method, the calibration of the conductivity ring sensor on the thickness measurement of the thin liquid film can be realized, the calibration range can be adjusted in a 0.1mm stepping mode from 0-2 mm, and the calibration device and the calibration method can be popularized to the calibration of conductivity sensors in other forms.

Claims

1. A calibration device for a conductive ring thin liquid film sensor comprises two planar strip electrodes, a stepping motor, a controller, a lead screw, an internal thread sleeve, a water tank, an excitation and acquisition module and an upper computer, wherein the length and the distance between the two planar strip electrodes are the same as the perimeter and the distance between two conductive rings of the conductive ring thin liquid film sensor to be calibrated; the excitation and acquisition module is used for applying voltage signals to the two plane strip electrodes, measuring current signals passing through the two electrodes and sending liquid film thickness information represented by the current signals to the upper computer.