CN104897737B - Eight electrode rotary Electric field conductivity sensor gas holdup measurement methods - Google Patents

Abstract

The invention relates to a method for measuring the gas holdup rate of an eight-electrode rotating electric field type conductance sensor. The sensor used includes four pairs of electrodes uniformly distributed on the same section of the inner wall of the pipeline, and each pair of electrodes is arranged oppositely; the following method is used for gas holdup Rate measurement: Sine signals with different initial phases are respectively applied to four pairs of electrodes to excite, and the phase difference between adjacent electrodes is 45°, so that a rotating measurement electric field can be synthesized on the cross section; when the gas-liquid two-phase fluid flows through When using the sensor, collect the output signal of the sensor; calculate the normalized conductance of the rotating electric field conductance sensor; use the normalized conductance of the rotating electric field conductance sensor to calculate the gas holdup rate. The invention has the advantages of relatively accurate measurement and simple structure.

Description

Measuring Method of Gas Holdup by Eight-electrode Rotating Electric Field Conductivity Sensor

Technical field

The invention belongs to the technical field of fluid measurement and relates to a conductivity sensor.

Background technique

Two-phase flow phenomena widely exist in traditional and emerging industries such as petroleum engineering, chemical engineering, metallurgical engineering, nuclear engineering, aviation and aerospace engineering. Gas-liquid two-phase flow refers to the mixed flow system of gas phase and liquid phase incompatible substances. Due to the differences in physical properties such as density and viscosity among the components in gas-liquid two-phase flow, under the influence of many factors such as flow rate, pressure, gravity and pipeline shape, the measurement of flow parameters of gas-liquid two-phase flow is very difficult. difficulty. Cross-sectional gas holdup is an important flow parameter in the gas-liquid two-phase flow industrial application system, and its accurate measurement is of great significance for the measurement, control and operation reliability of the production process.

Two-phase flow gas holdup measurement techniques mainly include ultrasonic method, optical method, ray method, capacitance method, conductometric method and so on. Since the conductivity sensor has many advantages such as clear principle, simple structure, and stable response, it has been widely used in the measurement of multiphase flow parameters. In the early stage of sensor development, flat electrodes were often used to measure the thickness of the liquid film. In order to avoid the disturbance of the sensor's convective pattern, Ring electrode sensors embedded in the inner wall of vertically rising pipelines have emerged, such as ring conductivity sensors and wall-to-wall ring conductivity sensors. However, the single-direction excitation and reception of the wall-type annular conductance sensor has limitations in the directionality of the electric field distribution and is easily affected by the flow pattern. In order to solve this problem, M.Merilo et al. proposed a rotating electric field conductance measurement method in "Void fraction measurement with arotating electric field conductance gauge" (Journal of Heat Transfer, 1997, Vol 99, P330), by applying three-phase alternating current around the tube wall Three pairs of electrodes are arranged to synthesize and generate a rotating measurement electric field, which eliminates the measurement error caused by the uneven distribution of the flow medium to a certain extent. However, it has not been demonstrated from the perspective of theoretical analysis and experimental verification whether the rotating measurement electric field generated by the combination of three pairs of electrodes is the best measurement method.

Contents of the invention

In view of the above problems, the object of the present invention is to provide a relatively accurate and simple and feasible two-phase flow gas holdup measurement method. The technical solution of the present invention is as follows:

A method for measuring gas holdup of an eight-electrode rotating electric field conductance sensor, the sensor used includes four pairs of electrodes evenly distributed on the same section of the inner wall of the pipeline, and each pair of electrodes is arranged oppositely; the four pairs of electrodes are sequentially A, B, C and D, A is adjacent to B, B is adjacent to C, C is adjacent to D, and D is adjacent to A. The gas holdup is measured by the following method:

(1) Sinusoidal signals with different initial phases are respectively applied to the four pairs of electrodes to excite, and the phase difference between adjacent electrodes is 45°, so that a rotating measurement electric field can be synthesized on the cross-section;

(2) When the gas-liquid two-phase fluid flows through the sensor, the output signal of the sensor is collected;

(3) Define the normalized conductivity G _e of the mixed fluid as the ratio of the conductivity σ _m of the mixed phase to the conductivity σ _w of the whole water, and the normalized conductance of the eight-electrode rotating electric field conductivity sensor is defined as four pairs of electrodes The average value of the normalized conductance calculates the normalized conductance value, calculates the normalized conductance of the rotating electric field conductance sensor

(4) Normalized conductance using rotating electric field conductance sensor Calculate gas holdup.

As a preferred embodiment, the gas holdup measurement method of the conductivity sensor is characterized in that the electrode opening angle θ is 22.5°. The axial height H of the electrode is 0.004m, and the radial thickness T of the electrode is 0.001m.

The gas holdup measurement method of the eight-electrode rotating electric field type conductivity sensor proposed by the present invention applies sinusoidal excitation signals with a phase difference of 45 degrees to the four pairs of electrodes to synthesize and generate a rotating electric field, and conducts theoretical analysis and calculation of the sensitivity of the section measurement electric field, The optimal geometric structure parameters of the eight electrodes are determined to achieve the best cross-sectional gas holdup measurement effect. Has the following advantages:

(1) The rotating electric field type conductivity sensor involved in the present invention has the advantages of simple structure, fast response speed, high stability, and convenient installation and measurement.

(2) The gas holdup measurement method of the present invention can be used for gas holdup measurement of gas-liquid two-phase flow at medium and low flow rates, and has simple calculation and high accuracy.

(3) The gas holdup measurement method of the present invention is applicable to the gas holdup measurement under vertical gas-liquid two-phase flow in bubbly flow, slug flow and mixed flow.

Description of drawings

Figure 1 is a schematic diagram of the geometric parameters of the rotating electric field conductivity sensor: (a) perspective view; (b) cross-sectional view; (c) front view

Fig. 2 is a schematic diagram of the excitation mode of the rotating electric field conductance sensor.

Fig. 3 is a finite element dissection structure diagram of the rotating electric field conductance sensor.

Figure 4 is the signal diagram of four pairs of electrodes for three flow patterns of gas-liquid two-phase flow, (a), (b) and (c) respectively represent bubbly flow, slug flow, and mixed flow.

Fig. 5 is the experimental chart between the normalized conductance value of the gas-liquid two-phase flow experimental measurement data and the water phase flow rate and gas phase flow rate calibrated by the simulation device.

Fig. 6 Effect diagram of gas holdup measurement for gas-liquid two-phase flow.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

The feature of the present invention is that by optimizing the structural size of the sensor, a relatively uniform measurement sensitive field is generated on the pipeline cross section. The structure and size optimization and measurement method of the gas-liquid two-phase flow rotating electric field type conductivity sensor of the present invention include the following steps:

(1) The structure of the eight-electrode wall-to-wall ring conductivity sensor is shown in Figure 1, which consists of four pairs of stainless steel electrodes. As shown in Figure 2, four pairs of electrodes are respectively excited by applying sinusoidal signals with different initial phases, A is 0°, B is 45°, C is 90°, and D is 135°, so that the rotation can be synthesized on the cross section Measure the electric field.

(2) The present invention uses the finite element method to optimize the structural size of the sensor, and uses the simulation software ANSYS to establish a rotating electric field conductance sensor model, as shown in FIG. 3 . When modeling, set the inner diameter of the vertically rising pipeline D = 0.02m, the length of the vertically rising pipeline L = 0.2m, the electrode radial thickness T, the electrode axial height H, the electrode opening angle θ, and the water phase resistivity δ _w = 1000Ω· m, electrode resistivity δ _s =1.7241e-8Ω·m. The free meshing method is used for mesh division, and the sinusoidal excitation is used when the load is applied. The simulation method is: when modeling in ANSYS, put a small ball with a diameter of 0.5mm on the measured section of the model to simulate the bubble movement. When the ball is in different positions, the voltage of the excitation electrode also changes accordingly, so the sensitivity of the conductivity sensor can be reflected by the voltage change of the excitation electrode. Every time the ball transforms a coordinate, the sensitivity value at that coordinate can be calculated. The coordinates of the ball traverse all positions of the vertically rising pipeline section to obtain the sensitivity distribution of the pair of electrodes.

The present invention uses the detection field uniformity error parameter (SVP) and the sensor relative sensitivity (S _avg ) as optimization targets. The relative sensitivity of the sensor (S _avg ) means the average value of the relative sensitivity of all positions of the section, defined as:

Define the uniformity error parameter (SVP) of the measurement section as:

In the formula, S _dev is the standard deviation of relative sensitivity at different positions on the measurement section, which is defined as:

Obviously, the larger the S _avg value, the higher the sensitivity of the sensor, and the smaller the SVP value, that is, the smaller the uniformity error.

The optimal design was carried out by single factor rotation method. Only one of the factors is changed, and the rest are fixed, and then a step-by-step collocation experiment is carried out to obtain a good collocation plan, which is called the single-factor rotation method, also known as the isolated factor method.

First, the electrode opening angle θ and electrode radial thickness T are fixed, the electrode axial height H is changed, the uniformity error parameter (SVP) and the relative sensitivity of the sensor (S _avg ) are shown in the table below. It can be seen that when the electrode axial height H is 0.004m, the distribution characteristics of the sensitivity field are the best. Therefore, the electrode axial height H is fixed at 0.004m.

Then, the electrode axial height H and the electrode radial thickness T are fixed, the electrode opening angle θ is changed, the uniformity error parameter (SVP) and the relative sensitivity of the sensor (S _avg ) are shown in the table below. It can be seen that when the electrode opening angle θ is 22.5°, the distribution characteristics of the sensitivity field are the best. Therefore, the electrode opening angle θ is fixed at 22.5°.

Finally, the electrode opening angle θ and the electrode axial height H are fixed, the electrode radial thickness T is changed, the uniformity error parameter (SVP) and the relative sensitivity of the sensor (S _avg ) are shown in the table below. It can be seen that when the electrode radial thickness T is 0.001m, the distribution characteristics of the sensitivity field are the best. Therefore, the electrode radial thickness T was fixed at 0.001 m.

By optimizing the size of the above sensor, the optimal parameters of the sensor can be obtained: the electrode opening angle θ is fixed at 22.5°, the axial height H of the electrode is fixed at 0.004m, and the radial thickness T of the electrode is fixed at 0.001m. At this time, _Savg is 75.9804%, and SVP is 0.2172.

At the same time, the sensitivity field simulation of the six-electrode rotating electric field conductance sensor (the structure proposed by M.Merilo) was carried out. The optimal parameters are that the electrode opening angle θ is fixed at 30°, the axial height H of the electrode is fixed at 0.003m, and the electrode diameter The thickness T is fixed at 0.001m. Its S _avg is 66.1191%, and its SVP is 0.2768. It can be seen that the eight-electrode rotating electric field conductivity sensor is superior to the six-electrode structure in terms of sensitivity and uniformity, so the eight-electrode structure is finally selected.

(3) Install the designed eight-electrode rotating electric field conductivity sensor on a vertically rising oil-gas-liquid phase flow pipeline with a pipe diameter of 20mm. When the gas-liquid two-phase fluid flows through the sensor, the sensor output signal is collected. The experimental verification method for the gas holdup measurement of gas-liquid two-phase flow is as follows: the fluid medium in the experiment is tap water and air, the industrial peristaltic pump and the air pump are used to transport the water phase and the gas phase respectively, and the flow rate of the gas phase is fixed to adjust the flow rate of the water phase to Get different ratios of working conditions. The flow rate of the water phase and the gas phase is set and passed into the vertical ascending pipeline at the same time. When the flow state of the two-phase flow is stable, the signal of the conductivity sensor is collected.

The method to obtain the measured normalized conductance value is as follows:

The normalized conductivity _Ge of the mixed fluid is defined as the ratio of the conductivity _σm of the mixed phase to the conductivity _σw of the whole water, and the expression is:

Among them, _σm and _σw are the conductivity of the mixed fluid and the conductivity of pure water, respectively, _Vm is the measured voltage of each pair of electrodes of the sensor, and _Vw is the measured voltage of the sensor in pure water. The normalized conductance of the rotating electric field conductance sensor is defined as the average value of the normalized conductance of four pairs of electrodes, which is defined as:

in, They are the normalized conductance values of phase A, phase B, phase C and phase D electrodes respectively.

(4) Experimental verification and results: Figure 4 shows the measurement signals of the sensor under three flow types (bubbly flow, slug flow, and mixed flow). It can be seen from the signals that there are four pairs of electrode signals at the same time These differences are caused by non-uniform distribution of the flowing medium. The measurement signals of the four pairs of electrodes are averaged to obtain the normalized conductance value, which can better solve this problem to a certain extent, and can also better reflect the flow characteristics of the gas-liquid two-phase flow. The experimental correlation chart between the normalized conductance value of the gas-liquid two-phase flow measurement results and the water phase flow rate and the gas phase flow rate is shown in Figure 5. It can be seen that the normalized conductance value has satisfactory sensitivity and resolution to the change of gas phase content.

In the experiment, the gas holdup rate y _g under each working condition was measured by using the quick-closing valve, and the gas-liquid two-phase flow gas holdup rate measurement statistical model was obtained by using the least square method:

The gas holdup measurement effect is shown in Figure 6, and the average absolute error of the gas-liquid two-phase flow gas holdup measurement results obtained by the rotating electric field conductance sensor is 0.023.

Claims (3)

1. A method for measuring gas holdup of an eight-electrode rotating electric field type conductance sensor, the sensor used includes four pairs of electrodes evenly distributed on the same section of the pipeline inner wall, and each pair of electrodes is relatively arranged; if the four pairs of electrodes are sequentially A, B, C and D, A is adjacent to B, B is adjacent to C, C is adjacent to D, and D is adjacent to A. Use the following method to measure gas holdup: (1) Sinusoidal signals with different initial phases are respectively applied to the four pairs of electrodes to excite, and the phase difference between adjacent electrodes is 45°, so that a rotating measurement electric field can be synthesized on the cross-section; (2) When the gas-liquid two-phase fluid flows through the sensor, respectively collect the output signals of the four pairs of electrodes of the eight-electrode rotating electric field conductivity sensor; (3) Define the normalized conductivity Ge of the gas-liquid two-phase fluid as the ratio of the conductivity of the gas-liquid two-phase fluid to the conductivity of the whole water, and the normalized conductivity Ge of the eight-electrode rotating electric field conductivity sensor is defined as four The average value of the normalized conductivity of the counter electrode; (4) The gas holdup was calculated by using the normalized conductivity of the eight-electrode rotating electric field conductivity sensor. 2. The method for measuring gas holdup of a conductivity sensor according to claim 1, wherein the electrode opening angle θ is 22.5°. 3. The method for measuring gas holdup of a conductivity sensor according to claim 1, wherein the electrode axial height H is 0.004m, and the electrode radial thickness T is 0.001m.