Design of measurement system based on microwave resonant cavity moisture measurement technology

1 Introduction

The last stages of large condensing steam turbines in conventional power plants and all stages of turbines in nuclear power plants work in a wet steam state. The liquid water content in wet steam has a great influence on the working medium and its efficiency. The existence of steam humidity not only reduces the operating efficiency of the turbine stage, but also causes severe water erosion of the blades, which brings hidden dangers to the economy and safety of the operating units in the power plant. Therefore, the liquid water content in the wet steam, that is, the degree of liquefaction, is accurately measured. It is of great significance to the long-term stable operation and life of the gas turbine.

Microwave resonant cavity moisture measurement technology is a popular technology that has emerged at home and abroad in recent years. The dielectric constant of the medium in the cavity is determined by the humidity of the flowing gas under a certain pressure and temperature. According to the characteristic that the resonant frequency in the resonant cavity shifts with the change of the dielectric constant of the dielectric in the cavity, if the change in the resonant frequency can be accurately measured, the humidity of the flowing gas can be measured. Compared with the currently used optical method and thermodynamic method, the use of this steam humidity measurement method can simplify the equipment, improve the measurement accuracy, and is conducive to online monitoring. It is a new method of moisture measurement with great development prospects. In this measurement method, the cavity structure, coupling structure and performance of the microwave resonant cavity are important factors affecting the humidity measurement accuracy. Here, a small-volume resonant cavity for a coaxial line coupling device is designed. Compared with rectangular waveguide coupling, its structure is more suitable for measuring the humidity of wet steam flowing in the turbine.

2 Principle of measuring the humidity of flowing wet steam in the steam turbine

It can be seen from the literature that the steam humidity relationship based on the theory of measuring the resonant frequency shift of the resonant cavity is:

In the formula: δf is the frequency deviation; f0 is the resonant frequency of the resonant cavity.

It can be seen from equation (1) that the humidity of wet steam circulating in the cavity can be obtained by measuring the offset of the resonant frequency fs of the resonant cavity sensor. Since the system operating frequency is f=9.6GHz, the designed f0=9.6 GHz. Figure 1 shows the relationship curve between humidity and frequency deviation calculated from the coaxial line characteristic impedance Z0 when the temperature is 30°C and the pressure is 0.02 MPa. It can be seen that the two are approximately linear within a small range, that is, within the turbine operating humidity range.

3 Microwave resonator structure design

3.1 Measurement system

To achieve wet steam measurement in a steam turbine, the resonant cavity needs to be placed inside the steam turbine cavity to achieve real-time data acquisition of humidity. The measured real-time data needs to be transmitted to the external processing circuit through the feeder. At the same time, the external processing circuit also needs to monitor the frequency offset in the resonant cavity in real time through the feeder. As a transmission medium, the feeder should meet the following requirements: ① When entering the steam chamber of the steam turbine, the damage to the cavity should be minimal; ② When used as a coupling device, the coupling with the resonant cavity must meet the requirements; ③ When transmitting signals, it must minimize attenuation.

3.2 Structural dimensions of cylindrical resonant cavity

Cylindrical resonant cavities are widely used due to their high quality Q number, solid structure, and easy processing and manufacturing. The cylindrical resonant cavity can be regarded as a cylindrical waveguide with both ends closed by conductor plates.

Since the steam humidity measurement system has high requirements for measurement sensitivity and accuracy, the TE011 mode with higher Q is selected as the working mode of the cylindrical resonant cavity. The electric field of the TE011 mode only has components along the φ direction; the magnetic field distribution has components along the r and z directions. There is only current along the φ direction on the inner surfaces of the side walls and both end walls of the cavity, and there is no current passing between the side walls and the two end walls of the resonant cavity. Therefore, non-contact pistons can be used for tuning to reduce cavity wear and weaken the influence of some interfering modes. Figure 2 shows the field structure of the TE011 mode cylindrical resonant cavity. The solid lines in the figure are electric power lines; the dotted lines are magnetic lines.

In the formula: a and l are the diameter and length of the resonant cavity respectively; c is the speed of light; ε and μ are the dielectric constant and magnetic permeability of the medium respectively.

The no-load quality factor of the TE011 mode cylindrical resonant cavity is expressed as:

In the formula: δ is the skin depth of the resonant cavity material at the resonant frequency.

The cylindrical resonant cavity is a key component in the humidity measurement system, which determines the operating frequency of the humidity measurement system. Its components can directly affect the accuracy and sensitivity of the system.

3.3 Coupling structure

The coupling method adopts ring coupling. The coaxial ring becomes a magnetic dipole under the action of the magnetic field, and the resonator and the coaxial line are cohered through the action of its magnetic moment. Therefore, the ring coupling is also called magnetic coupling. When using ring coupling, the small ring should be placed at the strongest magnetic field in the resonant cavity operating mode, and the ring surface should be adjusted to be perpendicular to the magnetic lines of force. Compared with the coupling between a rectangular waveguide and a resonant cavity, the coupling between a coaxial line and a chisel cavity can effectively reduce the size of the system, and the signal can be directly input to the data processing module without mode conversion.

(1) Structural analysis of coupling loop

Figure 3 shows the magnetic dipole. At point O at the center of the coil, the magnetic induction intensity of the magnetic dipole is:

The Hz component of the coupling loop is consistent with the Hz component of the TE011 mode mesh cylindrical resonant cavity, so this coupling method can excite the TE011 mode. The TE011 mode and the TM111 mode are degenerate wave types. The Hz component of the TM111 mode at the excitation port is zero and cannot be excited. Therefore, using this excitation method can cleverly suppress the generation of the TM111 mode. Place the coupling ring horizontally and perpendicularly to the magnetic field lines in the middle and outer side of the resonant cavity, which is where the magnetic field in the resonant cavity operating mode is strongest. According to the principle of symmetry, it can be known that the middle outside of the resonant cavity is also the strongest point of the magnetic field distribution of the coupling ring, so the maximum degree of coupling can be obtained.

(2) Determination of coaxial line size

The characteristic impedance of the coaxial line is:

In the formula: εr is the phase dielectric constant of the filling medium in the coaxial line; a is the radius of the inner conductor: b is the radius of the outer conductor.

In order to avoid transmission of higher-order modes, the operating wavelength in the coaxial line must be longer than the cut-off wavelength of the TE111 mode, that is:

4 HFSS simulation optimization

4.1 Optimal design of coupling loop

When using ring coupling, the small ring should be placed at the strongest magnetic field in the resonant cavity working mode, and the ring surface should be adjusted to be perpendicular to the magnetic lines of force. The quality of coupling has a great impact on the output signal. When the resonant cavity is in optimal coupling, the maximum output signal can be obtained. The advantages of placing the coupling ring horizontally in the middle of the outside of the resonant cavity are: 1) It meets the coupling requirements for ring coupling; 2) The impact of placement outside on the air flow in the resonant cavity can be minimized.

4.2 Optimized design of grid separator

The grid divider acts as an electrical short circuit, which can close the resonant cavity sensor and generate standing waves. The grid separator affects parameters such as fs, S11 attenuation and Q value. In order to effectively prevent electromagnetic radiation, the thickness Dg of the grid separator, the number of short-circuit rings Ng, the support material of the grid, etc. must be studied and analyzed. Theoretically, the larger Dg and grid width Wd, the denser the grid, the smaller its radiation, and the better the electromagnetic performance. Moreover, the grid separator has no surface current distribution along the radial direction, but because the resonator is separated from the grid The shape change between the devices causes field inhomogeneity, so in fact the grid will also generate radial current, so Wd cannot be too large, and the final size is subject to the best optimized value.

4.3 Optimization design of other characteristic parameters

The design is based on the best optimized values ​​of the length of the coaxial line, the inner and outer radii, the inner radius a, the outer radius b and the length l of the resonant cavity.

4.4 Simulation results and analysis

Simulation test conditions: subject to the optimized optimal value, assume the coupling ring radius b0=3 mm; the coaxial line inner radius a0=0.89 mm; b=2.65 mm; l=15.625 mm. Also assume a=20.598 mm; b=32 mm; l=41.196 mm. Let Dg=2 mm; Wd=1 mm; the number of short-circuit rings is 3, and there is no rotation angle between the grids. Figure 4 shows the performance parameter values ​​of the coaxial line coupling resonant cavity.